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Sublacustrine hydrothermal seeps and silicification of microbial bioherms in the Ediacaran Oued Dar'a caldera, Anti‐Atlas, Morocco

December 2018
Sedimentology 66(6)

December 2018
66(6)

DOI:10.1111/sed.12568

Authors:

José-Javier Alvaro

Institute of Geosciences

Laura González-Acebrón

Complutense University of Madrid

This paper presents a case study of the sublacustrine precipitation of hydrothermal silica ± TiO2 in the Ediacaran Mançour Group of the Saghro inlier, Anti‐Atlas, Morocco. Lacustrine carbonates containing stromatolitic mats and bioherms occur in ephemeral ponds developed within the Oued Da'ra caldera. Its syn‐eruptive infill consists of pyroclastites, ashflow tuffs and subsidiary lava flows and sills, whereas inter‐eruptive deposition is mainly represented by slope‐related debris‐flow breccias and landslides, alluvial fans and fluvial channels. Carbonate production took place in a mosaic of differentially subsiding, fault‐bounded intra‐caldera blocks controlled by episodic collapse‐induced drowning, pyroclastic blanketing and migration of alluvial/fluvial environments. After microbial carbonate production, the carbonates recorded several early‐diagenetic processes, punctuated by polyphase fissuring (controlling secondary permeability) locally linked to hydrothermal influx. Three generations of carbonate cements are recognisable: (i) fibrous, botryoidal and blocky/drusy mosaics of calcite; (ii) idiotopic mosaics of dolomite caused by flushing of hypersaline Mg‐rich brines; and (iii) euhedral to drusy calcite via dedolomitization. δ13C and δ18O values from carbonate cements broadly become successively isotopically lighter, as a result of meteoric and hydrothermal influence, and were probably overprint by the Panafrican‐3 phase that affected the top of the Mançour Group. Two mechanisms of silicification are involved: (i) early‐diagenetic occlusion of interparticle pores at the sediment/water interface of pyroclastic substrates and reefal core and flanks; and (ii) hydrothermal precipitation of silica ± TiO2 lining fissures and vuggy porosity encased in the host rock. Silica conduits crosscutting lacustrine mats and bioherms exhibit high potential of preservation in collapsed volcanic calderas. Primary fluid inclusions of hydrothermal silica contain brine relics with NaCl/CaCl2 ratios of 2.1 to 4.4, representing minimum entrapment temperatures of about 142 to 204°C, and abiotic hydrocarbons (heavy alkanes) related to serpentinization of the volcanic and volcanosedimentary basement of the Oued Dar'a caldera. This article is protected by copyright. All rights reserved.

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Sublacustrine hydrothermal seeps and silicification of microbial bioherms in the

Ediacaran Oued Dar’a caldera, Anti-Atlas, Morocco

J. JAVIER ÁLVARO* and LAURA GONZÁLEZ-ACEBRÓN†

* Instituto de Geociencias (CSIC-UCM), Dr. Severo Ochoa 7, 28040 Madrid, Spain (E-mail:

jj.alvaro@csic.es)

† Departamento de Estratigrafía, Facultad de Ciencias Geológicas (UCM), José Antonio

Novais 2, 28040 Madrid, Spain

Short Title – Silicification of microbial bioherms

ABSTRACT

This paper presents a case study of the sublacustrine precipitation of hydrothermal silica ±

TiO2 in the Ediacaran Mançour Group of the Saghro inlier, Anti-Atlas, Morocco. Lacustrine

carbonates containing stromatolitic mats and bioherms occur in ephemeral ponds developed

within the Oued Da’ra caldera. Its syn-eruptive infill consists of pyroclastites, ashflow tuffs

and subsidiary lava flows and sills, whereas inter-eruptive deposition is mainly represented

by slope-related debris-flow breccias and landslides, alluvial fans and fluvial channels.

Carbonate production took place in a mosaic of differentially subsiding, fault-bounded intra-

caldera blocks controlled by episodic collapse-induced drowning, pyroclastic blanketing and

migration of alluvial/fluvial environments. After microbial carbonate production, the

carbonates recorded several early-diagenetic processes, punctuated by polyphase fissuring

(controlling secondary permeability) locally linked to hydrothermal influx. Three generations

of carbonate cements are recognisable: (i) fibrous, botryoidal and blocky/drusy mosaics of

calcite; (ii) idiotopic mosaics of dolomite caused by flushing of hypersaline Mg-rich brines;

and (iii) euhedral to drusy calcite via dedolomitization. δ13C and δ18O values from carbonate

cements broadly become successively isotopically lighter, as a result of meteoric and

hydrothermal influence, and were probably overprint by the Panafrican-3 phase that affected

the top of the Mançour Group. Two mechanisms of silicification are involved: (i) early-

diagenetic occlusion of interparticle pores at the sediment/water interface of pyroclastic

substrates and reefal core and flanks; and (ii) hydrothermal precipitation of silica ± TiO2

lining fissures and vuggy porosity encased in the host rock. Silica conduits crosscutting

lacustrine mats and bioherms exhibit high potential of preservation in collapsed volcanic

calderas. Primary fluid inclusions of hydrothermal silica contain brine relics with NaCl/CaCl2

ratios of 2.1 to 4.4, representing minimum entrapment temperatures of about 142 to 204ºC,

and abiotic hydrocarbons (heavy alkanes) related to serpentinization of the volcanic and

volcanosedimentary basement of the Oued Dar’a caldera.

Keywords Caldera, fluid inclusions, Gondwana, lacustrine, microbial carbonate.

INTRODUCTION

Sublacustrine precipitation of hydrothermal silica is a common process recorded in present-

day rift lakes (e.g. Kerrich et al., 2002; Renaut et al., 2002; Balistrieri et al., 2007; Jones et

al., 2007; Wright, 2012; Della Porta, 2015; Stucker et al., 2016) but its potential of

preservation in the geological record is poor. This can be related to the onset of short-term

small-scale lacustrine depressions following explosive pyroclastic-dominant activity and

modification of hydrothermal silica undergoing diagenesis to quartz and chalcedony. In some

cases, volcanic calderas (the topographic manifestations of shallow magmatic chambers) can

preserve the conduits of sublacustrine hydrothermal seeps beneath weathering-resistant

microbial bioherms. There, silicification can induce pristine preservation of delicate

microbial textures. As a result, some Konservat-Lagerstätten controlled by hydrothermal

silicification occur in Recent and fossil volcanic landscapes despite the preservation of a

destructive combination of successive eruptive events and caldera collapses (Konhauser et

al., 2003; Smith et al., 2003; Kazmierczak & Kempe, 2006; Jones et al., 2008). The volcano-

tectonic collapse structures are also generally affected by growth faulting and epithermal

fluid circulation, where mineralization related to the transport of metal-bearing brines is

heterogeneously distributed (Stix et al., 2003; Martí et al., 2008; Todesco, 2008; Mueller et

al., 2009).

In the Anti-Atlas of Morocco (Fig. 1A), the end of the Panafrican (PA) Orogeny is

commonly associated with the onset of fracture cleavage associated with weak folding and

block faulting. The latter, which took place at about 564 to 560 Ma, has been referred to as

phase PA3 (Walsh et al., 2012; Álvaro et al., 2014a) or WACadomian phase (Hefferan et al.,

2014). Thomas et al. (2002) described several collapsed calderas associated with extensional

faulting and deposition of the Ouarzazate Supergroup in the Sirwa inlier, whereas, in the

neighbouring Saghro inlier, Harrison et al. (2008) and Walsh et al. (2012) reported the Oued

Dar’a caldera as a transtensional left-lateral pull-apart trough preceding the record of phase

PA3 (Fig. 1B).

The Ediacaran Oued Dar’a caldera displays an exceptionally well-preserved infill,

composed of pyroclastic (ashflow dominant) and epiclastic deposition and subsidiary lava

flows and sills, contemporaneous with episodic hydrothermal activity. Intramontane,

volcanic-dammed, fluvial-alluvial and lacustrine trough-like depressions locally recorded

episodes of carbonate production, which are at present exposed on the walls of gullies and

tributary systems of the Oued Dar’a. The aim of this paper is to distinguish the

hydrothermally induced silicification processes recorded in these ephemeral lacustrine ponds,

which were controlled by stepwise collapse episodes and preserved a distinctive record of

microbial textures. Two diagenetic processes will receive special attention: (i) diagenetic

evolution of carbonate host rock; and (ii) fracturing control on development of jasperoid

bodies.

GEOLOGICAL SETTING AND STRATIGRAPHY

In the Anti-Atlas, the Ouarzazate Supergroup (Thomas et al., 2002, 2004) consists of

volcanic and volcanoclastic rocks, up to 2 km thick, rich in encased subvolcanic plutons and

dykes (Fig. 2). The supergroup was deposited and intruded over approximately 65 Myr, from

about 615 to 550 Ma (Thomas et al., 2002; Gasquet et al., 2008; Walsh et al., 2012). In the

Saghro and Bou Azzer-El Graara inliers of the Anti-Atlas (Fig. 1A), the supergroup is

subdivided, from bottom to top, into: (i) the Mançour Group sensu Hawkins et al. (2001)

(labelled as ‘XIIIm’ on the 1:200,000 scale maps of Jaidi et al., 1970 and Hindermeyer et al.,

1977; and ‘NP3i’ in Walsh et al., 2012), which is represented by a volcanosedimentary

package of volcaniclastic, volcanic and granitic rocks; and (ii) the Imlas Group, largely

composed of volcaniclastic and volcanic rocks (labelled as ‘XIIIs’ and ‘NP3s’, respectively).

The Mançour Group (615 to 571 Ma), on which this paper is focused, was tilted, gently

folded and block-faulted during the final phase of Panafrican deformation (phase PA3). In the

Saghro inlier, the age of the angular discordance that marks the Mançour/Imlas contact is

about 560 Ma (Walsh et al., 2012; Álvaro et al., 2014a), but its precise age or even existence

is not well documented in other inliers.

Lithologies within the Mançour Group change over short distances both laterally and

vertically. They reflect the onset of scattered centres of volcanic activity and variations in

subaerial (alluvial and fluvial) depositional environments (Álvaro et al., 2010). Rocks of the

Mançour Group cropping out in the western Saghro inlier (1/50,000 Tizqui sheet) were

grouped by Harrison et al. (2008) and Walsh et al. (2012) in the so-called Oued Dar’a caldera

(Fig. 1B). The latter is a large (11 km x 18 km) rectangular-shaped volcanic structure,

elongated in a north-east direction and bounded by major strike-slip faults. It was filled with

trachytic, trachydacitic and rhyolitic ashflow tuffs. The preserved thickness of caldera fill

exposed along the Oued Dar’a oued and its tributaries is greater than 500 m and suggests a

minimum eruptive volume of volcanic material close to 100 km3 (Walsh et al., 2012). The

south-western margin of the caldera displays about 200 m of coarse-grained volcaniclastic

deposits and felsic tuffs and basaltic to andesitic lava flows and sills, including volumetrically

minor carbonate interbeds (Álvaro et al., 2010, 2014a; Walsh et al., 2012). Two U-Pb zircon

ages determined by SHRIMP (sensitive high-resolution ion microprobe) (Walsh et al., 2012)

are caldera fill at 574 ± 7 Ma and outflow facies at 571 ± 5 Ma (Fig. 1B).

The infill of the Oued Dar’a caldera, represented by the Mançour Group, is commonly

punctuated by unconformable contacts. These irregular surfaces represent palaeoreliefs

locally associated with syn-sedimentary faults and abrupt changes in thickness and facies.

These unconformity-associated strata are dominated by epiclastic sediments and, locally, by

carbonates. A decametre-scale lacustrine deposit rich in stromatolites, named Amane-

n’Tourhart (referred to as Amane Tazgart in Harrison et al., 2008) and traversed by the

Ouarzazate-Agdz road (at kilometre 464), represents a traditional stops for geological

fieldtrips (Choubert & Faure-Muret, 1970; Raaben, 1980; Devaere et al., 2014) and was

mapped as caldera fill moat deposits (labelled as ‘NP3ics’) capping an aphanitic andesite by

Harrison et al. (2008). The lacustrine stromatolites of this trough were taxonomically

assigned to Collenia by Choubert et al. (1952) and Harrison et al. (2008). The parataxon

Conophyton amantourartensis was defined by Raaben (1980).

Less known are other carbonate lenses, sandwiched between unconformable contacts,

which punctuate the ashflow tuff-dominated infill of the Oued Dar’a caldera. The facies and

geometries of two of them (OD1 and OD2 in Fig. 1B) are described below for the first time

in the following section (for a detailed facies description and sedimentological interpretation

of the Amane-n’Tourhart lacustrine deposits, see Álvaro et al., 2010).

MATERIALS AND METHODS

Sixty samples were collected from exposures along a north-east/south-west transect of the

Oued Dar’a caldera, for thin section and polished slab preparation and petrographic analysis

by standard optical microscopy. Sibley & Gregg (1987) were used for petrographic

descriptions of dolomite crystallinity and Le Bas & Streckeisen (1991) for igneous rocks.

Thin sections were partly stained with alizarin red-S and potassium ferricyanide in order to

facilitate the identification of ferroan and non-ferroan calcite, as well as dolomite phases.

Diagenetic minerals were studied by cathodoluminescence (CL), scanning electron

microscopy (SEM) and electron microprobe at IGME-Tres Cantos and the National Museum

of Natural Sciences (MNCN)-Madrid. The SEM-CL measurements were carried out using a

Hitachi S-2500 scanning electron microscope in the emissive and CL modes at 77 k (Hitachi

Limited, Tokyo, Japan). A liquid nitrogen cooled EO-817 germanium detector (North Coast

Scientific, Santa Rosa, CA, USA) was used for signal detection (details of the experimental

setup for spectral and panchromatic CL measurements are detailed in Méndez & Piqueras,

(1991). The SEM-CL textures of the hydrothermal quartz mosaics reveal otherwise

unobservable stepwise processes, such as precipitation-dissolution textures and

microfractures. These can be used to determine the relative timing of multiple generations of

quartz and coexisting fluid inclusions (Rusk, 2012).

The SEM-CL textures in hydrothermal quartz were described using nomenclature

after Götze et al. (2001) and Götze (2012). Cement phases were characterized based on

crystal size and shape, CL colour and brightness and cross-cutting relationships; SEM-CL

allowed discrimination of diagenetic quartz and cold-CL of carbonate cements; SEM and

electron microprobe provided backscattered electron (BSE) images from which diagenetic

parageneses were interpreted. Subsidiary minerals were identified by X-ray diffraction

(XRD).

Fluid inclusion microthermometry was used to characterize fluid composition and

entrapment temperature of hydrothermal quartz. These petrographic studies of fluid

inclusions on doubly polished thin sections of quartz were performed using a Linkam stage

(Linkam Scientific, Tadworth, UK) fitted on a binocular Olympus BX51 microscope

(Olympus, Tokyo, Japan) at the Complutense University of Madrid. Fluid inclusion

assemblage (FIA) approach of Goldstein & Reynolds (1994) was followed. The

microthermometric study was performed in a Linkam THMSG-600 heating and freezing

stage (Linkam Scientific). The stage was calibrated with synthetic fluid inclusions, including

triple point of CO2, melting point of H2O and critical point of H2O. In order to facilitate the

observations of the eutectic point and ice or hydrohalite melting, a cycling procedure was

applied following the methodology of Shepherd et al. (1985). Salinities were calculated using

the Steele-MacInnis et al. (2011) method. In addition, the non-intrusive and non-destructive

Raman spectroscopy technique (confocal Raman microscopy, Thermo Fisher DXR

spectrograph (Thermo Fisher Scientific, Waltham, MA, USA) of the Museo Nacional de

Ciencias Naturales (MNCN), Madrid was used to characterize fluid inclusions in quartz

crystals (see methodology in Frezzotti et al., 2012). The light at 532 nm of a frequency

doubled Nd: YVO4 DPSS solid laser (maximum power 30mW) was used for excitation. To

improve the spectral assignments of ten analyses, the deconvolution of the collected Raman

spectra was performed by using computational chemistry software Gaussian-16/2017

(Gaussian, Inc., 2017). Voigt functions have been used for the fitting procedure because this

function corresponds to the convolution of the phonon mode (Lorentzian profile) with the

resolution of the optical setup (Gaussian profile). The choice was made on the basis of sound

foundation, higher accuracy and reproducibility of the fit (see Zanatta & Ferri, 2007; Catelani

et al., 2014).

Twenty-six carbon and oxygen isotope analyses were carried out on carbonate

cements, which were removed by dental drill under a binocular microscope and analysed at

the Erlangen University. Carbonate powders were reacted with 100 % phosphoric acid at

75ºC using a Kiel III carbonate preparation line connected to a Thermo-Finnigan 252 mass

spectrometer (Thermo Fisher Scientific). All values are reported in ‰ relative to Vienna

PeeDee Belemnite (VPDB) by assigning a δ13C value of +1.95‰ and a δ18O value of –2.20‰

to NBS19; reproducibility was checked by replicate analyses of laboratory standards and it

was better than 0.01 to 0.02.

SEDIMENTARY GEOMETRIES AND FACIES

The Oued Dar’a caldera displays a complex emplacement history with respect to mapped

dome-flow complexes, dykes–sills and subvolcanic intrusions (Harrison et al., 2008; Walsh

et al., 2012). The infill of the caldera is dominated by thick stratified hyaloclastites (tuff,

lapilli tuff and lapilli tuff breccia) and subsidiary andesitic lava flows and breccias, which

represent episodes of high-rate effusive volcanism (Álvaro et al., 2010; Walsh et al., 2012).

Caldera collapse involved successive movements of numerous fault blocks. Collapse and

magmatic plumbing were deeply influenced by pre-tectonic and syn-tectonic faults, trending

dominantly north-east/south-west. Intra-caldera depocentres related to normal faulting during

collapse produced decametre to hectometre-scale grabens and half-grabens, which coincide

with the thicker parts of the volcano-sedimentary infill. Proximal coarse-grained

volcaniclastic tongues and aprons mark the change from predominantly primary volcanic

(near-vent volcanic rocks) to sedimentary (epiclastic) bodies, the latter mainly infilling the

depocentre of collapsed trough-like depressions. Three representative facies associations are

described in a proximal-distal transect from syn-sedimentary normal faults (Fig. 1B).

Debris-flow breccias and landslides

The deposits fringing the intra-caldera troughs consist of silicified chaotic breccias and

matrix-supported conglomerates (Fig. 3A and B). These include massive to weakly stratified,

unsorted to moderately sorted, pebble-dominated deposits of quartz and acidic volcanic rocks

and lesser amounts of feldspar and iron-oxide clasts and crusts. Contacts are scoured and

irregular, and grading is both normal and inverse. Breccias and conglomerates exhibit either

chaotic or amalgamated patterns and are interpreted as having undergone different types of

downslope transport by basal sliding of plastic to semi-rigid sediment masses (Fig. 3C);

volcaniclastic sediments were poorly consolidated, resulting in their rapid erosion and

redeposition. Amalgamation of debris-flow deposits suggests the presence of significant

slopes close to syn-sedimentary normal faults.

Alluvial fans and rivers

Debris-flow breccias and landslides commonly interfinger with packages of conglomerates

and pebbly sandstones that pinch out or change laterally abruptly. Conglomerates are both

channelized and infilling decametre-scale grabens and half-grabens (Fig. 3D). They show

massive stratification patterns grading upsection into cross-stratification. Clast-supported

conglomerates show massive to weakly imbricated pebbles and cobbles, which grade

upsection into litharenites interbedded with silicified oncoid packstones and microbial crusts,

the latter punctuated with mudcracks. The conglomerates and litharenites are mostly

volcanogenic, poorly sorted and commonly consist of subrounded pebbles and cobbles. The

uppermost part of these fining-upward packages is dominated by thin-bedded to medium-

bedded, oncoid-bearing laminae alternating with quartz-dominated sandy layers, which form

trough and planar, cross-laminated sets pinching out laterally in low-angle laminated layers

(Fig. 3E and F). The oncoids range in size from 1 to 3 cm; the core may be a quartz grain or

pyroclastic pebble and their well-laminated cortex displays a rough to crinkled surface,

completely silicified except for some carbonate relics.

The conglomerates were deposited as longitudinal in-channel gravel bars that graded

into straight to sinuous crested bedforms in shallow, semi-permanent braided systems.

Subsidiary carbonate production took place during episodes of abandonment or local

isolation of some stretches of active channels, which resulted in temporary isolation from

terrigenous input. During high-discharge events, sediments were reworked and accumulated

as channelized conglomerates. Dammed fluvial channels and floodplain ponds were filled

with sandy lenses capped by oncoids and microbial mats, which then became desiccated.

Lacustrine stromatolitic bioherms

A carbonate exposure at the top of a hill (OD2 in Fig. 1A) is underlain by ca 10 m of

greenish shales and purple pyroclastites, and rests on angular discordance on another

variegated package of pyroclastites (Fig. 3G). The carbonate lens, up to 1.2 m thick, is plano-

convex and up to 5.6 m in basal diameter. The build-up (core and flank; Fig. 3H and I) is

fractured and crosscut by several generations of syn-sedimentary faults and fissures, as a

result of which, some flanks occur as vertical strata fringing the subhorizontal build-up. Both

the build-up and the related fracture network are sealed by overlying (and onlapping)

pyroclastites (Fig. 3J and K).

The base of the build-up consists of a calcite-cemented pyroclastic conglomerate,

unsorted and up 10 cm thick. The overlying core is composed of stacked stromatolites; they

grew initially horizontally (tabular stromatolites) and became upsection domal, with

equidimensional to asymmetric structures. The top of the core contains scattered oncoids and

a massive fenestral limestone. In some contacts with the fringing flanks, the stromatolites

display contorted and overturned arrangements (Fig. 3I). The stromatolites consist of

submillimetric laminae of microsparry and sparry alternations. They typically contain

subsidiary fine-grained ash at episodic intervals, indicating the occurrence of episodic

explosive eruptions during deposition of microbial mats. The flank bedding surrounding the

core ranges from decimetre to centimetre-thick alternating layers of poorly sorted and well-

rounded intraclasts, oncoids and stromatoclasts interfingering with siltstone interbeds, which

dip up to 30º. Thicker slope beds appear to be preferentially located on the western side of the

build-up due to the onset of fractures fringing the core. Both the core and flanks are affected

by penecontemporaneous normal faults, responsible for tilting of laminated flanks, which

occur as vertical strata surrounding the core (Fig. 3J and K). The irregular top of the fractured

bioherm is onlapped by pyroclastites, sealing the bioherm and its fracture network.

The basal conglomeratic calcarenite served as the foundation for build-up growth.

Pyroclastic silty sedimentation occurred laterally to the build-up during its growth. The sharp

contact of the build-up with surrounding sediments, the medium-angle nature of the flanks

and, especially, the presence of basal conglomerates are indicative of early-diagenetic

consolidation of the carbonate build-up. The presence of scattered, metre-scale to hectometre-

scale carbonate deposits displaying frame-building fabrics, either bounded by lava flows

(Álvaro et al., 2010) or normal faults (this study), suggests that local ponds and lakes

developed in the topographic depressions remaining after intra-caldera tilting.

Caldera inter-eruptive environments

The Oued Da’ra caldera was a steep-walled, relatively flat-floored, closed trough that rapidly

filled with volcaniclastic sediments (Walsh et al., 2012). Caldera-filling processes induced

early-mass wasting from unstable caldera walls and intra-caldera syn-sedimentary faults, as

documented by breccia and conglomeratic debris-flow deposits intertongued with

pyroclastites and subsidiary emplacement of lava flows. Episodic caldera collapses generated

a complex framework of overlapping and tilted collapse structures, stratigraphically marked

by the onset of angular discordances (Fig. 3G) and/or erosive unconformities. The Oued

Dar’a caldera, as a closed intramontane basin, became a natural trap for various depositional

events, including contemporaneous ashflow and pyroclastic eruptions, concurrent with

landslides from overstepping sedimentary wedges flanking caldera walls. Effusion and multi-

step collapse structures were interspersed by short episodes of quiescence, marked by

carbonate deposition in scattered ponds.

Caldera collapse involved successive movements of numerous fault blocks, trending

north-east/south-west and subsidiary east–west. Intra-caldera depocentres developed

decametre-scale to hectometre-scale grabens and half-grabens. The changes in drainage and

sedimentation, as well as the development of debris-avalanche breccias, reflect tectonic

faulting during inter-eruptive episodes in an elevated mountainous terrain formed by

differential uplift and tilting of blocks.

During high-rate explosive volcanism (syn-eruptive episodes described in Álvaro et

al., 2010; Walsh et al., 2012), the volcaniclastic accumulations were sourced from scattered

volcanic centres that were progressively surrounded by volcaniclastic aprons and tongues,

deposited close to the volcanic centres. These deposits, composed of clast-rich and matrix-

supported debris-flow breccias and conglomerates, filled the pre-existing lower topographic

areas and depressions. The volcaniclastic apron deposits prograded toward the axes of fault-

bounded troughs onto older sediments and passed laterally into alluvial plain deposits, locally

fringed by fluvial-lacustrine settings. The latter indicate short-lived ponds or small, transient

and shallow lakes suggesting the onset of low relief and/or subdued palaeotopography.

When calderas, such as the Oued Dar’a case study, subside in a stepwise manner the

subsidence is characterized by horst and (half)graben structures. Fringing faults served as

conduits that facilitated both eruption of pyroclastic material and circulation of hydrothermal

fluids, causing significant permeability enhancement. Hydrothermal circulation has the

potential to produce more rapid and complete diagenetic cementation than progressive

compaction, by accelerating the successive dissolution and precipitation of diagenetic

minerals (Balistrieri et al., 2007; Jones et al., 2007). Enhanced chertification along fractures

and locally massive cementation of extensional fractures and fissures are evidence for brittle

deformation and pathways for fluid transport. Chert bodies of the Oued Dar’a caldera

adjacent to feeding fractures are irregular shaped and distributed in patches along fault traces

(Fig. 4A and B). These epithermal bodies commonly extend away from fissure zones,

following suitable carbonate layers, and resulting in stratabound-like chert bodies. Metal

sulphides (such as pyrite, galena, sphalerite and orpiment/realgar) and oxides (cuprite) occur

as reworked clasts, less than 1 mm in size, reflecting high-temperature hydrothermal

conditions in neighbouring areas of the studied Oued Dar’a caldera ponds.

EARLY DIAGENETIC AND HYDROTHERMAL CEMENTS

Based on the overall textural relationships of the cements identified in the above-reported

carbonate sediments, a reconstructed paragenetic sequence of the Oued Dar’a carbonate-

bearing troughs (including the neighbouring Amane-n’Tourhart lacustrine pond) can be

recognized (for a description of regional mineralization, see Harrison et al., 2008).

The paragenetic sequence reflects the onset of six major stages predating late-

diagenetic stylolitic emplacement. The relative timing of each stage and influence on porosity

evolution is summarized in Table 1. These include different stages of aggrading

neomorphism, calcite cementation, dolomitization and silicification.

 The first porosity-occluding phases represent precipitation of either silica or calcite on

pyroclastic vs carbonate substrates. On pyroclastic substrates, oncoidal packstones and

reefal core stromatolites, interparticle and interlaminae pores are partially filled with

cryptocrystalline to poikilotopic, equant to drusy mosaics of microquartz (si1; Fig. 5A to

C) that are blueish in CL. In vesiculated pyroclastites (scoria) and reefal flank interparticle

pores, primary porosity was partially occluded with nonluminescent fibrous to botryoidal

calcite (ca1; Fig. 5D). Both pyroclastic and carbonate substrates display a similar bright

orange-to-yellow luminescent, calcite cement (ca2), which developed as poikilitopic

(subeuhedral, blocky and drusy) mosaics filling interparticle and intercrystalline pore

spaces (Fig. 6A to D). Euhedral rhombs and idiotopic mosaics of dolomite, with bright to

dull red luminescence (do1), occur crosscutting mosaics of si1, ca1 and ca2 cements (Figs

5E to G and 6E).

 Subsequent porosity was induced by fracturing and related dissolution leading to the

formation of fissure porosity. The latter was occluded by calcite cements (ca3), exhibiting

bright orange to yellow luminescent. The crystals exhibit euhedral to drusy shapes and

occur lining fissuring walls and associated vugs. Precipitation of calcite and

dedolomitization were the by-product of surficial weathering and input of calcium-rich

fluids throughout tectonically enhanced permeability framework.

 Pulsed, repeated episodes of fissuring accompanied by input of hydrothermal fluids led to,

at least, two major episodes:

(i) Pulses of fissuring (Fig. 4A and B) are marked by the occurrence of silica (si2) ± TiO2,

the former as microquartz and macroquartz of (radial) fibrous and drusy mosaics and

concentric botryoids of chalcedony (Figs 5H 5I, 6F and 7A). The fronts of sharp

silicification are commonly accentuated by concentration of less soluble material (Fig.

7B), such as iron oxyhydroxides, clay stringers, or both. In some cases, distinction

between pore space infill (silicification) and complete replacement of host rock

(chertification) is ambiguous.

(ii) Marking the outlines of fissures and faults, and associated with the occurrence of si2,

scattered zeolite ± chlorite ± pyrite assemblages occur replacing previous minerals as

nonluminescent flakes and euhedral crystals. Clinoptilolite (Ca-Na zeolite) and

gismondine (Ca-zeolite) are estimated from XRD to amounts up to 5% of some

samples.

The paragenetic sequence reported above points to overprinting of several diagenetic and

hydrothermal processes. During or shortly after carbonate deposition, while the sediment was

still in an active phreatic zone, calcite cementation and presumably neomorphism resulted in

the formation of sparry calcite occluding primary (interparticle and intercrystalline) porosity.

Progressive depletion of Ca led to flushing of hypersaline Mg-rich brines resulting in the

supersaturation with dolomite. This led to partial replacement of calcite, subsequent increase

of porosity via dolomitization, and local obliteration of primary textures in microbial

carbonates. The increase in CL brightness exhibited, at least, by two generations of calcite

cements (ca1 to ca2; Fig. 6A to D) suggests a progressive increase in luminescence activators

in pore fluids, reaching peaks in MgO and MnO. This geochemical trend may be related to a

change from relatively oxidizing meteoric ground waters to burial (more reducing) pore

waters (Machel, 1985). Calcite (ca1), iron-stained jasper (si2) and accompanying minerals are

structurally related to branching fissures (Fig. 4A and B), which cut through the footwall and

up to the onlapping contact with overlying pyroclastites, but not above. This and the syn-

sedimentary reworking of silicified clasts (mainly stromatoclasts) suggest the

contemporaneity of hydrothermal activity at the end of stepwise collapse events. The extent

to which the rock adjacent to fissures is replaced reflects the modified permeability of the

host rock. Hydrothermal conduits for metal-poor, silica-bearing solutions are also identified

in the field by associated alteration patterns, which include zeolitization (for example, via

feldspar alteration) and chloritization.

The silica was therefore derived from two distinct sources: (i) early silicification (si1)

essentially developed at the sediment/water interface of volcanosedimentary substrates and

reefal flank interparticle pores by interaction of water that saturated with silica, as a result of

dissolution of volcanic glass and dewatering that promoted direct silica precipitation; and (ii)

late-stage silicification (si2) involving circulation of hydrothermal fluids from siliceous

sinters beneath the carbonate mats and bioherms that subsequently precipitated as fracture-

filling silica (silica veins) and in porous rocks of the discharge zone. Repeated movement

along fissures generated structure zones of high permeability close to syn-sedimentary faults

and landslide breccia aprons. Sulphide deposits are negligible in the study transect, although

As, Zn and Cu sulphide-bearing and oxide-bearing clasts (<1 mm across) occur as scattered

grains sourced from neighbouring vents.

GEOCHEMISTRY OF CARBONATES

Two geochemical datasets are considered below, including isotope and rare-earth element

(REE) compositions.

Isotope composition

Oxygen and carbon isotope analyses have been performed on carbonate samples

(stromatolites and the calcite and dolomite cements described above) from the Amane-

n’Tourhart lacustrine pond (Table 2). Lacustrine stromatolites and cements ca1, ca2, do1 and

ca3 yielded δ18O ratios between –2.1 to –3.2‰; –2.7 to –4.0‰; –3.9 to –4.9 ‰ and –3.8 to –

4.2‰; and δ13C ratios between –9.3 to –12.8‰; –11.5 to –12.3‰; –12.2 to –14.0‰ and –

11.1 to ––14.0‰, respectively (Fig. 8). A distinct trend is displayed from stromatolitic mats

to successive calcite and dolomite cements becoming progressively depleted in both δ13C and

δ18O. Cement ratios are distinct indicating that they precipitated from different fluids at

different temperatures. This is consistent with the inferred late origin of dolomite and may

reflect the influence of both meteoric waters and elevated temperatures of precipitation

(leading to lower δ18O values).

During Neoproterozoic times, the oxygen-isotope composition of sea water ranged

between –4‰ and –10‰ (Jacobsen & Kaufman, 1999) dropping to its lowest point in the late

Cambrian Age 2 to Furongian of –10 to –14‰ (Veizer et al., 1999). As a result, the

comparative δ18O depletion of sediments and cements from the Ediacaran Amane-n’Tourhart

case study could indicate an influence of either surface meteoric waters or hydrothermal

fluids, or both.

A comparison with lower Cryogenian and Terreneuvian carbonates of the Anti-Atlas

may yield some environmental interpretations; three Proterozoic episodes of carbonate

productivity are recorded in the Anti-Atlas, related to: (i) the Tonian(?)–early Cryogenian rift

that fringed the northern West African craton (Taghdout Group; Álvaro et al., 2014b); (ii) the

lacustrine episodes recorded in hectometre-scale ponds scattered in the Ediacaran Ouarzazate

Supergroup, which represents a voluminous pile of volcaniclastic and volcanic rocks related

to the onset of the PA3 orogeny; and (iii) the Terreneuvian Adoudou Formation, which

represents onlapping of an inherited Panafrican palaeorelief (Tabia Member) and

development of a peritidal-dominated carbonate platform (Tifnout Member; Álvaro et al.,

2014a) (Fig. 2). At the stratotype of the Taghdout Group, the δ13C background values of

carbonate interbeds range from –2‰ to +1‰, with a negative shift from background

conditions of 4 to 6‰, whereas the fluctuations of δ18O values range from –8.2 to –18.6‰. In

the Adoudou Formation, δ13C values vary between –4‰ and 0‰ in the Tabia Member, and

between –6 to +7‰ in the Tifnout Member, except some anomalous data from the Tagrara

inlier (Maloof et al., 2005; Álvaro et al., 2008). Hydrothermally affected carbonates of the

Tabia Member display δ13C values from –0.2 to –3.7‰, whereas their karstic infill

(speleothems) range from –8.2 to –8.6‰ (Álvaro, 2013). The Adoudou Formation displays

mean values of δ18O close to –6/–7‰, although some isolated data from the Tabia Member at

the Tagrara inlier is as low as –14‰ reflecting influence by hydrothermal alteration (Maloof

et al., 2005). Despite the depleted character of 18O, the wide variability of δ18O values and

weak covariance between δ13C and δ18O values, the carbonates of the Taghdout Group

recorded similar diagenetic behaviours and probably decreased oxygen isotopic values related

to temperature increase during burial. As a result, high-temperature leaching processes and

geothermally recirculated subsurface fluids lowered the 18O/16O isotope ratio at Amane-

n’Tourhart, in a similar way to the isotope ratios analysed in pre-Panafrican carbonate strata

(Fig. 7).

Excluding isotopic data from karstic speleothems from the Tabia Member, all the

Neoproterozoic–Terreneuvian data plot in a continuous domain of the δ13C/δ18O field, with

partial overlapping areas from higher values of post-Panafrican (Adoudou Group) to lower

values of pre- and syn-Panafrican (Taghdout Group and Mançour lacustrine carbonates). This

reflects a possible overprint of the isotopic signal by burial trend and the influence of

hydrothermal processes related to the onset of the Panafrican Orogeny.

Rare-earth element composition

Rare-earth element (REE) analyses of partly silicified, undolomitized and dolomitized

stromatolites from the Amane-n’Tourhart pond are found in Álvaro et al. (2010). These

analyses show low REE abundances, a slight negative Eu anomaly and a distinctive

enrichment in light rare-earth elements (LREE). Rare-earth element concentrations: (i)

document a lack of distinct sea water signatures; (ii) mimic the REE patterns displayed by the

rhyolitic tuffs that onlap the lacustrine infill; and (iii) show concentrations one to two orders

of magnitude greater than modern lacustrine stromatolites. As a result, the REE

concentrations in lacustrine carbonates of Amane-n'Tourhart reflect a strong contribution of

REEs from the surrounding pyroclastites which encase this carbonate body.

GEOCHEMISTRY OF HYDROTHERMAL QUARTZ

The characterization of hydrothermal quartz is made by linking SEM-CL textures to CL

spectroscopy, and analysing fluid inclusions with microthermometry and Raman spectra.

Scanning electron microscope – cathodoluminescence textures

Variations in the intensity of luminescence of quartz, caused by crystallographic

imperfections and cyclic trace element importation during silica precipitation, in response to

chemical gradients between the quartz and the hydrothermal fluid (Penniston-Dorland, 2001;

Frelinger et al., 2015), allow identification of: (i) concentric growth zoning of individual

crystals, less than 30 µm in thickness, revealing euhedral crystal shapes (Fig. 9A); (ii)

truncation of growth zones by regions of unzoned (homogeneous), CL-dull crystals, likely

due to recrystallization and affecting crystal size (mainly reflecting drusy textures) and

orientation (Fig. 9A to C); (iii) dissolution contacts modifying well-zoned quartz cores (Fig.

9B and D); and (iv) polyphase fissuring affecting zoned quartz crystals (Fig. 9C). Textural

relationships under SEM-CL indicate that these quartz mosaics (Fig. 9E) were precipitated

into open space and later, episodically fractured, locally dissolved and reprecipitated in

fracture/open spaces.

Cathodoluminescence spectroscopy, illustrated by their emission spectra, was made as

a test to corroborate the identification of several generations of quartz, controlled by changes

in thermodynamic conditions during precipitation (Götze et al., 2001). Two distinct quartz

generations were targeted (with six analyses per generation) to characterize the drusic

textures of vuggy occlusion: (i) CL emission of quartz crystals close to the host walls

displays a main shift centred at 415 nm, and lower intensities at 310 nm, 540 nm and 655 nm;

and (ii) quartz crystals occluding the centre of vug pores show a main peak at 478 to 505 nm

and a weak transition at 438 to 446 nm (Fig. 9F). These emission bands reflect several

anomalies, such as oxygen-deficient centres (290 to 310 nm), intrinsic emissions and carbon

impurities (415 to 420 nm), intrinsic defects and extrinsic impurities of silica (450 nm),

extrinsic emission by Al substitutions (500 nm) and oxygen vacancy and lattice defects (620

to 655 nm; Götze et al., 2001; Götze, 2012). Bands at 490 nm and 510 to 520 nm can also be

strengthened by incorporation of TiO2 (Devilliers & Mahé, 2010; Mikhailov & Yuryev,

2014), described above as a distinct accessory mineral in these hydrothermal precipitates

(Fig. 7B).

Microthermometric analysis

Drusic quartz mosaics encased in a stromatolitic carbonate host rock of Amane-n’Tourahrt

(Figs 5H and 9E) contain quartz crystals, 0.1 to 1.2 mm in size, displaying distinct concentric

growth zoning due to the presence of primary fluid inclusions (FIs). Several growth stages

can be observed even under transmitted light; primary FIs are oriented in the direction of

crystal growth and parallel to growth direction yielding a cloudy aspect (Fig. 10A); these are

commonly cut by narrow bands of clean quartz showing deformation patterns (Fig. 10B).

Rutile appears inside some quartz crystals (Fig. 10C).

Primary FIs are biphasic at room temperature with liquid:vapour ratios ranging from

95:5 to 85:15. They show irregular outlines and variable sizes, up to 30 µm. Some FIs present

unidentified birefringent solid phases of platy aspect (FI label with ‘b’ in Fig. 10C), which do

not change during heating and freezing runs. Scarce, very-rich gas inclusions (<10%) were

also identified.

During freezing, FIs darken suddenly and the bubble disappears at temperatures

between –62 to –88ºC, showing pale brownish colours (Tn in Table 3). Fluid inclusions failed

to totally freeze going down to liquid nitrogen temperature. This observation has been

checked allowing a single large ice crystal to grow. During reheating, the first melting of ice

is observed from –66 to –64ºC (Tfm(ice)). Final melting of ice temperatures occurs between –

27.8 to –20.8ºC (Tm(ice)). In some FIs, a clear bubble movement has been observed at

temperatures between 21ºC and 28.3ºC (Tm(halite?)). The temperature at which this movement

occurs sometimes varies from one freezing and heating run to another in the same fluid

inclusion. Neither hydrohalite nor clathrates are observed during freezing and heating runs.

Fluid inclusion homogenization occurs to the liquid at temperatures between 142ºC and

204°C (Th). Maximum and minimum Th in each fluid inclusion association (FIA) is

comprised in an interval of 19°C in FIA 1 and 33°C in FIA 2.

Fluid inclusions are interpreted as the result of homogeneous entrapment at high

temperatures. The variation of Th inside a single FIA (19°C for FIA 1 and 33°C for FIA 2) is

greater but not too far from what is consider as consistent and thus reliable for minimum

entrapment temperatures (up to 15°C; Goldstein & Reynolds, 1994). In absence of pressure

estimates, Th can be interpreted as minimum entrapment temperature of 142 to 204°C,

providing a minimum temperature for quartz crystallization. This palaeotemperature fits with

similar low-temperature synthesis of rutile (Jia, 2011; Chia Yan et al., 2014).

The scarce, very rich-gas fluid inclusions were probably formed by necking down

after bubble nucleation at high temperature (Goldstein & Reynolds, 1994) and have been

avoided for microthermometric purposes. The unidentified solid phases (Fig. 9C) are

interpreted as accidental minerals, based on the Goldstein & Reynolds (1994) criteria.

The FI darkening and the brownish colour after freezing point to a NaCl–CaCl2-

bearing fluid (Shepherd et al., 1985). The low first melting temperatures (from –66 to –64ºC)

are indicative of the presence of bivalent cations. They are close to the eutectic temperature

of a H2O–NaCl–CaCl2 system (–52ºC; Davis et al., 1990). Metastable behaviour is very

common in FIs in the H2O–NaCl–CaCl2 system (Davis et al., 1990; Samson & Walker, 2000;

Bodnar, 2003). The metastability explains the lower eutectic temperature, the failure in total

freezing and the bubble movement at positive temperatures (21.0 to 28.3ºC), which is

probably related to a metastable melting of halite. The low Tm(ice) are indicative of high

saline fluids, with calculated salinities of 27to 28 wt%, being NaCl 19 to 22 wt% and CaCl2 5

to 9 wt%. The high content in Ca should be related to leaching of the carbonate host rock.

Raman spectral analysis

Raman spectral analysis made in ten fluid inclusions displays a congestion of vibrational

modes in the region of C-H stretching vibrations, between 2800 cm-1 and 3100 cm-1, which

complicates spectral assignment. The calculated harmonic vibrational frequencies and the

corresponding Raman transition intensities are plotted in Fig. 11 (six Voigt functions have

been used for the fitting procedure of one composite Raman spectrum; see Repository Data).

Individual peaks are recognized at 3034 cm-1, 2882 cm-1, 2867 cm-1, 2856 cm-1 (maximum

shift), 2848 cm-1 and 2835 cm-1, all of them related to C-H stretching vibrations of CH3, CH2

and CH groups at room temperature and pressure (e.g. Qiao & Zheng, 2005; Yu et al., 2007;

Shi et al., 2008; Kar et al. 2009; Behler et al., 2013; Hoshina et al., 2015).

Inclusions including hydrocarbons should be related to three possible sources for

crustal and upper mantle rocks: (i) biological production; (ii) thermal cracking of higher

hydrocarbons; and (iii) inorganic (Fisher-Tropsch) synthesis (Sherwood Lollar et al., 2002,

2006, 2008; Potter et al., 2004; Proskurowski et al., 2008; Konn et al. 2009). In the Amane-n

Tourhart trough of the Oued Dar’a caldera, abiotic hydrocarbons (heavy alkanes) were

probably issued from serpentenized mafic rocks and mainly related to water–rock interactions

including low-temperature olivine hydrolysis (Fruh-Green et al., 2004; Sachan et al., 2007;

Sherwood Lollar et al., 2008; McCollom & Bach, 2009; Etiope et al., 2011; Neubeck et al.,

2011) affecting the underlying volcanosedimentary Mançour Group.

DISCUSSION

In the Oued Dar’a caldera, carbonates accumulated on a mosaic of differentially subsiding,

fault-bounded intra-caldera blocks. Contemporaneous deformation implies that local

topography was controlled by displacements on nearby faults. Landslides from the steep

unstable caldera walls accompanied by alluvial/fluvial sedimentation from high gradient

streams led to local development of small catchment areas that flowed between the caldera

rims and lakes that were filled with carbonates. Hydrodynamic conditions were in part

determined by surrounding structurally controlled topographies. Build-ups initiated in

shallow lacustrine settings, some bounded by faults on localized half-grabens and grabens of

sheltered caldera lakes (this paper) and others on ponds fringed by lava flows that acted as

physical barriers (Harrison et al., 2008; Álvaro et al., 2010). Their growth sharply ceased in

response to either collapse-induced drowning or pyroclastic flooding.

Related volcanic and geothermal activities were primary factors that controlled the

evolution of these carbonate-bearing ponds. The Mançour volcanosedimentary complex that

forms the lower part of the Oued Dar’a caldera dominated the subaerial landscape that

formed the basement that recorded alluvial, fluvial and lacustrine troughs and ponds.

Chemical weathering of this volcanic landscape provided the solutes that flowed through the

drainage basin. The dominant weathering process was silicate hydrolysis, during which

feldspars, volcanic glass and mafic minerals react with water charged with CO2 and carbonic

acid (H2CO3). Ions produced during silicate weathering (dominated by Na+, K+, Ca2+ and

Mg2+ and SiO2 as monosilicic acid: H4SiO4) dissolve in dilute surface waters and some

percolate downwards to the water table and groundwater trough permeable faulted rocks and

sediments (Schagerl & Renaut, 2016). As a result, new minerals were produced on site, such

as clay minerals (illite and chlorite), zeolites (clinoptilolite and gismondine) and earliest

diagenetic silica precipitation on pyroclastic substrates (si1 in Table 1).

The dilute waters that flowed through the reported alluvial-fluvial (pond OD1) and

lacustrine (ponds OD2 and Amane-n’Tourhart; Fig. 1B) depressions acquired alkalinity

through the hydrolysis of volcaniclastic sediment. The Ca/HCO3 ratio is key to understand

the geochemical precipitates in dilute waters: waters in which Ca < HCO3 will effectively

lose all calcium as calcite, both microbially induced (as stromatolites and microbial mats) and

precipitated occluding interparticle pores and vesiculated scoria (ca1 and ca2). After

elimination of Ca, Mg (abundant as by-product of hydrolysis of basic volcanics) came into

play, precipitating as dolomite (cement do1). Polyphase fissuring recorded in the collapsed

caldera led to modifications in the Ca/Mg ratio, leading to dedolomitization processes and

precipitation of secondary calcite (ca3) lining fissuring walls.

Another role of volcanism is its contribution to geothermal activity by fluids

discharged as hot springs, which can provide most of the annual recharge (Renaut et al.,

2013). Most of these spring fluids originated from surface waters that have permeated to

different depths along faults and fractures. Those penetrating to great depths became hot and

rich in SiO2 (si2) and TiO2. Fluid inclusions hosted in chert allow identification of both saline

brines with NaCl/CaCl2 ratios of 2.1 to 4.4 (microthermometric analysis) and abiotic

hydrocarbons related to low-temperature olivine hydrolysis (Raman spectral analysis).

The calderas and alkaline lakes of the East African Rift may be considered as Recent

analogues of the Oued Dar’a caldera due to their development on a volcanosedimentary

basement affected by silicate hydrolysis under subtropical conditions (Álvaro et al., 2000;

Deocampo & Jones 2014; Deocampo & Renaut, 2016). Soda lakes and shallow-dilute lakes

coexist in the same geographical area and can be classified as to whether they are

hydrologically open or closed. In the former, inflow from the surrounding drainage basin and

precipitation are balanced by evaporation and outflow (Deocampo, 2002; Renaut et al.,

2013). As a result, the lakes comprise relatively stable shorelines and water chemistry,

avoiding shifts toward increase salinity or alkalinity. In contrast, closed lakes have no regular

outlets and their base-level and chemistry are highly variable.

Carbonate precipitates occur around open lake margins, as cement in littoral sands,

such as in lakes Baringo, Eyasi, Naivasha and Turkana (Kenya and Tanzania; Finney &

Johnson, 1991; Deocampo, 2002, 2010). Microbialites occur when the lake is sufficiently

saturated precipitating CaCO3. Carbonates are preserved within a narrow vertical range

around the lake margins, but are absent in soda lakes (Renaut et al., 2013; Deocampo &

Renaut, 2016). With evaporative concentration and without replenishment in a closed basin,

Ca2+ becomes depleted and the formation of stromatolites ceases (Renaut et al., 2013).

CONCLUSIONS

The Ediacaran Oued Dar’a caldera of the Saghro inlier, Anti-Atlas, offers a unique

opportunity to study caldera-fill sedimentation, interrupted by multi-stage collapse structures.

The caldera infill is characterized by pyroclastic deposition with minor record of lava flows.

Autoclastic deposition consists of debris flows and landslides from the steep unstable intra-

caldera faults, accompanied by alluvial and fluvial sediments from high-gradient streams.

Sharp facies and bedding geometries suggest sharp changing drainage pathways. Active

faulting and caldera collapse efficiently promoted a net of interconnected faults that increased

secondary (structural) porosity over hectometric distances. As a result, circulation of

hydrothermal fluids was enhanced. Jasperoid cements are conspicuous in the silicified

microbial carbonates that form intramontanous lacustrine troughs and ponds.

The diagenesis of carbonate-bearing lacustrine ponds, developed under quiescent

conditions, recorded by two generations of carbonate cements: (i) fibrous, botryoidal and

blocky/drusy mosaics of calcite; and (ii) idiotopic mosaics of dolomite caused by flushing of

hypersaline Mg-rich brines. Secondary porosity was controlled by fracturing and related

dissolution, which was subsequently occluded by: (i) dedolomitization processes; (ii)

hydrothermal precipitation of silica ± TiO2; and (iii) associated alteration patterns, which

include zeolitization and chloritization. The δ13C and δ18O values from calcite and dolomite

stromatolites and cements broadly become successively isotopically lighter as a result of

meteoric and hydrothermal influence, probably overprint by the onset of the Panafrican-3

tectonic phase that affected the top of the Mançour Group. The silica was derived from two

distinct sources: (i) early silicification essentially developed at the sediment/water interface

by interaction of water that saturated with silica, as a result of hydrolysis of volcanic glass

and dewatering that promoted direct silica precipitation; and (ii) late-stage silicification

involving circulation of hydrothermal fluids that subsequently precipitated as fracture-filling

silica (silica veins). Fluid inclusions hosted in hydrothermal quartz include saline brine relics

with NaCl/CaCl2 ratios of 2.1 to 4.4 and minimum entrapment temperatures of about 142 to

204ºC. Identification of abiotic hydrocarbon-bearing fluids may be related to serpentinization

of the volcanic and volcanosedimentary basement of the Oued Dar’a caldera. Circulation of

metal-bearing fluids was negligible in the study transect, although As, Zn and Cu sulphide-

bearing and oxide-bearing clasts derived from neighbouring vents are locally abundant.

ACKNOWLEDGEMENTS

The authors warmly thank David Morrow (Natural Resources Canada, Ottawa), Brian Pratt

(University of Saskatchewan), Greg Walsh (US Geological Survey) and an anonymous

reviewer for their constructive criticism, Peir K. Pufahl (Acadia University) for technical

editing, Idoia Rosales (IGME, Tres Cantos) for help in carrying out CL analysis, and Alberto

Jorge (MNCN, Madrid) for SEM-BSE, SEM-CL and RAMAN analyses. This paper is a

contribution to project CGL2017-87631-P from Spanish MINECO.

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FIGURE CAPTIONS

Fig. 1. (A) Geological sketch map of the Moroccan High Atlas and Anti-Atlas showing their

major tectonostratigraphic units; modified from Gasquet et al. (2008). (B) Geological map of

the Oued Dar’a caldera in the western Saghro inlier – blue boxed area in (A), central Anti-

Atlas; modified from Walsh et al. (2012); OD1 – 30º49’41.08’’N, 6º39’51.58’’W; OD2 –

30º49’33.37’’N, 6º40’7.69’’W; Am: 30º47’31.12’’N, 6º43’15.68’’W. Abbreviations: Am and

OD1-2 – intra-Oued Dar’a troughs described in the text; JATF – Jebel Azouguiygh-n-Tazoult

fault; SFF – Sidi Flah fault; white triangles represent radiometric dating after Walsh et al.

(2012).

Fig. 2. Stratigraphic log of the Ediacaran in the Anti-Atlas; summarized from Álvaro et al.

(2014).

Fig. 3. Field aspect of the Oued Dar’a caldera. (A) Rhyolitic plug with vertical flow banding

marking the south-western edge of the caldera; symbol on the upper right represents strike

and vertical dip. (B) Detail of debris breccia deposit flanking the previous rhyolithic plug. (C)

Sliding strata composed of laminated and convoluted volcanosedimentary beds. (D) Half-

graben (setting of OD1) sandwiched between pyroclastites. (E) and (F) Detail of (D) showing

basal conglomerates and upper oncoids, respectively. (G) OD2 hill showing angular

discordance (arrows) at the pyroclastite/reefal complex contact. (H) Stromatolitic bioherm

with marked sole, core, flanks and sealing pyroclastite. (I) Contorted stromatolite fringing the

reefal core. (J) Perspective view of the core reef (subhorizontal) flanked by collapsed

(subvertical) flanks. Area outlined by box shown in (K). (K) Detail of (J) showing vertical

planar laminae of the reefal flank marking the outline of the collapsed trough. Abbreviations:

br – breccia; co – pyroclastic cover sealing reefal complex; cr – core of reef; fl – reefal flank;

py – pyroclastic beds; S – sole of reefal base; Sb – stratification plane of basal pyroclastites;

sh – onlapping shales; Src – stratification plane of reefal complex.

Fig. 4. Field aspect of quartz veining in the Amane-n’Tourhart lacustrine pond. (A)

Stromatolite bioherm cross-cut by a network of fissures, one of them (arrow) occluded with

hydrothermal quartz (false yellow). (B) Detail of impure limestone forming basal flat

stromatolites of a bioherm crosscut by quartz veins (arrow) encased in the lower bed but not

crosscutting the upper one, which allow relative dating of veining.

Fig. 5. Photomicrographs (taken in cross-polarized light) of biohermal microfacies. (A) Partly

silicified oncoid displaying two mosaics of microquartz surrounded by concentric mosaics of

macroquartz; OD1. (B) Detail of previous figure with relics of unsilicified microbial cortex

preserved in calcite. (C) Partially silicified domal stromatolite with alternating silicified (Si1)

and unsilicified laminae; fronts of silicification (arrows). (D) Radial-fibrous calcite mosaics

(Ca1) occluding vesiculated scoria; a (red circle) marks reddish calcite after alizarin staining.

(E) to (G) Stromatolitic laminae with arrowed fronts of silicification: (E) with parallel light;

(F) showing relics of undolomitized calcite (reddish); and (G) under cross-polarized light

showing authigenic crystals of zeolite (with artificial purplish-blue staining) punctuating the

microbial laminae. (H) Solution vug affecting the top of a columnar stromatolite, occluded

with drusy mosaics of quartz. (I) Detail of solution vugs hosted by stromatolites and

centripetally occluded by a crust of microquartz (Si1a), botryoidal chalcedony (Si1b) and a

mosaic of megaquartz (Si1c). Scales: (A) = 2 mm; (B) = 1 mm; (C) to (I) = 5 mm.

Fig. 6. Cold-cathodoluminescence photomicrographs of diagenetic cements. (A) Occlusion of

scoria pore with dull-brown, fibrous mosaics of calcite (ca1) outlined by bright-yellow

blocky mosaics of calcite (ca2). (B) Fractured cataclastite affecting the former ca1 fibrous

mosaics, subsequently cemented with calcite-2 blocky mosaics. (C) Sole of a microbial

boundstone formed by a framework of brownish sparry clasts (ca1), locally replaced to

dolomite (do1), with interparticle pore occluded with a bright-yellow drusic mosaic of ca2.

(D) Fissured dolomitized host with secondary pores occluded with yellowish ca2 mosaics.

(E) Mosaic of euhedral dolomite rhombs (do1) replacing ca1 mosaics. (F) Relics of yellowish

ca2 cements in a silicified groundmass (si2). Scales: (A), (B) and (D) = 300 µm; (C) = 400

µm, (E) and (F) = 500 µm; abbreviations after Table 1.

Fig. 7. Hot-cathodoluminescence (CL) and scanning electron microscopy (SEM)

photomicrographs. (A) Botryoidal chalcedony showing distinct silica zonation. (B) Solution

vug in a sericite-rich (se) calcarenitic infill occluded with silica (Q) and TiO2 (Ti) cements

(SEM-BSE). Scales: (A) = 1 mm, (B) = 30 µm.

Fig. 8. Plot of carbon and oxygen isotopes for stromatolites and carbonate cements of the

Amane-n’Tourhart lacustrine deposit, with indication of values from the lower Cryogenian

Taghdout carbonate interbeds (Álvaro et al., 2014a), the uppermost Ediacaran–basal

Cambrian Adoudou carbonates (Maloof et al., 2005; Álvaro et al., 2008; Álvaro, 2013) and

new data from the Tamjout Bed stratotype of the Tifnout Member (northern margin of

Igherm inlier).

Fig. 9. Scanning electron microscopy – cathodoluminescence (SEM-CL) photomicrographs

of selected hydrothermal quartz mosaics. (A) Mosaic of CL-grey quartz crystals with internal

growth zonation and CL-bright crystals outlining fissured-solved contacts (white arrows). (B)

Unzoned quartz (stained in yellow) occluding a fissure affecting zoned quartz mosaics. (C)

Quartz cores with outlines modified by dissolution (red arrows) and fissuring (yellow

arrows). (D) Dissolution contact (marked with red arrows) crosscutting a mosaic of well-

zoned quartz mosaics. (E) Slab of a partly silicified stromatolite; partly brecciated, with

setting of quartz mosaics (Q), where the SEM-CL analysis was made. (F) CL emission

spectra of hydrothermal quartz mosaics (for setting, see Figs 4H and 8E) from wall (blue) to

the centre (red) of a vuggy cavity; see text for explanation.

Fig. 10. Photomicrographs of silica mosaics encased in stromatolitic carbonates (shown by

red box in H) of the Amane-n’Tourhart pond. (A) Concentric zoning marked by primary fluid

inclusions; cross-polarized light; scale = 250 µm. (B) Deformation pattern manifested by

different stages of cloudy and clean quartz (arrows); cross-polarized light; scale = 250 µm.

phases (a to d) in fluid inclusion B, interpreted as accidental minerals; plane-polarized light;

ru – rutile; scale = 125 µm.

Fig. 11. Raman spectra of fluid inclusions at room temperature; the blue line represents

deconvolution framework by superposition of six Gaussian lines, performed by Gaussian-03

Software, marked by individual local vibrational modes; peak positions at 3034, 2882, 2867,

2856, 2848 and 2835 cm-1.

Table 1. Idealized early-diagenetic succession affecting the lacustrine carbonates of the Oued

Dar’a caldera (Mançour Group) at sites OD1, OD2 and Am (see setting in Fig. 1).

Table 2. Carbon and oxygen isotope data from the stromatolites and carbonate cements of the

Amane-n’Tourhart lacustrine pond, compared with the dolostone-hosted lead ore deposits of

the Tamjout Bed (basal Tifnout Member) at its stratotype of the northern Igherm inlier (see

Álvaro et al., 2014).

Table 3. Microthermometric results of fluid inclusions. FI – fluid inclusion; FIA – fluid

inclusion assemblage. Size in microns. L:V:(S): ratio of liquid:vapour:solid (solid is in

brackets because is not always present), liquid to vapour ratios estimated using the

comparison charts of Shepherd et al. (1985). Tm(ice) – temperature of final melting of ice; Tn

– nucleation temperature; Tm(Halite?) – temperature of melting of halite?; Tfm(ice) –

temperature of first melting of ice; Th – homogenization temperature. S wt% – salinity in

weight percent; wt%NaCl – weight percent of NaCl; wt%CaCl2 – weight percent of CaCl2;

salinities calculated using Steele-MacInnis et al. (2011) from Tm(ice) and Tm(Halite?);

temperature in °C.

REPOSITORY DATA

Voight deconvolution data of Raman spectra illustrated in Fig. 11, performed by using

Gaussian-03 Software.

δ13C ‰ VPDB δ18O ‰ V PDB

Mançour Group

stromatolitic host -2.13 -10.39

-2.76 -10.81

-2.95 -9.27

-3.16 -12.85

-2.22 -10.15

-3.04 -11.84

calcite-1 and 2 -2.67 -11.66

-3.21 -12.31

-3.83 -11.71

-3.98 -11.51

-3.61 -11.96

-3.88 -12.12

dolomite-1 -3.91 -13.52

-3.99 -12.17

-4.89 -12.22

-4.93 -13.99

-4.62 -12.98

-4.32 -13.22

calcite-3 -3.88 -11.12

-3.79 -12.39

-4.22 -13.69

-4.20 -13.91

-3.96 -12.21

-4.26 -13.82

Tamjout Bed -4.27 -9.01

-4.22 -9.09

FI FIA Size L:V:(S)

Tm(ice) TnTm(Halite?) T fm (ice) Th Swt% wt% NaCl wt% CaCl2

A 1 30 83:15:2 -22.8 -62 25-27 -64 185-186 27 21 6

B 1 20 85:10:5 -20,8 -71 190-191

C 1 16 95:5 -26.2/-25.7 -91 -66 183-184

D 1 17 90:8:2 -24.7 -82 28.1 -66 203-204 28 19 9

E 2 28 90:10 -27.8 -67 171-175

G 2 18 95:5 -23.4 -74 21-22 -65 143-145 27 21 6

H 2 9 90:5:5 -75 158-162

J 2 16 95:5 -22.8 -88 28.3 155-157 27 22 5

K 2 12 90:5:5 -24.9/-23.9 -84 142-162

Microbialites diversity from the Ediacaran of the Anti-Atlas (Morocco): A snapshot of microbial oases thriving in an alkaline volcanic lake

Article

Oct 2022
Ann Palaontol

The Moroccan Anti-Atlas belt preserves an exceptional record of an Ediacaran microbial biosphere. TheAmane Tazgart Formation of the Ouarzazate Group corresponds to an Ediacaran volcanic alkaline lakedepositional system (ca. 571 Ma) where microbial buildups accreted in an extreme environment. Thesemicrobial accumulations are exceptional not only for their diverse range of extreme conditions but alsofor their significance in understanding the early biosphere and earth’s habitability. A description of thesebuildups provides insights into their Spatio-temporal distribution. The lower part consists of thromboliticlimestones, usually displaying irregular to patchy mesoclots, associated with composite and stromatoliticbuildups. The upper part dominated by clastic stromatolites exhibits a variety of morphotypes rangingvertically from planar wrinkly laminated to large domes. The transitional morphotypes are made oflinked and vertically oriented or inclined columns, grading upward to cone-shaped domes. The increasein laminated fabrics and the decrease in clotted fabrics toward the top of the section indicate that environ-mental conditions were likely suitable for coexisting both fabrics during microbial carbonate accretion.The demise of carbonate production at the late stage coincides with riverine input of clastic sediments,subsequently followed during low sediment input by growth of siliciclastic stromatolites.

A 571 million‐year‐old alkaline volcanic lake photosynthesizing microbial community, the Anti‐atlas, Morocco

Article

Full-text available

Dec 2020

The Ediacaran period coincides with the emergence of ancestral animal lineages and cyanobacteria capable of thriving in nutrient deficient oceans which together with photosynthetic eukaryotic dominance, culminated in the rapid oxygenation of the Ediacaran atmosphere. However, ecological evidence for the colonization of the Ediacaran terrestrial biosphere by photosynthetic communities and their contribution to the oxygenation of the biosphere at this time is very sparse. Here, we expand the repertoire of Ediacaran habitable environments to a specific microbial community that thrived in an extreme alkaline volcanic lake 571 Myr ago in the Anti‐atlas of Morocco. The microbial fabrics preserve evidence of primary growth structures, comprised of two main microbialitic units, with the lower section consisting of irregular and patchy thrombolytic mesoclots associated with composite microbialitic domes. Calcirudite interbeds, dominated by wave‐rippled sandy calcarenites and stromatoclasts, fill the interdome troughs and seal the dome tops. A meter‐thick epiclastic stromatolite bed grading upwards from a dominantly flat to wavy laminated base, transitions into low convex laminae consisting of decimeter to meter‐thick dome‐shaped stromatolitic columns, overlies the thrombolitic and composite microbialitic facies. Microbialitic beds constructed during periods of limited clastic input, and underlain by coarse‐grained microbialite‐derived clasts and by the wave‐rippled calcarenites, suggest high‐energy events associated with lake expansion. High‐resolution microscopy revealed spherulitic aggregates and structures reminiscent of coccoidal microbial cell casts and mineralized extra‐polymeric substances (EPS). The primary fabrics and multistage diagenetic features, represented by active carbonate production, photosynthesizing microbial communities, photosynthetic gas bubbles, gas escape structures, and tufted mats, suggest specialized oxygenic photosynthesizers thriving in alkaline volcanic lakes, contributed toward oxygen variability in the Ediacaran terrestrial biosphere.

Microbially induced sedimentary structures from the Ediacaran of Anti-Atlas, Morocco

Article

Jul 2023
PRECAMBRIAN RES

The Eastern Anti-Atlas of Morocco hosts an early Ediacaran turbiditic series (Saghro Group) (630–600 Ma) unconformably overlain by thick late Ediacaran (580–550 Ma) terrestrial volcano-clastic formations (Ouarzazate Supergroup), with thin and geographically limited paralic shallow marine sedimentary rocks. This paper presents the first description of structures related to the former presence of extensive microbial mats developed in marine, fluvial, and lacustrine environments during the Ediacaran in the Eastern Anti-Atlas. These microbially induced sedimentary structures (MISS) are largely found in well-laminated fine- to coarse-grained sandstone and sandy-carbonate. They also cover vast bedding planes and occur in almost all sedimentary successions. MISS types include: gas domes, wrinkle structures, reticulate patterns, overflips and roll-ups, multidirectional linear ridges, sand cracks and biolaminated deposits. Based on morphology and petrographic macro and microfabrics, these microbially induced structures are very similar to those of modern photosynthetic cyanobacteria mats. Together with previously reported stromatolites, these newly reported microbial mat structures could have played an important ecological role in stabilizing siliciclastic sediments, as well as sustaining localized zones of high oxygen production in the Ediacaran marine and terrestrial paleoenvironments of the Anti-Atlas. The widespread geographic distribution and consistent occurrence of microbially induced sedimentary structures (MISS) in fluvial and lacustrine sediments present compelling paleontological evidence supporting the extensive greening of the land surface during the late Precambrian period.

A Pennsylvanian saline-alkaline lake in Gondwana mid-latitude: Evidence from the Piauí Formation chert deposits, Parnaíba Basin, Brazil

Article

Aug 2022
PALAEOGEOGR PALAEOCL

The Pangaea assemblage resulted in significant tectonic and eustatic changes, but also a well-known desertification triggered by the increasing continentalization. While the northern hemisphere basins have been thoroughly studied, several recent studies on southern hemisphere basins bring new insight on how these events affected the Gondwana hinterlands. In this study, sedimentological and geochemical analyses of chert deposits from the Pennsylvanian Piauí Formation of the Parnaíba Basin are examined as new evidence for the onset of continentalization and aridification in the northwestern Gondwana mid-latitudes. The chert deposits correspond to three facies (stromatolites, ooidal arenites and breccias). The chert features suggest deposition in a shallow water environment and biomarkers analysis of stromatolites indicated predominance of organic matter produced by cyanobacteria in an environment characterized by high salinity and elevated pH. Such evidence suggests that the Piauí Formation cherts deposited in a shallow, saline-alkaline lacustrine environment during a period marked by increasing arid climate conditions.

Late Pennsylvanian aridification in Gondwana mid-latitudes contemporaneous to high-latitudes ice cap expansion, Upper Piauí Formation, Brazil

Article

Jun 2022
J S AM EARTH SCI

The Pennsylvanian Epoch was marked by a well-known glaciation period documented in several basins across the Gondwana. However, stratigraphic data from the Parnaíba Basin suggest that drier climate conditions dominated mid-paleolatitudes simultaneously to the glaciation. To investigate the possible time equivalence and the causes of aridification in mid-paleolatitudes, high-resolution stratigraphic analysis of the Upper Piauí Formation were conducted in the eastern margin of the Parnaíba Basin. As a result, three stratigraphic intervals separated by key stratigraphic surfaces were identified: The first stratigraphic interval is dominantly composed of fluvial deposits progressively overlain by eolian facies; the second stratigraphic unit is essentially composed of lacustrine deposits and represent a regional flooding related to a period of lake expansion and finally, the third stratigraphic unit is characterized by three eolian genetic units and depicts cyclic expansion and contraction of a dune field. The overall vertical stratigraphic succession represents a progressive change from original fluvial sedimentation to essentially eolian accumulation and constitutes a drying upward trend that reflects an increasing aridification in the Parnaíba Basin during the Late Pennsylvanian age. The detailed analysis of the paleoclimatic conditions and the atmospheric circulation during the Late Pennsylvanian Epoch evidenced that the waning and waxing of the ice sheets controlled the development, expansion and contraction of the Upper Piauí Formation dune-field.

Le paradis des géologues : Un voyage à travers les temps dans le Royaume du Maroc

Book

Full-text available

Mar 2022

Ahmed El Hassani

Morocco is considered geologically among the countries that relate one of the globe's complete geological history (from the Archean to the Quaternary). Therefore, it is of significant interest to the international scientific community, and many geologists from the five continents regularly visit it. In addition to its geological terrains diversity, several stratotypes and fossil groups have been identified in Morocco, where they are the most representative on a planetary scale. Morocco also presents several geological curiosities (landscapes) rare in the world (example: the Devonian mud-mounds of Hamar Lakhdad in the Tafilalet, the Beni Bousera peridotites in the internal Rif, the numerous caves, ...). This wealth remains nowadays unknown by the majority of Moroccans, including managers of natural areas or even the scientific community not specialized. Inventory studies are encouraged to raise public awareness of this heritage's safeguarding, development, and rational use. Unfortunately, only the commercial value of this natural heritage currently prevails in Morocco; it generates massive exploitation, even abuse, of the mineral and fossiliferous riches that will inevitably lead to their disappearance in the short or medium term. This 350-page book is composed of four parts: a historical part that summarizes the first discoveries and monographs of the 19th and early 20th centuries, then in a second part this book shows the importance of the Scientific Institute in the development of Moroccan geology; a third part describes the different facets of the geology of Morocco; and finally, the last part discusses the concept of geological heritage and its importance in Morocco.

Calibrating δ13C and δ18O chemostratigraphic correlations across Cambrian strata of SW Europe and Morocco, West Gondwana

Chapter

Oct 2020

José-Javier Alvaro

The Cambrian remains one of the least constrained periods from the International Geological Timescale. Several global boundaries of series and stages are still awaiting formal definition and ratification. Particularly in West Gondwana, a major obstacle to the development of a common regional Cambrian timescale and its global correlation is the endemic character of its shelly fauna (mainly trilobites, archaeocyaths, echinoderms and brachiopods) due to the rare exposure of fossiliferous slope-to-basinal strata bearing cosmopolitan taxa. Over the past two decades, there has been a growing interest in the application of isotopic chemostratigraphic methods to regional studies, in an effort to solve and calibrate the timescale resolution in the Cambrian Atlas–Ossa-Morena rift axes and their lateral flanks preserved in Morocco, the Iberian Peninsula and South France. A critical re-appraisal of the resulting chemostratigraphic methods and results applied to the recognition and regional correlation of the Ediacaran–Cambrian (BACE event), Cambrian Series 2–Miaolingian (ROECE) and Miaolingian–Furongian (SPICE) boundaries is presented here. The reliability of isotopic data is mainly controlled by (i) the preferential analysis of bulk rock in carbonates and shales with total organic contents (TOC) lesser than 1 wt%; (ii) the unequal record of diagenetic (e.g., early vs. late diagenetic dolomitization) and metamorphic processes related to Variscan tectonothermal events; and (iii) dating uncertainty of chemostratigraphic fluctuations by chronostratigraphic and radiometric constraints.

The textural evolution and ghost matrices of the Cretaceous Barra Velha Formation carbonates from the Santos Basin, offshore Brazil

Article

Dec 2019

The Aptian lacustrine carbonates of the Barra Velha Formation, offshore Brazil, have undergone a highly unusual paragenetic and diagenetic history and present significant terminological problems if current textural classifications are applied. Existing limestone classifications invoke the concept of textural maturity, whereby the ratio of matrix to grain is a critical property and can be used to infer energy levels and the degree of transport prior to deposition. In the Barra Velha Formation, some critical carbonate grains grew in a matrix which later dissolved, congruently, producing what appears to be primary intergranular porosity. In addition, apparent boundstone textures occur, not due to organic trapping or binding, but ones which can be shown to be diagenetic, and were also related in part to the former presence of labile matrices. Thus, applying some terms in existing classifications can lead to false interpretations, and to avoid misinterpretations criteria for identifying ghost (former) matrices are provided.

Geochemistry of African Soda Lakes

Chapter

Full-text available

Aug 2016

Soda lakes are those in which sodium and carbonate species dominate the dissolved ions. They form in hydrologically closed lake basins, where inflow is balanced primarily by evaporation. They are important habitats, closely tied to water resource issues in arid lands, and they support important economic activities. Soda lakes differ from other saline lakes in that their drainage basins typically are dominated by volcanic terrains that produce very alkaline dilute inflow. In East Africa, soda lakes are commonly associated with volcanism and tectonic depressions, mostly along the East African Rift. Upon evaporative concentration, salinity increases, but many solutes are also lost to solid phases through the actions of abiotic mineral precipitation and biological utilization. Thus, soda lake waters tend to be dominated by sodium, bicarbonate and/or carbonate, with potentially significant sulfate and chloride concentrations as well. Alkaline earths (calcium and magnesium) are almost always negligible in concentration in evolved soda brines. Hydrothermal input is an important contributor of silica in soda lake waters and may also play an important role in deep subsurface environments (far below the sediment-water interface) by enriching CO2, enhancing silicate hydrolysis and generating high alkalinity.

Sedimentary processes and lithofacies in lake-margin groundwater-fed wetlands in East Africa

Chapter

Full-text available

Jan 2002

Daniel M Deocampo

Carte géologique au 1/50 000, Feuille Imtir

Article

Full-text available

Jan 2007

Dipping into the Soda Lakes of East Africa

Chapter

Aug 2016

Saline alkaline lakes are the focus of this book, but how do they form and how do they differ from freshwater lakes? We provide a brief introduction to soda lakes, their origin and their inhabitants, many of which are extremophiles. These closed lakes are highly sensitive ecosystems, and many experience the impacts of climate change. Some saline alkaline lakes provide economic resources, for both tourism (‘flamingo lakes’) and industry. A few lakes have long been commercially exploited but are ecologically damaged; others are threatened because of their potential mineral resources and changing land use in their drainage basins, including siltation and pollution from both agricultural and industrial sources. These unusual ecosystems must be protected (1) because they are ‘hot spots’ of haloalkaliphiles and rich genetic pools that might soon be tapped for biotechnological applications, and (2) for their exceptional natural environments, threatened increasingly by anthropogenic intervention. These environments must be preserved for future generations.

One-step synthesis of titanium oxide nanocrystal- rutile by hydrothermal method

Conference Paper

Sep 2014

Pure rutile phase titanium oxides ( TiO 2 ) nanocrystals were synthesized via hydrothermal method with titanium tetrachloride ( TiCl 4 ) and water ( H 2 O ) treated in an autoclave. The particle size and phase assemblages were characterized using Scanning electron microscopy (SEM) and X-ray diffraction (XRD) respectively. Band gap energy (Eg ) of the nanocrystals was estimated from the Ultra violet – visible light (UV-vis) absorption spectra. It was demonstrated that TiO 2 nanocrystals can be prepared through increasing of temperature and period of treatment. It is believed that the presence of acid chloride (HCl) as by-product during the hydrolysis played an important role in controlling the growth of morphology and crystal structures. The Eg of the samples were estimated from the plot of modified Kubelka-Munk function were in the range of 3.04 – 3.26eV for the samples synthesized at temperature ranging from 50 to 200°C for 16 hours.

Cathodoluminescence of TiO2 powders

Article

Oct 2014

Cathodoluminescence (CL) of titanium dioxide powder at 87 K was investigated and bands at 265, 275, 490, 515, 815, and 1080 nm were recorded. The maximum at 590 nm was found on the long wavelength wing of the 515 nm band. The bands in the visible region of the spectrum are the most intense. The band assignments are suggested. The band at 1080 nm was recorded for the first time and its nature is unknown. Irradiation with accelerated electrons results in the decrease in the band intensities. The increase in the 815 nm band intensity was observed at the beginning of irradiation. The luminescence enhancement is explained by the cleaning of the powder surface with the electron beam, and the decline is explained by the concentration quenching on radiation defects.

Carte géologique au 1/50 000, Feuille Bouskour

Article

Jan 2008

The area covered by the Bouskour map, scale 1/50 000, belongs to the eastern Anti-Atlas geographical domain constituted by a mountain range NE-SW oriented where culminate the highest peaks of the Anti-Atlas, which includes two inliers containing Precambrian rocks with their volcanic, volcano- clastic and sedimentary cover in the Ougnat to the east and the Saghro to the West. The Saghro itself is divided into eastern, central and western; Bouskour 1/50 000 sheet belongs to the southern part of the central Saghro. The lithostratigraphy has been reviewed in accordance with the international nomenclature, the very precise geochronology using the method SHRIMP U/Pb zircon and the many recent works published on the Anti-Atlas. The correlation tables are attached to this work. The geodynamic synthesis of the panafrican periods is based on geological mapping, satellite imagery and geochemical diagrams of major elements, trace elements and rare earths and their comparison with geodynamic sites known. The Jebel Saghro has a geological history that probably began in mid Neoproterozoic (Cryogénien or NP2) and continued during the upper Neoproterozoic (Ediacarien) and the Paleozoic. The lower Cryogénian (NP2i) is formed by volcano-sedimentary origin turbidites where are interbedded basic to intermediate volcanic flows, all suffered folding and metamorphism in the first panafrican phase. The upper Cryogénien is constitued of a molasse associated with volcanic acid rocks where the second panafrican phase printed a dissolution cleavage. A granitic episode took place between 640 and 680 Ma at upper Cryogénian as attested by granodiorites and tonalits SHRIMP ages in the North of Bou Azzer inlier (646 ± 8 Ma and 659 ± 7 Ma) and of eat Saghro (U/Pb Imider 675 ± 13 Ma, Igoudrane 645 ± 19 Ma, Tioulkhirine 645 ± 12 Ma). A second volcanic and detrital episode overlying diconformably NP2i or NP2s, took place at the lower Ediacaran (NP3i) between 600 and 560 Ma as attested by SHRIMP ages calculated on the granits cutting volcanic- detrital units whose ages range between 588 ± 6 Ma and 562 ± 5Ma). The upper Ediacarian records the latest Precambrian magmatic episode, associated with conglomerates and sandstones. The upper Ediacaran is discordant on the lower Ediacaran but structurally concordant with the Cambrian units. Upper Ediacaran ages found display between 558 ± 4 Ma and 555 ± 5 Ma. Trachyte-andésites interbedded in the first Cambrian transgression sediments, are dated at 531 ± 5 Ma The geochemical and geochronological studies have allowed us to propose a geodynamic model for the Cryogénian with a subduction toward the North between two oceanic crusts and formation of a volcanic arc and another model for Edicarian following the collision of the arc with West African Craton, the oceanic crust stoppage to the North and joined arc eventually break their embedding leading to the South under the newly expanded craton. In the Bouskour sheet the many mineralized copper seams in the Neoproterozoic units are probably related to the hydrothermalism induced by the many NP3s rhyolitic dykes N-S, recording the end of the panafrican orogen witch started 230 Ma before. The other mineralization are those of ocher and manganese. Two hydrogeological basins characterise Bouskour sheet, Dades basin to the north and the Dar’a basin to the South.

Carte géologique du Maroc au 1/50 000

Article

Jan 2007

Livret guide de l'excursion Anti-Atlas occidental et central

Article

Subaerial and sublacustrine hydrothermal activity at Lake Rotomahana

Article

Jul 2015
J VOLCANOL GEOTH RES

Lake Rotomahana is a crater lake in the Okataina Volcanic Centre (New Zealand) that was significantly modified by the 1886 Tarawera Rift eruption. The lake is host to numerous sublacustrine hydrothermal vents. Water column studies were conducted in 2011 and 2014 along with sampling of lake shore hot springs and crater lakes in Waimangu Valley to complement magnetic, seismic, bathymetric and heat flux surveys. Helium concentrations below 50 m depth are higher in 2014 compared to 2011 and represent some of the highest concentrations measured, at 6 × 10− 7 ccSTP/g, with an end-member 3He/4He value of 7.1 RA. The high concentrations of helium, when coupled with pH anomalies due to high dissolved CO2 content, suggest the dominant chemical input to the lake is derived from magmatic degassing of an underlying magma. The lake shore hot spring waters show differences in source temperatures using a Na–K geothermometer, with inferred reservoir temperatures ranging between 197 and 232 °C. Water δ18O and δD values show isotopic enrichment due to evaporation of a steam heated pool with samples from nearby Waimangu Valley having the greatest enrichment. Results from this study confirm both pre-1886 eruption hydrothermal sites and newly created post-eruption sites are both still active.

Sublacustrine hydrothermal seeps and silicification of microbial bioherms in the Ediacaran Oued Dar'a caldera, Anti‐Atlas, Morocco

Abstract

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