Quaternary geological map of the Bekaa Valley-Lebanon

Abstract

The first comprehensive Quaternary geologic map of the Bekaa Valley, Lebanon is prepared on a final scale of 1:100000. Both surface and subsurface (borehole) data are integrated. All mapping units mentioned in this paper, including one of pre-Quaternary age, are new to the geological literature of Lebanon. Three versions of the geological map are produced:
Version I uses tripartite classification comprising: i) genesis, G ii) landform and iii) lithology (structure symbol: IGt). This provided twelve mappable litho-geomorpho-genetic units.
Version II: is based on lithostratigraphical classification and dispalys ten newly recognized formations (including one of Miocene age)
Version III is based on chronostratigraphical classification. It shows seven chronostratigraphical units. This geological map of the Bekaa Valley contributes more data for the existing geological map of Lebanon.

Full text

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QUATERNARY GEOLOGICAL MAP OF THE BEKAA VALLEY-LEBANON
A.S.A.Lateef
Vrije Universiteit Brussel & Royal Museum for Central Africa
abdulsah@yahoo.com
ABSTRACT
The first comprehensive Quaternary geological map of the Bekaa Valley, Lebanon is
prepared on a final scale of 1:100000. Both surface and subsurface (borehole) data are
integrated. All mapping units mentioned in this paper, including one of pre-Quaternary age,
are novel to the geological literature of Lebanon. Three versions of the geological map are
produced:
Version I uses tripartite classification comprising: I) genesis, G ii) landform, l and iii)
lithology, t (symbolisation structure: lGt). This provided twelve mappable litho-geomorpho-
genetic units.
Version II is based on lithostratigraphical classification and displays ten newly recognized
formations (including one of Miocene age).
Version III is based on chronodstratigraphical classification. It shows seven
chronostratigraphical units.
This geological map of the Bekaa Valley contributes for the revision of the existing
geological map of Lebanon.
RESUME
La première carte compréhensive de la géologie du Quaternaire de la Vallée de la Beqaa,
Liban, est dressée à l‟échelle finale de 1:100000. Aussi bien les données de surface que ceux
de profondeur (sondages) y sont integrées. Toutes les unités cartographique mentionées dans

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cette étude, y compris une d‟âge pré-Quaternaire, sont nouvelles par rapport à la littérature
géologique du Liban. Trois versions de la charte géologique ont été produites:
Version I emploie une classification ternaire comprenant : i) génèse, G ii)
géomorphologie, l et iii) lithologie, t (structure de symbolisation : lGt). Ceci fournit douze
unités litho-géomorpho-génétique cartographiables. Version II est basée sur une
classification lithostratigraphique; elle montre dix nouvelles formations identifiée (y compris
une d‟âge Miocène). Version III est basée sur une classification chronostratigraphique. Elle
montre sept unités chronostratigraphique.
Cette carte géologique de la Vallée de la Beqaa peut contribuer à la revision de la carte
géologique actuelle du Liban.
INTRODUCTION
General
The geological map of Lebanon has been compiled half century ago on a scale
1:200000 (Dubertret, 1955). This was based on larger scale geological maps (1:50000)
prepared by Louise Dubertret and his collaborators. Like in other parts of the Middle East,
earlier geological mapping works concerned mostly with sequences older than those of
Quaternary age as these are the source for oil resources that formed the major target for
exploration geologists. The ultimate result of this situation is poor representation of
Quaternary deposits on the existing geological maps. Later developments in Quaternary
geology, particularly advancements in dating methods, and also the raise of interest in
Quaternary landscapes and Quaternary deposits called for tackling this deficiency of
information on geological maps. The present work is an attempt along this direction. On the
1:200000 geological map, one observes that significant Quaternary deposits in Lebanon are
present in the narrow coastal strip and the Bekaa Valley basin (Fig.1).

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Figure 1. Simplified geological map of Lebanon (after Walley, unpublished). Significant
Quaternary deposits are represented in the narrow coastal strip and the Bekaa Valley.
The present study is confined to the second region.
As mentioned previously, the Quaternary is represented as secondary theme, mostly
undifferentiated. Later sporadic and localized Quaternary investigations in Lebanon didn‟t
add to the spatial representation of the Quaternary cover. As a result, the Quaternary of the
Bekaa Valley is shown on the small-scale geological map of Lebanon (1:200000) as one
single unit. On the few available 1:50000 scale geological maps the legend of Quaternary
deposits reflect basic inconsistencies. Separate geomorphic, lithological, and genetic terms
are used (e.g. debris, ancient detachment, clay, gravels/cherty gravels, sand, land slide,
mudflow, mass wasting, superficial conglomeratic pavement, dejection cone, gravely slope).
Also, geologically non-meaningful terms such as „Terre arable” or chronologically non-
The current Geological Map of Lebanon

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supported term such as “alluvions moderne” are used. Terminology of this sort is not only
unable to meet the requirements of geological and stratigraphical procedures that aim to
delineate the spatial distribution and lateral relationship of the Quaternary deposits but also
their inability to establish temporal order of the deposits (i.e. the stratigraphy). In brief, there
is neither stratigraphical classification of the Quaternary deposits nor meaningful Quaternary
mappable units on the current geological map of Lebanon.
The present study attempts to review this situation. The work targets the largest basin of
continental deposits in Lebanon, the Bekaa Valley, by applying standard geological
approaches, including the basic principles of lithostratigraphy and Quaternary mapping
procedures, to identify these deposits and categorize them into mappable units that have both
spatial and temporal significance. The area of study (Fig.2) is structural depression that
evolved in association with the tectonics of the Lebanese segment of the Levant Fracture (the
Dead Sea Transform). It preserves significant amounts of continental deposits, most of which
are of Quaternary age, that vary in scale (thickness) from place to another. It is anticipated
that this work would stimulate interest in Quaternary geological mapping in Lebanon. The
novelties introduced in this study would be the subject of criticism, examination and revision
by future works.
Location and Method of Investigation
The Bekaa Valley is the middle zone of the three main morpho-structural divisions of
the Lebanese landscape. It is bounded by the western Lebanese range or Mount Lebanon from
west-northwest and by the eastern Lebanese range or Anti-Lebanon from east-southeast
(Fig.2).

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Figure 2. Location of the Bekaa Valley, shaded, within the topographic setting of
Lebanon.
Traditional mapping techniques have been applied to prepare the geological map of the
continental infill of the Bekaa Valley. A total of 23 toposheets of 1:20000 scale and one
1:50000-scale toposheet were used as base maps for fieldwork. This covers an area of 800 to
900 sq.km (Fig.3). Mapping accuracy is for 1:100000 final scale. Geological information is
based on the study of outcrops, artificial cuts (quarries, pits and trenches) and borehole
cuttings. Field data are supported by certain laboratory results including paleomagnetism,
chemistry, petrography, K/Ar & Ar/Ar dating, partial diatoms and pollen investigation, C14
dating, Th/U and Th/Th dating, and stable isotope analysis.

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Figure 3. Index map of the toposheets used in the geological mapping. 1:20000 scale
sheets are shown (shaded) beside part of one I: 50000 toposheet (lower stippled
rectangle). Heavy line boundaries represent borders of the enclosing 1:50000 toposheets.
Geomorphologic Setting of the Bekaa Valley
The Bekaa Valley is a high plateau standing, on average, 850 m a.s.l. In general
literature the Bekaa Valley is coined also as the Bekaa Plain probably as a result of the
impression of a levelled basin sandwiched between two overshadowing high mountain
ranges. But using the term “plain” is misleading as the Bekaa Valley exhibits variable micro
relief. It is recognized during the fieldwork of this study that the area is divisible into eight
physiographical units or zones (Fig.4).

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Mount Lebanon
Anti-Lebanon
0 5 10 15 20 km
Perspective view towards NE(N20E), tilt 30°
A
BC
DE
F
GH
I
Figure 4. Digital Elevation Model (DEM) of the Bekaa Valley based on 1:100000 scale
topographic map produced by Kriging gridding method with artificial illumination
from the northwest inclined at 43°. Letter symbols are: (A) is Bar Elias Plain, (B) is
ancient (fossil) tilted alluvial fans, (C) is the terraced merging fan gravels of Tamnine
area, (D) stands for Iaat Plain, (E) is the piedmont train (bajada), (F) is terraced
merging fan gravels of Kneisse-Laboue region, (G) stands for rock fans, (H) is
undulatory terrain, and (I) denotes Al Hermel/Al Qaa sloping plain. The bounding
heights of Mount Lebanon and Anti-Lebanon are partially represented (lower reaches
only). Tectonical-structural effects (tilting, subsidence, uplift and faulting) and
sedimentary processes have produced the observed geomorphological features.

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Unlike the case with pre-Quaternary mapping, geomorphology is an essential element
in mapping Quaternary landscapes. Whether landforms are ancient or of young age,
geomorphological aspect is fundamental in recognizing and delineation of Quaternary
mappable units. In fact, Quaternary deposits make the internal milieu of many Quaternary
landforms. Furthermore, in many Quaternary landscapes the geomorphological setting bears
indications to the sedimentary processes that have produced the sediments. Therefore,
frequently there is relationship between the recognized geomorphological feature and the
genesis of the Quaternary deposit. This vital role of geomorphology in Quaternary geological
mapping is reflected in this study by its incorporation in the structure of the mappable units of
the first version of the Quaternary geological map.
THE QUATERNARY GEOLOGICAL MAP
The Quaternary geological map of the Bekaa Valley is prepared in three versions. In the
following subsections, brief explanatory notes would accompany the geological map and the
associated legend.
1) Version 1
Symbolization structure of the mappable units in this version comprises three attributes or
entities:
Genesis (G), landform (I) and texture (t). The symbolization structure of mappable units has
the combined form (tGl) that defines litho-geomorpho-genetic units. Landform and texture
(Lithology) are field observable entities. Genesis, on the other hand, is an inferred attribute.
Examination of the sediments and geomorphology of the landscape indicate the presence of
the following genetic, textural and geomorphic categories:

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Genesis Symbol Texture Symbol Landform Symbol
Paludal-alluvial O Clastics c Plain p
Proluvial P Chalk-marl l Terraced t
Proluvio-Alluvial L Mudstone d Shallow depression d
Lacustrine T Gravels g Intermountain basin b
Lacustrine-Alluvial A Gravels-paleosols s Rolling terrain n
Paludal-Lacustrine U Basalt b Rock fan f
Volcanic V Mudstone- conglomerate m Ridged topography r
Plug-sill l
Slope o
Geological investigation enabled the identification of 12 mappable units. These are
tabulated in the legend below:
pOc
Paludal-Alluvial plain clastics
pLc
Proluvial-Alluvial plain clastics
tLc
Proluvial-Alluvial terraced clastics
dLc
Proluvial-Alluvial shallow depression clastics
bTl
Lacustrine intermountain chalk-marl
bPg
Proluvial intermontain gravels
nUd
Paludal-Lacustrine rolling plain mudstone
sPg
Proluvial bajada gravels
oPs
Proluvial slope gravels-paleosols
fAm
Lacustrine-Alluvial rock fan mudstone-conglomerate
rAm
Lacustrine-Alluvial ridge mudstone-conglomerate
lVb
Volcanic plug-sill basalt

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Two additional symbols are used on the map one for the caliche armour and the other
for older pre-Quaternary bedrock. The “ iVb” unit is not shown on the map as a result of scale
limitations. Spatially it is embedded within the “rAm” unit. The geological map of this
version is shown in Fig.5
Figure 5. Geological map of the Bekaa Valley, version I. The continental deposits are
classified into Litho-geomorpho-genetic units.
2) Version II
This version of the geological map is based on lithostratigraphical approach to classify the
continental infill of the basin. Subdividing rocks into lithostratigraphical units is fundamental

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practice in geological mapping. Application of the methodology to Quaternary deposits is not
without constraints as will be discussed later. To reconstruct the map, the continental deposits
of the Bekaa basin is divided, for the first time, into the following lithostratigraphical
mapping units:
1) Al Hermil Formation (upper Middle Miocene-basal Upper Miocene). Major Lithology is
mudstone (grainstone) and conglomerates. 2) Karak Noah Formation (Pliocene-Middle
Pleistocene). Major lithology is gravels-conglomerates with interspersed paleosols.
3) Al Asi Formation (basal-middle Middle Pleistocene). Major Lithology is mudstone and
interspersed paleosols. 4) Ain al Zarqa Caliche Formation (upper Middle Pleistocene).
Lithology: calcified debris-gravels, hard pans, nodular caliche, and splintery caliche. 5) Al
Taiba Conglomerate Formation (Upper Pleistocene). Major Lithology is unsorted
conglomerate. 6) Bar Elias Formation (Upper Pleistocene-Holocene). Major Lithology is silty
organic clay. 7) Iaat Formation (uppermost Upper Pleistocene-lower Holocene). Major
Lithology is unsorted gravels. 8) Al Yemmona Formation (Holocene). Major Lithology is
chalk and marl. 9) Marjheen Formation (Holocene). Major Lithology is unsorted gravel. 10)
Al Qaa Formation (Holocene). Major Lithology is admixture of unsorted gravels and loam.
The map shows also a conglomeratic-brecciated caliche blanket that starts in the south in the
region of Baalbak town and continues northeastward. Paleoclimatic and chronological
considerations (see Lateef, 2004) indicate that this surficial duricrust has been developed in
the middle Bekaa during the late Holocene or historical times. The possibility remains that
this young caliche may assume longer time span northeastward.
Description of stratotypes of these lithostratigraphical units (including also one sub-
surface formation not shown on the geological map) is provided in Lateef (2005). It suffices
here to state that Quaternary formations, which form the major bulk of the identified units,
have certain constraints peculiar for rock units of this system. This holds particularly true for

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the mapping boundaries that become, in some instances, diffused, blurred or conjectured
because of transition, poor exposure and/or masking by surficial calcrete crust. This problem
is partially ameliorated by use of geomorphological boundaries. Further discussion on
constraints of applying lithostratigraphical classification to Quaternary deposits in the Bekaa
basin is given elsewhere (Lateef, 2005).
Figure 6. Version II of the geological map of the Bekaa Valley based on
lithostratigraphical classification of the continental infill. The few pre-Quaternary
windows that appear (associated with transversal faulting) in the basin (represented on
the 1:200000 geological map of Lebanon) are not shown here.
Schematic longitudinal section in the Bekaa Valley from northeast to southwest
demonstrating the general distribution of the continental infill is shown in Fig.7. Inferred
tectonical effects in terms of localized subsidence, uplift and faulting are indicated also.

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Figure 7. Generalized NE-SW cross section demonstrating the distribution pattern of
the continental deposits in the Bekaa Valley basin and inferred local structures. Gravely
formations are grouped because of the difficulty to establish boundaries from available
borehole data.
2) Version III
Basically, the success in dividing the sediments of the Bekaa basin into chronostratigraphical
units should assist in the task of wider (regional and beyond) stratigraphical correlation.
However, in the Quaternary System, chronostratigraphical sub-divisions bear limited spatial
correlation potential in comparison to their counterparts in older geological systems, in other
words they are more of local application (see Lateef, 2004).
In the Bekaa Valley, recognition of the various sub-types of deposits and establishment
of their provisional stratigraphical hierarchy based on both chronological data and field
relations permit the endeavour to divide these deposits into chronostratigraphical units. The

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provisional chronostratigraphical divisions, on the series level, of the continental infill of the
Bekaa basin are tabulated below. The prepared geological map is presented in Fig. 8.
Figure 8. Version III of the geological map of the Bekaa Valley based on
chronostratigraphical classification (Series level).
Q3-Q4a
Q3-Q4
Q3
Q2
Pl-Q2
Mc
Holocene
Upper Pleistocene-lower Holocene
Upper Pleistocene-Holocene
Upper Pleistocene
Middle Pleistocene
Pliocene-Middle Pleistocene
upper Middle Miocene-basal Upper Miocene
Q4a-b

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CONCLUSIONS
This work introduced the first comprehensive geological map of the continental fill,
dominantly Quaternary facies, of the Bekaa Valley, Lebanon. The contribution of this work to
the geology of Lebanon becomes apparent from comparison with the existing geological map
of the country. The new geological map affords the following advancements:
1) Subdividing the Quaternary continental deposits of the Bekaa Valley into stratigraphically
meaningful mappable units.
2) Revision and modification of the spatial extent of the Quaternary cover in the Bekaa Valley.
Quaternary deposits, as presented in this study, have wider distribution than hitherto
portrayed on the existing geological map of Lebanon (compare Fig.1 with the new geological
map). Indeed, most of what was depicted as “Miocene” or “Neogene” in the middle and north
Bekaa Valley (particularly the piedmont trains) is re-established as Quaternary in this study.
The Quaternary deposits extend northeastward across the Lebanese-Syrian border. This
almost doubles the aerial extent of the Quaternary deposits.
3) Also of importance is the recognition of new Miocene formation in the northeast of the
Bekaa Valley (Al Hermil Formation). The stratigraphical and paleoenvironmental aspects of
this newly established formation and its equivalency and correlation to a previously identified
Miocene formation from the middle and south Bekaa (i.e.Zahla Formation) is provided in
Lateef (2005). Regional tectonical significance of the emplaced basalts within this formation
is provided elsewhere (Lateef, 2004).
The results of this research contribute for the revision of the existing geological map of
Lebanon.

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ACKNOWELEDGMENTS
This work was accomplished with support from the Geography Department, Vrije
Universiteit Brussel and Geology and Mineralogy Department, Royal Museum for Central
Africa, Belgium.
REFERENCES
Dubertret, L., 1955, Carte géologique du Liban au 1/200000. Beyruth.
Lateef, A.S.A., 2004, Quaternary terrestrial stratigraphic correlations between the Levant and
the circum-North Atlantic region: current knowledge and constraints. Studia Quaternaria,
20: 61-72.
Lateef, A.S.A., 2005, Lithostratigraphical framework for the continental deposits of the
Bekaa Valley-Lebanon. 1st International Conference on the Geology of the Tethys, Cairo
University.
Walley, W.C. (unpublished), The stratigraphy and geological history of Lebanon: An outline.
AUB, Beirut, 27p.

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