HYDROLOGICAL ANALYSIS OF NONCONFORMITIES: IMPLICATIONS FOR INJECTION-INDUCED SEISMICITY IN THE MIDCONTINENT UNITED STATES

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Solid Earth, 11, 1803–1821, 2020
https://doi.org/10.5194/se-11-1803-2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
Geologic characterization of nonconformities using outcrop and core
analogs: hydrologic implications for injection-induced seismicity
Elizabeth S. Petrie1, Kelly K. Bradbury2, Laura Cuccio2, Kayla Smith2, James P. Evans2, John P. Ortiz4,
Kellie Kerner3, Mark Person3, and Peter Mozley3
1Western Colorado University, Geology Department, 1 Western Way, Gunnison, CO 81231, USA
2Utah State University, Geology Department, 4505 Old Main Hill, Logan, UT 84322-4505, USA
3New Mexico Institute of Mining and Technology, 801 Leroy Pl., Socorro, NM 87801, USA
4Johns Hopkins University, Department of Environmental Health and Engineering,
3400 N. Charles St., Baltimore, MD 21218, USA
Correspondence: Elizabeth Petrie (epetrie@western.edu)
Received: 12 February 2020 – Discussion started: 26 March 2020
Revised: 25 June 2020 – Accepted: 8 July 2020 – Published: 12 October 2020
Abstract. The occurrence of induced earthquakes in crys-
talline rocks kilometers from deep wastewater injection wells
poses questions about the influence nonconformity contacts
have on the downward and lateral transmission of pore-fluid
pressure and poroelastic stresses. We hypothesize that struc-
tural and mineralogical heterogeneities at the sedimentary–
crystalline rock nonconformity control the degree to which
fluids, fluid pressure, and associated poroelastic stresses are
transmitted over long distances across and along the noncon-
formity boundary. We examined the spatial distribution of
physical and chemical heterogeneities in outcrops and core
samples of the Great Unconformity in the midcontinent of
the United States, capturing a range of tectonic settings and
rock properties that we use to characterize the degree of past
fluid communication and the potential for future communi-
cation. We identify three end-member nonconformity types
that represent a range of properties that will influence di-
rect fluid pressure transmission and poroelastic responses far
from the injection site. These nonconformity types vary de-
pending on whether the contact is sharp and minimally al-
tered (Type 0), dominated by phyllosilicates (Type I), or sec-
ondary non-phyllosilicate mineralization (Type II). Our ob-
servations provide geologic constraints for modeling fluid
migration and the associated pressure communication and
poroelastic effects at large-scale disposal projects by provid-
ing relevant subsurface properties and much needed data re-
garding common alteration minerals that may interact readily
with brines or reactive fluids.
1 Introduction
Deep wastewater injection near the nonconformity between
the Phanerozoic sedimentary sequence and Proterozoic crys-
talline basement in the midcontinent United States (Sloss,
1963) is the primary means by which produced formation
fluids are disposed of in Class II injection wells (Murray,
2015). Increased rates of seismicity in this region are asso-
ciated with large volumes of wastewater injection (Ellsworth
et al., 2015; Keranen et al., 2013; Nicholson and Wesson,
1990; Petersen et al., 2016; Zhang et al., 2013); the reduc-
tion of friction on preexisting faults and pressure diffusion
away from the injection point are controlled by the perme-
ability structure of the rocks in the subsurface (Goebel and
Brodsky, 2018; Yehya et al., 2018). Recent midcontinent
seismicity nucleates on faults in crystalline rocks kilometers
from injection sites (Keranen et al., 2014; Weingarten et al.,
2015; Zhang et al., 2016) and spans timescales of months
to years post-injection, indicating that pore-fluid pressures
and/or poroelastic loads are transmitted across or along the
nonconformity or through connected fracture systems (in-
cluding joints, faults, and veins) in the crystalline rocks (Or-
tiz et al., 2019). The depths of seismicity (up to 11 km) at
some injection sites suggest that crystalline basement per-
meability is perhaps moderate to high (10−16 to 10−14 m2)
(Zhang et al., 2016) and is dynamically increased by ele-
vated fluid pressures (Rojstaczer, 2008). The observations
presented in this paper are also relevant to Class VI injec-
Published by Copernicus Publications on behalf of the European Geosciences Union.

1804 E. S. Petrie et al.: Geologic characterization of nonconformities
tion wells used for the geologic sequestration of CO2; sev-
eral of our analog sites include deep reservoirs being evalu-
ated for CO2sequestration (Leetaru et al., 2009; Leetaru and
McBride, 2009; Plains CO2 Reduction (PCOR) Partnership,
2020; Thorleifson, 2008).
Numerical modeling of fluid flow and/or loading stresses
associated with poroelastic effects across nonconformities
indicate that (1) the presence of a high-storativity, low-
permeability basal seal reduces the potential for basement-
induced earthquakes; (2) poroelastic effects can trigger seis-
micity far away from the injection location; (3) the presence
of conductive faults, including those that cut the nonconfor-
mity and those that are isolated in the basement, can provide
direct fluid or fluid pressure pathways; and (4) permeable
cross-nonconformity faults may exhibit high rates of seis-
micity (Chang and Segall, 2016; Goebel and Brodsky, 2018;
Ortiz et al., 2019; Yehya et al., 2018; Zhang et al., 2013).
In this paper we summarize geologic observations made at
the nonconformity zone, the altered rock volume surround-
ing the nonconformable contact. This zone varies in thick-
ness and is defined by mineralogic and structural alteration
of the protolith rocks surrounding the nonconformity. We
characterize nonconformity zones associated with Precam-
brian granite, gabbro, gneiss, and schists that are overlain
by porous sedimentary rocks including sandstone and mixed
carbonate–clastic sequences. Study site locations were cho-
sen based on their distribution within the midcontinent region
and the suite of lithologies present (Fig. 1). These analogs
represent the diversity of the nonconformity in the United
States midcontinent region and are analogs for deep fluid
injection from produced waters (Class II) and sequestration
of CO2(Class VI). At each site we document the lithology
and structural features of the rocks on either side of the non-
conformity to characterize the range of rock types associ-
ated with the contact and identify any evidence of past cross-
contact fluid flow. We present data on the mineralogic and
structural heterogeneities observed in the outcrop and core,
and these observations serve as proxies for variation in min-
eral alteration and deformation surrounding subsurface non-
conformity zones, which may impact the future migration of
fluids along and across the contact.
We find three end-member types of nonconformity zones.
These zones range from diffuse to sharp (Type 0), can be
phyllosilicate-rich (Type I), or can be dominated by non-
phyllosilicate secondary minerals (Type II). Each contact
type observed in this study has a range of mineralized tex-
tures and structural discontinuities. Due to weathering, defor-
mation, diagenesis, and fluid–rock interactions, the noncon-
formity zone may be hydraulically heterogeneous at scales
of millimeters to tens of meters and influence the migra-
tion of fluid and fluid pressures away from the injection
well. Characterizing variations in rock properties at the non-
conformity zone is critical for safe implementation of deep
fluid injection, as the dimensions and hydraulic properties of
the rocks in the nonconformity zones impact the subsurface
flow regimes (Ortiz et al., 2019). The lithologic character of
the nonconformity zone has implications for hydraulically
connected regions by allowing direct fluid communication,
changes in pore-fluid pressure, and/or poroelastic loads. Be-
cause pressure diffusion and fluid migration depend on the
permeability structure at a given location, our work can be
used to improve hydrogeologic models that test the impact
of lithologic changes and cross-nonconformity fractures on
the transmission of pore fluids and/or poroelastic stress. We
present results from hydrogeologic models based on obser-
vations of nonconformity zone characteristics, thereby test-
ing the impact various nonconformity zone types have on the
transmission of pore fluids.
2 Geologic setting
The North American craton, Laurentia, includes the Precam-
brian shields, the platforms and basins of the North Ameri-
can interior, and the reactivated Cordilleran foreland of the
southwestern United States (Fig. 1). The craton includes
Archean blocks, the Yavapai–Mazatzal and Grenville accre-
tionary belts, and failed rifts (Hoffman, 1988; Marshak et al.,
2017; Whitmeyer and Karlstrom, 2007). Precambrian ex-
humation produced erosional surfaces on the top of the crys-
talline basement, which were buried by Phanerozoic clastic
and marine sedimentary rocks (Marshak et al., 2017; Sloss,
1988). The nonconformities studied in this paper are lo-
cated within the Superior craton, an Archean basement com-
plex of granite–greenstone or higher-grade equivalent over-
lain by erosional remnants of early Proterozoic platform fa-
cies (Hoffman, 1988), the Yavapai–Mazatzal province, 1.76–
1.65 Ga juvenile arc terrane that includes the Central Plains
Orogen (Karlstrom and Humphreys, 1998; Sims, 1985; Sims
and Peterman, 1986), the Grenville province, 1.3–1.0Ga im-
bricate thrust slices formed during continent–continent colli-
sion (Rivers, 1997), and the Midcontinent Rift, an approxi-
mately 1.1 Ga failed rift system dominated by volcanic rocks
and basin-fill sedimentary rocks (Ojakangas et al., 2001)
(Fig. 1).
2.1 Study areas
2.1.1 Outcrop locations
Exposed at Presque Isle and Hidden Beach along the south-
ern shore of Lake Superior, Michigan, the nonconformity is
defined by Proterozoic Jacobsville Sandstone overlying early
Proterozoic altered peridotite crystalline basement (Fig. 2).
The geologic history of the serpentinized peridotite is not
well-constrained; it is thought that the peridotite was serpen-
tinized between 1.80 and 1.1 Ga (Gair and Thaden, 1968).
The overlying Jacobsville Sandstone is a dominantly fluvial
sequence of feldspathic and quartzose sandstones (Malone
et al., 2016), and at this study locality it consists of a vari-
ably indurated pebble to cobble conglomerate and a lentic-
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E. S. Petrie et al.: Geologic characterization of nonconformities 1805
Figure 1. Precambrian tectonic elements map with the location of the nonconformity analog study sites (after St. Onge et al., 2009). (a) Lake
Superior, Presque Isle, Michigan outcrop, (b) Gallinas Canyon, New Mexico outcrop, (c) R.C. Taylor 1 core, (d) CPC BD-139 core, and
(e) BO-1 core.
Figure 2. (a) Schematic lithologic log at Lake Superior, Michigan, where altered peridotite is overlain by the Jacobsville Sandstone at
Presque Isle and mineralized conglomerates of the Jacobsville Sandstone overlie the Compeau gneiss at Hidden Beach. (b) Presque Isle –
outcrop of small fault cutting the contact between the mineralized conglomerate of the Jacobsville Sandstone and the underlying altered
basement; the inset shows stockwork jasperoid veins in the underlying serpentinized peridotite basement and (c) Hidden Beach outcrops.
At this locality the Jacobsville Sandstone overlies the Proterozoic altered peridotite basement rocks. (d) Geologic map of the Marquette,
Michigan, field area showing the locations of Hidden Beach and Presque Isle; modified from Gair and Thaden (1968).
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1806 E. S. Petrie et al.: Geologic characterization of nonconformities
ular planar to cross-bedded light red quartz arenite (Fig. 2).
The Presque Isle outcrops are analogs for geologic seques-
tration of CO2in the deep saline Jacobsville Sandstone reser-
voir (Leetaru and McBride, 2009).
The nonconformity at Gallinas Canyon is exposed along
a 4 km long section in the southernmost Sangre de Cristo
Mountains, New Mexico (Figs. 1 and 3). The outcrop ex-
posure consists of the crystalline rocks of the Yavapai
province, highly deformed compositionally layered quartzo-
feldspathic gneiss, amphibolitic gneiss, felsite, biotite schist,
and granitic pegmatite (Lemen et al., 2015) overlain by the
Devonian to Mississippian shallow marine transgressive se-
quence of carbonate and clastic rocks of the Espiritu Santo
Formation. The Espiritu Santo Formation primarily consists
of limestone and dolomitic limestone, with a basal conglom-
eratic sandstone known as the Del Padre Member (Baltz and
Myers, 1999). The rocks are exposed within north-trending
fault-bounded blocks uplifted during the Neogene Laramide
Orogeny (Baltz and Myers, 1999; Lessard and Bejnar, 1976).
This location provides an analog for the Raton Basin to the
east where injection in Class II wells has been linked to base-
ment earthquakes that began in 2001 (Nakai et al., 2017; Ru-
binstein et al., 2014).
2.1.2 Core samples
The R.C. Taylor 1 core samples the Cambrian La Motte
Formation sandstone and sheared Proterozoic granitoids in
the Central Plains Orogen of the 1.6 Ga Yavapai–Mazatzal
province (Marshak et al., 2017; Sims, 1985; Whitmeyer and
Karlstrom, 2007) (Fig. 1). The borehole was drilled adja-
cent to the Cambridge Arch and is associated with a se-
ries of northwest-trending transpressional faults of the Cen-
tral Plains Orogen (Sims, 1985; Whitmeyer and Karlstrom,
2007) (Fig. 4). In this core, the arkosic La Motte Forma-
tion, regionally called the Reagan and Sawatch sandstones,
is a fine-grained, well-sorted glauconitic sandstone deposited
during a transgression and is an analog for Cambrian sand-
stones being evaluated for sequestration of CO2(Carr et al.,
2005; Miller, 2012).
The CPC BD-139 core, recovered from the Michigan
Basin, samples the contact between the Cambrian Mount
Simon Sandstone and Precambrian altered granitoid gneiss
of the Grenville Front Tectonic Zone (Figs. 1 and 5). The
Precambrian crystalline rocks captured in this core are char-
acterized as granitic to tonalitic gneiss (Easton and Carter,
1995) that form the basement of the Michigan Basin. The
Michigan Basin is a thermally complex intracratonic basin
situated over the lower peninsula of Michigan. Unexpectedly
high levels of thermal maturity in the Paleozoic strata of the
basin are thought to be attributed to elevated basal heat flow
occurring up until Silurian time and the prior existence of
∼2 km of Pennsylvanian and Permian strata that has since
been eroded (Everham and Huntoon, 1999). The Mount Si-
mon Sandstone reservoir is a unit of deep wastewater injec-
tion in Oklahoma, and it is also targeted for CO2storage
(Barnes et al., 2009; Dewers et al., 2014; Leetaru et al., 2009;
Liu et al., 2011).
The BO-1 core samples the lower Cambrian Mount Simon
Sandstone overlying a Precambrian layered intrusive com-
plex of altered gabbro and other mafic intrusions and fel-
sic dikes (Fig. 6) (Smith et al., 2019). The crystalline base-
ment rocks are part of the Northeast Iowa Intrusive Complex
and are associated with the Midcontinent Rift system (An-
derson, 2012). The Midcontinent Rift system extends from
Kansas to Lake Superior and then southward through Michi-
gan (Fig. 1). The geologic features associated with the Mid-
continent Rift system, include axial basins filled with basalt
and immature clastic rocks along with evidence of crustal
extension (Ojakangas et al., 2001). The BO-1 core is anal-
ogous to several geologic settings anticipated in the subsur-
face of the midcontinent region where lower Cambrian rocks
directly overly Precambrian mafic igneous rocks of the Mid-
continent Rift system along the Great Unconformity (Gilbert,
1962; Mossler, 1995). Northwest-trending fault systems near
the borehole were identified by magnetic lineaments and are
likely part of the regional NW–SE Belle Plaine Fault Zone
(Drenth et al., 2015). The Midcontinent Rift system is being
studied for deep injection of CO2(Abousif, 2015; Wickstrom
et al., 2010).
3 Characterization of the nonconformity
Given the recognized importance of direct fluid transmission,
variation in pressure, and poroelastic loads on induced seis-
micity (Chang and Segall, 2016; Ortiz et al., 2019; Yehya
et al., 2018; Zhang et al., 2013), we provide an overview
of rock properties observed at the nonconformity using inte-
grated outcrop-based studies in Michigan and New Mexico,
as well as analyses of cores from Michigan, Minnesota, and
Nebraska (Fig. 1).
3.1 Methods
To describe the nonconformity zone in the core and outcrop
and document structures and mineralogy across the bound-
ary, we use a variety of microscale to mesoscale methods
including lithological and structural logging of outcrop and
core, optical thin-section petrography and X-ray diffraction
(XRD) mineralogic studies, whole-rock X-ray fluorescence
(XRF) elemental analysis, and gas or air permeability mea-
surements, when possible. We evaluated fracture distribu-
tion in the outcrop and core, noting fracture types, the pres-
ence of fault and/or shear zones, and associated mineral-
ogy. XRD and XRF were carried out at Utah State Uni-
versity (USU); XRD analysis was carried out at the West-
ern Colorado University (WCU) petrography laboratory. At
USU, XRD analyses were done using a Panalytical X’Pert
Pro X-ray diffraction spectrometer (40 mA and 45 kV) with
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E. S. Petrie et al.: Geologic characterization of nonconformities 1807
Figure 3. (a) Gallinas Canyon, New Mexico, outcrop lithology log. (b) Precambrian granitic gneiss and schist are overlain by the Missis-
sippian Espiritu Santo Formation; red arrows mark the nonconformity, and blue lines mark the boundary of phyllosilicate alteration. (c) The
4 km long exposure in Gallinas Canyon; the nonconformity is cut by several centimeter to meter displacement faults. Red arrows mark the
nonconformity, and the fault is shown by the green line. (d) Geologic map of the Gallinas Canyon study area; modified from Hesseltine
(2019). Faults that cut the nonconformity are shown in red with a ball on the downthrown side; at the map scale the nonconformity is
relatively planar and parallels topographic contour lines.
monochromatic CuKαradiation utilizing X’Pert Highscore
software for phase analysis. Whole-rock XRF analyses were
conducted at the Washington State University Peter Hooper
GeoAnalytical Lab using a Thermo-ARL automated X-ray
fluorescence spectrometer. At WCU, XRD analysis was done
using a Brucker D8 X-ray diffraction spectrometer (40 mA
and 45 kV) with monochromatic CuKαradiation utilizing
DIFFRAC.SUITE software for phase analysis.
A total of 25 samples from the BO-1 drill core were se-
lected for gas permeability testing through Schlumberger
Rock Mechanics and Core Analysis Services. Profile per-
meability measurements were made in steady-state condi-
tions with a mini-permeameter whereby gas is injected di-
rectly onto the core slab surface. The profile permeame-
ter has a measurable permeability range of 0.1 mD to 3 D
(∼9.9 ×10−17 to 3.0 ×10−12 m2).
To illustrate the effects of reduced permeability above
the nonconformity on fluid migration we compare three hy-
drogeologic models of basal reservoir injection that con-
sider continuous and discontinuous zones of altered low-
permeability rocks above the basement. We develop three-
dimensional models to assess fluid migration along crys-
talline basement faults using MODFLOW, a public domain
finite-difference groundwater flow code (Harbaugh and Mc-
Donald, 1996; Harbaugh et al., 2000) that solves the follow-
ing groundwater equation:
∂
∂x Kx
∂h
∂x +∂
∂y Ky
∂h
∂y +∂
∂z Kz
∂h
∂z 
=Ss
∂h
∂t +Q(x, y, z, t ), (1)
where his the hydraulic head (L), Kis the hydraulic con-
ductivity tensor (L T−1), Ssis the specific storage (L−1), Qis
the fluid injection source term (i.e., injection well; T−1), and
tis time (T). Equation (1) represents single-phase, constant-
density groundwater flow in a three-dimensional Cartesian
coordinate system. Hydraulic conductivity is a lumped pa-
rameter that includes the influence of fluid and medium prop-
erties and is defined as K=krfg(m−1), where Kis hy-
draulic conductivity (ms−1), kis intrinsic permeability (m2),
rfis fluid density (997 kg m−3; water), gis the accelera-
tion due to gravity (9.81 m s−2), and mis the dynamic vis-
cosity of the fluid (8.9×10−4kg (m s−1)−1). Multiple re-
searchers have implemented similar groundwater flow mod-
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1808 E. S. Petrie et al.: Geologic characterization of nonconformities
Figure 4. (a) Lithologic log of the R.C. Taylor 1 core, Nebraska,
from 3984 to 4038 ft (1214–1231 m) of measured depth. The non-
conformity occurs at 4018 ft (1225 m). Fracture density in the
core is based on the number of fractures per meter of core. Four
lithologic units are identified in the core, including sandstone,
sedimentary-rock-hosted shear zone, altered basement shear zone,
and minimally altered basement. The nonconformity occurs be-
tween the altered basement shear zone and overlying sandstone of
the La Motte Formation. (b) Photographs of the R.C. Taylor 1 core.
Both the basement shear zone (SZ) and overlying sandstone are cut
by veins (V) of quartz, calcite, and Fe-oxides. (c) The borehole loca-
tion is shown on the Precambrian basement map from Sims (1985).
Figure 5. (a) Lithologic log of the CPC BD-139 core, Michi-
gan, from 1404–1412.1 m of measured depth. There are five main
lithologic units identified, including sandstone, dolomitized and un-
dolomitized finely foliated gneiss, and dolomitized and undolomi-
tized gneiss with subhorizontal white veins. (b) Photographs of
the CPC BD-139 core showing the core between ∼1404.5 and
1405.5 m as well as contact between the Cambrian Mount Simon
Sandstone (light tan) and the underlying Precambrian gneiss. The
gneiss directly at the contact is fine-grained, tan, and dolomitized.
This is underlain by green altered gneiss with subvertical pink frac-
tures. This lithology grades into a dark grey gneiss with subhorizon-
tal white veins (core between 1411.5 and 1412.5 m), which extends
through the bottom of the logged section. (c) Geologic map St. Clair
County, Michigan; modified from Milstein (1987).
els in MODFLOW to investigate pore pressure propagation
associated with basal reservoir injection (Zhang et al., 2013,
2016), and a more exhaustive description of cases and bound-
ary conditions can be found in Ortiz et al. (2019). Each of
our model simulations includes a 100 m thick basal reser-
voir (3 ×10−15 m2) underlain by 9.9 km of relatively low-
permeability (kx=kz=3×10−17 m2) crystalline basement
rock. A 20 m wide conduit-barrier fault (kz/kx=105; kz=
3×10−10 m2) is present in all simulations as is an injection
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E. S. Petrie et al.: Geologic characterization of nonconformities 1809
Figure 6. (a) BO-1 lithologic log with select representative core samples of each of the major lithologic units. Above the nonconformity,
the analog reservoir, or the injection unit, the Cambrian Mount Simon Sandstone is porous with evidence for both dissolution and oxidation
front. The crystalline basement rock consists of foliated, intensely altered, and altered metagabbro with localized faulting, variably altered
and faulted diabase localized intrusions, pegmatite dikes, and, at greater depths, relatively unaltered and less-deformed metadiorite. Gas
permeability measurements were made on 25 core samples spanning the nonconformity interface. For each core sample tested, five spot
measurements were made (locations shown by white circles). For relative comparison across the contact and within the various lithologic
units, permeability data (millidarcy, mD) are plotted using a log scale and are the averaged values for each sample. (b) Geologic map modified
from Mossler et al. (1995).
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1810 E. S. Petrie et al.: Geologic characterization of nonconformities
well located 150 m from the fault zone. Wellhead pressures
reached over 50 m excess hydraulic head after 4 d in response
to 5000 m3d−1of continuous injection.
3.2 Results
3.2.1 Lake Superior, Michigan
Outcrops of the nonconformity between late Proterozoic Ja-
cobsville Sandstone and early Proterozoic altered peridotite
crystalline basement are exposed at Presque Isle and Hid-
den Beach along the southern shore of Lake Superior, Michi-
gan (Fig. 2) (Lewan, 1972). At Presque Isle the topographic
relief of the basement nonconformity varies by 2.5 m over
a distance of 1100 m of outcrop (Cuccio, 2017), is locally
cut by small-offset faults (30 cm of throw), and is com-
posed of mineralized conglomerate in direct contact with
the underlying serpentinized peridotite or is transitionally
interbedded with the overlying sandstone (Fig. 2b). Where
present, the conglomerate consists of subangular to rounded
chalcedony, gneiss, and greenstone cobble clasts with fine-
grained, poorly sorted, hematite-cemented angular quartz
grains.
At Hidden Beach, poorly consolidated basal conglomer-
ates of the Jacobsville Sandstone are in contact with the
Precambrian Compeau Creek Gneiss. The quartz arenite
consists of fine-grained, angular, moderately sorted quartz
with some feldspar. Distinctive bleached open-mode frac-
tures or reduction spots are associated with the lower Ja-
cobsville Sandstone and range in orientation from near-
vertical to bedding-parallel. The near-vertical reduction frac-
tures (Fig. 2c) are not observed to extend into the basement.
Optical petrography across the transition from red sand-
stone protolith to a bleached fracture zone at Hidden Beach
reveals a reduction in hematite grain coatings and cements.
Whole-rock XRF analysis of the bleached areas of Jacob-
sville Sandstone indicates a minor depletion of K2O and a
minor enrichment of FeO and MgO relative to the unaltered
Jacobsville Sandstone (Fig. 7). At Presque Isle, mineral alter-
ation products in the conglomerate include nontronite, with
trace zeolites and iron oxides (Fig. 7). The underlying ser-
pentinized peridotite is black to brown, with abundant white
carbonate mesh veinlets and localized stockwork jasperoid
veins up to 10 cm wide (Fig. 2). Jasperoid mineralization oc-
curs along a few small faults that cross the nonconformity
(Cuccio, 2017).
3.2.2 Gallinas Canyon, New Mexico
Devonian to Mississippian carbonate and clastic rocks of
the Espiritu Santo Formation deposited on the Proterozoic
quartzo-feldspathic and amphibolitic gneiss, biotite schist,
and granitic pegmatite (Lemen et al., 2015) are exposed
along a 4 km long section in Gallinas Canyon, eastern San-
gre de Cristo Mountains, New Mexico. The nonconformity
is cut by centimeter to meter displacement faults; at this lo-
cation we characterize both the faulted and un-faulted non-
conformity zone (Hesseltine, 2019; Kerner, 2015). The top
of the basement is defined by a phyllosilicate-rich zone with
variable thickness, 0 to >5 m, that is truncated by the Del
Padre Sandstone. Locally the Del Padre Sandstone is laterally
discontinuous (Hesseltine, 2019) but is reported to be up to
15 m thick, filling depressions in underlying crystalline rock
elsewhere in New Mexico (Armstrong and Mamet, 1974).
The carbonate and clastic rocks of the Espiritu Santo For-
mation include 1 m thick massive, fine-grained, rounded to
sub-rounded sandstone with calcite nodules, ∼1 m of mi-
crocrystalline dolomite that transitions upward into a chert
nodule limestone, interbedded mudstone and limestone, and
a massive microcrystalline limestone bed. A phyllosilicate-
rich zone directly below the nonconformity is approximately
60 cm thick and is a poorly lithified zone that marks the tran-
sition from highly altered (weathering and hydrothermal al-
teration) to minimally altered crystalline rock (Fig. 8). The
Precambrian crystalline rocks are cut by large thrust faults
and smaller-scale normal faults (Baltz and Myers, 1999;
Lessard and Bejnar, 1976) with some faults juxtaposing sed-
imentary and crystalline rock (Fig. 8d).
The predominant lithology of the crystalline basement
is gneiss, with minor schist, pegmatitic granite, and basalt.
Mineral alteration is greatest directly below the noncon-
formity. This zone is enriched in sericite within feldspars
and clay minerals (mixed with hematite and associated with
replacement of micas) (Fig. 8). Where cut by faults the
nonconformity-associated phyllosilicates form a matrix that
surrounds more rigid grains such as quartz, suggesting that
deformation in this unit was accommodated by granular flow,
a process associated with high pore-fluid pressure (Paterson,
2012). Microscopic fracturing has occurred within the crys-
talline basement; these fractures are mineralized with iron
oxide, sericite, chert, and calcite. The majority of fractures
within the crystalline basement occur along weak grains such
as sericitized feldspar and altered mica or cut across quartz
and feldspar grains. Authigenic calcite is rare within the crys-
talline basement, though it commonly occurs as coarsely
crystalline calcite cement within grain fractures in feldspar
and sericitized feldspar.
Where faults cut the altered crystalline basement locally,
cataclasites are found throughout the fault core. Where
faulted, the sedimentary rock damage zone includes large
twinned calcite grains in fracture-filling cements and cata-
clasites that lie along the edges of the calcite veins. The cata-
clasites include pulverized quartz and feldspar grains, chert,
pulverized protolith, and clay- and iron-oxide-rich minerals.
Quantitative microprobe analyses of the carbonate and fine-
grained matrix composition within the sedimentary and base-
ment fault cores reveal that all calcite vein elemental values
have a slightly more reduced level of Fe and Mg substitution
for Ca than the calcite matrix. The fine-grained matrix within
the sedimentary fault core is nearly pure silica, whereas the
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E. S. Petrie et al.: Geologic characterization of nonconformities 1811
Figure 7. Petrographic summary figure, photomicrographs, and X-ray diffraction results of nonconformity units studied at Presque Isle,
Michigan. (a) Jacobsville Sandstone arenite (100×, ppl), (b) Jacobsville Sandstone altered conglomerate (200×, ppl), (c) basement calcite-
rich slip surface in dolomitized, serpentinized peridotite (200×, xpl), (d) basement serpentinized peridotite (100×, ppl), (e) basement slip
surface within Fe-rich serpentinized peridotite (red) (100×, ppl).
fine-grained matrix within the crystalline basement fault core
is aluminum-rich (Fig. 8).
3.2.3 R.C. Taylor 1 Core, Nebraska
Core from the R.C. Taylor 1 wildcat well was obtained in
1953 in south-central Nebraska (Table S1 in the Supple-
ment). We examined 19.2 m of core recovered over the Cam-
brian La Motte Formation sandstone and sheared Protero-
zoic granitoids in the Central Plains Orogen of the Yava-
pai province (Marshak et al., 2017; Sims, 1985; Whitmeyer
and Karlstrom, 2007). The arkosic La Motte Formation, re-
gionally called the Reagan and Sawatch sandstones, is a fine-
grained, well-sorted glauconitic sandstone (Fig. 6).
The basal La Motte Formation and uppermost basement
are cut by quartz, calcite, dolomite, and iron-oxide veinlets
(Fig. 9). Iron-oxide veins cut quartz veins, and both are cut
by calcite veins, providing evidence for fracture reactivation
(Fig. 9). Below the La Motte Formation is a phyllosilicate-
rich zone composed of a 40 cm thick highly altered base-
ment shear zone that overlies a minimally altered basement
shear zone; both are comprised of fine crystalline sericitized
feldspar and chlorite-rich shear zones and overlie the coarse-
crystalline, minimally altered granitic basement containing
some sericitized feldspar (Fig. 9).
The altered basement shear zone is composed of quartz,
feldspar, biotite, chlorite, and dolomite (Fig. 9). Quartz
and feldspars are disintegrated; well-developed chlorite,
hematite, and magnetite are altered from biotite, and granular
disintegration has resulted in clay development. Open pore
space occurs between host-rock grains and neo-formed clays.
The basement shear zone is characterized by feldspar, quartz,
mica, and the alteration minerals chlorite and dolomite
(Fig. 9). The shear zones contain chlorite-lined slip surfaces
and S–C fabrics within chloritized zones. The shear fab-
rics are cut by open-mode quartz, sparry calcite, iron-oxide,
and dolomite veins. The basal, moderately altered basement
unit is a coarse-crystalline granite composed of feldspar,
quartz, biotite, and hornblende (Fig. 9). Chlorite is present
and associated with minor shear fabrics. Open-mode calcite,
dolomite, and quartz veins parallel and cross-cut the chlorite-
rich shear fabrics and cut quartz and feldspar crystals (Fig. 9).
In the coarse-crystalline granite, altered feldspars contain
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1812 E. S. Petrie et al.: Geologic characterization of nonconformities
Figure 8. Petrographic and elemental analysis summary of nonconformity units at the Gallinas Canyon site. Thickness is measured in meters
from base of outcrop section; the red line represents a fault. Elemental analysis shows a similar calcite composition of the veins within
the sedimentary sequence and the Precambrian basement faults. (a, b) The Espiritu Santo sandstones are clay-rich calcite-cemented quartz
sandstones; in the fault core, (b) the sandstones are cut by twinned calcite veins and stylolitic textures and contact cataclastic. (c) Adjacent
to the nonconformity the granitic basement contain fractures in the micas and sericitized feldspars. (d, e) Basement alteration decreases
away from the nonconformity, with a phyllosilicate matrix surrounding quartz grains and sericitization of feldspars occurring 10 m from the
nonconformity.
sericite that has formed adjacent to twin planes. Open-mode
fractures mineralized with dolomite, calcite, and hematite oc-
cur in the lower 7 m of the La Motte Formation and are ob-
served through the underlying granitic shear zone covering
12.5 m of core.
3.2.4 CPC BD-139 Core, Michigan
The CPC BD-139 core, obtained in 1964 for the design of
a brine disposal well, samples the contact between the Cam-
brian Mount Simon Sandstone and Precambrian altered gran-
itoid gneiss of the Grenville Front Tectonic Zone (Table S1
in the Supplement). We divide the CPC BD-139 core into
three lithologic units: a laminated sandstone, a finely foliated
gneiss, and a gneiss with subhorizontal white veins.
Sandstone grains are rounded to sub-rounded and moder-
ately to well-sorted. The Cambrian Mt. Simon Sandstone in
the Michigan Basin is characterized as a porous (5 %–15 %
pore space) arenite to sub-arkosic sandstone (Leeper, 2012).
Permeability in the basal Mt. Simon Sandstone is reported
to be between 1 ×10–16 and 1 ×10–12 m2(Frailey et al.,
2011).
A discrete boundary separates the Mount Simon Sand-
stone from the underlying altered granitoid gneiss (Fig. 10).
The uppermost 30 cm of the basement is composed of a tan,
fine-grained dolomite horizon that grades into a dark green
foliated gneiss cut by pink subvertical fractures over a span of
∼5 cm (Fig. 10). The basal meter of the Mount Simon Sand-
stone is a tan, finely laminated arenite with minor amounts of
iron-rich clay. The quartzo-feldspathic granitoid gneiss near
the contact contains the following alteration products: zeo-
lites, vermiculite, Fe- and Mn-oxides, and carbonates includ-
ing dolomite (Fig. 10). Dolomitization of the basement host
rock reappears ∼2 m below the nonconformity. The origi-
nal basement foliation is preserved and is associated with
micrometer-scale crystalline dolomite grains, radiating sil-
ica crystals, and subhorizontal calcite and dolomite open-
mode veins (Fig. 10). Trace amounts of ankerite, clinochlore,
and vermiculite are also present in the dolomitized basement
rocks (Fig. 10).
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E. S. Petrie et al.: Geologic characterization of nonconformities 1813
Figure 9. Petrographic summary, photomicrographs, and X-ray diffraction results of nonconformity units studied in the R.C. Taylor 1 core;
(a) La Motte Formation sandstone, rounded to sub-rounded, poorly sorted quartz sandstones (100×, ppl and xpl); (b) lower La Motte For-
mation, opaque Fe-oxide cements and vein fill, porosity shown by blue epoxy, (100×ppl); (c) top Precambrian crystalline altered basement
shear zone. Syntaxial veins are mineralized with quartz, reactivated, and mineralized with Fe-oxide then sparry calcite. There is some poros-
ity between rigid grains and neo-formed clays (50×PPL); (d) altered basement shear zone, chlorite-lined shear planes, sericitization of
feldspars along twining lamella (100×ppl); (e) coarse-crystalline sericitization of feldspars adjacent to twin lamellae (150×XPL).
3.2.5 BO-1 Core, Minnesota
The BO-1 core was originally collected in 1962 as part of
an exploratory mining project in Fillmore County, south-
east Minnesota (Gilbert, 1962) (Table S1 in the Supplement).
This core provides a continuous 300 m section of altered and
mineralized rocks of lower Cambrian Mount Simon Sand-
stone overlying a Precambrian layered intrusive complex of
altered and unaltered gabbro, other mafic intrusions, and fel-
sic dikes (Smith et al., 2019; Fig. 11).
Sedimentary sequences in BO-1 extend to ∼1.2 km where
the nonconformity is marked by an approximately 12 cm
zone of pervasive leaching and iron-hydroxide staining
(goethite). Intense alteration extends into the basement rocks
for ∼21 m, with ∼50 m of argillitic and propylitic alter-
ation and/or fracture mineralization observed to ∼402 m of
depth (Fig. 11). Localized faults as well as hybrid and open-
mode fracture surfaces intersect the sampled basement core
from within ∼1 cm of the nonconformity contact and extend
to 475.5 m; fracture density decreases with depth (Fig. 11).
Slip surfaces exhibit oblique to dip-slip slickenlines, range
from millimeters to centimeters thick, and are either coated
in clay or contain mineral infillings (±carbonate, ±silica,
±chlorite, ±iron oxides).
The Mount Simon Sandstone contains a ∼0.5 m zone of
intense iron-hydroxide (goethite) alteration at the noncon-
formity (Fig. 11). This iron-hydroxide oxidized zone ex-
tends for several meters into the slightly altered and meta-
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1814 E. S. Petrie et al.: Geologic characterization of nonconformities
Figure 10. Petrographic summary, depth in meters measured, photomicrographs, and X-ray diffraction results of nonconformity units studied
in the CPC BD-139 core. (a) The basement sample at the contact is an argillaceous dolomitized gneiss with dolomite veins (XPL); (b) folia-
tion defined by quartz–feldspar–clinochlore fabric with iron alteration of potassium feldspar grains (XPL); (c) banded quartz–feldspar gneiss
with common sericitization of potassium feldspar grains (XPL); (d) carbonate vein with colloform zeolite rim and euhedral quartz crystals
(XPL); (e) dolomitized gneiss with common quartz and carbonate veins (XPL).
morphosed crystalline basement rock. From petrographic
and XRD analyses, we identify mineralogical assemblages
(dolomite, siderite, iron oxides, iron hydroxides, illite, smec-
tite, kaolinite–serpentinite, vermiculite) and textures that are
indicative of weathering, diagenesis, and multiple episodes
of fluid–rock interactions coupled with deformation within
the broad ∼50 m zone of intense alteration also marked by
abundant structural discontinuities (fractures, faults, veins)
across the nonconformity zone (Fig. 11).
Measured gas permeability values are highest above the
nonconformity within the porous Mount Simon sedimentary
reservoir (up to 1000 mD) and vary significantly from 0 to
500 mD below the nonconformity contact. Locally perme-
ability increases in direct correlation with the presence of
structural discontinuities (Fig. 6).
3.3 Hydrogeologic models
The first model (Fig. 12a) is a Type 0 nonconformity, rep-
resented by a sharp contact between the basement and
overlying injection reservoir. The second model simula-
tion, a Type I nonconformity, includes a 20 m thick, low-
permeability (kx=kz=3×10−18 m2) zone (Fig. 12b); this
layer is 1 order of magnitude less permeable than the base-
ment host rock and a further 1 order of magnitude less per-
meable in the fault core. The continuous low-permeability
zone reduces the permeability of the basement fault dam-
age zone by 4 orders of magnitude, making the fault dam-
age zone nonconductive. Pressure does not propagate into
the crystalline basement although there was some diffusion
of the 2 m excess hydraulic head front to depths ≤500 m. In
the third simulation, a discontinuous low-permeability zone
is present (Fig. 12c). Where this zone is absent, the pressure
front propagates into the basement along the fault damage
zone to a depth of 2.5 km. The fault zone was not blocked
by the low-permeability zone, and elevated pore pressures
propagated downward to depths of 2.5 km via the fault zone
(Fig. 12c). Elevated fluid pressures likewise appeared to be
forced down in other areas where the low-permeability zone
pinches out, such as towards the right-hand side of Fig. 12c.
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E. S. Petrie et al.: Geologic characterization of nonconformities 1815
Figure 11. X-ray diffraction mineralogy and photomicrographs of BO-1 drill core samples showing representative compositions and textures
across the nonconformity interface contact. Samples within centimeters of the contact (1–3) are strongly weathered, altered, and slightly
metamorphosed gabbro-norite. Alteration and diagenesis assemblages include iron-oxide hydroxides with chlorite, ankerite, and dolomite.
Alteration extends for ∼50 m into the basement. Sample 3 illustrates millimeter-scale offset across the argillite layer. Note the fracture
permeability (blue epoxy) parallel to the slip surface. Fracture surfaces within Sample 4 at 34 m below the contact are several millimeters
of mixed chlorite–clay alteration and fine-scale permeability (blue epoxy). Sample 5 at 1304.9 m shows multiple phases of fluid–rock in-
teractions coupled with dilation, serpentinization, and dolomitization. There is a multi-layered clay-rich fault core gouge within Sample 6
at 1332 m or ∼70 m below the contact. Note the open fractures within the central portion of the fault core gouge (blue epoxy). At 1472 or
272 m below the contact within the meta-grano-diorite unit, clay alteration is observed within feldspar grains at the microscale.
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1816 E. S. Petrie et al.: Geologic characterization of nonconformities
Figure 12. Cross-sectional views of pore pressure envelope propa-
gation resulting from injection into a reservoir underlain by a low-
permeability altered zone. Excess hydraulic heads after 4 years of
constant-rate injection are presented for a Paleozoic conduit-barrier
fault scenario. (a) Faulted Type 0 nonconformity with no low-
permeability altered zone; (b) Type I nonconformity with an altered
zone present as a 20 m thick confining layer (represented by two
horizontal grey lines) that is continuous such that the reservoir and
basement fault zones do not connect; (c) Type I nonconformity with
a discontinuous altered zone that pinches in and out in 20 m hori-
zontal intervals (i.e., undulating) but with the reservoir and base-
ment fault zones fully connected. Results are zoomed in to the top
3 km ×3 km of the model domain. Vertical grey lines indicate the
location of the fault zone. The injection well location is indicated
on top of each panel. Transition from grey to dark blue (and all
subsequent contour lines) denotes a 2 m increase in hydraulic head
(0.02 MPa). Adapted from Ortiz (2017; for details of the modeling
approach, see Ortiz et al., 2019).
4 Discussion
The nonconformities examined in this study range from
sharp contacts to zones several meters thick and exhibit a
range of mineralized textures and structural discontinuities
(Table S2 in the Supplement). We observe mineralogic alter-
ations across the nonconformity that are expected to impact
diffusivity and storativity, and the sites evaluated provide ge-
ological and hydrogeological analogs that aid in understand-
ing the impact circulating fluids may have on altering rock
properties at depth (Oliver et al., 2006). Based on observa-
tions made in this work, we divide nonconformities into three
end-member types (Table 1): Type 0 – a sharp contact be-
tween sedimentary strata and basement rocks; Type I – an
interface dominated by phyllosilicates; and Type II – an inter-
face dominated by non-phyllosilicate secondary mineraliza-
tion (Table 1). All the nonconformity types observed in this
study are cut by structural discontinuities; therefore, several
possible contact subtypes exist within these three proposed
end-members (Fig. 13). Based on our observations, struc-
tural and mineralogical heterogeneities at the sedimentary–
crystalline rock nonconformity are thought to control the de-
gree to which fluids, fluid pressure, and associated poroe-
lastic stresses are transmitted over long distances across and
along the nonconformity boundary. The structural elements
Figure 13. Proposed geologic schematics of the nonconformity
contact region. (a) Type 0 – sharp contact expected to prevent di-
rect fluid pressure communication across the contact while promot-
ing migration parallel to the contact distributing fluids laterally.
(b) Type I – phyllosilicate-dominated zone above the crystalline
basement is expected to inhibit fracture propagation across the non-
conformity, prevent fluid migration due to permeability contrast,
and promote lateral migration; downward fluid migration can occur
at a permeable fault zone. (c) Type II – secondary-mineralization-
dominated zone with lateral migration due to permeability contrast;
mineralization due to fluid–rock interactions suggests that deep
fluid circulation occurs even without enhanced permeability from
fractures. All nonconformity types may be cut by structural discon-
tinuities. Blue arrows indicate potential flow paths of injected fluids.
and fluid-related alteration patterns observed in these analog
sites support the hypothesis that the nonconformity interface
zone influences or controls the potential for cross-contact
fluid flow and distribution of fluids within the crust.
Our collective field and core observations in various base-
ment tectonic settings document the occurrence of signifi-
cant variations in altered or mineralized zones that lead to
contrasts in permeability across the nonconformity. Where
present, the structural discontinuities include small-offset
faults, shear fractures, and veins. In thin section we note evi-
dence for dissolution, recrystallization, new mineral growth,
and veins that reflect mineralization or deformation at depth
and are not the result of alteration due to weathering alone.
Crack–seal textures and calcite twinning lamella suggest vein
mineralization at depth (Burkhard, 1993), and the reacti-
vation of preexisting fractures documents episodic fracture
growth (Davatzes and Hickman, 2005; Laubach et al., 2004).
At a Type 0 nonconformity, the nonconformity zone is ex-
pected to prevent direct fluid pressure communication across
the contact due to a significant contrast in rock permeabil-
ities that would hinder cross-contact fluid migration while
promoting migration parallel to the contact, distributing flu-
ids laterally away from the injection site (Fig. 13a). At a
Type I nonconformity, a phyllosilicate-dominated contact is
expected to inhibit fracture propagation across the noncon-
formity (Ferrill et al., 2012; Larsen et al., 2010; Schöpfer
et al., 2006) and therefore maintain a significant permeabil-
ity contrast, preventing direct fluid migration. In such cases,
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E. S. Petrie et al.: Geologic characterization of nonconformities 1817
Table 1. Summary of nonconformity types and associated features.
Nonconformity type Site location Summary features
Type O Hidden Beach; Marquette, Michigan Sharp, discrete nonconformity contact ±topographic
variations between porous sedimentary sequences and
nonporous crystalline basement
Type I Gallinas Canyon, New Mexico
R.C. Taylor core
Altered contact with phyllosilicate mineralization;
faults within basement only and/or cross-cutting non-
conformity contact into overlying sedimentary rocks,
open-mode veins in basement, and overlying sedimen-
tary rocks
Type II Presque Isle; Marquette, Michigan
CPC BD-139 core
BO-1 core
Altered contact with non-phyllosilicate secondary min-
eralization and hydrothermal mineralization; faults
within basement only and/or cross-cutting nonconfor-
mity contact into overlying sedimentary rocks; open-
mode and hybrid veins in basement and overlying sedi-
mentary rocks
nonconformities result in a poor hydrologic connection be-
tween the sedimentary section and deeper basement rocks
(Fig. 13b).
However, repeated brittle failure and mineralization, ob-
served in Type I nonconformities, suggest that phyllosilicate-
dominated shear zones can act as a zone of mechanical weak-
ness that can be reactivated, allowing for the development
of fracture permeability. In this fractured nonconformity we
observed alteration as deep as 5 m below the nonconformity
in the crystalline rocks examined; however, previous work
highlights the potential for fractures and connectivity to base-
ment fault zones at much greater depths (Duffin et al., 1989).
Preexisting basement shear zones that are reactivated may al-
low future fluid circulation during injection scenarios.
Type II nonconformities (Table 1, Fig. 13c, Table S2) are
mineralized contacts that include secondary alteration miner-
als found within 10 cm to several meters below the noncon-
formity. The mineralization due to fluid–rock interactions at
Type II nonconformities suggests that deep fluid circulation
occurs even without enhanced permeability from fractures
(Cuccio, 2017) (Fig. 12c). This nonconformity type could
prevent brittle deformation but may be more influenced by
poroelastic loads. The impact of these contacts on hydrogeo-
logic properties is not yet well-understood or modeled.
The impact the morphology of the nonconformity has on
the downward propagation of fluid pressures into the crys-
talline basement has been shown by several numerical hy-
drogeologic studies (Ortiz et al., 2019; Segall and Lu, 2015;
Yehya et al., 2018; Zhang et al., 2016). Models suggest that
direct pore-fluid pressure communication (Ortiz et al., 2019;
Segall and Lu, 2015; Yehya et al., 2018) and significant
changes in poroelastic stress (Goebel and Brodsky, 2018;
Zhang et al., 2016) can occur well way from the injection
zones. Numerical models predict that nonconformities with
throughgoing fractures distribute fluid deeper into the base-
ment rocks and that direct pore pressure communication can
destabilize faults at depth (Ortiz et al., 2019; Segall and Lu,
2015; Yehya et al., 2018). All the nonconformity types ob-
served here are cut by structural discontinuities, and sev-
eral possible contact subtypes exist within these three pro-
posed end-member scenarios (Fig. 12). Fractures, especially
fault zones, are expected to distribute fluids and propagate
fluid pressures to a greater depth regardless of nonconformity
type (Yehya et al., 2018). Because nonconformity interface
zones with preexisting deformation fabrics may be preferen-
tial flow pathways that distribute fluid pressure away from
the injection zone, high-permeability damage zones transmit
fluid pressure to greater depths than non-conduit fault zones
(Yehya et al., 2018).
To illustrate the effects of reduced permeability above the
nonconformity and the impact of permeable fault zones on
fluid migration, we compare three models of basal reser-
voir injection that consider continuous and discontinuous
zones of altered low-permeability rocks above the basement
(Fig. 12). The Type 0 nonconformity, represented by a sharp
contact between the basement and overlying injection reser-
voir, results in lateral migration away from the injection well
and downward migration whereby it encounters a fault zone
(Fig. 13a). In the second model simulation, a Type I non-
conformity, the presence of a 20m thick, low-permeability
zone and no connection between basement and sedimentary
fault zones result in lateral migration and pressure does not
propagate into the crystalline basement (Fig. 12b). The third
simulation models a Type I nonconformity with a discontin-
uous low-permeability zone; where this zone is absent, the
pressure front propagates into the basement along the con-
nected fault damage zone to a depth of 2.5 km and elevated
fluid pressures appear to be forced downward where the low-
permeability zone pinches out (Fig. 13c).
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1818 E. S. Petrie et al.: Geologic characterization of nonconformities
Our collective field and core observations document the
occurrence of significant variations in altered or mineralized
zones, which would impact permeability values associated
with the nonconformity zone, and indicate that alteration
coupled with abundant structural discontinuities can result in
relatively higher permeability that extends for tens of meters
into the crystalline basement rock below the nonconformity.
5 Conclusions
We define key rock types and structural elements of the
nonconformity zone and split the analog nonconformities
into three end-member types. The three nonconformity end-
member types provide a broad hierarchy of nonconformi-
ties in the midcontinent (Table S1) and are observed at non-
conformity sites elsewhere. We expect these nonconformity
types to either distribute fluid pressure away from the injec-
tion point or provide direct communication with basement
rocks, moving fluids to a greater depth across the nonconfor-
mity. We observe that fractures cut all nonconformity types
and expect in these cases that changes in fluid pressure or
poroelastic loads could result in triggered earthquakes within
basement rocks (Chang and Segall, 2016; Zhang et al., 2013).
Numerical modeling of Type 0 and Type I end-members that
include fault zones predicts downward propagation of fluid
pressure and changes to poroelastic loads. The data presented
here can be used to improve model inputs for evaluation
of cross-contact fluid and pressure communication, whether
through the creation or modification of existing permeability,
poroelastic pathways, or rheological changes associated with
fluid–rock interactions. We show that conditions along the
nonconformity zone vary, and the data from outcrop and core
observations also suggest that injection of brines at depth
may drive mineralogical alteration and potential fault zone
weakening; these data can also be used to understand the im-
pact that long-term storage of chemically reactive fluids has
on rock properties (Callahan et al., 2020). Once fluids pene-
trate the basement, flow is likely controlled by fracture and
fault systems, and the reactivation of preexisting structures
is possible. However, microporosity within basement rocks
may enhance mineralogical changes over the long term and
transmit fluids deeper in the basement while promoting short-
term lateral migration along the nonconformity.
Our observations illustrate that the contact should not be
treated as an impermeable barrier to fluid flow nor as one
cut by faults of various permeabilities but should instead be
evaluated on a site-by-site basis prior to injection of large
fluid volumes.
Data availability. Underlying and ongoing research data related
to this paper are listed in the Supplement in Table S1. Access
to underlying data associated with student theses can be found
through university websites: https://digitalcommons.usu.edu/etd/
6889/ (last access: 17 September 2020, Cuccio, 2017) and https://
digitalcommons.usu.edu/etd/7497/ (last access: 17 September 2020,
Hesseltine, 2019).
Supplement. The supplement related to this article is available on-
line at: https://doi.org/10.5194/se-11-1803-2020-supplement.
Author contributions. This work was collaborative, and author con-
tributions include but are not limited to the following. ESP collected
data and characterized the R.C. Taylor 1 core (Nebraska), conceived
and designed the analysis, provided funding support for sample col-
lection and analysis, and wrote the paper. KKB collected field data
at the Presque Isle outcrop, conducted data analysis, provided fund-
ing support for sample collection and analysis, and wrote the paper.
LC collected field data at the Presque Isle outcrop and the CPC BD-
139 core (Michigan), conducted data analysis, and provided funding
support for sample collection. KS collected data from the BO-1 core
(Minnesota), conducted data analysis, and provided funding sup-
port for sample collection. JPE conceived and designed the analy-
sis, contributed to data analysis and data collection for the CPC BD-
139 core (Michigan), provided funding support for sample collec-
tion and analysis, and wrote the paper. JPO conceived and designed
the analysis as well as hydrogeologic modeling based on outcrop
and core observations. KK collected and characterized data at the
Gallinas Canyon, New Mexico, outcrop. MP contributed analysis
tools, conceived and designed the analysis, and conducted hydroge-
ologic modeling. PM conceived and designed the analysis for Gal-
linas Canyon, conducted analysis of mineral assemblages and data,
and provided funding support for sample collection and analysis.
Competing interests. The authors declare that they have no conflict
of interest.
Special issue statement. This article is part of the special issue
“Faults, fractures, and fluid flow in the shallow crust”. It is not as-
sociated with a conference.
Acknowledgements. This work was improved by thoughtful and
thorough review, the authors thank the reviewers and Topical Ed-
itor, Roger Soliva, for their comments.
Financial support. This work was supported by the collabora-
tive U.S. Geological Survey (USGS) National Earthquake Haz-
ards Reduction Program (NEHRP) (grant nos. G15AP00080 and
G15AP00081) through an award to James P. Evans, Kelly K. Brad-
bury, Mark Person, and Peter Mozley, a Western Colorado Univer-
sity Professional Activity Fund grant to Elizabeth S. Petrie, and a
United States Geological Survey–Utah State University coopera-
tive (agreement no. G17AC00345) grant to Kelly K. Bradbury and
James P. Evans. Additional student support was obtained from stu-
dent research grants from the Geological Society of America (GSA)
and J.S. Williams Utah State University (USU) Geosciences grants
Solid Earth, 11, 1803–1821, 2020 https://doi.org/10.5194/se-11-1803-2020

E. S. Petrie et al.: Geologic characterization of nonconformities 1819
awarded to Laura Cuccio and Kayla Smith, the GSA Stephen E.
Laubach Structural Diagenesis Award to Kayla Smith, and an Insti-
tute of Lake Superior Geology grant to Laura Cuccio.
Review statement. This paper was edited by Roger Soliva and re-
viewed by Owen Callahan and two anonymous referees.
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