Geologic characterization of nonconformities using outcrop and whole-rock core analogues: hydrologic implications for injection- induced seismicity

The occurrence of induced earthquakes in crystalline rocks kilometres from deep wastewater injection wells poses 15 questions about the influence nonconformity contacts have on the downward and lateral transmission of pore fluid pressure and poroelastic stresses. We hypothesize that structural 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 nonconformity boundary. We examined the spatial distribution of physical and chemical heterogeneities in outcrops and whole-rock core samples of the great nonconformity in the midcontinent of the United States, 20 capturing a range of tectonic settings and rock properties that we use to characterize the degree of historical fluid communication and the potential for future communication. We identify three end-member nonconformity types that represent a range of properties that will influence direct fluid pressure transmission and poroelastic responses far from the injection site. These nonconformity types vary depending on whether the contact is sharp and minimally altered, or if it is dominated by phyllosilicates or secondary non-phyllosilicate mineralization. We expect the rock properties associated with the presence or 25 absence of secondary non-phyllosilicate mineralization and phyllosilicates to either allow or inhibit fractures to cross the nonconformity, thus impacting the permeability of the nonconformity zone. Our observations provide geologic constraints for modelling fluid migration and the associated pressure communication and poroelastic effects at large-scale disposal projects https://doi.org/10.5194/se-2020-20 Preprint. Discussion started: 26 March 2020 c © Author(s) 2020. CC BY 4.0 License.

by providing relevant subsurface properties and much needed data regarding common alteration minerals that may interact readily with brines or reactive fluids. 30

Introduction
Deep wastewater injection near the nonconformity between the Phanerozoic sedimentary sequence and Proterozoic crystalline basement in the mid-continent 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 associated with large 35 volumes of wastewater injection (Ellsworth et al., 2015;Keranen et al., 2013;Nicholson and Wesson, 1990;Petersen et al., 2016;Zhang et al., 2013), reduction of friction on pre-existing faults, and pressure diffusion away from the injection point controlled by the permeability structure of the rocks in the subsurface (Goebel and Brodsky, 2018;Yehya et al., 2018). Recent mid-continent seismicity nucleates on faults in crystalline rocks km's 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 40 and/or poroelastic loads are transmitted across or along the nonconformity zone or through connected fracture systems in the crystalline rocks (Ortiz et al., 2019). The depths of seismicity (up to 11 km) at some injection sites suggest that crystalline basement permeability is perhaps moderate to high (10 -16 to 10 -14 m 2 ; (Zhang et al., 2016) and is dynamically increased by elevated fluid pressures (Rojstaczer, 2008).
The nonconformity zone is the rock volume surrounding the nonconformable contact. This zone may range from diffuse to 45 sharp, be phyllosilicate rich, or dominated by non-phyllosilicate secondary minerals. Each contact type observed in this study has a range of mineralized textures and structural discontinuities. Characterizing variations in rock properties at the nonconformity 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). Due to weathering, deformation, diagenesis and fluid-rock interactions, the nonconformity zone may be hydraulically heterogeneous at the mm to 10's m scales 50 and influence the migration of fluid and fluid pressures away from the injection well. 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.
Numerical modelling 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 potential for basement induced earthquakes; 2) 55 poroelastic effects can trigger seismicity far away from the injection location; 3) the presence of conductive faults, including those that cut the nonconformity 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 seismicity (Chang and Segall, 2016;Goebel and Brodsky, 2018;Ortiz et al., 2019;Yehya et al., 2018;Zhang et al., 2013). https://doi.org/10.5194/se-2020-20 Preprint. Discussion started: 26 March 2020 c Author(s) 2020. CC BY 4.0 License.
We document the lithology and structural features of the rocks on either side of the nonconformity in outcrops and whole-rock 60 core to characterize the range of rock types and geologic settings associated with the contact and identify any evidence of past cross-contact fluid flow. The sites evaluated here provide geological and hydrogeological analogues that aid in understanding controls on cross-contact fluid flow and the impacts deep circulating fluids may have on altering rock properties at depth (Oliver et al., 2006). Because 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 65 fractures on the transmission of pore fluids and/or poroelastic stress.
In this paper we characterize nonconformities that are associated with Precambrian granite, gabbro, gneiss, and schists, and are overlain by porous sedimentary rocks. We present data on the mineralogic and structural heterogeneities observed in outcrop and core, and these observations serve as proxies for variation in mineral alteration and deformation at the nonconformity which may impact the migration of fluids along and across the contact. 70

Characterization of the nonconformity
Given the recognized importance of direct fluid transmission, variation in pressure, and poroelastic loads on induced seismicity (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 integrated outcrop-based studies in Michigan and New Mexico, and analyses of core from Michigan, Minnesota and Nebraska (Fig. 1). 75

Methods
To describe the nonconformity zone, we analyse outcrop sites and core samples. The outcrop sites are from the southern shore of Lake Superior, Michigan, where the late Proterozoic Jacobsville Sandstone overlies Archean and Proterozoic crystalline rocks, and the eastern Sangre de Cristo Mountains, New Mexico, where Devonian to Mississippian Espiritu Santo Formation overlies the Proterozoic Gallinas Canyon gneiss. We examine core from Nebraska that samples the Cambrian Lamotte 80 Sandstone and its contact with the granitic Yavapai-Mazatzal Precambrian complex (Whitmeyer and Karlstrom, 2007), core from the Michigan basin that samples Precambrian granitoid gneiss near the Grenville Front, overlain by the Cambrian Mt.
Simon Sandstone, and core from south-eastern Minnesota that samples Precambrian metagabbro, diabase, and metadiorite also overlain by the Cambrian Mt. Simon Sandstone.
To describe rock properties observed at the nonconformity interface zone we use a variety of micro-to meso-scale methods 85 including detailed lithological and structural logging of outcrop and core, optical petrography and x-ray diffraction (XRD) mineralogic studies, whole-rock x-ray fluorescence (XRF) elemental analysis, and gas permeability measurements.

Lake Superior, Michigan
Outcrops of the nonconformity between late Proterozoic Jacobsville Sandstone and early Proterozoic altered peridotite 90 crystalline basement are exposed at Presque Isle and Hidden Beach along the southern shore of Lake Superior, Michigan (Lewan, 1972). At these localities the Jacobsville Sandstone (Hamblin, 1958) consists of a variably indurated pebble to cobble conglomerate and a lenticular planar to cross-bedded light red quartz arenite (Fig. 2).
At Presque Isle, a mineralized conglomerate is in direct contact with the underlying serpentinized peridotite or is transitionally interbedded with the overlying sandstone (Fig. 2B). Where present, the low-porosity conglomerate consists of sub-angular to 95 rounded chalcedony, gneiss, and greenstone cobble clasts with fine-grained, poorly sorted, hematite cemented angular quartz grains.
At Hidden Beach, poorly consolidated basal conglomerates 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 near-vertical to bedding-parallel bleached fractures or reduction spots are associated with the lower Jacobsville 100 Sandstone and are not observed to extend into the basement (Fig. 2C). Locally, basement-hosted slip surfaces are coated with epidote-iron oxide and roughly align with the vertical bleached fracture zones in the overlying Jacobsville Sandstone (Fig.   2C).
Optical petrography across the transition from red sandstone 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 Jacobsville Sandstone 105 indicates a minor depletion of K2O, and a minor enrichment of FeO and MgO, relative to the unaltered Jacobsville Sandstone ( Figure 3). At Presque Isle, mineral alteration products in the conglomerate include nontronite, with trace zeolites and iron oxides (Fig. 3). The underlying serpentinized peridotite is black to brown, with abundant white carbonate mesh veinlets and localized stockwork jasperoid veins up to 10 cm wide. Jasperoid mineralization occurs along a few small faults that span the contact. 110

Gallinas Canyon, New Mexico
Devonian to Mississippian carbonate and clastic rocks of the Espiritu Santo Formation deposited on the Proterozoic quartzofeldspathic and amphibolitic gneiss, biotite schist, and granitic pegmatite (Lemen et al., 2015) are exposed along a 4-km long section in Gallinas Canyon, eastern Sangre de Cristo Mountains, New Mexico. The nonconformity is cut by cm-to m'sdisplacement faults, where we characterize both the faulted and the adjacent un-faulted nonconformity zone (Hesseltine, 2019;115 Kerner, 2015). The carbonate and clastic rocks of the Espiritu Santo Formation include: 1-m thick massive, fine-grained, rounded to sub-rounded sandstone with calcite nodules, ~1-m of microcrystalline dolomite that transitions upward into a chert nodule limestone, interbedded mudstone and limestone and a massive microcrystalline limestone bed. A phyllosilicate-rich https://doi.org/10.5194/se-2020-20 Preprint. Discussion started: 26 March 2020 c Author(s) 2020. CC BY 4.0 License. zone directly below the nonconformity is approximately 60-cm thick and is a poorly lithified zone that marks the transition from highly altered (weathering and hydrothermal alteration) to minimally altered crystalline rock (Fig. 4). 120 The predominant lithology of the crystalline basement is gneiss, with minor schist, pegmatitic granite, and basalt. Mineral alteration is greatest directly below the nonconformity. This zone is enriched in sericite within feldspars, and clay minerals (mixed with hematite and associated with replacement of micas) (Fig. 5). Where cut by faults the nonconformity-associated phyllosilicates form a matrix that surrounds more rigid grains such as quartz, suggesting deformation in this unit was accommodated by granular flow, a process associated with high pore-fluid pressure. Microscopic fracturing has occurred 125 within the crystalline 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 crystalline basement, though commonly occurs as coarsely crystalline calcite cement within grain fractures in feldspar and sericitized feldspar.
Fractures cut the altered crystalline basement locally and cataclasites are found throughout the fault core in crystalline basement 130 but are absent within protolith crystalline basement. Where faulted, the sedimentary rock damage zone includes large twinned calcite grains in fracture-filling cements, and cataclasites that lie along the edges of the calcite veins. These cataclasites include: pulverized quartz and feldspar grains, chert, pulverized protolith, as well as clay-and iron oxide-rich minerals. Quantitative microprobe analyses of the carbonate and fine-grained matrix composition within the sedimentary and basement fault cores reveals that all calcite vein elemental values have a slightly reduced level of iron and Mg substitution for Ca than the calcite 135 matrix. The fine-grained matrix within the sedimentary fault core is nearly pure silica, whereas the fine-grained matrix within the crystalline basement fault core is aluminium-rich (Fig. 5).

R.C. Taylor 1 Core, Nebraska
Core from the R.C. Taylor 1 wildcat well was obtained in 1953 in south-central Nebraska. We examined a total of 19.2 m of core recovered over the Cambrian Lamotte Formation sandstone and sheared Proterozoic granitoids in the Central Plains 140 Orogen of the Yavapai Province (Marshak et al., 2017;Sims, 1990;Whitmeyer and Karlstrom, 2007). The arkosic Lamotte Formation, regionally called the Reagan and Sawatch Sandstones, is a fine-grained, well-sorted glauconitic sandstone (Fig. 6).
The basal Lamotte Formation is cut by quartz, calcite, dolomite, and iron-oxide veinlets. Iron-oxide veins cut quartz veins, and both are cut by calcite veins, providing evidence for three mineralization events here (Fig. 7). Below the Lamotte Formation is a phyllosilicate-rich zone composed of 40 cm thick highly altered basement shear zone that overlies a minimally altered 145 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.7).
The altered basement shear zone is composed of quartz, feldspar, biotite, chlorite, and dolomite (Fig. 7). 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 neoformed clays. The basement shear 150 zone is characterized by feldspar, quartz, mica, and the alteration minerals chlorite and dolomite (Fig. 7). The shear zones https://doi.org/10.5194/se-2020-20 Preprint. Discussion started: 26 March 2020 c Author(s) 2020. CC BY 4.0 License. contain fractures, slip surfaces, and S-C fabrics within chloritizied zones. The shear fabrics are cut by 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. 7). Chlorite is present and associated with minor shear fabrics. Calcite, dolomite, and quartz veins parallel and cross-cut the chlorite-rich shear fabrics and cut quartz and feldspar crystals (Fig. 7). In the coarse-155 crystalline granite altered feldspars contain sericite that has formed adjacent to twin planes. Veins of dolomite, calcite, and hematite occur in the lower 7 m of the Lamotte Formation and are observed through the underlying granitic shear zone covering 12.5 m of core.

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 Cambrian 160 Mt. Simon Sandstone and Precambrian altered granitoid gneiss of the Grenville Front Tectonic Zone. We divide the CPC DB-139 core into three lithologic units: a laminated sandstone, a finely foliated gneiss, and a gneiss with sub-horizontal white veins. Both gneiss units have zones of dolomitization. The Mount Simon Sandstone reservoir is a unit of deep wastewater injection in Oklahoma and it is also targeted for CO2 storage (Dewers et al., 2014). Measured porosity values of the Mount Simon Sandstone range from 11-18% (Wisconsin Geological Natural History Survey, 2019). 165 Sandstone grains are rounded to sub-rounded and moderately to well-sorted. A discrete boundary separates the Mt. Simon Sandstone from the underlying altered granitoid gneiss (Fig. 8). The uppermost 30 cm of the basement is composed of tan, fine-grained, dolomite horizon which grades into a dark green foliated gneiss cut by pink sub-vertical fractures over a span of ~5 cm (Fig. 8). The basal meter of the Mt. Simon Sandstone 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: zeolites, vermiculite, 170 and Fe-, Mn-oxides, and carbonates including dolomite (Fig. 9). Dolomitization of the basement host rock re-appears ~2 meters below the nonconformity. The original basement foliation is preserved and is associated with micrometer-scale crystalline dolomite grains, radiating silica crystals, and sub-horizontal calcite and dolomite veins (Fig. 9). Trace amounts of ankerite, clinochlore, and vermiculite are also present in the dolomitized basement rocks (Fig. 9).

BO-1 Core, Minnesota 175
The BO-1 core was originally collected in 1962 as part of an exploratory mining project in Fillmore County, southeast Minnesota (Gilbert, 1962).This core provides a continuous 300 m section of altered and mineralized rocks of lower Cambrian  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).
Sedimentary sequences in BO-1 extend to ~ 1.2 km where the nonconformity is marked by an ~ 12 cm zone of pervasive 185 leaching and iron-hydroxide staining (goethite). Intense alteration extends into the basement rocks for ~ 21 m, with ~ 50 m of argillitic and propylitic alteration, multi-layered veins, and/or fracture mineralization observed to ~ 402 m depth (Fig. 10).
Localized fault and 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. Slip surfaces exhibit oblique to dip-slip slickenlines and range from mm's to cm's thick and are either coated in clay or contain mineral infillings (±carbonate, ±silica, ±chlorite, ±iron-190 oxides).
The porous Mt. Simon Sandstone contains a ~ 0.5 meter zone of intense iron-hydroxide (goethite) alteration at the nonconformity (Fig. 10). This iron-hydroxide oxidized zone extends for several m's into the slightly altered and metamorphosed crystalline basement rock. From petrographic and X-ray diffraction analyses, we identify mineralogical assemblages ( Fig. 11; dolomite, siderite, iron-oxides, iron-hydroxides, illite, smectite, kaolinite-serpentinite, vermiculite) and 195 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 marked by the abundant structural discontinuities (Fig. 11).
Measured gas permeability values are highest above the nonconformity within the porous Mt. Simon sedimentary reservoir (up to 1000 millidarcy) and vary significantly from 0-500 millidarcy below the nonconformity contact, locally permeability increases in direct correlation to the presence of structural discontinuities (Fig. 10). 200

Discussion
The nonconformities we examined range from sharp contacts to zones several m's thick and exhibit a range of mineralized textures and structural discontinuities. These include small faults, in-situ mineral growth, dissolution, recrystallization, foliations and veins that reflect mineralization or deformation at depth and are not the result of alteration due to weathering alone. 205 We divide nonconformities into three types: Type 0a sharp contact between sedimentary strata and basement rocks (Hidden Beach); Type Ian interface dominated by phyllosilicates (Gallinas Canyon, and Nebraska core); and Type IIan interface dominated by non-phyllosilicate secondary mineralization (Michigan Basin core, BO-1 core, and Presque Isle).
We expect the non-fractured Type 0, I and II subtype nonconformities (Fig. 12) to each impact fluid migration and the distribution of poroelastic stress differently. We note that all three nonconformity sub-types could promote long-distance lateral 210 (along-nonconformity) fluid and/or pressure migration away from the borehole.
For Type 0, a sharp contact with minimal alteration (Fig. 12A), the non-conformity region is expected to prevent direct fluid pressure communication across the contact due to a significant contrast in rock permeabilities and would hinder cross-contact fluid migration while promoting migration parallel to the contact. The Type 0 contact type is observed elsewhere (Armstrong https://doi.org/10.5194/se-2020-20 Preprint. Discussion started: 26 March 2020 c Author(s) 2020. CC BY 4.0 License. and Carter, 2010; Easton and Carter, 1995), and has been observed at the Hidden Beach locality. At Hidden Beach a 0.3 to 1 215 m thick portion of poorly cemented basal Jacobsville conglomerate overlies the Archean Campeau Gneiss. The hydrogeologic impact of the unfractured Type 0 nonconformity type would likely be to distribute fluids laterally away from the injection site ( Fig. 12A).
In the R.C. Taylor 1 core, a Type I nonconformity, the vein mineralogy and cross-cutting relationships provide evidence for cross-nonconformity flow. The altered basement shear zone is dominated by chlorite mineral foliations cut by brittle fractures 220 and vein fill of iron-oxide, calcite, dolomite, and we observe porosity between neo-formed clays and rigid grains. Iron-oxides, calcite, dolomite, and quartz veins cut both the overlying Lamotte Formation and underlying crystalline basement (Fig. 12B).
Repeated brittle failure and mineralization suggest that the altered shear zone is a zone of mechanical weakness that can be reactivated allowing development of fracture permeability. In the fractured nonconformities we note evidence for alteration as deep as 5 m below the nonconformity in the crystalline rocks (Duffin et al., 1989). 225 We observe mineralogic alteration across the nonconformity that impacts diffusivity and storativity. At the Gallinas Canyon The mineralization due to fluid-rock interactions at the Type II nonconformities suggests that deep fluid circulation occurs even without enhanced permeability from fractures (Fig. 12C). The mechanical/rheologic changes at these nonconformities 240 could prevent brittle deformation but may be more influenced by poroelastic loads. The impact of these contacts on hydrogeologic properties is not yet well understood or modelled.
The structural elements and fluid-related alteration patterns observed in analogue 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. Once fluids penetrate the basement, flow is likely controlled by fracture and fault systems and reactivation of pre-245 existing structures is possible. However, micro-porosity 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. The impact the morphology of the nonconformity has on the downward propagation of fluid pressures into the crystalline basement has been shown by several numerical hydrogeologic studies (Ortiz et al., 2019;Segall and Lu, 2015;Yehya et al., 2018;Zhang et al., 2016). These models suggest that direct pore-fluid pressure communication (Ortiz et al., 2019;Segall and 250 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 through-going fractures distribute fluid deeper into the basement 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 observed here are cut by structural discontinuities, and several possible contact sub-types exist within these 3 proposed end member scenarios (Fig. 12). Fractures, 255 and 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 pre-existing deformation fabrics may be preferential 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). Our collective field and core observations document the occurrence of significant lateral variations in altered or mineralized zones that are associated with 260 a relatively wide range of permeability values, and that alteration coupled with abundant structural discontinuities can result in relatively higher permeability that extends for 10's of m's both laterally and vertically into the crystalline basement rock below the nonconformity.
To illustrate effects of a reduced permeability above the nonconformity on fluid migration we compare three models of basal reservoir injection that consider continuous and discontinuous zones of altered low permeability rocks above the basement 265 ( Fig. 13). Each model run includes a 100 m-thick basal reservoir (3x10 -15 m 2 ) underlain by 9.9 km of relatively low permeability (kx = kz = 3×10 -17 m 2 ) crystalline basement rock. A 20 m-wide conduit-barrier fault (kz/kx=105; kz = 3 × 10 -10 m 2 ) is present in all simulations as is an injection well located 150 m from the fault zone. Wellhead pressures reached over 50 m excess hydraulic head after 4 days in response to 5000 m 3 /day of continuous injection. The first model (Fig. 13A) is a Type 0 nonconformity, represented by a sharp contact between basement and overlying injection reservoir. In the second model 270 simulation, a Type I nonconformity, we included a 20-m-thick, low-permeability (kx=kz=3×10 -18 m 2 ) zone (Fig. 13B); this layer is 1 order of magnitude less permeable than the basement host rock and a further 1 order of magnitude less permeable in the fault core. The continuous low permeability zone reduces the permeability of the basement fault damage zone by 4 orders of magnitude, making the fault damage zone non-conductive. 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 275 discontinuous low permeability zone is present (Fig. 13C). 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. 13C). Elevated fluid pressures likewise appeared to be forced down other areas where the low permeability zone pinches out, such as towards the right-hand side of Fig. 13C. 280 https://doi.org/10.5194/se-2020-20 Preprint. Discussion started: 26 March 2020 c Author(s) 2020. CC BY 4.0 License.

Conclusions
We define key rock types and structural elements of the nonconformity zone and split the analogue nonconformities into three end-member types. The geologic conditions associated with the nonconformities documented here can be used to help constrain the permeability architecture that impacts both diffusivity and storativity at and across the nonconformity. We expect these nonconformity types to either distribute fluid pressure away from the injection point or provide direct communication 285 with basement rocks, distributing fluids to a greater depth across the nonconformity. 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 modelling of Type 0 and Type I end members that include fault zones predict 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 290 whether through creation or modification of existing permeability or poroelastic pathways or through rheological changes associated with fluid-rock interactions. We show that in-situ conditions along the nonconformity zone vary, and observe common, localized, high permeability zones. 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 impact that long-term storage of chemically reactive fluids has on rock properties. Our observations illustrate that the contact 295 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.

Acknowledgements
This work was supported by collaborative NEHRP grants #G15AP00080 and #G15AP00081 awarded to Evans, Bradbury, Person, and Mozley, a Western State Colorado University Professional Activity Fund grant to Petrie, and a USGS-USU 300 cooperative agreement #G17AC00345 to Bradbury and Evans. Additional student support obtained from student research grants from the Geological Society of America (GSA) awarded to Cuccio, Hesseltine, and Smith, the GSA Stephen E.
Laubauch Structural Diagenesis Award to Smith, and a GDL Foundation grant to Cuccio.