Exposed at Presque Isle and Hidden Beach along the southern 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 serpentinized 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 variably indurated pebble to cobble conglomerate and a lenticular planar to cross-bedded light red quartz arenite (Fig. 2). The Presque Isle outcrops are analogs for geologic sequestration of CO2 in the deep saline Jacobsville Sandstone reservoir (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 exposure 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 sequence of carbonate and clastic rocks of the Espiritu Santo Formation. The Espiritu Santo Formation primarily consists of limestone and dolomitic limestone, with a basal conglomeratic 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 basement earthquakes that began in 2001 (Nakai et al., 2017; Rubinstein et al., 2014).

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 adjacent to the Cambridge Arch and is associated with a series of northwest-trending transpressional faults of the Central Plains Orogen (Sims, 1985; Whitmeyer and Karlstrom, 2007) (Fig. 4). In this core, the arkosic La Motte Formation, 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 sandstones 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 characterized 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 Simon Sandstone reservoir is a unit of deep wastewater injection in Oklahoma, and it is also targeted for CO2 storage (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 complex of altered gabbro and other mafic intrusions and felsic dikes (Fig. 6) (Smith et al., 2019). The crystalline basement rocks are part of the Northeast Iowa Intrusive Complex and are associated with the Midcontinent Rift system (Anderson, 2012). The Midcontinent Rift system extends from Kansas to Lake Superior and then southward through Michigan (Fig. 1). The geologic features associated with the Midcontinent 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 analogous to several geologic settings anticipated in the subsurface of the midcontinent region where lower Cambrian rocks directly overly Precambrian mafic igneous rocks of the Midcontinent 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).

Outcrops of the nonconformity between late Proterozoic Jacobsville Sandstone and early Proterozoic altered peridotite crystalline basement are exposed at Presque Isle and Hidden Beach along the southern shore of Lake Superior, Michigan (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 composed 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 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 bleached open-mode fractures or reduction spots are associated with the lower Jacobsville Sandstone and range in orientation from near-vertical to bedding-parallel. The near-vertical reduction fractures (Fig. 2c) are not observed to extend into the basement.

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 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 alteration products in the conglomerate include nontronite, with trace zeolites and iron oxides (Fig. 7). The underlying serpentinized 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 occurs along a few small faults that cross the nonconformity (Cuccio, 2017).

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 Sangre de Cristo Mountains, New Mexico. The nonconformity is cut by centimeter to meter displacement faults; at this location we characterize both the faulted and un-faulted nonconformity 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 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 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. 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 sedimentary 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 nonconformity. 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 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 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 cataclasites that lie along the edges of the calcite veins. The cataclasites 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 basement 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 fine-grained matrix within the crystalline basement fault core is aluminum-rich (Fig. 8).

Core from the R.C. Taylor 1 wildcat well was obtained in 1953 in south-central Nebraska (Table S1 in the Supplement). We examined 19.2 m of core recovered over the Cambrian La Motte Formation sandstone and sheared Proterozoic granitoids in the Central Plains Orogen of the Yavapai province (Marshak et al., 2017; Sims, 1985; Whitmeyer and Karlstrom, 2007). The arkosic La Motte Formation, regionally 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 basement 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 fabrics 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 sericite that has formed adjacent to twin planes. Open-mode fractures mineralized with dolomite, calcite, and hematite occur in the lower 7 m of the La Motte Formation and are observed through the underlying granitic shear zone covering 12.5 m of core.

The CPC BD-139 core, obtained in 1964 for the design of a brine disposal well, samples the contact between the Cambrian Mount Simon Sandstone and Precambrian altered granitoid 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 moderately 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 Sandstone 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 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, Fe- and Mn-oxides, and carbonates including dolomite (Fig. 10). Dolomitization of the basement host rock reappears ∼ 2 m below the nonconformity. The original basement foliation is preserved and is associated with micrometer-scale crystalline dolomite grains, radiating silica 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).

The BO-1 core was originally collected in 1962 as part of an exploratory mining project in Fillmore County, southeast 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 Sandstone overlying a Precambrian layered intrusive complex of altered and unaltered gabbro, other mafic intrusions, and felsic 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 alteration 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 nonconformity (Fig. 11). This iron-hydroxide oxidized zone extends for several meters into the slightly altered and metamorphosed crystalline basement rock. From petrographic and XRD analyses, we identify mineralogical assemblages (dolomite, siderite, iron oxides, iron hydroxides, illite, smectite, 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 permeability increases in direct correlation with the presence of structural discontinuities (Fig. 6).