Fine-Scale Structure of the Precambrian Beneath the Illinois Basin John H

Research Paper

GEOSPHERE Fine-scale structure of the Precambrian beneath the Illinois Basin John H. McBride1, Hannes E. Leetaru2, R. William Keach, II1, and Eleanor I. McBride1 1Department of Geological Sciences, P.O. Box 24606, Brigham Young University, Provo, Utah 84602, USA GEOSPHERE; v. 12, no. 2 2Illinois State Geological Survey, 615 East Peabody Drive, Champaign, Illinois 61820, USA

doi:10.1130/GES01286.1 ABSTRACT best geophysically constrained observations of such reflectivity in a classic 13 figures area of geological interest. Recent advancements in 2D and 3D seismic im- Increasing our understanding of the heterogeneity of Precambrian crust aging, applied as part of a carbon sequestration program in the Illinois Basin CORRESPONDENCE: john_mcbride@byu.edu continues to be a focus for deep seismic reflection studies. High-resolution (Couëslan et al., 2009, 2013, 2014a, 2014b; Leetaru et al., 2009; Alvi et al., 2013; two-dimensional (2D) seismic profiles and a high-resolution 3D seismic vol- McBride et al., 2014), provide a unique opportunity to employ state-of-the-art CITATION: McBride, J.H., Leetaru, H.E., Keach, ume, all centered on Decatur (Illinois, USA), provide new insights on the struc- techniques to studying Precambrian geology beneath the basin. R.W., II, and McBride, E.I., 2016, Fine-scale structure of the Precambrian beneath the Illinois Basin: ture and composition of Precambrian basement beneath the Illinois Basin of Outstanding questions that can be addressed by high-resolution 2D and 3D Geosphere, v. 12, no. 2, p. 585–606, doi:10.1130 the central USA midcontinent. The new data reveal a pattern of strong and co- seismic data are: (1) what is the degree of heterogeneity in Precambrian crust, /GES01286.1. herent reflections and associated diffractions deeply buried within the eastern including that related to isolated strong reflectors interpretable as tabular ig- Granite-Rhyolite Province. This pattern is dominated by a thick seismic strati- neous intrusions (e.g., mafic igneous sills); (2) is there evidence of bimodal Received 29 October 2015 graphic sequence, which is wedge or bowl shaped in cross section and has volcanism beneath the Paleozoic Illinois Basin; (3) what is the extent of strati- Revision received 15 January 2016 Accepted 5 February 2016 an angular unconformity with the overlying Paleozoic section. Deeper intra graphic sequences in what has traditionally been called basement beneath the Published online 9 March 2016 basement bowl-shaped sequences or series are also observed in the same Illinois Basin; and (4) is there anything new to be learned about the Precam- area. We interpret these features to be a northward continuation of analogous brian from fine-scale 2D and 3D seismic imaging? basement sequences located 75 km to the south below the southern part of the Illinois Basin. This correlation indicates a vast Precambrian province with GEOLOGICAL BACKGROUND a north-south dimension of >200 km. Although multiple explanations are ad- missible for the Precambrian reflectivity, the most likely for our study area is The Illinois Basin (Fig. 1A) is filled with as much as 7 km of Paleozoic igneous intrusion of broad mafic igneous (diabase?) sills possibly underlain by sedimentary rocks ranging from early or middle Cambrian to early Permian small plutons. The concentration of such mafic (or bimodal) igneous activity (Collinson et al., 1988). Although the Illinois Basin has been characterized as a within or coeval with the eastern Granite-Rhyolite Province suggests an epi- classic sag basin (Buschbach and Kolata, 1991), the geology of the deep sub- sode of Proterozoic crustal extension and rifting. surface has a complex history of faulting, Precambrian basement uplifts, and folding (Nelson, 1995; McBride and Nelson, 1999; Leetaru and McBride, 2009). INTRODUCTION Contractional deformation, as observed from borehole and sparse geophysical data, occurred over a broad span of Paleozoic time, culminating in Late The expansion of major national geophysical investigations into the United Pennsylvanian and early Permian and corresponding to the Alleghenian orog- States midcontinent has increased the focus on intracratonic deep-Earth struc- eny in the Appalachians (Kolata and Nelson, 1991). ture in areas like the Illinois Basin (e.g., EarthScope, 2015, www.earthscope.org). Geophysical interpretation of the lower Paleozoic and deeper crustal struc- However, the application of petroleum industry strategies such as high-resolu- ture beneath the basin has been constrained by regional seismic profiles tion two-dimensional (2D) and 3D seismic reflection data has lagged. While (Bertagne and Leising, 1991; Heigold and Oltz, 1991; Pratt et al., 1992; Bear et al., classic seismic refraction, seismic tomography, and potential field studies have 1997; Potter et al., 1995, 1997; McBride and Kolata, 1999; McBride et al., 2003), increased our understanding of broad-scale structure of the Earth’s crust and potential field data (Pratt et al., 1992; Heigold and Kolata, 1993; Hildenbrand upper mantle beneath the Illinois Basin and the surrounding region (e.g., Pratt et al., 2002; McBride et al., 2002; Okure and McBride, 2006), regional seismic et al., 1992; Bedle and van der Lee, 2006; Liang and Langston, 2008; Yang et al., refraction profiles (Heigold, 1991; Catchings, 1999; Chulick and Mooney, 2002), 2009; Chu et al., 2012; Hamburger et al., 2011b; Foster et al., 2014; Gallegos et al., and by analysis of earthquakes (e.g., Kim, 2003; Hamburger et al., 2011a). 2014), detailed data for the deeply buried Precambrian are usually not available. The Cambrian Mount Simon Sandstone has for many years been the pri- A persistent challenge for crustal geophysical studies in general is how mary target in the basin for gas storage and for carbon dioxide sequestration For permission to copy, contact Copyright to explain strong subhorizontal seismic reflections (sometimes termed bright (Morse and Leetaru, 2005; Leetaru et al., 2009; Leetaru and McBride, 2009). Permissions, GSA, or editing@geosociety.org. spots) in the deep continental Precambrian. Our study provides one of the Accordingly, much is known about this unit, especially compared to the under

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lying Precambrian basement (Couëslan et al., 2009, 2014a, 2014b; Leetaru Lake A et al., 2009). The Precambrian beneath the basin comprises heterogeneous Ia. Wi. Michigan granitic composition igneous rocks and/or related metasedimentary strata, all presumably belonging to the eastern Granite-Rhyolite Province (EGRP; Sar- SCPO Ill. 42° gent, 1991; Pratt et al., 1992; Van Schmus et al., 1996; McBride et al., 2003, Mich. 2010; Fig. 1A). The EGRP extends as a band of diachronous basement rocks EGRP from northern Mexico to Quebec (Karlstrom et al., 1999). These rocks, where exposed in the St. Francois Mountains (Ozark dome of southeastern Missouri), are 1480–1460 Ma (Bickford et al., 1986; Van Schmus et al., 1996; Rohs and Van Schmus, 2007; Thomas et al., 2012); this means that the unconformity at the base of the Paleozoic section represents almost 1 b.y. of missing geologic time. Regional and local drill-hole data for our study area (Figs. 1B, 1C) indicate that Ind. 40° area of Fig. 1B Oh. these same rocks extend beneath the Illinois Basin (McBride et al., 2010). a crust 6 G The origin of the EGRP is not well understood. Because the province in- pre-1.

cludes A-type granites, the rocks are thought to have been emplaced in an ex- crust tensional plate tectonic setting, as opposed to along an active convergent mar- 6 Ga gin (Dall’Agnol et al., 2005). Lidiak (1996) argued for an intraplate extensional post-1. tectonic setting based on geochemical data. Van Schmus et al. (1987) inter- Illinois Basin preted the EGRP to have been derived from partial melting of older Proterozoic lower continental crust. Van Schmus et al. (1996) used geochemical model- 38° ing based on rare earth elements (samarium and neodymium) to postulate a major geological boundary cutting diagonally across the central midcontinent, Rough CreekGraben rough including the EGRP (Fig. 1A). This boundary, which is interpreted to separate T Paleoproterozoic lower crust to the northwest from Mesoproterozoic lower St. Francois Ky. crust to the southeast, is thought to have once marked the edge of the older Mts. Rome Mo. Pascola part of the Laurentian continental margin (Van Schmus et al., 1996; see also Tenn. Ark. Hoffman, 1989; Karlstrom et al., 1999). Arch 36° ? t Z

PREVIOUS STUDIES 50 km SGRP GFT The new 2D and 3D seismic data are situated over the northwestern flank Reelfoot Rif of the Illinois Basin, where the depth to the Precambrian basement is locally 90°88° 86°

2183 m below sea level, as measured from the CCS1 well (Fig. 2), and region- Figure 1 (on this and following two pages). (A) General location map for the central United ally ~1000–2500 m below sea level as known from deep drill holes within the States midcontinent centered over the Illinois Basin. SCPO—southern Central Plains orogen; basin (Fig. 1B). Acquisition and processing of the 2D seismic profiles were per- EGRP—eastern Granite-Rhyolite Province; SGRP—southern Granite-Rhyolite Province; GFTZ— Grenville Front tectonic zone (Van Schmus et al., 1996). Blue dashed line is the geochemically formed during a U.S. Department of Energy study on the Cambrian–Ordovician defined boundary delineating differing ages for Proterozoic deep crust (Van Schmus et al., 1996). strata of Illinois and Michigan (Leetaru, 2014). For reference, the site of the 3D seismic survey, where the three 2D profiles intersect (Fig. 1C), is located 75 km north of the nearest comparable regional seismic profiles (McBride et al., 2003, METHODS 2010). Previous geological interpretation of 3D seismic data in the Illinois Basin has been based on small surveys, including from the Tonti and Stewardson The IBDP study area is centered on Decatur, Illinois, which has been the site Dome east oil fields (McBride et al., 2009, 2014) in central Illinois, and from of intensive testing of carbon dioxide injection and monitoring using dedicated the U.S. Department of Energy National Energy Technology Laboratory Illinois drill holes (Figs. 1C and 2) and 2D and 3D seismic reflection data (Couëslan Basin–Decatur Project (IBDP) in Decatur, Illinois (Couëslan et al., 2014a, 2014b). et al., 2014a, 2014b). This means that the Decatur site has one of the best sets These studies focused primarily on the lowermost Paleozoic section and top of geophysical and geological constraints in the Illinois Basin for studying Pre- of Precambrian basement. Our study is focused solely on the Precambrian. cambrian basement. It is ideal for integrating fine-scale interpretations of deep

contoursare depthto B Precambrian basement (ft(m)) –600 zero contour from 0

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Figure 1 (continued). (B) Location map for new seismic reflection data discussed in this report (not all data are shown), including county lines (gray), structural contours for the Precambrian orange)( in thousands of feet (meters shown in parentheses) below sea level, based primarily on deep drill-hole data, and associated faults (black) mapped on top of the Precambrian (modified from Busch bach and Kolata, 1991; Wheeler, 1997; McBride et al., 2010). Note that the contours do not incorporate new data; inclusion of the CCS1 well data would shift the –7000 ft (2133 m) contour located east of the well ~11 km to the west. The pairs of black and green brackets identify the surface projections of the shallow and deep bowl-shaped reflection features, respectively, discussed in the text (see Figs. 3A, 3C). Line 101 is ~149.6 km in length, line 601 is ~9.7 km, and line 501 is 40.2 km. 3D—three dimensional.

Precambrian rocks from 2D and 3D seismic data, the former providing a broad ern and western portions of lines 601 and 501, respectively, suffered from the regional context and the latter revealing high-resolution information, unprece- effects of decreasing CMP fold and noise from industrial and cultural installa- dented for the Precambrian beneath the basin. tions as they entered northeastern Decatur (Fig. 1C). Although both migrated The new regional profiles were acquired in 3 separate lines totaling almost and unmigrated data were examined for this study, the 2D profiles are shown 200 km (Fig. 1B; Leetaru, 2014). In order to optimize the utility of the 2D seismic unmigrated in order to preserve information on diffraction structure. Because data, all three profiles intersected one another within or close to the 3D seismic detailed seismic velocity information needed to convert the seismic data from volume (Figs. 1B, 1C). The seismic source for the 2D profiles consisted of a traveltime to depth are only available for the 3D volume (Couëslan et al., 2013), Hemi-44 20,412 kg vibrator (4 sweeps, 4–100 Hz, and a sweep length of 18 s), all seismic data are shown as traveltime sections. with a source interval of 36.6 m, a receiver interval of 3 m, a listening time The 3D data volume at the IBDP site had 10.17 km2 of surface coverage. The of 5 s, and processed into a common midpoint (CMP) interval of 6.1 m. The seismic source was 2 AHV-IV Buggy vibrators (fundamental ground force was close spacing of receiver and source elements (especially the latter) furnish 19,845 kg) per station with 2 sweeps per vibrator, a sweep length of 12 s, and a high-resolution image, unlike anything previously available in the Illinois a listening time of 5 s (sweeping 2–100 Hz). The source line and point intervals Basin. CMP data processing included prestack and poststack noise reduction, were 219.5 m and 24.4 m, respectively. The receiver line and single-sensor in- refraction and residual statics corrections, surface-consistent deconvolution, tervals were 195.1 m and 3.0 m, respectively. Nominal CMP fold is 68 with a bin automatic gain control, velocity analysis for normal move-out corrections, size of 12.2 m × 12.2 m and bin density of 53,858/km2. All parameters combine radon multiple attenuation, and stacking with a nominal fold of 60. The south- to provide a high degree of spatial resolution.

C 2D Line 101

500 Figure 1 (continued). (C) Map enlargement centered over area of the 3D seismic vol- VW2 ume with location of extracted vertical views used in this report shown by red lines. CCS1, VW1, and VW2 are locations

3 2D Line 501 of drill holes penetrating Precambrian basement (portion of well log for CCS1 VW1 shown in Fig. 2). The dashed rectangle is CCS1

area shown in Figures 7A and 7B.

e01 6

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ldip 2D

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RESULTS the three deep wells (Fig. 1C) (Couëslan et al., 2014a, 2014b) drilled into the 3D seismic volume (the lower part of the CCS1 well log is shown in Fig. 2). Primary Observations Well logs show the contact as clastic sediments overlying a rhyolite and/or its eroded products (Fig. 2). U-Pb isotope dating of zircons indicates the age The primary result of the seismic data is that the typically highly reflective, of the rhyolite formation to be 1467 ± 25 Ma (Leetaru and Freiburg, 2014). As well-layered, and relatively flat-lying Paleozoic section overlies a more complex discussed in detail by Couëslan et al. (2013), results from a vertical seismic and structured Precambrian basement (basement surface is approximately the profile (VSP) were used to correlate the top of Precambrian basement with base of Cambrian Mount Simon Sandstone) (Fig. 2), as observed further south the 3D reflection data, which can then be correlated into the intersecting 2D in the Illinois Basin (Heigold and Oltz, 1991; Pratt et al., 1992; McBride et al., profiles (Fig. 1C). Unlike many of the Paleozoic seismic stratigraphic markers 2003). Locally, the Paleozoic-Precambrian basement contact is defined from (e.g., the base of the Cambrian–Ordovician Knox Group), the top of basement

30 Porosity (%) 0 Gamma Ray (API) Resistivity Laterolog (Ohm-m) 0 PEF 10 0 150 0.2 2000 2103.1 10% porosity cut-off 2118.4 Mount Simon Sandstone 2133.6 Figure 2. Excerpt of well log from lower- NPHI most section of the CCS1 drill hole (see Fig. 1C for location). PEF—photoelectric DPHI effect log; NPHI—neutron porosity log; 2148.8 DPHI—density porosity log; depths are in meters. See Couëslan et al. (2014a, 2014b) for more information on the drill hole and pre- PEF Couëslan et al. (2013) for discussion of the 2164.1 VSP (vertical seismic profile) results that Mt. Simon were correlated to the well data.

2179.2 Precambrian

2194.6 Granite

Total Depth=2205.5 m

in this area is not always a strong reflector. Our study focuses on the reflec- top of the basement (see western part of Fig. 3A). The westernmost part of line tivity structure beneath the top of basement (~1000–4000 ms for 2D data and 501 (Fig. 3C), due to its oblique orientation with respect to the geometry of the 1000–2000 ms for 3D volume). sequence, only shows a weak expression of the basal reflector. Nevertheless, line 501 pinpoints the location of the pinchout of the sequence on the east 2D Seismic Profile Observations (see IpC markers in Fig. 3C), confirming the overall bowl shape. Line 601, de- spite a severe noise problem near its south end where the CDP fold is low and The regional 2D profiles (Figs. 3A–3C) below ~1.1 s are dominated by two interference from ground infrastructure is high, extends the observations from broad, concave-upward or bowl-shaped reflectors or thin series of reflectors, lines 101 and 501 by revealing a prominent component of southward thinning each of which become shallow to the west and to the east. This pattern is of the sequence. best seen on the east-west line 101 and an intersecting north-south line 601 The interior of the intrabasement sequence on the 2D profiles is poorly (see IpC markers in Figs. 3A and 3B, respectively). The rocks corresponding reflective, especially with respect to the overlying Paleozoic section. Some to the traveltime interval between the base of the Mount Simon Sandstone horizontal events beneath the base of the Mount Simon Sandstone could be and the basement reflector are referred to as a sequence, even though it is not interpreted as primary reflections (horizontal arrows, Fig. 3A); however, they necessarily internally reflective. Based on a P-wave velocity of 6 km/s, typical may only be multiple reflections generated in the strongly reflective and well- for granitic igneous basement rocks (McBride et al., 2003), the maximum thick- layered Paleozoic section. ness of the sequence along line 101 (Fig. 3A) is ~2280 m. Along the western Below the base of the intrabasement sequence (IpC) are long and short edge of the sequence, the apparent dip of the basal reflector (IpC) is ~10° to segments of reflectivity, including diffractive zones, which are less coherent the east. The shape of the basal reflector is complex, with abrupt terminations than the base of the sequence. These reflections (DpC in Figs. 3A–3C) are (e.g., below station 214300, line 101; Fig. 3A) and lateral changes in the number subhorizontal or east dipping (in the plane of the section), mimicking, but not of cycles (e.g., below station 22500, line 101; Fig. 3A). The shallowest part of parallel to, the base of the overlying sequence. A series of deep reflections the basal reflector is truncated by the relatively horizontal or gently structured on line 501 can be interpreted to describe a deeper bowl-shaped sequence,

Line 601 W 17500 18000 18500 19000 19500 20000 20500 21000 21500 22000 22500 23500 23500 24000 24500 25000 25500E 0 0

NA NA 1 Kx Kx 1 pC [ pC 2200 m

(s) [ 4225 m IpC 2 IpC 2 traveltime DpC DpC DpC 3 3

Line 101 4 4 10 km V. E. =1.85@6km/s

Figure 3 (on this and following page). (A) Excerpt from two-dimensional seismic line (unmigrated) 101 (see Fig. 1B for location). NA—base of Devonian New Albany Shale; Kx—base of Ordovician Knox Supergroup; pC—top of Precambrian basement (approximately the base of the Mount Simon Sandstone); IpC—intra-Precambrian reflector (base of shallow Precambrian sequence discussed in text); DpC—deep Precambrian reflector. Horizontal arrows show possible primary stratal reflectors cut off by the IpC reflector. The black brackets identify the shallow bowl-shaped reflection series discussed in the text (see Fig. 1B). V.E.—vertical exaggeration.

analogous to, but much deeper than, the sequence on line 101. Note that this by 3D seismic migration. North-south profiles from the volume (Fig. 4) indicate series of reflectors does not form an angular unconformity with the base of a strong north-dipping component to the basal reflector. An east-west profile the Paleozoic section. The 3D orientation of these reflectors and/or diffractors (Fig. 5) provides a purely strike view of the same reflector. A dip profile (with is unknown; they are not covered by the 3D seismic data volume (discussed respect to the dip of the basal reflector) (Fig. 6) shows the basal reflector with a in the following). true dip of 16° to the northwest (using the VSP for depth conversion; Couëslan et al., 2013). This profile also reveals the structural behavior of the reflector as 3D Seismic Volume Observations it flattens abruptly to the north, before plunging deeper where it intersects 2D line 101 (Fig. 3A). The three 2D regional profiles are tied together within or near the 3D seis- Horizon mapping from the 3D volume furnished detailed information on the mic volume (Fig. 1C), thereby allowing an accurate image of Precambrian re- structure of the top- and infra-Precambrian surfaces (Figs. 7A and 7B, respec- flectivity that can be extended well beyond the volume. Optimally oriented tively). The top of basement reflector, as mapped only from the 3D volume, vertical extracts from the 3D volume reveal the same prominent basement dip- defines two prominent oblong or circular domes (in plan view) (Fig. 7A) sepa- ping reflector, which ties with the dipping basal reflector imaged on the three rated by a narrow east-west–oriented trough (see also Couëslan et al., 2014a). 2D profiles (note that the deeper basement series on line 501 is not covered by The surface for the dipping basement reflector indicates fine-scale complexity, the volume). On the volume, coherency and location accuracy are improved including small offsets and disruptions (Fig. 7B). As observed on the 3D vol-

C Line 501 3D

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traveltime (s) DpC traveltime (s) 3 [ 3 3 3 DpC [ 4 4 DpC 4 Line 601 4 V. E. = 1.85 @ 6 km/s 10 km

Line 501 5 5 10 km V. E. = 1.85 @ 6 km/s

Figure 3 (continued). (B) As in A, but showing all of line 601. (C) As in in A, but showing all of line 501. The green brackets identify the deep bowl-shaped reflection series discussed in the text (see Fig. 1B). The two depths indicated by the two horizontal arrows are derived from the CCS1 drill hole (Fig. 2) and assuming a bulk upper Precambrian crust seismic velocity of 6 km/s (this applies to all subsequent vertical views). Note that all profiles are shown unmigrated in order to preserve the effects of diffraction.

ume and the 2D profiles, the bowl shape of the Precambrian sequence includes On vertical views generated from the volume, the bowl-shaped sequence is some fine-scale undulations (e.g., see line 101, especially to the east; Fig. 3A). expressed as wedge shaped owing to the location of the volume along the Medium-scale structure on the dipping basal reflector includes a ridge that southern margin of the larger structure (cf. Figs. 6 and 8). plunges to the northwest (Fig. 7B). Combining mapping for the basal reflector A noticeable difference between 2D and 3D vertical views is a fabric of ap- from the 2D and 3D data is somewhat of a challenge due to the sparse, but parent horizontal reflectivity on the latter that is absent or much less observed broad, coverage for the former and detailed, but restricted, coverage from the on 2D views (e.g., 1.1–1.5 s; Fig. 6). At first glance, the horizontal reflections ap- latter. Nevertheless, an integrated gridded map of the horizon clearly shows a pear to represent intrabasement layering, suggesting a possible stratigraphic bowl-shaped sequence with a component of north and northeast dip (Fig. 8). origin. In particular, the basal reflector (IpC) that defines the eastern and

S Line 501 N S Line 501 N 0.000 0.000 0.000 0.000 A B 0.100 0.100 0.100 0.100

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2.000 2.000 2.000 2.000 1500 m Line 7640 1500 m Line 7640, unmigrated No vertical exaggeration @ 5 km/s No vertical exaggeration @ 5 km/s

Figure 4. (A) Extracted migrated vertical north-south view (inline 7640) from three-dimensional seismic volume (see Fig. 1C for location). (B) Same as in A, but unmigrated. Abbreviations as in Figure 3.

western margins of bowl-shaped Precambrian sequence on line 101 appears basement reflector. The result shows a strong match of positive and negative to truncate a series of reflection events within the sequence (two horizontal traveltime features (Fig. 9). This suggests that the mapped intrabasement hori- arrows in Fig. 3A). However, in order to examine the idea that the horizontal zon is actually part of a multiple reflection series generated in the well-layered reflectivity could be a complex series of multiple reflections, we used the 3D Paleozoic section above. Autocorrelation tests (Yilmaz, 2001) computed on the volume to carefully map one event within this series (Fig. 9) between the dip- stacked data show evidence of residual multiple contamination that could be ping reflector and the base of the Mount Simon Sandstone (top of basement). propagating into the section after ~1.1 s, which could account for the spurious We then compared the traveltime structure of this reflection to the top of horizontal reflectivity there (Fig. 10).

Line 601 W E W Line 601 E 0.000 0.000 0.000 0.000 A B 0.100 0.100 0.100 0.100

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2.000 2.000 2.000 2.000 X-Line 4153 1500 m X-Line 4153 1500 m No vertical exaggeration @ 5 km/s No vertical exaggeration @ 5 km/s

Figure 5. (A) Extracted migrated vertical east-west view (cross line 4153) from three-dimensional seismic volume (see Fig. 1C for location). (B) Same as A, but unmigrated. Abbreviations as in Figure 3.

INTERPRETATION cal of the overlying Paleozoic sedimentary section (Fig. 3A). The deep reflectivity is distinguished from the Paleozoic section by indicating dipping and The new 2D and 3D seismic reflection data provide the most detailed view structurally complex surfaces. The basement reflectivity documented in this available of Precambrian reflectivity beneath the Illinois Basin, and for the study, centered around Decatur, Illinois, is likely part of (or closely related to) EGRP in general. The reflectivity in deep basement rocks is as relatively strong another broad, regional pattern of reflective basement sequences, expressed as, or even stronger and more coherent than, the well-layered reflectivity typi as a large basinal depression at least 225 km wide (east-west) and 120 km long

Line 7368 6976 X-line 2065 4753 SSE Line 501 Line 601 NNW 0.000 0.000

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0.800 0.800 Figure 6. A migrated, nonorthogonal verti- 0.900 0.900 cal view extracted from three-dimensional Kx seismic volume (see Fig. 1C for location). 1.000 1.000 This view is oriented optimally to approxi- mate a true dip profile with respect to the

traveltime (s) 1.100 1.100 basal reflector in Figure 7B (see Fig. 1C for 2200 m location). Abbreviations as in Figure 3. 1.200 pC 1.200

1.300 1.300

1.400 ? 1.400

1.500 1.500

1.600 1.600

1.700 IpC 1.700 4225 m

1.800 IpC 1.800

1.900 1.900

2.000 2.000 1500 m true dip profile No vertical exaggeration @ 5 km/s

(north-south), located 75 km south of our study area (Fig. 11). The western There are, however, significant differences between the Precambrian re- tapered edge of the shallower bowl-shaped sequence on line 101 matches the flectivity of the two areas (Fig. 11): the sub–Mount Simon Sandstone to the northward extrapolation of the western edge of the basement sequences to north shows little or no reflectivity other than the strong isolated reflectors, the south (Fig. 11). A correlation between the two areas suggests a vast Pre- whereas to the south, the sub–Mount Simon is strongly layered and shows ap- cambrian province underlying the Illinois Basin with a north-south dimension parent stratigraphic features in addition to strong isolated reflectors (McBride of >200 km. and Kolata, 1999). Furthermore, the northern area is dominated by a broad,

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closed-contour and high-amplitude magnetic anomaly (Fig. 12), while only a dle Run Formation (Drahovzal et al., 1992). A Keweenawan age for the Middle portion of the southern area shows anomalies, which are less well defined. We Run Formation would indicate that the sediments are considerably younger suggest that the Precambrian of the northern area may have a relatively high than the EGRP rocks found in drill holes beneath the study area. Furthermore, concentration of magnetic igneous plutons or sills that can account for the sedimentary strata correlative to the Middle Run Formation have not been major anomaly there. observed from deep drill holes in the study area or vicinity (McBride et al., Three explanations can be proposed for the origin of the strong Precam- 2010). Available information from drill holes into the Precambrian basement brian reflectivity within the EGRP beneath the Illinois Basin: (1) part of a thick of Illinois indicates granitic-rhyolitic compositions and textures (Sargent, 1993; undocumented sedimentary basin beneath the Illinois Basin Paleozoic; (2) part McBride et al., 2010). of a layered sequence of felsic volcaniclastic deposits; (3) mafic igneous sills From observations on regional seismic profiles in the southern Illinois intruded into a granitic country rock. Basin, McBride et al. (2003) described a stacked series of seismic stratigraphic Thick Precambrian sedimentary strata have been proposed to lie beneath sequences of interpreted volcaniclastic origin for the uppermost Precambrian. a portion of the Illinois Basin of east-central Illinois and eastward into Indi- These sequences, which include the Centralia sequence (Pratt et al., 1992), are ana, Ohio, Kentucky, and adjoining areas (Shrake et al., 1990, 1991; Drahovzal mapped over a large area of southern Illinois and western Indiana (Fig. 11), et al., 1992; Drahovzal, 1997). These deposits, which include the Middle Run where regional seismic reflection data are available. However, we do not ob- Formation (Shrake et al., 1990), are estimated to be 1.2–1.0 Ga (Keweenawan serve a well-developed layering on the new seismic profiles and 3D data. Most age) based on limited drill-hole control and seismic reflection profiles located layering appears to be only apparent, most prominently on the 3D volume east of Illinois (Drahovzal et al., 1992; Baranoski et al., 2009). Layers of basalt (Fig. 9). Instead, the reflection pattern is defined by two broad, bowl-shaped have been observed in drill holes in Kentucky within and overlying the Mid- basal surfaces, the shallowest of which continues up to and is truncated by

Base of Mt. SimsonSandstone reflection

–1437 Figure 9. Comparison of base of the Cam- –1460 brian Mount Simon Sandstone reflection –1465 (approximately the top of Precambrian basement) traveltime structure and ap- –1470 parent subhorizontal intra-Precambrian reflection arriving before the dipping basement reflection (see Fig. 7B). NAD—North American Datum. Vertical arrows show –1475 correspondence of traveltime highs and

lows between the two reflections, which Easting suggests that the subhorizontal reflection –1480 is actually a multiple arrival. See text for more discussion. –1485 –1490 –1495 –1500 –1505 –1513 travel-time below datum (ms)

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Figure 10. Autocorrelation tests for a portion of an unmigrated stacked section extracted from the three- dipping, dimensional volume. Autocorrelation function was computed for the input and output length shown. Note the apparent peg-leg multiples (red arrows on autocorrelation) possibly generated from the horizons at intra-basement ~430 and 800 ms. reflector ()IpC

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the base of the Mount Simon Sandstone, without obvious internal reflectivity. 1990; Litak et al., 1991; Pratt et al., 1992; BABEL Working Group, 1993; Papasikas Likewise, the deeper bowl-shaped series imaged only on line 501 (Fig. 3C) ap- and Juhlin, 1997). Most studies of mafic igneous sills, as expressed on seismic pears to be an isolated feature with no layered reflectivity above or below it. reflection data, describe intrusion into the deep strata of a sedimentary basin We conclude that the most acceptable interpretation for the Precambrian (Planke et al., 2005; Thomson and Hutton, 2004; Magee et al., 2015), but diabase reflectivity beneath our study area is mafic igneous (e.g., diabase) sills intruded sills are also interpreted from a granitic basement host rock in other geologic into a granitic-rhyolitic country rock. Strong intrabasement Precambrian reflec- settings (Juhlin, 1990; Litak et al., 1991). Detailed interpretations of diabase (or tors and sequences of reflectors have been imaged previously on long-record other mafic igneous) sills and intrusive complexes were made from 2D and seismic profiles from a variety of settings outside the Illinois Basin (e.g., Juhlin, 3D seismic data from the North Sea and the Rockall Trough by Polteau et al.

3D volume 5 8 1.7 6 1. 5 .8 1.6 1 1. 1. 1. 1.0 1. 1.5 1.5 Decatur

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39°N Figure 11. Regional traveltime structural ? contour map for base of Precambrian 4 .0 reflection sequences in southern Illinois (McBride et al., 2003, 2010) and for this study (3D—three dimensional). Enlarge- ment shows detail of contours of shallow Precambrian sequence discussed herein. Contours are in seconds, two-way travel 3. 0 time. Blue dashed line represents geochemically defined crustal boundary postu- lated by Van Schmus et al. (1996) (Fig. 1A).

1. 0 2.0 1.5 Ill. Ind. 38°N Ky. Mo.

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(2008), Thomson and Hutton (2014), Magee et al. (2015), and others. In one of Diabase intrusions are typically interpreted on seismic profiles on the basis the best-documented cases, high-amplitude subhorizontal reflections from the of high amplitude (from the impedance contrast between high P-wave velocity Siljan Ring impact structure in central Sweden have been shown to correlate diabase and granitic or sedimentary country rock), strong lateral continuity, with diabase sills intruded into Precambrian granite at depths of 4500 m and nonconcordant relationships with the country rock (e.g., crosscutting relation- greater, based on a deep drill hole (Juhlin, 1990; Papasikas and Juhlin, 1997). ships with other reflectors, if present), cutting upsection within Precambrian These deep reflections are similar to those observed on the Illinois profiles, in basement or in a deep part of a sedimentary section, abrupt terminations, and terms of high-amplitude, coherency, and showing an overall concave-upward perhaps most important, an overall bowl shape in profile (Litak et al., 1991; (bowl) shaped structure. Malthe-Sørenssen et al., 2004; Planke et al., 2005; Polteau et al., 2008; Magee

92° 90° 88° 86° DISCUSSION

Lake An almost ubiquitous feature of deep seismic reflection profiles of conti- nT/km nental crust is relatively isolated horizontal or gently dipping high-amplitude Michigan reflectors. In many cases, these reflectors are so strong and coherent as to be 42° 42° considered bright spots. The occurrence of high-amplitude reflections in deep 30 Precambrian crust, traditionally considered to be more or less homogeneous, suggests significant lithologic heterogeneity. Such features are described 25 from many disparate geological environments worldwide. Explanations span 20 a broad range: an active shear zone or the brittle-ductile transition in the crust (Ryberg and Fuis, 1998; Liotta and Ranalli, 1999; Brogi et al., 2005), detachment 15 faults (Allmendinger et al., 1983; Reston, 1996), aqueous fluids or molten material (de Voogd et al., 1986; Makovsky et al., 1996; Makovsky and Klemperer, 10 1999), or mafic igneous sills (Goodwin et al., 1989). Any of these features have sufficient acoustic impedance contrasts to produce high-amplitude reflections 40° 5 40° or sequences of reflections. Without good 3D volumetric constraints, such 0 reflections cannot be interpreted uniquely. For example, the Surrency bright spot, a prominent isolated deep crustal reflection observed in the core of the –5 known southern limit of late Paleozoic suture between Laurentian and African proto-Atlantic terranes, Centralia sequence –10 was originally interpreted as evidence of fluids in crystalline igneous crust (Pratt et al., 1991). This feature was, however, later interpreted as more likely a –15 diabase sill related to the Mesozoic crustal extension that followed continental suturing (Pratt et al., 1991; Barnes and Reston, 1992). The interpretation of the –20 Death Valley bright spot as partially molten material in deep continental crust 1.00 s 1.50 s 38° –25 was supported by an apparent spatial association with Cenozoic volcanoes at 38° the Earth’s surface (de Voogd et al., 1986). Both of these cases were, like almost –30 all deep seismic reflection studies, limited by access to only one long, regional profile. It is rare to have imaging of such isolated bright reflectors on multiple –35 intersecting 2D seismic profiles that are validated by a 3D seismic volume, as –40 in our study. A significant concentration of mafic igneous rocks buried within the upper and/or middle crust should produce a significant potential field anomaly. Comparing results from the seismic data with a reduced-to-pole first vertical 92° 90° 88° 250 km derivative magnetic intensity map (Figs. 1C and 12) shows a correspondence of the shallow bowl-shaped sequence on line 101 with a prominent, isolated Figure 12. First vertical derivative of reduced-to-pole magnetic intensity for the central United States midconti anomaly. The Precambrian reflectivity centered on the strong anomaly is not nent (modified from McBride et al., 2010). Dashed black rectangle shows area of Figure 1B, with seismic data found elsewhere on the 180-km-long transect. An isostatic gravity anomaly coverage. Long dashed white line is geochemically defined boundary from Figure 1A. The known southern limit map (Daniels et al., 2008) also shows an anomaly, albeit less prominent rela of the Centralia sequence is taken from Figure 11. Magnetic intensity data for this area without the derivative filter are available in Daniels et al. (2008). tive to nearby gravity anomalies. This correspondence is consistent with a zone of intrusion of mafic igneous rock with expected higher magnetization, as described elsewhere for magnetic anomalies within the Illinois Basin (McBride et al., 2003). Given this context, the enigmatic zones of prominent, high-ampli- et al., 2015). Many of these characteristics are present in our data set, espe- tude reflections and diffractions (DcP in Figs. 3A–3C) below the bowl-shaped cially the overall bowl or wedge shape. The width of a bowl-shaped sill in- sequence on line 101 may be expressions of deeper isolated mafic igneous sills creases with increasing depth of intrusion below the Earth’s surface (Planke and small plutons. Note that the deeper bowl-shaped reflection series on line et al., 2005); this accords well with the broad width of the two interpreted sill 501 located to the east does not correspond to a significant magnetic anom- sequences in our study. aly; this suggests that the deep crust directly beneath the large closed-contour

anomaly was intruded by a pluton that could have been the source for sills or tubular intrusions in the shallower crust to the east. Forward modeling of the observed magnetic intensity pattern was considered, but deemed impractical due to the limited spatial coverage of the seismic data, relative to the much larger anomaly (and anomalies); furthermore, although the seismic images provide constraints on the uppermost parts of S the interpreted sills or plutons, they cannot constrain the lower limits of these bodies. Planke et al. (2014, p. 15) noted that “if sills are buried below more than a few kilometers of sediments,” magnetic anomaly modeling may have poor resolution. Nevertheless, we can call upon analogous relationships between better studied (or exposed) mafic igneous intrusions and magnetic anomalies in order to provide guidance for interpreting the associations we observe. approx. depth 4.2 km Perhaps the most relevant example is that from the St. Francois Mountains in southeast Missouri (located a little more than 250 km southwest of our study area), where strong positive magnetic anomalies have been conclusively correlated to mafic igneous rocks (including diabase) intruded within a granitic host rock (Kisvarsanyi, 1974; Hildenbrand et al., 1982, 1996). As described here, this granitic host rock is part of the EGRP, which extends beneath our study area in central Illinois (Fig. 1A). Cases of magnetic anomaly modeling of mafic igneous sills can be found approx. 10 km for very well constrained geologic settings. For example, Krassay et al. (2013) demonstrated the utility of forward magnetic anomaly modeling of interpreted mafic igneous sills or tabular intrusions within sedimentary basins when high-resolution magnetic intensity data and a coincident grid or network of Figure 13. Conceptual illustration of the base of the bowl-shaped seismic sequence (Fig. 3A) intruded into Precambrian basement, along with small plutons beneath it, interpreted from the seismic profiles are available. In the southern Illinois Basin, occasional positive deeper reflection and diffraction patterns (Figs. 3A, 3C). Feeder dikes (not imaged by the seismic magnetic anomalies have been shown to be related to shallow ultramafic igne- data) are purely speculative. The concave-upward shape is meant to recall the shallow bowl- ous sills intruded into Paleozoic sediments by forward modeling of high-reso- shaped reflection sequence observed in Figure 3A. The north-dipping component is recalled from line 601 and the three-dimensional (3D) volume (Figs. 3B and 6). This figure is partially lution magnetic intensity data (Sparlin and Lewis, 1994). In both of these cases, redrawn from the Chevallier and Woodford (1999) 3D model of the Karoo diabase sill. the sills or series of sills, which individually ranged in thickness from 72 m to a few hundred meters, produced magnetic anomalies of 100–250 nT. A direct interpretation of the exact geometry of hypothetical igneous bod- Due to our limited data coverage, the full 3D distribution of the inter- ies is not possible, because the seismic method cannot easily record reflec- preted sills is unknown; however, in order to derive dimensions, for com- tions from a steep or vertical interface. Furthermore, imaging the base and in- parison purposes, we have calculated a rough hypothetical areal extent, ternal structure of an igneous body is generally not possible due to attenuation based on the projection up to the ground surface assuming the maximum of signal within the body and the lack of strong impedance contrasts within north-south width and east-west extent of the gridded horizon (Fig. 8) to be it, including its base (Luke, 2012). Given these caveats, we envision a group 9.3 km × 27. 5 km = 255.8 km2. The actual area is likely to be much larger, so of deep mafic igneous plutons and sills that are genetically related and con- this is a conservative estimate. Next we used the high-resolution 3D volume centrated beneath the bowl-shaped sequence best expressed on line 101 (Fig. to measure a traveltime interval for the basal reflector to be typically ~50 ms 13). The dimensions of the bowl-shaped sequence in our study area are larger and assumed this value to represent the interpreted sill thickness. Assuming than those of most mafic igneous sill sequences described from sedimentary a typical upper crustal P-wave seismic velocity (6 km/s), this traveltime con- basins (see discussion here); however, our feature, which is almost 30 km wide verts to 150 m. This is not greatly different from the thickness of the Siljan in an east-west cross section (Fig. 8C), is comparable to the dimensions of the Ring diabase sills (as much as 60 m thick), as confirmed by drilling and im- Golden Valley diabase sill of the Karoo Basin of South Africa (Galerne et al., aged as strong subhorizontal crustal reflectors (Juhlin, 1988). The estimated 2011). The Golden Valley diabase sill has a long dimension of ~20 km (Che- thickness of the interpreted sill within our seismic volume, comparable to vallier and Woodford, 1999). The Great Whin and Midland Valley diabase sill values cited here for mafic igneous sills, would not be expected to alone complexes of northern England and southern Scotland crop out over several cause the large closed-contour magnetic anomaly of >600 nT in amplitude tens of kilometers (Goulty, 2005). (Daniels et al., 2008).

From our estimated area and thickness values, we have a conservatively SUMMARY AND CONCLUSIONS estimated volume of 38.4 km3. These estimates are comparable to outcrop-based observations of diabase sills from many locations around the New 2D and 3D seismic data furnish an unprecedented view of reflectivity world. For example, the Great Whin Sill is reported to have thicknesses of as in Precambrian rocks of the EGRP beneath the Paleozoic Illinois Basin. The much as 100 m or greater (Goulty, 2005). Diabase sills within the Karoo Basin new coverage and the high resolution of the data reveal significant fine-scale have maximum thicknesses of 100 m (Chevallier and Woodford, 1999) and features not previously possible with limited 2D data available. The results 150 m (Galerne et al., 2011). The Ferrar igneous province of Antarctica has of our study extend observations of Precambrian seismic stratigraphic se- multiply stacked diabase sills of ~300 m in thickness (Muirhead et al., 2012) quences from the southern Illinois Basin northward over a distance of 75 km. and an estimated volume approaching 230,000 km3 (Elliot and Fleming, 2004; The basement seismic reflectors exhibit geometric properties of mafic igne- Muirhead et al., 2012). For a broad area of the San Rafael Swell of Utah (USA), ous sill complexes by analogy with better constrained examples of sills from Richardson et al. (2015, p. 2) measured diabase sill thicknesses between 5 and several locations (e.g., Rockall Trough, North Sea, Siljan Ring). We observe 40 m and estimated these to represent a “magma plumbing system in a tabu a discrete, bowl-shaped basement sequence, which corresponds to a promi- lar block approaching 25 km3 in volume.” Thus, our conservative estimates nent magnetic intensity anomaly, with dimensions of almost 30 km width and of sill thickness, area, and volume are consistent with direct measurements ~2.3 km maximum observed thickness. The thickness represents a minimum from outcrop. value, as the upper part of the sequence may have been beveled off by up- In the midwestern USA, subordinate mafic igneous sills and dikes have lift and erosion. Apparent internal horizontal reflectivity appears copiously on intruded into granitic country rock in the St. Francois Mountains (Ozark dome) the 3D sections, but is likely mostly or entirely multiple reflection. A less-well- in southeast Missouri (Kisvarsanyi and Kisvarsanyi, 1990; Walker et al., 2002). defined series of bowl-shaped reflections is also seen deeper to the east. Three According to Van Schmus et al. (1996), mafic igneous plutonic rocks in the possible explanations may be proposed for the origin of the strong basement St. Francois Mountains are related to the main 1470 ± 30 Ma igneous event reflectivity. Our preferred explanation, that best fits the new profiles centered of the EGRP. These mafic igneous rocks were derived from the same source around Decatur, Illinois, is a complex of mafic igneous sills intruded into (or rocks that produced the dominant granites and rhyolites (Van Schmus et al., with) the felsic igneous basement of the EGRP. Emplacement of such a bimodal 1996). Our interpretation is that the relationship between the granitic and ba- igneous complex is consistent with crustal extension and provides further evi saltic basement components of our study area is similar to that of the EGRP of dence of a major, but poorly understood, Proterozoic rifting and/or magmatic the St. Francois Mountains. If correct, this suggests that the reflectors of our event in the central USA midcontinent. study area represent mafic igneous sills or plutons that are synchronous with the felsic igneous host rocks. Such bimodal volcanism is consistent with an ACKNOWLEDGMENTS interpretation of subduction along an active continental margin or lithospheric This research was supported by the U.S. Department of Energy, Office of Fossil Energy, through extension (Walker et al., 2002). Although igneous sills may form in any tectonic their Regional Carbon Sequestration Partnership Program under contract DE-FC26–05NT42588 environment, major sills tend to be diabase and intrude in extensional tectonic and the Illinois Office of Coal Development with the participation of the Illinois State, Indiana, and environments (Kavanagh et al., 2006; Magee et al., 2015). As pointed out by Kentucky Geological Surveys. Support was also received from the National Energy Technology Laboratory via contract FE0002068 (H.E. Leetaru, lead principal investigator). We gratefully ac- Lidiak (1996), basalts encountered in deep boreholes further to the east (but knowledge the Landmark (Halliburton) University Grant Program, the IHS Kingdom Educational not in Illinois) are interpreted to be associated with crustal rifting; however, Grant Program, and the Schlumberger Worldwide University Software Program for software although these basalts tend to be assigned a Keweenawan age, isotopic dates grants that made processing and visualization possible. Reviews by Sallie E. Greenberg, Ernest C. Hauser, and an anonymous reviewer significantly improved the final version of the paper. We also are not available, and the basalts could either be synchronous with or postdate thank W.J. Stephenson and E.E. Wolfe for valuable advice on data processing. the granitic basement. 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