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EVAPORITE KARST IN THE BLACK HILLS, SOUTH DAKOTA AND WYOMING, AND THE OIL PLAY IN THE WILLISTON BASIN, NORTH DAKOTA AND MONTANA Jack B. Epstein, Daniel H. Doctor U.S. Geological Survey, 12201 Sunrise Valley Drive, MS 926A, Reston, Virginia, 20192, [email protected], [email protected] Abstract Many of the evaporite intervals lie within or between Subsurface red beds of the Permo-Triassic age petroleum-bearing horizons, and karstic development Spearfish Formation in the Williston Basin has recently within them are of concern to oil and gas exploitation. been touted as “another Bakken Oil Boom for North In this paper we discuss two areas of interest: Dakota”. The senior author, totally uninformed about 1) surficial evaporite karst in the Black Hills of the subsurface geology of North Dakota, was requested Wyoming and South Dakota, and its extension into by INFOCAST* to discuss petroleum potential in the the subsurface in the Williston Basin of North Dakota, Spearfish based on his field experience in the outcrop and 2) a paleokarst horizon in the upper Madison belt of the Black Hills in neighboring Wyoming and Limestone of Wyoming and South Dakota, and its South Dakota. That request was extended to a discussion possible extension into the Williston Basin. of the surface and subsurface evaporite-karst features in four formations ranging from Pennsylvanian to Jurassic Oil and gas potential in the Williston Basin of age. Dissolution of these rocks, which has resulted in North Dakota and adjacent Saskatchewan, Canada, sinkholes, caves, springs, breccia pipes, and subsurface has recently attracted considerable attention, with collapse, has apparently gone on since the Black Hills particular interest in the organic shales of the Bakken was formed during the Early Tertiary, and continues Formation of Devonian age. With the use of hydraulic today. The formation of salt-dissolution paleokarst in fracturing to increase the release of oil and gas from the Williston Basin and adjacent Powder River Basin these source rocks, other stratigraphic units in the has been documented to have occurred many times in basin, such as the Spearfish Formation of Permian the geologic past, between the mid-Paleozoic through and Triassic age, have been touted as possibly being the Tertiary. Reported subsurface collapse has affected additional unconventional oil plays in the subsurface rock characteristics, including local structure, fracturing, of North Dakota (LeFever, 2011). In contrast to the porosity, and permeability. These significant effects reducing environment of deposition of the dark of evaporite karst in the Williston Basin, as well as in organic-rich rocks in the Bakken, the Spearfish subsurface evaporite-bearing sequences nationally, is a red-bed sequence deposited in an oxidizing should be of concern to any oil exploration efforts, as environment. The Spearfish has been prospected in well as for surface infrastructure development such as northernmost North Dakota, more than 500 km (340 pipeline right-of-way. mi) from the nearest outcrops in the northern Black Hills (Figure 2). It is not exposed at the surface in Introduction North Dakota, so one of us (Epstein) was requested to Large areas of the United States are underlain by discuss the geologic characteristics of that formation evaporite deposits that mainly consist of gypsum, in its nearest outcrop belt of the northern Black Hills anhydrite, and salt (Figure 1). Holocene dissolution of South Dakota and Wyoming. has created abundant karstic features at the surface and near-surface in many areas. Dissolution has also The Spearfish Formation and occurred during intervals in the geologic past, thereby evaporite karst in the Black Hills creating many intervals of paleokarst throughout The Black Hills is an asymmetric uplift about 210 km Paleozoic-lower Mesozoic-age strata in the Rocky (130 mi) north-south and 100 km (60 mi) east-west. It Mountains and adjacent mid-western United States. is cored by Precambrian age metamorphic rocks that are ________________________________________________________________________ *http://www.infocastinc.com/downloads_pdf/bakken11_pre.pdf (accessed 11/5/2012) 13TH SINKHOLE CONFERENCE NCKRI SYMPOSIUM 2 161 Figure 1. Map showing the distribution of outcropping and subsurface evaporite rocks in the United States and areas or reported evaporite karst. The 32.5-in. mean-annual- precipitation line approximates a diffuse boundary between eastern and western United States karst terrains (from Epstein and Johnson, 2003). surrounded by shallow-marine to nearshore terrestrial predominantly of red, planar-bedded and laminated sedimentary rocks of Paleozoic and Mesozoic age shale, siltstone, and very fine-grained sandstone, with which dip away from the center of the dome (Figure 3). interbedded gypsum abundant at varying horizons (Figure Gypsum and anhydrite are found in rocks ranging in age 5). Bedding is generally sheet-like, although lenticular from Mississippian to Jurassic (Figure 4). The “Limestone beds and ripple laminations are not uncommon. Salt casts Plateau” located outbound from the central Black Hills have been reported (Sabel, 1984). These sedimentary core, comprises rocks of the Madison Limestone (Pahasapa features indicate that the Spearfish was deposited in a hot Limestone of other reports) and the Pennsylvania age and arid climate on low-gradient coastal plains and near- Minnelusa Formation. Most surface exposures of the shore hypersaline mudflats bordering evaporite basins. Minnelusa contain abundant karst features resulting from dissolution of anhydrite at depth. The Madison contains Sinkholes are abundant in the northern Black Hills. They brecciated rocks resulting partly from similar gypsum range in size from small shallow pits and solutionally- dissolution, as well as world-class limestone caves. The widened joints (Figure 6) of as little as several feet to “Red Valley” encircles the Black Hills resulting from the as much as much as 140 m (460 ft) across (Figure 7). erosion of siltstones, shales, and gypsum in the Spearfish The Vore Buffalo Jump shown in Figure 7 is rimmed Formation. Resistant sandstone of Cretaceous age forms by several convoluted, disjoined, and disrupted gypsum the hogback that encircles the Black Hills and defines its beds 2.5-3.0 m (10 ft) thick. No gypsum is seen at the outer physiographic perimeter. Below those sandstones bottom of the sinkhole, which is probably less than 15 m is a thin gypsum bed in the Gypsum Spring Formation (50 ft) above the underlying Minnekahta Limestone. The of Jurassic age. Much of the information about the karst Minnekahta crops out along the service road about one in the Black Hills presented here is summarized from mile to the west where a 1.2 m (4 ft) thick bed of gypsum Epstein (2003) and Epstein and others (2005). lies at the base of the Spearfish (Figure 8). The Spearfish Formation in the Black Hills ranges to The largest sinkhole in the northern Black Hills is slightly more than 240 m (800 ft) in thickness, consisting about 140 m (460 ft) across, occurs at the base of the 162 NCKRI SYMPOSIUM 2 13TH SINKHOLE CONFERENCE Figure 2. Geologic map of parts of North Dakota, South Dakota, Wyoming, and Montana. The outcrop area of the Spearfish Formation in the Black Hills is located approximately 530 km (330 mi) from Figure 3. Generalized diagram showing the geology the Spearfish oil play in Bottineau County, North and geomorphology of the Black Hills, Wyoming and Dakota. The Black Hills is a domal structure cored by South Dakota. Modified from Strahler and Strahler (1987), Precambrian rocks in the center (orange). It is encircled with permission. by uplifted and erosionally breached Paleozoic to Cretaceous rocks. Red beds and evaporite deposits of the Spearfish Formation (red) form a valley nestled between the hogback of Cretaceous age sandstones and plateau of underlying carbonate rocks, including the Madison Limestone of Mississippian age. The surface rocks in the western part of North Dakota are dominated by Tertiary rocks (solid green) overlying Cretaceous sediments to the west. Spearfish Formation (Figure 8). It is underlain by a 1.2 m (4 ft) thick bed of gypsum which in turn overlies the Minnekahta Limestone; both dip gently underneath the sinkhole. The limestone outcrop closest to the sinkhole contains a small collapse opening in a blind valley. A smaller sinkhole (inset of Figure 8) opened in 1985, and was observed by local ranchers who heard running water in a cavern that extended horizontally beyond the limits of their flashlight beam (Ted Vore, oral communication, 1999). This running water at the base of the Spearfish is deemed to be partly responsible for dissolution and collapse. However, the four-foot-thick gypsum bed in the basal Spearfish seems to be too thin to account for all Figure 4. Stratigraphic column showing distribution the observed depth of the collapse. of gypsum and anhydrite in the northern Black Hills. 13TH SINKHOLE CONFERENCE NCKRI SYMPOSIUM 2 163 Figure 5. Typical exposures of the Spearfish Formation in the Black Hills, South Dakota and Wyoming. (A) White gypsum interbedded with red shale and siltstone about 15 km (10 mi) southeast of Newcastle, Wyoming. (B) Planar- bedded red beds overlain by shales of the Stockade Beaver Member of the Sundance Formation (dashed line), one mile northeast of Beulah, Wyoming. A bed of gypsum, about 3 m (10 ft) thick, comprising the entire Gypsum Spring Formation a few miles to the east of this locality, is missing here. This is the closest exposure of the Spearfish Formation to the Spearfish oil play in northern North Dakota. (C) Fining-upward sequences of very fine-grained sandstone, siltstone and shale in Devil’s Tower National Monument, Wyoming. (D) Spearfish mudcracks 13 km (8 mi) northwest of Spearfish, South Dakota. The Spearfish Formation in the northern Black Hills is wells. Perched water tables with springs below zones as much as 250 m (820 ft) thick and contains several of gypsum in fractured red beds are present in several intervals of gypsum in the lower 60 m (200 ft).

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