For more information visit: Geology of wyoparks.state.wy.us/index.php/places-to-go/ keyhole Wyoming State Geological Survey

P.O. Box 1347 Laramie, WY 82073-1347 www.wsgs.wyo.gov phone: (307) 766-2286 email: [email protected]

A typical outcrop of the Cretaceous Fall River Formation in Coulter Bay. Modern-day erosion causes the large sandstone blocks to tumble into the reservoir. Photo Credit: Cory Reeves INTRODUCTION Keyhole State Park, home to Keyhole Reservoir, is in the Black Hills region of northeastern Wyoming. The rocks in and around the park record more than 150 million years of Earth history and preserve evidence of a large inland seaway that once covered much of the state. GEOLOGIC HISTORY More than 100 million years ago, during a geologic timespan known as the Cretaceous Period, the North American plate deformed as the Farallon plate subducted under the continent. This created a low-lying area that stretched from Canada to Mexico, and from what is now Wyoming east through the Great Plains. Sea level also began rising around this same time, and ocean water flooded onto the continent over millions of years, forming an inland sea. Geologists named this ancient sea the Western Interior Seaway, and at its largest, it connected the Arctic Ocean to the Gulf of Mexico. As sea level rose and fell through time, the Keyhole State Park area alternated between being beneath and above water. The variety of rocks now exposed along the shore of Keyhole Reservoir are remnants of this sea level fluctuation. About 70 million years ago, near the end of the Cretaceous Period, the seaway began retreating toward the edges of the continent, and the modern-day Rocky Mountains began to form. The Rocky Mountains, including the Age Name Environment The Pierre Shale, exposed west of Wind Creek Bay near U.S. Highway 14 ( 7 ), records the deepest Black Hills, developed during a mountain- 66 waters of the Western Interior Seaway. Much of the oil and gas exploration west of the park is in million years Lance 7 building event known as the Laramide Formation Deltaic the Pierre Shale. The youngest rocks in the area—the Fox Hills Sandstone and Lance Formation— orogeny, which faulted, folded, and tilted are exposed west of U.S. Highway 14. These formations record the retreat of the Western Fox Hills the originally flat-lying rocks in the park. 7 Interior Seaway across the continent and the early uplift of the Black Hills and modern-day Rocky This event exposed older strata near Sandstone Mountains. the eastern end of the reservoir and younger strata near the reservoir inlet. HYDROGEOLOGY Two major folds—the arch-shaped Pine Ridge Anticline and the step-like Black Hills Keyhole Reservoir sits behind an earthfill dam constructed in 1952 on the Belle Fourche River. Monocline—are visible between the dam The reservoir is the largest body of water in northeastern Wyoming, covering approximately Pierre 7 and inlet. Shale 13,700 acres, or 85 percent of the park’s area. Most of its water is from regional surface runoff. THE ROCKS OF KEYHOLE STATE PARK Shallow Marine Keyhole Reservoir and the local groundwater form an integrated hydrogeological system. This means water moves between aquifers and the reservoir through pore spaces between sand and When rock layers fold into the shape of an mineral grains. Similar water levels in the reservoir and nearby groundwater wells are evidence arch, the oldest layers are in the middle of of this integrated system. The Lakota and Fall River formations and the Newcastle Sandstone the fold. The oldest rocks found in Keyhole Limestone- serve as important aquifers throughout northeastern Wyoming. Many wells completed in these Niobrara 6 Forming State Park—the Morrison and Lakota Upper Cretaceous Formation formations provide water for irrigation and watering livestock. formations—are in the core of the Pine Ridge Anticline (1 on the geologic map Carlile Map Symbols Kfr Js and geologic column). These formations 6 r Shale Geologic contact e

Kb Kla v

Keyhole i

existed before the Western Interior Approximate anticline, dotted where concealed P Jm R State Park IN E Ksc Kfr Seaway and were formed by ancient rivers Approximate syncline, dotted where concealed e h R c WYOMING I r Greenhorn Deep Marine Approximate monocline, dotted where concealed D Kfr that coursed across the landscape 150 B u G e 5 ll o U.S. or state highway Kcu E e million years ago. Formation Qu F Kg A River N Ksc T I Belle Fourche 6 Keyhole Reservoir Kpu C Kla Kn L Jm I The large tan, blocky sandstone cliffs Shale 5 Keyhole State Park boundary N E Kla above the Lakota Formation near Coulter, 100.5 Mowry Shale 5 Geologic Units Ksc Cenozoic (Present–66 Ma) Kmr Kla Eggie Creek, Cottonwood, and Mule million years Newcastle 6 Qu Alluvial deposits, undivided Knc Sandstone 5 Dam Kfr Creek bays (22 , 3 ) are part of the Fall Kfh QTt Terrace, pediment, and gravel deposits Qu Skull Creek Jm River Formation. Shallow ocean waters 4 Mesozoic (66–252 Ma) deposited this sediment as the Western Shale Kl Lance Formation Kla Kl Kb Fox Hills Sandstone Interior Seaway moved into the region. As Fall River 3 Kfh

Lower Formation 2 Kpu Pierre Shale, undivided Kfr

the seaway deepened, it left behind the Rd Kn Niobrara Formation n

Cretaceous Keyhole Reservoir e Ksc dark-black Skull Creek Shale and light- Carlile Shale v Lakota Kcu a Ksc 1 H gray Newcastle Sandstone that are now Formation Kg Greenhorn Formation

145 e n

exposed in Mule Creek Bay (44 ). Kb Belle Fourche Shale Qu i million years P Morrison River Kmr Mowry Shale Knc 1 B Jurassic L Formation Knc Newcastle Sandstone Qu A Around Wind and Deep Creek bays (55 , 66 ), C 113 K Ksc Skull Creek Shale Kmr Qu the remnants of deep marine waters are H Fall River Formation I The geologic formations exposed in Keyhole Kfr L L preserved in the Mowry, Belle Fourche, S State Park range in age from Jurassic (Morrison Kla Lakota Formation and Carlile shales. As the Western Interior Jm Morrison Formation M QTt Formation) to Upper Cretaceous (Lance Formation). O Js Sundance Formation N Seaway warmed, hard-shelled organisms O Kn Blue circles with numbers correspond to locations C Kpu Kg 0 1 2 3 Miles L Kb like oysters and clams left behind the Kl I described in the text and on the geologic map. N Kmr 0 1 2 3 4Kilometers E limestones of the Greenhorn and Niobrara Rivers deposited the oldest formations, later Kfh Qu Kcu formations found west of the bays. flooded by ocean waters of the Western Interior Limestone is a rock composed of calcium Seaway. The seaway retreated at the end of the Bedrock geologic map of Keyhole State Park. Age ranges of rock are in millions of years (Ma). Blue circles with numbers carbonate, the same material found in the Cretaceous, allowing shallow-water deltas and correspond to locations described in the text and in the geologic column. shells of these organisms. rivers to return to the area.