Geological Survey Bulletin 1493 The Geologic Story of the Great Plains

The Interior Plains, of which the Great Plains is the western, mostly unglaciated part (fig. 2), is the least complicated part of our continent geologically except for the Coastal Plain. For most of the half billion years from 570 million (fig. 5) until about 70 million years ago, shallow seas lay across the interior of our continent (fig. 6). A thick sequence of layered sediments, mostly between 5,000 and 10,000 feet thick, but more in places, was deposited onto the subsiding floor of the interior ocean (table 1). These sediments, now consolidated into rock, rest on a floor of very old rocks that are much like the ancient rocks of the Superior Upland.

Figure 5.—Geologic time chart and progression of life forms. Note Cretaceous Triceratops, Oligocene Titanotheres, and Miocene Moropus. (click on image for an enlargement in a new window)

Figure 6.—Generalized paleogeographic map of the United States in Late Cretaceous time (65 to 80 million years ago), when most of the Great Plains was beneath the sea.

Table 1.—Generalized chart of rocks of the Great Plains.

About 70 million years ago the seas were displaced from the continental interior by slow uplift of the continent, and the landscape that appeared was simply the extensive, nearly flat floor of the former sea.

Warping and stream deposition

Most of these rocks of marine origin lie at considerable depth beneath the land surface, concealed by an overlying thick, layered sequence of rocks laid down by streams, wind, and glaciers. Nevertheless, their geologic character, position, and form are exceptionally well known from information gained from thousands of wells that have been drilled for oil. The initial, nearly horizontal position of the layers of rock beneath the Interior Plains has been little disturbed except where mountains like the Black Hills were uplifted about 70 million years ago. At those places, which are all in the northern and southern parts of the Great Plains, the sedimentary layers have been warped up and locally broken by the rise of hot molten rock from depth. Elsewhere in the Interior Plains, however, earth forces of about the same period caused only a reemphasis of gentle undulations in the Earth's crust.

These undulations affected both the older basement rocks and the overlying sedimentary rocks, and they take the form of gentle basins and arches that in some places span several States. (See sketch map, figure 7.) A series of narrow basins lies along the mountain front on the west side of the Great Plains. A broad, discontinuous arch extends southwest from the Superior Upland to the Rocky Mountain front to form a buried divide that separates the large Williston basin on the north from the Anadarko basin to the south.

Figure 7.—Structural setting of the Great Plains. Williston basin and Anadarko basin are separated by a midcontinental arch.

While the flat-lying layers of the Interior Plains were being only gently warped, vastly different earth movements were taking place farther west, in the area of the present Rocky Mountains. Along a relatively narrow north-trending belt, extending from Mexico to Alaska, the land was being uplifted at a great rate. The layers of sedimentary rock deposited in the inland sea were stripped from the crest of the rising mountainous belt by erosion and transported to its flanks as the gravel, sand, and mud of streams and rivers. This transported sediment was deposited on the plains to form the rocks of the Cretaceous Hell Creek, Lance, Laramie, Vermejo, and Raton Formations. Vegetation thrived on this alluvial plain, and thick accumulations of woody debris were buried to ultimately become coal. This lush vegetation provided ample food for the hordes of three-horned dinosaurs (Triceratops) that roamed these plains. Their fossilized remains are found from Canada to New Mexico.

As the mountains continued to rise, the eroding streams cut into the old core rocks of the mountains, and that debris too was carried to the flanks and onto the adjoining plains. The mountainous belt continued to rise intermittently, and volcanoes began to appear about 50 million years ago. Together, the mountains and volcanoes provided huge quantities of sediment, which the streams transported to the plains and deposited. The areas nearest the mountains were covered by sediments of Late Cretaceous and Paleocene age (table 1) — the Poison Canyon Formation to the south, the Dawson and Denver Formations in the Denver area, and the Fort Union Formation to the north (fig. 8). Vegetation continued to flourish, especially in the northern part of the Great Plains, and was buried to form the thick lignite and subbituminous coal beds of the Fort Union Formation (fig. 9). The earliest mammals, most of whose remains come from the Paleocene Fort Union Formation, have few modern survivors.

Figure 8.—Progressive southward expansion of areas covered by Paleocene, Oligocene, and Miocene-Pliocene sedimentary deposits.

Figure 9.—Big Horn coal strip mine in Fort Union Formation at Acme, Wyo. Photograph by F W Osterwald, U.S. Geological Survey.

Beginning about 45 million years ago, in Eocene time, there was a long period of stability lasting perhaps 10 million years, when there was little uplift of the mountains and, therefore, little deposition on the plains. A widespread and strongly developed soil formed over much of the Great Plains during this period of stability. With renewed uplift and volcanism in the mountains at the end of this period, great quantities of sediment again were carried to the plains by streams and spread over the northern Great Plains and southeastward to the arch or divide separating the Williston and Anadarko basins (fig. 8). Those sediments form the White River Group, in which the South Dakota Badlands are carved. In addition to the Titanotheres, huge beasts with large, long horns on their snouts who lived only during the Oligocene (37 to 22 million years ago), vast herds of camels, rhinoceroses, horses, and tapirs—animals now found native only on other continents—grazed those Oligocene seimarid grassland plains.

Sometime between 20 and 30 million years ago the streams began depositing sand and gravel beyond the divide, and, for another 10 million years or more, stream sediments of the Arikaree and Ogallala Formations spread over the entire Great Plains from Canada to Texas, except where mountainous areas such as the Black Hills stood above the plains. Between 5 and 10 million years ago, then, the entire Great Plains was an eastward-sloping depositional plain surmounted only by a few mountain masses. Horses, camels, rhinoceroses, and a strange horselike creature with clawed feet (called Moropus) lived on this plain.

Sculpturing the land

Sometime between 5 and 10 million years ago, however, a great change took place, apparently as a result of regional uplift of the entire western part of the continent. While before, the streams had been depositing sediment on the plains for more than 60 million years, building up a huge thickness of sedimentary rock layers, now the streams were forced to cut down into and excavate the sediments they had formerly deposited. As uplift continued—and it may still be continuing—the streams cut deeper and deeper into the layered stack and developed tributary systems that excavated broad areas. High divides were left between streams in some places, and broad plateaus were formed and remain in other places. The great central area was essentially untouched by erosion and remained standing above the dissected areas surrounding it as the escarpment-rimmed plateau that is the High Plains.

This downcutting and excavation by streams, then, which began between 5 and 10 million years ago, roughed out the landscape of the Great Plains and created the sections we call the Missouri Plateau, the Colorado Piedmont, the Pecos Valley, the Edwards Plateau, and the Plains Border Section. Nearly all the individual landforms that now attract the eye have been created by geologic processes during the last 2 million years. It truly is a young landscape.