Geology of South Dakota

The Western Interior Seaway flooded the region, creating vast shale, chalk and coal beds in the Cretaceous as the Laramide orogeny began to form the Rocky Mountains.

The Black Hills were uplifted in the early Cenozoic, followed by long-running periods of erosion, sediment deposition and volcanic ash fall, forming the Badlands and storing marine and mammal fossils.

The Deadwood Formation is beach sand and gravel at its base, but as the ancient shoreline moved eastward, thin layers of mudstone and limestone formed in the west, in deeper waters.

Intraformational conglomerate is also common, resulting from fragments of limestone broken up in waves The Silurian and Devonian were marked by more carbonate deposition, particularly within the Williston Basin, east to central South Dakota.

Beach sand coal sequences left behind as the up to 200 feet thick Fox Hills Formation after the Western Interior Seaway began to recede, 15 million years after the Pierre shale first started to form.

Low permeability shale yielded a poorly drained landscape, cut streams and rivers, which brought in large amounts of sediment—clay, silt and sand up to 500 feet thick—which deposited as the Hell Creek Formation.

Like many other locations around the world, South Dakota preserves the inch thick boundary clay, left by the asteroid impact of the K-Pg mass extinction 66 million years ago.

The Ludlow Formation is up to 350 feet thick and in the northwest contains no obvious break in continental deposition at the time of the mass extinction, except for being sandier and less well-cemented.

In the northwest, the Ludlow, Cannonball and Tongue River formations are categorized as the Fort Union Group, with thick sandstone layers sourced from the Rocky Mountains.

Bird bones and even fossil duck eggs have been found in association with clams, snails and the remains of algal mats in temporary ponds.

It contains light-green bentonite clay, with a high shrink-swell capacity, as well as lenses of limestone containing snail, clam and algae remains from ponds.

The overlying Brule Formation encompasses clay laid down by water, windblown sand, silt and volcanic ash, with a maximum thickness of up to 450 feet.

Desert caliche soils are sometimes preserved as lime nodules and small clay buttes are often capped by well-cemented channel sands and gravels.

At the end of its five million year deposition, the formation was capped with the brilliantly white Rocky Ford ash, the windfall from powerful rhyolite eruptions in the Western Cascades in Oregon.

[5] The oldest glacial drift deposits are in the southeast, at Sioux Falls and Newton Hills, overlain by deeply weathered Illinoisan, or potentially pre-Illinoisan age material.

[6][7] The Black Hills were extensively explored and periodically mined for silver, gold, mica, tungsten, feldspar, bentonite, beryl, lead, zinc, uranium, lithium and sand, as well as oil, beginning in the 1870s.

[9] South Dakota has oil and gas production in the Williston Basin in the northwest, although it produces only one percent of the US total, primarily from traditional vertical wells.