Lower Cretaceous Albian Rocks in Northern Great Plains

Abstract

Rocks of Lower Cretaceous Albian age in Montana, North and South Dakota, and northeastern Wyoming have an average thickness of about 1,000 feet and can be divided into three major facies: (1) the Dakota facies in eastern South Dakota, (2) the Blackleaf facies in northwestern Montana, and (3) the basin facies lying between and intertongued with the other two. A study of the lithofacies, paleontology, and tectonic aspects of the basin facies suggests that two major transgressive-regressive marine cycles occurred during Albian time. After a period of non-deposition, marine waters from the Gulf region invaded the northern Great Plains. As this sea transgressed into Wyoming and Montana, the continental Aptian sands were reworked and redeposited into a blanket type sand called Fall River and First Cat Creek. The Fall River Sandstone has been and still is called Dakota; this terminology should be avoided because the Fall River is the basal sandstone of the basin facies and is not correlative with the entire Dakota facies on the east. As waters became deeper black shale (Skull Creek) was deposited, and finally the sea retreated, and a vast delta was built out from central South Dakota into eastern Wyoming and southeastern Montana. The Newcastle Sandstone is a typical delta distributary sand, and the Birdhead Sandsto e is a shelf deposit marginal to the Newcastle delta complex. A study of the sedimentary structures in the Newcastle suggests that at least four delta lobes can be ascertained. Much of the northern Great Plains then became land for a short period of time, and a very thin conglomerate layer was deposited at the top of the Birdhead sand. Before much erosion took place, a sea from the north invaded the area. In eastern Montana and western North Dakota a blanket type sand called Dynneson (new name; previously called Newcastle) was deposited. Some of the sand probably was derived from the upper part of the Newcastle delta, but much of it no doubt came from land areas on the east. The upper part of the Dynneson was deposited as a series of northeasterly trending features resembling off-shore bars. Then the waters became deeper, and the sea spread farther east and south. Much volcanic ash fell and apparently changed the muds to highly siliceous clays resulting in t e distinctive gray Mowry shale. Near the end of Albian time the northern sea retreated, but probably not all areas became emergent before another marine cycle began in Cenomanian (possibly late Albian) time.