Abstract: Cyclic Sedimentation in Jurassic Sequences, Northern Portion of the Western Interior, USA

Robert L. Brenner, James A. Peterson

The interaction of tectonism with eustatic sea level changes, resulted in the deposition of 6 unconformably-bounded sequences that comprise the Jurassic System in the northern portion of the Western Interior. Bounding unconformities were previously reported and named from the base of the Jurassic upward as the J-0 through J-5, plus a basal Cretaceous unconformity (K-1). These sequences contain 6 transgressive-regressive cycles: (1) Lower Continental (J-0 to J-1), (2) First Marine (J-1 to J-2), (3) Second Marine (lower portion J-2 to J-3), (4) Third Marine (upper portion J-2 to J-3), (5) Fourth Marine (J-4 to J-5), and (6) Upper Continental (J-5 to K-1). They consist of marine and continental siliciclastics, carbonate and evaporite rocks ranging in composite thickness from more than 5, 00 ft (1,524 m) in northern Utah and southeastern Idaho to less than 1,000 ft (305 m) elsewhere.

The Jurassic paleogeography of the northern Western Interior was characterized by shallow seas with dune fields, deltaic sedimentary point sources, and restricted saline basins. Marine invasion of the Rocky Mountain shelf in the United States began in early Bajocian time and continued into Oxfordian time. As the period drew to an end, tectonism to the west resulted in increased supplies of siliciclastics. Sediment accumulation rates were higher than the growth of accommodation space created by subsidence and rising sea levels, resulting in regional regression.

Jurassic sea level changes may have resulted in part from ocean-volume changes related to ocean-spreading pulses associated with the continuing breakup of Pangea. Similarities in the Jurassic sequence framework are recognizable in many parts of the world.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994