Evaluation of Unconformities in Marine Jurassic Rocks of Western Montana: Preliminary Report

Karen W. Porter

New understanding of unconformity surfaces and transgressive-regressive sequences associated with global eustasy is now being applied to ancient sediments in cratonic basins, causing revision of interpreted sequences and of local and regional basin history. Of particular interest are unconformities contained within marine strata. Two recognized regional unconformities in marine Jurassic rocks of western Montana are currently being evaluated with respect to (1) evidence for subaerial or submarine erosional conditions, (2) development of significant erosional relief, and (3) depositional environments of basal transgressive marine units. These unconformities are mid-Bajocian and upper Callovian surfaces associated with the regionally extensive Sawtooth, Rierdon, and Swift Fo mations. Goals of the research are (1) to establish the events recorded across these unconformities and (2) to develop an event-stratigraphic model for the time interval. The present report is preliminary, and focuses primarily on outcrop data.

AAPG Search and Discovery Article #91033©1988 AAPG Rocky Mountain Section, Bismarck, North Dakota, 21-24 August 1988