Print this pageEvaluation 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