APERIODIC CYCLOTHEMS IN A PALEOCENE LACUSTRINE SEQUENCE IN THE FORT UNION FORMATION OF THE BIGHORN BASIN, MONTANA AND WYOMING

A 400 m-thick sequence characterized by prominent tabular sandstone beds and a significant carbonate content occurs in Paleocene strata of the northern part of Bighorn Basin (Clarks Fork subbasin) of Wyoming and Montana. This sequence, currently designated as the Belfry Member of the Fort Union Formation, consists of two separate but related lithogenetic units. The lower one, which includes the Belfry Member stratotype, shows a gradual upward increase in tabular sandstone and carbonate content and is inferred to have been deposited on a drowning flood plain under paralacustrine conditions. The upper unit, as yet unnamed, is characterized by the presence of six asymmetrical, basin-wide cyclothems. Each begins abruptly with an accommodation surface overlain by a laterally extensive tabular sandstone, followed by a micrite-dominated interval that together represent the lacustrine phase of the cycle. These are succeeded by lenticular interbeds of mudstone and sandstone inferred to have been deposited as a prograding fluvio-deltaic and flood plain sequence. Serial variation in thickness of the cyclothems is inferred to represent their unequal duration.

Detailed stratigraphic mapping and correlation with the paleomagnetic and vertebrate biostratigraphic framework for the Bighorn Basin places all six cyclothems within a portion of one vertebrate zone, Ti4, of the middle Tiffanian Provincial Age (57.81 to 57.50 Ma) with a duration of 210,000 ± 230,000 yrs. The cyclothem length of 35-38,000 yrs does not fit Milankovitch periodicities. Instead, they appear to be driven by aperiodic tectonic pulses, most likely related to the nascent Beartooth thrust fault or the Nye-Bowler Lineament zone.