Depositional patterns and stratal relationships on the distal margins of a foreland basin: Middle Jurassic Gypsum Spring and lower Sundance Formations, Bighorn Basin, WY

Abstract

Depositional patterns and stratal relationships in the Middle Jurassic Gypsum Spring and lower Sundance Formations were influenced by a combination of paleotopographic highs and eustatic changes in the eastern Bighorn Basin. Most of these highs are of tectonic origin related to an island arc collision to the west and assumed to be reactivations of crustal weaknesses from earlier western North American orogenies. Although overall basin geometry was affected by the encroaching tectonic load, the study area is far enough away from the orogenic front that it behaves like a passive margin in a ramp setting in response to relative sea level changes. The Middle Jurassic section in the Bighorn Basin records three major transgressive regressive cycles. Many of these contain high-order cycles identifiable by evidence of subaerial exposure in outcrop or by a geophysical log signature showing cyclic alternating peaks and troughs within a given lithologic unit. Chert pebble lag deposits have been used by previous workers to locate regional unconformities in the Bighorn Basin and throughout the Western Cordillera. One of these unconformities, the J-2 surface, is particularly enigmatic. If these lag deposits do in fact mark the J-2 unconformity the surface in the Bighorn Basin is localized and only present in the vicinity of paleotopographic highs, the Black Mountain High being the most prominent. The chert pebbles were shed off of this high and deposited locally and sporadically across two lithofacies units. The combination of paleotopography, tectonics and eustatic changes all contributed to the stratigraphy of the Middle Jurassic section. A sequence stratigraphic model was developed to gain insight into the timing of these tectonic and eustatic events in relation to deposition.