TWO EPISODES OF LARAMIDE SHORTENING AS REVEALED BY PALEOGENE UNCONFORMITIES NEAR HEART MOUNTAIN, NORTHWESTERN WYOMING

Mapping of Cretaceous and Paleogene strata near Heart Mountain shows that this area experienced two separate episodes of Laramide deformation during the Paleocene-Eocene. Pronounced angular unconformities at the base of Paleogene sedimentary rocks are two different ages and overlie folded strata with two different orientations. In previous studies, only one of these angular unconformities was observed in the Heart Mountain area: the unconformity west of Heart Mountain between the gently-dipping Willwood Formation and underlying tilted Paleocene and Cretaceous rocks. Pierce (1965) mapped this as an angular unconformity and recently, Lillegraven (2009) suggested that the contact was an east-directed thrust fault that was active along a 30-km segment of the western edge of the Bighorn Basin. We excavated the Paleogene-Cretaceous contact and did not find evidence for thrust faulting, but instead our mapping supports folding of Cretaceous strata before deposition of Paleogene sediments began.

Near Heart Mountain, the Paleocene Fort Union Formation consists of coarse-grained sandstones and locally black chert pebble lags. These lags distinguish the Fort Union from the overlying Willwood Formation, which contains tan-to-brown sandstones commonly interbedded with paleosols. West and northwest of Heart Mountain, chert pebble conglomerate of the Fort Union is conformably tilted with underlying Cretaceous rocks, which are unconformably overlain by gently dipping Late Paleocene-Eocene Willwood Formation. South of Heart Mountain, steeply dipping Cretaceous rocks are overlain by gently dipping black chert pebble conglomerate of the Fort Union Formation. This angular unconformity suggests that at least one folding event occurred before deposition of the chert-pebble-bearing member of the Fort Union Formation. Cretaceous rocks in this area are striking WNW, while Cretaceous rocks in the northern Heart Mountain area strike almost due north. This suggests that two differently oriented episodes of deformation occurred in the vicinity of Heart Mountain. One possibility is that a first episode of deformation occurred as a result of uplift of the Pat O’Hara mountain block, which strikes roughly WNW, followed by a second episode of deformation during the uplift of the Beartooth block, which strikes roughly north.