LATERAL VARIATION IN THE RECORD OF TRANSGRESSIONS IN NONMARINE SUCCESSIONS: CLOVERLY FORMATION, WYOMING, USA

Increasing accommodation on fluvial coastal plains during rising relative sea level implies that nonmarine areas are expected to accumulate sediment landward of the transgressing shoreline, yet nonmarine deposits are seldom described as the landward equivalents of transgressing marine facies. The Cretaceous Cloverly Formation of Wyoming is a nonmarine unit ranging from lacustrine deposits in its lower Little Sheep Mudstone Member (LSMM) to fluvial facies and well-drained paleosols in its upper Himes Member (HM). The overlying Sykes Mountain Formation (SMF) consists of backstepping parasequences that pass upwards into the offshore Thermopolis Shale. Here, we offer a sequence-stratigraphic interpretation of this succession and its relationship to the transgression of the Thermopolis Seaway, emphasizing local facies variation in the northeastern Bighorn Basin. Stratigraphic columns 50–75 m thick were described at 15 localities, with multiple columns at some localities to characterize lateral variation. We find the LSMM–SMF interval records three periods of deposition. Chronostratigraphy implies an unconformity between the LSMM and the HM. Amalgamated channels and well-drained paleosols dominate the lower HM, and in some areas, this interval is partially incised by valley fills 15 m thick, which contain increasingly brackish channel-dominated estuarine facies. This supports the presence of a subaerial unconformity separating it from the upper HM, which is dominated by brackish-water paralic facies. These are conformably capped by a regional transgressive ravinement surface, which is in turn overlain by a retrogradational parasequence set of lower shoreface to offshore facies. This upper HM–SMF succession is the corresponding nonmarine record of the Thermopolis Shale transgression. Other nonmarine formations underlying regional marine shales may similarly represent the nonmarine portion of transgressions and may warrant reanalysis.