Abstract

During deposition of the late Campanian Mesaverde Group (about 70-80 million years before present) extensive regressions and transgressions of the Western Interior sea crossed southwestern Wyoming and northwestern Colorado. Regressions resulted in successive offlapping of younger wedges of upward-coarsening non-marine and marginal marine sedimentary sequences, and transgressions resulted in onlap of marginal marine sediments onto coastal and continental beds. Extensive seaward progradations during offlap are represented by high constructional river-dominated deltas whereas onlap sequences consist mainly of destructional delta front, chandeleur island- or barrier bar-type deposits.

During early Mesaverde time (Rock Springs and Iles Formations) in the western Washakie-Green River basins area of southwestern Wyoming, sedimentation was slow as detritus derived from highlands west and northwest of the Green River basin bypassed the area and was transported either north into the Great Divide and eastern Washakie basins or southeast into Colorado. Northwestern Colorado, on the other hand, was mainly an area of subsidence and thick sediment accumulation in delta front and delta plain environments. Subsidence was produced both by major regional downwarp in the combined Washakie-Sand Wash basins and by local tectonism. During middle Mesaverde time (Ericson Sandstone, Allen Ridge and Williams Fork Formations), uplift in west-central and western Wyoming resulted in erosion and extensive southeast to east spreading of coastal swamp and marsh deposits followed by fluvial floodplain (braided stream) deposits that reached as far east as the Powder River basin. At this time, development of a local north-trending paleohigh in the eastern Washakie and Sand Wash basins profoundly affected sedimentation in the central parts of those basins. Late Mesaverde time (Almond Formation) was characterized by an extensive marine transgression with development of coastal swamp, marsh, lagoon, and destructional delta front facies on a broad shelf that extended across northwestern Colorado and southwestern Wyoming. By this time subsidence had slowed in northwestern Colorado, whereas southwestern Wyoming began to subside more rapidly.

Eustatic sea level changes overprinted in several areas by tectonic controls were mainly responsible for major changes in shoreline position in southwestern Wyoming. In northwestern Colorado, however, high sedimentation rates combined with eustatic sea level fall and subsequent rise caused extensive eastward progradation succeeded by westward transgressions. The timing of eustatic events was the same in both areas, but the events are more easily recognized in northwestern Colorado where the stratigraphic record is more complete. In general, southwestern Wyoming was an area of uplift and relatively thin sediment accumulation during most of Mesaverde time, whereas northwestern Colorado was an area of subsidence and thick sediment accumulation. Subsidence there at times accelerated as a result of regional uplift to the north during early Mesaverde time. Deposition in southwestern Wyoming and northwestern Colorado during latest Mesaverde time was influenced by subtle topographic highs and lows. Barrier bars developed near uplifts. The relief was produced by differential structural movement along trends established as early as Precambrian time.