Non-marine sedimentation in the overfilled part of the jurassic-cretaceous Cordilleran foreland basin: Morrison and Cloverly Formations, central Wyoming, USA

Lithostratigraphic, chronostratigraphic, sedimentological and petrological data from the Upper Jurassic-Lower Cretaceous Morrison and Cloverly Formations in central Wyoming allow detailed characterization of the early history of the central part of the Cordilleran foreland basin. The Morrison is divisible into three informal members: (1) a lower sandstone, deposited by a complex coastal dune-foreshore-fluvial system during retreat of the Sundance sea; (2) a middle mudstone, deposited by muddy fluvial and ephemeral lacustrine systems during a period of regional, seasonal aridity; and (3) an upper sandstone, deposited by a sandy fluvial system of variable sinuosity. The overlying Cloverly Formation is divisible into two informal members: (1) a lower mudstone (previously considered as part of the Morrison Formation), deposited by muddy fluvial and lacustrine systems; and (2) an upper chert-pebble conglomerate and sandstone, deposited primarily by gravel-dominant braided rivers. Palynological data and a single fission-track date indicate that the lower part of the middle Morrison mudstone is early to middle Oxfordian and the upper part of the lower Cloverly mudstone is Valanginian. Morrison sandstones are subarkosic, with average %QFL = 91,6, 3 and %QmFLt = 83, 6, 11. Cloverly sandstones are cherty litharenites and sublitharenites, with average %QFL = 99.6, 0,0.4 and %QmFLt = 82,0,18 (Gazzi-Dickinson point-counting method). Palaeocurrent data and sandstone compositions indicate a complex provenance including exirrabasinal sources in lower Mesozoic and upper Palaeozoic sedimentary and volcanic rocks of the Cordillera and intrabasinal sources of Proterozoic clasts in south-central Wyoming. Cloverly sandstone compositions in the eastern part of the study area were influenced by short-term fluvial reworking within the basin. The thickness of the composite Morrison-Cloverly succession is practically constant over a distance of several hundred km east of the Idaho-Wyoming thrust belt, and its internal chronostratigraphic zones are subparallel. On the other hand, equivalent strata in the Gannett Group of the thrust belt are at least three times thicker. This indicates that the Morrison and Cloverly in central Wyoming were deposited within the overfilled part of the foreland basin. Preliminary regional correlation indicates that coarse-grained lithofacies in these rocks are significantly time-transgressive, generally becoming younger toward the E and NE. Overfilling of the early Cordilleran foreland basin in central Wyoming was accomplished by progradation from the W and S. In spite of their three-dimensional (3D) complexity, the Morrison and Cloverly Formations generally confirm theoretical model predictions for overfilled foreland basins.