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Middle and Upper Cambrian deposits in Wyoming represent a classic transgression or onlapping sequence of clastic and non-clastic rocks. However, sedimentologic and paleontologic criteria indicate deposition in a variety of shallow-marine, nearshore environments rather than in a simple progressively deepening marine setting. The basal, coarse-clastic Flathead Sandstone was deposited in fluvial and nearshore marine environments as evidenced by sedimentary and biogenic structures. Braided stream deposits are characterized by alternating sets of planar cross-stratified and horizontally-bedded conglomerate and sandstone. Intertidal deposits consist of ripple cross-stratified sandstone and silty mudstone. Herringbone cross-stratification and desiccation features are common. Subtidal deposits consist of tabular and lenticular sandstone with both large-scale compound cross-stratification and small-scale trough cross-bedding. Younger, fine clastic and carbonate sequences (Gros Ventre and Gallatin Formations) represent more offshore environments where ooids and stromatolites formed on carbonate shoals. The shoreline was oriented north-south throughout the Cambrian and was characterized by numerous embayments and islands of Precambrian basement. A number of local, as well as regional, regressions are recorded within this easterly transgressive sequence. End_of_Article - Last_Page 750 |
