Sharp Base Research Articles

Identification and Interpretation of Upper Cretaceous Fluvial and Deltaic Sandstones: ABSTRACT

Well-developed fluviatile and deltaic characteristics are present in the Upper Cretaceous Ferron Sandstone Member of the Mancos Shale in east-central Utah. Detailed stratigraphic relations and sedimentary structures not only provide criteria for paleoenvironment identification, but also form the basis for semiquantitative estimation of paleogeographic parameters. The fluviatile facies consists of 100-700 ft of alternating sandstone, shale, and coal in units of inconstant lateral continuity and thickness. The fine- to medium-grained sandstone beds are in distinct channels or more laterally continuous sheets as thick as 60 ft. The sandstone beds have sharp bases, fining-upward sequences, dominantly trough cross-lamination, and large-scale, point-bar-migration cross-beds. A generally northerly transport direction is shown by decreasing unit thicknesses, decreasing grain size, and by current-oriented sedimentary structures. The deltaic facies is mainly an inclined series, 20-40 ft thick, of delta-front sheet-sandstone beds that thin and become finer grained down the 5-10° depositional foreslope. Beds in the sequence are thin, End_Page 713------------------------------ consist of alternating very fine- to medium-grained sandstone, and are characterized by even, parallel laminae. Detailed analysis of Ferron fluvial sandstone, following relations developed by Schumm, Simons, and others, provides reasonable paleogeographic estimates. Large rivers carried mixed sediment loads under lower flow-regime conditions in meandering channels of intermediate sinuosity northward to a deltaic plain, debouching into a shallow embayment in the Late Cretaceous coast. For selected channel sandstones, ranges can be specified for channel depth, current velocity, rate of discharge, channel sinuosity, and other flow parameters. End_of_Article - Last_Page 714------------