Intertongued Dakota Sandstone-Mancos Shale Members of Surface and Subsurface in San Juan Basin, New Mexico and Colorado: Eustatic, Tectonic, and Sedimentologic Effects on Depositional Systems: ABSTRACT

Three wedges of transgressive marine Mancos Shale, deposited off-shore, intertongue with four Dakota Sandstone units, largely shoreface deposits. Contacts between shoreface and offshore lithologies trend approximately north-south, with nonmarine Dakota main body sandstones to the west. Submarine bar sandstones trending northwest-southeast are locally developed near the shoreface-offshore transition. This succession was deposited during approximately 4 m.y. of the transgressive phase of the Greenhorn cyclothem. Each Dakota-Mancos sandstone-shale pair may represent (1) a fourth-order eustatic cycle or (2) regional tectonic and local sedimentologic overprints of two third-order Vail cycles. Detailed analyses of trace elements within shales and primary sedimentary structures and trace fossils of sandstones, combined with geophysical logs and regional paleontologic control, were invaluable in this first, basin-wide, surface and subsurface paleoenvironmental study of these complex strata. Dakota main body and basal fluvial sandstones were deposited by easterly flowing, meandering stream system. Overlying sets of extensively burrowed shoreface sandstones were deposited under prevailing southerly paleocurrents. The uppermost Dakota tongue (Twowells Sandstone) illustrates a dominant, but complex, regressive character and a seaward pinch-out, as well as a thin, upper transgressive phase in nearshore locations. Significant petroleum accumulations are contained in this sandstone.