Abstract

The coastal sandstones of the La Ventana Tongue in the San Juan Basin, New Mexico, were deposited along the western margin of the Cretaceous Western Interior seaway and form the thickest "bench" of the Cliff House Sandstone (Mesaverde Group). The Cliff House Sandstone represents the transgressive phase of the Clagget cycle sandstones of the thick (up to 800 ft, 240 m) La Ventana Tongue stacked within a narrow zone due to a gross balance among basin subsidence, eustatic sea level changes, and sediment supply rates. Regional net-sandstone distribution, determined from 920 electric logs, highlights the elongate geometry and depositional strike (N50 degrees W) of the unit. Examination of 760 of the logs from a part of the study area allowed the delineation, correlation, and net-sandstone mapping of eight genetic sandstone packages (subunits) within the La Ventana Tongue. Subdued, dip-oriented bulges in shoreline deposits represent deltaic headlands. Shoreline areas between headlands are considered to be strand plains or coastal barriers. Relatively isolated, elongate sandstones in offshore positions represent shelf sandbar systems. Depositional processes inferred to have been important during deposition of the La Ventana Tongue include: (1) delta switching and abandonment, (2) transport of coast-parallel "plumes" of sand downdrift from deltaic headlands by coastal currents, (3) formation of shelf sandbar systems by deflection of coastal currents from the headlands, and (4) reworking of deltaic sands into thin sand sheets during local or regional transgressions. Sedimentary facies and depositional processes are believed to be similar to the modern Nile Delta.--Modified journal abstract.

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