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Steven L. Dorobek1, Christine Iannello1, Michael F. Hoffmann2, Steven L. Bachtel3, Steven K. Reid4
(1) Texas A&M University, College Station, TX
(2) Burlington Resources Inc, Houston, TX
(3) Exxon Production Research Company, Houston, TX
(4) Morehead State University, Morehead, KY

Abstract: Carbonate Gravity-flow Processes and Resultant Depositional Features in Outer-Ramp Facies: Mississippian Lake Valley Formation, South-central New Mexico

Coarse-grained skeletal limestone facies in the Nunn and Tierra Blanca members of the Lake Valley Formation (Mississippian, Sacramento and San Andres Mountains, south-central New Mexico) were deposited by a variety of sediment gravity-flow processes. Gravity-flows were largely unconfined and ultimately created large (10’s of meters thick, up to km-scale laterally) lobes of skeletal sand. Synoptic relief from the top of the lobate sand bodies to the adjacent basin floor was generally small (<20 m) and small lobes constructed larger, composite lobes. Very low-angle downlapping geometries are the most common stratigraphic expression of lobe progradation, although clinoforms are found locally. Regional dip on the Lake Valley ramp was likely <1°, so seismic events may have been responsible for triggering sediment gravity-flows and for creating soft-sediment deformation features within these strata.

Stratal geometries and sedimentary structures reflect deposition by grain flows, high-density turbidity currents, and sand-rich debris flows (<10% lime mud matrix). Structureless beds with little internal grain size variation are most common, although transport processes for these beds are enigmatic. Turbidity currents clearly transported some sediment, however, as indicated by fining-upward beds, traction-related stratification, and large-scale (100s of meters laterally) lateral facies changes. Unconfined to locally channelized debris flows were initiated from intrastratal failure events. Some debris flows left large, ‘outsized’ lithoclasts in updip deposits, suggesting significant runout distances for these flows. Sediment volumes and the erosive power of individual flows were highly variable (both spatially and temporally), although stratal geometries suggest both parameters generally increased as the sand lobes prograded.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana