Depositional Facies and High-Resolution Sequence Stratigraphic Analysis of a Mixed-Process Influenced Deltaic System in a Stormy Ramp Setting: The Cretaceous Gallup System, New Mexico, U.S.A.

Abstract

The Cretaceous Gallup Sandstone has been studied extensively owing to the petroliferous nature of the San Juan Basin, but the depositional environments and sediment processes of this progradational system are not well understood. The Gallup Sandstone was previously interpreted as wave-dominated shorefaces, strand plains, or barrier islands. Our interpretation shows that the Gallup is a mixed-process influenced deltaic system with strong river and storm signature on an epicontinental ramp setting. We measured 69 sections and walked out key surfaces and facies along 60 km of laterally continuous cliffs and covered slopes that are oblique to paleo-depositional dip around Northwest New Mexico. Detailed facies analysis based on sedimentological sections and numerous photomosaics shows 9 major facies associations, including marine shelf/offshore, river-dominated delta, storm-flood-dominated delta, lower shoreface, upper shoreface, distributary channels and bars, fluvial, nonmarine coastal plain, and tide-influenced deposits. Abundant gutter casts and hummocky cross-stratification indicate a low accommodation/sediment supply ratio and a strongly storm-influenced depositional environment. Numerous erosional-based channelized features, coupled with low bioturbation, suggest river-influenced conditions. Abundant distinctive soft sediment deformation at various scales also indicates a mixed storm and river influenced setting. The lateral transition from deltaic and lagoonal/bay deposits to shorefaces may be indicative of an asymmetrical delta. Depositional facies are closely dependent on systems tracts and shoreline trajectory, which are related to allogenic relative changes of sea level. Within parasequences, facies also change due to autogenic controls. The vertical and lateral changes in depositional facies manifest the evolution of depositional environments over time.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018