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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.


Paleodepositional reconstruction is the key to deciphering Earth’s history and evolution. However, such work requires detailed vertical facies analysis and lateral correlation and plan-view mapping with isochronous controls. 3D paleogeographic mapping is critical to understand the lateral variability of ancient depositional systems, as to test facies models. 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 depositional system are poorly understood. The Gallup Sandstone was previously interpreted as wave-dominated shorefaces, strand plains, or barrier islands. Facies analysis with sufficient sample spacing in the sequence stratigraphic context allows the interpretation of the depositional system. We measured 71 sedimentological sections along 60 km of laterally continuous cliffs and covered slopes that are oblique to paleo-depositional dip around the Northwest New Mexico. Our interpretation shows that the Gallup clastic wedge is a mixed-process influenced progradational deltaic system with strong river and storm signature on an epicontinental ramp setting. We identify 16 major facies associations, including marine shelf/offshore, deltaic, shoreface, shoreface, distributary channels and bars, fluvial, coastal plain, and tide-influenced facies. Abundant gutter casts and hummocky cross-stratification indicate a low accommodation/sediment supply ratio and strongly storm-influenced depositional environment and define a new type of delta, termed “storm-flood-dominated delta.” Numerous erosional-based channelized features coupled with low bioturbation suggest river-influenced conditions. Abundant soft sediment deformation structures in various scales also indicate a mixed storm and river influenced setting. Paleogeographic maps of parasequences decode lateral facies variations. The transition between deltaic, lagoonal/bay, and shoreface deposits may suggest the asymmetrical delta model. We also define the difference between wave-dominated deltas and shorefaces based on vertical and lateral facies analysis.