AAPG Annual Convention and Exhibition

Datapages, Inc.Print this page

High-Resolution Sequence Stratigraphy, Shoreline Trajectory, Accommodation Successions, and Facies Association in the Cretaceous Gallup System, Shiprock, New Mexico, USA


Although the Cretaceous Gallup system in New Mexico, US has been studied extensively owing to the petroliferous nature of the San Juan Basin, a high-resolution sequence stratigraphy has not been well documented, especially from the perspective of shoreline trajectory and accommodation successions. We measured 45 sections over 58 km of laterally continuous cliffs and covered slopes along depositional dip around the Shiprock area. Two laterally continuous bentonite beds provide reliable datums. The “Upper Bentonite”, used as a regional datum, divides the Gallup section into upper and lower parts. Five stratigraphic sequences, consisting of sixteen parasequence sets that are constituted of thirty-two parasequences are identified in the lower Gallup interval; six sequences with twenty-three parasequence sets and sixty-one parasequences compose the upper Gallup. Parasequence sets show aggradation, progradation, retrogradation, and degradation, reflecting high-frequency relative sea-level changes. Abundant gutter casts and storm beds indicate a low accommodation / sediment supply ratio and strongly storm-influenced deltaic deposits. Channelized features and soft sediment deformation, coupled with low bioturbation also suggest river-influenced deltaic conditions. These deltaic deposits show along-strike facies changes into dune-scale stratified sandstones with abundant trace fossils interpreted as shoreface deposits. This suggests asymmetrical wave-influenced deltaic systems, rather than barrier-shorefaces as previously interpreted. Inoceramids and ammonite fossils suggest a depositional duration of Late Turonian to Early Coniacian time, which is approximately 2 Ma. Therefore, the average duration of each parasequence was about 21,500 years, and this may indicate eustatic control by precession-dominated Milankovitch Cycles.