Sedimentology of a Sequence Boundary Within The Nonmarine Torrivio Member, Gallup Sandstone (Cretaceous), San Juan Basin, New Mexico
Andrew D. Miall, 1991. "Sedimentology of a Sequence Boundary Within The Nonmarine Torrivio Member, Gallup Sandstone (Cretaceous), San Juan Basin, New Mexico", The Three-Dimensional Facies Architecture of Terrigenous Clastic Sediments and its Implications for Hydrocarbon Discovery and Recovery, Andrew D. Miall, Noel Tyler
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The Gallup Sandstone of northwest New Mexico consists of a series of sandstone tongues, deposited in strandplain environments, interbedded with fine-grained marine sediments of the Mancos Shale. The uppermost tongue includes shales and thin sandstones of the "nonmarine Gallup" and a prominent sandstone unit, the Torrivio Member. The latter has been interpreted as fluvial in origin. Earlier work by other researchers interpreted the Torrivio as the basal member of a transgressive sequence, which includes in its upper part open-marine shelf deposits of the upper Mancos Shale.
The nonmarine Gallup contains some evidence of marine influence and is interpreted here as a back-barrier tidal-flat or lagoonal deposit The overlying Torrivio Member is fluvial in its lower part, but some sections along the west and south flank of San Juan Basin contain tidal sand waves in the upper part of the member. A prominent bounding surface separates fluvial from tidally influenced beds in some localities, and it is suggested here that this is the position of the main sequence boundary, marking the time of maximum regression. The succession up to this bounding surface is that of a regressive, strandplain to fluvial parasequence. Above the surface, tidal deposits, probably representing estuarine sand shoals, form the base of a transgressive sequence.
Maximum regression may record the time of lowest relative sea level. However, there is no independant evidence of sea-level change. Current models that purport to explain changes in fluvial accomodation space with reference to changes in base level are flawed, and an alternative model of regional tectonism and resulting increases in sediment supply cannot be ruled out as a mechanism for the generation of fluvial tongues in foreland basins, such as the Torrivio Member.