Depositional Environments of Unstable Shelf-Margin Deltas of the Oligocene Vicksburg Formation, McAllen Ranch Field, South Texas

Cores and supporting petrophysical data from 20 wells in McAllen Ranch field were analyzed. This Vicksburg field lies in northern Hidalgo County near the south end of the Texas Gulf Coastal Plain on the downthrown block of a major growth fault that merges with a glide plane dipping 5 degrees to the east beneath the field. The analyses provided a base for interpreting depositional environments.We grouped cored strata into four facies: (1) prodelta shales, (2) distal delta-front sandstones and shales, (3) proximal delta-front sandstones, and (4) distributary-channel sandstones. We found that prodelta and distal delta-front deposits contain thinly interbedded sandstone and silty mudstone, which form dip elongate features that we interpreted as density-flow deposits formed within a fluvial-dominated deltaic setting. The proximal delta-front sandstones consist of thick (10- to 120-ft) sandstone beds containing crude, subhorizontal laminae or massive bedding interstratified with less common trough crossbeds, deposited as broad, strike-elongate bodies in a shallower, higher energy, wave-dominated depositional environment. Reworking and northward transport of sediment by longshore current from the southern fluvial-dominated setting created strike-elongate sand bodies in the north. Distributary-channel deposits are common in the apical southwest and west parts of the deltas. These depositional patterns indicate a history of southern fluvial-dominated deltaic deposition and concurrent and subsequent reworking of the distal and north parts of the delta complex into a wave-dominated regime. The lower Vicksburg deltaic deposits exhibit a complicated history of development of interrelated sedimentation, growth faulting, and diapirism within an overall transgressive environment. Growth faulting localized progradational deltaic deposition into rapidly rotating and subsiding wedges. Rotation and expansion of the section along the growth fault and glide plane also controlled the thickness of the deltaic deposits. The deltaic sediments are thickest along the glide-plane contact and thin dramatically down depositional dip. Transgressions shifted the foci of deltaic deposition and associated growth faulting in a landward direction. Eventually transgressions moved deltaic deposition well to the west of McAllen Ranch field, ending deposition of the unstable shelf-margin deltas. Shale diapirism increased in importance as the depositional loci shifted. An erosional unconformity formed across the distal ends of many deltaic wedges as a diapir rose beneath the field. Subsequent deflation of the diapir produced a depression that was filled by distal shelf siltstones and mudstones. Understanding this style of deposition can lead to locating and recovering hydrocarbons in Texas because, although relatively common, shelf-margin deltaic reservoirs are difficult to characterize because of their structural and depositional complexity. Because the basal Vicksburg deltas were deposited on distal shelf and upper slope mudstones of the underlying Eocene strata, they are important examples of unstable shelf-margin deltas that accumulated in a setting of active tectonism and deposition.