Viento Formation: a syndepositional deltaic system recording adjacent passive salt diapir rise, La Popa basin, northeast Mexico
The Paleocene/Eocene Viento Formation, the youngest shallow marine unit in La Popa mini-basin, Mexico, is exposed adjacent to a surface outcropping salt diapir that is partially welded. Viento Formation offers a rare insight into the facies distribution and stacking pattern of siliciclastic strata exposed adjacent to the welded salt diapir in the margin of La Popa mini-basin, and are analogous for subsurface hydrocarbon-bearing reservoirs. The depositional facies distribution, structural geometry and stratigraphic architecture of the Viento Formation was influenced by Hidalgoan shortening across the basin, on-going passive rise of La Popa salt diapir and eustatic sea level changes. Viento Formation strata are exposed along a ~5 km belt adjacent to La Popa salt weld, which consists of discontinuously outcropping remnant gypsum. The Viento Formation displays thinning from SE toward NW over ~900 m to pinch-out. Viento strata form a series of five stacked sequence stratigraphic wedges ranging 25-200 m thick. Beds dip vertically at the weld but dip orientations decrease to ~30° basinward. Sharp, subaqueous erosional surfaces and abundant, low-angle unconformities that becomes conformable within tens of meters distance from the diapir characterize wedge halokinetic sequence boundaries. Seventeen identified sand-prone lithofacies indicate deposition in a tide-influenced delta system and include prodelta, delta front and platform, and fluvio-deltaic facies assemblages. Overall shallowing upward depositional facies successions are comprised of shale, or rarely limestone buildup, oyster facies, horizontal heterolithic stratification, inclined heterolithic bedding, bioturbated sandstone with Ophiomorpha and oyster-shells, mud-draped ripple-laminated sandstone, spheroidal weathering sandstone, trough-cross bedded sandstone, herringbone sandstone, sandstone with wood fragments and mud ripped-up clasts, and conglomerate and conglomeratic sandstone. Conglomerate and conglomeratic sandstone contain abundant well-rounded and well-sorted black chert and diapir-derived detritus - Jurassic metaigneous boulders that decrease in abundance, size, and angularity in a basinward direction. Cyclical tectonic compressional events coeval with continuous sediment loading controlled La Popa salt diapir rise and drove episodic diapir extrusion. Viento stratal composition, distribution, and distinct geometry of halokinetic drape folding forms a tapered composite halokinetic sequence that reflects an overall increase in local third-order net sediment accumulation rates relative to salt rise rates.