Geological Controls on Evaporite — Carbonate Facies Transition in Permian Seven Rivers Formation, SE New Mexico

Abstract

Regional evaporite-carbonate transitions control stratigraphic trapping in many carbonates, yet basin-scale controls on the position of evaporite-carbonate facies changes are not well understood. The Seven Rivers Formation changes from a carbonate near the Delaware Sea to evaporite to the north and northwest. The objectives of this study are to describe the nature of this lateral facies change and interpret its geological controls. In the subsurface of eastern Eddy Co., NM, the transition is a fine-scale interfingering of southerly thickening dolomite beds with northerly thickening anhydrite beds within a narrow (< 1.5 km) E-W striking transition zone. The evaporite – carbonate transition does not shift as the shelf margin progrades to the south about 10 km. On outcrop near Rocky Arroyo, NM, the gypsum-dolomite transition is abrupt, but it systematically moves ∼ 5 km NW during Seven Rivers deposition as the shelf margin prograded 2.5 km SE. Cenozoic? gypsum dissolution modified the shape of the preserved gypsum-dolomite transition. Shelf-interior, dolomitized carbonates are salinity-restricted, subtidal, marine deposits with no evidence for intertidal or supratidal deposition except near major siliciclastic units. Carbonates are interbedded with thin (2 – 30 cm) siltstones that are subaerial sheet-flow and eolian sand-flat deposits. The carbonate – siltstone couplets form meter-scale parasequences. Carbonate facies changes within each parasequence are autocyclic. Parasequences do not show systematic thickening- or thinning-upwards patterns that can be interpreted as lower-order sequences. Gypsum and carbonate are synchronous, subaqueous deposits from a salinity-stratified water column. Carbonate was deposited in mildly hypersaline marine water at shallow depth whereas gypsum was precipitated from brine deeper in the water column. Both carbonate and evaporite accumulation rates decrease near the pycnocline, and the paleobathymetry therefore steepens near the facies contact. The steeper bathymetric gradient at the transition narrows the transition zone width so that minor water-balance or sea-level changes do not significantly shift the facies boundary. The longer-term position of the facies boundary is controlled by the shelf interior water balance. Thus, shelf-interior processes control the position of the evaporite facies boundary independent from shelf margin progradation patterns.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016