Stratigraphy of the Lower Cretaceous (Albian) Salmon Peak Formation of the Maverick Basin, South Texas

Abstract The Lower Cretaceous (Albian) Salmon Peak Formation of the Maverick basin is divided into surface-near surface and subsurface occurrences separated by a transition zone within which electric log character is very poor. Correlations through this zone are based on stratigraphic projections. Surface-near surface stratigraphic units include a lower lime mudstone unit with a burrowed zone in the upper half, and an upper lime grainstone unit. In the subsurface, the Salmon Peak is divided into upper and lower unit equivalents, projected from surface units, the Pryor Member (Winter, 1962), based on electric log character, and the equivalents of the West Prong beds of the Del Rio Formation. Three facies are defined: lime mudstone facies; burrowed zone facies; and lime grainstone facies. The lime mudstone facies is calcisphere lime mudstone to wackestone. Rocks of this facies overlie the pellet, intraclast lime packstone of the McKnight Formation throughout the area studied. The entire Salmon Peak in the subsurface and basal part of the lower stratigraphic unit in surface-near surface occurrences are of this facies. The burrowed zone facies is burrowed, echinoderm fragment lime wackestone. The upper part of the lower stratigraphic unit in surface sections has this facies. The unit changes northward, toward the margin of the basin, to lime grainstone facies and basinward to lime mudstone facies. The lime grainstone facies varies from lime wackestone and packstone to grainstone and is characteristic of the upper stratigraphic unit of surface-near surface sections. It lies beneath calcisphere, shell fragment lime mudstone of the West Prong beds of the Del Rio Formation. In general, the upper unit thins southward into the basin by progressive change, from the base upward, to burrowed zone facies. To the north it changes to rudist limestone of the upper Devils River Formation. Stratigraphic and paleontological studies indicate the lime mudstone facies was deposited in a partially restricted shelf basin environment behind the Stuart City margin. The lime grainstone facies was deposited as part of a platform bank which prograded southward into the basin. Variations in lithology in this facies may be related to changes in shelf to basin slope. The burrowed zone facies was formed in a transitional environment of deposition between the prograding bank and basin. The burrowing within this zone could be related to an increase in oxygen content, a decrease in the rate of sedimentation, or an increase in nutrients along the basin edges.