ABSTRACT

Late Cretaceous volcanic activity along the northern rim of the Rio Grande Embayment resulted in the growth of a number of cones that form an arcuate trend in south central Texas. Some of these cones grew to sea level and served as nuclei for shallow-water carbonate sedimentation. Resulting limestones are known as the Anacacho Formation. Significant hydrocarbon accumulations have been found in the limestones associated with many of the volcanoes such as the Elaine Field in Dimmit County, Texas. Facies distribution and diagenetic fabric were analysed from core and electrical logs.

Shallow water on the flanks of the emergent volcano favored rudist-reef development, which, with other marine fauna, supplied abundant shell material for reworking into shoals. Diagenetic fabrics of the resulting grainstones include precipitation of bladed and mosaic calcite as well as limpid dolomite, indicating cementation during meteoric-phreatic conditions. This evidence supports subaerial exposure of the shoals as beaches, allowing the development of a lagoonal environment landward. Seaward of the beach, red-algal ridges and a muddy sand halo developed. The muddy sand halo was supplied with shell material from high-energy areas, whereas the mud was a product of deeper water accumulation. With increasing water depth off the flanks of the volcano these facies grade into a mud-rich open-shelf environment dominated by burrowing organisms.

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After this initial carbonate buildup the volcano subsided and the facies onlapped the volcano as a result of the changing relative sea level. Subsidence continued until the volcano was completely submerged. Faults reflecting readjustments of strata over the plug are recorded in sediments as young as the Navarro Group.

Porosity in Elaine Field carbonates occurs in areas where, a fresh-water lens developed in association with subaerial exposure. In these areas dissolution of grains and limited cementation produced excellent quality hydrocarbon reservoirs.

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