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Domal Thrombolitic Microbial Biostromes in the Upper Albian Devils River Limestone Along the Northern High-Energy Margin of the Maverick Basin


The Lower Cretaceous Devils River Trend on the northeastern margin of the Maverick Basin (Edwards County, TX) in southwest Texas contains a unique deposit of large (up to 7 ft high) domal thrombolitic microbial biostromes that have not been identified elsewhere in Texas. These microbialites are overlain by microbial dominated, moderate-energy grainstones. Above the grainstone section is a hiatus that is transgressed by large caprinid bioherms. The overall setting for these carbonate deposits is the northern rim of the intraplatform Maverick Basin that is approximately 150 miles behind the Stuart City Reef Trend. Four levels of description are necessary to describe the biostrome: (1) Megalevel (overall external form)—domal biostrome, (2) Macrolevel (larger internal form or layering)—sinuous convex layers, (3) Mesolevel (basic growth morphology—thrombolitic (subvertical to vertical), and (4) Microlevel (microbialite basic texture)—peloids and mesoclots (clots of microbial-mediated material). The finer scale texture is complex. The thrombolitic peloids and mesoclots are arranged in digitate to randomly branching growth forms. The thrombolites are very microporous. Cavities with sediment fill are very common and are filled with mesoclots, peloids, clasts, miliolids, ostracods, echinoid fragments, bivalve fragments, and gastropods. The microbial biostrome is not continuous, but has gaps filled with microbial-rich grainstone. The grainstone strata above the microbial biostrome is continuous, but irregular, and is composed of fossiliferous peloidal intraclast grainstone with abundant allochems associated with microbial growth. Evidence of a caprinid bioherm at the top of the section are toes of talus debris. The debris ranges from mud-rich rudist-fragment packstones to grainstones with some whole rudists. The main body of the rudist bioherm itself is approximately 0.1 mi to the northwest of the outcrop. The lower domal thrombolitic microbialite biostrome is interpreted to have been deposited on a moderate-energy, intraplatform rim at the northeastern edge of the Maverick Basin during a higher order transgression. The above microbial-rich grainstones were laid down in a moderate-energy environment during a highstand. Another higher order cycle is composed of a thin couplet of another thrombolite unit and microbial-rich grainstone. A sharp break exist with the shelf-interior rudist mound transgressive cycle above.