OCEAN STRATIFICATION, INTERNAL WAVES, MISSISSIPPIAN MUDMOUNDS, AND THE PERMIAN REEF

Controls on the localization of carbonate buildups by heightened organic productivity and carbonate production is an ongoing topic in reef research. The historical explanation has invoked autotrophic growth in a shallow-water phototrophic environment. Many ancient and modern deep-water reefs, however do not fit the phototrophic paradigm. Recent oceanographic investigation of the vertical structure of ocean water masses and density boundaries shows how internal waves and tides shape the substrate, move sediment, and enable the cycling of nutrients. Ocean stratification should inform the development of ancient reef models because understanding the oceanographic conditions that resulted in the accumulation can help to make sense of stratal relationships and reveal community dynamics and evolution.

Mississippian mounds of the Sacramento Mountains, New Mexico provide an example of the effects of upwelling, oxygen minimum zones (OMZ), and internal waves and provide criteria by which these mechanisms of buildup formation may be identified in the geologic record. Initial mound facies indicate a succession from passive accumulation of sponge-rich mudstones and wackestones upward into cement crusted fenestellid framestone fabrics. Accretionary and clotted microbial fabrics are noticeably absent. Flanking strata have relationships that indicate a continuing increase in accommodation, have wavy bedding with up-slope dipping cross beds, and contain submarine red beds with microbial fabrics similar to examples cited as indicative of edge effects at redox boundaries. These are overlain by glauconite rich sediments similar to those observed near the base of modern OMZs.

The Permian Reef of West Texas has been studied extensively and details of its stratigraphy are well established; however, the depth at which the reef formed and factors responsible for its facies distribution and sustained growth are uncertain. A nutrient model of stratified basin waters provides a mechanism and scaffolding for the reef system being the product of the oceanographic conditions created by a pycnocline and internal waves interacting with the substrate near the base of the photic zone. New data from the reef to fore-reef transition in the Guadalupe Mountains include onlapping wedges of coated grain grainstone with oriented particles and several decimeter scale wave forms and cross bedding dipping toward the platform. These data support this hypothesis and that these and numerous ancient reef and mound systems may inhabit similar ecological space.