Oxygen minimum zone and internal waves as potential controls on location and growth of Waulsortian mounds (Mississippian, Sacramento Mountains, New Mexico)

Abstract

Lower Mississippian Waulsortian mounds in the Sacramento Mountains grew on a south-dipping homoclinal ramp at depths ranging from approximately 110 to 250 m, a setting in which nutrient sources and pathways are poorly understood in ancient carbonate depositional systems. Lithologic, biotic, and chemical data suggest that the mounds grew in an area on the ramp within a dysoxic oxygen minimum zone (OMZ) resulting from the concentration of organic matter below a thermocline. The abundance of organic matter there, perhaps a consequence of upwelling, enhanced the productivity of the major sediment contributors to the mounds—sponges, pelmatozoans, bryozoans, and heterotrophic carbonate-producing microbes. The siting and growth of individual mounds within the OMZ is best explained by two factors. One is the positive low-relief intra-ramp topography on and around which the mounds grew. The other is that nutrient supply for carbonate producing microbes was concentrated at the sites of mounds growth by internal waves on the pycnocline coincident with the thermocline. The internal waves mixed the nutrient-rich water of the OMZ with the better-oxygenated adjacent water mass, stirred the substrate and resuspended organic matter so that it was more available for the primarily suspension-feeding macrofauna both above and below the pycnocline, and generated local vertical mixing. Because of initial intra-ramp topography and subsequently increased relief as the mounds grew, internal waves focused and localized these processes, thus enhancing carbonate production at individual mound sites in a positive feed-back loop.