Microbial boundstone margins—different style of carbonate slope with important implications

Seismic and well data from Tengiz, one of the larger fields in the Pricaspian Basin, and outcrops in Asturias of Northern Spain show margin progradation of up to 5 km and more than 10 km, respectively, despite their high-relief (up to 600 m) and steep (~20–32°) nature. These Carboniferous examples share a highly productive microbial boundstone factory extending from the platform/shelf break down the slope to nearly 300 m (or more) depth and a lower slope dominated by (mega)breccias and grain flow deposits derived from the margin and slope itself. Utilizing a concept of a self-nourishing slope potentially leads to a rethinking of the development of other margin-slope systems, as is suggested for the Permian Capitan Margin of the Delaware Basin and the modern Tongue of the Ocean in the Bahamas. Elements of this “model” (Fig. 1) have also been recognized in numerous other examples representing a wide range in age and geographic location.

The broad depth range of a microbial and cement boundstone factory increases the potential for carbonate production during both lowstands and highstands of sea level and thereby facilitates progradation. Slope shedding has important implications for slope readjustment, slope architecture, sequence stratigraphic models, and reservoir characterization. When the dominant factory is microbially induced as in the examples considered herein, high rates of carbonate production extend down the slope to considerable depths therefore new carbonate production-water depth profiles should be considered. Other important implications of the subsurface and outcrop examples are: 1) progradation and accretion are a function of slope boundstone growth vs. off platfrom (highstand) shedding, 2) generalized concepts of leeward progradational vs. windward aggradational margins need rethinking, and 3) microbial slope reservoirs differ from their adjacent platform/shelf counterparts in seismic response (lack of throughgoing reflectors) and character (dominantly fractures and dissolution).