SEQUENCE STRATIGRAPHY OF UPPER JURASSIC MICROBIAL BEARING STRATA, NORTHEASTERN GULF OF MEXICO

In the northeastern Gulf of Mexico, Upper Jurassic microbial buildups developed in an inner carbonate ramp setting. These thrombolites attained a thickness of 58 m and were present in an area of as much as 13 km2. Sequence stratigraphic studies of the Upper Jurassic subsurface strata that contain these buildups indicate these microbiolites developed in deposits of the transgressive to lower regressive systems tracts of a transgressive-regressive sequence. These microbes nucleated on a cemented and encrusted substrate or on a crystalline rock surface associated with a Paleozoic basement paleohigh during a rise in sea level under initial zero to low background sedimentation rates in low-energy paleoenvironments characterized by fluctuating conditions. Colonization on a basement paleohigh corresponds to the transgressive surface that develops in association with marine flooding. The buildup continues to grow in response to sea level rise. With a decrease in the rate of sea level rise, loss of accommodation, and the initiation of higher energy conditions, the microbes cease propagation. In parts of the northeastern Gulf of Mexico, buildups developed in the absence of basement paleohighs. Microbes nucleated on an encrusted and/or cemented surface that corresponds to the surface of maximum sediment starvation. The buildup continues to grow with continued rise in sea level. In these carbonate systems, the surface of maximum transgression occurs stratigraphically higher than the surface of maximum sediment starvation. A reduction in growth of the microbiolites post dates the surface of maximum transgression, which separates the transgressive and regressive systems tracts. Thus, the buildups occur in deposits of the transgressive and lower regressive systems tracts. With a continued reduction in the rate of sea level rise and either the initiation of higher energy conditions or a high increase in background sedimentation rates, thrombolite growth ceases. Therefore, the initiation of microbial growth corresponds to a transgressive surface associated with crystalline rock or surface of maximum sediment starvation, and the demise of the buildup post dates the surface of maximum transgression. These Upper Jurassic buildups occur in deposits of transgressive to lower regressive systems tracts in the northeastern Gulf of Mexico.