MORPHOLOGY OF THE MONROE GAS ROCK: A SMALL CARBONATE PLATFORM SITTING ATOP A FOUNDERED LATE CRETACEOUS SHELF SURFACE

The Monroe Gas Rock (MGR) in northeastern Louisiana represents the last stage of Mesozoic carbonate platform development in the north-central Gulf province. The morphology of the MGR shows a well developed platform margin sitting at least 30m above a surrounding foundered shelf surface. The pinch-out margin of the MGR consists of platform-bounding carbonate slope debris. The platform carbonates are economically productive, containing the giant Monroe Gas Field, whereas the slope debris is non-productive. Local topography on the platform surface is demonstrably neotectonic rather than of depositional or dissolution origin as commonly interpreted. The platform is capped by the basal shales of the Tertiary clastic sequence. The underlying and surrounding foundered shelf surface is an erosional surface developed across the Late Cretaceous Monroe Uplift. Primary topography of the erosional surface is largely masked by the Cenozoic tectonic deformation, but it appears to have been shallowest where overlain by the MGR. This surface is not covered with post-erosional pre-MGR sediment, so it can be assumed that the MGR developed atop a topographic high directly following a sea-level rebound.