The Other Lower Cretaceous Carbonate Shelf-MarginTrends in the Gulf of Mexico: Winn and Calvin Limestones
Loucks, Robert G.; Sullivan, Peter A.; Zahm, Laura; Kerans, Charles
Three well-documented Lower Cretaceous shelf-margin carbonate complexes are present in the Gulf of Mexico: Knowles, Sligo, and Stuart City. Documentation of two other thick Lower Cretaceous carbonate-shelf margins between the Knowles and Sligo shelf margins comes from wells drilled in northern Louisiana and a cored well, the ARCO #1 Huffman McNeely (15,500-17,500 ft), in Natchitoches Parish, Louisiana. These are the Winn (Valanginian in age?) and Calvin (Berriasian to Valanginian in age?) carbonate shelf margins. The stratigraphic positions of the Winn and Calvin shelf margins are documented by a wireline-log stratigraphic cross section in northern Louisiana and a seismic line in East Texas. The Lower Cretaceous Knowles Limestone dips into the deeper subsurface before reaching the outboard position of either the Winn or Calvin margins. The Calvin carbonate margin (>1500 ft thick) is farther downdip than the Knowles and Winn margins. The Calvin is separated from the Winn and Knowles by the Calvin Shale. The Winn shelf margin (600 ft thick) is younger and updip of the Calvin shelf margin. Above the Winn Limestone is the Hosston Sandstone. Several cores over a 2000-ft interval were cut in the Huffman McNeely well. Three general depositional environments are documented: (1) slope, (2) reef, and (3) back reef. The Calvin slope system is distinguished by carbonate debris with fragmented components, including sparse corals and stromatoporoids that were deposited in front of the reef proper. Reefal buildup of the Calvin is dominated by framework stromatoporoids and scleractinians (some in growth position) with major amounts of Lithocodium. Early peloidal cement is abundant. The Calvin backreef apron is characterized by encrusting stromatoporoids and molluscan material and fewer corals. This facies has a grainer texture than the reef facies, and some Lithocodium coats individual grains. The contact between Calvin Shale and Winn Llimestone was cored. The transition consists of calcareous, siliciclastic mudstone grading up into ooid/intraclast/skeletal packstones in the Winn Limestone, which has some massive corals and stromatoporoids. Both the Calvin Shale and Winn Limestone are interpreted to have been deposited on the slope. The only pore networks in these deeply buried limestones are micropores after the transformation of Mg-calcite allochems. Porosity ranges from <1% to nearly 9%, and permeability ranges from 0.01 to 1 md.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013