Morphology and Evolution of Mesozoic Carbonate Paleoshelf Edge, Eastern Baltimore Canyon Trough
Gary M. Edson
Deep-water petroleum exploration on the U.S. Atlantic continental slope provided information about a Late Jurassic-Early Cretaceous shelf-edge carbonate trend. Well data and seismic interpretation indicate a ridgelike limestone buildup of probably Valanginian age at the crest. One well, penetrating over 1,200 ft of this unit, shows that the buildup consists of bioclastic grainstone, packstone, wackestone, rudstone, and possible minor boundstone. Lobate metazoans several centimeters in size may be framework elements. However, more than 80% of this unit is bioclastic debris consisting mostly of coated grains, suggesting that the buildup is dominantly a shelf-edge carbonate debris accumulation.
Immediately shoreward, a topographically lower tabular facies, locally 8-10 mi wide, interfingers landward with terrigenous clastics. This back-bank carbonate facies is lithologically, biologically, and genetically similar to the shelf-edge buildup, but appears to be Berriasian-Berjocian.
Overlying the back-bank unit, lithologically immature Barremian-Valanginian continental clastic rocks form a seaward-thinning wedge, whose apex meets the top of the shelf-edge carbonate bank. In places, this terrigenous unit may have covered and spilled over the buildup onto the paleoslope during the Barremian and Hauterivian.
Unconformably overlying the clastic wedge, an Aptian blanketlike grainstone, packstone, and wackestone unit 200-400 ft thick contains a distinctive biotic assemblage, including calpionellids, tubiphytes, and sparse hexactinellids. It is concluded that during the Aptian marine regression, the terrigenous clastic unit was beveled by erosion to its present wedge configuration. The blanketlike carbonate unit was deposited on the resulting topographic surface during the subsequent Aptian transgression.
AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.