ABSTRACT

The lower unnamed member of the Upper Cretaceous (Lower Campanian) Mooreville Chalk, 500 feet (152 m) thick, is comprised of hemipelagic sediments with minor amounts of sand. Five sedimentary facies have been delineated through detailed analysis of outcrops and subsurface samples. In approximate stratigraphic order these are: (1) tan to greyish green, calcareous, glauconitic, intraclastic sandstone with interbedded sandy foraminiferal chalk and calcareous clay (interpreted as shallow inner shelf deposits); (2) light-colored, bioturbated, Thalassinoides-bearing, silty marl containing a macrofauna of infaunal bivalves, ostreids, inoceramids, and calcareous worms (open shelf, below storm wave base); (3) dark grey to olive, fissile, laminated, inoceramid-bearing calcareous clay and marl including layers of bioturbated marl with tiered trace-fossil sequences that suggest periodic rapid deoxygenation events (open shelf, well below storm wave base); (4) tan to brown sand and sandy marl occurring in sand bodies up to 30 feet (9 m) thick and displaying a coarsening upward sequence with hummocky cross-bedding and megaripples at the top (shelf sand bodies, well above storm wave base); and (5) micaceous, silty, and sandy Exogyra-bearing marl in which some Exogyra are disarticulated, abraded, and imbricated and in which some sands occur in thin veneers, several meters wide, on concave-upward scour surfaces (open shelf, above storm wave base). The vertical and lateral arrangement of these sedimentary facies, especially the shelf sand bodies, suggests a sea-level curve with three cycles of sea-level rise and fall. These cycles are not represented in coeval rocks of the Western Interior Seaway where the Lower Campanian is comprised of only one cycle. A more complicated, perhaps rift-related, history of sea-level change is suggested for the northern Gulf rim during the Early Campanian.