ABSTRACT

Three depositional sequences, associated with cycles of change in relative sea level and coastal onlap, can be identified in the Upper Cretaceous (Santonian, Campanian, and Maastrichtian) strata of the Mississippi-Alabama area. These depositional sequences have an event spacing of 2 to 11 million years and are herein designated the UZAGC-3.0 (Upper Zuni A, Gulf Coast), UZAGC-4.0, and UZAGC-5.0 sequences. The UZAGC-3.0 cycle comprises a lower type 1 sequence boundary (Tuscaloosa-Eutaw contact); lowstand systems tract (Eutaw Formation); first transgressive surface or disconformity (Eutaw-Tombigbee contact); transgressive systems tract (Tombigbee Sand Member of the Eutaw Formation and the lower Mooreville Chalk); surface of maximum transgression or submarine disconformity (within the Mooreville Chalk); and highstand systems tract (upper Mooreville Chalk and its Arcola Limestone Member, basal Demopolis sandy beds, Coffee Sand, Tupelo Tongue of the Coffee Sand). The UZAGC-4.0 cycle includes a lower type 2 sequence boundary (Coffee or Tupelo Tongue contact with the Demopolis or Sardis Formation or a contact recognized within the Demopolis), which is coincident with the first transgressive surface; transgressive systems tract (Demopolis marls, Sardis Formation); surface of maximum transgression within the Demopolis Chalk; and highstand systems tract (Demopolis Chalk and its Bluffport Marl Member, Coon Creek Formation, Ripley Formation, McNairy Sand). The UZAGC-5.0 cycle includes a lower sequence boundary (McNairy Sand contact with the Owl Creek Formation or Chiwapa Sandstone Member of the Ripley Formation or a disconformity recognized within the Ripley Formation), which can he coincident with the first transgressive surface; transgressive systems tract (Chiwapa or Ripley calcareous sands, Prairie Bluff or Owl Creek marts); surface of maximum transgression within the Prairie Bluff or Owl Creek; and highstand systems tract (Prairie Bluff or Owl Creek beds).

The component systems tracts and defining physical surfaces of these sequences have been recognized and traced from Selmer (Tennessee) to Selma (Alabama), a distance of about 420 km. The sequence boundaries and transgressive surfaces are diachronous along their traces. The transgressive systems tract deposits of a given sequence become progressively younger in age from the basin proper to the basin margin. Only the maximum flooding surfaces and/or condensed section strata have chronostratigraphic significance for regional and worldwide correlation. The synchronous nature of the maximum flooding surface is illustrated by the fact that from the basin proper to the basin margin the beds immediately above this surface rest with the same biostratigraphic zones. Therefore, depositional cycles should be dated by using the synchronous surface (maximum flooding surface) rather than the diachronous surface (sequence boundary or transgressive surface) associated with them. Three such maximum flooding events are evident in the Santonian through Maastrichtian strata of the Mississippi Embayment area. They occur in lower Campanian, upper Campanian, and middle Maastrichtian strata.