A unique Lower Paleocene shelf-sea chalk in the eastern US Gulf coastal plain (Clayton Formation, western Alabama): Implications for depositional environment, sea-level dynamics and paleogeography

摘要

The deposition of carbonate oozes in shelf-sea settings so prevalent during the Cretaceous generally ceased by the onset of the Paleogene in response to sea-level fall. However, the Lower Paleocene (Danian) Clayton Formation exposed at Moscow Landing in western Alabama, U.S.A., contains a thin chalk bed-the Clayton chalk-that records a later, final phase of shelf-sea carbonate ooze deposition in the U.S. eastern Gulf coastal plain. Sedimentologic character, ichnology, and foraminiferal assemblages indicate deposition in an outer shelf setting, comparable to that for Cretaceous chalks exposed in the Gulf region (i.e., Campanian Demopolis Chalk), at depths of 90 m or more, during a brief transgressive pulse peaking circa 63.8-63.2 Ma. This transgression occurred similar to 1 Myr after the maximum expansion of the Danian Cannonball Sea in the North American western interior, presumably reflecting different controls on sea-level dynamics (eustasy vs. Laramide tectonism and isostasy). Although regional paleogeography for the Early Paleocene is poorly constrained, deposition of the Clayton chalk suggests that the eastern Gulf seaway at times extended much further north than implied by the presently mapped updip limit of preserved Paleocene marine strata. The apparent geographic restriction of this unique Paleocene shelf-sea chalk indicates that relative sediment starvation and associated carbonate ooze deposition may have been influenced by local structural controls operating along the eastern margin of the Mississippi embayment.