Geochemical and paleoenvironmental variations across the Cretaceous/Tertiary boundary at Braggs, Alabama

The Cretaceous/Tertiary (K/T) boundary in southern Alabama occurs in a sequence of interbedded shallow-marine limestones and marls deposited during a Late Maastrichtian regression and subsequent Danian transgression. The presence of a diverse assemblage of Cretaceous and Paleocene benthic micro- and macrofossils has allowed detailed examination of paleoenvironmental changes in this shallow-marine setting at the time of the K/T boundary extinctions. Although extensive diagenesis has resulted in the recrystallization and cementation of whole rock carbonate, a few molluscan macrofossils have retained their original calcitic structure. The oxygen isotopic record of these macrofossils shows a gradual cooling of 2-3°C over a 3 m.y. period which began in the late Maastrichtian and continued into the Danian. The characteristic depletion in δ¹³C across the K/T boundary displayed by planktonic microfossils from pelagic sequences is not recorded at Braggs. Major changes in whole rock trace and minor element chemistry reflect the decrease in terrigenous mineral fluxes to this location brought about by the rapid Early Paleocene transgression.