NEW CAMPANIAN-MAASTRICHTIAN ATLANTIC AND GULF COASTAL PLAIN AMMONITE BIOSTRATIGRAPHIC SCHEMES AND THEIR CORRELATION WITH THE WESTERN INTERIOR BIOZONATION

The Atlantic and Gulf Coastal plains (ACP and GCP) of North America preserve an extensive stratigraphic succession, which offer an excellent record of evolutionary patterns, faunal change, depositional systems, and ocean-climate change along a passive margin setting during the Cretaceous greenhouse. Despite this excellent record, correlation and age dating of these successions have been hampered by a paucity of dateable ash beds, condensed sections, and limited biostratigraphic study. To improve correlation and age constraints for ACP and GCP Cretaceous strata and faunas, this project re-assesses and refines their lower Campanian to upper Maastrichtian ammonite biostratigraphic frameworks with the objective of correlating these biozones to those erected for the very refined bio- and chronostratigraphy of the Western Interior (WI). Both the ACP and GCP have many ammonite taxa in common with the WI. These shared taxa promote high-resolution correlation, which are used in this compilation to bracket the age and duration of ACP and GCP ammonite biozones. The biozonations for the ACP and GCP are nearly identical and are characterized by 14 biozones. Four of these ammonite biozones are applied to the ACP and GCP for the first time. They also provide a biostratigaphic scheme that fills many of the gaps for intervals that have previously been unzoned due to a limited understanding of ammonite ranges in these provinces and facilitate correlation across the region as well as with the WI scheme. The ACP and GCP ammonite biozones range from 0.3 to 3.3 Ma in duration and have a mean length of 1.2 ± 1.0 Ma. The durations of these ACP and GCP ammonite biozones are longer and more variable than the 0.5 ± 0.2 Ma durations for the WI, which likely relates in the ACP and GCP to reduced evolutionary rates for index taxa, limited exposure of fossiliferous strata, the occurrence of many condensed horizons, and a paucity of high-resolution biostratigraphic studies using ammonites below the upper Maastrichtian. This study is the first step in a major re-evaluation of the Cretaceous molluscan-based biozones along the ACP and GCP.