--> --> A New Atlantic and Gulf Coastal Plain Campanian–Maastrichtian Ammonite Zonal Scheme and Its Correlation with the Classic Western Interior Biozonation

AAPG Rocky Mountain Section Meeting

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A New Atlantic and Gulf Coastal Plain Campanian–Maastrichtian Ammonite Zonal Scheme and Its Correlation with the Classic Western Interior Biozonation

Abstract

This project is aimed at re-evaluating and refining the ammonite-based Campanian–Maastrichtian zonal scheme for the Atlantic and Gulf Coastal plains (ACP and GCP, respectively) with the over-arching goal of correlating these zones to those erected for the U.S. Western Interior (WI) by William A. Cobban in conjunction with his collaborators. Both the ACP and GCP share numerous ammonite taxa with the WI; the latter having one of most refined molluscan-based biostratigraphic frameworks globally, which has been temporally calibrated by dating of numerous ash beds. The shared species between these regions allow for high-resolution correlations, which are used in this compilation to constrain (or bracket) the age and duration of GCP and ACP ammonite zones. The ACP and GCP ammonite zones range from 0.3 to 3.3 Ma and have a mean duration of 1.2 ± 1.0 Ma. The durations of these ammonite zones are considerably 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 the chosen index taxa, limited exposure of fossiliferous strata, the occurrence of numerous condensed beds, and a paucity of highresolution ammonite-based biostratigraphic studies below the upper Maastrichtian. Our ammonite-based zonations for the ACP and GCP are virtually identical and are represented by 14 zones, respectively. Four Campanian ammonites zones extend from the WI into the ACP and GCP based on the shared occurrence of index species. Many of these ammonite zones are applied to these areas for the first time and provide a more comprehensive framework that fills many of the biostratigraphic gaps for intervals that have previously been unzoned due to a poor understanding of ammonite range data. These ammonite zones in conjunction with their associated ammonite faunas provide an important link for broad-scale correlation between North America and other parts of the globe.