SEQUENCE STRATIGRAPHIC SETTING OF THE CRETACEOUS-TERTIARY BOUNDARY IN CENTRAL ALABAMA

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Abstract
In central Alabama, near the town of Braggs, a complete section across the Cretaceous-Tertiary (K-T) boundary is present within the lower portion of the Clayton Formation. The K-T microfauna and microfloral transition occurs within a 2.5-m (8 ft) section of interbedded sandstones and limestones that directly overlies a sequence boundary, marked by regional truncation of the underlying Prairie Bluff Formation. This sequence boundary is related to a major eustatic fall in the late Maastrichtian (67 Ma). The interbedded sandstones and limestones in the basal Clayton Formation are interpreted as two backstepping marine parasequences deposited on the inner shelf during the subsequent relative rise in sea level. These two backstepping parasequences are overlain, in turn, by 1.5 m (5 ft) of glauconite-rich strata representing a condensed section produced during a period of slow terrigenous deposition, continued par-asequence backstepping, and shoreline retreat. Three small iridium anomalies have been identified at the Braggs locality. These anomalies occur at marine-flooding surfaces, interpreted to be parasequence boundaries, in the uppermost Prairie Bluff and basal Clayton formations. The uppermost of these anomalies also coincides with the base of the well-developed condensed section in the basal Clayton Formation. The concurrence of iridium concentrations with marine-flooding surfaces at Braggs suggests that iridium was present in the open ocean during the latest Maastrichtian through earliest Danian but concentrated only during periods of terrigenous-sediment starvation. Thus, variations in sediment supply and possibly basin location are critical factors controlling iridium enrichment across the K-T boundary.

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