ESTIMATING THE DURATION OF STRATIGRAPHICALLY CONDENSED MIOCENE MARINE SHELL BEDS USING STRONTIUM ISOTOPE STRATIGRAPHY (CALVERT CLIFFS, MD): IMPLICATIONS FOR PALEOECOLOGICAL ANALYSIS

The Calvert Cliffs (Maryland, USA) is a thin Miocene sedimentary record that has been studied for more than a century, with the diverse molluscan fauna of four major complex shell beds in the Calvert and Choptank Formations receiving the greatest paleontologic attention. The origin of these laterally extensive and massive shell deposits has been the subject of long-standing debate. Each exhibits a similar deepening-up trend in molluscan paleoecology, internal stratification, and taphonomy. In a sequence-stratigraphic context, these trends suggest that each shell bed is a condensed set of parasequences comprising the transgressive systems tract of a 3^rdorder sequence. The duration of accumulation for each major complex shell bed was inferred as ≥10 ky and <1 My based on geologic and paleontologic inference; however, the exact period of time is still disputed.

We applied strontium isotope stratigraphy to the Camp Roosevelt major complex shell bed (Shattuck Zone 10) to determine the length of bed deposition. Valves of the bivalve Chesapecten nefrens were newly collected from the top and bottom of the shell bed at three localities; valves from a fourth locality had been collected in the 1970s. After screening shells for diagenetic alteration, we selected 30 left valves for Sr⁸⁷/Sr⁸⁶ analysis. Using the marine strontium isotope curve to convert our data to geologic ages, we then estimated bed duration as the difference in the top and bottom ages at each locality.

The average duration for the 3 m thick Camp Roosevelt shell bed (16.5 Ma) was 600 ky and is consistent among sampled localities. These findings indicate that the Camp Roosevelt shell bed does represent long-term accumulation, corroborating previous interpretations of the major complex shell beds as the condensed parasequences of transgressive systems tracts, but the absolute duration on the order of 10⁵ years is an order of magnitude longer than considered likely, magnifying the degree of condensation. Given paleontological and sedimentological similarities among major complex shell beds in the Maryland Miocene, long-term accumulation likely does characterize other Neogene examples of 'hiatal' shell concentrations globally. Such records present completely different challenges for paleobiological analysis than shell beds from ordinary sedimentary records.