CHALLENGES OF CORRELATION IN VERY THIN STRATIGRAPHIC RECORDS: EXTENDING G.B. SHATTUCK'S CLASSIC MIOCENE SUBDIVISIONS AWAY FROM THE TYPE CALVERT CLIFFS (Invited Presentation)

Outcrops of Miocene strata along the shoreline of Chesapeake Bay, especially the Calvert Cliffs of Maryland, have been studied in detail since G.B. Shattuck’s classic work in 1904. However, recognizing his lithostratigraphic subdivisions in isolated outcops and cores has been challenging. His three formations, the Calvert, Choptank, and St. Marys (~160 m of strata representing ~10 m.y. of time), include a nearly identical spectrum of shallow-subtidal siliciclastic facies and are distinguished primarily by their stratigraphic position, even in large exposures such as the Calvert Cliffs. Correlation with Miocene strata elsewhere has thus typically required biostratigraphic evidence, focusing initially on mollusks (Shattuck) and increasingly on microfossil groups, especially diatoms and dinoflagellates. Detailed physical stratigraphic analysis of the Calvert Cliffs has revealed the existence of many thin (less than 10 m) unconformity-bounded sequences with associated incised valleys, stratigraphic condensation of both proximal-onlap and distal-backlap records, shaving of regressive tracts, and lateral shingling of fundamentally lenticular units. Although these features are no longer controversial, the practical consequences for correlation, mapping, paleobiology, and historical analysis (including recognition of warping) is still under-appreciated. We will highlight several key examples of miscorrelation and resulting nomenclatural confusion, verified by new dinoflagellate biostratigraphic data, especially around (1) the Calvert-Choptank formational boundary (“Calvert Beach Member”, “Governor Run sand”), (2) the “Boston Cliffs Member” of the Choptank Fm (Shattuck’s Zone 19 shellbed), and (3) the Choptank-St. Marys formational boundary (“Conoy Member”, “Little Cove Point Member”). Molluscan assemblages provide useful provisional correlations in the field, but dinoflagellates provide a more finely resolved and less facies-dependent basis for “final” determinations. We are nonetheless now reaching the limits of even that resolution in order to age-discriminate all physically discrete lenticular units within this thin Miocene record.