MIDDLE MIOCENE CLIMATIC OPTIMUM (CA. 15.6 MA) AND ANTARCTIC ICE SHEET EXPANSION (CA. 13.9 MA): RECORDED IN UPPER CALVERT FORMATION EXPOSED ALONG WESTERN SHORE OF CHESAPEAKE BAY?

Shallow marine sediments of the Miocene Chesapeake Group crop out along the Calvert Cliffs, a ca. 50 km long series of ca. 10-40 m high eroding bluffs along the western shore of the Chesapeake Bay. The strata, subdivided into 21 numbered beds, have been shown by borehole transects to be the landward fringe of regional Mid-Atlantic margin successions (Browning et al., Bull. GSA, 2006). The latter find a quasi-cyclic stack of sequences from 24 to 10 Ma, averaging 1.27 Ma per sequence (Cape May)—probably glacioeustatic responses to ca. 1.2 Ma modulation of the 41 ka obliquity cycle.

We posit that the two most pronounced global climate events during the 20-10 Ma interval recorded in the Calvert Cliffs strata are uniquely recognizable as sea level-related features. These events are the Middle Miocene Climatic Optimum (MMCO; peaking ca. 15.7-15.5 Ma) and the Antarctic Ice Sheet Expansion (AISE; ca.13.9 Ma). We use these dates to recalibrate the ages of upper Calvert Formation beds 11-14.

Miocene warmth peaked during the anomalous MMCO, with ice-free areas along the Antarctic coast. The locally unique “Parkers Creek Bone Bed” (Bed 12) may record this event because its marine fauna show the deepest shelf waters of the ca. 20-10 Ma Calvert Cliffs exposures (Kidwell, J. Sed. Res., 1997). Terrestrial vertebrate remains introduced from adjacent lands also indicate subtropical climates. The temporal correlation between the MMCO and peak eruption rates of the Columbia River Flood Basalts suggests the MMCO was at least partly caused by increased volcanogenic CO₂ from those voluminous, volatile-rich eruptions. The following AISE occurred ca. 13.9 Ma, and we associate this cooling, which lowered eustatic sea levels by ca. 40 m, with the only known buried channel within in the Calvert Cliffs strata—the Kenwood Beach Channel, filled by sand by Bed 17 time, is 5-7 m deep and 3 km wide in exposure. We thereby interpolate ages of ca. 15.5-14.5 Ma for Bed 13 and 14.5-14.4 Ma for Bed 14. While within the range of earlier age estimates, our assignments are up to ca. 1.5 Ma older than Sr86/Sr87 ages reported for mollusk shells (Browning et al., 2006), a discrepancy we attribute to shell alteration. Our ages imply that lithic erratics (previously described from beds 12-14) were deposited during warm climates, strengthening the case for transport by driftwood, not ice.