Driftwood dropstones in Middle Miocene Climate Optimum shallow marine strata (Calvert Cliffs, Maryland Coastal Plain): Erratic pebbles no certain proxy for cold climate

Abstract

Sparse lithic erratics (pebbles to cobbles) sampled from three shallow marine strata deposited during the Middle Miocene Climate Optimum (MMCO, ca. 16–14Ma) along the western Atlantic margin (exposed in the Calvert Cliffs, Maryland) suggest transport and deposition not from ice but from the roots of trees uprooted during floods and carried out to sea. Evidence for driftwood transport includes carbonized wood in the same strata. More than half the ca. 225 erratics were quarried in the largely metamorphic Piedmont province (including a few from the Port Deposit Gneiss, still outcropping on the lower Susquehanna River).The lowest sampled bed (Parker Creek Bone Bed) is assigned to the ca. 15.7–15.5Ma peak warmth of the MMCO, which we attribute in part to CO2 from the coevally erupted Grande Ronde flood basalts (GRFB), the peak effusiveness episode of the Columbia River Flood Basalts (CRFB) The three sampled beds predate the ca. 13.9Ma Antarctic cryosphere expansion, which may be recorded in the Calvert Cliffs by a unique buried channel.