THE BASAL CALVERT FORMATION: MISSING ZONES, REWORKED DINOCYSTS, PALEOCHANNELS, AND THE DYNAMICS OF THE ATLANTIC COASTAL PLAIN MARGIN

The Miocene Calvert Formation forms spectacular cliffs on the western side of Chesapeake Bay in Maryland and occurs as scattered outcrops inland of these cliffs. “Shattuck Zones” have been used for over 100 years to distinguish specific parts of the Calvert. Since the 1970s, it has been known that the age of the basal Calvert varies from place to place. Here, we concentrate on the oldest and second-oldest units of the Calvert, which include similar basal shelly to gravelly lag sands updip (and landward) to finer grained downdip (and seaward). The oldest Calvert (“Dunkirk beds” or Shattuck Zones 1 and 2 plus a diatomite now called “Zone 3A”) occurs in an arcuate band that thins from New Jersey, through Delaware to Maryland and is present in cores in southern Virginia. Stratigraphically above this, or as the basal Calvert where the “Dunkirk beds” are not present, are deposits unrecognized by Shattuck called the Popes Creek Sand Member, overlain unconformably by fine-grained, diatomaceous "Zone 3B.” Paleontologically, the “Dunkirk beds” are characterized by dinocyst subzone DN2a and a microvertebrate fish fauna lacking Lagodon (pinfish) teeth. The Popes Creek Sand Member is characterized by dinocyst subzone DN2b and abundant Lagodon teeth. Controls on the extent of preserved Dunkirk-equivalent deposits include sediment source and accommodation space, complicated by erosion, faulting, and paleochannels, plus differential subsidence around the buried Chesapeake Bay impact structure. Dinocyst biostratigraphic studies benefit from examination of reworked specimens. The pre-existing formation(s) that contributed to the basal lags is highly variable, as is the condition of the reworked specimens. Distinctively preserved dinocysts, sourced from the lower Eocene Nanjemoy Formation, are found in basal Calvert Fm. lags of different ages only a mile or two apart. Well preserved dinocysts sourced from upper Oligocene formations have their own patterns of occurrence.