THE DINOCYSTS OF THE MORAN LANDING CORE, CALVERT CLIFFS, MARYLAND: NEW EVIDENCE FOR A HIGH-ANGLE REVERSE FAULT AFFECTING MIOCENE STRATA

The western shore of the Chesapeake Bay in Calvert County, Maryland, is a ~40 km-long series of 15-30 m-high cliffs that provide exceptional exposures of Miocene shallow-marine deposits. Strata are gently dipping, generally to the south to southeast at 1-3 m/km, and gaps between cliffs are typically drainages. Dip reversals and anomalous elevations of key contacts on either side of one such gap located at Moran landing, between the cliffs at Camp Conoy and those of Rocky Point, have been known since the 1970s and well documented since the 1990s. However, the nature of these structural complications has remained ambiguous. We postulated the existence of a fault or fault zone at this site, where strata are displaced upward on the south side by several meters. To test this, we examined outcrop material from both sides of the gap and samples from a core acquired in 2015. USGS drillers Gene Cobbs III, Jeff Gray, and Keith Moody achieved excellent recovery from a 20-m long core taken along the south edge of the gap using a hollow-stem auger rig.

Both outcrops and core yielded abundant and diverse dinoflagellate cyst (dinocyst) assemblages. On the north side of the gap, the outcrop exposes Shattuck Zone 19 from the upper half of the Choptank Formation (dinocyst zones DN6, DN7) at water level and the base of the St. Marys Formation at an altitude of 8 m. In contrast, the upper part of the core yields Choptank strata but only from its lower part (DN6), underlain by a thick section of the upper part of the underlying Calvert Formation (DN5). The dinocyst DN5-6 zone boundary (and the Calvert-Choptank boundary) in the core occurs at 3 m below sea level, in contrast to 7 m or deeper that would be projected on the basis of the full thickness of the Choptank Formation exposed nearby.

Dinocyst correlations have revealed abrupt truncation, thinning, and thickening of various Cenozoic units over short distances in previous USGS subsurface investigations north of Moran landing, including both drill cores (Tilghman Elementary, Knapps Narrows) and auger holes (Wye Mills; Washington East 1:100,000 quadrangle). Such observations support the interpretation that the folds and anomalous elevations at Moran landing signify a high-angle reverse fault located along that drainage, most likely related to reactivation of faults along a deeply buried Mesozoic basin boundary.