COASTAL PLAIN FAULTS WITHIN WASHINGTON EAST 1:100,000 QUADRANGLE

New USGS core and auger-hole data were used as a guide to our detailed compilation of borehole, paleontological, and geophysical data within the Washington East Geologic 1:100,000 scale quadrangle, revealing the complex, faulted stratigraphic framework of the Coastal Plain. We recognize new fault zones that best explain areas where strata greatly steepen in dip or show dramatic thickening, thinning, or truncation, especially where these changes are vertically stacked. The faults strike NE but trend more easterly with steeper southward dips across the northern part of the quadrangle. Beneath the Eastern Shore of Maryland, the steeper dip includes northward and westward truncation of some Paleocene through Miocene units. A few faults strike NW; these result in abrupt truncations (not pinch-outs) of the southern limits of several Upper Cretaceous units across the southern part of the quadrangle, creating a Pliocene or Pleistocene fault-bounded valley on the Eastern Shore of Maryland. Both NW and NE striking faults have influenced Cretaceous through Neogene depocenters and buried paleo- and modern-drainage patterns (geomorphic expressions that locally correlate with straight segments and large meanders of coastal-plain rivers).

Proposed fault zones include (youngest interpreted movement in parenthesis): beneath the Anacostia River (early Cretaceous); near and just east of the Branch Avenue Metro Station (Miocene); beneath the lower Patuxent River (middle Eocene); from Chesapeake Beach to south Kent Island (early Miocene); from Moran Landing to north Easton (middle Miocene); and the NW striking Cordova paleovalley-Swan Farm fault zone (Pliocene or Pleistocene).

Coastal Plain fault zones in this region are rooted in pre-Cretaceous basement fault systems, as they overlie and align with Triassic basin border faults. The Brandywine fault zone (middle Miocene) includes both reverse and normal faults and coincides with the buried western boundary of the Taylorsville Triassic basin. Decreasing offsets in younger strata (top of basement up to 80 m, top of Paleocene up to 30 m, Miocene 5-12 m, Pliocene and Pleistocene 1-7 m) document recurrent fault movement and indicate prolonged Cretaceous and Cenozoic reactivation of Paleozoic and early Mesozoic fault systems in the North American mid-Atlantic Coastal Plain.