Reports
Stratigraphy, sedimentology, and Cambro-Ordovician paleogeography of the Frederick Valley, Maryland
1974, Reinhardt, J.
Report of Investigations 23
Abstract
Recent mapping in the Frederick Valley of central Maryland has clarified the stratigraphic relationships among lithologies along the western margin of the Piedmont province. A conformable sequence from a Lower and Middle Cambrian metasiltstone, previously mapped as Lower Cambrian Antietam Formation, through the Upper Cambrian Frederick Limestone and Lower Ordovician carbonates of the Grove Limestone is proposed. The Frederick Limestone has been subdivided into three mappable members. The total carbonate section, about 1350 meters (4555 feet) thick, is dominated by two distinct lithotypes: 1) a thinly bedded, laminated, dark gray limestone, composed primarily of silt and clay-sized carbonate, and 2) a thickly bedded to massive, planar or cross-bedded, light gray limestone, composed primarily of peloids and coarse quartz sand.
A shoaling upward pattern from Frederick to Gove Limestone is indicated by changing sediment types, sedimentary structures and the development of well defined sedimentary cycles. An increasing diversity and abundance of trace and body fossils plus the appearance of cryptalgal structures in the sediment sequence indicate a gradual transition from “deeper water carbonates” (>400 meters) to inter- and supratidal carbonate sedimentation.
The stratigraphic and petrologic framework in the Frederick Valley combined with regional stratigraphic relationships indicates a time transgressive clastic-carbonate boundary becoming younger from west to east during the Cambrian. The transition from deep to shoaling water sedimentation within the carbonate sequence in the Frederick Valley indicates an eastward progradation of the Cambro-Ordovician carbonate platform. The conformable relationships and the shoaling upward pattern necessitates a low angle slope rather than an abrupt platform to basin transition for the edge of the carbonate bank during late Cambrian and early Ordovician time.
