Reports
The lithology and distribution of pre-Cretaceous basement rocks beneath the Maryland Coastal Plain
1986, Hansen, H.J. and Edwards, J.
Report of Investigations 44
Abstract
The Maryland Coastal Plain is underlain by a basement complex consisting of Precambrian to Paleozoic crystalline rocks and Mesozoic rift-basin sedimentary and igneous rocks. This report contains a compilation of cuttings and core descriptions from significant Coastal Plain wells that were drilled into the basement complex. These data help in the interpretation of regional magnetic and gravity maps and provide a basis for preliminary speculations concerning the distribution of lithotectonic terranes in the buried basement complex.
Three distinct belts of crystalline rocks have been tentatively mapped in the buried basement complex of Maryland . These belts strike with increasing angularity to the Fall Line from northeast to southwest. The Inner Belt exhibits lithologic and geophysical patterns similar to the adjacent, outcropping Piedmont. It is dominated by rocks of the Wissahickon Group, apparently overlain in places by stacked thrust sheets of the Baltimore Mafic Complex and James Run Formation. The Middle Belt, which does not outcrop in Maryland, strikes southwestward where it appears to correlate with rocks associated with the Fredericksburg Complex and Petersburg Granite that outcrop along the eastern margin of the Virginia Piedmont. In Maryland the Middle Belt generally yields gneissic and granitoid basement rock samples, which is consistent with the generally featureless magnetic gradient associated with it. The Outer Belt in Maryland is marked by a series of en echelon gravity and magnetic anomalies that are apparently truncated by the Middle Belt. The occurrence of low-grade metamorphic rocks in the Outer Belt suggest that it may have been accreted to the continental margin subsequent to the Ordovician (Taconic) orogeny that tectonically transported and metamorphosed the exposed Piedmont. If so, the boundary between the Middle and Outer Belts may represent a major, Late Paleozoic suture zone.
Buried Mesozoic rift-basin sedimentary rocks have been penetrated by wells drilled in the Middle Belt at a few localities in Charles, Prince George's and Anne Arundel Counties. Along strike in Virginia similar occurrences have been correlated with the outcropping rocks of the Late Triassic Taylorsville basin. Consolidated "red beds" have also been encountered in several deep wells drilled on the Delmarva Peninsula. It is suspected, however, that these rocks may be younger than the Mesozoic rift-basin sediments, occurring rather at the base of the Coastal Plain wedge above the break-up unconformity.
In Maryland structural contours drawn on the pre-Cretaceous basement rock surface show a pronounced strike change near the boundary between the Middle and Outer lithotectonic Belts . This change in strike from northeast, north of the boundary, to north-south, south of the boundary, defines the south flank of the Salisbury Embayment and suggests that it may be controlled by reoccurring movements along a Paleozoic suture zone. A reflection seismic survey in St. Mary's County recorded a 250-foot, up-to-the-south offset of the top-of-basement surface near the suspected suture zone.
