Reports

Upper Cretaceous (Senonian) and Paleocene (Danian) pinchouts on the south flank of the Salisbury Embayment, Maryland, and their relationship to antecedent basement structures

1978, Hansen, H.J.

Report of Investigations 29

Abstract

Upper Cretaceous (Santonian to Maestrichtian) and Lower Tertiary (Danian) pinchouts on the southern flank of the Salisbury Embayment in Maryland coincide broadly with a change in aeromagnetic pattern believed to be associated with the contact between two distinct crustal blocks. On regional maps the central part of the Embayment is dominated by a “featureless” aeromagnetic belt called the Richmond-Brandywine trend. In Virginia, Maryland, and Delaware, this trend is characterized by a generally planar, down-dip decrease in total intensity, punctuated by broad-amplitude closures. It is bordered on the west and north by a complex, ropy pattern of steep-sided highs and lows which correlates with the schistose, ultramafic and gabbroic rocks of the outcropping eastern Piedmont. Southward the NE to NEE Richmond-Brandywine trend abuts against a N- to NNE-striking en echelon series of aeromagnetic highs and lows, dominated by the Sussex-Currioman Bay anomaly which extends from Sussex County, Virginia, into St. Mary’s County, Maryland.

In Virginia the Richmond-Brandywine aeromagnetic trend strikes oblique to the Fall Line where it is coincident with an extensive granite-gneiss pluton containing one large and several small, downfaulted Triassic basins. Beneath the Coastal Plain, the association of the Richmond-Brandywine trend with Triassic rocks has been confirmed by basement core data as far north as Prince George’s County, Maryland. Extension of the Richmond-Brandywine trend across the Delmarva Peninsula suggests that downfaulted Triassic rocks may occur there as well, although several vibratory seismic lines failed to record primary reflections from below top-of-basement.

It is proposed that general downwarping of the continental margin during late Jurassic, Cretaceous, and early Tertiary times was accentuated in the Salisbury Embayment and Baltimore Canyon Trough because those areas correspond broadly with Triassic rift basins functioning as antecedent zones of crustal weakness. The Upper Cretaceous and Lower Paleocene pinchouts described herein coincide broadly with the southern margin of the Salisbury Embayment (as defined by the Richmond-Brandywine aeromagnetic trend). The association of Upper Cretaceous and Lower Paleocene pinchouts with the boundary of the Richmond-Brandywine trend suggests that the Salisbury Embayment was rooted in a residually active Triassic rift system, sporadically readjusting to stress during the downwarping of the continental margin.

Faulting within the Richmond-Brandywine trend in Prince George’s County, Maryland, was first proposed by Jacobeen (1972) who mapped a high-angle, up-to-the-SE reverse fault system affecting strata as young as Lower Tertiary. Seventy miles of reflection seismic lines were run across the margins of the Richmond-Brandywine trend in Kent, Queen Anne’s, Talbot, Dorchester, and St. Mary’s Counties, Maryland to investigate the possibility of Coastal Plain structures associated with the edges of juxtaposed basement blocks.

A potentially significant fault was recorded on a seismic profile in central St. Mary’s County on the southern margin of the trend near its contact with the Sussex-Currioman Bay anomaly. The fault, herein called the Hillville Fault, is a high angle, up-to-the-south reverse fault with an estimated offset of about 250 feet. A marked change in the seismic signature from the top-of-basement occurs across the fault, suggesting that two distinct basement terranes are involved. Conversely, seismic profiles run in Talbot and Dorchester Counties on the Eastern Shore of Maryland failed to establish a comparable fault on the southern edge of the Richmond-Brandywine trend, even though Coastal Plain pinchouts along the same traverse are well documented These apparently conflicting data appear to eliminate the possibility of a single, transcurrent border fault but may be compatible with an en echelon series of up-to-the-south, scissor faults oblique to regional strike.

Faults, involving at least the lower part of the Coastal Plain, were mapped on seismic profiles near the northern margin of the Richmond-Brandywine trend in Kent and Queen Anne’s Counties. These were consistently up-to-the-north faults and had offsets of less than 100 feet, imparting a subdued stair-step configuration to the top-of-basement. Stage-level Upper Cretaceous and Lower Tertiary pinchouts are not associated with this faulting in Maryland.