Reports
New data bearing on the structural significance of the upper Chesapeake Bay magnetic anomaly
1979, Edwards, J. and Hansen, H.J.
Report of Investigations 30
Abstract
Three test borings have been drilled to basement in the upper Chesapeake Bay area of Maryland to clarify the structural significance of a deep, flat, low magnetic anomaly bordered on the southeast by a steep, northeast-striking linear magnetic gradient. This magnetic pattern has previously been modeled as an abnormally thick Coastal Plain half graben bordered on the southeast by a normal or high-angle reverse fault.
Boring HAR-Dg3 was drilled in Harford County on Spesutie Island, west of the linear magnetic feature but within the low magnetic anomaly. Borings CE-Dc2 at Turkey Point and CE-Ec17 on Grove Neck, both in Cecil County, were drilled east of the magnetic gradient in an area dominated by a series of closed high and low magnetic anomalies.
Boring HAR-Dg3 encountered saprolite at an elevation of about -700 feet, which conforms reasonably well to the regional dip of the basement surface as projected from the Fall Zone and does not indicate an unusually deep basin. Boring CE-Ec17, located on the alleged up-thrown block of the proposed fault, penetrated basement at an elevation of about -1,020 feet, again conformable to the regional dip. The average slope of the basement surface between Spesutie Island and Grove Neck is approximately 88 feet per mile, a figure that is incompatible with major vertical offset of the basement surface between the two sites. Boring CE-Dc2 encountered basement about 50 feet higher than estimated from projection of the regional dip. However the unusual thickness of saprolite at this site, about 118 feet, suggests the occurrence of a buried hill on the basement surface.
Data obtained from the Cretaceous Coastal Plain strata indicate that faulting penecontemporaneous with sedimentation seems unlikely insofar as the direction of depositional transport characteristic of palynostratigraphic Zone I (Patuxent Formation) and Subzone IIB (Patapsco Formation) is roughly normal to the trend of the magnetic gradient. Broadly similar lithologic packages occur on both sides of the alleged fault trend without abrupt thinning or lithofacies change. The apparent absence of Zone III (Elk Neck Beds) of the Patapsco Formation west of the anomaly further diminishes any likelihood of a down-faulted Coastal Plain basin at the Spesutie Island test site.
The basal 17 feet of core recovered at Spesutie Island is coarse-grained felsic gneiss similar to gneisses such as the Port Deposit Gneiss in the adjacent exposed Piedmont. This is overlain by chloritic greenstone and greenstone saprolite which resembles the Gilpins Falls member of the James Run Formation in Cecil County. There is a reasonable possibility that the large, flat low magnetic anomaly beneath the Upper Chesapeake Bay is the magnetic signature of the felsic gneiss cored at Spesutie Island, and that the small-amplitude irregularities associated with the western flank of the low anomaly are caused by infolds of the James Run Formation within the gneiss terrane.
Both of the Cecil County test borings cored schistose rocks in the area east of the linear magnetic feature. These rocks are correlated with the pelitic gneiss facies of the Wissahickon Formation. Ultramafic and mafic rocks may be associated with the high magnetic anomalies elsewhere in this terrane but none were encountered in these two test borings.
Evidence of a post-metamorphic shear zone within the basement was found at Grove Neck where the schist in the core is extensively mylonitized. Mild cataclastic effects are also present in the Spesutie Island core. However, there is no clear-cut relationship of the rocks at these two sites with the linear magnetic gradient beneath the Upper Chesapeake Bay. Fresh bedrock beneath the thick saprolite mantle at Turkey Point showed no evidence of cataclassis, although of the three test borings this was nearest to the alleged fault. It is concluded that the magnetic anomaly pattern in the upper Chesapeake Bay area largely reflects magnetic susceptibility contrast within the basement complex. Faults undoubtedly exist within the buried basement complex, but there is no evidence of any significant (greater than 50 feet) displacement of the basement surface in this area since the onset of sedimentation in Early Cretaceous time.
