BASEMENT TERRANES OF THE CHESAPEAKE BAY REGION – EVIDENCE FROM BOREHOLE SAMPLES AND GEOPHYSICAL SURVEYS

Studies of borehole samples and potential field data from the Chesapeake Bay impact structure (CBIS) and surrounding areas provide new insights into the geology of coastal-plain basement from the Chesapeake Bay suture zone (CSZ) east to the Delmarva Peninsula. The CSZ, including the Sussex terrane, coincides with the Sussex-Leonardtown (Salisbury) anomaly (gravity high, magnetic high) and separates NW-trending anomalies to the east from NE-trending anomalies typical of Appalachian Piedmont terranes to the west. However, the lithostratigraphy, age, and kinematics of the CSZ are poorly constrained. Recent USGS drill cores from the York-James and Middle Peninsulas west of Chesapeake Bay recovered Neoproterozoic granites (in situ) and lower greenschist-facies metavolcanic rocks (reworked CBIS ejecta) similar to those of exposed peri-Gondwanan terranes. In contrast, cores from the southern Delmarva Peninsula (Eyreville site) recovered a lower section of multiply-deformed, middle to upper amphibolite-facies graphitic schists with calc-silicate gneiss, mylonite, and pegmatitic granite. Whether or not these rocks represent a northern extension of the Hatteras terrane, contrasting metamorphic grades suggest that the CBIS straddles a crustal boundary between lower greenschist-facies rocks on the west and amphibolite-facies rocks on the east. The Eyreville cores also recovered a slab of Neoproterozoic gneissic granite intruded by Permian (late Alleghanian) granite similar to that of the Portsmouth pluton (also Permian), a block of orthoamphibolite, and clasts that include greenschist-facies metamorphic rocks, undated granitoids, and subgreenschist-facies gray and black shales. The latter are too thermally mature to be derived from coastal-plain sediments or Mesozoic rift basins and are interpreted to represent a previously unknown sequence of uncertain (Paleozoic?) age and tectonic affinity. Crystalline basement is cut by NE- and N-trending extensional faults related to Triassic/Early Jurassic rift basin formation; the northeast end of the Taylorsville basin is bounded by the CSZ, suggesting tectonic inheritance. Cretaceous/Cenozoic passive-margin tectonism deformed the basement and overprinted the rift basins, producing the Salisbury embayment and the James River structural zone.