CRYSTALLINE ROCKS FROM THE FIRST COREHOLE TO BASEMENT IN THE CHESAPEAKE BAY IMPACT STRUCTURE, HAMPTON, VIRGINIA

Crystalline rocks from the first corehole to basement in the buried Eocene Chesapeake Bay impact structure provide insight into the types of rocks concealed beneath the Atlantic Coastal Plain. Furthermore, the core reveals how these rocks were affected by the impact event. The corehole at NASA’s Langley Research Center, Hampton, Va., lies about 7 km inside the outer rim of the 90-km-wide structure. Metagranite below 626.3 m is overlain by weakly to strongly impact-disturbed Cretaceous sediments followed by a 31.6-m-thick polymict diamicton (wash-back deposit), which is blanketed by 236.0 m of post-impact sediments. The metagranite at Langley is pale red, medium-grained, homogeneous, and peraluminous. It has no penetrative foliation, and lower greenschist-facies metamorphism is pervasive. Tabular masses of chlorite, the principal mafic mineral, suggest replacement of biotite. No shocked minerals or other features clearly attributable to the impact were seen. A SHRIMP U-Pb zircon age of 614±9 Ma (2 sigma) indicates Neoproterozoic crystallization. Peraluminous granite of this age is unusual in the exposed Piedmont but is present in the Hatteras terrane beneath the Coastal Plain to the south, suggesting a tectonic affinity. Zircon and apatite fission-track cooling ages of 375±44 Ma and 184±32 Ma (2 sigma) from the metagranite indicate no discernible impact-related thermal disturbance at this location near the outer rim. Most veins, fractures, and faults in the metagranite contain lower-greenschist-facies minerals and are inferred to predate the impact. A sharp unconformable contact separates the weathered metagranite from overlying Cretaceous sediments, and detrital pebbles of the metagranite are abundant at the base of the sedimentary section. Normal faults occur in these sediments, but the core shows no evidence for a decollement zone just above the metagranite. Most crystalline clasts in and just below the polymict diamicton are pre-impact detrital clasts. However, a few have cataclastic textures, and some of these have shock lamellae in quartz. All rock fragments known to contain shocked quartz, and thus interpreted as impact ejecta, consist of porphyritic felsite (peraluminous rhyolite). Similar felsite clasts having microspherulitic matrix without shocked minerals suggest a possible impact melt.