COMPARISON OF MAGNETIC SIGNATURES OF PRECAMBRIAN CATOCTIN METABASALT IN FREDERICK COUNTY, MARYLAND WITH MESOZOIC BASALT FLOWS OF THE FAUQUIER AND CULPEPER COUNTIES, VIRGINIA

Although highly metamorphosed, the Precambrian rift-related Catoctin metabasalt has a magnetic signature similar to that of unmetamorphosed Jurassic basalt flows. Data from the Catoctin Formation and related sedimentary units were collected through three traverses on South Mountain in Frederick County, Maryland by means of a proton precession magnetometer. These magnetic values were then compared to published data from the Culpeper Basin of northern Virginia, where Jurassic basalt flows and interlayered sedimentary rocks were analyzed using a fluxgate magnetometer (Awosika 1980). Units on South Mountain were identified through outcrop analysis and a geologic map of the western Blue Ridge compiled by David Brezinski. An average value of approximately 53150 gammas was calculated as being characteristic of the Catoctin metabasalt. The sedimentary units surveyed included arkosic sandstones of the Swift Run Formation and quartzites from the Harpers Formation, which both averaged approximately 52500 gammas. Thus, there is a 650 gamma difference between the magnetic signatures of the metabasalt and sediment. In the Culpeper Basin, three different Jurassic basalt flows were found to have an average value of approximately 700 gammas, while the interlayered sediment was only slightly magnetic, averaging approximately -180 gammas. The difference in magnetic signatures between the Mesozoic basalt and related sediment was 880 gammas. The differences in magnetic values of the basalts and sediments of both studies only vary by approximately 230 gammas. This suggests that the Paleozoic regional metamorphism did not significantly reset the original magnetic signature of these rocks.