PETROGRAPHIC ANALYSIS OF SEDIMENTARY INTERBEDS OF THE CATOCTIN FORMATION IN CHARLOTTESVILLE, VIRGINIA

Within the Neoproterozoic Catoctin Formation metabasalts in the central Piedmont region, near Charlottesville, VA, are interbeds of clastic sedimentary rock. The samples are from the east side of the Rivanna River and are stratigraphically located between the upper Lynchburg Group to the west and the overlying Catoctin Formation metabasalts of the eastern limb of the Blue Ridge anticlinorium. Previous investigators have described these interbeds as sandstones (Bloomer et al. 1947; Reed et al. 1955; Dilliard et al. 1999; Bailey et al. 2017), but little detailed petrographic or textural investigation has been reported.

Preliminary examination of the seven samples reveals two groups of clastic rocks. The stratigraphically lower group (4 samples) are mostly tan, beige and pink in color, clast or grain supported and finely bedded at the base but more massive toward the upper contact. The minerals (quartz, alkali and plagioclase feldspar, opaque minerals, tourmaline, zircon) are angular to subrounded and fine to medium grained. Some of the feldspars exhibit exsolution, as well as argillite (clay mineral) alteration. Quartz + feldspar rock fragments (0.5 to 0.75 mm) are common. Hematite staining may be related to weathering of the opaque minerals. Based upon the mineral composition and grain size, these rocks are an arkosic sandstone.

The other group, which is stratigraphically above the arkosic sandstone, appears to be the contact zone with the overlying Catoctin Formation metabasalts. This rock ranges in color from dark green and maroon with angular to subangular clasts that are largely matrix supported. These samples are primarily composed of metabasalt rock fragments (chlorite, epidote, plagioclase feldspar) and matrix material (opaque minerals, feldspar, little quartz). These metabasalt rock fragments (0.7 to 20 mm) are much coarser grained than the arkosic sandstone. Some feldspars show exsolution as well as sericite (white mica) alteration. Most of the mineral grains are subangular and the rock may be a breccia, possibly related to either fracturing or faulting found in the area. Much of the quartz is secondary in the form of a fracture or vein fill. These rocks have more opaque minerals, metabasalt rock fragments and less quartz, suggesting that their provenance is different from the arkosic sandstone.