An unconformity in the Carolina slate belt of central North Carolina: new evidence for the areal extent of the ca. 600 Ma Virgilina deformation

Abstract

Detailed mapping in the Ramseur, N.C. 7 1/2' quadrangle has shown that lithostratigraphic units of the Virgilina sequence, units 11 and 111 (Glover and Sinha, 1973) in the Roxboro-Durham, N. C. area can be extended into central North Carolina. The volcanic stratigraphy is composed of the Hyco, Aaron and Uwharrie Formations, all of which have been subjected to greenschist facies metamorphism. The oldest map unit, the Hyco Formation, consists of intermediate(?) lava flows, pyroclastic and volcaniclastic rocks. Deposition of these units was in a subaqueous environment although some units are indicative of transient subaerial conditions. The Aaron Formation is a volcanic epiclastic sequence composed of conglomerate, pebbly and feldspathic arenite with intercalated siltstone, argillite and vitric tuff. The arrangement of sedimentation packages in the Aaron Formation are analogous to those of a coarse grained retrogradational submarine fan sequence. In the western part of the map area the Uwharrie Formation unconformably overlies the Hyco Formation. The Uwharrie consists of a bimodal (felsic-mafic) sequence of lava flows, pyroclastic and volcaniclastic rocks, all of which were deposited in a subaqueous environment. Structural data indicates that the older units of the Hyco and Aaron Formations were folded (F₁) and faulted during the Virgilina deformation (D₁). The entire volcanic sequence of Hyco, Aaron and Uwharrie Formations was subsequently folded (F₂) and metamorphosed during the Taconic deformation (D₂). Associated with the Taconic event is the development of a pervasive spaced anastomosing cleavage (S₂) in the volcanic lithologies. Previous regional correlations preferred by Wright and Seiders (1980) are thought to be incorrect. It is proposed in this paper, as first suggested by Glover (1974), that an angular unconformity separates the older volcanic strata of the Virgilina sequence from the younger units of the central N.C. sequence. The presence of an unconformity is indicated by 1) the truncation of lithologies comprising the Hyco Formation at the contact between the Hyco and Uwharrie Formations, 2) the deviation of macroscopic fold trends from the Hyco and Aaron Formations to the adjacent Uwharrie Formation and 3) the intrusion of felsic dikes equivalent to those comprising the Uwharrie Formation, which crosscut the older units and structures of the Virgilina sequence. The Virgilina deformation is probably correlative in time with the Late Precambrian Monian, Cadomian and Pan-African orogenies which effect similar age volcanic terranes like the older Carolina slate belt. In this study it is proposed that the name Virgilina deformation should have precedence over the term Avalonian event, because of the relation of the former 'to compressional tectonics versus the extensional tectonism prevalent in the latter. The Virgilina deformation may be attributed to active plate margin tectonics associated with a former volcanic arc.