THE NATURE OF THE HYCO AND AARON FORMATIONS CONTACT WITHIN THE CAROLINA TERRANE IN CHATHAM COUNTY, NORTH CAROLINA – INSIGHTS FROM DETAILED GEOLOGIC MAPPING

The North Carolina Geological Survey is currently mapping portions of the Bear Creek and Coleridge 1:24K Quadrangles located in Chatham County with partial STATEMAP funding. These quadrangles lie within what is traditionally accepted as the Carolina terrane, a large component of the Neoproterozoic to Cambrian superterrane known as Carolinia.

Lithologies within this portion of the Carolina terrane are part of two lithotectonic assemblages, the older Hyco and younger Aaron Formations, that consists of various metamorphosed intrusive, extrusive, and volcanogenic sedimentary rocks. The contact between the Hyco and Aaron Formations is characterized by a 37 Ma disconformity and can be difficult identifying due to similar sedimentary lithologies.

Recent mapping has identified a marker unit in the lowermost Aaron Formation that has aided in the distinction between Hyco and Aaron Formations. The marker unit varies from conglomerate that is composed of abundant rounded to sub-rounded quartz and volcanic clasts that range from 2 cm – 15 cm to conglomerate and conglomeratic sandstone that contains rip-up clasts and sparse quartz.

Complications that arise in deciphering this contact are the presence of similar lithologies within both the Hyco and Aaron Formations. Both formations contain bedded mudstone, siltstone, and volcanic rocks that appear to have very similar mineralogical assemblage but differ in age. Furthermore, two deformational events are interpreted to have affected this portion of the Carolina terrane. The two foliation-forming events have similar orientations, which make the identification between the two difficult at the outcrop scale.

Recent mapping in surrounding areas suggests a presence of folds that have not been recognized by previous mappers. Newly identified map patterns seem to reveal a macroscale anticline and syncline with associated smaller scale parasitic folds that complicate the contact and span from the Bear Creek 1:24K Quadrangle northwestward into the Coleridge 1:24K Quadrangle. To better understand the contact between the Hyco and Aaron Formations and to validate interpretive map patterns, Matlab-modeling of fold interference patterns is accompanied with structural analysis to present several interpretations for problematic areas within this portion of the Carolina terrane.