CHARACTER OF THE REGOLITH - BEDROCK TRANSITION AT A PIEDMONT SITE UNDERLAIN BY QUARTZ DIORITE, IREDELL COUNTY, NORTH CAROLINA

Seven continuous cores have been collected from a ground-water research station at the Davidson College Lake Campus on Langtree Peninsula in southwestern Iredell County. These cores provide subsurface information for site characterization purposes and allow for the proper placement of casing and screen for permanent observation wells. All of the cores extend through overlying regolith into quartz diorite and other igneous and metaigneous rocks assigned to the Charlotte belt. An examination of the cores reveals that regolith at the site ranges in thickness from 10 to 73 feet. Textural and compositional changes with depth in regolith are gradational and are similar in each core. The transition from basal saprolite to fresh rock is quite variable. In some cores, basal saprolite is in direct contact with relatively fresh quartz diorite. In other cores, up to 15 feet of highly weathered quartz diorite are present above fresh quartz diorite. There appears to be a positive relationship between the intensity of fracturing in the uppermost bedrock and the thickness of the transition zone; the orientation of the fractures and the type and degree of fracture mineralization also appear to play a role. Discrete weathered rock zones ranging in thickness from 0.5 to 1.5 feet also can be observed in most of the cores. Dissolution of amphibole, feldspar and other minerals has increased porosity and permeability in these zones; the relative importance of these zones is unknown. Fracture logging of rock cores and downhole geophysical logging of bedrock wells indicate that the density of fractures beneath the site generally decreases with depth. The variability and discontinuity of a transition zone defined by weathered bedrock and the decrease in fracturing with depth are consistent with observations made during drilling at nearby contaminated sites that are underlain by quartz diorite and with the distribution of contaminants at those sites.