SAND INJECTITES IN THE CASTLE HAYNE AQUIFER OF EASTERN NORTH CAROLINA: IMPLICATIONS FOR FLUID MIGRATION IN THE CENTRAL COASTAL PLAIN CAPACITY USE AREA
Removal of the overburden during the mining operation created pillars (4 m) of limestone capped by the Spring Garden Member. The pillars show three-dimensional configurations of the proposed sand injectites. The faces of the pillars are commonly coincident with irregular, sand-filled fractures (composition 70% quartz, 30% glauconite). Subhorizontal layers or sills of siliciclastic sand occur in the contact zone between the members. Above this, sand injectites form irregular bodies, sills, and dikes. Since the Spring Garden Member is middle Eocene, we speculate that a seismic event associated with the Chesapeake Bay Impact Structure in late Eocene time caused the injection.
Sand injectites are highly permeable, interconnected networks that form rapid vertical pathways for fluid migration. For the Castle Hayne Aquifer, injectites may enhance the internal permeability of the aquifer because they bypass occluded, cemented zones, and less permeable strata. Injectites also could enhance transport of polluted wastewater from the ground surface to deeper aquifers, in areas lacking shallow confining units. Deeper brines could move rapidly and easily upward along these features, contaminating shallow aquifers.