SAND INJECTITES IN THE CASTLE HAYNE AQUIFER OF EASTERN NORTH CAROLINA: IMPLICATIONS FOR FLUID MIGRATION IN THE CENTRAL COASTAL PLAIN CAPACITY USE AREA

Features that resemble sand injectites occur in the Eocene Spring Garden Member of the Castle Hayne Formation, in a quarry near New Bern, North Carolina. In the lower Neuse River Basin, the Castle Hayne Aquifer includes the Castle Hayne Formation and the overlying Oligocene River Bend and Belgrade Formations. At the quarry, both the Comfort and overlying Spring Garden Members of the Castle Hayne Formation are exposed in pit walls, but the Oligocene section is thin to absent. The Comfort Member consists of cemented, bioclastic, burrowed, massive to rubbly, packstones and grainstones. The Spring Garden Member is a cemented caprock with secondary moldic dissolution porosity. It consists of clast to matrix supported bivalves in a mixed siliciclastic/bioclastic matrix. The top of the Spring Garden is marked by a bench or hardground, with black phosphate and an oyster pavement. Above the Spring Garden is a thin, discontinuous, sliver of Yorktown Formation. This is overlain by Quaternary flaser-bedded sand and massive mud. Here the Castle Hayne Aquifer is connected to the surficial, unconfined aquifer.

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.