GEOLOGY OF THE ONSLOW QUARRY, RICHLANDS, NORTH CAROLINA

The Eocene age Castle Hayne Limestone Formation located on the coastal plain of North Carolina is found throughout Onslow County and is mined at a quarry south of Richlands. This study characterizes the geology of the rock units present at the quarry.

The site has a variable thickness of sandy clay overburden ranging from less than five feet to almost sixty feet. Limestone immediately underlies the overburden. It varies in thickness from approximately two feet to greater than one hundred feet. In areas where the limestone is thin, usually less than 10 feet, there is a marl layer. The marl layer thickens from east to west and is at its greatest thickness in the north and northwest where it is over fifty feet thick. The marl is rich in siliciclastics and fossils.

The limestone is divided into several distinct layers and in some areas is interrupted by very loosely consolidated carbonate sand. Some of the limestone layers consist of porous grainstone and packstone while others are dense wackestone. There are structural features evident in the limestone layers which include cross-beds, lamination and graded bedding. The limestone layers contain a variety of fossils including bryozoans, echinoderms, gastropods, foraminifera and a few coral fragments. Below approximately 100 feet the limestone is replaced by either sand or marl signifying the likely base of the Castle Hayne Formation.

The limestone layers correspond to the inner and mid-ramp zones of a homoclinal carbonate ramp model. The specific sequences include the inner ramp shoal, inner ramp restricted/open marine and inner ramp open marine facies. The carbonate sand layers are indicative of a carbonate sand shoal/bank. The various layers have evidently shifted between these facies. The mechanism may be either changes in sea level or migration of near shore sediments.

Diagenetic features evident in the rocks include dissolution of mollusc shells, replacement of original shell material, neomorphism of micrite with microspar and spar, syntaxial overgrowths of echinoderms, filling of molds and voids with micrite, microspar and spar, micritic rims, authigenic formation of pyrite and quartz and breakage and layering of bioclasts. The diagenetic features change with depth; there is more dissolution of micritic matrix with depth and more growth of authigenic minerals.