Phosphorite deposits in Beaufort County North Carolina were first formally described by Brown in 1958. He designated the phosphorite unit as being middle Miocene in age on the basis of foraminifera from the upper part of the unit that are correlative with foraminifera from the middle Miocene Calvert Formation of Maryland. Kimrey (1964) proposed the name, Pungo River Formation, for the phosphorite unit. The name is derived from the Pungo River near Belhaven, North Carolina. The formation is not known to outcrop, so a core hole drilled near Belhaven in northeastern Beaufort County was used to designate the type section.

The Pungo River Formation is primarily composed of inter-bedded phosphatic sands, and clays, limestones and dolostones. Riggs (1983) noted that individual lithologic horizons in the formation can be traced laterally and correlated based on grain size and percent P2O5 content.

The Pungo River Formation unconformably lies above the Castle Hayne Limestone of Eocene age and is unconformably overlain by the Pliocene Yorktown Formation. The contact with the overlying Yorktown Formation as observed in cores, is sometimes gradational due to the reworked phosphatic material at the base of the Yorktown Formation.

The thickness of the formation ranges from a thin edge in the western part of Beaufort County to more than 120 feet in eastern Beaufort County. The top of the formation dips generally to the east at a rate of about 10 feet per mile.

The Pungo River Formation contains inter-bedded phosphatic sands, phosphatic/calcareous/diatomaceous clays, dolosilts and indurated limestones and dolostones.

The phosphatic sands of the formation are comprised of fine to medium-grained phosphate (francolite mineral species of the apatite family) and quartz with varying percentages of silt and clay sized material along with phosphatized fossil fragments. The phosphatic sand grains are typically smooth, glossy, black to brown in color, and spheroidal to ovate in shape. Surfaces of individual grains commonly show concentric rings or bandings. Pebble-sized phosphate grains (+10 mesh) normally comprise less than 5 percent volume of the phosphatic sand matrix. The quartz occurs typically as clear, flat sided, angular to subrounded grains. Accessory minerals in the phosphatic sands include calcite, garnet and ilmenite. The more clayey phosphatic sands often contain weathered shell material.

The clays are principally phosphatic and diatomaceous clays; there are some calcareous clays in the formation. The clays (-200 mesh) comprise about 20 percent volume of the phosphatic sand matrix. The phosphatic clays are often high in dolomite content and are olive drab green in color. Often, after drying, they appear fissile. The diatomaceous clays are light gray-green in color, they are composed of up to 90 percent diatom shells and fragments in a silt to clay sized groundmass. The calcareous clays are usually light green to light gray to white in color.

The primary Pungo River formational sequence consists of dolosit, coquinoid limestone, phosphatic sandy clay, dolstone and phosphatic doloclaystone.

The capping unit in the formation locally overlies the indurated limestone is discontinuous unit of bryozoan dolosilt hash known as the Chartreuse Bed named so for it’s greenish yellow color. Occasionally diatomaceous clays are inter-bedded with the coquinas and calcareous clays. The principle phosphatic ore matrix of sandy clay lies above dolmitic sandstone and phosphatic doloclaystone at the base of the formation.

Indurated zones in the formation include several rock types: phosphatic limestones, silty claystones, coquinas and dolostone. The phosphatic limestones are usually white to dark gray in color. Typically, they are highly inter-bedded dense vuggy limestones that contain varying amounts of marly phosphatic clay, quartz sand and pebble phosphate. They contain casts and molds of shell material. Locally, this rock is composed entirely of cast and moldic limestone. The dolomitic sandstones at the base of the phosphatic ore matrix that are well indurated are very competent rocks. These indurated rocks grade both laterally and vertically into poorly indurated rocks so that competent zones usually have a limited lateral extent.

The silty claystones, derived from cementation of the phosphatic clays, are usually light to dark gray in color. Usually, their occurrence is localized and they grade both laterally and vertically in the phosphorite. Typically, they are friable, but locally are well cemented and competent. The occurrence of coquina limestones in the section is confined to Beaufort County on the south side of the Pamlico River. The coquinas, creamy white to light gray in color, are composed of shell fragments, whole shells, and re-crystallized calcite; very locally, they contain significant amounts of pebble and cobble sized phosphate. They vary in degree of induration from very poorly cemented to well cemented, competent rocks. The coquinas are inter-bedded with calcareous clays that are white to light gray. The Pungo River Formation was deposited during a transgressive - regressive cycle of the sea into a northeast-south-west trending basin. Southeast of Beaufort County, in the deeper part of the basin, available drilling data indicates that the formation is comprised mainly of clay.

Content for this page provided by Tex Gilmore, Superintendent of Mine Planning and Chief Geologist, PotashCorp - Aurora, Aurora, N.C.