Stop 1: CGS 2011

Pekin Formation - Boren Clay Pit

Boren Clay Products Pekin Formation near Gulf, NC

Disclaimer: The following field stop was originally published as part of a Field Trip Guide for the 50th Annual meeting of the Southeastern Section of the Geological Society of America, April 2001, p. 27-50. It has been included in this guidebook with minor editorial changes to the text. In addition, figures were replaced with new photographs.

The Boren Clay Products pits are located about 1.5 miles east of the western border of the Sanford basin on both sides of US 421. Written permission must be gained from Boren Clay Products before entering the property. The pits expose strata of the middle Pekin Formation (Fig. A) that are being mined to produce bricks and drainpipes (Gore, 1986). The Boren operations consist of several old pits northeast of US 421, as well as the old Pomona Pipe Works on the southwestern side of US 421. At present, quarrying is concentrated on the southwestern side of US 421.

Boren Pit - Pekin Formation

Figure A. Reddish-brown siltsone and sandstone of the Pekin Formation exposed in the inactive Boren Clay Products pit.
Photograph by Jeffrey C. Reid

The rocks in the Boren pits are dominantly reddish- brown siltstone and sandstone. Tan to white, arkosic channel sands and purple mudstones are also present in lesser amounts. Plant fragments are present in some of the finer-grained units. Most units are overprinted by Scoyenia bioturbation, including large backfilled burrows, probably attributable to a decapod such as a crayfish (Gore, 1986). Vertebrate tracks are also present. Invertebrate fossils are scarce, but present locally, including conchostracans or clam shrimp and small freshwater bivalves.

Thin diabase dikes are present in the pits on both sides of US 421. These dikes have thermally metamorphosed the sediments, accentuating the bioturbation. Near the surface, the diabase weathers to a yellowish orange color, contrasting with the surrounding grayish red and reddish-brown strata. Drag folding, faulting,and intense fracturing are common near the dike.

Field trips led by Gore (1986) and Olsen and others (1989) visited the quarry on the northeastern side of US 421, which was active at the time, but which is now abandoned. This pit is one of the premier sites for Triassic plant fossils in the eastern US. The plant fossils are found in gray siltstone and shale units and yellow tan siltstones, which are not exposed in the new pits on the southwestern side of US 421. The old pits contain abundant stems, roots, cones, and leaves of a variety of seed and non-seed plants (Fig. B).

Boren plant fossils

Figure B. Examples of plant fossils from the Boren Clay pits. Car key for scale.

Gensel (1986) provided a thorough description of these fossil plants, which include ferns, horsetail rushes, cycads, cycadeoids, and conifers. One of the most unusual plant fossil finds is the only known intact specimen of Leptocycas gracilis, one of the oldest known cycads, a gymnosperm sometimes called the sago palm (News release, NC State University, 2000). The plant fossils suggest a tropical to subtropical climate (Gensel, 1986). Fern spores and conifer pollen are present in the gray shales and siltstones. These palynomorphs were interpreted by Traverse (1986) as Julian (middle Carnian) in age.

The Pomona Pipe quarry on the southwestern side, of US 421 (now filled with water) has yielded vertebrate fossils from reddish-brown clayshales. The most abundant vertebrate is a crocodile-like phytosaur, Rutiodon, known from teeth and bones. Also present are: Typothorax, a 2.5 meter-long armored pseudosuchian; teeth of a large carnivorous theropod dinosaur; and several specimens of Placerias, a herbivorous, dicynodont, mammal-like reptile (Baird and Patterson, 1967; Patterson, 1969). Fish scales and bones also occur (Olsen and others, 1989).

The Pomona Pipe quarry has also yielded the oldest vertebrate track assemblage in the Late Triassic of eastern North America (Olsen and Huber, 1997). Tracks include both three- and five- toed forms, ranging in size from 10 to 30 cm (Olsen and Huber, 1997). The tracks are apparently dinosaurian, making them among the oldest known dinosaurian tracks in the world (Olsen and Huber, 1997). The vertebrate assemblage indicates an early Tuvalian (early Late Carnian) age (Huber and others, 1993).

Link to: The oldest Late Triassic footprint assemblage from North America (Pekin Formation, Deep River Basin, North Carolina, USA) online paper.

Link to References