CONODONTS AND PALEOENVIRONMENTS OF THE PRUITT RANCH MEMBER, CRINER HILLS, SOUTHERN OKLAHOMA

The purpose of this undergraduate research project is to describe sedimentary rocks, paleoenvironments, and the conodont fauna from the Pruitt Ranch member of the Oil Creek Formation (Ordovician Age) located in the Criner Hills region of southern Oklahoma. The field and laboratory work in our project allowed us to apply classroom concepts to real world experiences.

The Pruitt Ranch is composed of 24 m of peritidal limestone. The Pruitt Ranch overlies the lower Oil Creek Formation and underlies the McLish Formation. During our field work, we visited the type section of the Pruitt Ranch and identified the lower contact just west of a low ridge (34¢ª 7’ 39.6” N, 97¢ª12’ 21.94”W). The base of the Pruitt Ranch is marked by a transition from fossiliferous, oolitic limestone to fine grained, fenestral or “birdseye” limestone. The upper contact with the McLish Formation is marked by a transition from fine-grained limestone to coarse, fossiliferous limestone. Pruitt Ranch beds strike at N54W and dip 45¢ª NE. Using a Jacob Staff, we measured the thickness of the Pruitt Ranch and collected 16 samples (1-2 kg) at 1.5 m intervals. Samples (8) of the lower Oil Creek and the McLish Formation were also collected. One-kg samples were crushed and placed in an acetic acid bath. Sample residues were rinsed and collected on a number 140 USA Standard Testing Sieve. Residues were further processed using heavy liquid separation.

Sample residues were examined microscopically and conodonts were picked and placed on micropaleontology slides. One-kg samples collected from the lower Oil Creek and McLish formations yielded large collections of several hundred conodont elements. As expected, the peritidal Pruitt Ranch samples yielded few conodonts (0-8 per kg). Pruitt Ranch conodonts included Neomultioistodus angulensis, Parapanderodus striatus, and Leptochirognathus quadratus. The difference in yield reflects the shift from subtidal to very shallow subtidal and intertidal environments.