MISSISSIPPIAN (LATE OSAGEAN) AMMONOIDS FROM THE NEW PROVIDENCE SHALE MEMBER OF THE BORDEN FORMATION, NORTH-CENTRAL KENTUCKY

Journal of Paleontology, Nov 2004 by Work, David M, Mason, Charles E

ABSTRACT-

The Mississippian ammonoids Polaricyclus conkini new species, P. ballardensis (Gordon, 1965), Winchelloceras knappi new species, and Cantabricanites? greenei (S. A. Miller, 1892) occur with Gnathodus texanus Zone conodonts near the base of the New Providence Shale Member of the Borden Formation in north-central Kentucky. Both ammonoids and conodonts indicate an early late Osagean (lower Keokuk equivalent) age corresponding to the middle or, more probably, the late Chadian (latest Tournaisian or early Viséan) portion of the Fascipericyclus-Ammonellipsites Zone.

INTRODUCTION

THE NEW Providence Shale Member of the Borden Formation in north-central Kentucky and southern Indiana contains two distinct Osagean ammonoid assemblages belonging to two different f'aunal zones. Lower Osagean strata at the base of the New Providence Shale at Rockford in Jackson County, southern Indiana, contain the classic Rockford ammonoid assemblage characterized by Muensteroceras oweni (Hall, I860), M. paralleluin (Hall, I860), lmitoceras ixion (Hall, 1860), Protocanites lyoni (Meek and Worthen, 1860), and Prodromites praematurus Smith and Weller, 1901 (Lineback, 1963; Rexroad and Scott, 1964; Manger, 1979; Gordon and Mason, 1985). This interval was referred to the Muensteroceras oweni Assemblage Zone by Gordon and Mason (1985, p. 195) and Gordon (1986, p. 10) and indicates correlation to the lower Ivorian Stage of the Belgian upper Tournaisian succession. A second, distinctly younger, undescribed Osagean ammonoid assemblage with Polaricyclus conkini new species, P. ballardensis (Gordon, 1965), Winchelloceras knappi new species, and Cantabricanitesl greenei (S. A. Miller, 1892) occurs a few meters above the base of the New Providence Shale at Coral Ridge in Jefferson County, north-central Kentucky, and is the basis for the current report.

The Coral Ridge locality was discovered in the early 1950s by J. E. Conkin, then of the University of Kansas, in the course of thesis field work in the area. He secured a small collection of ammonoids from Coral Ridge which were identified by the late A. K. Miller, and were listed by Conkin (1957, p. 125, 127, table 1) as Pericyclus sp., Beyrichoceras sp., and Merocanites sp. Ammonoids from this initial collection were included along with additional specimens collected by Conkin within a larger Ph.D. study of Eastern Interior Mississippian ammonoids by Knapp (1965) which was never published. In recent years, more extensive collections of Coral Ridge ammonoids, totaling more than 200 specimens, have been assembled, largely through the efforts of CEM. Nevertheless, although Gordon and Mason (1985, p. 195), Kammer (1985, p. 90), and Work et al. (2000, p. 26) have published faunal lists and brief biostratigraphic summaries of the Coral Ridge assemblage (see synonymies under Systematic Paleontology for details), it has never been formally described.

STRATIGRAPHY AND AGE RELATIONSHIPS

Representatives of Polaricyclus conkini n. sp., P. ballardensis, Winchelloceras knappi n. sp., and Cantabricanitesl greenei were recovered from the New Providence Shale Member of the Borden Formation in the General Shale Products Company's Coral Ridge quarry, 1.8 km (1.1 mi) east-southeast of Coral Ridge, Jefferson County, Kentucky (Carter Coordinates NE NW SE, 24-T-46, Brooks 7.5' quadrangle, lat. 38°05'27'', long. 85°43'15''). The New Providence sequence at Coral Ridge consists of 60 m of greenish-gray to dark gray clay shale with a persistant, 0.1-mthick zone of sideritic cone-in-cone 6 m above the base. Ammonoids occur as pyritic casts 4-6 m above the base of the shale sequence, immediately below the zone of cone-in-cone (Fig. 1). The New Providence Shale unconformably overlies the New Albany Shale, a dominantly black, fissile, phosphatic shale unit which contains the Devonian-Mississippian boundary (see Kammer, 1985 for additional discussion of the Coral Ridge section and associated fauna).

The Coral Ridge ammonoid assemblage indicates correlation with the Fascipericyclus-Ammonellipsites Zone (as emended by Riley, 1991) and, on the evidence of Polaricyclus ballardensis and Winchelloceras knappi, with the P. ballardensis Assemblage of southeastern Kansas (see below). Polaricyclus Riley, 1991 and Winchelloceras Ruzhencev, 1965 appear first in the middle Osagean (upper Burlington equivalent) but extend into the late Osagean (lower Keokuk equivalent) in the Midcontinent, which is correlative with the middle or, more probably, the late Chadian (latest Tournaisian or early Viséan) portion of the Fascipericyclus-Ammonellipsites Zone. Conodonts associated with the ammonoids in the Coral Ridge section (Fig. 1) were identified by T. L. Thompson (personal commun., 2003) and include Gnathodus texanus Roundy, 1926, the nominate species of the texanus conodont Zone (Faunal Unit 7) of Lane (1974, p. 277-279) and Lane and Ormiston (1982) [= texanus Zone of Lane et al. (1980)]. Gnathodus texanus appears first at the base of the late Osagean Keokuk Limestone in the Mississippi Valley (Lane and Brenckle, 2001, p. 89, figs. 2, 4), and recovery of this species between 1 and 2 m below the ammonoid-bearing interval in the New Providence Shale at Coral Ridge (see Fig. 1) establishes a maximum age. A diverse assemblage of crinoids from the middle one-third of the New Providence Shale, 8-26 m above the ammonoid-bearing interval, also indicates Keokuk (late Osagean) affinities, as determined by Kammer (1984, p. 117, table 2; 1985, p. 94-95).