CONODONTS FROM THE GLEN DEAN FORMATION 

 (CHESTER) OF THE ILLINOIS BASIN 



CARL B. REXROAD 



ABSTRACT 



The conodont fauna of the Glen Dean Formation in its outcrop area around the 

 margin of the Illinois Basin is described. Samples were collected from thirteen uniformly 

 spaced sections in southern Illinois, Indiana, and Kentucky, including the type section, 

 and virtually every sample from limestone and many from shale yielded conodonts. 



Twenty-seven species, seven new, were recognized and are referable to thirteen genera: 

 Cavusgnathus, Gnathodus, Hibbardella, Hindeodella, Kladognathus (new name for 

 Cladognathus), Ligonodina, Lonchodina, Neoprioniodus, Ozarkodina, Roundya, Spathog- 

 nathodus, Trichonodella, and one new genus Lambdagnathus. 



The fauna is unaffected by iithologic variations and is remarkably uniform geo- 

 graphically, although a southern faunal province based on abundance of certain species 

 is recognized. 



Six species appear restricted to the formation but their ranges cannot be considered 

 fixed because the remainder of the Chester Series has not been sampled in comparable 

 detail. 



INTRODUCTION 



Purpose and Scope 



Studies of conodonts from the Chester 

 Series in the type area of southwestern IIH- 

 nois (Rexroad, 1957) showed a diversified 

 fauna from eight formations and indicated 

 that conodonts will be useful in strati- 

 graphic investigations of Chester strata. 

 Although the earlier study established the 

 Chester conodont fauna as a whole, that 

 study was confined to one area and mainly 

 to the shale beds. Thus it did not allow 

 evaluation of limestone faunas, effects of 

 facies changes, nor faunal variations within 

 a single formation over a broad area. Such 

 details, along with the more precise delim- 

 iting of stratigraphic ranges within the se- 

 ries, were the objectives of the present 

 study. 



The Glen Dean Formation was chosen 

 for study because: (1) it contains relatively 

 abundant conodonts in the type area of the 

 Chester Series; (2) it is well exposed around 

 the margin of the Illinois Basin; (3) in 

 much of its outcrop area it consists of a 

 lower massive limestone unit and an upper 

 unit of mixed lithology, dominantly shale, 

 which would be desirable for facies analysis; 



and (4) its position in the middle Chester 

 might show whether several species, which 

 in the initial study appeared to have a 

 range limited to the lower or to the upper 

 part of the series, are truly restricted. 



Thirteen sections from Illinois, Indiana, 

 and western Kentucky, including the type 

 section of the formation, were sampled. 

 Only sections where stratigraphic positions 

 are well established were chosen as collect- 

 ing localities and these were spaced as uni- 

 formly as was consistent with the choice of 

 complete or nearly complete Glen Dean 

 sections. 



Methods of Study 

 From each exposure, channel samples of 

 all lithologies except sandstone were col- 

 lected. The thickness represented by each 

 sample was commonly one or two feet but 

 depended upon the thickness of the Iitho- 

 logic units. Initial samples averaged about 

 700 grams. The samples were processed, 

 then bulk samples were collected from each 

 zone that contained abundant conodonts. 

 In addition, a sample of approximately 150 

 pounds of crushed rock was collected from 

 the lower massive limestone unit in the 

 Mulzer Brothers quarry, location 12. 



[7] 



