Biostratigraphic problems of Morrowan and Derryan (Atokan) strata in the Pennsylvanian System of western United States D. L. DUNN Gulf Research and Development Company, H.T.S.C., P.O. Box 36506, Houston, Texas 77036 ABSTRACT CONODONT ZONES Although both foraminiferal and conodont zonations have been Lane and Straka (1974) proposed a conodont zonation (Fig. 1) proposed for Lower and Middle Pennsylvanian strata, the current for Arkansas and Oklahoma utilizing slightly different terminology state of the art indicates that several proposed Morrowan cono- from that proposed by Lane and others (1971). They did not rec- dont zones and one important Derryan(?) fusulinid zone (21) and ognize the Streptognathodus expansus—S. suberectus, the the ranges of several important associated conodonts and Idiognathodus humerus—I. sinuosis, and the Streptognathodus fusulinids are in question. The conodont zones in question are the parvus—Adetognathus spathus Zones proposed earlier (Dunn, Gnathodus girtyi simplex, the Streptognathodus expansus-S. sub- 1970b), apparently because they did not believe these zones to be erectus, the Idiognathodus humerus—I. sinuosis, and the adequately documented in northeastern Oklahoma (Lane and Streptognathodus parvus— Adetognathus spathus Zones of Dunn Straka, 1974, p. 31). Lane and Straka (1974) have failed to recog- and the Idiognathodus sinuosis, I. klapperi, and 1. convexus nize the significance of some of the most important upper Mor- Zones of Lane and Straka. The conodonts whose ranges are in rowan index fossils now known — namely, those of the first and question are Gnathodus girtyi simplex, Rhachistognathus, third zones cited above — and, consequently, have demonstrated muricatus, Declinognathodus noduliferus, Adetognathus spathus, inconsistency in the correlation of Morrowan strata and in the Neognathodus bassleri bassleri, and Idiognathoides convexus. placement of their Morrowan-Derryan boundary. This seemingly is Evidence based on conodonts suggests that there is a time over- due to their lesser reliance on conodonts and greater emphasis on lap between uppermost type Morrowan and lowermost type Der- foraminiferal ranges as related to Zone 21 of Mamet and Skipp's ryan strata. Because of this, there is as yet no precise paleontologi- (1970) foraminiferal zonation. cal definition of a single Morrowan-Derryan (or Morrowan- Lane and others (1972, p. 554) and Lane and Straka (1974, p. 1) Atokan) boundary. Key words: micropaleontology, Carboniferous, stated that the zonal scheme of Lane and others (1971, p. conodonts, Paleozoic, Pennsylvanian. 398-403), and as modified by Lane and Straka (1974) and shown in Figure 1 here, is applicable in west Texas and Nevada. Analysis of their data, however, indicates that they have not documented INTRODUCTION their Idiognathodus sinuosis and Idiognathoides convexus Zones in west Texas or Nevada. Moreover, their Idiognathodus klapperi Recent contributions on Mississippian and Pennsylvanian bio- Zone is known at only one locality in Arkansas and is not stratigraphy in the western United States (Mamet and Skipp, 1970; documented in Nevada. Lane and others, 1971, 1972; Lane and Straka, 1974) dealing with foraminifers and (or) conodonts have presented biostratigraphic QUESTIONABLE CONODONT ZONES data dealing with zonations for these fossil groups. Lane and others (1972) and Lane and Straka (1974) presented their interpretation Gnathodus girtyi simplex Zone. Webster (1969, p. 22, 23) pro- of the relationships of their proposed conodont zones to the posed the Gnathodus girtyi simplex Zone and considered it as foraminiferal scheme of Mamet and Skipp (1970). Seemingly, they chronologically equivalent to the Rhipidomella nevadensis have shed light on understanding Chesterian, Morrowan, and Der- brachiopod zone. Although Webster did not specifically define the ryan correlations and the Mississippian-Pennsylvanian and zone on the basis of conodont limits, a restricted Gnathodus girtyi Morrowan-Derryan boundaries. simplex Zone was recognized in the Indian Springs Formation at The purpose of this paper, however, is to note problems involv- Lee Canyon in Nevada (Dunn, 1970b). Contrary to Webster ing some of the conclusions reached in the papers cited above and (1969) and Lane and Straka (1974, p. 21, 22, 29), I found that to offer alternative conclusions and interpretations. In particular, Gnathodus girtyi simplex succeeds G. girtyi girtyi in the Indian more research is needed on the problem of a Morrowan-Derryan Springs (Dunn, 1970a, p. 331), with the lowest occurrence of the (and Morrowan-Atokan) boundary before we can determine former above that for Adetognathus unicornis. On this basis, I whether or not it is only one horizon, as we have been led to recognized the zone as embracing the range of G. girtyi simplex believe. That is, the upper type Morrowan boundary is not neces- prior to the lowest occurrence of Rhachistognathus muricatus sarily correlative with the lower type Derryan (or Atokan) bound- (Dunn, 1970b). ary. The initial appearance and extinction point of G. girtyi simplex In addition, the validity of several proposed Morrowan cono- seemingly rests on interpretation of this subspecies. Webster (1969, dont zones is questioned because separate zonal schemes have been PI. 5, fig. 10), for example, includes forms under G. girtyi simplex proposed by me (Dunn, 1970b) and Lane and Straka (1974) for the that I would consider as G. girtyi girtyi. Moreover, because G. same part of the Lower Pennsylvanian (Morrowan) section (Fig. 1). girtyi simplex evolves to Declinognathodus noduliferus at the Geological Society of America Bulletin, v. 87, p. 641—645, 1 fig., May 1976, Doc. .. 60501. 641 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/87/5/641/3418251/i0016-7606-87-5-641.pdf by guest on 30 September 2021 5 CONODONT ZONES üü (f) »- UJ co a: RANGES OF KEY ZONE CONODONTS >- tu LANE $ STRAKA co THIS PAPER co 1974 z 2 ë Si Idiognathoides n sp. I UJ o ? Streptognathodus parvus I. convexus Adetognothus spathus Q. I. klapperl 10 in Idiognathodus humerus c in a> a> o I. sinuosi« a> TJ XJ Q. '5 o I. sinuosi» a. -C o O Z Streptognathodus expansus c o < CJl o O 2 S. suberectus £ u> T3 v> o 10 </> 3 3 < o o TJ T3 ÎZ 3 in O O >.t E C 3> -J o O 3 JC. >- a tr £ to O 0 N. basslerl bassleri N. bassleri bassleri 3 O c co~ oc a. c Q. <31 01 o m </> </j O O o 3 3 0) Q. XL .c -C •a fa- CO (0 TJ o il) (/) 3 D UJ 0 a n 3 "O Q- a. c c 3 (0 o cn N. bassleri symmetrlcus N. bassleri symmetricus 01 CP a U) 3 a> .c o o c 05 TJ o> CO o E o <2 o a -C 3 c £ ja c J2 CT o o a) o o o o 75 c Ê 3 o> (0 D. noduliferus I. noduliferus -C .c cu 3 Q- a: CE Q E in g •o a> >> a> c T3 k. (A o o i/> Q. -C 3) o X o -Q as c en R. primus R. primus to O 3 CO in 3 -C 3 10 3 -X. o _T3 . J8 -o a> -n- O Q. > -C Z R. muricatus c < R. muricatus o u) o < 10 o 3 c po i a. 3 TJ en tr (G.girt. simplex ? ) W o Q. a> v Q. o "O a> co £ ti A. unicornis c O CO (/) o A. unicornis o> £5 C o5 UJ o o> </> X D o co o a> C <D <T 3 o> S Pre A. unicornis g TJ Figure 1. Comparison of conodont zonal schemes and ranges of key zone conodonts. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/87/5/641/3418251/i0016-7606-87-5-641.pdf by guest on 30 September 2021 BIOSTRATIGRAPHIC PROBLEMS OF MORROW AN AND DERRYAN STRATA 643 Mississippian-Pennsylvanian boundary, differentiation of these two distinctly different species of Idiognathodus in their upper Mor- conodonts at the boundary is difficult. Whether or not a broad rowan material. interpretation of G. girtyi simplex is the reason for Lane and Other authors (Koike, 1967; Webster, 1969; Webster, in Lane Straka's (1974, p. 28) reported lower Morrowan occurrence of this and others, 1971, p. 405) may have recovered I. sinuosis and I. subspecies at Arrow Canyon in Nevada is not known. In any case, humerus in their material and, if so, possibly identified them to- the difficulties of recognizing any sort of G. girtyi simplex Zone, as gether with other forms of Idiognathodus as I. delicatus Stauffer pointed out by Lane and Straka (1974), may preclude its useful- and Plummer. ness. Further research is necessary to answer this question. Redefinition in this paper of the underlying Streptognathodus Idiognathodus sinuosis Zone versus Streptognathodus ex- expansus—S. suberectus Zone raises the lower limit of the pansus-Streptognathodus suberectus Zone. In proposing the Idiognathodus humerus—I. sinuosis Zone to the stratigraphic level Idiognathodus sinuosis Zone, Lane and Straka (1974, p. 31) seem- of extinction of S. expansus and S. suberectus. ingly discredited my evidence for the S. expansus-S. suberectus Idiognathoides convexus Zone versus Streptognathodus par- Zone in the Arkansas-Oklahoma area (Dunn, 1970b), and recog- vus-Adetognathus spathus Zone. Lane and Straka (1974) ap- nized the Idiognathodus sinuosis Zone as immediately succeeding parently have unraveled the knotty problem of nomenclature con- the Neognathodus bassleri bassleri Zone. They are technically cor- cerning Idiognathoides sulcatus, I. sinuatus, I. sulcatus parvus, I. rect in regard to my original definition of the S. expansus—S. sub- macer, and I. convexus. On the basis of their determined range for erectus Zone (Dunn, 1970b, p. 2970). Nevertheless, these impor- Idiognathoides convexus, they have proposed an uppermost Mor- tant middle Bloyd (Arkansas) conodonts are present in Lane and rowan conodont zone tentatively defined by the range of that Straka's faunal material, although invariably they seem to occur species below the level of appearance in so-called "Derryan" strata with Idiognathodus sinuosis (and [or] I.