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Late Dresbachian (Late Cambrian) Biostratigraphy of North America

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Late Dresbachian (Late Cambrian) Biostratigraphy of North America CHRISTINA LOCHMAN-BALK Department of Geosdence, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801 Late Dresbachian (Late Cambrian) Biostratigraphy of North America ABSTRACT bachian Aphelaspis Zone (Howell and Dresbachian time in the central Texas others, 1944) into an older Aphelaspis Zone section, with only a very short hiatus? before An examination of the taxonomic com- and a younger post-Aphelaspis Zone. the appearance of the Elvinia Zone fauna. position of the North American late Trilobites and brachiopods assigned by The short hiatus? refers to the well-defined Dresbachian faunules described to date Palmer to the Aphelaspis Zone are listed in faunal break and disconformity between the indicates that there is an Aphelaspis Zone Table 1. In the basal beds of the zone, Palmer Lion Mountain Sandstone Member of the faunal assemblage characteristic of the early Riley Formation and the Welge Sandstone half of the late Dresbachian Stage and a Member of the Wilberns Formation (Loch- TABLE 1. TRILOBITES ÄND BRACHIOPODS ASSIGNED TO Dunderbergia Zone faunal assemblage APHELASPIS ZONE, RILEY FORMATION, TEXAS, man, 1938; Freeman, 1969). characteristic of the later half. The diagnos- BY PALMER (1955) From 1951, Palmer was engaged in the tic elements of each of the assemblages are Aphelaspis walcotti study of the Late Cambrian faunas of the listed, and the biozones of the common Labiostria (now Aphelaspis) conveximavginata Great Basin and published late Dresbachian Labiostria (now Aphelaspis) platifrons trilobite genera are noted so that correct Dunderbergia variagranula faunal lists in Nolan and others (1956) and Eaaschella (now Glaphyraspis) omata intracontinental correlations may be made. Cheilooephalus brevilobus in Drewes and Palmer (1957) to document Aphelaspis constriota the existence in the Great Basin of a large INTRODUCTION Aphelaspis spinosa Aphelaspis longifrons faunal assemblage younger than the Dytremacephalus granulosus Aphelaspis Zone and older than the Elvinia Recent discussion of the late Dresbachian Dytremacephalus laevis lithostratigraphy and biostratigraphy Pseudagnostus comrunis Zone assemblage. In 1958, Lochman-Balk Pseudagnostus josephus (Palmer, 1971; Kurtz, 1971) indicates the Geragnostus cf. G. tumidosus and Wilson published on the Cambrian Angulotreta triangularis biostratigraphy of North America and need for a clarification of the exact Dictyonina perforata composition of the faunal assemblages cf concluded that the term "post-Aphelaspis" the Aphelaspis, Dunderbergia, and Elvinia was inadequate for the large faunal Zones and the need for a listing of the assemblage which was being studied by currently known ranges of the trilobite and reported Aphelaspis walcotti in association Palmer. In analyzing Palmer's faunal lists, brachiopod genera within these zones. with Coosia cf. C. albertensis, Lochman-Balk and Wilson (1958, p. 337) Confusion concerning the occurrence and Crepicephalus? (now Coosella) perplexa, noted that Dunderbergia appeared to be an range of the common genera appears to be Maryvillia cf. M. (now Coosina) ariston, abundant and widespread genus in the hampering accurate correlation of the late Llanoaspis pecularis, and Tricrepicephalus younger late Dresbachian assemblage. Dresbachian faunules in the United States. A coria. The trilobites and brachiopods Lochman-Balk and Wilson (1958, p. 333) review is made of all the faunules of this age assigned by Palmer to the post -Aphelaspis were also aware that Dunderbergia did described to date in the United States. Zone are listed in Table 2. Palmer placed occur in association with Aphelaspis and During this time, the stock of the Aphelas- stated that it first appears near the top of the pididae was evolving rapidly and gave rise Aphelaspis Zone, but it continues into and becomes more abundant in higher beds: both to several closely related genera that TABLE 2. TRILOBITES AND BRACHIOPODS ASSIGNED TO are now placed in that family and to four POST-APHELASPIS ZONE, RILEY FORMATION, "The passage beds between the Aphelaspis TEXAS, BY PALMER (1955) other genera, Prehousia, Dytremacephalus, and the Elvinia faunizones in both Texas Dunderbergia, and Pterocephalia that are Labiostria (now Aphelaspis) convezimarginata and Nevada occupy a position spanning the Labiostria (now Apehlaspis) sigmoidalis regarded as the ancestral genera of the Dytremacephalus granulosus median level of the Dunderbergia biozone. It Housiidae, the Dokimocephalidae, the Cheilooephalus brevilobus is proposed that this biostratigraphic unit be Dunderbergia Variagranula Elviniidae, and the Pterocephaliidae, respec- Pterocephalia cf. P. ocaidens (now Pterocephalia oonaava) known as the Dunderbergia faunizone" Angulotreta triangularis tively. By the later half of late Dresbachian Apsotreta expansa (Lochman-Balk and Wilson, 1958, p. 333). time and the earliest part of Franconian Dysoristus lochmanae By including Texas in this statement, time, Aphelaspis and the closely related Lochman-Balk and Wilson (1958) were genera had become extinct, and genera and accepting Palmer's 1955 concept that the species of the four descendent families were post-Aphelaspis Zone faunal assemblage of diversifying and dominating the faunules. Labiostria in synonymy with Aphelaspis in Texas (1) was distinct from the Aphelaspis 1962, and he described Pterocephalia Zone faunal assemblage of Texas and (2) FAUNAL ASSEMBLAGES concava in 1960c. was to be correlated with the large faunal In 1965, p. 6, Figure 3, Palmer showed assemblage in Nevada younger than the Aphelaspis Zone that he considered the Aphelaspis and the Aphelaspis Zone which Palmer listed in Palmer (1955), in his study of the Riley post-Aphelaspis Zones and their assem- 1956 and 1957. The faunule lists in Tables 1 Formation of Texas, split the late Dres- blages to occupy nearly all of late and 2 show that the two zones in Texas have Geological Society of America Bulletin, v. 85, p. 135-140, January 1974 135 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/85/1/135/3428756/i0016-7606-85-1-135.pdf by guest on 01 October 2021 136 C. I.OCHMAN-BALK five genera and species in common, and the similar to the two Texas faunules listed by the time span of the late Dresbachian and synonymizing of Labiostria with Aphelaspis Palmer (1955) as representing the early Franconian Ages. Palmer's range chart (Palmer, 1962) demonstrates that the Aphelaspis and post-Aphelaspis Zones. The (1965, PI. 21) shows the composition and faunule of the post-Aphelaspis Zone of Texas faunal lists in Tables 1 and 2 show range of genera and species in each of the Texas is actually a slightly younger that Aphelaspis occurs in the faunules of zones. The 13 trilobite taxa assigned to his Aphelaspis Zone faunule. both zones, and is associated in both zones new Aphelaspis Zone in the Great Basin are Palmer (1960a, 1960b, 1962) presented a with Dunderbergia variagranula, Dy- listed in Table 4. The 13 trilobite taxa number of Dresbachian sections from the tremacephalus granulosus, Cheilocephalus assigned to the succeeding Dicanthopyge Great Basin, indicated that the Eureka, brevilobus, and Angulotreta triangularis. Zone are listed in Table 5. Of the 23 species Nevada, section is incomplete for the late These ranges and associations indicate that Dresbachian Stage, and showed the position the faunule of Palmer's post-Aphelaspis TABLE 4. TRILOBITES ASSIGNED TO APHELASPIS ZONE, of the Crepicephalus, Aphelaspis, Zone of Texr.s certainly is NOT GREAT BASIN, BY PALMER (1965) post-Aphelaspis, that it is only slightly Dunderbergia, and Elvinia Zones in these Aphelaspis buttsi sections. He recognized (1960a) an upper younger than the faunule assigned to the Aphelaspis haguei Aphelaspis brachyphasis and a lower subzone in both the Aphelaspis Aphelaspis Zone in Texas by Palmer, and Aphelaspis subdita and the Dunderbergia Zones. that it represents the same time span as the Aphelaspis longispina Olenaapella separata In June 1965, Rasetti published an faunule of the middle subzone and possibly Olenaspella regularis Cheilocephalus brevilobus analysis of the Upper Cambrian trilobite the base of the upper subzone of the Glaphyraspis orncta faunas of northeastern Tennessee. Very Aphelaspis Zone of Tennessee (see Table 9). Terranovella brevis Blountia bristolensis detailed collecting was done through the Consequently, in all my recent papers Listroa toxoura Cedaria faunules in the upper Maryville (Lochman-Balk, 1970, 1971, 1972), I have Hardyoides minor Limestone and basal Nolichucky Forma- considered that only faunules of the tion, and through the Crepicephalus and Aphelaspis Zone time span are present in the TABLE 5. TRILOBITES ASSIGNED TO DICAHTHOPiGE ZONE, Aphelaspis faunules in the upper Texas sections. The faunules "spanning the GREAT BASIN, BY PALMER (1965) Nolichucky Formation to the highest median level of the Dunderbergia biozone" Aphelaspis subdita fossiliferous beds. Aphelaspis occurs in all (Lochman-Balk and Wilson, 1958, p. 333), Tumicephalus depressus Dicanthopyge quadrata faunules overlying the Crepicephalus as- as developed in Nevada, are all absent from Dicanthopyge convergens Dicanthopyge reaucta semblage, and Rasetti placed all the faunules central Texas; the disconformity between Olenaspella regt.laris in the Aphelaspis Zone (see Table 3). Rasetti the Lion Mountain Sandstone member and Olenaspella paucisegmenta Listroa toxoura divided the zone into lower, middle, and the Welge Sandstone Member occupies the Dunderbergia brcvispina time span of
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