James H. Stitt

Cambrian-Ordovician boundary interval extinctions; implications of revised trilobite and brachiopod data from Mount Wilson, Alberta, Canada

The type section of the Basal Silty Member of the Survey Peak Formation spans the Cambrian–Ordovician boundary (North American usage) at Mount Wilson in the southern Canadian Rocky Mountains. The zonal and subzonal terminology through the boundary interval developed in Texas and Oklahoma is applicable to the trilobite faunas recovered from the section. The oldest trilobites recovered in this study occur in the top of the underlying Mistaya Formation and are assigned to the Saukiella serotina Subzone of the Upper Cambrian Saukia Zone. Trilobites and brachiopods of the S. serotina and Eurekia apopsis Subzones of the Saukia Zone occur in the lower half of the Basal Silty Member; trilobites and brachiopods assigned to the Lower Ordovician Missisquoia Zone and the Symphysurina brevispicata Subzone of the Symphysurina Zone occur in the upper half of the Basal Silty Member. The S. brevispicata Subzone extends an unknown distance into the Putty Shale Member of the Survey Peak Formation. The extinction horizons at the base of the Eurekia apopsis Subzone and at the base of the Missisquoia depressa Subzone (the Cambrian–Ordovician boundary) occur within the Basal Silty Member of the Survey Peak Formation, not at the formational contact with the underlying Mistaya Formation. This leaves hypotheses linking immigration of the replacement trilobite faunas to major lithofacies changes through the boundary interval as untenable. Critical review of the evidence for the extinctions at the end of the Ptychaspid Biomere suggests that they were caused by an invasion of the shelf region by cold, anoxic water. Forty-seven taxa are illustrated and 18 of those which provide new taxonomic information are discussed. One new genus, Rampartaspis Loch, is described in addition to four new species: Eurekia plectocanthus Loch, Highgatella wilsoni Derby, Macronoda punctata Derby, and Rampartaspis dissimulosulcus Loch. The identifications of trilobites and brachiopods in this paper revise those of Aitken and Norford (1967) and Derby et al. (1972) and result in minor changes in the reported positions of the bases of the Missisquoia and Symphysurina Zones. Revision of the identification of some trilobites in Dean (1989) changes the biostratigraphic interpretation of the Basal Silty Member at Wilcox Pass, Albert, Canada.

Carbon isotope evidence for deep-water invasion at the Marjumiid-Pterocephaliid biomere boundary, Black Hills, USA: A common origin for biotic crises on Late Cambrian shelves

An integrated carbon isotope, sedimentologic, and biostratigraphic study across a Late Cambrian mass-extinction boundary (Marjumiid-Pterocephaliid biomere boundary) in the Deadwood Formation, Black Hills, South Dakota, indicates invasion of the carbonate shelf by deep, cold, marine waters. Carbonate samples stratigraphically below the extinction boundary show an apparent trend of upwardly increasing δ 13 C values, from −0.4‰ to +0.1‰ and relate well to secular δ 13 C trends of Late Cambrian limestones. Coincident with the extinction horizon is a negative shift in δ 13 C values of 0.4‰ to 0.7‰. This shift can be attributed to introduction of cold, deep, 12 C-enriched seawater accompanying invasion of opportunistic, cold-water trilobites onto depopulated shallow-water continental shelves. A similar abrupt negative shift in δ 13 C values has been observed at the suprajacent Pterocephaliid-Ptychaspid biomere boundary. Others have found that decreases in 87 Sr/ 86 Sr values synchronous with increases in δ 13 C values accompany renewed sea-level rise in this part of the Late Cambrian. Abrupt negative shifts in δ 13 C values at biomere boundaries require a companion process responsible for the extinction events. This process may be impingement of a shallow thermocline on the shelf or overturning of a stratified Cambrian ocean. Coupling of sea-level rise with location of the thermocline and shelf geometry may create conditions responsible for recurrent extinctions of shallow-shelf communities throughout the Late Cambrian.

Upper Cambrian and lowest Ordovician articulate brachiopods from the Arbuckle and Wichita Mountains, Oklahoma

Brachiopods assigned to eight genera and 15 species have been recovered from Upper Cambrian (Franconian and Trempealeauan Stages) and lowest Ordovician strata in the Arbuckle and Wichita Mountains of Oklahoma. Species of Ocnerorthis and Eoorthis occur in the Reagan Sandstone and Honey Creek Limestone of the Timbered Hills Group. Species of Billingsella appear in the Honey Creek and range upward into the overlying Fort Sill and Signal Mountain Limestones of the Arbuckle Group. Species of Cymbithyris, Finkelnburgia, Nanorthis , and Apheoorthis succeed each other upsection in the Signal Mountain Limestone. Five brachiopod zones and one subzone have been established that can be used to correlate the measured sections in the Arbuckle and Wichita Mountains, and to correlate with varying degrees of confidence from Oklahoma to similar brachiopod occurrences in other areas in the United States.

Trilobites, biostratigraphy, and lithostratigraphy of the Crepicephalus and Aphelaspis zones, lower Deadwood Formation (Marjuman and Steptoean stages, Upper Cambrian), Black Hills, South Dakota

Abstract Trilobites assigned to 25 genera and 39 species are reported from the Crepicephalus Zone (Marjuman Stage) and Aphelaspis Zone (Steptoean Stage) in the lower part of the Deadwood Formation in the Black Hills of South Dakota. Six taxa are left in open nomenclature, and one new species, Glaphyraspis newtoni, is described. Analysis of the lithologies for this interval from the best exposed measured sections on a southeast-northwest transect reveal a nearshore, shallow subtidal, siliciclastic dominated environment to the southeast, succeeded offshore by a shallow subtidal to lowest intertidal carbonate shoal environment, and then a transitional shaly limestone interval into a more shaly distal intrashelf basin to the northwest. Specimens of species of Coosia, Crepicephalus, Tricrepicephalus, Kingstonia, Pseudagnostina, and Coosina comprise more than 75 percent of the fauna of the Crepicephalus Zone. Coosina ariston, Crepicephalus snowyensis, Tricrepicephalus tripunctatus, Arcuolimbus convexus, and some species of Blountia had a strong preference for the shallow-water siliciclastic facies present in the southeastern sections closest to the paleoshoreline. Crepicephalus rectus, Tricrepicephalus coria, Agnostogonus, cf. A. incognitus and the genera Coosella and Uncaspis preferred the farther offshore, deeper-water, shaly intershelf basin located in the northern Black Hills. Trilobites from the Crepicephalus Zone are used to correlate the lower part of the Deadwood Formation with coeval strata elsewhere in North America.

Jujuyaspis borealis and associated trilobites and conodonts from the Lower Ordovician of Texas and Utah

Jujuyaspis borealis is reported from earliest Ordovician (North American usage) limestones in central Texas and western Utah, the first time this species has been recognized in the United States. Jujuyaspis is a widespread olenid trilobite that occurs near the base of the Tremadoc Series in a variety oflithologies in North and South America, Europe, and Asia. When international agreement is reached on the exact horizon at or near the base of the Tremadoc Series that is to be used as the Cambrian-Ordovician boundary, Jujuyaspis will likely prove to be a very useful taxon for recognition of the boundary interval.

Trilobites from the Cedarina dakotaensis zone, lowermost part of the Deadwood Formation (Marjuman Stage, Upper Cambrian), Black Hills, South Dakota

Trilobites assigned to 14 genera and 14 species are reported from basal part of the Deadwood Formation in the Black Hills of South Dakota. Six additional taxa are left in open nomenclature. One new species, Cedarina dakotaensis , is described. These trilobites are assigned to a new zone, the Cedarina dakotaensis Zone, named after the most abundant trilobite species. Species of Cedarina and Modocia are the most abundant in the Cedarina dakotaensis Zone, accompanied by less common specimens of species of Arapahoia, Menomonia, Hardyoides, Welleraspis, Kormagnostus , and Kingstonia. Cedarina dakotaensis and Modocia centralis are the most abundant species in the nearshore sandstone lithofacies, whereas Arapahoia spatulata is the predominate taxon in the offshore limestone lithofacies. The fauna of the Cedarina dakotaensis Zone (which lacks species of Cedaria ) occupies the biostratigraphic interval of the traditional Cedaria Zone of the Marjuman Stage. Trilobites from the Cedarina dakotaensis Zone can be used to correlate the basal part of the Deadwood with coeval strata elsewhere in North America.

Geological implications of Late Cambrian trilobites from the Collier Shale, Jessieville area, Arkansas

Late Cambrian shelf trilobites characteristic of the Elvinia and Taenicephalus Zones (Franconian Stage) have been recovered from seven localities in newly recognized outcrops of the Collier Shale near Jessieville, Arkansas. Deposition of the Collier Shale spanned the Cambrian-Ordovician boundary; Late Cambrian trilobites occur in the lower part and Early Ordovician conodonts occur in the upper part of the formation. The trilobites are well known from many shallow-water shelf localities in North America, and their presence indicates that the Benton uplift is not an exotic terrane but an original part of North America. The thin, dark limestones containing the trilobites were deposited in a deep-water, outer-shelf or continental-slope location, and they establish an approximate position for the Late Cambrian shelf edge along the southern margin of North America.

ADDITIONAL INFORMATION ON LOWEST ORDOVICIAN TRILOBITES FROM THE UPPERMOST DEADWOOD FORMATION, BLACK HILLS AND BEAR LODGE MOUNTAINS, SOUTH DAKOTA AND WYOMING

Trilobites from the Missisquoia Zone and the Symphysurina brevispicata Subzone of the Symphysurina Zone (Ibexian Series, lowest Ordovician) were collected from measured sections in the uppermost Deadwood Formation in the Black Hills of South Dakota and Bear Lodge Mountains in northeasternmost Wyoming. These collections were made by Christina Lochman-Balk and her students, and turned over to the author to complete the project. They are compared with previous reported occurrences of this fauna from this area. No trilobites from the underlying Sunwaptan Stage (Upper Cambrian) occur with the lowest Ordovician trilobites, suggesting that the sharp faunal extinction at the base of the Ordovician (North American sense = Eurekia apopsis Zone, Ibexian Series) occurred in the Deadwood Formation as it did over all of the North American continent. The Black Hills is a domal uplift cored by Precambrian granites and quartzitic and schistose metamorphic rocks that was formed during the Laramide orogeny (Stitt, 1998). The Deadwood crops out as an oval belt of siltstones, sandstones, shales, and limestones unconformably overlying the Precambrian rocks. Specimens used in this study were collected from a series of measured sections located in the northernmost outcrop belt of the Deadwood. Hu (1973) identified lowest Ordovician trilobites from one section (Bridal Veil Falls) in the northern Black Hills and from three sections (Reuter Canyon, Bearlodge Ranch, and Sheep Mountain) in the nearby Bear Lodge Mountains. He also described one new genus and four new species. His composite faunal list, and subsequent taxonomic reassignments in brackets, include the following trilobites: Euloma cordilleri [= Highgatella cordilleri Winston and Nicholls, 1967], Euptychaspis trematocus, Highgatella facila [= Apoplanias rejectus Ludvigsen, 1982], Hystricurus sp., Missisquoia graphica [= Missisquoia typicalis Westrop, 1986], Missisquoia cyclochila [= Missisquoia typicalis Ludvigsen, 1982], cf. Missisquoia typicalis, Olenaspella elongata, Symphysurina brevispicata, and the brachiopod …

New Elvinia Zone (Upper Cambrian) trilobites from new localities in the Collier Shale, Ouachita Mountains, Arkansas

Three new species of trilobites, Erixanium lacunatum, Cernuolimbus monilis , and Anechocephalus aphelodermus , have been recovered from two new fossiliferous localities in the normally unfossiliferous Collier Shale in the Ouachita Mountains of west-central Arkansas. These new taxa occur with other trilobites previously reported from the Elvinia Zone in the Collier and elsewhere. Like the other trilobites reported from the Collier, these new taxa were originally deposited on the outer shelf, and were later eroded and redeposited in the adjacent, deep-water basin. Other species assigned to these genera have also been reported from outer shelf or continental slope sites. Species of Erixanium were widely distributed in equatorial and subtropical paleolatitudes in the Late Cambrian.