James D. Loch

Paleoceanographic events and faunal crises recorded in the Upper Cambrian and Lower Ordovician of west Texas and southern New Mexico

A revised lithostratigraphy for Lower Paleozoic strata in New Mexico and west Texas was developed through detailed sedimentological study of the Bliss and Hitt Canyon Formations within a refi ned temporal framework assembled from precise biostratigraphic (trilobite and conodont) and chemostratigraphic (carbon isotope) data. Member boundaries within the Hitt Canyon now correspond with mappable and essentially isochronous horizons that represent major depositional events that affected sedimentation in basins throughout Laurentian North America. This trip is designed to examine these and other important intervals, such as the extinction horizons at the base and top of the Skullrockian Stage, and to demonstrate the utility of associated faunas and isotopic excursions for correlation within and beyond the region.

New trilobite taxa from Upper Cambrian microbial reefs in the central Appalachians

Abstract Previously undescribed trilobites associated with microbial patch reefs have been recovered from the Upper Cambrian (upper Steptoean Stage) Ore Hill Limestone Member of the Gatesburg Formation in south-central Pennsylvania. Reefs in the Imler Quarry and Drab-Beaverton measured sections yielded low-diversity assemblages of trilobites that we assign to the uppermost Cliffia lataegenae Subzone of the Elvinia Zone. The faunas are dominated by the catillicephalid Buttsia drabensis Wilson, 1951, and include several new taxa that are not known from coeval off-reef facies. Fine-scale correlation between the two sections reveals that the reefs in the Drab-Beavertown section are slightly younger than those at Imler Quarry and represent the youngest Steptoean fauna described from the Appalachian region. New taxa include Imlerella n. gen. (type species Imlerella praecipita n. sp.), Stittaspis n. gen. (type species Stittaspis loria n. sp.), and Dellea rogersi n. sp. Stigmacephalus? distorta Wilson, 1951, is reassigned to Stittaspis and is restricted to the type. Additional material of Buttsia drabensis illustrates features not discernible in previous treatments of this variable species.

Biostratigraphy and Chronostratigraphy of the Cambrian–Ordovician Great American Carbonate Bank

The carbonate strata of the great American carbonate bank (GACB) have been subdivided and correlated with ever-increasing precision and accuracy during the past half century through use of the dominant organisms that evolved on the Laurentian platform through the Cambrian and the Ordovician. Trilobites and conodonts remain the primary groups used for this purpose, although brachiopods, both calcareous and phosphatic, and graptolites are very important in certain facies and intervals. A series of charts show the chronostratigraphic units (series and stages) currently in use for deposits of the GACB and the biostratigraphic units (zones, subzones, and biomeres) whose boundaries delineate them. Older and, in some cases obsolete, stages and faunal units are included in the figures to allow users to relate information from previous publications and/or industry databases to modern units. This chapter also provides a brief discussion on the use of biostratigraphy in the recognition and interregional correlation of supersequence boundaries within the Sauk and Tippecanoe megasequences, and the varied perspectives on the nature of biostratigraphic units and their defining taxa during the past half century. Also included are a concise update on the biomere concept, and an explanation of the biostratigraphic consequences of a profound change in the dynamics of extinction and replacement that occurred on the GACB in the Early Ordovician when the factors responsible for platformwide biomere-type extinctions faded and ultimately disappeared. A final section addresses recent and pending refinements in the genus and species taxonomy of biostratigraphically significant fossil groups, the potential they hold for greatly improved correlation, and the obstacles to be overcome for that potential to be realized.

A new genus of aporrhaid gastropod from Southern California

A paleoecological study of the Point Loma Formation (Upper Cretaceous, southern California) resulted in the discovery of a new genus assigned to the gastropod family Aporrhaidae. The individuals recovered provide the basis for the designation of Teneposita n. gen. based upon T. laeva n. sp. Teneposita is characterized by two delicate labral digitations, a short opposing digitation, and elongate anterior and posterior canals. Teneposita laeva is one of the smallest aporrhaids yet described.

Biostratigraphy of Cambrian and Lower Ordovician Strata in the Llano Uplift, Central Texas

Decimeter-scale sampling of the Cambrian and the lowermost Ordovician (Sauk megasequence) rocks of the Llano uplift, Texas, has produced a finely resolved biostratigraphic framework based primarily on trilobites and conodonts. Systematically collected trilobites of the Llano Uplift allow recognition of 13 biozones that extend from the Bolaspidella Biozone (Cambrian System, Marjuman Stage) through the Symphysurina Zone (Ordovician System, upper Skullrockian Stage). Systematic collection of conodonts has produced specimens assignable to 13 zones that range from the Proconodontus tenuiserratus Zone (Cambrian System, Sunwaptan Stage) through the Rossodus manitouensis Zone (Ordovician System, upper Skullrockian Stage). The base of the Ordovician System in the Llano uplift, as elsewhere, has been identified by the lowest occurrence of the conodont Iapetognathus fluctivagus and is closely approximated by the lowest occurrences of the cosmopolitan trilobite Juyjuyaspis and the Laurentian trilobite Symphysurina “bulbosa.” Although the overlying Ordovician strata of the Ellenburger Group have not been systematically sampled, scattered trilobite collections do establish the approximate positions of the base of the Stairsian Stage (based on Paraplethopeltis) and the base of the Jeffersonian Stage (based on Rananasus and Jeffersonia) in the Tanyard and Honeycut Formations, respectively.

An integrated trilobite and conodont biostratigraphy across the base of the Laurentian Whiterockian Series (lower Middle Ordovician) at its stratotype, Whiterock Canyon Narrows, Nevada

Abstract. The outcrop at Whiterock Canyon Narrows, Nevada, is the stratotype for the Middle Ordovician Whiterockian Series in Laurentia. Contrasts in the distribution of trilobites and conodonts between two parallel sections at the stratotype demonstrate the presence of an unconformity separating Ibexian faunas (the historic trilobite Zone J, the more recent restricted “Pseudocybele nasuta” trilobite Zone, the Reutterodus andinus conodont Zone) from overlying Whiterockian faunas (Zone L, the Psephosthenaspis pseudobathyurus trilobite Zone, the Orthidiella brachiopod Zone, the Tripodus combsi conodont Zone). The unconformity represents the erosional loss of a minimum of 10m of shale and bedded limestone within the upper Ninemile Formation. In comparison to more continuous section in the Ibex region, Utah, the equivalent to the thin Psephosthenaspis microspinosa trilobite Zone and, possibly, the uppermost interval of the underlying “P. nasuta” Zone are missing. Illaenus welchi new species is described from the Whiterockian faunas of the upper Ninemile and Antelope Valley formations.