Middle Cambrian articulate brachiopods from the Southern New England Ford Belt, Northeastern N.S.W., Australia

Journal of Paleontology, Jul 1998 by Brock, Glenn A

ABSTRACT-Calcareous articulate brachiopods are rare components of the high diversity, phosphatic, silicified, and epidote coated shelly fauna derived from Middle Cambrian (Floran-Undillan) allochthonous limestone clasts from the Murrawong Creek Formation, southern New England Fold Belt, northeastern New South Wales, Australia. Three taxa are described, the kutorginids Nisusia metula n. sp., and Yorkia sp. indet., and the protorthid Arctohedra austrina n. sp. Yorkia is documented from Australia for the first time. An unusual valve (possibly a brachial valve) of enigmatic affinity is also reported and illustrated. Generically, the taxa provide broad regional paleobiogeographic links with the "first discovery limestone" Member of the Coonigan Formation, western New South Wales, and the Current Bush Limestone in the Georgina Basin, northern Australia, and globally, with broadly contemporaneous sequences in western North America, Siberia, and South China.

INTRODUCTION

MIDDLE CAMBRIAN fossils were first reported from basal sequences of the southern New England Fold Belt by Cawood (1976) (Fig. 1). Allochthonous limestone clasts recovered from the lowermost unit, the Murrawong Creek Formation (Cawood, 1980), have yielded a high diversity phosphatic, silicified and epidote coated Middle Cambrian (Floran-early Boomerangian using the Australian Cambrian Stage Scale of Shergold (1995)) shelly fauna dominated by miomeroid and polymeroid (corynexochide and ptychoparine) trilobites (Cawood, 1976; Sloan, 1991), lingulate brachiopods (Engelbretsen, 1996) and mollusks (Brock, in press). Other faunal elements including an enigmatic coral-like organism (Engelbretsen, 1993) and a diverse array of material, including coeloscleritophorans, poriferan spicules, hyoliths, hyolithelminths, conodonts, echinoderm fragments, and a number of problematic small shelly fossils.

The objective of this contribution is to document the calcareous articulate brachiopods from these clasts, to discuss their morphology and phylogenetic relationships, and make preliminary comments on their paleobiogeographic significance.

GEOLOGICAL SETTING AND DEPOSITIONAL ENVIRONMENT

The New England Fold Belt extends for approximately 1,300 km along the east coast of Australia (Fig. 1.1) and is interpreted as the site of an ancient convergent plate margin (Cawood, 1980, 1983; Leitch et al., 1988; Lindsay, 1990; Leitch and Cawood, 1996). Evidence for the earliest phase of convergence, during the Cambrian-Early Ordovician, is represented by a thick sequence (1,700 m) of generally poorly exposed volcaniclastic detritus located west of the Peel Fault, in the Copes Creek-Murrawong Creek area (Fig. 1.2; Cawood, 1976, 1980; Leitch et al., 1988).

The Murrawong Creek Formation (Cawood, 1980), the oldest of three lithostratigraphic units in this area, crops out as a 4.5 km north-south trending tract of interbedded siltstone, sandstone and conglomerate, some 25 km S.S.E. of Tamworth (Fig. 1.3). In the type section, along Murrawong Creek, the sequence consists of 450 m of poorly outcropping, interbedded coarse to fine debris flow conglomerates, turbiditic sandstone, siltstone, siliceous mudstone and ash-fall tuffs (Fig. 2). The base of the section is faulted against undifferentiated ?Devonian metasediments. Leitch and Cawood (1987, fig. 2) recognized three conglomerate horizons in the Murrawong Creek Formation, informally designated units 1-3. Unit 1, the coarsest of the three units (Fig. 2), is a poorly outcropping, approximately 85 m thick, polymictic paraconglomerate with angular to subrounded, mostly volcanic, clasts (0.01-1.50 m in diameter) set in a poorly sorted sand- to granule-grade volcaniclastic matrix. Engelbretsen (1996, p. 71) has previously commented on petrological aspects of the fossiliferous limestone clasts. Essentially the limestone clasts are coarse grained and consist of a high proportion of shelly allochems forming highly fossiliferous coquinites. Cavities between shelly allochems are infilled with fine carbonate mud, partially recrystallised to microspar and pseudospar. The noncarbonate component consists of silty and sandy silicieous/intermediate volcanic rock fragments, plagioclase and feldspar and abundant beta form (i.e., volcanic) quartz. Plagioclase is partially replaced by epidote, and chlorite in some clasts, suggesting lowgrade post-depositional metamorphism. The Murrawong Creek Formation fines upwards gradationally and is conformably overlain by approximately 900 m of argillites, cherts, tuffs, sandstones and fine conglomerates of the Pipeclay Creek Formation (Cawood, 1980; Leitch and Cawood, 1987).

Leitch and Cawood (1987) provided a detailed provenance study of the cobble and pebble sized igneous clasts from the Murrawong Creek Formation and found that the chemical composition of the clasts (low-K orogenic suite), and the absence of detritus characteristic of continental crust was consistent with derivation from an intra-oceanic island arc rather than a continental margin arc. Limited directional data from the Murrawong Creek suggests derivation from a westerly source (Cawood, 1980). The intermediate to silicic volcanic origin of the small siliciclastic component of the limestone clasts reveals no evidence of a continental origin.