The Sedimentary Record 2000; Shen and Schidlowski, 2000). Due to The Precambrian- endemic biotas and facies control, it is diffi- cult to correlate directly between siliciclas- Cambrian Transition in the tic- and carbonate-dominated successions. This is particularly true for the PC-C boundary interval because lowermost Southern Great Basin, USA Cambrian biotas are highly endemic and Frank A. Corsetti James W.Hagadorn individual, globally distributed guide fossils Department of Earth Science Department of Geology are lacking (Landing, 1988; Geyer and University of Southern California Amherst College Shergold, 2000). Los Angeles, CA 90089-0740 Amherst, MA 01002 Determination of a stratigraphic bound- [email protected] [email protected] ary generates a large amount of interest because it provides scientists with an oppor- ABSTRACT:The Precambrian-Cambrian boundary presents an interesting tunity to address a variety of related issues, stratigraphic conundrum: the trace fossil used to mark and correlate the base of the including whether the proposed boundary Cambrian, Treptichnus pedum, is restricted to siliciclastic facies, whereas position marks a major event in Earth histo- biomineralized fossils and chemostratigraphic signals are most commonly obtained ry. Sometimes the larger-scale meaning of from carbonate-dominated sections.Thus, it is difficult to correlate directly between the particular boundary can be lost during many of the Precambrian-Cambrian boundary sections, and to assess details of the the process of characterization. This is timing of evolutionary events that transpired during this interval of time.Thick demonstrated in a plot of PC-C boundary sections in the White-Inyo region of eastern California and western Nevada, USA, papers through time (Fig. 2): an initial contain mixed siliciclastic-carbonate lithofacies, and therefore promote correlation “gold rush” to publish on the boundary between these classic, well-studied lithologic end-members.An integrated occurred after the formation of the working stratigraphic approach was applied to the White-Inyo succession, combining group on the PC-C boundary in the early lithologic, paleontologic, and chemostratigraphic data, in order to address the 1970s; papers trailed off though the 1980s, temporal framework within the basin, and to facilitate worldwide correlation of the and plummeted after the GSSP was ratified boundary. Results from the southern Great Basin demonstrate that the negative by the International Union of Geological δ13C excursion that is ubiquitous in carbonate-dominated successions containing Sciences (IUGS) in 1992 (Rowland and small shelly fossils occurs within stratigraphic uncertainty of the first occurrence of Corsetti, 2002). As our understanding of T. pedum.This global geochemical marker thus provides a link with the primary this interval of Earth history grows, we biostratigraphic indicator for the Precambrian-Cambrian boundary. focus more on the “bigger picture” issues (e.g., evolution and diversification of the Metazoa), and focus less on the “boundary INTRODUCTION the GSSP (Gehling et al., 2001). issues.” However, we will inevitably seek tie The Precambrian-Cambrian (PC-C) transi- Approximately 70% of all PC-C boundary points with which to link fossils from silici- tion records one of the most important successions are siliciclastic (Landing, 1994). clastic sections to the geochemical and cli- intervals in the history of life, because it However, many carbonate successions mate-change data from carbonate-dominat- encompasses the appearance and diversifica- around the world have been more intensely ed sections so that we can improve our tion of metazoans, the invasion of the infau- studied because they record the advent of understanding of this critical interval. nal realm, the advent of biomineralization widespread biomineralization and easily Mixed siliciclastic-carbonate successions and predation, as well as dramatic isotopic obtainable δ13C chemostratigraphic records become crucial in the search for stratigraph- and atmospheric changes (Lipps and Signor, (summarized in Kaufman et al., 1997; ic tie points. Although a number of impor- 1992; Bengtson, 1994; Knoll and Carroll, Shields et al., 1997; Bartley et al., 1998; tant lower Cambrian sections containing 1999; Bottjer et al., 2000; Knoll, 2000; Shields, 1999; Corsetti and Hagadorn, mixed siliciclastic-carbonate successions are Babcock et al., 2001; Fig. 1). Here we draw a distinction between the PC-C transition, represented by the post-glacial terminal Proterozoic through the early Cambrian (ca. 600-520 Ma), and the PC-C boundary, a chronostratigraphic boundary represented by a point in rock (Global Standard-strato- type Section and Point, or GSSP) in the Fortune Head, Newfoundland, section (Landing, 1994). The GSSP section is com- posed predominantly of siliciclastics (Narbonne et al., 1987), and the fossil cho- sen to coincide with the boundary, Figure 1. Summary of pertinent features associated with the PC-C transition (data compiled principally Treptichnus (or Phycodes) pedum, is restricted from Droser and Bottjer, 1988; Crimes, 1992; Droser et al., 1999, 2002; Knoll and Carroll, 1999; to siliciclastic facies. T. pedum recently has McIlroy and Logan, 1999), demonstrating the biospheric changes represented in this interval. SSFs: been demonstrated to occur ~ 4 m below small shelly fossils. Arrow denotes the first appearance of vertically-oriented bioturbation. 4 | May 2003 The Sedimentary Record GEOLOGIC BACKGROUND From the time of Walcott (1908), the thick, superbly exposed, and highly fossiliferous Lower Cambrian strata from the southwest- ern United States (Figs. 3, 4, cover photo) have proved instrumental for understanding the Cambrian biotic explosion, and have even been suggested for a potential basal Cambrian stratotype (Cloud, 1973). Proterozoic—lower Paleozoic strata in the southern Great Basin were deposited on a thermally subsiding trailing margin created Figure 2. PC-C boundary publications through through rifting of the Laurentian craton in Figure 3. A. Map showing distribution of four time (constructed by searching GEOREF for the Neoproterozoic (e.g., Stewart, 1966, interfingering facies successions that span the ‘Precambrian-Cambrian,’ ‘Proterozoic- 1970; Stewart and Suczek, 1977; Armin PC-C interval in the southern Great Basin; suc- Cambrian,’ and ‘Neoproterozoic-Cambrian’ in and Mayer, 1983; Bond et al., 1985). cessions become progressively thicker to the the title). Neoproterozoic-Cambrian strata in the northwest, the offshore direction (after Nelson, southwestern United States thicken from 1978; Corsetti et al., 2000; Fedo and Cooper, known (e.g., Mongolia, Brasier et al., 1996; southeast to northwest, and can be grouped 2001). B. Shaded area represents the approxi- mate PC-C outcrop belt in the southern Great Olenik uplift, Siberia, Knoll et al., 1995; into four distinct but interfingering succes- Basin (after Stewart, 1970); the Mt. Dunfee Mackenzie Mountains, northwestern sions: Craton, Craton Margin, Death Valley section was offset to its present position by post- Canada, Narbonne and Aitken, 1995; (proximal-shelf), and White-Inyo (proxi- Cambrian transtensional faulting. southern Great Basin, Corsetti and mal- to mid-shelf) successions (Stewart, Hagadorn, 2000), not all of these contain 1970; Nelson, 1976, 1978; Mount et al., ous with respect to earliest Cambrian guide the PC-C boundary. Also, not all the sec- 1991; Corsetti and Hagadorn, 2000; Fedo fossils (e.g., Signor and Mount, 1986). tions contain an appropriate juxtaposition and Cooper, 2001; Fig. 3). An erosional dis- Trilobites, which are important guide fossils of carbonate- and trace-fossil-rich siliciclas- conformity removed the PC-C boundary in Lower Cambrian sections in the Great tic strata. Thick, relatively complete, well- interval in the Craton and Craton Margin Basin (e.g., Nelson, 1976; Palmer, 1981, exposed, and easily accessible successions in successions (Fedo and Cooper, 1990, 2001). 1998; Hollingsworth, 1999), make their the southern Great Basin contain siliciclastic This sequence boundary is traceable from first appearance well above this interval units and the boundary-marking fossil, T. the Craton Margin through the Death (Hollingsworth, 1999). pedum, in association with carbonate units Valley succession to the White Inyo succes- recording a complete δ13C chemostrati- sion, and probably represents the “Sauk I” White-Inyo Succession graphic profile (Corsetti and Kaufman, disconformity (Palmer, 1981). The bound- In ascending order, the White-Inyo 1994; Corsetti et al., 2000; Corsetti and ary interval resides below this unconformity Succession consists of the Wyman Hagadorn, 2000; Hagadorn and Waggoner, where incision was limited. The Death Formation, Reed Dolomite, Deep Spring 2000). The southern Great Basin sections Valley succession records both appropriate Formation, Campito Formation, Poleta provide an excellent opportunity to com- trace fossil biostratigraphy and carbon iso- Formation, Harkless Formation, and the pare results from a variety of stratigraphic tope data (Corsetti and Hagadorn, 2000). Mule Spring Limestone (Nelson, 1962; Fig. approaches that have emerged as useful for However, the sections are relatively thin and 4). The White-Inyo Mountains have been correlating potential stage and series bound- the carbonates from which the δ13C record the focus of intense PC-C boundary study, aries within the Cambrian (e.g., Geyer and was recovered are particularly thin.