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New Approaches to Understanding the Mechanics of Burgess Shale

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New Approaches to Understanding the Mechanics of Burgess Shale The Sedimentary Record platforms, at sharp shelf-slope breaks (Conway New Approaches to Morris, 1998; Rees, 1986). Classic models for the Burgess Shale have considered the deposi- tional environment to be fully anoxic, due to Understanding the the exquisite preservation of fossils, and the dark color of the mudrocks (e.g., Conway Morris, 1986). This implies that the faunas Mechanics of Burgess were transported, yet some assemblages of fos- sils, such as the well-known Ogygopsis trilobite beds in the Burgess Shale and horizons that Shale-type Deposits: contain delicate sponges, clearly occur in situ. Important questions, key to a first-order understanding the biotas in an ecological sense, have remained: Can discrete, paleoeco- From the Micron Scale to logically meaningful assemblages be resolved from within the homogeneous sediments? the Global Picture Were bottom water oxygen conditions suffi- Robert R. Gaines, Department of Geology, Pomona College, Claremont, CA 91711; cient to permit episodic, frequent, or sustained [email protected] benthic colonization by in situ faunas? Mary L. Droser, Department of Earth Sciences-036, University of California, Because of the extraordinary importance of Riverside, CA 92521 these biotas, a better knowledge of the rela- tionships of the organisms to the paleoenvi- ABSTRACT ronments that they inhabited and/or facilitat- Cambrian Burgess Shale-type (BST) deposits are among the most significant deposits for ed their preservation is desirable. understanding the “Cambrian explosion” because they contain the fossilized tissues of In order to begin to address these interrelat- nonmineralized organisms and provide a substantially different window on the radiation ed problems, we focused on the middle of the Metazoa than is afforded by the more “typical” fossil record of skeletal parts of Cambrian Wheeler Formation of western biomineralized organisms. Despite nearly a century of research, BST deposits remain Utah, which contains abundant Burgess Shale- poorly investigated as sedimentologic entities largely because they comprise fine-grained type preservation of nonmineralized fossils, mudrocks. Here, we describe a new, integrative approach to understanding a single BST including common macroscopic algae and a deposit, the middle Cambrian Wheeler Formation of Utah, which reveals a dynamic diverse fauna of more than 20 genera of more interplay of paleoenvironmental, paleoecologic, and sedimentologic/diagentic factors rarely occurring arthropods, priapulids, within a superficially homogeneous lithofacies.This millimeter-scale microstratigraphic sponges, and cnidarians (Robison, 1991). It and paleontologic approach is augmented by both outcrop and microscopic study.These also contains a well-described and diverse types of data are applicable to issues of quite different scales, including micron-scale dia- skeletal fauna (Robison, 1964, 1991). The for- genetic processes involved in fossil preservation, organism-environment interactions and mation is well exposed and readily accessible paleoecology of the early Metazoa, and regional and global controls on the distribution over a broad area in the House Range (39°15’ of BST deposits. N, 113°20’ W) and immediate vicinity. The study area is arid and also has not been sub- BURGESS SHALE-TYPE time of the “Cambrian explosion.” In the jected to extensive compressive stress. As a DEPOSITS:THE GLOBAL rock record, this interval is marked by the rela- result, mudrocks of the Wheeler Formation PICTURE tively sudden appearance of fossils represent- are less strongly altered by the effects of tec- Cambrian lagerstätten (Conway Morris, 1989) ing most animal phyla, the acquisition of tonics and weathering than are other impor- provide a fortuitous window into one of the skeletal hard parts by a number of animal tant BST deposits. most dramatic episodes in evolutionary histo- groups, and the advent of predation and com- Mudrocks present unique challenges to ry. Represented most famously by the Burgess plex metazoan in ecosystems (e.g., Seilacher, study. Unlike other types of sedimentary rocks Shale (middle Cambrian of British Columbia), 1997; Zhuravlev and Riding, 2001; Babcock, that are more readily interpreted in the field, these deposits are noteworthy for two major 2003). In this paper, we present a unified mudrocks most often appear featureless and reasons. First, they provide an excellent record framework for understanding the dynamic massive. Thus, in a stratigraphic context, of ancient biodiversity because they contain interplay of sedimentological, paleoecological, mudrocks are commonly interpreted to repre- fossils that are not preserved under normal cir- and paleoenvironmental factors that con- sent static, monotonous conditions, or are cumstances. Whereas the “normal” fossil trolled the preservation and distribution of occasionally subdivided on the superficial basis record is almost exclusively a record of hard, fossils in a single Burgess Shale-type deposit, of color. Mudrocks comprise the bulk (60%) mineralized skeletal parts, these deposits pre- the Wheeler Formation (middle Cambrian) of of the sedimentary record, yet remain poorly serve some labile, nonmineralized tissues of western Utah. understood (Potter et al., 1980). However, organisms, including those lacking hard skele- recent methodological advances in the study tal parts. Secondly, these deposits, which are NEW APPROACHES of mudrocks have demonstrated that inten- rare in the geologic record as a whole, are Many BST deposits, including the Wheeler sive, fine-scale approaches can yield much moderately abundant in lower and middle Formation and the Burgess Shale, represent information about depositional processes and Cambrian strata, an interval spanning the deposition just offshore of broad carbonate paleoenvironments (e.g., Schieber 2003). The 4 | June 2005 The Sedimentary Record suite of depositional environments represented by mudrocks is typically characterized by low- energy deposition, relatively continuous sedi- mentation, and, often, little current reworking in comparison to coarser-grained deposits. Furthermore, mudrocks often contain in situ fossil assemblages, making them particularly good candidates for paleoecological and pale- oenvironmental study. An integrative, millimeter-scale, methodol- ogy, incorporating aspects of sedimentology and paleontology was applied. In the field, short (1-5 m) intervals were targeted for inten- sive study and sampled continuously in dupli- cate. In the laboratory, these short sections were reconstructed and slabbed parallel to bedding, then etched lightly with dilute HCl to reveal primary and secondary sedimentary features. Using direct analysis of the slabbed samples in combination with thin section study, the sections were logged on a millimeter scale. The duplicate sets of samples were split along bedding planes for paleontological analysis, and the microstratigraphic positions of fossils were incorporated into the logs. These techniques were complemented by compositional (XRD, EDX, coulometry), geo- chemical (δ13C; δ18O), microscopic (SEM, flu- Figure 1. Microstratigraphic logs from the Wheeler Formation illustrating the context of nonmineralized orescent light, cathodoluminescent), and preservation. A. 10-cm interval with complete absence of bioturbation and Burgess Shale-type preservation in most beds, typical of sustained anoxic benthic conditions in the Wheeler Formation. B. imaging (X-ray) analyses. 20-cm interval showing cm-scale oscillations in bioturbation, bottom water oxygen, and, accordingly, fossil content. Most nonmineralized metazoan fossils occur where favorable preservational environments PALEOENVIRONMENTS are closely interbedded with other types of beds. AND BOTTOM-WATER OXYGEN “couplets” ranging from 1 to 12 mm in thick- primary sedimentary structures (Savrda et al. The Wheeler Formation accumulated in what ness, with no apparent size or compositional 1984). Extrapolating ichnological models to has been interpreted as a fault-bounded grading; and 4, uninterrupted vertical persist- Cambrian sediments requires caution, because trough, termed the House Range embayment, ence for tens of meters of section, indicating the infaunal habitat was incompletely devel- at the edge of an expansive carbonate platform sustained low-energy deposition below the oped at this time (Droser and Bottjer, 1988). (Rees, 1986). At most localities, the Wheeler influence of storm waves. The “couplets” of Thus, this model was used as a relative tool, to Formation is composed of homogenous, fine- the Wheeler Formation are interpreted as the compare oxygen levels among beds within the grained mudrocks, with smaller thicknesses of distal expression of gravity-dominated deposi- Wheeler Formation. This model was applied thin-bedded, fine-grained carbonates. Burgess tional events spilling off of the adjacent ramp- on a bed-to-bed basis in order to develop rela- Shale-type biotas occur only in the mudrock however, they also may contain a hemipelagic tive oxygen curves at a millimeter scale for facies. Wheeler mudrocks, which represent the component. Some evidence for bottom-flow- each of the continuously sampled intervals. lowest energy deposits found in the region, ing currents is present, including transported The Wheeler Formation is inferred to have represent deposition in a basin adjacent to the (see below) and current aligned fossils (Rees, had a complex redox history during deposi- distal end of a carbonate ramp (Rees, 1986). 1986), and rare tool
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