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Authors requiring further information regarding Elsevier’s archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright Author's personal copy Palaeogeography, Palaeoclimatology, Palaeoecology 264 (2008) 100–122 Contents lists available at ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo Microstratigraphy, trilobite biostratinomy, and depositional environment of the “Lower Cambrian” Ruin Wash Lagerstätte, Pioche Formation, Nevada Mark Webster a,⁎, Robert R. Gaines b, Nigel C. Hughes c a Department of the Geophysical Sciences, University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60637, United States b Geology Department, Pomona College, 185 E. Sixth Street, Claremont, CA 91711, United States c Department of Earth Sciences, University of California, Riverside, CA 92521, United States ARTICLE INFO ABSTRACT Article history: The uppermost 43 cm of Dyeran strata at the Ruin Wash Lagerstätte (Chief Range, Lincoln County, Nevada) Received 13 November 2007 contain nonmineralized invertebrates and exceptionally preserved, articulated olenelloid trilobites. However, Received in revised form 4 March 2008 the environmental factors responsible for the preservation of olenelloids in this unusual state at Ruin Wash Accepted 3 April 2008 have received little study and are therefore poorly understood. Microstratigraphic analysis of this interval reveals that the strata, comprised almost exclusively of clay-sized particles and rather monotonous and Keywords: featureless in outcrop, were deposited as a series of thin event beds interpreted as tempestites. The energy of Cambrian Trilobita deposition progressively waned then waxed through the interval, and benthic redox conditions shifted Taphonomy concurrently. Biostratinomic data for more than 500 trilobite specimens encountered in the measured Biostratinomy section reveal that the trilobite fauna was autochthonous, although winnowed surfaces rich in bioclasts Microstratigraphy occurring at the base and top of the section indicate intervals of prolonged pre-burial exposure time and/or Ichnology in situ reworking. Trilobite biostratinomy was strongly influenced by subtle environmental shifts: the degree of disarticulation and sclerite fracturing correlate positively with event bed energy and inferred bottom- water oxygen content. These results demonstrate that (1) preservation of articulated trilobites is favoured near the distal limit of scouring associated with depositional events, where rapid sedimentation occurs in the absence of substrate reworking; and (2) differential taphonomic overprint on trilobite preservation can occur within a monofacial succession, driven by subtle environmental change insufficient to leave a distinct lithological signature. This highlights the need for careful microstratigraphic, sedimentological, and ichnological analysis prior to comparison of palaeobiological or palaeoecological parameters inferred from fossil assemblages, even when the assemblages occur within the same lithofacies. © 2008 Elsevier B.V. All rights reserved. 1. Introduction order to carefully document the occurrence of the fossil biota and to recover a large number of specimens for research. Articulated Strata of the uppermost 43 cm of the Combined Metals Member trilobites can provide a wealth of palaeobiological and palaeoecolo- (Pioche Formation, uppermost Dyeran Stage, Cambrian) at the Ruin gical information, and the material collected during this and earlier Wash locality, Chief Range, Lincoln County, Nevada (Fig. 1) contain a excavations at Ruin Wash has already provided insight into issues of diverse fauna of excellently preserved olenelloid trilobites (Palmer, taphonomy (Webster and Hughes, 1999), systematics and phylogeny 1998a; Fig. 2) and other invertebrates (Lieberman, 2003). This short (Webster et al., 2001; Paterson and Edgecombe, 2006), and palaeo- stratigraphic interval qualifies as both a concentration Lagerstätte (on biology (Palmer, 1998a; Webster et al., 2001; Webster and Zelditch, account of the abundance of trilobite remains) and a conservation 2005; Webster, 2007a). However, the depositional setting of the Lagerstätte (on account of the preservation of nonmineralized Lagerstätte, and consequently the environmental controls responsible organisms and of olenelloid trilobites in an unusual, articulated for the exquisite fossil preservation, has received little attention. state). In March 1999, a multi-institutional consortium involving the The present paper documents the microstratigraphy, sedimentology, University of California, Riverside (UCR), the Museum of Comparative ichnology, and trilobite biostratinomy of the Ruin Wash Lagerstätte, Zoology at Harvard University (MCZ), and the Peabody Museum at which together permit detailed inference of the depositional environ- Yale University (YPM) in collaboration with the Bureau of Land ment in which this unit accumulated. Here, we aim to improve Management (BLM) conducted an extensive excavation of the site in understanding of the relationship between depositional setting and trilobite preservation; environmental factors responsible for the ⁎ Corresponding author. preservation of the nonmineralized organisms will be discussed else- E-mail address: [email protected] (M. Webster). where. Our integrative study demonstrates a strong dependence of 0031-0182/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2008.04.002 Author's personal copy M. Webster et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 264 (2008) 100–122 101 Fig. 1. A, Map showing location of the Ruin Wash Lagerstätte on the western flank of the Chief Range, Lincoln County, Nevada, southwestern United States. Other localities mentioned in the text are shown (see also Palmer,1998a). Black line with triangles mark eastern limit of thrusting of Sevier Orogenic Belt (overthrust block to west). B, Detailed map of the Ruin Wash locality showing the trenches from which the invertebrate fauna has been recovered. The detailed excavation described herein was conducted at the “taphonomy trench” (Fig. 3). Sites at Klondike Gap to the south represent the closest well-studied outcrop of correlative strata (Webster, 2007b), but yield fossils of unremarkable preservational quality. Map created with TOPO! software (© National Geographic, 2002). trilobite preservation on subtle changes in depositional conditions (Fig. 1b). The material collected is now housed in the Geology Museum within this short, monofacial section. Resolving such environmental at UCR, the MCZ, the YPM, the Field Museum of Natural History, controls over trilobite preservation is important for understanding the Chicago (FMNH), and the Institute for Cambrian Studies, Department distribution of trilobite Lagerstätten and of complete trilobites in of the Geophysical Sciences, University of Chicago (ICS). general, and for identifying potential taphonomic bias when comparing The “taphonomy trench” refers to a pit systematically excavated for palaeobiological and palaeoecological parameters inferred from fossil the specific purpose of collecting biostratinomic data during the 1999 assemblages. operation. The pit exposed a rock surface approximately 2 m by 2 m in area (Fig. 3). The stratigraphically highest bed within the trench was 2. Materials and methods the ribbon limestone marking the base of the Comet Shale Member (basal Delamaran Stage; Palmer, 1998b; Sundberg and McCollum, The taphonomic data presented herein stem from the “taphonomy 2000), which immediately overlies the Ruin Wash Lagerstätte (Fig. 4). trench” dug by MW and NCH as part of the major excavation in 1999 Over the course of a week, the trench was deepened by progressively Author's personal copy 102 M. Webster et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 264 (2008) 100–122 Fig. 2. Exceptionally preserved olenelloid trilobites from the Ruin Wash Lagerstätte. A, B, Clusters of large, articulated cephalothoraces of Olenellus gilberti and Olenellus chiefensis. Specimens on each slab show a hint of preferred orientations in two orthogonal directions (roughly top/bottom and left/right as illustrated). Several of the specimens retain a hypostome in life position, and are interpreted as minimally disturbed carcasses. A, Internal moulds; ICS-1044.50. B, Latex cast of ICS-1044.10. C, Slightly disarticulated dorsal exoskeleton of O. chiefensis; internal mould; UCR 9945.70. D, Complete dorsal exoskeleton of O. gilberti, including opisthothorax and pygidium; internal mould; UCR 9945.43. E, Minimally disrupted articulated exoskeleton of Nephrolenellus geniculatus with hypostome retained in life position, interpreted as a carcass; internal mould; FMNH PE57943. F, Minimally disrupted articulated exoskeleton of N. geniculatus with hypostome and rostral plate rotated 180° ventrally (pivoting at the base of the genal spines) relative to life position and now