Microstratigraphy, trilobite biostratinomy, and depositional environment of the 'Lower Cambrian' Ruin Wash Lagerstatte, Pioche Formation, Nevada

摘要

The uppermost 43 cm of Dyeran strata at the Ruin Wash Lagerstatte (Chief Range, Lincoln County, Nevada) contain nonmineralized invertebrates and exceptionally preserved, articulated olenelloid trilobites. However, the environmental factors responsible for the preservation of olenelloids in this unusual state at Ruin Wash 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 featureless in outcrop, were deposited as a series of thin event beds interpreted as tempestites. The energy of deposition progressively waned then waxed through the interval, and benthic redox conditions shifted concurrently. Biostratinomic data for more than 500 trilobite specimens encountered in the measured section reveal that the trilobite fauna was autochthonous, although winnowed surfaces rich in bioclasts occurring at the base and top of the section indicate intervals of prolonged pre-burial exposure time and/or 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 ichnoiogical analysis prior to comparison of palaeobiological or palaeoecological parameters inferred from fossil assemblages, even when the assemblages occur within the same lithofacies.