MID-PALEOZOIC TRILOBITE LAGERSTATTEN: MODELS OF DIAGENETICALLY ENHANCED OBRUTION DEPOSITS

Abstract

Spectacular trilobite Lagerstatten occur in distinctive offshore calcareous mudstone facies through the Late Ordovician to Devonian, and reflect a combination of mass mortality or molting and burial, coupled with early diagenetic enhancement. Evidence indicates two distinct modes of burial, Type I and II assemblages, which show evidence for burial without or with seafloor disturbance, respectively. Type I assemblages suggest rapid (hours to days), but not instantaneous burial, without bottom disruption, enabling preservation of in situ behavior, including mass aggregations and molt ensembles. Most occurrences contain bedding planes in which trilobites exhibit incipient disarticulation. These assemblages were buried by cascades of flocculated sediment from hypopycnal, detached flows. Type II assemblages show well-articulated, enrolled, semi-enrolled, and outstretched trilobites in varied orientations relative to bedding. In such cases, bottom flows and seafloor disruption by storm or seismic disturbances in shallow waters suspended large amounts of flocculated muds as viscous slurries, which developed into hyperpycnal flows that entrained carcasses of trilobites and other organisms. In many cases, both Type I and II obrution was followed by additional sedimentation, geochemical zones moved upward through the sediment column, and there was little tendency to form diagenetic overprints. Alternatively, if burial was followed by an interval of sediment starvation, the sediments were bioturbated and very early diagenetic mineralization was superimposed, first, in rare cases, as mineralized soft parts in entombed carcasses, and later as pyritization of burrow linings. Development of the concretionary layers required more prolonged periods of stability of the sulfate reduction zone. Cementation of sediment shielded organism bodies from most or all effects of compaction. Thus, ironically, the best preservation of delicate remains required rapid burial, associated with mass mortality, and very low rates of background sedimentation following the event.