Mid-Paleozoic Trilobite Lagerstatten: Models of Diagenetically Enhanced Obrution Deposits

Abstract

Spectacular trilobite Lagerstätten 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.

The depositional environment and taphonomy of the Homerian “Aulacopleura shales” fossil assemblage near Loděnice, Czech Republic (Prague Basin, Perunican microcontinent)

“…Isolated sclerites of A. koninckii are rare, and on almost all bedding surfaces exoskeletons are predominantly partially articulated and lack both alignment and sclerite fragmentation. The occurrence of A. koninckii conforms in many ways to the characteristics of a Type I trilobite lagerstätte of Brett et al (2012). The presence of enrolled A. koninckii suggests that final burial may have resulted from relatively rapid obrution, although the condition of partial articulation indicates that many carcasses or exuviae partially disaggregated before burial.…”

“…Episodes of deposition yielded enough material to entomb and preserve articulated specimens (and presumably also kill those specimens preserved in enrolled posture), but to do so without significantly disrupting, fragmenting, aligning, or winnowing shelly material at the sediment-water interface. The Na Černidlech assemblage thus belongs to the Type I trilobite lagerstätten of Brett et al (2012) that are characterized by clusters of partially articulated exoskeletons. Such assemblages have been interpreted as the product of burial events induced by hypopycnal flows (see Kohrs et al 2008).…”

“…The mean length of articulated dorsal shields from both collections is also comparable. The pattern of articulation described above suggests that bedding planes within the 1.4 m interval represent a relatively narrow range of taphonomic states compared to the broad spectrum seen amongst trilobites as a whole (see Brett et al 2012). Nevertheless, analyses of individual slabs show that bedding surfaces do vary notably in certain characteristics.…”

“…The preservation of both body and moult clusters have been linked to environments in which benthic oxygen levels were generally low, and in some cases the preservation of body clusters has been suggested to relate to mass mortality due to insufficient oxygen (Brett et al 2012, Chatterton & Ludvigsen 1998, Gutiérrez-Marco et al 2009. A particularly apt comparison may be that made with a study of the occurrence of the "ptychopariid" trilobite Elrathia kingii from middle Cambrian rocks of the Wheeler Shale of Utah (Gaines & Droser 2003).…”

A Formação Serra Alta, Permiano, no centro-leste do Estado de São Paulo, Bacia do Paraná, Brasil

“…Simões e Torello (2003) interpretaram níveis fossilíferos semelhantes, presentes na base da Formação Corumbataí, como depositados por tempestades, durante as quais correntes de retorno carreariam fósseis para águas profundas e distais (Bondioli et al no prelo). Nessa interpretação, níveis de coquina ou horizontes delgados contendo fragmentos fósseis (fácies Sl) são produto de material retrabalhado por tempestades em ambientes de águas rasas e transportado para o interior da bacia por correntes de alta energia, com deposição rápida por decantação devido à desaceleração do fluxo, num processo semelhante à remobilização de material bioclástico para áreas mais profundas da bacia interpretada por Brett et al (2012aBrett et al ( , 2012b.…”