The Cambrian Radiation and the
Diversification of Sedimentary Fabrics
Mary L. Droser and Xing Li
The Cambrian represents a pivotal point in the history of marine sedimentary rocks. Cambrian biofabrics that are directly a product of metazoans include ichnofabrics, shell beds, and constructional frameworks. The development and distribution of biofabrics is strongly controlled by sedimentary facies. In particular, terrigenous clastics and carbonates reveal very different early records of biofabrics. This is particularly obvious with ichnofabrics but equally important with shell beds. Ichnofabrics in highenergy sandstones (e.g., Skolithos piperock) and fine-grained terrigenous clastic sediments can be well bioturbated at the base of the Cambrian, whereas other settings show less well developed bioturbation in the earliest Cambrian. Nearly all settings demonstrate an increase in extent of bioturbation and tiering depth and complexity through the Cambrian. Shell beds appear with the earliest skeletonized metazoans. Data from the Basin and Range Province of the western United States demonstrate that shell beds increase in thickness, abundance, and complexity through the Cambrian. The study of biofabrics is an exciting venue for future research. This is particularly true of the latest Precambrian and Cambrian, where biofabrics have been relatively underutilized in our exploration to find the relationships between physical, chemical, and biological processes and the Cambrian explosion. Biofabrics provide a natural link between these processes.
WITH THE CAMBRIAN RADIATION of marine invertebrates, sedimentary rocks on this planet changed forever. The advent of skeletonized metazoans introduced shells and skeletons as sedimentary particles, and the tremendous increase in burrowing metazoans resulted in the partial or complete mixing of sediment and/or in the production of new sedimentary structures. Whereas constructional frameworks formed by stromatolites were common in the Precambrian (e.g., Awramik 1991; Grotzinger and Knoll 1995), metazoan reef builders first appeared near the PrecambrianCambrian boundary, initiating complex reef fabrics in Early Cambrian time (Riding