The importance of inherited shelf bathymetry, pycnoclines, and wave-enhanced sediment dispersal perturbations as part of the taphonomic process

Cambrian sedimentary rocks on the Yangtze Platform host a treasure trove of paleogeographic and paleobiologic information. The latter, in form of exceptionally well-preserved soft-bodied fossils, provide critical insights into the origins of modern life. Studies of the taphonomic processes governing the preservation of these fossils have largely focused on the primary chemical conditions in ocean basins and the diagenetic alterations through time, all of which influence the information that can be extracted from these fossils. Here we present results from integrated sedimentologic and geochemical analyses of the Cambrian Maotianshan Shale in Yunnan Province, China. We propose that episodic, storm-derived, highly concentrated intermediate nepheloid layers (INL) developed along a pycnocline over intra-shelf basins on the Yangtze Platform. This interpretation is based on the observation that massive, clay-mineral-rich mudstones (structureless mudstones) in many instances show no direct relationship to coarser-grained (silt- and sandstones) facies that are interpreted as density flow deposits. Rather the structureless mudstones are embedded within other mudstones (biotic mudstones) that represent the background sedimentation in the basin. These INL events provide a possible mechanism to explain the apparent near simultaneous encapsulation of epifaunal, benthic, and nektonic fossils of the Chengjiang fauna.