Development of the sediment mixed layer in deep time: a reading of the Ediacaran-Cambrian boundary type section

The mixed layer is a zone of fully homogenized sediment resulting from intense bio-turbation. Below this zone, a transitional layer is characterized by discrete burrows. In modern settings, the mixed layer thickness varies according to depositional environments and controlling factors impacting on the benthos. The mixed layer was originally thought to appear at the start of the Fortunian (540 Ma), but recent studies estimate its development at the base of the Cambrian Age 2 (530 Ma) or after the Silurian (420 Ma). The Cambrian type section in Newfoundland, Canada, contains exquisitely preserved trace fossils. High resolution sedimentologic and ichnologic datasets were gathered in this section to assess depositional environments and ichnofabric development. Three lines of evidence allow the identification of a sediment mixed layer at the base of the Cambrian Stage 2. (1) Intervals of fully bioturbated sediment are identified in polished samples. (2) Sandstone tempestites host sharp-walled burrows, have diffuse tops, and are capped by homogeneous sandy silt-stone, which is interpreted as being totally bioturbated. (3) Reconstruction of ichnofabrics allows the identification of mid-and deep-tier trace fossils with sharp outlines typical of the transitional layer overprinting on the mixed layer. The mixed layer in this succession is best identified below the reach of storm waves (shelf environment), but also in the lower and upper offshore. These results illustrate how empirical approaches and robust description and interpretation of sedimentologic and ichnologic datasets from various depositional settings are necessary to unravel the colonization of the marine seafloor by early animals.