The geological context of the Lower Cambrian (Series 2) Emu Bay Shale Lagerstatte and adjacent stratigraphic units, Kangaroo Island, South Australia

Abstract

The lower Cambrian (Cambrian Series 2, Stage 4) Emu Bay Shale Lagerstätte, which is by far the most important Burgess Shale-type (BST) deposit in Australia, occurs mainly in the bottom 10 m of the Emu Bay Shale at Big Gully on the north coast of Kangaroo Island, South Australia. In this area, the exposed Cambrian succession commences with the White Point Conglomerate, the bulk of which comprises a crudely cross-bedded cobble to boulder conglomerate with minor mudstone and sandstone facies. The conglomeratic horizons thin markedly to the south. The White Point Conglomerate was deposited as coalesced fan deltas derived from an uplifted tectonic margin immediately to the north of the present coastline. The White Point Conglomerate is overlain by the sandstone, siltstone and conglomerate beds of the Marsden Sandstone (new name), the basal 3 m of which is a distinctive fossiliferous argillaceous limestone and shale, the Rouge Mudstone Member (new name). Syndepositional folding and faulting affected both the White Point Conglomerate and Marsden Sandstone prior to the deposition of the Emu Bay Shale, the base of which represents a sequence boundary. The Lagerstätte occurs within dark grey to black laminated micaceous mudstone facies, some of which show evidence of syndepositional disturbance, and are interpreted to have been deposited in isolated stagnant, anoxic to oxic depressions on the sea floor, beneath a normally oxic water column, with a sharp redox boundary at the sediment-water interface; below this boundary the pore water was anoxic. Thin (up to 20 cm) structureless fine sandstone horizons within the mudstone are interpreted as either sediment gravity flow or storm deposits. The Lagerstätte-bearing mudstone beds thin southwards and disappear 500-600 m south of the coast. The Emu Bay Shale coarsens upwards; arthropod tracks are abundant in fine sandstone beds towards the top of the Emu Bay Shale. In coastal sections the sandstone facies of the Boxing Bay Formation rest conformably on the Emu Bay Shale; inland the contact is channelled. © 2011 Geological Society of Australia.