HANGING CRYPTIC TUBULAR SPONGES IN THE RED CRUST OF THE LOWER CAMBRIAN FLINDERS RANGES, SOUTH AUSTRALIA

The Flinders Unconformity (FU) is a regional unconformity that can be traced across the central Flinders Ranges in the lower Cambrian (Series 2, Stage 3) carbonate successions of the Arrowie Basin, South Australia. It separates the underlying biohermal Wilkawillina Limestone from overlying slope and turbidite facies of the Mernmerna Formation and is easily recognized in the field by a “reddened crust” directly capping the Wilkawillina Limestone. This red crust is a decimeter-thick layer with hematitic laminations, ferruginous microstromatolites, cryptobionts, and sponges. Past studies have described the FU as an erosional surface associated with subaerial exposure, while some recent studies have suggested that it is a marine hardground. Though the laminated ferrimicrobialites and associated have been documented in some detail, associated sponge and cavity-fill fauna have received minimal prior study. Here, we present preliminary findings on a new sponge taxon from a cavity within the Wilkawillina Limestone formed in association with the FU. We reconstruct the gross morphology in three dimensions through serial grinding and imaging, followed by segmentation. These methods reveal clusters of elongate tubular sponges growing downward from the roof and walls of a cavity below the hematite-rich laminae. The sponges are distinct from contemporaneous calcareous Gravestockia pharetronensis documented within the laminated red-crust. While acid maceration of a subsample reveals no obvious spicules, EDS maps show the presence of rounded silica grains within the sponge forms, suggesting it is a demosponge.