SHALLOW SHELF BIOSILICEOUS SEDIMENTATION: A TERRESTRIAL ARIDITY RECORD?

Shallow shelf biosiliceous sedimentation occurs periodically throughout the rock record, and is dominated by demosponges that today inhabit deep water environments. Across 2000 km of the Late Eocene southern Australia margin, deep water siliceous demosponges populated estuarine, and protected shoreface to inner shelf environments. These sediments grade outboard to open shelf habitats with a typical cool-water calcareous biota. Scarce calcareous fossils in nearshore habitats, plus authigenic mineral assemblages, confirm that sponges were able to outcompete calcareous organisms because of enhanced dissolved silica concentrations. The facies distribution points to a terrestrial silica source. Marine salinities up to 200 km upstream from estuary mouths indicate that despite the humid Late Eocene climate, silica did not come from active run-off, but instead was sourced through groundwater outflow. Groundwaters in humid climates, however, are generally silica-poor. In contrast, groundwaters from mid-latitude arid zones are of high salinity with enhanced silica solubility and concentrations (up to 100 ppm Si in modern southern Australian saline groundwater). In Middle and early Late Eocene time a vast area of interior Australia was subjected to a prolonged episode of mid latitude aridity and therefore salt and silica-rich groundwater generation, now represented by the Colville silcrete. We hypothesize that the onset of latest Eocene humidity flushed Colville saline, silica-enriched groundwaters into nearshore environments, which aided biosiliceous sponges without eutrophication leading to photoautotroph competition. Pangean and Tethyan sponges also thrived in mid latitude marine settings during shallow shelf biosiliceous spiculite sedimentation, suggesting that arid phases and concomitant saline weathering also influenced their production. Shallow shelf biosiliceous sedimentation may, therefore, be a record of terrestrial silica export associated with mid latitude arid to humid climate fluctuations.