Organic facies of holocene carbonates in North Stromatolite Lake, Coorong region, South Australia

Abstract

Numerous small, shallow, ephemeral lakes are scattered along the seaward margin of the extensive Pleistocene beach-dune ridge system that comprises the coastal plain of southeastern South Australia. Fed primarily by seasonal inflow of alkaline ground water, these lakes are sites of Holocene carbonate deposition. North Stromatolite Lake is part of a chain of such lakes located immediately south of Salt Creek, within the first interdunal corridor landward of the Coorong Lagoon. Here four different sedimentary facies (basal quartzose skeletal packstone; organic-rich mudstone with thin sapropel layers; laminated pelletal mudstone; and massive pelletal wackestone/mudstone) occur within a shoaling-upwards carbonate cycle (~3 metres thick) and define the following vertical succession of environments: estuarine, density-stratified lacustrine, perennial lacustrine and, finally, ephemeral lacustrine. Acquisition of X-ray diffraction, total organic carbon (TOC), Rock-Eval pyrolysis, visual kerogen and biomarker hydrocarbon data on sediment samples (n = 35) from a single core taken near the centre of the lake has allowed the recognition of five discrete organic facies, eachwith a distinctive mineralogy. The organic-rich unit (6− 12% TOC) may be subdivided into organic facies 1 (Type I/II kerogen) and 2 (Type II kerogen), whereas the organically leaner laminated and massive units are distinctly bimodal hosting both organic facies 3 (Type II/III kerogen) and 4 (Type III kerogen). The latter two facies together define an inverse relationship between hydrogen index and TOC content, a geochemical signature that may be attributed to differences in the extent of pelletisation of carbonate muds by a diverse salt-tolerant fauna including brine shrimp, gastropods and ostracods during the shallowing perennial and ephemeral phases of the Lake's history. The basal unit hosts organic facies 5 (Type IV kerogen). The aliphatic hydrocarbon distributions of these lacustrine sediments are dominated by C20 and C25 highly branched isoprenoids; and C12− C33 n-alkanes displaying marked odd/even predominance above, and even/odd predominance below, C20. This biomarker assemblage reflects the respective major contributions of bacillariophyceae (diatoms), chlorophyceae (green algae) and eubacteria (including cyanobacteria) to their preserved organic matter. Its passage through the guts of the aforementioned grazers and excretion as faecal pellets has dramatically enhanced the relative abundance of cholest-2-ene, thereby imparting to the pelletised upper sapropel, laminated and massive units a hitherto unrecognised molecular signature. This signature of ingestion may remain in such micritic limestones, even where their original pelleted texture has been obliterated by the physical compaction that accompanies early burial and diagenesis. Of wider significance is our finding that lacustrine sediments containing high levels of hydrogen-rich protokerogen may have accumulated beneath relatively shallow bottom waters (<5 m deep) that were not perennially anoxic.