Quaternary stratigraphy, palaeowinds and palaeoenvironments of carbonate aeolianite on the Garden Island Ridge and in the Naturaliste-Leeuwin region, southwest Western Australia

Deposition of carbonate aeolianite has long been constrained by research models based on the isolated carbonate platforms of Bermuda and the Bahamas; however, models for Quaternary carbonate aeolianites differ depending on the interplay of several key factors that reflect location-specific circumstances. Consequently, real steps forward in carbonate aeolianite research require that the depositional system profile (e.g. steeprimmed platform, ramps), source (e.g. sediment supply) and processes (e.g. transportation, deposition and preservation of sediment) be understood and the timing of aeolianite deposition be constrained. Accordingly, these parameters are targeted in this research of carbonate aeolianite in the distally steepened ramp-like setting of Western Australia. The research conducted here is focussed on the Tamala Limestone of Western Australia, which is known for the thickest intervals of Pleistocene carbonate aeolianite in Australia and the most extensive aeolianite deposit of this age in the world (Ward, 1975; Brooke, 2001). Quaternary deposits in Western Australia form a complex depositional system composed of carbonate aeolianite, protosols, palaeosols, calcretes and marine deposits formed during different glacioeustatic sea levels. Two regions within the Tamala Limestone are investigated in this research, the Garden Island Ridge within the Perth region and the Naturaliste-Leeuwin region of the Margaret River area. The laterally discontinuous nature of the dune packages and the lack of distinctive marker horizons make unravelling the sedimentary sequence of the aeolian deposits complicated. To improve understanding of carbonate aeolianite transportation, deposition and preservation and the impact of these processes on the depositional model, the strategy employed here involves interrogation of the dune packages set within a chronostratigraphic framework. The temporal framework for this study is acquired through the application of whole-rock amino acid racemisation (AAR) dating. The calibrated age estimates of dune formation on the Garden Island Ridge show that major episodes of dune formation are attributed to interglacial highstands Marine Isotope Stage (MIS) 5 and MIS1 – more specifically during MIS5e/5d, MIS5a and MIS1. The depositional system profile of the Rottnest Shelf and the AAR dates of aeolian samples indicate that the formation of dunes on the Garden Island Ridge were linked to stands of sea level higher than at least ii the present 40 m isobath. AAR dating refines the palaeo-sea level during the MIS1 dune depositional episode to a sea level higher than at least the 10 m isobath. The Pleistocene-Holocene succession on the Garden Island Ridge comprises four broad facies – dune accumulation deposits, palaeosols, calcretes and marine deposits – and records three successive depositional episodes (MIS5d/5e, MIS5a and MIS1) that are separated by hiatal intervals. The MIS5d/5e depositional episode consists of transgressive-phase carbonate aeolianites, a marine highstand unit (Rottnest Limestone Member) and regressive-phase carbonate aeolianites bounded on top by a calcrete horizon with deep solution pipes and a terra rossa palaeosol. The MIS5a depositional episode comprises mostly composite sets of regressive-phase carbonate aeolianites that are usually separated by erosional contacts and/or protosols. The long hiatial period in dune development between MIS5a and MIS1 is marked by a discontinuous calcrete marker horizon and a discontinuous black palaeosol. The youngest depositional episode of MIS1 includes transgressive-phase aeolianites, marine limestones (Hershcell Limestone Member) and other unconsolidated deposits (i.e. swamp and beach sand deposits). The carbonate aeolianites of these depositional episodes exhibit a similar mix of skeletal and shell grains of Bryozoa, foraminifera, coralline algae, Mollusca, corals and echinoderms with a minor proportion of quartz grains. The approach taken in evaluating the stratigraphy and its palaeoenvironmental implications for the Tamala Limestone on the Garden Island Ridge in the Perth Basin was extended to the Tamala Limestone within the Naturaliste-Leeuwin region. The sedimentary succession at Whitecliff was deposited from frequent and short periods of carbonate dune sand accumulation and soil development. At Shelly Beach, a large, single set of aeolian cross-strata was deposited upon a red palaeosol and conglomeratic limestone that is most probably emplaced during MIS11 and is comparable to the Cowaramup Conglomerate. The calibrated AAR age estimates of dune formation at Whitecliff and Shelly Beach in the Naturaliste-Leeuwin region show that episodes of dune formation are attributed to the overall regressive phase of MIS5 at Whitecliff and phases during MIS8e, MIS9 or the regressive phase of MIS11 at Shelly Beach. The depositional system profile of the Naturaliste-Leeuwin shelf and the AAR dates of aeolian samples indicate that the formation of dunes was linked to stands of sea levels higher than the present 30 m isobath. The carbonate aeolianites in both regions are characterised by moderately sorted, medium sand with grainsize distributions that are finely skewed and mesokurtic, much like modern carbonate dune sediment in other world locations. The carbonate aeolianites display sedimentological characteristics that are consistent with an origin by aeolian transportation sourced from a marine environment and provide a detailed insight into sediment supply, transportation and deposition of sediment and its subsequent preservation. The carbonate aeolianites were sourced from the upwind beach environment, which was continually refreshed by sediment from the adjoining shallow marine shelf. The dominant source of sediment to the carbonate aeolianites was an enduring, highly productive, Foramol subtropical-temperate carbonate province on the Rottnest and Naturaliste shelves that was sensitive to the relative strength and position of the warm Leeuwin Current. The insoluble fraction of the carbonate aeolianite is dominated by terrigenous particles attributed to the fluvial discharge of nearby rivers and was not derived from dust from the mainland carried by offshore winds. Partial cementation of aeolianites occurred under freshwater or mixed vadose conditions, as indicated by the dominance of pendant meniscus, grain contact meniscus and partially rimmed cements. The foreset dips of large- and very-large scale tabular crossbeds and sedimentological analysis show that carbonate dune formation and migration occurred from persistent "fresh" winds (>9 m/s) and record the influence of the westerly wind belt of the Ferrel cell. However, on the Garden Island Ridge, dune formation and migration also records the influence of the southeast trade wind belt of the Hadley cell during MIS5e. The movement of the Hadley cell over the Garden Island Ridge is consistent with that expected to accompany the change from a glacial to an interglacial climate at the interglacial highstand of MIS5e. The Quaternary deposits in the distally steepened ramp-like morphologies of the Perth Basin show that the depositional model is controlled by a very sensitive relationship between climate, depositional system profile and pattern of sea-level change. The amount and composition of the sediment available for aeolian deposition is ultimately controlled by sea level – when the regions were adjacent to a shallow sea, 10-30/40 m deep that formed a highly productive Foramol subtropical to temperate carbonate province. However, the size and location of the aeolian accumulations are influenced by palaeoclimate, the depositional system profile and in the Leeuwin Region by the topography of the Leeuwin Block. The very sensitive relationship between climate, depositional system profile and the pattern of sea-level change controlled sediment production, mobilization, deposition and preservation