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Seismic characterization of Eocene-Oligocene heterozoan carbonate strata from the Browse Basin, Northwest Shelf of Australia, defines marked progradation of nearly 10 km. Stratal terminations and stacking subdivide the succession into mappable seismic units. Stratal architecture and seismic geomorphology varies systematically through the succession. Individual surfaces, discerned by toplap, onlap, and truncation, outline sigmoidal to tangential oblique clinoforms with heights of ranging from 350 to 650 m and maximum gradients between 8 and 18°. Sigmoidal clinoforms can include aggradation in excess of ∼200 m, prograde more than 500 m, and have slopes characterized by inclined, wavy to discontinuous reflectors that represent ubiquitous gullies and channels. In contrast, the overlying tangential oblique clinoforms include downstepped shelf margins, limited on-shelf aggradation (<100 m) and toplap, subdued progradation (<500 m), and continuous parallel inclined reflectors on the slope. Wedges of basinally restricted reflectors at toe of slope onlap surfaces of pronounced erosional truncation or syndepositional structural modification. The succession includes repeated patterns of seismic units that onlap, aggrade, and prograde, interpreted to represent sequence sets and composite sequences. The associations of shelf aggradation, shelf-margin progradation, and slope channeling within sigmoidal seismic units and the less marked progradation and channeling within tangential oblique seismic units contrast with the classic sequence model in which sediment delivery to the slope and pronounced progradation is favored by limited shelf accommodation. This distinct divergence is interpreted to reflect the prolific heterozoan production across the shelf during periods of rising and high base level when the shelf is flooded, perhaps enhanced by downwelling. Comparison with purely photozoan systems reveals similarities and contrasts in seismic stratigraphic heterogeneity and architecture, interpreted to be driven by distinct characteristics of heterozoan sedimentary systems. [ABSTRACT FROM AUTHOR]

The North West Shelf is an ocean-facing carbonate ramp that lies in a warm-water setting adjacent to an arid hinterland of moderate to low relief. The sea floor is strongly affected by cyclonic storms, long-period swells and large internal tides, resulting in preferentially accumulating coarse-grained sediments. Circulation is dominated by the south-flowing, low-salinity Leeuwin Current, upwelling associated with the Indian Ocean Gyre, seaward-flowing saline bottom waters generated by seasonal evaporation, and flashy fluvial discharge. Sediments are palimpsest, a variable mixture of relict, stranded and Holocene grains. Relict intraclasts, both skeletal and lithic, interpreted as having formed during sea-level highstands of Marine Isotope Stages (MIS) 3 and 4, are now localized to the mid-ramp. The most conspicuous stranded particles are ooids and peloids, which 14C dating shows formed at 15.4-12.7 Ka, in somewhat saline waters dining initial stages of post-Last Glacial Maximum (LGM) sea-level rise. It appears that initiation of Leeuwin Current flow with its relatively less saline, but oceanic waters arrested ooid formation such that subsequent benthic Holocene sediment is principally biofragmental, with sedimentation localized to the inner ramp and a ridge of planktic foraminifera offshore. Inner-ramp deposits are a mixture of heterozoan and photozoan elements. Depositional facies reflect episodic environmental perturbation by riverine-derived sediments and nutrients, resulting in a mixed habitat of oligotrophic (coral reefs and large benthic foraminifera) and mesotrophic (macroalgae and bryozoans) indicators. Holocene mid-ramp sediment is heterozoan in character, bill sparse, most probably because of the periodic seaward flow of saline bottom waters generated by coastal evaporation. Holocene miter-ramp sediment is mainly pelagic, veneering shallow-water sediments of Marine Isotope Stage 2, including LGM deposits. Phosphate accumulations at 200 m water depth suggest periodic upwelling or Fe-redox pumping, whereas enhanced near-surface productivity, probably associated with the interaction between the Leeuwin Current and Indian Ocean surface water, results in a linear ridge of pelagic sediment at 140 m water depth. This ramp depositional system in an arid climate has important applications for the geological record: inner-ramp sediments can contain important heterozoan elements, mid-ramp sediments with bedforms created by internal tides can form in water depths exceeding 50 m, saline outflow can arrest or dramatically slow mid-ramp sedimentation mimicking maximum flooding intervals, and outer-ramp planktic productivity can generate locally important fine-grained carbonate sediment bodies. [ABSTRACT FROM AUTHOR]

The acoustic properties of heterozoan and photozoan carbonates are compared and interpreted in the light of their distinct patterns of diagenetic evolution. The combined role of depositional and diagenetic processes on the petroacoustic signatures of both heterozoan and photozoan carbonates has been assessed on the basis of a multi-disciplinary and multi-scale study of Oligo-Miocene limestone outcrops from the Cape Range peninsula (NW Australia). This study integrates a detailed diagenetic analysis (using sedimentology, petrography and stable isotopes geochemistry), as well as physical properties characterization (through acoustic velocity laboratory and porosity measurements, petroacoustic modelling and 3D CT-scan imaging). The three formations studied are characterized by distinct patterns of diagenetic and petroacoustic evolution: heterozoan, non-tropical carbonates from the Mandu and Tulki formations are characterized by significant development of microporosity and pressure-solution compaction features which led to decreasing the pore network stiffness. Photozoan, sub-tropical to tropical carbonates from the Trealla formation are dominated by moldic, vuggy and partially cemented intergranular pores which tends to increase the pore network stiffness. The distinct petroacoustic signatures of heterozoan and photozoan limestones have been shown to be mainly related to (1) differences in compaction during burial resulting from an emersion phase and (2) differences in the mineralogy of dominant carbonate producers. Temperate High-Mg calcite dominated carbonate typically poorly cemented allowed for greater compaction degree, reducing the ratio of intergranular macropore. Furthermore, these dominant calcite producers (large benthic foraminifers, red algae ...) from the heterozoan Mandu and Tulki formations favoured the development of intragranular microporosity whereas less-stable aragonite dominated biota from the photozoan Trealla Limestone favoured the development of molds and vugs, during early meteoric diagenesis. Finally, petroacoustic modelling approaches such as α K inversion have proved to be efficient for quantifying the impact of diagenetic processes and associated pore types on acoustic properties. These results, emphasizing the strong variability in depositional and diagenetic features for distinct carbonate fabrics could have interesting implications in terms of reservoir properties 3D modelling and upscaling approaches. • We studied the petroacoustic signature of Oligo-Miocene limestones from NW Australia. • Heterozoan & photozoan carbonates are characterized by distinct diagenetic processes. • Distinct diagenetic paths led to different dominant pore types and acoustic signatures. • 2 main causes explain it: drastic change in carbonate factory type and emersion phase. • Pore types were successfully retrieved by equivalent pore aspect ratio modelling. [ABSTRACT FROM AUTHOR]