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Miocene Depositional Architecture in the Mixed Clastic-Carbonate Rankin Platform: Implications for Clinoform Development and Morphodynamic Diversity

Abstract

A study of slope-confined gullies that dissect a prograding mixed clastic-carbonate, Miocene-age clinothem succession on the Northwest Shelf of Australia, has enabled a better understanding of the linkage between the geometry and stacking pattern of upper-slope gullies and the processes which took place between the shelf-break and upper-slope which gave rise to them. By integrating a 1300 km2 three dimensional seismic reflection survey, well logs and core data, we define the scale and spatial distribution of slope-confined gully systems in the middle Miocene. The evolution of the wedge-shaped progradational sequences dissected by stacked slope gullies has been constrained by detailed seismic facies mapping and shelf-edge trajectory analysis. High-resolution seismic-reflection sections and slices were used in Petrel® to measure variations in width, depth, and inter-gully spacing of these submarine gullies over a sample area covered by five profile lines 4.5 Km apart. Measurements of variations in gully width, depth and inter-gully spacing within clinoform sets suggests that fluctuations in relative sea level (which correspond to obliquity-driven Milankovitch sea-level cycles), and changes in latitude and sediment supply-productivity are the key controls impacting their development. Gullies are usually straight, U-shaped and evenly-spaced, and extend from the shelf-edge to the 400-m isobath. Gully-confined episodes of deposition resulted in aggrading gully systems with a few asymmetrical forms. Average gully width is between 150 – 400 m and gullies commonly display relief of 25 – 110 m Lateral variability in the geometry and stacking patterns of the gullies appears to correlate with along-strike variations in clinoform dip. The clinoforms are relatively steep and locally associated with concave upwards updip slope failures. Gullies are absent where the gradient is less than 6o; 100 – 300 m wide where the gradient is 3 o to 5 o, and between 300 m to 800 m wide where the gradient exceeds 8 o. The recognition of an erosional surface and gully-confined episodes of deposition on the upper-slope suggests that gullies may have been maintained by dispersive sedimentation, and sheds more light on the role slope gullies play in the construction basin margins. We propose that gullies were initiated and maintained by erosive downslope-directed sediment gravity flows concentrated on facies heterogeneities close to the rollover.