Significance of hummocky cross‐stratification in the Permian of the Carnarvon Basin, Western Australia

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Abstract The Lower Permian Byro Group and lower Coolkilya Sandstone of the Carnarvon Basin comprise a 1400 m thick, marine‐shelf sequence of sandstone, siltstone and shale. The sequence is repetitive and is characterised by six facies: 1. black‐shale facies (representing the lower offshore environment);2. grey‐siltstone facies (upper offshore environment);3. bioturbated‐sandstone facies (transitional environment);4. laminated‐to‐burrowed facies (lower shoreface environment);5. hummocky‐cross‐stratified sandstone facies (middle shoreface environment); and6. swaley‐cross‐stratified sandstone facies (middle to upper shoreface environment). The hummocky‐cross‐stratified facies is characterised by low‐angle (0–15°), inclined laminations that show both concave‐upwards and convex‐upwards curvature. The hummocky cross‐stratification (HCS) is interpreted as resulting from sub‐polar storms, although the intensity of the storms is uncertain. The development and preservation of HCS in the sequence is attributed to a delicate balance between storm activity and biogenic processes. Turbidites, although commonly associated with HCS in other basins, are poorly represented in the Byro Group and lower Coolkilya Sandstone. The paucity of turbidites is attributed to (a) strong bioturbation in the offshore zone, which obliterated the thinner turbidites, and (b) a general inability of storm‐generated turbidity currents to carry significant amounts of sand into water depths below storm wave base. Variations in the abundance and character of HCS between formations is principally due to variation in water depth during deposition, amalgamated sets of HCS developing in shallow water, inner shelf environments (e.g., Mallens, upper Nalbia and lower Coolkilya Sandstones). More subtle variations in the type and proportion of HCS within certain formations appear to reflect variations in depositional rate (e.g., Nalbia Sandstone) or palaeoclimate (e.g., Wandagee Formation). Although a complex origin is suggested for HCS in the Byro and Kennedy Groups, the overall facies association and palaeogeography suggest that a stable tectonic setting, low palaeoslope and low to moderate rate of sediment input were important factors in the evolution of the sequence.