Siliciclastic sedimentation on a storm- and tide-influenced shelf and shoreline: the Early Devonian Roxburgh Formation, NE Lachlan Fold Belt, southeastern Australia

Abstract

The Early Devonian (late Lochkovian) Roxburgh Formation of the northeast Lachlan Fold Belt, New South Wales, southeastern Australia, is a >750 m thick, regressive, largely siliciclastic sequence deposited on a volcanically active platform (the Capertee High) which flanked a deep marine basin (the Hill End Trough). Facies analysis reveals ten lithofacies representing a variety of depositional environments, ranging from storm-dominated middle to outer shelf (Facies A and B) through inner shelf (Facies D) to amalgamated hummocky shoreface (Facies E), along with a shoaling wave-dominated shoreface (Facies F), tide-dominated shoreface to?tidal inlet complex (Facies G), and foreshore (Facies H). Minor contemporaneous silicic volcanism is indicated by sporadic occurrences of subaqueously-deposited accretionary lapilli and ashfall tuff (Facies C), together with gravelly volcaniclastic deltas (Facies J) which prograded into the nearshore zone, probably following eruptions. Rare bodies of biostromal limestone also occur (Facies I). Facies A, D, and I contain locally rich shallow marine faunas which taphonomic studies show to have been strongly influenced by storm events. Facies sequences are characterised by ubiquitous, high-frequency cyclic alternations of facies (parasequences); these typically comprise stacked, highly asymmetric, partial shelf progradational sequences which are 3.5–15 m thick and dominantly regressive in character. Sections devoid of facies cyclicity are also present. The setting for deposition was a west-sloping shelf flanked to the immediate east by a coastal plain and further east by a mixed silicic volcanic and sedimentary basement source terrain. Storm waves approached the NNE-SSW-trending shoreline from the northwest, generating a coastal set-up that produced offshore-directed bottom return currents; these were deflected consistently to the left (flowing to the southwest and west) as they flowed across the shelf probably due to geostrophic veering caused by Coriolis Force. During post-storm conditions, asymmetric shoaling waves on the shoreface drove lunate megaripples (3-D dunes) east-southeastward, towards the shoreline, balanced by oblique offshore rip currents and longshore currents. Tidal currents became predominant on the shoreface in the upper parts of the Roxburgh Formation. The study are had a low palaeolatitude and was probably positioned propitiously to receive frequent long-period, hurricane-generated waves approaching, down the Hill End Trough, from the north.