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Fluvial Lateral Accretion Deposits, Scroll Bars and Stacking. How Much Do We Know?


Fluvial lateral accretion deposits provide a record of past river activity, and are an important target for hydrocarbon exploration in continental successions. Thousands of lateral accretion deposits (or lateral accretion packages; LAPs) have been interpreted in the ancient record. Surprisingly few (less than 10) detailed investigations of Quaternary scroll bar and lateral accretion deposit sedimentology have been found in the published literature. Studies of Quaternary depositional systems as analogues for the ancient record are of great importance, given our ability to link the characteristics of deposits to known environmental controls. Considering the resource importance of LAP, the lack of modern analogue investigations is surprising. In light of this, it is unsurprising that the control of depositional variations on subsurface fluid-flow behaviour remains poorly understood. We present a review of published literature, and present new findings from two recent studies, led by the authors, of Quaternary scrolled LAPs (on the Fitzroy River, north-eastern Australia, and the Mitchell River upper delta plain, northern Australia). Both of these studies have uncovered different aspects of the studied deposits that vary substantially from described scrolled lateral accretion packages in the literature. One presents lateral variations in sedimentology that do not fit with current scroll bar models, and describes the stacking of a secondary channel LAP over the primary channel migration ∼ 20 ka previously. The other describes an internal deposit architecture that is interpreted to have been modified (compared with existing models) by development of chute channels through swales in this highly seasonal system. Based on our research findings and literature review, we conclude by adding a new model summarising key research questions raised and proposing an explanation for the internal sedimentology of fluvial lateral accretion deposits under different conditions to those previously described.