Abstract

The Boss Point Formation is of Westphalian A age and was deposited in a strike-slip basin undergoing rapid subsidence. The formation is about 800 m thick and comprises alternating braidplain and lacustrine (LFA) facies associations. Sixteen megacycles, bounded by sharp but non-erosive lacustrine flooding surfaces, each comprise a lower lacustrine and an upper braidplain package. Braidplain deposits invariably scour deeply into lacustrine strata. The LFA consists of very fine sandstone, mudstone claystone, coal and limestone which were deposited in very shallow, hydrologically open freshwater lakes. Lakes were filled by small, elongate river-dominated deltas. Sandy deltaic facies represent mouth bar, subaqueous levee, crevasse channel and splay and straight and sinuous distributary channel subenvironments. Evidence of wave-reworking is very scarce. Interdeltaic areas were up to 10 m deep and accumulated platy gray mudstone. As interdistributary bays aggraded to water level they were subject to oxidation and pedogenic modification, forming bright colours, prismatic (desiccation) fabrics, pseudo-anticlines and calcretes. Thin but laterally-extensive (up to ?40 km) "abandonment facies" comprising black, organic rich mudstones, coals and ostracod limestones, suggest periods of clastic starvation. Lacustrine flooding surfaces commonly preserve dunes, current ripples and rarely adhesion ripples beneath a blanket of mud. Rare oscillation ripples attest to localised reworking of the fluvial sand during or soon after flooding. Flooding surfaces can be traced for up to 40 km and are attributed to instantaneous subsidence along the NW margin of the basin during major earthquakes. One locality preserves evidence of abrupt, 1800 paleoflow reversals above and below a lacustrine unit. This is interpreted to record abrupt basin tilting, first to the NW, forming a lake which filled from the SE, followed by tilting to the SE, causing marginal streams to flow in the same direction. We suggest that Boss Point megacycles (mean duration 62,500 yrs) reflect the interaction of tectonic subsidence and sediment supply controlled by cyclical climatic changes in the Milankovitch band. Pulses of earthquake-related subsidence were probably very common during Boss Point deposition. During periods of pronounced wet-dry seasonality, clastic production and supply were very high; braidplain deposition prevailed and any lakes formed by subsidence abrupt were rapidly infilled by sandy deltas. During semi-arid periods, clastic supply was much diminished and tectonically-generated lakes persisted for many thousands of years, accumulating mainly suspended and biogenic sediment. Hydrologic changes accompanying the transition from semi-arid to wet-dry periods might have resulted in deep fluvial erosion into lacustrine deposits.

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