Abstract
ABSTRACT The Boss Point Formation (Westphalian A) was deposited in a rapidly subsiding strike-slip basin. The formation, about 800 m thick, comprises alternate braidplain and lacustrine associations. Sixteen megacycles, defined by sharp but nonerosive lacustrine flooding surfaces, each comprise a lower lacustrine and an upper braidplain package. Braidplain deposits invariably scour deeply into lacustrine strata. The lacustrine association consists of very fine sandstone, mudstone, claystone, coal, and limestone deposited in shallow, hydrologically open freshwater lakes. Lakes were filled by small, elongate, river-dominated deltas, and by suspended and biogenic sediment. Sand-rich deltaic 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 accumulated platy gray mudstone. As interdistributary bays aggraded to water level they were subject to oxidation and pedogenic modification, forming bright colors, prismatic (desiccation) fabrics, pseudo-anticlines, and calcretes. Thin but laterally extensive (possibly up to 40 km) abandonment facies comprising black, organic-rich mudstones, coals, and ostracod limestones suggest periods of clastic starvation. Dunes, current ripples, and rarely adhesion ripples are common on the top surfaces of braidplain sandstone packages, preserved beneath a blanket of lacustrine mudstone. Rare oscillation ripples attest to localized reworking of the fluvial sand during or soon after flooding. Flooding surfaces can be traced for 12 km, and possibly up to 40 km, and are attributed to instantaneous subsidence along the northwest margin of the basin during major earthquakes. One locality preserves evidence of abrupt, 180° paleoflow reversals above and below a lacustrine unit. This is interpreted to record abrupt basin tilting, first to the northwest, forming a lake that filled from the southeast, followed by tilting to the southeast. We interpret Boss Point megacycles to reflect the interaction of episo ic tectonic subsidence with longer-term changes in sediment supply controlled by cyclical climatic variation in the Milankovitch band. During periods of pronounced wet-dry seasonality when clastic supply was high, sandy and gravelly braidplain deposits prevailed, and any lakes that formed by earthquake-related subsidence were rapidly filled. During more humid periods, higher vegetative cover greatly reduced clastic supply, and lakes persisted for many thousands of years. Hydrologic changes accompanying the transition from relatively humid to strongly seasonal wet-dry periods might have resulted in the deep fluvial erosion observed at the base of each braidplain package.
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