TECTONIC EVENT STRATIGRAPHY IN A FLUVIOLACUSTRINE, STRIKE-SLIP SETTING - THE BOSS POINT FORMATION (WESTPHALIAN-A), CUMBERLAND BASIN, MARITIME CANADA

The Boss Point Formation (Westphalian A) was deposited in a rapidly su bsiding strike-slip basin. The formation, about 800 m thick, comprises alternate braidplain and lacustrine facies associations. Sixteen mega cycles, defined by sharp but nonerosive lacustrine flooding surfaces, each comprise a lower lacustrine and an upper braidplain package. Brai dplain deposits invariably scour deeply into lacustrine strata. The la custrine facies association consists of very fine sandstone, mudstone, claystone, coal, and limestone deposited in shallow, hydrologically o pen freshwater lakes. Lakes were filled by small, elongate, river-domi nated deltas, and by suspended and biogenic sediment. Sand-rich deltai c facies represent mouth bar, subaqueous levee, crevasse channel and s play, and straight and sinuous distributary-channel subenvironments. E vidence of wave reworking is very scarce. Interdeltaic areas accumulat ed platy gray mudstone. As interdistributary bays aggraded to water le vel they were subject to oxidation and pedogenic modification, forming bright colors, prismatic (desiccation) fabrics, pseudo-anticlines, an d calcretes. Thin but laterally extensive (possibly up to 40 km) ''aba ndonment facies'' comprising black, organic-rich mudstones, coals, and ostracod limestones suggest periods of clastic starvation. Dunes, cur rent ripples, and rarely adhesion ripples are common on the top surfac es of braidplain sandstone packages, preserved beneath a blanket of la custrine mudstone. Rare oscillation ripples attest to localized rework ing of the fluvial sand during or soon after flooding. Flooding surfac es can be traced for 12 km, and possibly up to 40 km, and are attribut ed to instantaneous subsidence along the northwest margin of the basin during major earthquakes. One locality preserves evidence of abrupt, 180-degrees paleoflow reversals above and below a lacustrine unit. Thi s is intrepreted to record abrupt basin tilting, first to the northwes t, forming a lake that filled from the southeast, followed by tilting to the southeast. We interpret Boss Point megacycles to reflect the in teraction of episodic tectonic subsidence with longer-term changes in sediment supply controlled by cyclical climatic variation in the Milan kovitch band. During periods of pronounced wet-dry seasonality when cl astic supply was high, sandy and gravelly braidplain deposits prevaile d, and any lakes that formed by eartquake-related subsidence were rapi dly filled. During more humid periods, higher vegetative cover greatly reduced clastic supply, and lakes persisted for many thousands of yea rs. Hydrologic changes accompanying the transition from relatively hum id to strongly seasonal wet-dry periods might have resulted in the dee p fluvial erosion observed at the base of each braidplain package.