DISTAL SEDIMENTATION IN A PERIPHERAL FORELAND BASIN - ORDOVICIAN BLACK SHALES AND ASSOCIATED FLYSCH OF THE WESTERN TACONIC FORELAND, NEW-YORK-STATE AND ONTARIO

Black shale is characteristic of early siliciclastic fill along the di stal (western) flank of the Middle to Late Ordovician Taconic peripher al foreland basin. This facies, referred to as the Utica black-shale m agnafacies, includes at least five intervals bounded by unconformities and/or condensed beds. Each unit records a pulse of subsidence and su bsequent siliciclastic sedimentation in the foreland basin. Progressiv e episodic shifts from carbonate to organic-rich mud deposition may be due to steepening and subsidence of the western carbonate ramp follow ed by onlap of axially dispersed sediment. In general, silt-poor, orga nic-rich mud was deposited on a subsided carbonate ramp that sloped ea stward toward the nearest siliciclastic sediment source. This mud may have accumulated from the pelitic tails of axial fan turbidites derive d from source areas to the east, and possibly the south. The resultant black shale does not necessarily represent the deepest part of the ba sin; the deepest part of the basin (the basin axis) contained coarser- grained, axial, submarine fan deposits. Throughout the later part of t he Middle Ordovician and the first half of the Late Ordovician, the ba sin axis migrated westward over 100 km, yet the orientation of the axi s and the basin margins were persistent. This basin orientation (gener ally parallel to the orogenic belt), along with the large-scale basin shape, resulted from a combination of deformational loading of the con tinental margin and progressive flexure in the foreland. Smaller-scale structural elements, normal fault-bounded basement blocks, were super posed on large-scale Taconic foreland-basin geometry. Abrupt shifts fr om carbonate ramp sedimentation to deeper-water, organic-rich mud depo sition may reflect movement along these faults during the orogeny.