Abstract

Black shale is characteristic of early siliciclastic fill along the distal (western) flank of the Middle to Late Ordovician Taconic peripheral foreland basin. This facies, referred to as the Utica black-shale magnafacies, includes at least five intervals bounded by unconformities and/or condensed beds. Each unit records a pulse of subsidence and subsequent siliciclastic sedimentation in the foreland basin. Progressive episodic shifts from carbonate to organic-rich mud deposition may be due to steepening and subsidence of the western carbonate ramp followed by onlap of axially dispersed sediment.

In general, silt-poor, organic-rich mud was deposited on a subsided carbonate ramp that sloped eastward toward the nearest siliciclastic sediment source. This mud may have accumulated from the pelitic tails of axial fan turbidites derived from source areas to the east, and possibly the south. The resultant black shale does not necessarily represent the deepest part of the basin; the deepest part of the basin (the basin axis) contained coarser-grained, axial, submarine fan deposits.

Throughout the later part of the Middle Ordovician and the first half of the Late Ordovician, the basin axis migrated westward over 100 km, yet the orientation of the axis and the basin margins were persistent. This basin orientation (generally parallel to the orogenic belt), along with the large-scale basin shape, resulted from a combination of deformational loading of the continental margin and progressive flexure in the foreland. Smaller-scale structural elements, normal fault-bounded basement blocks, were superposed on large-scale Taconic foreland-basin geometry. Abrupt shifts from carbonate ramp sedimentation to deeper-water, organic-rich mud deposition may reflect movement along these faults during the orogeny.

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