The Quassaic Group, a Medial to Late Ordovician arenite sequence in the Marlboro Mountains Outlier, mid‐Hudson Valley, New York, U.S.A

Abstract The Quassaic Group occurs as an outlier in the north‐south trending Marlboro Syncline which extends 40 km between Kingston and Newburgh in southeastern New York. The group is comprised primarily of 3050 m of late Medial to medial Late Ordovician marine arenites. It is subdivided (oldest to youngest) into five formations: Creek Locks, Rifton, Shaupeneak. Slab Sides, and Chodikee. On the western limb of the syncline the group overlies a minimum of 610 m of the early late Medial Ordovician Bushkill Shale and the bottom three formations grade north and south to Bushkill Shale. Abundance of graded but occasionally conglomeratic beds, common tabular cross‐lamination, much less common trough cross‐lamination, comparative lack of channelling, general lack of shales, and scarcity of fossils indicate that the average depositional environment of the formations was near the base of the slope of a sedimentary apron or delta which lay to the southeast. The environment, however, probably extended from a basin plain, in the marginal Bushkill Shale, through the lower and middle slope and, rarely, to the upper slope. The occasional molasse‐like deposits in the Shaupeneak Formation may indicate local transport of molasse to an upper slope position. The Quassaic Group thus records an initial, local extension of arenites of the lower slope of an apron or delta onto a downwarping basin plain in the Bushkill Trough. This was followed by progressive enlargement and extension of the delta slope with attendant shallowing, culminated by the appearance of molasse‐like sediments. Subsequent deposition involved greater slope extension followed by recession with gradual deepening. These activities were prelude to the Hudson Valley Phase of the Taconian Orogeny which downfolded the Quassaic Group into the Marlboro Syncline during latest Ordovician times.