Abstract

The Givetian Plattekill and Manorkill Formations (680 m thick) exposed in the Catskill Front, New York State, record fluvial deposition on a broad alluvial plain adjacent to the rising Acadian Mountains. Channel-belt deposits are represented mainly by single storey sandstone bodies; overbank deposits are represented by interbedded sandstone and mudstone. The overall sequence is marine regressive: the proportion, maximum storey thickness and mean grain size of the sandstone bodies generally increase upsection, as does the proportion of sandstone in the overbank deposits. The abundance and maturity of calcareous paleosols generally decrease upsection. These regional trends are associated with increase in alluvial plain slope, deposition rate, and channel size and discharge. Local 100 m scale cyclic variations in proportion of channel belt deposits may be related to local variation in avulsion/ deposition rate, floodplain/channel-belt width, or tectonism; they cannot be related to regional sea level changes.

The Frasnian part of the Catskill sequence also shows episodic variations in fluvial depositional style, and they become more marked with distance from the paleoshoreline. Comparison with the Frasnian parts of the Catskill sequence suggests that the Givetian vertical trends reflect increasing distance from the paleoshoreline with time. Increasing channel sizes with distance from the paleoshoreline may reflect a distributary channel system. Progradation of the alluvial plain, associated with regression, is related to episodic variations in tectonic uplift, sediment supply and basin subsidence. Unambiguous evidence for climatic or eustatic sea level changes in these transgressive-regressive cycles is lacking. However, there is a critical need for more extensive examination of adjacent coeval deposits, and more refined age dating, before detailed examination of extrabasinal controls can be undertaken.