Abstract

The Frasnian Catskill magnafacies of the Catskill region, New York State, represents alluvium deposited in a foreland basin adjacent to the Acadian fold-thrust belt. Facies characteristics vary in space and time in association with a series of transgressive-regressive sequences on the order of 10 2 m thick. Away from the paleoshoreline and upsection, the channel-belt sandstone bodies within a given regressive sequence show 1) an increase in mean grain size from fine to medium and coarse sand; 2) an increase in amount of large-scale cross-stratification relative to small-scale cross-stratification and planar stratification; 3) a decrease in the inclination of lateral-accretion surfaces and more common truncation of their upper parts by chute channel fills; 4) an increase in the proportion of sandstone-filled channels relative to those filled with sandstone plus mudstone. The overbank sandstone-mudstone beds concomitantly increase in mean grain size. The proportion of channel-belt deposits relative to overbank deposits increases slightly with distance from the paleoshoreline, and the degree of overlap (connectedness) of channel-belt sandstone bodies increases markedly. Paleohydraulic analysis shows that channels had low sinuosity (less than 1.25) and were predominantly unbraided. Channels ranged from tens to hundreds of meters wide, and were meters in depth. Channel slopes ranged from approximately 0.0001 to 0.0003, increasing with mean grain size of channel deposits, but decreasing as bankfull discharge increased. Bankfull mean velocity varied little from 0.5 to 0.7 m/s. With distance from the paleoshoreline, channels decreased in sinuosity and increased in sediment size, slope, and braiding index. Increase in proportion and connectedness of channel-belt deposits with distance from the paleoshoreline probably reflects upvalley increase in channel-belt-width/floodplain-width ratio rather than variation in deposition rate/avulsion frequency ratio. During regression and transgression these spatial facies changes are reflected in, respectively, coarsening-upward and fining-upward vertical sequences. Eustasy probably had a minor influence on Frasnian regressive-transgressive sequences compared to the effects of varying rates of tectonic uplift, sediment supply, and subsidence. There is no evidence of long-term climatic change, either in the distribution of river-channel geometries or in the distribution of paleosols and plant and animal activity on the extensive floodplains.

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