The geometry, flow patterns and sedimentary processes of Devonian rivers and coasts, New York and Pennsylvania, USA

Abstract

AbstractThe Middle-Upper Devonian strata of New York and Pennsylvania were deposited in a foreland basin adjacent to the Acadian fold-thrust belt. Fluvial (Catskill) strata in the east thin to the west into marine (Chemung) strata. The overall sequence is marine regressive, but there numerous superimposed marine transgressions. The Catskill river channels changed character with distance from the palaeoshoreline. Sinuous, single-channel rivers (widths tens of metres, maximum bankfull depths 4–5 m, sinuosity 1.1–1.3, mean bankfull flow velocity 0.4–0.7 m s−1) occurred near the coast. With increasing distance from the coast, rivers increased in slope, became wider (up to hundreds of metres) and deeper (up to 15 m), coarser grained, and possibly braided. This suggests that rivers were distributive near the coast. Furthermore, muddy floodplain areas were more extensive near the coast than up-valley, such that the proportion of channel deposits increased up-valley. Coastal areas comprised tide-influenced channels and mouth bars, wave-influenced sandy shoals and washovers, sandy and muddy tidal flats, and muddy interdistributary bays and lakes. Major marine transgressions are thus represented in wholly fluvial successions by a decrease in the proportion of channel sandstone bodies, and a decrease in channel size and grain size. Marine transgressions in near-coastal successions are represented in places by an increase in sandstone proportion associated with progradation of coastal sands over muddy backswamps and interdistributary areas. Small-scale marine transgressions and regressions in coastal deposits (i.e. metres-thick parasequences) are probably associated with switching of coastal channels or delta lobes. There is no evidence for the preservation of incised valleys, nor for any changes in coastal channel patterns associated with these parasequences: therefore, major regional sea-level changes were not responsible. However, large-scale (tens to hundreds of metres) transgressive-regressive sequences were due partly to eustatic sea-level change (e.g. in mid-Givetian time) but also due to changes in the balance between sediment supply and subsidence associated with tectonic uplift of the hinterland and peripheral bulge. Such uplift may have induced some kind of climate change. However, climate changes in the depositional basin are not supported by sedimentological evidence. Further understanding of the origin of these strata will require higher precision in the biostratigraphic correlation of marine and non-marine strata (using miospores), and quantitative interpretation of depositional environments from other regions.