The Middle to Late Eocene Chumstick Formation represents a thick sequence of fluvial and lacustrine rocks that filled a tectonically active basin under humid-tropical paleoclimatic conditions. Four facies associations are recognized in the Chumstick Formation: gravel-bedload, sand-bedload, and mixed-load stream deposits and lacustrine deposits. The depositional system is interpreted as a humid-tropical alluvial-fan system. Five sets of basinal characteristics are believed to result from the combination of humid-tropical paleoclimate, active basin-margin faulting, and high (0.2 to 1.2 m/k.y.) basin subsidence rates. 1) Fluvial architecture was strongly affected by basin-margin faulting. Proximal regions of the fan consist of broadly lenticular, laterally stacked (i.e., offlapping ribbons), gravel-bedload stream deposits representing fanhead channels. Distal regions of the fan consist of gravel- and sand-bedload stream deposits with a sheet-like geometry. The basin-fill is dominated by multistory, vertically stacked, sand-bedload and mixed-load stream deposits and lacustrine deposits. 2) Rapid subsidence has resulted in the preservation of compositionally immature sediments (sublithic, feldspathic arenites) with thin, shallow paleosols. 3) Paleohydraulic reconstructions indicate that streams carried coarse-grained bedload through channel reaches with paleoslopes < 5 m/km. 4) Rapid vegetation growth resulted in the bank stabilization of sandy sediments, development of paleosols, leaf-litter layers on emergent bar-tops, and the introduction of woody debris into channels. Even coarse-grained, proximal fluvial deposits have a significant overbank component and show evidence for bank stability, cutbanks, and cohesive bank failures. 5) Debris-flow deposits are relatively minor, except for local talus-cone deposits. The rarity of debris flows throughout the basin may be due to hillslope stability imparted by vegetation.

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