ABSTRACT

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 arenite ) 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.