Origin of Megaphytoclast Concentrations In Coarse-Grained Turbidites, Cretaceous of Northern California

Abstract

Coarse-grained, medium- to thick-bedded turbidites in the Franciscan Complex contain carbonized plant debris concentrated in the upper parts of some of the beds. These concentrations typically occur at the top of a thick, graded to massive sand subdivision, and may be covered by a thin, parallel-laminated silt or silty sand subdivision. Plant fragments are usually angular, vary greatly in size and shape, and do not display obvious signs of current orientation. The largest fragments (> 1 cm in longest dimension) are here termed megaphytoclasts. Accumulations of plant debris in sandy turbidites suggest a complex interaction of water currents with heterogeneous sediment loads in high-concentration turbidity currents. Phytoclast lamina probably are formed by hydrofoil stripping, in which plant fragments are wafted out of sediment gravity flows by currents moving forward and upward through the head regions of the flows, followed immediately by settling of the fragments directly onto newly deposited carpets of turbidite sand. Phytoclast concentrations formed in this way may be buried by fine-grained sediment in the last stage of turbidite emplacement, or plant material may accumulate without further clastic deposition resulting in lamina of deepsea terrigenous coal.