Depositional processes on an ancient and modern muddy shelf, northern California

Abstract

ABSTRACTThe modern Eel River shelf and analogous Pleistocene Rio Dell Formation in northern California provide an ideal opportunity to combine the advantages of studying a modern environment with those of studying an ancient sequence, and thereby enables further understanding of muddy‐shelf processes. The modern shelf is the site of accumulation of a thick deposit of Holocene mud. Both large‐scale sediment distribution patterns and small‐scale stratigraphy on the shelf indicate that river floods play an important role in sediment accumulation, even on a high‐energy, ‘storm‐dominated’ coast. The major factors in preservation of this flood ‘signature’ are the cohesive behaviour of fine‐grained sediments and episodically rapid rates of sediment input.The Rio Dell Formation includes approximately 400 m of mostly fine‐grained shelf deposits that accumulated offshore from a palaeo‐Eel River mouth. The shelf sediments comprise four depositional sequences. Sequence 1 records progradation from outer to inner shelf depths. Facies trends closely resemble across‐shelf trends on the modern shelf, suggesting that processes were similar. Detailed examination of these deposits provides insight into the nature and role of various processes on both the ancient and modern shelf.Muddy facies of the Rio Dell sequence are characterized by bioturbated, clayey silts, interbedded with event layers of several types. Clay‐rich silt layers are interpreted as flood deposits and physically stratified, coarse‐silt layers are interpreted to record transport and deposition of coarse silt on the midshelf during storms. Sediment‐transport calculations and consideration of grain‐size distributions of bioturbated sediments, which form the bulk of the Rio Dell sequence, suggest that these sediments are the result of biological homogenization of the fine‐grained flood deposits and of the coarser‐grained storm deposits. The results of this study in general indicate that fine‐grained shelf deposits do preserve a distinguishable, if subtle, record of depositional processes.