Emplacement, modification, and preservation of event strata on a flood-dominated continental shelf: Eel shelf, Northern California

Floods on the Eel River in Northern California during the winter of 1995 deposited a sediment layer 5–10 cm thick on the adjacent shelf, north of the river mouth and seaward of the 50-m isobath. Additional thinner oceanic flood layers were deposited over the same region in 1996 and 1997. Physical and biological modification of the 1995 event layer has been studied over a 2.5-year period and examined by means of sedimentary-fabric, radiotracer, and macrofaunal analyses of box cores collected across the shelf from 50- to 70-m water depth. Depositional sedimentary fabric of the 1995 flood deposit (as determined here and by Drake, 1999; Sommerfield and Nittrouer, 1999; Wheatcroft and Borgeld, 2000) indicates that it was deposited by three separate events over a period <3 months, each of which deposited a basal layer of silty sediment (containing crossbeds in some locations) and an overlying layer of clay-rich sediment that displays no obvious grading. Following each depositional event, biological mixing of the upper few centimeters resumed within a matter of weeks. This mixing produced millimeter-scale burrow mottling that replaced depositional sedimentary fabric in the upper 3–5 cm over a period of <6 months. Physical modification of the upper few cm includes reworking and deposition of coarse silt and fine sand on top of the uppermost clay-rich bed, which, along with shallow biodiffusive mixing, has produced coarse-grained bioturbated sedimentary fabric similar to that of the pre-flood seabed. Below ∼5 cm, the event layer has been modified by deep burrowing activity (including incipient Teichichnus traces), which can mix the upper ∼20 cm of the seabed over time scales of 6–350 years. Tiered ichnofabrics are produced by a vertically zoned macrofaunal community, which includes significant numbers of both surface- and deep-deposit feeders. Analysis of preservation potential for event layers indicates that basal portions of layers significantly thicker than 5 cm (such as the 1995 deposit) are likely to be preserved, especially when buried rapidly by subsequent depositional events, as occurred in 1997. Piston-core study shows that such partially preserved event layers compose ∼10% of the upper 3 m of the seabed, and have mean recurrence intervals of ∼100 years over the past ∼500 years.