The sandy inner shelf as a repository for muddy sediment: an example from Northern California

To understand the sedimentary history of a siliciclastic, tectonically active inner-shelf environment, 50 vibracores were collected in water depths ranging from 20 to 55 m on the Eel margin, Northern California. Sediments exhibit changes in grain size along- and across-shelf, as well as vertically. The mean grain size of the surface sand fraction is 2.2–3.6φ11φ=−log2(diameter(mm)/1mm) 82.5 μm), with 7–40% of the sediment finer than 4φ (64 μm). The along-shelf distribution of sand is controlled predominantly by proximity to the sediment source and by the prevailing oceanic conditions (e.g., waves and currents). Fine sand dominates within ∼10 km of the Eel River mouth, which is consistent with sediment settling velocities out of the Eel River plume. Coarser sediment is found farther (>10 km) north of the Eel River mouth, and may have a source other than the modern Eel River. Cores tend to demonstrate upward fining (except cores collected at ∼30 m) and other vertical grain-size variations, which preserve the record of floods on the Eel margin. Muddy layers, which are assumed to record large floods, are found interbedded with sand. Two types of muddy layers are observed. “Distinct” mud layers have >70% mud and are identifiable through visual observation and a decrease in bulk density. “Diffuse” mud layers contain only 5–15% mud due to reworking caused by wave activity during or after deposition. Diffuse mud layers are not visually obvious, and poor sorting increases their bulk density. Recent flood layers have detectable 210Pb and 137Cs, and the deepest detectable 137Cs is 120 cm below the surface. Assuming that the mud layers result from major floods (e.g., 1964), and using 210Pb and 137Cs radioisotopes to establish chronological control, the accumulation rate of sand on the inner shelf since 1964 ranges from 1.3 to 3.3 cm/yr. Approximately 6–13% of the fine-grained sediment discharged by the Eel River over the past 36 yr is accumulating interspersed with the inner-shelf sand. An additional ∼1% of the fine-grained sediment discharged since 1964 may be accounted for as distinct flood layers (containing >70% mud) interbedded with the inner-shelf sand. In all, 40–50% of the sand discharged by the Eel River, both in suspension and as bedload, can be accounted for on the inner shelf. An additional 2–5% of the sand accumulates in the mid-shelf mud deposit. Transport farther seaward (e.g., Eel canyon), landward (beaches or Humboldt Bay), or along shelf should account for the remainder of the sand. Thus, the inner-shelf region plays a significant role in controlling the fate of sand and mud supplied from terrestrial sources.