Post-depositional alteration and preservation of sedimentary event layers on continental margins, I. The role of episodic sedimentation

The degree of post-depositional alteration and hence the preservation potential of sedimentary event beds and transient signals is determined by the outcome of a ‘race’ between biological (and to a lesser extent physical) processes that conspire to destroy a signal and sediment accumulation which advects the layer or signal out of the surface mixing zone. Preservation potential specifically depends on the relative magnitudes of the (1) biological mixing intensity (sometimes parameterized as a biodiffusivity, Db), (2) mixing-layer thickness, (3) layer or signal thickness, and (4) sediment accumulation rate. These terms control the dissipation time (i.e. time required to destroy a signal) and the transit time (i.e. time necessary to advect a signal through the surface mixing layer). On fine-grained, upper continental margins in general, and the Eel River shelf (northern California) in particular, biological mixing intensity is high (i.e. Db ranges from 10 to 100 cm2/yr), mixing-layer thickness is large (>10 cm), and sediment accumulation rates are rapid (0.1–1 cm/yr). Despite the high sediment accumulation rates, transit times through the surface mixing-layer range from decades to a century. Signal dissipation times are considerably shorter: (1) <3 yr for beds up to 6 cm thick imaged in X-radiographs, and (2) <15 yr for the grain-size signature of beds up to 8 cm thick. Therefore event layers and their corresponding grain-size signature have a low probability of preservation. However, short-lived episodic sedimentation events (e.g. oceanic floods) can instantaneously advect material through the surface mixing layer, thereby preserving event beds and transient signals. On the Eel River shelf the sequential timing of episodic sedimentation events has exerted a first-order control on the resultant stratigraphic record (presence/absence of layers and grain-size fluctuations). Episodic sedimentation – a hallmark of continental shelf settings – is key to understanding strata formation and preservation on margins.