A MULTI-FACETED APPROACH FOR RECOGNIZING DISCRETE BIOTURBATION EPISODES IN MASSIVE DEEP-WATER SEDIMENT GRAVITY FLOWS, PLIOCENE RIO DELL FORMATION, ONSHORE EEL RIVER BASIN, CA

Featureless, massive beds could be caused by 1) suspension fall-out of concentrated sands within a turbiditic flow, 2) post- or syn-depositional liquefaction, or 3) obliteration of original sedimentary fabric by bioturbation. Although massive beds are a common element of high- and low-density turbidites, they can also occur in densely bioturbated slope deposits. It is therefore important to accurately interpret massive sandy beds in deep-water successions in order to create reliable depositional/reservoir models.

A case study in the Pliocene Rio Dell Formation demonstrates the need for a multi-faceted approach to understand the origin of massive beds. The onshore Eel River basin in California contains over 3,000 feet of mud-rich bathyal deposits with discrete deep-water sand packages up to 150 feet thick (20-40% net sand). Cores that contain sand/mud pairs were previously interpreted as “linked-debrites” consisting of a lower sandy turbidite linked with an overlying muddy debrite. However, computer-aided tomography (CT) x-ray scans, combined with petrography and SEM data, suggests each sand/mud pair records multiple episodes of bioturbation.

Individual sedimentation units (36 units total) range between 0.3-5 feet thick and commonly have a lower sandy portion (Zone 1) overlain by an upper muddy portion (Zone 2). Zone 1 is an ungraded, structureless, clay-rich, fine- to very fine-grained sandstone that is partially to fully bioturbated. It has a sharp or scoured basal contact and an irregular bioturbated upper contact. Zone 2 is a poorly-sorted, densely bioturbated muddy siltstone with mud- and sand-filled burrows descending down to a consistent level into Zone 1 creating a sharp grain size contrast between the two zones.

We interpret each sand/mud pair to record a single upward-fining turbidite that was extensively bioturbated in at least two phases. Following deposition, infauna first burrowed down and occupied the lower sandy portions (Zone 1). Tracemakers that preferred the more muddy portions then churned the upper muddy portion (Zone 2) and proceeded to burrow down to a particular level where living conditions were still preferable. This degree of bioturbation implies increased oxicity and nutrient levels and a lower sedimentation rate relative to the previous depositional model.