ICHNOLOGIC FABRICS IN DEEP-WATER SLOPE CHANNEL DEPOSITS, PLIOCENE RIO DELL FORMATION, ONSHORE EEL RIVER BASIN, NORTHWESTERN CALIFORNIA

For a variety of reasons, few detailed studies exist that describe deep-water bioturbation fabrics in structureless (massive) sediment gravity flow deposits. Almost all massive deep-water sands are interpreted to indicate flow conditions at the time of deposition (as opposed to post-depositional bioturbation destroying primary fabrics), the sub-mm scale of the burrows cannot be identified on core photos or weathered outcrop surfaces, and the cryptic nature of massively bioturbated sandstone can only be recognized by underutilized petrographic analyses of sand grain distribution and orientation.

A correlated core-to-outcrop case study in the Pliocene Rio Dell Formation of the onshore Eel River basin (NW California) helps distinguish the types and sequence of bioturbation in deep-water deposits and how proximity to channel axes can control the extent of bioturbation in the sandy portions. Each sedimentation unit has an ungraded, sandy bottom half and a muddy upper half. Computer-aided tomography (CT) x-ray scans, combined with petrographic data suggest each sand/mud pair records multiple episodes of bioturbation. Both the sandy and muddy lithofacies are often fully bioturbated with little to no primary structures preserved. The interface between the two lithofacies is chaotic and burrowed.

By characterizing broad ichnofacies and cross-cutting relationships, we interpret the following sequence responsible for the bioturbated textures observed in the sand/mud pairs. After a very fine-grained sand turbidite is deposited, mud-prone tracemakers burrow downward to a critical level creating a homogeneous mix of clay, silt, and sand grains. The more sand-prone deposit feeders burrow down past the muddy lithofacies to the sandy section of the turbidite and subsequently produce a cryptobioturbated, massive, sandy texture. This texture contains sub-mm burrows with high porosity, well-sorted, quartz and feldspar fills that are surrounded by aligned, lithic/opaque grains with poor porosity.

Based on comparisons to underdeveloped bioturbated textures in the correlative higher energy areas of deep-water channels, we hypothesize these areas are too energetic for the full bioturbation sequence to evolve. Conversely, the overbank areas contain a better developed bioturbation sequence as described above.