The significance of deep-water cryptic bioturbation in slope-channel massive sand deposits of the lower Rio Dell Formation, Eel River basin, California

Abstract

Deep-water massive (featureless) sands are commonly interpreted as indicators of flow conditions during or shortly after deposition and are almost never interpreted to reflect post-depositional homogenization due to bioturbation. Deep-water deposits of the lower Rio Dell Formation (Pliocene), Eel River basin (NW California), however, reveal previously unrecognized cryptically bioturbated fabrics in massive sandstone and mudstone. This study represents the first detailed documentation of cryptically bioturbated massive-appearing sandstone in deep-water channelized environments. Although fluid escape contributed to some of the massive-appearing textures in the Rio Dell Formation, we present criteria for recognizing cryptic bioturbation fabrics: (1) tubes with lithologically distinctive concentric linings or infillings, (2) meniscate backfill in burrows, (3) lithologically distinctive spreite, (4) concentric grain orientations, and (5) grains realigned adjacent to tubular structures. Using biostratigraphic data, detailed stratigraphic descriptions, core photos, and photomicrographs, we document ichnofabrics associated with two main lithofacies and their bounding surfaces: (1) diffuse muddy siltstone lithofacies (e.g. Planolites, Phycosiphon, Schaubcylindrichnus), (2) massive sandstone lithofacies (e.g. cryptic bioturbation , Planolites), (3) lithofacies crossers, (4) burrows that prefer the top-of-sand/base-of-mud interface (e.g. Planolites, Skolithos), and (5) a Glossifungites-demarcated discontinuity at the base of the massive sand lithofacies. We also establish two main lithofacies associations with characteristic sedimentologic traits. We use relative percentages of the lithofacies associations to interpret slope-channel depositional facies: distal channel margin, proximal channel margin, and channel axis. By comparing ichnologic characteristics to depositional facies, we conclude that proximity to the channel axis appear to control the degree and type of bioturbation observed in the massive-appearing deposits. Distal channel margin deposits preserve the smallest burrows, highly developed cryptic bioturbation fabrics, and the most complete sequence of background bioturbation. Proximal channel margin and channel axis deposits record larger burrows but incomplete cryptic bioturbated fabrics and an overall smaller proportion of preserved background bioturbation.