Bioturbation Patterns in a Channel-Levee-Overbank Sequence of Paleocene Submarine-Canyon Fill, Point Lobos, California: ABSTRACT

Abstract

Bioturbation patterns in the Paleocene submarine canyon fill (Carmelo Formation) at Point Lobos, California, differ for channel, levee, and overbank deposits. Variation in (1) such ichnoassemblage characteristics as taxonomic composition, diversity, abundance, and behavioral/preservational types, and (2) the overall degree of biogenic reworking of the sediment are particularly significant. The ichnoassemblage of the channel-levee-overbank sequence includes Arenicolites, ?Aulichnites, Chondrites, ?Helminthoida, ?Neonereites, Ophiomorpha, Scolicia, Thalassinoides, escape structures, and two unidentified traces. All the trace fossils were produced by infaunal organisms burrowing at various depths below the sediment-water interface. The channel deposits are characterized by relatively low diversity and density (in comparison with the overbank deposits), and mainly consist of traces of deep-burrowing animals (e.g., Ophiomorpha). The overbank deposits have a relatively diverse and dense ichnoassemblage produced by both deep- and shallow-burrowing animals. The levee deposits are similar to the overbank deposits in trace diversity, but are intermediate between the channel and overbank deposits with respect to their overall degree of bioturbation. Bioturbation patterns in these three subenvironments differ as a consequence of the chance of preservation of biogenic sedimentary structures. Each bed type (e.g., mudstone) contains similar ichnoassemblages regardless of the depositional subenvironment in which the bed type occurs. This pattern indicates that the distribution of the infaunal organisms producing the traces was influenced more by factors associated with a particular lithology (e.g., texture or organic content) than by environmental factors peculiar to a specific subenvironment . Therefore, the relative abundance of the various bed types ultimately preserved within each subenvironment corresponds to the bioturbation patterns characteristic of the channel-levee-overbank sequence. For example, physical sedimentary structures (e.g., Bouma a-b intervals in thick sandstone beds) and biogenic sedimentary structures (e.g., escape structures and truncation of traces) in the channel environment indicate frequent episodic events of extensive erosion, followed by rapid deposition of sand bed several tens of centimeters thick. Thus, traces produced by deep-burrowing organisms in thick sandstone beds are most frequently preserved. In the overbank deposits, an abundance of traces left by shallow-burrowing organisms (e.g., Arenicolites) in mudstone beds, the type of behavior (e.g., feeding burrows) represented by such traces, and the higher degree of bioturbation of all bed types indicate relatively slow, continuous deposition. Therefore, the change in conditions of sedimen ation (e.g., frequency of significant erosional/depositional events, amount and/or type of sediment eroded/deposited, ratio of erosion to deposition) results in the preservation of a different bioturbation pattern within the more tranquil overbank subenvironment. The bioturbation pattern characteristic of the levee deposits represents End_Page 483------------------------------ a transition between the bioturbation patterns of the channel and overbank deposits--that is, it results from a transition in the factors that dictate the preservation of biogenic sedimentary structures. End_of_Article - Last_Page 484------------