Trace fossil zonation in Cretaceous turbidite facies, northern California

Abstract

The differences between trace fossil assemblages of “proximal”; and “distal”; turbidites (terms used here in the broadest senses) are often attributed to varied preservation processes, although some workers have suspected that trace fossil distribution also represents regional deployment of benthic organisms. Thick‐bedded, thin‐bedded, and muddy turbidites (representing a proximal‐to‐distal environmental gradient) were studied at Point Saint George, northern coastal California, to determine which aspects of trace fossil zonation result from trace preservation and which aspects reflect the distribution of benthic tracemakers.

Thin‐bedded turbidites contained nearly all of the trace fossils found in the sequence: predepositional traces (totalling 17 ichnotaxa and other distinctive biogenic structures) were preserved as casts on the soles of sandstone layers; postdepositional traces (9 forms) were preserved in full relief throughout beds. In contrast, muddy turbidites had only three ichnotaxa preserved in flattened full relief. The difference in trace fossil assemblages between these two kinds of distal deposits appears to result mainly from sand‐casting in the thin‐bedded deposits. In this particular comparison, preservation controls zonation more strongly than the original distribution of tracemaking organisms. However, it is difficult to ascribe differences between the thick‐ and thin‐bedded turbidites simply to different modes of preservation. Four of the five ichnotaxa found in thick‐bedded turbidites were preserved as sand‐casts, and most of these traces did not occur in the thin‐bedded deposits. In fact, the majority of all the ichnotaxa at Point Saint George were confined to a single facies, including most of the postdepositional (opportunistic) traces. Only postdepositional Phycosiphon occurred throughout the sequence. Because the mode of preservation was generally the same in the thick‐ and thin‐bedded facies, original distribution of trace‐makers is probably reflected in this type of zonation.

Although facies‐level ecologic interpretations of ichnoassemblages should be made with caution, especially when body fossils are not available for comparison, patterns at Point Saint George appear to support the notion that both preservation processes and original deployment of benthos are reflected in trace fossil zonation. The common denominator here, controlling both large‐scale taphonomic processes and development of benthic ecosystems, was the regional disturbance regime, characterized primarily by the nature, intensity, and frequency of sediment gravity flows.

Key words:

• trace fossil zonation
• deepsea sand fan
• turbidite facies
• disturbance regime
• mid‐Cretaceous
• Franciscan Complex
• California