Trace Fossils and Bathymetry of the Upper Cretaceous Point Loma Formation, San Diego, California

Abstract

Distinctive biogenic sedimentary structures (trace fossils) in sedimentary rocks reflect the burrowing behavior of the animals that made them. Because the distribution of these animals is influenced by environmental factors such as water depth and substrate type, trace fossils have been used widely as paleoenvironmental guides. The massive, ungraded sandstones in the Point Loma Formation have sharp upper and lower contacts, thick lenses of mudstone clasts, and common load-deformation structures, suggesting deposition largely by grain-flow processes. Foraminifers and most of the trace fossils suggest deposition in a deep, probably bathyal environment. However, two abundant trace fossils, Qphiomorpha and Thalassinoides , are widely held to represent very shallow-water to intertidal paleoenvironments, and they usually occur in such facies. Two morphologic forms of a third trace fossil, Zoophycos , have been thought to reflect different environmental conditions, but they occur here in the same strata. These occurrences emphasize the point that paleoenvironmental interpretations should not be based on individual trace-fossil taxa, but that such traces should be considered only in conjunction with other available evidence.