Assemblages of Biogenic Structures in Marine Shale-Forming Environments and Deep-Water Basins
This final section of the volume concerns trace fossil assemblages from deeper water environments that often are oxygen deficient and the sites for significant accumulation of fine-grained sediment. Douglas Jordan's paper describes trace fossil assemblages from Devonian black shales in Kentucky and then illustrates how these assemblages can be used to inteipret paleodepositional environments in shaly basins. The different assemblages are shown to vary with sediment type and color and are thought to be controlled by oxygen availability at the time of deposition. Table 1 presents a synthesis of the occurrence of trace fossils in black shales and the interpreted paleoenvironments and O2 content, and Figure 12 illustrated Jordan's proposed model for the occurrence of trace fossil assemblages in a marine, deep-water basin.
The enigmatic depositional origin of the Ordovician “trilobite shales” of Ohio is the topic of the paper by Danita Brandt Velbel Her study combines ichnologic taphonomic and sedimentologic data to show that these shales were deposited rapidly but not necessarily by turbidity currents. Trace fossils particularly various types of Chondrites are shown to be useful in determining the relative timing of events of deposition.
The paper by Brian Edwards is a comprehensive study of modern bioturbation in sediments at oxygen-deficient, bathyal depths in the Santa Cruz Basin off the southern California coast. Although the basin floor sediments support a high density and diversity of benthic macrofauna traces preserved in the sediment are low in density and diversity Based on his findings from the Santa Cruz Basin Edwards
Figures & Tables
Biogenic Structures: Their Use in Interpreting Depositional Environments
Organisms of one sort or another today inhabit virtually every sediment environment on Earth, and the rock record tells us that this has been the case through the greater part of our planet’s history. Furthermore, organisms leave their mark in most sedimentary settings, either directly in the form of body fossils or indirectly as biogenic structures. In addition to their often profound modifying effects on substrates, ancient biogenic structures preserve a record of organism behavioral activity in response to substrate and other paleoenvironmental controls. Thus, biogenic structures can be highly useful as facies indicators and can provide valuable clues to the interpretation of paleodepositional environments. The purpose of this volume is to present a broad spectrum of case-book examples of the use of biogenic structures in the interpretation of depositional environments.