This part of the volume contains papers that deal with biogenic structures found in intertidal to shallow subtidal environments. Microbial endoliths (microorganisms that live within a rock or shell substrate) can be significant agents of bioerosion, particularly within the intertidal and supratidal zones of carbonate rocky coasts. The paper by E.J. Hoffman documents a clear zonation in carbonates of microbial endoliths within the intertidal and supratidal zones of Bermuda. The zones established by Hoffman are represented in the same manner from site to site on Bermuda and appear to be very similar to those found along Mediterranean coasts and in some areas of the Caribbean. The distribution of microbial endoliths along the Bermuda coast is summarized in Hoffman's Figure 3, and Table 1 presents a carefully constructed list of criteria for the recognition of different endolith forms. Hoffman concludes by suggesting that fossil endolith assemblages could be most useful for identifying and subdividing ancient carbonate coastal sequences.
Gary Hill’s study of the modern ichnofacies of the sediment size graded shelf of the northwestern Gulf of Mexico shows that bioturbation varies significantly and systematically across the shelf. The study reveals that the diversity and density of Iraces decrease across the shelf as substrate grain size becomes finer and more uniform and as infaunal assemblages become less dense and diverse. Hill concludes with some useful observations on how geologists might recognize better zonation patterns in shelf sediments based on patterns and degree of bioturbation.
Algal mats and stromatolites commonly are thought to
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.