Shallow Marine Environments — A Comparison of Some Ancient and Modern Examples
Shallow marine environments are commonly more complex and varied than the depositional settings of the prograding shoreline sequences of Chapter 7. Rather than begin with a general discussion of shallow marine systems, we will keep to the theme of these notes and examine two contrasting sequences of sedimentary structures. Both can reliably be interpreted as shallow marine, but they lead to different interpretations. The two we have chosen are the Campanian Shannon, Sussex, and Hygiene sandstones of Wyoming and Colorado and the Turonian Cardium Formation of Alberta; both are from the Late Cretaceous seaway of the Western Interior of North America.
Having thus set the stage by examination of these two types of ancient rock sequences we will review some modern shallow seas and, finally, examine additional ancient examples that each represent slightly different conditions. One of the recurring problems in the interpretation of shallow marine sequences is the absence of a clear reference point or level--the beach or shoreline served as such a reference point in the sequences of the previous chapter. In ancient shallow marine sequences, sand was apparently spread more broadly than we would infer from modern analogs. However, lacking clear evidence of positions of shorelines or sea levels, interpretation is difficult and elusive.
Figures & Tables
These notes are for a course on the use of primary structures and stratification sequence as tools for interpretation of depositional environments. The emphasis is to provide a concise review of the factors that had led to the renaissance in clastic sedimentology during the decade leading up to 1975. The attempt is to provide an organized summary of both experimental studies and ideas on bed forms and primary sedimentary structures that was then relatively new and to show how this information could be applied to solving geologic problems. A second broad objective of the course is more philosophical, in that there is an attempt to outline some general approaches to interpretation and convey the goals of interpretation. The authors believe that there are a fairly small number of general depositional settings but that numerous environmental and process variables within each general setting lend considerable variation to the deposits themselves. The emphasis is at the scale of features and sequence that can commonly be observed in individual outcrops or cores. Interpretation begins with data collected at this level.