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.