The Western Interior Seaway : An introduction to the general history, Paleogeography, and Ammonite record
Abstract: Throughout most of the Cretaceous, rapid sea floor spreading rates contributed to a rise in global sea levels culminating in a Phanerozoic all time high. Regionally this allowed flooding to occur in the lower lying regions of central North America in a predominately north/south axis forming the Western Interior Seaway.
Minor sea level fluctuations, tectonic events, and variations between deposition and subsidence rates formed complex cyclic deposits primarily composed of clastics (chiefly sandstone and shale) or carbonates (limestones). These fluctuations also acted as “gates” controlling entry into, or exit from the seaway of marine organisms by physical barriers or changes in ocean temperature and/or chemistry.
Ammonites left an impressive 50 million year record in the seaway and are the principal tools used in the regions biostratagraphic correlation. Additionally Ammonites are educational in evolutionary studies and are highly regarded by amateur & commercial fossil collectors alike.
Studies in progress are now unraveling their complex evolutionary history. Related websites are suggested as well as text. Selected specimens from the N.S. Brown collection are figured.Continue reading →