Paleobiologic Evidence for Cretaceous Tides, Western Interior Basin, North America: ABSTRACT

Abstract

Modern intertidal environments contain a suite of diagnostic physical sedimentary structures and macro-invertebrate organisms. The former are often used in the identification of ancient intertidal paleoenvironments; the latter are rarely used. Yet the conservative evolution of intertidal organisms and their distinctive adaptive morphologies, expressed as preservable biogenic structures and/or shells, facilitate their identification in ancient strata. The habitat ranges of modern intertidal organisms are narrowly restricted within the intertidal zone. Groups of organisms with roughly equivalent habitats constitute distinct, low-diversity communities. In general, the total number of taxa in the intertidal biota and, accordingly, the number of recognizable communities increa e with increasing tidal range. We propose that: (1) fossil intertidal organisms and their traces can be used to recognize Mesozoic and Cenozoic intertidal paleoenvironments; and (2) the diversity and number of distinct paleocommunities recognizable among such faunas can be used to determine, at least qualitatively, paleotidal range. We have applied these concepts in a study of Cretaceous nearshore and marginal-marine strata in the western interior basin. Obligate or habitat-selective intertidal organisms occur along both shorelines of the interior Cretaceous seaway from northern Alberta to the Gulf of Mexico. Different groups of paleocommunities characterize exposed, wave-dominated, sandy coasts and low-energy, vegetated shorelines in marginal-marine settings. The compositions of the pal ocommunities along low-energy shorelines further vary with paleosalinity, permitting recognition of broad salinity gradients in Cretaceous estuarine settings. Faunal evidence indicates that Cretaceous tides extended throughout the interior Cretaceous seaway. The tidal range was generally greater along the western shoreline than along the eastern shoreline. Figure End_of_Article - Last_Page 731------------