Abstract

New biostratigraphic data obtained from measured stratigraphic sections of Santonian through Maastrichtian age located along the west coast of North America necessitate changes to the currently accepted chronostratigraphic framework for this region of the North Pacific biotic province. We recognize and/or define 12 molluscan zones over this interval of the Upper Cretaceous and propose revisions to the currently accepted integration of ammonite zones with global Upper Cretaceous magnetochrons. Our findings demonstrate that there was significantly more faunal interchange between the North American Pacific Coast and both the Western Interior and Gulf Coast regions of North America during the Late Cretaceous than has previously been recognized, and because of this, novel and direct biostratigraphic correlations can be made. These new faunal correlations are augmented with the magnetostratigraphic record from Pacific Coast localities to arrive at better interregional correlation for the Upper Cretaceous globally. The new integration of the global polarity time scale with the local, west coast ammonite zonation now allows better correlation between sections both within the North Pacific province (but geographically far from our study areas) as well as to sections outside of the province itself. However, we note here that previous correlations between biostratigraphy and the top and bottom of magnetochron 33r in west coast North American sections appear to have been in error due to unrecognized, modern-day normal-field overprint of originally reversed polarity in Upper Cretaceous sections. We reinterpret the position of this chron based on this new information.

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