Abstract

Bioerosive, marine ichnofossils excavated by pholadid and teredinid bivalves and preserved in late Maastrichtian deposits of southcentral Wyoming and southeastern Colorado indicate regional marine influence on fluvial systems and the presence of brackish-water environments as late as 67 Ma. The upper Medicine Bow Formation of southcentral Wyoming and the Vermejo Formation of southeastern Colorado are chronostratigraphically equivalent to the Lance and Hell Creek Formations and are usually interpreted as continental deposits free of marine influence. The recovery of teredinid calcite tubes from heterolithic, fluvial barforms in the upper 240 m of the approximately 2000 m thick Medicine Bow Formation suggests a late phase of marine transgression at ~67 Ma. Similarly, the presence of Teredolites clavatus and T. longissimus in the Vermejo Formation, approximately 5–10 m above the top of the 67 Ma, marine Trinidad Sandstone reveals approximately time-equivalent transgression and marine influence on local fluvial systems 580 km to the south. These ichnofossils add to the growing body of evidence suggesting that late Maastrichtian formations, generally considered to be continental in origin, in fact contain records of varying degrees of marine influence on coastal plain settings. The previously unrecognized phase of sea level rise and transgression indicated by these ichnoassemblages raises questions about the development of a late Maastrichtian, southern land bridge across the Western Interior Sea and the purported paleobiogeographic interchange between eastern North America (Appalachia) and western North America (Laramidia).

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