Unusual trace fossil assemblage from the Upper Cretaceous Paguate Member of the Dakota Formation in the Ojito Wilderness, Sandoval County, New Mexico

Abstract

A recent paleontological inventory of Cretaceous strata in the Ojito Wilderness near San Ysidro, Sandoval County, New Mexico, discovered a slab of sandstone of the Upper Cretaceous (Cenomanian) Paguate Member of the Dakota Formation with a highly unusual ichnoassemblage preserved on the surface . The sandstone is a 0.1 to 0.3 m thick bed that is medium-grained and quartzose. It has symmetrical ripples on the trace-fossil-bearing surface and is from a stratigraphic level ~ 3 m above the base of the Paguate Member. The traces are: (1) abundant Thalassinoides that form branching, polygonal networks; (2) several trails of Cruziana ; (3) four specimens of Zoophycos; and (4) an area with undertrack fallout of the horseshoe crab walking trace Kouphichnium . Some Thalassinoides crosscut Cruziana and one of the Zoophycos traces, so this suggests at least two episodes of trace formation. This ichnoassemblage from the Paguate Member clearly was preserved in a shallow, well oxygenated marine setting at or slightly above wave base. Indeed, the assemblage can readily be assigned to the Cruziana ichnofacies, which is characteristic of shallow marine settings. Nevertheless, Zoophycos has long been accepted as an indicator of deep and/or poorly oxygenated marine waters, so it is “out of place” among traces typical of the Cruziana ichnofacies. The Paguate ichnoassemblage thus presents evidence that Zoophycos is a facies crossing ichnotaxon that should not be associated with an archetypal ichnofacies.