PALEOENVIRONMENT OF A TRACE FOSSIL MARKER UNIT IN THE LATE CRETACEOUS NORTH HORN FORMATION, UTAH
The marker overlies a 3-m dark gray mollusk and ostracod-bearing shale. From the base, it includes (1) ~25 cm of stacked symmetric ripple structures of rapidly changing orientation superposed with abundant straight to meandering diminutive (1-3 mm), sometimes backfilled invertebrate traces in horizontal, vertical and oblique orientations, and infrequently, reburrowed, horizontal burrows (5-10 mm) with lengthwise striae; (2) 0-2 cm of mudstone; and (3) ~25 cm of sandstone with scattered dinosaur tracks (up to 37 cm deep). The unit was subsequently penetrated by large (up to 25 mm) backfilled burrows, which gently curve and frequently extend downward through the rippled layers. Variegated smectitic mudstones interbedded with sandstone lenses overlie the marker unit. Provisionally the small traces include Planolites, and the large ones are Ophiomorpha-like.
The dark shales are interpreted as organic-rich lacustrine deposits and the sandstone as associated shoreline with near-shore features formed by frequently shifting wind-driven currents. Small, diverse, aquatic organisms heavily burrowed the thin, successive, rippled deposits in shallow water and/or ephemeral wet subaerial exposures, suggesting they rapidly reoccupied successive deposits during periodic lulls in ripple formation. As a fluvial floodplain replaced the lake, dinosaurs walked along receding shorelines and arthropods burrowed the dinosaur tracks and sands, occasionally cutting into the lowest shoreline sands while they were unconsolidated.