A DAY IN THE LIFE OF A JURASSIC BEACH: THE STUDY OF A BATHONIAN LIMESTONE OUTCROP IN NORTHERN WYOMING

The Lower Sundance Formation in the Bighorn Basin of Wyoming preserves several locations of dinosaur tracksites. The University of Wyoming Geological Museum’s undergraduate research program in paleontology allowed for ready access to a previously undescribed location to be studied.  The site (Melinda 2, UW-V-2002-003) is in the uppermost layers of the Canyon Springs Member of the Lower Sundance Formation, a few centimeters below the Stockade Beaver Shale.  This 60 square meter site contains numerous theropod tracks and trackways of various sizes.  The tracks currently mapped range in length from 11 to 33 cm. There are many individual tracks, as well as several short trackways found at Melinda 2.  The tracks are preserved on at least three separate layers.  The state of preservation of the tracks within these layers differs with the upper (#3) and lower (#1) layers showing the poorest preservation.  There are also many ripples preserved at the site which show orientations that differ from those of other track sites in the area.  The two layers that show clear orientations of ripples agree with each other while internal ripples imply tidal events. The ripples in layer #2 steepen to the north, implying a northerly flow direction.  The ripples in layer #3 are too poorly preserved to provide clear data about wave direction.  The undertracks in layer #3, along with unrelated tracks below, show that the activity of dinosaurs here was not a unique event, but occurred repeatedly over time, during at least three tidal events.  Overprinting of tracks on tracks within layers #1 and #2 along with variations in track size implies the activity of dinosaurs over time within the period of each tidal event.

 The cycle of tide and the variations in direction of ripples in the layers, as well as the variations in direction between these and others in the area help to illustrate the complex system and the changing world of the Middle Jurassic.  The opportunity to work with a tracksite in the Bighorn Basin near my former home allowed me to expand my prior knowledge of the unique geological and paleontological resources of the region.