BELEMNITE ROSTRUM USED AS AN INDICATOR OF MARINE FLOODING SURFACES IN THE JURASSIC SUNDANCE FORMATION: SEMINOE RESERVOIR WYOMING, USA
Belemnite rostra occur in a variety of lithologies in the Sundance, but appear to be most abundant in beds where they occur with other mollusks and bored cobbles. Belemnites are a prehistoric Cuttlefish-like creature that lived in these ancient seas, the fossil remains from these soft tissue squids are a conical spear-like feature called a Rostrum that protrudes from the top of their heads or their phragmocone. Some soft parts are found in the fossil record although they are very rare. In one bed where Belemnites are particularly abundant we see evidence of very low rates of deposition, which leads us to believe that this was an indication of a Maximum Flooding Surface. The accumulations of skeletal remains in this case appear to be stratigraphic or sedimentological rather than the result of mass mortality or any other biological accumulation. Also, after this interval the overlying strata grade upward into progressively shallower water facies and eventually terrestrial facies indicating a marine regression and eventual demise of the Sundance Sea.