BELEMNITE ROSTRUM USED AS AN INDICATOR OF MARINE FLOODING SURFACES IN THE JURASSIC SUNDANCE FORMATION: SEMINOE RESERVOIR WYOMING, USA

Belemnites literally cover the ground near ridges of the Sundance Formation around the Seminoe Reservoir in Southeastern Wyoming. A series of samples were collected from beds of the Sundance in hopes of better understanding the accumulations of this cephalopod. Many of the interbedded sandstones, mudstones, and limestone in the lower and middle parts of this interval are the results of a marine transgression that took place in the Middle Jurassic with the expansion of the Sundance Sea. The goal of the research was to better understand what was happening to sea levels during this time and what was the environmental setting that deposited these layers in the now present day Seminoe Reservoir. We expected to find a transgression followed by a regression that eventually regressed enough to give us the paleosols and fluvial deposits of the famous Morrison Formation.

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.