Siphuncular fragments from arminocerid cephalopods are abundant along 350 m of a well defined rocky shore of late Ordovician or early Silurian age exposed near Churchill in northeastern Manitoba, Canada. The paleoshore is composed of the Precambrian Churchill Quartzite, which is dramatically abutted by carbonate strata gently dipping north beneath Hudson Bay. At one end of the exposure, a former sea stack or sea arch extends prominently outward for 15 m perpendicular to the otherwise linear paleoshore. Probably it acted as a natural groin with some influence on local currents and waves. The orientations of 189 siphuncular fragments were recorded and found to be dominantly perpendicular to the trend of the ancient shore. They are far more cylindrical in shape than conical, and are interpreted to have been rolled to their final resting place pushed by long‐shore currents. Concentration of nearly 80 of the fragments towards one end of the shore is consistent with transport by long‐shore currents. Ripple marks preserved at a different stratigraphic level form a 45‐degree angle to the trend of the paleoshore, providing additional evidence that wave trains must have set up long‐shore currents from time
展开▼
