A storm surge origin for sandstone beds in an epicontinental platform sequence, Ordovician, Norway

Abstract

The upper Ashgillian (Ordovician) sediments of the Oslo area were deposited on an extensive open marine platform. Within the sequence thin sandstones (0.5–10 cm thick) are interbedded with bioturbated silty shales. The beds commonly have sharp lower surfaces and more diffuse tops which suggest sand was rapidly introduced into a tranquil environment. Estimates suggest that the deposition of a sand bed was a very infrequent event, recurring every few thousand years. The sandstones show lateral changes in frequency of occurrence, and total cumulative thickness, which, taken together with palaeocurrent evidence, indicate derivation from the west. Vertical changes in thickness of sandstone per metre, mean bed thickness, bedforms and grain size all reflect an increasing proximality to a westerly sediment source and can be related to a shoreface with eastward longshore drift developed during the upper Ordovician regression. The general reconstruction of Ashgillian palaeogeography in the Oslo region suggests that the sand beds were deposited by currents flowing offshore, obliquely down a low palaeoslope. Most normal currents on modern continental shelves flow parallel to the shoreline, so we have considered other mechanisms which might have produced low-frequency, high-energy currents which flowed offshore and which could have been responsible for the deposition of the thin, discrete sandstones. Three models are discussed for the formation of the thin sandstones, and an ebb current generated by a storm surge is considered the most likely explanation.