CIRCULATION IN LARGE ANCIENT EPICONTINENTAL SEAS: WHAT WAS DIFFERENT AND WHY?

Abstract

At times in the geological past, vast epicontinental seas flooded the continents. Almost everything we know about pre-Jurassic (200 million years ago) marine life and environmental change originates from such settings. Most of the truly oceanic sediments deposited before this time have either been subsequently subducted or metamorphosed (Allison and Briggs, 1993). An understanding of ancient epicontinental seas is clearly essential to our interpretation of past ecological and environmental change. Epicontinental seas, however, really have no suitably scaled modern counterparts. They were typically shallow, on the order of 10 to 200 m deep, but of vast extent, covering areas of up to ∼106 km2 (Wells et al., 2005a, 2005b). Such modern seas as the Baltic are of similar depths but are considerably smaller. Without suitably scaled modern analogues our understanding of these important water bodies is handicapped. Since the depth and scale of these ancient seaways makes them so different, their response to storm and tidal waves has been questioned (Hallam, 1975). Specifically, it has been argued that wave energy was attenuated in ancient epicontinental seas by the large distances traveled by waves (Keulegan and Krumbein, 1949; Shaw, 1964; Irwin, 1965). This contention can be rejected in the case of wind and storm waves on the basis of wave height data from modern isolated seas. Wind waves are affected by fetch, duration, and intensity of wind (e.g., see Jonsson et al., 2002, 2005). Substantial waves are regularly documented in even the small epicontinental seas of today. Wave heights—crest-to-trough height—of larger than 6 m are documented in the Baltic Sea almost annually. They result from strong winds (15–20 m/s) blowing over deep water for at least 6 hours. Annual storms can generate wavelengths of 80 m, leading to sediment reworking down to 40 m water …