MODELLING TIDES IN THE LATE PENNSYLVANIAN MIDCONTINENT SEAWAY USING THE IMPERIAL COLLEGE OCEAN MODEL

We used a new “next-generation” finite element model, the Imperial College Ocean Model (ICOM) to hind-cast tidal range in an ancient (c. 300Ma) epeiric sea, the Late Pennsylvanian Mid-continent Seaway (LPMS). The model has been validated by comparison with the modern North Sea, Baltic Sea, and Mediterranean Sea.

Two ‘base-case' Pennsylvanian paleogeographies were tested; (1) sea-level highstand and (2) sea-level lowstand through early transgression. Sensitivity tests determined the importance of bathymetric uncertainty on tidal range prediction. During the highstand, tidal ranges in the craton interior (Midcontinent Shelf, Illinois and Appalachian basins) are consistently predicted as micro-tidal (<2 m tidal range). Such low tidal ranges would have inhibited water-body mixing in the LPMS, promoting stratification. This, with other factors, such as influx of oxygen-poor water from the Permian Basin to the west and high organic input from tropical rivers may have contributed to the deposition of ‘core' black shales in LPMS cyclothems. Conversely, meso- to macro-tidal ranges are predicted for lowstand through early transgression in a large-scale embayment in eastern Kansas (Douglas Group). The rocks here include cyclic rhythmites and other tidal indicators and have been ascribed to a meso- to macro-tidal setting.

It may seem intuitive to expect high tidal ranges during sea-level highstands, when there is more water in the basin. However, funnelling, shoaling and resonance in the smaller, shallower lowstand seaway plays an important role in tidal amplification. Is this model of micro-tidal highstands with stratified water-bodies vs. meso- to macro-tidal lowstands with increased volumes of tidalites applicable to other ancient epeiric seas?

Most of the pre-Jurassic (200 million years ago) marine sedimentary record was deposited in shallow epeiric seas. Despite their obvious importance, our understanding of these ancient water-bodies is hindered by a lack of suitably scaled modern analogues. As a result, the hydrodynamics of ancient epeiric seas, including their tidal influence, has long been questioned. Numerical modelling provides a quantitative means of investigating tidal influence in the geological record that does not require uniformitarian analogues.