HYPERTIDAL RANGES AND SEDIMENTATION IN CARBONIFEROUS AND MODERN TIDAL RHYTHMITES

Widespread use of the term “macrotidal” includes tidal ranges higher than 4 m; however, no upper limit for this term has ever been defined. Herein, a proposed classification would include a “hypertidal” range, which would be applied to tidal ranges above 6 m. For subdivisions, “hypertidal A” would include ranges from 6 to 8 m, “hypertidal B” would include 8 to 10 m, “hypertidal C” would include tidal ranges of 10 to 12 meters and “hypertidal D” would include ranges from 12 to 14 m. Tidal ranges greater than 14 m could be referred to as “hypertidal E,” however, such ranges do not normally occur in modern oceans. Hypertidal D ranges do occur in two places in Canada, the Bay of Fundy and the Leaf Lake area in the southern part of Ungava Bar.

Study of modern hypertidal settings indicates a variety of laminar and rippled bedforms can be deposited within fluvio-estuarine bars that occur near the inland tidal limit. The rates of vertical accretion can be very high and each flood tide can deposit a distinct sedimentation unit. Similar sedimentary structures occur within the upper reaches of Turnagain Arm in Cook Inlet of Alaska, hypertidal estuaries along the Atlantic Coast of Patagonia (southern Argentina) and in the estuarine reaches of the Bay of Mont Saint Michel, in northeastern France.

Within strata of the Carboniferous Period of the east-central U.S., virtually identical tidally modulated sedimentation is widespread and sequences containing fine-scale lamina and rippled bedforms are commonplace. Similar to the modern analogs discussed above, nearly continuous sedimentation (“tidal rhythmites”) has been recorded from a number of sites. One unusual aspect is the occurrence of rhythmitesdirectly overlying geographically widespread coal seams. These rhythmites also exhibit features related to high rates of sediment accumulation, most notably the occurrence of in place, 1 to 5 m tall lycopsids associated with other upright Carboniferous tree-sized plants. The association of widespread coals directly overlain by tidal facies is unique in the Phanerozoic rock record. A hypothesis that is currently being evaluated is that there was widespread and essentially “catastrophic” flooding of the coastal coal swamps. This allowed the rapid influx of tidally modulated sedimentation over the mires and the concurrent entombing of upright plants.