Recurring Assemblages of Biogenic and Physical Sedimentary Structures in Modern and Ancient Extreme Macrotidal Estuaries

Archer, Allen

Cook Inlet (Alaska, USA), the Bay of Fundy (Nova Scotia, Canada), the Bay of Mont Saint Michel (France), and the Amazon River mouth (Brazil) have a recurring assemblage of unique physical and biogenic-sedimentary structures. The major control affecting these occurrences appears to be the very high rates of localized sedimentation. Locations of these estuaries range from equatorial up to 61° N. Lat. Because of this great latitudinal range, there are great differences among the climates. Thus, as a significant control on the recurring assemblages, the effects of climate can be eliminated. Similarly, there are extreme differences among the biota and ecology of the various sites. Thus, these biological factors also appear to be of little consequence. Apparently, the high rates of deposition and sedimentary dynamics of the sites strongly constrain the type of biological activity and thus sedimentary processes are the primary control on the recurring assemblages. These modern macrotidal estuaries appear to be useful modern analogs for specific occurrences of laminated siltstones of Carboniferous age in the eastern USA. Many Carboniferous facies have traditionally been interpreted to have formed within fluvial and lacustrine settings; however, similar to the modern macrotidal estuarine analogs, the Carboniferous siltstones contain upright trees (lycopods, calamites, and pteridosperms) and exhibit a variety of unique physical and biogenic sedimentary structures. Detailed analyses of series of successive laminae thicknesses reveals periodicities indicative of tidal controls on deposition ("tidal rhythmites"). Moreover, a number of other sedimentary features also support a tidal interpretation. Based upon the similarities described herein with the modern analogs, the Carboniferous rhythmites can be hypothesized to have formed within low salinity to freshwater estuarine tidal flats.

Recurring Assemblages of Biogenic and Physical Sedimentary Structures in Modern and Ancient Extreme Macrotidal Estuaries

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