Abstract

Prograding clastic tidal coastlines in embayed areas, lagoons, and open coasts generate a graded, fining-upward sequence of sediments. This graded sequence is produced by a distinct zonation of sediment transport processes across a clastic intertidal flat. These transport zones are (1) tidal bedload transport (producing subtidal sand), (2) combination of tidal bedload transport and emergence runoff prior to exposure (producing low tidal flat sand), (3) alternation of bedload and suspension sedimentation (producing midflat interbedded sands and muds), and (4) suspension sedimentation (producing high tidal flat muds and clays). The distinction between subtidal and low intertidal flat sands is achieved by identifying sedimentary structures indicating late-stage emergence runoff prior to exposure in the low tidal flat sands. The contact between these two sands, one with and the other without emergence runoff sedimentary structures, coincides with the postion of mean low water. The top of the graded, fining-upward sequence consists of clay-sized sediment and coincides with the level of mean high tide.

Such prograded fining-upward sequences contain within them a preserved record of tidal range. In Holocene sediments, the thickness of the interval representing low tidal flat, midflat and high tidal flat sediments coincides with mean tidal range. In fossil equivalents, the thickness of similar sequences gives a quantitative measurement of paleotidal range.

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