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The presently transgressing sea has a marginal low and intermediate energy zone of tidal marshes and lagoons, protected from the wave energy of the open ocean by barrier beaches. Peats and muds are being deposited in the tidal marshes, sands and muds in the lagoons. The sands are largely derived from the ocean, and are deposited as tidal deltas, which start at gaps in the barrier beaches, and grow across the lagoon toward the mainland. The lagoons have biotas of variable character, depending on bottoms and salinity, but distinct from those of the open sea.
With the rise in sea-level, the lagoons transgress over the land, and the barrier beaches follow behind them, transgressing over older lagoonal deposits. The waves breaking at the face of the barrier are cutting into older lagoonal sediments, and are reworking "fossil" lagoonal shells into the sands of the open-shore and near-shore bottoms. The aragonitic members of the lagoonal fauna have been diagenetically eliminated, but distinctive calcitic lagoonal shells are thus being mixed into the open-sea assemblage. It is not known how much, if any, of the lagoonal sedimentary record escapes this erosion by the advancing surf zone.
Transgressive seas of the past, advancing over a lowland surface, are likely to have had a marginal belt of brackish or saline lagoons. The sediments which come first to be deposited on the former land surface are low-energy sediments of non-marine or semi-marine character. Insofar as these primary deposits of the transgression are not entirely removed by the surf zone advancing behind them, their remnants come to be disconformably overlain by the higher-energy, offshore deposits of the main seaway. Thus transgressive sequences are likely to contain a marked intra-sequential disconformity.
Transgressive sequences of this sort, and matching regressive sequences, which are longer and lack the disconformity, are found in the Cherokee (Pennsylvanian) cyclothems of southeastern Kansas and adjacent parts of Oklahoma and Missouri.
The classical Pennsylvanian cyclothems of the Illinois basin can also be interpreted in a parallel manner. Here the middle and upper shales are considered as the sediments of transgressive and regressive lagoons respectively--lagoons in an extended sense, namely, more or less brackish seas occupying the whole of the Illinois basin, and barred in part tectonically, in part by limestone deposits (calcarenites and algal banks).
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