Late Triassic Tidalflats

Towards the end of the Triassic, the first phases of marine transgression began and this first established sabkha shorelines with evaporating lagoons and eventually produced a set of restricted carbonate-shelf environments over parts of central and southern Britain. These latest Triassic (Rhaetic) deposits characteristically

Fig. 62 Late Triassic Tidal-flats a algal mounds

Fig. 62 Late Triassic Tidal-flats a algal mounds

section through part of growing algal mound

contain low diversity-high density communities like those of the Muschelkalk and, like these, also reflect life under high environmental stress conditions.

The illustration shows the probable mode of formation of one of the more famous of the British Rhaetic beds, namely the Cotham (or Landscape) 'Marble'. This consists of irregular 'bun-shaped' mounds of banded muddy and shelly limestone with well developed internal laminations that probably resulted from algal growth (Hamilton, 1961).

Recent analogues of such structures have been found in modern hypersaline bays such as Shark Bay, Western Australia. In

Shark Bay the stromatolitic structures are composed of sand-sized shell debris that has been trapped and bound by filamentous algae. The Cotham Marble stromatolites are associated with lenses of tiny gastropods that were presumably grazing upon the algal surfaces.

Not all of the Cotham Marble presents the typical laminated and arborescent structures. In places, the rock has a brecciated appearance and this 'Crazy Cotham' was formed by the sediment deposited within channels that existed between the growing mounds. Individual flakes and fragments within the breccias were themselves derived from the erosion of stromatolitic material.

Both the Cotham and Shark Bay mounds are penetrated by desiccation cracks and in contemporary Shark Bay the height of the mounds approximates to the present day tidal range. It is possible to extend this analogy and to infer that the height of the Cotham mounds (200mm) gives an approximation of the palaeo-tidal range.

Rhaetic Beds outcrop on the coast in southern Devon, Avon and northwestern Scotland. Inland exposures occur in southwestern England, Glamorgan and the Midlands. Poorer outcrops exist in Yorkshire. The Cotham Limestone facies is confined to the more southerly areas.

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