Triassic palaeogeography of the British Isles

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Abstract
By means of a series of lithological-isopachous maps the palaeogeography of the British Isles is illustrated for six divisions of the Triassic period. In the absence of sufficient biostratigraphical and chronostratigraphical control these six divisions of the Trias are defined in lithostratigraphical terms. Evidence is produced to show that the grabens and most of the principal basins in which the Triassic sediments accumulated were structurally controlled. These fault-controlled regions of deposition were formed before the beginning of the Triassic period; they were probably initiated at the end of the Hercynian orogeny. Three major tectonic trends appear to have influenced the orientation of the grabens: (i) Caledonoid trend in the Irish Sea graben, Cardigan basin, Carlisle basin and partly in the Cheshire graben; (ii) Charnoid trend in the Cheshire graben, Needwood basin and Bosworth trough; (iii) A north–south or Mal vernian trend in the Worcester graben, Eccleshall basin, Wessex basin, and possibly in the St Georges graben. Faulting during the Triassic period appears to have had important influence on the thickness and nature of the sediment deposited. The activity and influence of the faulting waned towards the end of the Triassic and appears to have ceased by the beginning of the Rhaetic. Faulting also controlled to some extent the sources of sediment by periodically causing the floor of the northern end of the Worcester graben to subside more than the southern end, so that this structure remained the principal channel through which passed much of the sediment deposited in central and northern England. In this way the Midlands were supplied with detritus derived from the Hercynian uplafids throughout most of the Trias. Faulting was probably also responsible for the periodical relative uplift of the other sediment source areas of Scotland, Wales and the London–Brabant massif. The majority of the Triassic sediments deposited in the British Isles appear to have formed in a continental environment under a variety of conditions: torrential, fluviatile, lacustrine and possibly aeolian. Towards the middle of the Triassic succession a widespread marine transgression affected large parts of the British Isles and resulted in the deposition of the Waterstones Formation. The climate of the British Isles throughout the Triassic period was probably hot with wet and dry alternating seasons. The sedimentary structures of much of the Triassic sequence testify to the presence of rivers and lakes, and so indicate that water was abundant at least periodically. With the progressive reduction of relief of the landscape throughout the Trias and the establishment of extensive bodies of standing water, in which the Waterstones and the Keuper salt were deposited, there must have been a correspondingly progressive change in the climate during the Trias. The gradual advance of marine conditions towards the British Isles during the Trias may also have influenced the climatic pattern. At the beginning of the Trias the landscape had considerable relief with uplands in Wales, Scotland and parts of Ireland. The Pennines, Lake District, Gornubia and the London–Brabant massif also formed uplands but probably less elevated. Between these uplands huge braided rivers flowed during the deposition of the rocks of divisions 1 & 2. As the relief was reduced these rivers changed their nature, they probably formed meanders and occasionally lakes during the deposition of division 2. The rocks of division 3 seem to have been deposited largely in lakes and meandering rivers and partly under the influence of the Muschelkalk sea which invaded the broad flat plains of the British Isles for a short interval. The rocks of division 4, which contain the thick salt and saliferous mudstones, were deposited mainly in lakes covering a wide flat surface with surrounding uplands of low relief. The large quantities of gypsum, anhydrite, dolomite and rock-salt in the strata of division 4 may have been derived from Triassic sea-water or from the reworking of older evaporite deposits. Whatever the source of the chloride and sulphate evaporite minerals, the accumulation of rock-salt in considerable quantities in division 4 seems most likely to have been the result of two main factors: high evaporation associated with high temperatures and very low rainfall, and topographical depressions subsiding in the grabens into which ground-water brines drained with the development of salt lakes. The sulphate minerals, which are much more widespread than the rock-salt in division 4, must also have been produced under high evaporation conditions. It seems likely that the mechanism of their formation is related to ground-water evaporation similar to processes described by Shearman (1966) . The rocks of division 5 were also deposited in ephemeral lakes that covered large flat areas of the British Isles. The upland areas must have been of very low relief. The large quantities of gypsum and anhydrite were probably formed by evaporation of ground-water solutions under conditions of inland drainage. Whether the sulphate solutions in the ground waters were derived from contamination with Triassic sea-water or from reworking of older evaporite deposits in the North Sea basin is uncertain. Finally the widespread and important marine transgression occurred which deposited the Rhaetic and brought the Triassic period to a close.