The Wealden environment: Anglo-Paris Basin

Abstract

The Wealden cyclothems in the Weald are distinct from other sedimentary cycles so far reported. Their re-interpretation is based on recent sedimentological studies of analogous modern environments. The largely arenaceous lower part of each of the two megacyclothems in the Hastings Beds records the several environments of a growing and subsiding delta pile. Levee, crevasse and backswamp alluvium, pond-and-mere muds, channel sediments, shoreface sands, distributary mouth-bar and pro-delta deposits, etc., seem to be recognizable. The deltas advanced into lakes with fairly steady or falling water levels. Rates of deposition, up to 1 cm per 5 to 6 years, are suggested by certain of the fine-grained shoreface sands. ‘Classical' rather than birdfoot types of delta seem to be represented. The exposed portions were surrounded by extensive shallow water platforms. No major breaks of underwater slope are detectable: large-scale foresets are probably absent. The upper argillaceous parts of the megacyclothems, together with the back-delta (northern) portions of the arenaceous members immediately benath , are now regarded as having been formed during times of rising lake level. The clays and their basal beds are transgressive delta-front and pro-delta lakesediments, while the back-delta sands and silts below seem to have been valley-plug, levee and crevasse alluvium formed earlier behind the retreating coast in direct response to the rising base level. Alluvium also constitutes the more argillaceous parts of certain minor cyclothems of a different type. Extensive horsetail reedswamp grew offshore during times of retreat. The reeds were not able to establish themselves everywhere during periods of deltaic advance, owing to rapid silting, more frequent scour and the greater depth of suitable bottom sediments. Bigger plants as well as scattered horsetails grew on the surfaces of the deltas. Estuaries-of-inundation must have characterized these periods. The first major (Wadhurst) transgression advanced quickly, the reed beds in any one place lasting only a few years. Birdfoot deltas may have formed during its early stages for a short while. Close lithological, faunal, floral and petrological similarities between the cyclothems show that the same changes of environment were repeated several times. Probably the major fluctuations of lake level were in part relative only, being due either to changes in the rate of subsidence or to repeated river diversions causing the periodic abandonment of deltas and their consequent inundation. But, at certain horizons, continuity of deposition between the Wealden of Sussex and that of northern France is indicated by the petrology of the detritus. Hence the long-term fluctuations in lake level were probably also due in part to distant movements of the sea back and forth across the Paris basin. These in turn would have influenced channel ‘fixing’ and abandonment in the English alluvial plain. Short-term fluctuations of lake level were numerous. Some, near the base of the Wealden, may have been tidal in origin. Others took place mostly during times of deltaic advance. Several lines of evidence suggest the existence of well-marked seasons. Fundamental changes occurred during Weald Clay times, when the Wealden environments gradually coalesced and became transformed into a brackish arm of the advancing sea.