Pursuit of Wealden models

Abstract

Abstract: SUMMARY: Qualitative basinal models presented previously (Allen 1975, 1976) are developed. Outwash plain models for Weald-Wessex depict the freshwater arenaceous formations as low-angle watery coalescent fans (`fan-deltas’ auctt .), ‘moving lakes', flood-basins, static lakes, interfan depressions, etc. Five major rivers dominate, spreading out from fault-scarps on the basin margins. There are also minor streams, extra-basinal and intra-basinal in origin. During wet periods vigorous floods mobilize large areas of fine sand, silt and clay; during dry spells there are still large expanses of water around, especially in the distal facies, acting as floral and fauna1 refuges. Pedogenic processes frequently extend to the distal silts and clays. Gravel is scarce, especially in the Weald sub-basin, even at the fan-apexes, and suspension load is abundant. The component fans are therefore atypical and the model has resemblances to sand- and mud-dominated proglacial outwash plains, with ingredients from arid and sheetflood plains. Lake-lagoon-bay-estuary models describe the argillaceous formations. Shallow-water environments transgress the alluvial plains and salinities rise locally, but plant colonization and exposure of most lithofacies remain common. Floods sweep the basin from the same directions as before, but less frequently. They account for most of the shoreline aggradation in the lakes and lagoons. Therefore beach deposits are hardly differentiated except where wave erosion in the larger bodies attacks the higher parts of the fans. To what extent the Weald-Wessex models are applicable to the Western Approaches and Celtic Sea basins is uncertain. In the provincial model for S England the basin is bordered by tectonically active source-massifs and featured internally by horst and step-fault swells. Marginal upfaulting and high relief on the massifs generate the outwash plains; down-faulting and low relief generate the lake-lagoon-bay environments. Outcrops on the massifs are predominantly of sedimentary rocks underlying deep soils or exposed in deeply cut river courses. ‘Londinia’ exposes mostly Old Red Sandstone, Lower Carboniferous and Upper Jurassic; Cornubia-New Red Sandstone and Upper Jurassic; Armorica-Precambrian, New Red Sandstone and Jurassic (with Middle Jurassic igneous rocks or pyroclastics); Hibernia-Lower and Upper Palaeozoic. Contemporary volcanicity and associated mineralization affect Cornubia and probably the W Channel basin. The climate is warm, with periodic heavy rains producing leached upland soils supporting forests frequently swept by fires. Soft and intensely weathered rocks on the massifs account for the scarcity of gravel in the Wealden outwash. Widespread removal of Portlandian and Kimmeridgian enhances the sand and mud contents respectively; and, broadly, in that order. Salinities in the S England basin are controlled by the effectiveness of coastal protection against advances by the Boreal sea into the Londinia-Cornubia gap. During arenaceous phases (low salinities) the alluvial plain is well protected, aided by high inputs of freshwater from the elevated massifs. During muddy lake-lagoon-bay phases (higher salinities) the coastal ‘barrier’ leaks, or sometimes breaks, aggravated by reduced freshwater input from the sunken massifs. Common tectonic control of coastal barriers and massif relief is indicated. The final phase (Hauterivian-Barremian clays) records more serious breaches than any before. Sandy shoals of mixed marine and alluvial compositions invade the muddy sub-basins, tidal influences appear, and salinities locally rise higher than at any time since the start of the Cretaceous. Tectonic subsidence on this occasion was possibly aided by rising sealevel. The regional model for the S British Isles area comprises a framework of active massifs loosely linked by coastal deposits similarly protecting the intervening basins against the Boreal and Tethys seas. Three out of nine possible palaeogeographies are illustrated. In the first, Cornubia, Armorica and Hibernia join at the present continental shelf edge to form a massif bordering Tethys and from which the drainage in all three basins runs NE to the Boreal sea. The second depicts the Celtic and Approaches basins as draining SW into Tethys; S England drains to the Boreal sea as before. In the third, the three basins are silted-up gulfs formerly connecting the Tethys and Boreal seas. Each model is initiated by early Cretaceous elevation of the Jurassic seafloor and degraded massifs. Highest in the W, it presages the revival of true ocean spreading in the N arm of Tethys. Collapse by rifting, listric faulting, etc. near the present continental margin then ends the Wealden regime. This is thought to have begun earlier than sometimes proposed, viz. during the Hauterivian, when subsidence of the western massifs cut off the supply of bedload to wide areas, and the N and S seas both made notable advances. Control of Wealden sedimentation is therefore ultimately plate-tectonic. But, `Having burned my fingers so consumedly with the Wealden, I am fearful for you...' (Darwin, writing to Lyell on 25 November, 1860).