The provenance of upper Jurassic and lower cretaceous coarse-grained detritus in southern Britain and Normandy

Abstract

The provenance of coarse-grained detritus from the Upper Jurassic Corallian Group and Lydite Beds and Lower Cretaceous Purbeck Formation, Spilsby Sandstone Formation, Wealden Group, Claxby Ironstone Formation, Lower Greensand Group and Gault Clay Formation of southern Britain and Normandy (northern France) has been studied. The coarse-grained detrital suites contain extrabasinal phenoclasts derived from Palaeozoic and Precambrian massifs, and Jurassic and Cretaceous clasts from intrabasinal highs. The former assemblage is dominated by chert derived from Carboniferous limestones and quartz. Sandstones, radiolarian cherts and tourmalinites are locally abundant. The intrabasinal detritus is composed principally of chert, phosphorite and ironstone.Progressive changes in phenoclast suites provide a means for studying both local variations associated with uplift of intrabasinal structures and regional variations associated with basin development.Local variations in the intrabasinal pebble suites of Dorset provide evidence of major fault-associated uplift of the South Dorset High during earliest Cretaceous times. Post-faulting subsidence resulted in marine onlap and further erosion of the structure. The South Dorset High is structurally similar to other fault-associated highs in the Wessex and Paris Basins, and it is probable that they have all undergone extensive Early Cretaceous uplift in conjunction with basin rifting.The Upper Jurassic and Lower Cretaceous phenoclast suites of southern Britain and Normandy are separable into three stratigraphical assemblages. The oldest assemblage, Oxfordian to Early Berriasian in age, is dominated by chert derived from Carboniferous limestones. The two younger assemblages (Late Berriasian to Barremian and Aptian to Middle Albian) are distinguished by a generally higher proportion of quartz. The Aptian to Middle Albian phenoclast assemblage is separable into two subassemblages (Aptian and Lower Middle Albian) and the latter shows lateral changes which enable it to be divided into three geographical provinces and five subprovinces. In the Wessex Basin, a change from chert-dominated to quartz-dominated phenoclast suites occurred at the beginning of the Cretaceous System. This resulted from uplift and erosion of the marginal massifs adjacent to the rifting basin. On the East Midlands Shelf, however, modifications in the detrital suites did not occur until mid-Cretaceous times. The delayed change was caused by limited uplift of the adjacent massifs during Early Cretaceous times and reworking of slightly older pebble beds on the shelf. The marginal highs were then uplifted and eroded during the mid-Cretaceous phase of thermal subsidence in the Southern North Sea Basin.

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