Changes in the provenance of pebbly detritus in southern Britain and northern France associated with basin rifting
Authors: Garden I. R. 
Published: April 2008

Stratigraphical and geographical variations in the composition of pebble suites in Upper Jurassic and Lower Cretaceous rocks in southern Britain and northern France provide a means for studying both local and regional changes in tectonic conditions associated with basin development. Local variations in pebble suites provide evidence of major fault-associated uplift and erosion of intrabasinal and basin-margin highs during earliest Cretaceous times, and the subsequent post-faulting subsidence and marine onlap in mid-Cretaceous times.

Regional provenance studies have demonstrated that the Upper Jurassic and Lower Cretaceous pebble suites of southern Britain and Normandy are separable into six assemblages.

Assemblage 1
is dominated by Carboniferous shelf chert, and is typical of Upper Jurassic pebble suites in Oxfordshire and Wiltshire, and Cretaceous pebble suites reworked from these older pebble beds. This detritus was probably derived from the western margin of the Anglo-Brabant Massif.

Assemblage 2
which is quartz dominated, includes Carboniferous basinal chert and tourmalinite, and is restricted to the Cretaceous pebble beds of Dorset. This assemblage was sourced principally from the Cornubian Massif.

Assemblage 3
contains subequal proportions of quartz and Carboniferous shelf chert and is present in the Lower Greensand pebble beds of the Isle of Wight, Hampshire, Wiltshire and Oxfordshire and Wealden pebble beds in the western and southern Weald. The distribution of this assemblage implies its derivation from the eastern margin of the Welsh Massif.

Assemblage 4
is characterized by a high proportion of sandstone clasts and is restricted to the Cretaceous pebble suites of Kent and East Sussex. Material in this assemblage was derived principally from the southern margin of the Anglo-Brabant Massif.

Assemblage 5
is compositionally akin to assemblage 3 but is restricted to the Albian pebble beds of the East Midlands Shelf. This material was derived from the northern margin of the Anglo Brabant Massif, with minor amounts of material from the Market Weighton High.

Assemblage 6
is quartz dominated and includes clasts of deformed sandstone and vein rock. This assemblage is restricted to the Aptian and Albian pebble beds of Normandy which are derived from the northern margin of the Armorican Massif.

In the Weald and Central Channel Basins, a change from chert-dominated assemblage 1 suites to quartz-rich pebble suites (assemblages 2 to 5) occurred at the beginning of the Cretaceous as a result of uplift and erosion of the marginal massifs adjacent to the rifting basin, accompanied by a regional fall in relative sea level. In contrast, on the East Midlands Shelf modifications to the detrital suite did not occur until the mid-Cretaceous, due to the Early Cretaceous reworking of Mesozoic detritus on the Shelf, probably from the uplift and erosion of the Sole Pit area. Subsequently, with the onset of subsidence in the Sole Pit area and a regional marine transgression during Albian times, the adjacent massifs were rejuvenated causing an input of fresh clastic material onto the Shelf.