Basement weathering at the Lower Palaeozoic unconformity in the Channel Islands and northern Brittany

Deep weathered profiles are developed in igneous basement rocks immediately beneath the Lower Palaeozoic elastic sequences of Alderney, Jersey and northern Brittany. The weathering features are interpreted to have developed during the early Palaeozoic. In western Alderney, homogeneous quartz diorite host rocks are relatively uniformly decomposed whereas jointed porphyry dykes preserve spectacular corestone weathering morphologies. In central southern Alderney the diorite basement exhibits only a thin weathered skin but is mantled in a degraded sequence of rotten corestone boulders, interpreted to represent fossil colluvium. At an inland locality in northern Alderney, degraded diorite profiles feature in-situ corestones. Rotten corestones are also present at the unconformity surface and are
included in the overlying fluvial sediments. Basement weathering at the unconformity surface in Jersey can only be positively identified at one locality. Here angular fragments of rhyolitic breccia are preserved spalled from weathered rhyolite profiles and are interbedded with alluvial fan sediments. On the Côtes du Nord, northern Brittany, a 7m thick sequence of homogeneously degraded granodiorite occurs beneath alluvial fan deposits. Although the evidence is not fully conclusive, this weathering profile might also have developed in the early Palaeozoic.