Tectonic controls on the variation in thickness and mineralogy of pebble-beds in the Lower Greensand Group (Aptian-Albian) of the Isle of wight, southern England

Pebble-rich horizons occur throughout the Aptian-Albian Lower Greensand Group of the Isle of Wight. The four distinctive pebble-beds selected for detailed analysis in this study occur above transgressive and/or erosive surfaces and have previously been used as stratigraphie markers. The Perna Member in the Atherfield Clay Formation marks the first Aptian transgression across the area. Pebble-beds at the bases of Members XI/XII and XIII, in the Ferruginous Sands Formation are associated with the regional manlniodes regression and the subsequent nutfieldieiuis transgression. The topmost pebble-beds in the Lower Greensand Group occur in the Carstone Formation which transgresses and oversteps across the Isle of Wight-Portsdown High to the north of the Channel Basin. The pebble-beds show different mineralogical contents, thickness distributions and palaeocurrent patterns to the surrounding sediments. The pebble-beds comprise fartravelled clasts derived from the distant Welsh Massif to the north and locally derived detritus eroded from Jurassic strata exposed on the Isle of Wight-Portsdown High. Most Lower Greensand Group formations thicken from north to south across the island, whereas the pebble-beds are typically thickest in the north and thin southwards. The pebble-beds are also associated with increases in kaolinite abundance relative to other parts of the Lower Greensand Group. The introduction of locally derived detritus together with evidence of local variations in the thickness, palaeocurrents and clay content of pebble-beds provides a clear indication of rejuvenation of topographic highs in the area and shedding of locally derived detritus into the adjacent basins. These changes resulted principally from contemporaneous movement along the Isle of Wight fault, uplift and erosion of the footwall and subsidence and deposition in the hangingwall basin. © 1997 Geologists' Association.