Willesden Green field is a long narrow sandstone body completely encased in marine mudstones. The field is Late Cretaceous (Turonian) in age, and contains as many as seven stacked, sandier upward sequences. The sequences are as much as 12 m (39.3 ft) thick, are stratigraphically distinct, and do not offlap systematically. Each sequence begins with marine bioturbated mudstones and passes upward into storm-dominated shallow marine sandstones. Each sequence shows a lateral shaling-out from hummocky cross-stratified sandstone into bioturbated mudstone. Isopach maps show that the sequences thin basinward at a rate of 15-67 cm/km (9.45-42.21 in./mi), and suggest dispersal directions ranging from north-northeast to southeast.
The stack of marine sequences is dissected by a basin-wide erosion surface that initially formed subaerially, following a major relative lowering of sea level. During ensuing transgression, a series of stepped shorefaces was cut into the stack of sequences, removing all evidence of subaerial exposure. The shorefaces in most places are overlain by a thin pebble veneer, which in turn is buried by thick transgressive marine mudstones. In places, the pebble veneer thickens into conglomerates as much as 20 m (65.6 ft) thick that form reservoirs at Carrot Creek, Pembina, and Ferrier, as well as Willesden Green. These conglomerates are interpreted to have been deposited at river mouths, and locally reworked along the shoreface by waves. During continued transgression, the stacked sequences and shoreface conglomerates were buried by marine mudstones. Overall reservoir characteristics are determined by the geometry and stacking of the sequences. Stratigraphic traps have been formed partly by lateral shaling-out and partly by mud-draped incised shorefaces. Locally, shoreface gravels are good reservoirs, and the overlying transgressive mudstones form the traps.
We believe that the long, narrow geometry of the field represents an erosional remnant of a formerly continuous sand sheet; the field is not an offshore bar.