7 o MIDDLE ALBIAN STRATIGRAPHY 



(b) Relationship of Winchester to Portsdown, the Weald, and the Isle 



of Wight 



The correlation of the Gault sequence in the Winchester No. i boring with those of 

 Selborne, Nyewood, and Compton Bay, is shown as far as is possible in text-fig. 30. 

 The lithological succession shown by the Winchester borings is comparable to that of 

 the outcrop at Selborne on the western border of the Weald. They both show in the 

 spathi Subzone sediments a lower pyritic facies overlain by clays in which the shells 

 are preserved and in which a good benthos is present (Owen 1963a ; 43-44). More- 

 over, at Bradshott Hall clays of basal Upper Albian age were seen by Osborne- White 

 (1910 ; 20) to overlie clays classified with the ' interruptus Zone ' in Jukes-Browne's 

 sense. The lower clays could be of intermedins Subzone age (Owen 1963a ; 51). 



From the map given in text-fig. 18, it is apparent that the borings at Winchester 

 and Portsdown, and the natural exposure at Culver Cliff in the Isle of Wight all lie 

 roughly on the same NNW-SSE. line. The total thickness of the Gault at Win- 

 chester No. 1 is 250 feet (76-2 m.) compared with 163 feet (49-68 m.) at Portsdown 

 situated 15 miles to the SSE., and just over 100 feet (30-4 m.) at Culver Cliff. No 

 information on the degree of ammonite subzonal representation in the chipped 

 sequence of the British Petroleum Portsdown boring in known (Taitt & Kent 1958), 

 but, in Hampshire, West Sussex, and in the Isle of Wight, there is a similar change in 

 facies within the spathi Subzone from pyritic clays below to shelly clays above. 

 Superficially it seems that as one proceeds SSE. from Winchester the decreasing 

 thickness of the sediments suggests the shallowing of a basin in this direction. How- 

 ever, at Winchester the lower part of the Upper Albian is represented by at least 

 128 feet (39-01 m.) of silty Gault, more than the total thickness of the Gault at 

 Culver Cliff in which the lower part of the Upper Albian is also represented (p. 43). 

 The decrease in thickness of Middle Albian sediments from Winchester to Culver 

 Cliff is, therefore, not particularly well marked. It is also apparent that the detailed 

 lithological sequence in the spathi Subzone sediments which is recognisable for a 

 distance of over 35 miles at the outcrop in the south western part of the Weald is 

 totally different from that of the Isle of Wight (text-fig. 30). The common distri- 

 bution of the pyritic facies reflects the presence of a common sea environment 

 which affected different depositional areas, and it is apparent from the pre-Albian 

 sequence at Portsdown that this area formed a ridge separating the area of the south 

 western Weald from that of the Isle of Wight. 



The Lower Greensand in the south-western area of the Weald thins rapidly towards 

 Portsdown (Falcon & Kent i960). Continuing SW. into the Isle of Wight the Lower 

 Greensand as a whole thickens from Redcliff NE. of Sandown to reach a known 

 maximum in the southern part of the Island and this increase in thickness can be 

 correlated with an increase in finer grade sediments. It is apparent from the dis- 

 tribution of lyelli and spathi Subzone sediments that the Portsdown ridge affected 

 Middle Albian sedimentation. By Middle Albian times this submarine feature con- 

 sisted of an elongated swell (text-fig. 52), and its extent can be determined by the 

 presence of the ' Iron Grit ' beneath the Gault and the absence of tardefurcata and 

 mammillatum Zone phosphatic nodule beds. The Lyelli Subzone sediments are 



