148 MIDDLE ALBIAN STRATIGRAPHY 



active erosion during this period, and the Middle Albian sea may well have extended 

 further during later Subzones. In the writer's opinion all the evidence points to a 

 source in the clays of the Jurassic, west of the London Platform, for the sediment 

 which was redeposited as the Lower Gault. 



A Jurassic source to the west in Wales is suggested by the work of Jones (1955 ; 

 348-50), and by the borings at Port More, Antrim (Robbie & Manning 1966), and 

 Mochras, Merioneth (Wood & Woodland 1969) in which Lias is preserved. At 

 Port More, the incomplete remnant of Lower Lias is overlain by Upper Chalk 

 indicating a major intra-Mesozoic hiatus, the exact nature and extent of which is 

 uncertain at present. 



Much the same state of affairs existed in the Paris Basin and it is an interesting 

 fact that here also the Middle Albian sediments are coarser in the west and finer in 

 the eastern areas of the Basin. 



(d) The cristatum Subzone disturbance 



After the commencement of the cristatum Subzone, a major disturbance affected 

 the whole of the Anglo-Paris Basin and adj oining areas. In fact a break in sedimenta- 

 tion associated with erosion occurs widely throughout the Earth at about this time, 

 and is sometimes accompanied by folding. The disturbance caused the partial 

 planing-off of Middle Albian sediments over the whole area of the Anglo-Paris Basin . 



The writer considered (i960 ; 377) that the planing-off of the upper surface of the 

 Lower Gault in southern England was due to a tilting movement up towards the west. 

 This may be true for the eastern half of England where definite early Upper Albian 

 faulting has been proved (Owen in press), but it is not necessarily the explanation 

 for the southern part of the country as a whole. In France, there is some evidence 

 of a similar tilting movement towards Morvan and Armorica. Although the effects 

 on the sediments is readily apparent, the main cause is much more obscure and may 

 be connected with faulting at the margin of Europe and America before the later de- 

 velopment of the Atlantic Ocean (p. 138). 



Tectonic features are few in number, and there is certainly no evidence of anything 

 but a slight broad warping of the Basin as a whole, except for the faulting mentioned 

 above which removed Lower Gault sediments at least from the southern part of 

 Essex. The turbulent water conditions are reflected in the nektonic fauna ; for 

 example Inoceramus concentricus quite rapidly develops the far stronger sulcatus 

 form, and does not revert back to a concentricus form until the varicosum Subzone 

 when thick, little condensed sequences are seen again. 



On the resulting planed-off Middle Albian surface Upper Albian sediments were 

 laid down in an entirely different pattern to that seen in the Middle Albian. More- 

 over, there are two intergrading facies ; the Upper Greensand and the Upper Gault. 

 This change in pattern renders meaningless isopachyte maps based on the sediments 

 of the whole Stage (Wooldridge & Linton 1938). It appears at present that the 

 Upper Albian depositional pattern is more closely related to that of the Upper 

 Cretaceous. 



