n8 MIDDLE ALBIAN STRATIGRAPHY 



(B) The Zonal Scheme of the Middle Albian in the Anglo-Paris Basin 

 (i) DEFINITION OF THE BASE OF THE MIDDLE ALBIAN 



Towards the top of the mammillatum Zone (puzosianus Subzone) in the Anglo- 

 Paris Basin the ammonite genus Pseudosonneratia (s.s. non Casey 1 ) develops as a 

 minority element subordinate to such hoplitid genera as Protohoplites s.s., P. (Hemi- 

 sonneratia) , Otohoplites, and Sonneratia as well as the common element consisting of 

 Douvilleiceras, Beudanticeras etc. The lyelliceratids are represented by very rare 

 specimens of Tegoceras. 



Pseudosonneratia of the pusosianus Subzone is the direct forerunner of the simple- 

 ribbed non-lautiform species of Hopiites of the dentatus group. Casey (1954b ; 112, 

 1961a ; 599) has separated off those species which are transitional between these two 

 genera as a subgenus Hopiites (Isohoplites). They are characterised by a partial 

 interruption in the strength of each rib along the siphonal line as it sweeps across the 

 venter, but no general en echelon offsetting of the ventro-lateral rib terminations 

 occurs although the tendency towards this is often apparent. Casey has demon- 

 strated that Hopiites (Isohoplites) characterises an horizon above the development of 

 the typical mammillatum Zone fauna and corresponds approximately to Spath's 

 inaequinodum Subzone (Casey 1961a ; 498 : Spath 1923b ; 73). He included the 

 Subzone in the dentatus Zone and subsequent work in France (P., & J. -P. Destombes 

 1965 : and herein) and England support this reading. 



The junction between the mammillatum Zone and the eodentatus Subzone has not 

 yet been seen in an uncondensed sequence in England. Sediments spanning the zonal 

 boundary have, however, been exposed in France at St. Martin in the Pays de Bray 

 (p. 100) and at Les Cotes Noires in the Haute Marne (p. 91). At both localities there is 

 a marked change in the ammonite fauna with the virtual disappearance of Otohoplites 

 at the top of the mammillatum Zone, and the appearance in bulk of Hopiites (Iso- 

 hoplites) at the base of the eodentatus Subzone above. In England the available 

 exposures of the sequence at this level show only condensed phosphatic nodule beds 

 in which the sudden change in the character of the ammonite fauna is probably 

 accentuated. The base of the Middle Albian in the Anglo-Paris basin is marked, 

 therefore, by the appearance of the genus Hopiites, and as is shown below by the 

 appearance of Lyelliceras. 



1 Dr. P. Destombes has collected a very fine ammonite fauna from two localities in the Bois de Perchois, 

 Aube, which I have visited in company with him. It is apparent from the material he has shown me 

 that Pseudosonneratia iserensis figured by Casey (1965 ; 541 text-fig. 203c, f) is associated in a very little 

 condensed sequence — the ' Red Bed ' — at Perchois Ouest with species of Cleoniceras (Neosaynella) and 

 Cleoniceras s.s., indicating afloridum Subzone age. It contains species of Otohoplites earlier than any yet 

 known in England, showing morphological gradations to the Pseudosonneratia-\ike forms which accom- 

 pany them in the same bed. The collection also contains species of Sonneratia with distinct kitchini 

 Subzone affinities, although higher in the succession, as well as Douvilleiceras, Beudanticeras and Pro- 

 tanisoceras. The fauna of Perchois Est, although preserved in a similar manner, occurs higher in the 

 mammillatum Zone sequence. The very important fauna obtained from these two sections is to be 

 described in due course by Dr. P. Destombes (see also Destombes 1965). The area is also of interest in 

 that at Perchois Ouest the grey clays above the ' Red Bed ' contains an ammonite fauna of distinct 

 tethyan aspect. 



