Sternotaxis plana Zone
Base: The base of the zone in southern England has traditionally been defined at a lithostratigraphical feature, namely the base of the Bridgewick Flints or their lateral correlatives, following Jukes-Browne & Hill (1904). The latter was based partly on the apparently greater abundance of the index species in and above the flints than below, and partly because the Bridgewick Flints appeared to mark the junction between the traditional Middle and Upper Chalk subdivisions (Mortimore & Wood, 1986). In East Anglia, the Bridgewick Flints equate with part of the Brandon Flint Series, and in northern England, with part of the succession of large tabular and nodular flints that occur adjacent to the North Ormsby Marl (Mortimore & Wood, 1986).
Top: The top of the zone is also defined by a lithological feature, namely the top of the Navigation Hardgrounds in the Lewes Nodular Chalk of southern England and their lateral equivalents (Lake et al., 1987), which is slightly above the top of the range of the index species (Mortimore, 1986). The Navigation Hardground was inferred to be present in the BGS Trunch Borhole [TG 2933 3455], in East Anglia (Wood & Morter, 1994), but in northern England, where the correlative horizon appears to be only locally developed, the top of the zone is placed at the Kiplingcotes Marls (Gaunt et al., 1992; Whitham, 1991); these marls are thought to equate with the Navigation Marls that closely overlay the Navigation Hardground in southern England.
Range of index species: In southern England, the index ranges from the upper T. lata Zone to just below the top of the nominate zone (Mortimore, 1986), and in northern England, from around the middle of the T. lata Zone to just above the middle of the nominate zone (Gaunt et al., 1992).
|abundant pycnodonteine oysters*|
|Micraster corbovis (lata & plana Zone types)|
(*:occurs in the S. plana Zone in northern England)
Faunal abundance & preservation: The zone is characteristically fossiliferous, locally abundantly so in the middle and upper parts. Ammonites and other aragonitic molluscs are typically much commoner in this zone than at any other level in the White Chalk Subgroup.
Bio-markers: In southern England, ammonites, small forms of Micraster leskei, Spondylus spinosus and Inoceramus costellatus are common in the lower and middle parts of the zone. A rich aragonitic mollusc fauna, named the Reussianum Fauna after the characteristic ammonite Hyphantoceras reussianum, occurs towards the middle of the zone, where there is also an acme of large forms of Micraster leskei in the Lewes Marl (Mortimore, 1986). Above the Lewes Marl, species of Mytiloides become common, and there is a rich Micraster fauna comprising M. precursor and M. normanniae (Mortimore, 1986). In the top of the zone in southern England, there is an acme of the trace fossil Zoophycos (= Cuilfail Zoophycos of Mortimore, 1986), associated with common Sternotaxis placenta (Mortimore, 1986).
The plana Zone fauna is slightly less diverse in northern England, but some biomarkers, such as Hyphantoceras reussianum, S. placenta and common Zoophycos still occur, and in both successions Echinocorys gravesi becomes abundant in the top of the zone (Mortimore, 1986; Whitham, 1991). However, some aspects of the plana Zone fauna in northern England are specific to this region of the UK, suggesting that the area at this time was part of a separate biogeographical province. Such features include two acmes of pycnodonteine oysters in the upper part of the zone, named the Ulceby Oyster Bed and Enthorpe Oyster Bed, and an acme of the bivalve Didymotis, which may equate with a similar abundance in German successions (Gaunt et al., 1992). Infulaster excentricus and the sponge Cystispongia bursa are also features of the T. lata and S. plana zones in northern England, although they also occur locally near Swaffam in East Anglia, which Peak & Hancock (1970) suggested might be the dividing line between northern and southern faunal provinces.
Age: Late Turonian & Earliest Coniacian