Micraster cortestudinarium Zone

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Base: In southern England and East Anglia, the base of the M. cortestudinarium Zone is placed at the top of the Navigation Hardground (Lake et al., 1987). In northern England, where the equivalent hardground is possibly only locally developed, the base of the zone is tentatively placed at the level of the Kiplingcotes Marls, which are inferred to equate with the Navigation Marls that immediately overlay the Navigation Hardground in southern England (Gaunt et al., 1992).

Top: The top of the zone is marked by Shoreham Marl 2 in southern England, and its correlative, Little Weighton Marl 2, in northern England (Mortimore, 1986; Gaunt et al., 1992). In East Anglia, a low resistivity spike at 411.5 m in the BGS Trunch Borehole [TG 2933 3455] was inferred to equate with Shoreham / Little Weighton Marl 2. However, the top of the zone in Norfolk has previously been defined by the lowest limit of Micraster with 'divided' or 'strongly divided' ambulacra, which would include strata equivalent to the lowest third of the M. coranguinum Zone of southern England (Peake & Hancock, 1970; Drummond, 1983, Fig. 5).

Range of index species: In southern England, the index ranges within the zone, but does not occur near the base or top of the zone (Mortimore, 1986). Poor exposure in East Anglia means that accurate range data is lacking, but in northern England, the index (sometimes referred to as Micraster decipiens) seems to mostly occur in the lower and middle part of the zone (Whitham, 1991).

Key fauna:

Brachiopoda: Cretirhynchia subplicata
Bivalvia: Cremnoceramus spp.
Echinoidea: Echinocorys gravesi
E. scutata
Infulaster spp.*
Micraster bucaillei*
M. cortestudinarium
Micraster normanniae
M. aff. turonensis
Tylocidaris sp.

(*: characteristic of the zone in northern England)

Faunal abundance & preservation: The zone is typically less fossiliferous than the S. plana Zone below, especially in parts of East Anglia (Peake & Hancock, 1970), and the fauna is much less diverse.

Bio-markers: The lower part of the zone in southern England and parts of northern England is characterised by common Micraster normanniae and Echinocorys gravesi (Mortimore, 1986). In southern England, the replacement of M. normanniae by M. cortestudinarium occurs at the Hope Gap Hardground (Mortimore, 1986). Cremnoceramus spp. is common through the greater part of the zone in all regions, including the Trunch Borehole [TG 2933 3455] in East Anglia, however, the lowest Cremnoceramus are usually C. ex gr. waltersdorfensis and C. deformis erectus, whilst higher records mostly comprise C. crassus and C. deformis (Mortimore, 1986; Wood & Morter, 1994; Gaunt et al., 1992). Recently, Walaszczyk & Wood (1998) recorded that C. waltersdorfensis occurred in the latest Turonian (presumably uppermost S. plana Zone equivalent) in the expanded Salzgitter-Salder quarry in Germany. Micraster bucaillei is a specific feature of the middle and higher parts of the zone in northern England (Whitham, 1991; Gaunt et al., 1992), demonstrating the presence of faunal provincialism at this time. M. bucaillei does occur in southern England, but not until the middle part of the M. coranguinum Zone (Mortimore, 1986). The top of the zone in southern England is characterised by Micraster aff. turonensis (Mortimore, 1986).

Age: Coniacian