Micraster coranguinum Zone/Hagenowia rostrata Zone
In northern England, the equivalent of the M. coranguinum Zone of southern England and East Anglia has been referred to by some previous workers as the Hagenowia rostrata Zone (Whitham, 1993), while others have used M. coranguinum Zone (Gaunt et al., 1993). Since the differences between the zones appear to be nomenclatural rather than conceptual, they are treated herein as a single interval. The commonly occurring species at this level in northern England is actually Hagenowia anterior and not H. rostrata (Gale & Smith, 1982)
Base: The base 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.
Top: In southern England, the top of the zone is marked by Buckle Marl 1, and in northern England, the top of the coeval interval (= Hagenowia rostrata Zone; see above) is c. 9.5 m above Enthorpe Marl 1 of Mitchell (1994), between marls DD15 and DD16 of Whitham (1993; NB marls not shown on published log of succession). In the BGS Trunch Borehole [TG 2933 3455] in East Anglia there is no lithological marker for the top of the zone, which is placed immediately below the first record of Uintacrinus socialis (Wood & Morter, 1994).
Range of index species: In southern England, Micraster coranguinum ranges from slightly below the middle of the coranguinum zone, to within the overlying U. socialis Zone (Mortimore, 1986). Hagenowia anterior (i.e. the true index of the H. rostrata Zone of northern England; see above) ranges from the upper part of the M. coranguinum Zone in southern England, through the U. socialis and M. testudinarius zones, and into the U. anglicus Zone (Gale & Smith, 1982).
In northern England, Hagenowia anterior (= H. rostrata & H. blackmorei of Whitham, 1993) occurs in the Lower and Upper H. rostrata zones, the U. socialis Zone, and sporadically in the I. lingua Zone (Mitchell, 1994), while M. coranguinum is apparently confined to the Lower Hagenowia rostrata Zone (Whitham, 1993).
Exposure of the zone is poor in East Anglia, but Peake & Hancock (1970) recorded that M. coranguinum did not range above its nominate zone.
|Hagenowia anterior *|
|Micraster (Isomicraster) gibbus|
(*: more characteristic of the zone in northern England than in southern England)
Faunal abundance and preservation: In southern England, the lower and higher parts of the zone are fairly fossiliferous, but the central part is characteristically almost barren of fauna. Fossiliferous parts of the zone are usually dominated by laterally extensive bands of abundant comminuted inoceramid shell. In northern England, the lower part of the zone, with common inoceramids and sponges, seems more fossiliferous than the upper part, but generally the fauna is rather sparse and of low diversity (Whitham, 1991, 1993). There are scattered faunal records from infrequent exposures of this zone in East Anglia.
Bio-markers: All regions show an acme of Volviceramus involutus, and locally also V. koeneni, in the lower part of the zone. In southern England, this acme reaches its maximum at the Seven Sisters Flint, the possible correlative of which in northern England is the De-la-Pole Flint, adjacent to which V. involutus is similarly common (Gaunt et al., 1992). In southern England, an undescribed species of Cremnoceramus occurs in the basal M. coranguinum Zone, below the appearance of Volviceramus. Platyceramus is long-ranging in the zone, but in southern England, the shells appear to be thickest (i.e. greater than 5 mm) in the lower part of the zone (to just above the Seven Sisters Flint), and near the top of the zone. An acme of sponges occurs above the De-la-Pole Flint in northern England (Whitham, 1991), and in southern England and East Anglia, the Coniacian / Santonian boundary is indicated in the middle of the zone by the lower of two acmes of Cladoceramus undulatoplicatus (Bailey et al., 1984; Wood & Morter, 1994). There is an increase in faunal diversity in the Santonian, particularly near the boundary with the Coniacian, where Gibbithyris ellipsoidalis has a restricted range in southern England (Mortimore, 1986). Cordiceramus and Sphenoceramus occur in the higher part of the zone in all regions, and Conulus albogalerus is locally abundant near the top of the zone in southern England and East Anglia, being a major component of Rowe's Echinoid Band (Mortimore, 1986; Robinson, 1986; Wood & Morter, 1994). Whitham (1993) recorded acmes of Hagenowia blackmorei near the top of the zone in northern England, but these are in fact Hagenowia anterior (Mitchell, 1994).
Age: Late Coniacian & Early Santonian