Category:Standard zonation

Overview

Macrofossils are rather rare in the Chalk, and with the exception of the lower beds, ammonites, used as the basis of standard zonations elsewhere in the Mesozoic, are particularly uncommon and poorly preserved. For this reason, the standard biozonation of the Chalk is based on a mixture of ammonites, brachiopods, bivalves, crinoids and echinoids (see: standard macrofossil zonation & correlation of the UK Chalk Group), but correlation between this and the international ammonite zonation is not totally clear. More recently, inoceramid bivalves have been shown to have great utility in Chalk biostratigraphy (Jarvis and Woodroof, 1984; Gale, 1995, 1996). Microfossil biozonations for the Chalk (Carter and Hart, 1977; Hart et al., 1989; Swiecicki, 1980) can be indirectly related to the macrofaunal zones, and are valuable where other faunal data is absent or ambiguous.

Characteristic features of standard macrofossil zones/subzones and bio-marker horizons in the Chalk Group

The key features of the standard macrofossil zones and subzones are systematically described from oldest (Cenomanian) to youngest (Maastrichtian). Alternatively, to find out information about a specific zone or subzone, select it from the correlation table (see: standard macrofossil zonation and correlation of the UK Chalk Group). In the descriptions of zones and subzones that follow, 'northern England' means Lincolnshire and Yorkshire and 'southern England' is the remainder of the UK except East Anglia and Northern Ireland.