Chalk Group Lithostratigraphy: Northern England - Welton Chalk Formation
The Welton Chalk Formation (stratotype: Welton Wold Quarry [SE 970 282]), c. 44-54 m thick, comprises marly, thin bedded, flintless, locally shell-rich chalk in the basal few metres, passing upwards into hard, massive bedded chalk with sporadically developed, sometimes common, nodular flints and common, regularly developed marl seams, capped by thinner-bedded chalk (Gaunt et al., 1992; Whitham, 1991). The base of the Welton Chalk Formation is the sub-Black Band erosion surface, and the top is marked by the first bedding plane beneath the Ravendale Flint (Wood & Smith, 1978).
The basal sub-Black Band erosion surface is overlain by a thin bed of hard chalk and silty sediments containing a further erosion surface (Gaunt et al., 1992). Above this is up to 0.5 m of marly sediments, with a dark, carbonaceous marl at its centre, known as the Black Band, passing up into pebbly and shell-rich chalks, containing abundant broken shells of the bivalve Mytiloides (Gaunt et al., 1992). The shell-rich chalk interval is terminated by a group of three marls (Chalk Hill Marls), between which rare, small (c. 10 mm) flints occur, although the main entry of flint in the succession is slightly higher (Gaunt et al., 1992). The remainder of the succession mostly comprises very pure chalk, relatively softer than the overlying Burnham Chalk Formation, and also much less flinty (Gaunt et al., 1992). The flints are rather sporadic in their distribution, and show a concentration over particular intervals (eg. adjacent to the Grasby Marl, and between Barton Marl 2 to the Ferruginous flint; the First Main Flint is a conspicuous marker a couple of metres above the Chalk Hill Marls, although smaller, less well developed nodular flints occur just beneath it (Gaunt et al., 1992). The marls, green when fresh, but weathering orangey-brown, are conspicuous and very common in the sequence, typically vary from 10mm-100 mm thick, and form an important framework for correlation of exposures and borehole geophysical logs.
Apart from the abundant Mytiloides in the lower part of the succession, fossils from the remainder of the Welton Chalk (particularly above the Grasby Marl) include common brachiopods ( including Terebratulina lata) and inoceramids (mostly Inoceramus cuvieri, I. lamarcki and I. inaequivalvis), and echinoids (eg. Infulaster excentricus and Sternotaxis plana) are increasingly common in the higher part of the succession (Gaunt et al., 1992).
The key named markers in the Welton Chalk are as follows (many other marls and flints occur apart from those named):
DIAGRAM HERE
Macrofossil Biozonation: M. geslinianum (pars), N. juddii, Mytiloides spp. & T. lata Zone
Correlation: see Correlation with other UK Chalk Group successions
References
GAUNT, G D, FLETCHER, T P & WOOD, C J. 1992. Geology of the country around Kingston-upon-Hull and Brigg. Memoir of the British Geological Survey.
WOOD C J & SMITH, E G.1978. Lithostratigraphical classification of the Chalk in North Yorkshire, Humberside and Lincolnshire. Proceedings of the Yorkshire Geological Society, Vol. 42(2), 263-287.
See: marl, flint, Black Band