Chalk Group Lithostratigraphy: Marlborough Downs/Berkshire Downs/Chilterns - Chalk Rock: Bottom Hardground Suite
The bottom hardground suite of the Chalk Rock comprises the Ogbourne Hardground overlain by the Pewsey Hardground (Bromley & Gale, 1982).
The Ogbourne Hardground is well developed in the Marlborough Downs and further south and west; norteastwards of Marlborough it quickly fades, and eventually disappears (Bromley & Gale, 1982). Where present it is typically a highly convolute, strongly glauconitised hardground (Bromley & Gale, 1982), the formation of which Gale (1996) linked to a major fall in sea level and ensuing erosion across a shallow marine platform in the late C. woollgariZone; deeper water in regions adjacent to this platform (e.g. parts of Dorset and Hampshire) developed much thicker coeval successions.
Above the Ogbourne Hardground is the Pewsey Hardground, a convoluted, weakly phosphatised hardground that like the Ogbourne Hardground, fades and disappears norteastwards of the Marlborough Downs (Bromley & Gale, 1982). This hardground was formed by transgression in the early S. neptuni Zone, following the regression responsible for the formation of the Ogbourne Hardground (Gale, 1996).
Macrofossil Biozonation: late C. woollgari and early S. neptuni ammonite zone (= T. lata Zone)
Correlation: see Correlation with other parts of the UK
BROMLEY, R G & GALE, A S. 1982. The lithostratigraphy of the English Chalk Rock. Cretaceous Research, Vol. 3, 273 - 306.
GALE, A S. 1996. Turonian correlation and sequence stratigraphy of the Chalk in southern England. In HESSELBO, S P & PARKINSON, D N (eds), Sequence Stratigraphy in British Geology, Geological Society Special Publication, No. 103, pp. 177-195.
See: C. woollgari Zone, S. neptuni Zone, T. lata Zone