Lithology of the Chalk Group - Marls
Clay-rich horizons in the Chalk Group are called marls. They have higher water retention than the adjacent chalk, and are often preferentially vegetated in outcrops. Where visible in weathered outcrops, marls may appear as shaley horizons and/or be preferentially eroded, but in fresh exposures their darker colour might be the only distinguishing feature. Away from basin margins, many marls are geographically extensive and are of value in long range correlation. In borehole geophysical logs, they appear as spikes in resistivity profiles, and generally, as peaks in gamma profiles, and are important for the sub-surface correlation of the Chalk. Some marl seams also have distinct patterns of trace element geochemistry (Wray and Gale, 1993), augmenting their usefulness for correlation. In the lower part of the Chalk, the relatively thick beds of marl appear to represent phases when suppressed carbonate sedimentation allowed clay sediments to concentrate and accumulate (Ditchfield and Marshall, 1989). The exact origin of the thinner marls in the higher part of the Chalk is unresolved, but some have been interpreted as decomposed volcanic ashes (e.g. Shoreham Marl 2) and others as the result of an enhanced influx of land-derived sediment (Wray, 1999; Wray and Wood, 1995).
WRAY, D S. 1999. Identification and long-range correlation of bentonites in Turonian-Coniacian (Upper Cretaceous) chalks of northwest Europe. Geological Magazine, 136, 361-371.
WRAY, D S & GALE, A S. 1993. Geochemical correlation of marl bands in Turonian chalks of the Anglo-Paris Basin. In HAILWOOD, E A & KIDD, R B (eds), High Resolution Stratigraphy, Geological Society Special publication No. 70, 211-226.
WRAY, D S & WOOD, C J. 1995. Geochemical identification and correlation of tuff layers in Lower Saxony, Germany. Berliner geowiss. Abh., E16, 215-225.