Lithology of the Chalk Group - Trace fossils

From Earthwise
Jump to navigation Jump to search

The main types of trace fossils recognised in the Chalk Group are : Zoophycos, Bathichnus paramoudrae, Chondrites, Trichichnus, Thalassinoides, Spongeliomorpha and Ophiomorpha. Mortimore & Pomerol (1991) recognised that acmes of one or two of these trace fossil types occurred throughout the Chalk Group, and could be used for correlation as well as a guide to relative sea level change. The most useful trace fossil events for correlation are as follows:

Southerham Tubular and Finger Flints: An abundance of tubular and finger shaped Thalassinoides flints in the Late Turonian (upper T. lata Zone). They are concentrated into a band below Southerham Marl 1, in the lower part of the Lewes Nodular Chalk Formation.

Lewes Tubular Flints: These flints form an interconnected network of hollow-centred tubes in the Late Turonian (middle S. plana Zone), representing the trace fossil Thalassinoides. They occur through as much as 3 m of chalk, a short distance below the Lewes Marl in the middle of the Lewes Nodular Chalk Formation.

Cuilfail Zoophycos: Named after the Cuilfail Tunnel at Lewes in Sussex, where the trace fossil event is well exposed in cliffs adjacent to the south portal. The abundant Zoophycos are preserved as dark marl-filled or silicified lenses. It occurs in a soft, marly chalk interval in the latest Turonian (top S. plana Zone) part of the Lewes Nodular Chalk Formation.

Beachy Head Zoophycos: In the Mid Coniacian (M. cortestudinarium Zone), near the top of the Lewes Nodular Chalk Formation, Zoophycos is abundant as dark marly chalk infilled lenses. This event is well developed in the cliffs at Beachy Head (Sussex), and can be traced eastwards to the London area, and westwards into south-east Devon (Mortimore et al., 2001)

Mid to Late Santonian Zoophycos Event: Zoophycos, typically preserved as spikey nodular flints, is abundant at the base of the M. testudinarius Zone, near the base of the Newhaven Chalk Formation.

Warren Farm Zoophycos Event: Abundant Zoophycos, usually preserved as thin, brown-skinned flint lenses, occurs in the Early Campanian (upper G. quadrata and basal B. mucronata zones), near the base of the Portsdown Chalk Formation. The event is well exposed at the Warren Farm chalk pit [SU 604 068], near Portsmouth, and at scattered inland localities near Dorchester.

Paramoudra Chalk: The trace fossil Bathichnus paramoudrae is locally so abundant in the Late Campanian (B. mucronata Zone s.l.) Chalk of north Norfolk that it has given its name to a chalk interval exposed on the coast between West Runton and Cromer, and fomerly exposed in old quarries near Norfolk.

References[edit]

MORTIMORE, R N & POMEROL, B. 1991. Stratigraphy and Eustatic Implications of Trace Fossil Events in the Upper Cretaceous Chalk of Northern Europe. Palaios, Vol. 6, 216-231.

MORTIMORE, R N, WOOD, C J & GALLOIS, R W. 2001. British Upper Cretaceous Stratigraphy, Geological Conservation Review Series, No. 23, Joint Nature Conservation Committee, Peterborough.