Southern Region Chalk Group Lithostratigraphy: Sussex lithostratigraphy of Mortimore (1986a) - Navigation Beds

The Navigation Beds, c. 6 m thick at the stratotype locality (Navigation Pit, Sussex [TQ 4250 1000]), are marked by the Snowdrop Flint 1 at the base, and the upper of the two Navigation Marls at the top (Mortimore, 1986a). The lower part of the succession comprises markedly soft, smooth-textured chalks compared to the usual hard, nodular textured chalk of the Lewes Member (Mortimore, 1986a). The trace fossil Zoophycos is abundantly preserved in the soft chalks, usually as millimetre-thin darker grey marly lenticles in transverse section, and compressed discs in plan view, and the interval is named the Cuilfail Zoophycos (Mortimore, 1986a). Sometimes the Zoophycos are silicified, forming distinctively shaped, scattered thin, small flints (Mortimore, 1986a). Sheet flints also occur, and conspicuous nodular flint horizons are named (in ascending order) Snowdrop Flint 2 , Snowdrop Flint 3 and the Navigation Flint (Mortimore, 1986a). In contrast, the upper part of the Navigation beds consists of three hard, conspicuously nodular chalk beds (locally merged into a single, hard, massive horizon), each surmounted by a hardground, and separated from adjacent hardgrounds by slightly softer chalk (Mortimore, 1986a). These are the Navigation Hardgrounds, which elsewhere, locally merged with other closely overlying hardgrounds, form a unit that has traditionally been referred to as the Top Rock (Mortimore, 1983, 1986a). Two closely spaced marl seams, named the Navigation Marls, usually overly the Navigation Hardgrounds (Mortimore, 1986a) although they may be absent in condensed successions.

The fauna of the Navigation Beds is characterised by echinoids, notably Micraster normanniae, abundant Echinocorys gravesi, and the large, inflated Sternotaxis placenta (Mortimore, 1986a; Lake et al., 1987). The acme of Zoophycos, in the context of the adjacent lithostratigraphical markers, is valuable for long range correlation.

Macrofossil Biozonation: top S. plana Zone

Correlation: see Correlation with other Southern Region Chalk Group classifications

see Correlation with other UK Chalk Group successions

References

LAKE, R D, YOUNG, B, WOOD, C J & MORTIMORE, R N. 1987. Geology of the country around Lewes. Memoir of the British Geological Survey.

MORTIMORE, R N.1983. The stratigraphy and sedimentation of the Turonian - Campanian in the Southern Province of England. Zitteliana, Vol. 10, 27-41.

MORTIMORE, R N.1986a. Stratigraphy of the Upper Cretaceous White Chalk of Sussex. Proceedings of the Geologists' Association, Vol. 97(2), 97-139.

See: Flint, Sheet Flint, Marl, Nodular Chalk, Hardground, Trace Fossils, Zoophycos, Cuilfail Zoophycos, Top Rock