Recognition of Chron C25n in the upper Paleocene Upnor Formation of the London Basin, UK

Abstract

Lithostratigraphic studies of four borehole cores drilled through the late Paleocene Lambeth Group (Upnor, Woolwich and Reading Formations) and basal London Clay Formation of central London have been supplemented with palaeomagnetic, calcareous nannoplankton and palynological data. The Woolwich and Reading Formations and the lower London Clay Formation are reversely magnetized and were deposited during the early part of Chron C24r. The first record of both NP9 and Chron C25n, hitherto missing from the Paleogene record in southern England, has been identified in the Upnor Formation (formerly the Woolwich Bottom Bed). It provides a key reference marker for linking events associated with the Paleocene-Eocene boundary (positioned within Chron C24r) to the type area of the internationally recognized Thanetian and Ypresian Stages. Work carried out to establish the chronostratigraphy of the Paleogene in southern England (Fig. 1) has been concerned principally with the late Paleocene Thanet Sand Formation and the early Eocene London Clay Formation (e.g. Aubry et al. 1986; Ali et al. 1993; Knox et al. 1994). The Thanet Sand Formation lies within calcareous nannoplankton zones NP6-NP8 and magnetochrons C26r to C25r (magnetochron nomenclature from Cande & Kent 1992). The base of Division B of the London Clay Formation (sensu King 1981) coincides with the start of Chron C24n.3n. The intervening Lambeth Group (Upnor, Woolwich and Reading Formations), Harwich Formation (Ellison et al. 1994) and lower part of the London Clay Formation are allocated to Chron C24r. Aubry et al. (1986) indicated that an hiatus in excess of 0.6 million years, marked by the absence of Chron C25n, separates the Thanet Sand Formation from the Lambeth Group. In recent years the drilling of fully cored boreholes (Fig. 2) through the Lambeth Group in central London has provided a unique opportunity for multidisciplinary study. One borehole in particular, Jubilee 404T (BGS Registration number TQ 37 NW 2118; National Grid Reference TQ 3363 7960), has become a key reference section for the Lambeth Group in central London (Ellison et al. 1994). The main reasons for its importance are firstly the wide variety of Lambeth Group facies, including marine sands, brackish lagoonal muds and pedogenic horizons, and secondly the presence 55