Detrital zircon U-Pb ages and source of the late Palaeocene Thanet Formation, Kent, SE England

Thomas Stevens,Yunus Baykal

doi:10.1016/j.pgeola.2021.01.003

Abstract

The sources of the Paleocene London Basin marine to fluviodeltaic sandstones are currently unclear. High analysis number detrital zircon U-Pb age investigation of an early-mid Thanetian marine sand from East Kent, reveals a large spread of zircon age peaks indicative of a range of primary sources. In particular, a strong Ediacaran age peak is associated with the Cadomian Orogeny, while secondary peaks represent the Caledonian and various Mesoproterozoic to Archean orogenies. The near absence of grains indicative of the Variscan orogeny refutes a southerly or southwesterly source from Cornubia or Armorica, while the strong Cadomian peak points to Avalonian origin for a major component of the material. Furthermore, the relatively well expressed Mesoproterozoic to Archean age components most likely require significant additional Laurentian input. Comparison to published data shows that both Devonian Old Red Sandstone and northwesterly (Avalonia-Laurentia) derived Namurian-Westphalian Pennine Basin sandstones show strong similarities to the Thanetian sand. This pattern is consistent with derivation of Thanetian material via a SE draining proto-Thames River system that was initiated in the Paleocene due to uplift of western and northwestern Britain. This river system would have incised and eroded cover sandstones and potentially also Avalonian basement of mid to north Wales and England. However, the possibility of a contribution of Laurentian grains directly from the north via longshore drift cannot be excluded by the data, and the extent to which the sediment source signatures of Paleogene sands of the London Basin are variable both geographically and over time remains unclear.

Highlights

The late Paleocene Thanet Formation of the London Basin comprises up to 30 m of shell rich, buff-grey coloured, glauconitic silty fine sand deposited in a shallow marine open water setting (Ellison et al, 1994)
Armorican basement exhibits a characteristic age peak centred around 1.9 Ga, which is absent in the Thanet Bed sample and the other potential source areas
Analysis of U-Pb age for a large number (302) of detrital zircons of a Thanetian age shallow marine sandstone from Pegwell Bay in East Kent reveals a large range of age peaks that suggest ultimate derivation from both Avalonian and Laurentian basement rocks

Summary

Introduction

The late Paleocene (early-mid Thanetian; 59.2À56 Ma) Thanet Formation of the London Basin comprises up to 30 m of shell rich, buff-grey coloured, glauconitic silty fine sand deposited in a shallow marine open water setting (Ellison et al, 1994) The provenance of this sandstone, along with other rocks of the early Paleogene in the London Basin, and its relationship to palaeogeography and river drainage, has been a considerable source of debate. It has been hypothesised that at the end of the Cretaceous this area was covered by up to 3 km of post-Triassic sediment, subsequently eroded during the early Paleogene uplift (Lewis et al, 1992; White and Lovell, 1997) Under this scenario, a proto-Thames River would have transported eroded sediment from these uplifting and denuding areas, which may imply that Thanetian age marine sediments are derived from this former, mostly removed, post Triassic cover, or underlying rocks currently exposed in the area today. As part of a wider study into the origin of Quaternary loess sediments in southern Britain (Stevens et al, 2020), we present here results of high n (Pullen et al, 2014) detrital zircon U-Pb age analysis of a sample of shallow marine sands of the Thanet Formation at one of the stratotype sections, Pegwell Bay, Isle of Thanet, East Kent, UK

Site and methods

Results and discussion

Conclusions