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FULLTEXT01.Pdf G Model PGEOLA-885; No. of Pages 9 Proceedings of the Geologists’ Association xxx (xxxx) xxx–xxx Contents lists available at ScienceDirect Proceedings of the Geologists’ Association journal homepage: www.elsevier.com/locate/pgeola Detrital zircon U-Pb ages and source of the late Palaeocene Thanet Formation, Kent, SE England Thomas Stevens*, Yunus Baykal Department of Earth Sciences, Uppsala University, Villavägen 16, Uppsala, 75236, Sweden A R T I C L E I N F O A B S T R A C T Article history: The sources of the Paleocene London Basin marine to fluviodeltaic sandstones are currently unclear. High Received 25 November 2020 analysis number detrital zircon U-Pb age investigation of an early-mid Thanetian marine sand from East Received in revised form 14 January 2021 Kent, reveals a large spread of zircon age peaks indicative of a range of primary sources. In particular, a Accepted 15 January 2021 strong Ediacaran age peak is associated with the Cadomian Orogeny, while secondary peaks represent the Available online xxx 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 Keywords: strong Cadomian peak points to Avalonian origin for a major component of the material. Furthermore, the Proto-Thames relatively well expressed Mesoproterozoic to Archean age components most likely require significant Provenance Thanetian additional Laurentian input. Comparison to published data shows that both Devonian Old Red Sandstone Pegwell Bay and northwesterly (Avalonia-Laurentia) derived Namurian-Westphalian Pennine Basin sandstones show Paleogene strong similarities to the Thanetian sand. This pattern is consistent with derivation of Thanetian material Wales-Brabant Massif 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. © 2021 The Geologists' Association. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). 1. Introduction changing source areas through time can certainly not be discounted as a major factor in these differences (Moffat and The late Paleocene (early-mid Thanetian; 59.2À56 Ma) Thanet Bateman, 1983), even the proposed specific sources to the Thanet Formation of the London Basin comprises up to 30 m of shell rich, Formation are starkly contrasting between different studies. buff-grey coloured, glauconitic silty fine sand deposited in a Sources as diverse as Armorica (Groves, 1928; Blondeau and shallow marine open water setting (Ellison et al., 1994). The Pomerol, 1968; Weir and Catt, 1969), ‘northern seas’ (Blondeau and provenance of this sandstone, along with other rocks of the early Pomerol, 1968), the Scottish Highlands (Morton, 1982), Cretaceous Paleogene in the London Basin, and its relationship to palae- Greensand and Cornubia (Thomas, 2007), an unknown amphibo- ogeography and river drainage, has been a considerable source of lite-facies metamorphic province (Weir and Catt, 1969), or a debate. Interpretations of heavy mineral analyses undertaken on combination of many of these, have all been proposed (Fig. 1). the Thanet Formation have differed not only in terms of source Taken as a whole, currently available heavy mineral data are assignment, but also in terms of reported mineralogy, with (for consistent with any of these areas/terranes acting as Thanet example) Morton (1982) describing considerably more mineral- Formation sediment sources (Thomas, 2007). ogical variation than Thomas (2007). These differences may reflect Uncertainty over the origin of these deposits has wider sampling of different sequences with gradually evolving sources, implications in understanding the drainage as well as uplift and the varying effects of post depositional dissolution, or count denudation history of Cenozoic NW Europe. The Thanetian shallow number and methodology (Thomas, 2007). While the effect of marine sediments in the London Basin are restricted to the eastern part of the basin (extending into Belgium and France) but overlying sediments of the Reading Formation are comprised of floodplain and delta complexes indicative of a large prograding river entering * Corresponding author. E-mail address: [email protected] (T. Stevens). the London Basin in the area of the modern Chilterns (Gibbard and https://doi.org/10.1016/j.pgeola.2021.01.003 0016-7878/© 2021 The Geologists' Association. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Please cite this article as: T. Stevens and Y. Baykal, Detrital zircon U-Pb ages and source of the late Palaeocene Thanet Formation, Kent, SE England, Proc. Geol. Assoc., https://doi.org/10.1016/j.pgeola.2021.01.003 G Model PGEOLA-885; No. of Pages 9 T. Stevens and Y. Baykal Proceedings of the Geologists’ Association xxx (xxxx) xxx–xxx Fig. 1. Late Paleocene (Thanetian) palaeogeography of the North Sea region based on Gibbard and Lewin (2003, 2016) with possible proposed source areas and transport pathways of the Thanet Formation sediments (arrows). Coastline shapefile downloaded from eea.europa.eu. Lewin, 2003). It is likely that Paleocene uplift of W and NW Britain hinterland to the W and NW in what is now North Wales and associated with compression driven by Alpine uplift, North Central Northern England (Fig. 1). It has also been hypothesised Atlantic rifting, and the rise of the Iceland mantle plume, formed that at the end of the Cretaceous this area was covered by up to 3 or rejuvenated this ‘proto-Thames’ river system, setting up NW to km of post-Triassic sediment, subsequently eroded during the SE drainage patterns over the UK (Gibbard and Lewin, 2003, 2016). early Paleogene uplift (Lewis et al., 1992; White and Lovell, 1997). River systems would have transported considerable volumes of Under this scenario, a proto-Thames River would have transported sediment towards the subsiding SE of England and into a major eroded sediment from these uplifting and denuding areas, which delta complex extending eastwards (Fig. 1). Thus, it is plausible may imply that Thanetian age marine sediments are derived from that the marine sands of the Thanet Formation would have been this former, now mostly removed, post Triassic cover, or sourced by this proto-Thames River and delta complex. Gibbard underlying rocks currently exposed in the area today. Alternatively, and Lewin (2003, 2016) suggest a Paleocene proto-Thames would uplifted areas to the S and SW (Armorica and Cornubia) may have have drained uplifting Paleozoic-Precambrian massifs in the contributed to marine sands of the Thanetian if drainage was more 2 G Model PGEOLA-885; No. of Pages 9 T. Stevens and Y. Baykal Proceedings of the Geologists’ Association xxx (xxxx) xxx–xxx 204 202 oriented W-E or SW-NE, while the Scottish Highlands may have was corrected for Hg using the mass Hg and the natural 202 204 contributed sediment via longshore transport southwards along Hg/ Hg ratio of 4.3. This Hg correction was typically not the western margin of the Paleocene North Sea Basin. significant because most Hg backgrounds were low. Initial Pb was 206 204 As described above, currently available provenance data alone corrected on the basis of the measured Pb/ Pb and the are inconsistent, suggest multiple possible pathways, or are unable assumed composition of Pb based on Stacey and Kramers (1975). 206 238 206 207 208 232 to test between these different scenarios. Here we propose that Fractionation of Pb/ U, Pb/ Pb, and Pb/ Th was detrital zircon U-Pb dating of Thanet Formation sands may provide corrected via a sliding-window average of eight reference material further insight into their source. While complicated by the effects analyses, accounting for instrumental drift. Measurement uncer- 206 238 208 232 of recycling of zircon grains between different episodes of tainties for Pb/ U and Pb/ Th were based on scatter sandstone formation (Morton et al., 2016), zircon age populations around a regression line of the measured values. Uncertainties for 206 207 206 204 from different potential source areas of NW Europe should show Pb/ Pb and Pb/ Pb were based on the standard deviation differences reflecting the relative importance of numerous of measured values. The sum of these uncertainties, and over orogenic events in different terranes (Hallsworth et al., 2000; dispersion factors, are reported as the internal uncertainty for each Rainbird et al., 2001; Samson et al., 2005; McAteer et al., 2010; analysis. These uncertainties are reported at 1s. U and Th Morton et al., 2013; Morton et al., 2015; Fairey et al., 2018). As part concentrations were estimated from published concentrations 206 238 of a wider study into the origin of Quaternary loess sediments in for FC-1. External (systematic) uncertainties for Pb/ U, 206 207 208 232 southern Britain (Stevens et al., 2020), we present here results of Pb/ Pb, and Pb/ Th included scatter of the reference high n (Pullen et al., 2014) detrital zircon U-Pb age analysis of a material analyses, age uncertainties for reference materials, sample of shallow marine sands of the Thanet Formation at one of uncertainties in common Pb composition, and decay constants 235 238 the stratotype sections, Pegwell Bay, Isle of Thanet, East Kent, UK. uncertainties for U and U. However, following convention, external uncertainties are not reported here with the ages. 206 238 206 238 206 207 2.
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