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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 marine to fluviodeltaic are currently unclear. High

Received 25 November 2020

analysis number detrital zircon U-Pb investigation of an early-mid 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 age peak is associated with the Cadomian , while secondary peaks represent the

Available online xxx

Caledonian and various Mesoproterozoic to Archean . 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 Old Red

Pegwell Bay and northwesterly (-) 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 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 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),

Paleogene in the London Basin, and its relationship to palae-

Greensand and Cornubia (Thomas, 2007), an unknown amphibo-

ogeography and river , 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 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/ 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 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 and France) but overlying

sediments of the 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

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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- 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- massifs in the contributed to marine sands of the Thanetian if drainage was more

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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

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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

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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

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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

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of a wider study into the origin of Quaternary loess sediments in for FC-1. External (systematic) uncertainties for Pb/ U,

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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, , East Kent, UK. uncertainties for U and U. However, following convention,

external uncertainties are not reported here with the ages.

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2. Site and methods Pb/ U ages for Pb/ U ages <900 Ma and Pb/ Pb ages

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for Pb/ U ages >900 Ma were used for plotting and

Sampling was conducted in early March 2019 at the S-SE facing interpretation. Analyses with >10 % uncertainty (1s) in

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cliff section below Cliffs End at the site of the former hover port Pb/ U age were not included. Analyses with >10 % uncertainty

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(Fig.1; N51 19.651’, E001 22.200’; TR 3464 8811). A 2 kg sample of (1s) in Pb/ Pb age were not included, unless the Pb/ U

sands was taken from the Silts Member of the Thanet age was <400 Ma. For ages younger than 900 Ma, the discordance

207 235 206 238 207 235

Formation, c. 2.5 m below the boundary with the Quaternary loess was defined as ( Pb/ U- Pb/ U)/ Pb/ U*100; for ages

and 60À90 cm below the second exposed layer of comminuted older than 900 Ma, the discordance was defined as

207 206 206 238 207 206

bivalve shells from the top of the exposure (S2 in Pitcher et al., ( Pb/ Pb- Pb/ U)/ Pb/ Pb*100. Analyses with >15 %

1954). The Reculver Silts is an informal unit of significance only in discordance and with >5% reverse discordance were not included

East Kent, characterised by yellowish grey fine silty sometimes in further analyses.

glauconitic sands with bands of drifted bivalve shells (Ellison et al., Detrital zircon U-Pb dating has been extensively used in

1994). provenance analyses of sedimentary rocks of Britain and

The sample taken for detrital zircon U-Pb analysis was and in many instances can be extremely diagnostic of specific,

disaggregated using ultrasonic disruption to remove minerals multiple sources (Hallsworth et al., 2000; Rainbird et al., 2001;

and break apart aggregates. Zircon grains from the sample were Samson et al., 2005; McAteer et al., 2010; Morton et al., 2013, 2015,

first separated using a Wilfley table, then a Frantz magnetic 2021; Fairey et al., 2018). However, in general, analyses numbers

separator and then heavy liquids. A Hitachi 3400 N scanning per sample have been <100-150 grains, which makes it difficult to

electron microscope (SEM) was used to generate high-resolution compare relative heights of peaks in provenance assignment in a

backscattered electron (BSE) images of the grain mounts providing statistically robust way (Pullen et al., 2014), and in some cases may

a guide for locating analysis pit optimal sites and to assist in miss significant age peaks entirely (Vermeesch, 2004). Here we use

interpreting results. Dating of detrital zircons was undertaken with a relatively new approach whereby high analysis numbers (large n

a small diameter beam (12 mm) during laser ablation inductively datasets) are generated (>300 grains) that overcome this limita-

coupled plasma mass spectrometer analysis (Gehrels et al., 2008; tion (Pullen et al., 2014). However, it should be stressed that this

Gehrels and Pecha, 2014; Pullen et al., 2014). Measurements were was only conducted on one Thanet Formation sample, giving no

made using a Teledyne Photon Machines G2 solid state NeF indication of how variable the sources of the Thanet Formation are

excimer laser ablation system coupled to a Thermo Fisher Scientific geographically or through time.

ELEMENT 2 single collector inductively coupled plasma mass

spectrometer at the Arizona LaserChron Center, University of 3. Results and discussion

Arizona.

Isotope fractionation was corrected for using a suite of zircon The distribution of Thanet Formation zircon U-Pb ages (Fig. 2;

reference materials: FC-1 (Black et al., 2003), R33 (Black et al., Appendix A) shows a wide range of age peaks, with the largest peak

2004), and SL (Gehrels et al., 2008). Signal intensities were strongly centred at c. 600 Ma. Other important peaks occur centred

measured with a secondary electron multiplier that operates in on c. 420 Ma,1.05 Ga, 1.16 Ga, 1.37 Ga, 1.5 Ga, 1.65 Ga, 1.78 Ga, 2.0 Ga

pulse-counting mode for signals less than 4 million counts per and 2.7 Ga. However, zircon ages between 1.5 and 2.0 Ga are

second (cps) and in analog mode above 4 million cps. The relatively diffusely spread, although concentrated between 1.6 and

calibration between pulse-counting and analog signals was 1.8 Ga. The relatively high number of zircon ages in the sample

determined line by line for signals between 50,000 and 4 million permit discussion of gaps in the data distribution, and there are

cps and was applied to >5 million cps signals. Because of the very few grains present younger than 400 Ma, between 700 Ma and

nonlinearity reported for secondary electron multipliers in isotope 1.0 Ga, between 2.1 and 2.7 Ga, and older than 2.8 Ga.

mass spectrometry (Richter et al., 2001; Hoffmann et al., 2005) and The dominant zircon age peaks match well with peaks in

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the desire for the highest accuracy, Pb/ U ratios with U >5 Caledonian age (420 Ma), Armorica-Ganderia-Megumia-

235

million cps were calculated from U*137.88. Data were reduced Avalonia basement (600 Ma) and Laurentia basement (Mesopro-

using an Arizona in-house Excel spreadsheet (E2ageclac). E2age- terozoic to Archean peaks), as assembled from published zircon U-

204

calc calculates the average intensities for each isotope. Mass Pb Pb age data from these terranes (Fig. 3). However, the near total

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absence of Variscan ages precludes direct sourcing from uplifting zircons derived from varying source terranes and transported

Cornubia (Neace et al., 2016) or Armorica terranes to the south and along multiple, contrasting trajectories (Morton et al., 2015, 2016,

southwest. Moreover, Armorican basement exhibits a characteris- 2021). Under a Paleocene proto-Thames scenario, the source rocks

tic age peak centred around 1.9 Ga, which is absent in the Thanet of the Thanet Formation ought to be located in central and mid-

Bed sample and the other potential source areas. Direct sourcing North Wales and England and the central Basin (Fig. 1) as

from Laurentian terranes such as the Scottish Highlands alone is the system would likely have drained uplifting Paleozoic-

also highly unlikely, given the dominant Cadomian age (600 Ma) Precambrian massifs to the W and NW of the London Basin, and

peak in the zircon age data. Zircons of this age are absent in the provided sediment to an extensive delta lagoon complex (Gibbard

basement of Laurentia, but common in peri-Gondwanan terranes and Lewin, 2016), and ultimately, marine sediment to the SE.

accreted during the Caledonian and Varsican orogenies (Fig. 3). Morton et al. (2015; 2021) conducted a detailed detrital zircon U-

However, given the low abundance of Mesoproterozoic to Archean Pb age study of sandstones broadly in or adjacent to

age zircon grains in these peri-Gondwanan terranes, and the this area, in the southern and central part of the Pennine Basin,

substantial and broad spread of these ages in the Thanet Formation fringing the northwestern margin of the Ganderian-Avalonian

sample (Fig. 2), a considerable contribution from Laurentia must Wales-Brabant Massif. Previously, a wide range of provenance

also have occurred. indicators have suggested Carboniferous sandstones in the

While direct sourcing of the Thanet Formation sample from northern and central part of the Central Pennine Basin were

Cornubia, Armorica or (solely) the Scottish Highlands is precluded derived from N and NE sources, while in the southern part of the

by the zircon age data, the wide range of peaks is consistent with a basin, sediments were shed from the south, from the western part

range of sources, probably including grains recycled from older, of the Wales-Brabant Massif (Morton et al., 2015). However,

pre-existing sedimentary sources. Indeed, large areas of Britain are detailed zircon analyses indicate a complex range of temporally

covered by extensive sandstone formations comprised of recycled and geographically shifting sources (Morton et al., 2021).

Fig. 2. Kernel density estimate (KDE) plots (Vermeesch, 2012) of detrital zircon ages from the Thanet Formation sediments (this study) compared to potential source rocks:

Carboniferous sandstone in the Pennine Basin subdivided according to proposed provenance by Morton et al. (2021) from southeast (Hallsworth et al., 2000; Morton et al.,

2021), west (Morton et al., 2021), north (Hallsworth et al., 2000; Morton et al., 2015, Morton et al., 2021; Lancaster et al., 2017), the Wales-Brabant High (Hallsworth et al.,

2000; Morton et al., 2015) and the combined sample; New Red Sandstone in the Wessex Basin (Morton et al., 2016); samples from the northern margin of

the Wales-Brabant High and the Dingle Peninsula (Morton et al., 2015; Fairey et al., 2018). Left column displays an expansion of the 200 - 800 Ma age range, right column

displays 0 - 3000 Ma age range.

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Namurian sandstones generally were derived from northern peak is relatively much more important and strongly centred on

sources in Laurentia or local western Wales-Brabant Massif 600 Ma, while the Caledonian age peak (c. 420 Ma) is reduced in

(Avalonia) basement rocks to the immediate south. The latter importance but more tightly clustered in comparison to the

are represented by a strong Ediacaran age peak while the former grouped Carboniferous sandstone data (Morton et al., 2021).

are witnessed by Caledonian orogeny age and Mesoproterozoic- Furthermore, the Mesoproterozoic to Archean age grains in the

Archean grains. It is possible that the Laurentian grains themselves Thanet sample are relatively more strongely expressed than those

in the Carboniferous sandstone were mostly reworked from in the Carboniferous data (Fig. 2). Overall, the westerly

recycled upper Old Red Sandstone resting on Avalonian basement, (Avalonian) derived early Westphalian sandstones or western

the latter in turn sourced from the Caledonides of Northern Wales-Brabant High derived sandstones of the Namurian most

Scotland and Ireland. In the Westphalian (Langsettian-Duckman- closely match the Thanetian zircon data (Fig. 2). Indeed, the

tian) there is a shift in dominant source to more westerly derived narrow 600 Ma peak in the Thanet data is coincident with one of

grains, still represented by mainly recycled Laurentian compo- the main phases of magmatism in the western part of the Wales-

nents, but also with a clearly defined Avalonian basement Brabant Massif (Compston et al., 2002). The relatively young

component. Morton et al. (2021) argue for a western-southwestern Caledonian age peak in the Thanet Formation (c. 420 Ma), as

Ireland source of these grains via Avalonian basement and recycled compared to the main Grampian orogeny in the Scottish

Laurentian-derived Devonian sandstones. In the late Duckman- Highlands (463–493 Ma; Druat et al. (2009)), is also better

tian-Bolsovian there is another marked shift to a dominant reconciled with an Irish Caledonide source, where main phases of

southeastern sediment source, represented by late Devonian- activity occur between 430 and 380 Ma (Buchwaldt et al., 2001;

Carboniferous grains derived from the advancing Variscan front. Feely et al., 2003, 2011; Condon et al., 2004; Mange et al., 2010;

Many of the zircon age peaks seen in the various Pennine Chew et al., 2009). This is consistent with the provenance of a

Namurian-early Westphalian sandstones presented in Morton Pennine Measures Group sandstone of Duckmantian age in

et al. (2015, 2021) are also present in the Thanetian age Thanet the Derbyshire coalfield (Morton et al., 2015). However, despite

Formation sample (Fig. 2). However, there are considerable these similarities we note that the relative importance of the

differences in their relative peak heights and different age Caledonian and Cadomian age peaks in the Thanetian sample

Carboniferous samples show greater or lesser affinity to the contrast with those in the Avalonia-derived Carboniferous

Thanet Formation sample. In this Thanet sample, the Cadomian sandstones of the Pennines (Fig. 2).

Fig. 3. KDE plots of published detrital zircon ages from potential source terranes (expanded after Fairey et al. 2018): Laurentia (Cawood et al., 2003, 2012; Friend et al., 2003;

Kirkland et al., 2008; Waldron et al., 2008, 2014; McAteer et al., 2010; Strachan et al., 2013; Johnson et al., 2016); Ganderia (Fyffe et al., 2009; Waldron et al., 2014, 2019;

Willner et al., 2014); Megumia (Waldron et al., 2009, 2011; Pothier et al., 2015; White et al., 2018); East Avalonia (Collins and Buchan, 2004; Murphy et al., 2004b; Strachan

et al., 2007; Linnemann et al., 2012; Willner et al., 2013); West Avalonia (Keppie et al., 1998; Thompson and Bowring, 2000; Barr et al., 2003, 2019; Murphy et al., 2004b,

2004a; Satkoski et al., 2010; Dorais et al., 2012; Thompson et al., 2012; Willner et al., 2013; Henderson et al., 2016); Cadomia-Armorica (Fernández-Suárez et al., 2002; Samson

et al., 2005; Gerdes and Zeh, 2006; Linnemann et al., 2008; Strachan et al., 2014; Lin et al., 2019). Inset map shows simplified arrangement of potential source terranes based

on Fairey et al. (2018). Coastline shapefile modified from thematicmapping.org.

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The Mesoproterozoic to Archean age peaks in the Thanet in the Thanetian, as hypothesised by Gibbard and Lewin (2003,

Formation sample are also mostly expressed in the Carboniferous 2016), although some additional material may have been trans-

sandstone data, as well as in Devonian Old Red Sandstone from the ported via longshore drift input from the north. The drainage route

English Midlands and Ireland (Fig. 2), the latter in turn likely a of this system would have been set up by early Paleogene uplift of

source to the westerly-derived Westphalian sandstones (Morton W and NW Britain, which in turn led to denudation of the cover

et al., 2015, 2021). However, again the relative size of these peaks is rocks of the western part of the Wales-Brabant Massif. However,

different, with much stronger expression in the Thanet Formation some aspects of this proposal require further consideration. As

data, especially for the Palaeoproterozoic-Archean age grains. This noted above, the analysed sample of Thanetian sand cannot have

is also true when comparing the Thanetian sands to purely been derived from all the Namurian-Westphalian sandstone cover

Laurentia-derived Namurian sandstones of the Pennines (Fig. 2). of the southern Pennine Basin because upper Duckmantian-

The dominance of zircons related to the Caledonian orogeny is Bolsovian sandstones there are sourced from the Variscan

greatly reduced in the Thanetian sands. Comparison to Triassic age mountains to the south (Fig. 2; Morton et al., 2021). These

New Red Sandstone detrital zircon ages from southern and Duckmantian-Bolsovian sandstones overly the Namurian-lower

southwestern England shows a similar prominent Cadomian age Westphalian sandstones that have strong Laurentian affinity, and

peak to the Thanetian (Fig. 2). This peak in the New Red Sandstone although their regional extent is not fully constrained, they are

likely has its origins in Armorica (Morton et al., 2013, 2016), which known to extend into southern Scotland (Morton et al., 2010).

shares similar Cadomian granitoid ages to Ganderia-Avalonia. Additionally, Triassic sandstones that extend north well into the

However, the abundance of Variscan ages and paucity of Irish Sea Basin are known to have southerly origin, and therefore a

Mesoproterozoic to Archean age grains in the New Red Sandstone large Variscan-derived component (e.g., Morton et al., 2013, 2016).

contrasts with their expression in the Thanet Formation sample Although no zircon data is yet available from these northern

and demonstrates that the Thanetian sands from this sample Triassic sandstones, Pb isotopic composition of K-feldspars of

cannot have been sourced from the south or southwest. This is Triassic sandstones from the East Irish Sea Basin and Northern

further reinforced by the observed differences between the Ireland strongly suggest that they are formed from Variscan-

Thanetian sample and the south-easterly-derived (Morton et al., derived detritus (Tyrrell et al., 2012; Franklin et al., 2020). Thus,

2021) upper Duckmantian-Bolsovian Pennine sandstone zircon Variscan-derived upper Westphalian and Triassic sandstones

age distribution (Fig. 2). existed in or at least near a Palaeocene proto-Thames catchment,

Overall, the data here are consistent with the sources of northwest of the London Basin, and at least some of these rocks

Paleocene Thanet Formation sand in the rocks or pre-existing cover overlie the Laurentia-Avalonia derived Namurian-lower West-

of the western part of the Wales-Brabant Massif and of the Pennine phalian sandstones of the southern Pennine basin. However, given

Basin, including the Namurian-early Westphalian and Devonian the absence of a significant Variscan zircon component in the

sandstones (Fig. 2). This is consistent with sediment transport to Thanetian sample, these rocks cannot have been sources for at least

the London Basin via an early proto-Thames River system. the uppermost Thanet Formation sands. This may seem incom-

Sediment routing from the SW, S or SE appears highly unlikely, patible with a proto-Thames sourcing sediment via of the

as does a purely northern supply of Laurentian material. However, Avalonian basement and Namurian-Westphalian sandstones of the

the significantly more prominent Cadomian (Ediacaran) and Pennine Basin. However, the areal exposure of Variscan-derived

Paleoproterozoic-Archean peaks in the Thanet Formation com- Triassic and upper Westphalian sandstones during the Thanetian is

pared to both the Old Red Sandstone and Carboniferous sandstones unknown, and conversely, our data may suggest that at least for

demands additional sources relatively rich in zircons of these ages. part of the Thanetian they were not eroding within a proto-Thames

Direct sourcing of material from the western edge of the Avalonian catchment. The Thanetian age sample here is taken from relatively

Wales-Brabant Massif basement would account for the Cadomian high up in the Thanet Formation, and if the proto-Thames scenario

peak, but the abundant Palaeoproterozoic-Archean grains point to is correct, the overlying Triassic and upper Westphalian cover

a significant contribution of sediment originating from Laurentia, could have been eroded first, with the river then incising into lower

to the north. A potential source would be the Dalradian of the Westphalian and Namurian material during the formation of these

Scottish Highlands, where grains of this age are abundant (Cawood upper Thanet Formation sands. If this proposal is correct, then

et al., 2003), and this may indicate partial derivation of Thanetian Variscan grains derived from the Triassic and upper Westphalian

sands via longshore drift southwards down the Paleocene North rocks should be found lower down in the Thanet Formation. The

Sea (Morton, 1982). However, given the strong indication of Irish lack of contemporary magmatic zircons in the sample here may

Caledonide sources for the Caledonian age peak in the data, and the also suggest some temporal variability in the Thanet Formation.

highly dominant Cadomian peak, a source in an, as yet, Tuffaceous material has been found previously in the Thanet sands

unidentified sandstone that is comprised of significant proportions at Pegwell Bay (Knox, 1979), and alkaline igneous minerals have

of westerly-derived, recycled Laurentian grains is likely. Moreover, been noted from Thanet sands by Morton (1982), making the lack

this unidentified sandstone must include abundant Cadomian age of Thanetian age zircons in the Thanet Formation sample here

grains of Avalonian origin, or requires an additional input of notable. However, the volcanic indicators noted by Knox (1979)

material shed directly from the western Wales-Brabant Massif and Morton (1982) were only found in the basal part of the

basement itself, yet not include Variscan age material. Given these formation.

requirements, it seems likely that this sandstone must lie (or have Alternatively, if the sample here can be taken as more widely

existed) in North-Mid Wales, Northern England and the Central representative of Thanet Formation sediments, it might lend

Irish Sea Basin, and have a similar composition to the westerly support to a scenario where this material was recycled from a now

derived lower Westphalian sandstones of the Pennines (Morton removed post Triassic cover over the Irish Sea Basin and adjacent

et al., 2021) and the Devonian Old Red Sandstone (Morton et al. areas. Indeed, it seems plausible that considerable volumes of

2015). This sandstone also likely contained material from Laurentia material may have been eroded and redistributed by the river

that was derived from erosion of the Irish Caledonides. However, system, at least partly consistent with the hypothesis of

without more detailed study we cannot test between this and the widespread erosion of a pre-existing post Triassic cover in NW

possibility of input from the north via longshore drift. Britain (Lewis et al., 1992). This post Triassic cover is a good

In summary, the above proposed scenario is consistent with candidate for the missing source rocks needed to fully explain the

routing of material to the London Basin via a proto-Thames system origin of the Thanet Formation sample here. While the nature of

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this cover is unknown, it is here speculated that it would have had a that this river system may have eroded substantial volumes of

similar source assemblage to the western Wales-Brabant Massif Devonian, Carboniferous and even younger sandstone cover of

and southern-central Pennine Devonian and Namurian-West- central Britain and redistributed them in the London Basin and

phalian sandstones, but with smaller Caledonian components North Sea Basin. This may have been a major mechanism in

(Fig. 2), pointing to Laurentian and Avalonian derivation. removal of material denuded during the Paleogene uplift, and

We emphasisethatthese inferencesare based ononlyone sample would have been a significant transport route for now eroded

from the early-mid Thanetian of the Paleogene London Basin. At hypothesised extensive post Triassic cover over Britain. However,

present it is not possible to tell if these results are more widely we note that our analysis is restricted to one sample, and at present

applicable both spatially in the basin and through the Paleogene, there is no knowledge about temporal or spatial variability in

especially because of conflicting results over temporal variability in sources in the Paleogene of the London Basin. Indeed, some London

potential source rocks reported from heavy mineral data (Morton, Basin sands heavy mineral data may suggest the possibility of some

1982; Moffat and Bateman, 1983; Thomas, 2007). This potential for substantial changes in sources (Moffat and Bateman, 1983). We

spatial and temporal source variability is clearly shown in the hope that this study provokes a detailed provenance investigation

remarkable variation seen in sources to the Pennine Basin of these extensive Paleogene sands.

Carboniferous sandstones in Morton et al. (2021). Our findings

above set up two competing hypotheses over the nature of the cover Declaration of Competing Interest

eroded by a proto-Thames system and recycled into the Thanet

Formation sands, testable by examination of temporal source The authors report no declarations of interest.

variability through the Thanet Formation. Furthermore, our results

suggest a need to conduct detrital zircon U-Pb provenance work on Acknowledgements

fluvial sediments of the London Basin, which would be more

unambiguously representative of proto-Thames detritus, such as the This work was funded by the Swedish Research Council (VR

overlying Paleocene ReadingFormation.Clearlya systematicstudyof grant 2017-03888) and the Quaternary Research Association (QRA)

the zircon U-Pb age assemblage of Paleogene rocks throughout the Quaternary Research Fund. We are grateful to Thanet District

London Basin is needed to fully test the hypotheses presented here, Council for access to the old hover port site at Pegwell Bay, as well

and we hope our study triggers such an undertaking. as to Ragna Orbe for help in the field, and Phil Gibbard for

discussions of Thames and proto-Thames drainage. We also thank

4. Conclusions Andrew Morton and one anonymous reviewer for their construc-

tive reviews, which have greatly improved this manuscript.

Analysis of U-Pb age for a large number (302) of detrital zircons

of a Thanetian age shallow marine sandstone from Pegwell Bay in Appendix A. Supplementary data

East Kent reveals a large range of age peaks that suggest ultimate

derivation from both Avalonian and Laurentian basement rocks. Supplementary material related to this article can be found, in the

This, and the lack of grains associated with the Variscan orogeny in online version, at doi:https://doi.org/10.1016/j.pgeola.2021.01.003.

the Thanetian sample, implies northern or western sources for the

material and is inconsistent with suggestions of sources to the

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