Geo chem i cal anal y sis for palaeoenvironmental in ter pre ta tions – a case study of the Eng lish Wealden (Lower Cre ta ceous, south-east Eng land)
Oladapo AKINLOTAN1, *
1 Uni ver sity of Brigh ton, School of En vi ron ment and Tech nol ogy, Lewes Road, Brigh ton, BN2 4GJ, UK
Akinlotan, O., 2017. Geo chem i cal anal y sis for palaeoenvironmental in ter pre ta tions – a case study of the Eng lish Wealden (Lower Cre ta ceous, south-east Eng land). Geo log i cal Quar terly, 61 (1): 227–238, doi: 10.7306/gq.1328
The el e men tal com po si tion of terrigenous sed i ments is ex tremely im por tant for in ter pret ing the depositional con di tions of sed i men tary bas ins es pe cially when tra di tional data are not avail able or in suf fi cient. For the first time in the pub lished lit er a - ture, the el e men tal com po si tion of the Lower Cre ta ceous Weald Ba sin (south-east Eng land) is pre sented via X-ray flu o res - cence (XRF) and Spec tral Gamma-ray (SGR) meth ods. The con cen tra tions of Zr, Cr, Ni, Rb, and Sr in vary ing quan ti ties sug gest that the sed i ments may have been sourced from a sta ble craton while the en riched re dox-sen si tive el e ments such as Mo and Co con firm the pres ence of anoxic and re duc ing con di tions at the sites of de po si tion. The spec tral gamma-ray data show that the sand stone fa cies are more ra dio ac tive than the ad ja cent mudstone and shale and this trend is con sis tent with a fresh wa ter or i gin. The higher quan tity of tho rium in re la tion to ura nium in di cates hu mid and hot palaeoclimatic con di - tions at the source ar eas that fa voured in tense weath er ing. The re sults of these anal y ses re in force the im por tance of the geo chem is try of sed i men tary rocks as use ful tools for un der stand ing the depositional con di tions of sed i men tary bas ins.
Key words: el e men tal com po si tion, X-ray flu o res cence, spec tral gamma-ray, palaeoenvironments, Lower Cre ta ceous, south-east Eng land.
INTRODUCTION
The use of the geo chem i cal com po si tion of sed i men tary rocks for in ter pret ing palaeo- and depositional en vi ron ments is well-doc u mented (e.g., Alvarez and Roser, 2007; Pe-Piper et al., 2008; Ghasemi-Nejad et al., 2010; González-Álvarez and Kerrich, 2010; Eker et al., 2012; Garnit et al., 2012; Pi et al., 2014; Wójcik-Tabol, 2015). The el e men tal com po si tion of terrigenous sed i ments is ex tremely im por tant for in ter pret ing the depositional con di tions of sed i men tary bas ins and may be more use ful than ex pected when tra di tional data such as field ob ser va tions and palaeontological ex am i na tions are not avail - able or in suf fi cient. The geo chem i cal com po si tion of terrigenous sed i men tary rocks is a func tion of tec tonic set ting, prov e nance, weath er ing, trans por ta tion and diagenesis (e.g., Bhatia and Crook, 1986; Alvarez and Roser, 2007;
González-Álvarez and Kerrich, 2010). Many ma jor el e ments such as Fe, K, Ca, Ti and Mn in de creas ing or der of abun dance may be af fected by diagenesis or bi o log i cal pro cesses af ter de - po si tion (e.g., Abanda and Hannigan, 2006; Brumsack, 2006;
Wójcik-Tabol and Œl¹czka, 2009). Some trace el e ments such as Mo, U, Cr, Ni, Co and Zn are en riched in re duc ing sed i ments and are highly sen si tive to re dox changes (e.g., Lipinski et al., 2003; Pi et al., 2014). As a re sult, they are im por tant prox ies for palaeoredox re con struc tion (e.g., Algeo and Rowe, 2012; Xu et al., 2012a) in var i ous depositional en vi ron ments (e.g.,
Elbaz-Poulichet et al., 2005). Trace el e ments such as Th, Zr, Co, Ti and Sc are the most im mo bile el e ments dur ing weath er - ing, trans por ta tion, de po si tion, diagenesis and are con sid ered the most use ful for de ter min ing prov e nance and the na ture of tec tonic set tings of sed i men tary rocks (e.g., Bhatia and Crook, 1986; Marques et al., 2011). The ra tio of Th/U in creases in sed i - men tary rocks with in creas ing al ter ation (Pe-Piper et al., 2008).
Po tas sium (K), ura nium (U) and tho rium (Th) are the dom i - nant sources of gamma-rays in rocks. The con cen tra tions and vari a tions of these three el e ments in rocks de ter mined by spec - tral gamma-ray method (SGR) pres ent im por tant geo log i cal and geo chem i cal in for ma tion on source rock com po si tion, sort - ing dur ing trans port and de po si tion, grain size, clay con tent, modal com po si tion of sand stone, po ros ity, or ganic car bon con - tent, weath er ing at source and palaeoclimate con di tions (e.g., Ruffell and Worden, 2000; Šimíèek et al., 2012; Šimíèek and Bábek, 2015). Po tas sium is com mon in many sed i ments that con tain feld spar, mica, clays or chlo ride salts. Ura nium and tho - rium have many host phases in sed i men tary rocks and these may in clude clays, feld spars, heavy min er als, phos phates and or ganic mat ter. In mudrocks and ar gil la ceous lime stones, most of the U and Th are sourced from clays, or ganic mat ter, and heavy min er als (Šimíèek et al., 2012; Šimíèek and Bábek, 2015). Po tas sium is leached from feld spars and mus co vite dur - ing the for ma tion of kaolinite un der hot and hu mid cli mates while tho rium is con cen trated dur ing weath er ing. K and U are gen er ally more sol u ble than Th and U is prone to mo bi li sa tion dur ing weath er ing and clay min eral diagenesis (Ruffell and Worden, 2000; Schnyder et al., 2006). De ple tion of K and U dur ing weath er ing and palaeoweathering re flects the du ra tion and se ver ity of weath er ing (e.g., McLennan et al., 1983;
Alvarez and Roser, 2007; Ghasemi-Nejad et al., 2010). In ter - pre ta tion of gamma-ray emis sion of sed i men tary rocks may
* E-mails: o.akinlotan@brighton.ac.uk; dapo_akinlotan@yahoo.com Received: June 22, 2016; accepted: November 15, 2016; first published online: November 23, 2016
some times be come prob lem atic where spe cific heavy min er als have high Th or U con tents, re sult ing from high amounts of zir - cons, uraninites or thorites (Schnyder et al., 2006) while siliciclastic fa cies can also be com pli cated be cause of their multi-com po nent na ture (Šimíèek and Bábek, 2015). It may not be pos si ble to ap ply a global set of el e men tal dis crim i nate for the iden ti fi ca tion of terrigenous sed i ment sources with out a sys - tem atic in ves ti ga tion to eval u ate the ef fects of diagenesis or sort ing (e.g., Abanda and Hannigan, 2006; Pe-Piper et al., 2008; Wójcik-Tabol and Œl¹czka, 2009).
The Early Cre ta ceous rep re sents an im por tant phase in the geo log i cal re cord which pro duced ep i sodes of ac cel er ated global change in cli mate and sea level and con se quently changed sed i men ta tion style (e.g., Sla den, 1980; Ziegler, 1981;
Sla den, 1983; Raw son, 1992; Tay lor and Ruffell, 1993;
Vandycke, 2002; Hopson et al., 2008; Föllmi, 2012). The pe riod wit nessed a change from arid/semi-arid cli mate and rel a tively high sea level in the Ju ras sic to hu mid con di tions and a low sea level in the Cre ta ceous (e.g., Sla den, 1980, 1983; Hal lam, 1984; Hal lam et al., 1991; Raw son, 1992; Tay lor and Ruffell, 1993). These changes en sured that sed i men ta tion oc curred in non-ma rine con di tions in the Early Cre ta ceous: south-east Eng land (e.g., Allen, 1975, 1981; Stew art, 1981a, b, 1983;
Allen, 1989), south Amer ica (e.g., Anjos and Carozzi, 1988;
Rodrigues et al., 1988; Legarreta et al., 1989; Scherer and Lavina, 2006; Garrido and Salgado, 2015; Ferreira et al., 2016) and China (e.g., Sha et al., 2003; Jiang and Sha, 2007; Li et al., 2013; Chen et al., 2014; Ju and Sun, 2016). The Eng lish Lower Cre ta ceous (Fig. 1) is used as a case study to show the geo - chem i cal com po si tion of sed i ments as ef fec tive in di ca tors of palaeoenvironments be cause the fa cies is very im por tant geo - log i cally as de scribed be low. The Lower Cre ta ceous of the Weald in south-east Eng land ex poses some prin ci pal out crops and some of the most com plete suc ces sions of the low er most Cre ta ceous fa cies in north-west Eu rope and be yond (e.g., Stew art, 1981a; Booth, 2005; Hopson et al., 2008; Radley and Allen, 2012). The Ashdown and Wadhurst Clay For ma tions (Fig. 2) pro vide ex cel lent op por tu nity for the study of the Lower
Cre ta ceous non-ma rine and flu vial-re lated sed i men ta tion pat - terns in va ri ety of con di tions in clud ing la goonal, deltaic and tidal that per sisted prior to the Late Cre ta ceous green house Earth (Booth, 2005; Hopson et al., 2008). These two for ma tions also pres ent the op por tu nity to study the depositional, cli ma tic, tec - tonic, eustatic changes wit nessed in the Lower Cre ta ceous in north-west Eu rope (Sla den and Bat ten, 1984; Hal lam et al., Fig. 1. Geographical locations of the Wealden strata in south-east
England showing the study locations
1 – Rock-a-Nore, Old Town, Hastings; 2 – Pett Level (Had dock’s Cot tages and Cliff End), Fair light, near Hastings; 3 – Northiam Quarry, Northiam; 4 – Hastingford Cut ting, Hadlow Down; 5 – West Hoathly Quarry, Sharpthorne;
50°N and 51°N re fer to the lat i tude while 2°W to 1°E re fer to the lon gi tude; mod i - fied af ter Goldring et al. (2005: fig 1)
Fig. 2. Generalized stratigraphy of the Wealden sediments (part of the Lower Cretaceous) in the Weald and Wessex
basins (after Allen and Wimbledon, 1991: fig. 2)
1991). Their su perb ex po sures also make them po ten tial an a - logues for study ing com pli cated ba sin tec ton ics in clud ing ba sin in ver sion and for hy dro car bon res er voirs. Al though the sedimentology and stra tig ra phy of the Lower Cre ta ceous Eng - lish Wealden sed i ments have been stud ied since the early 19th cen tury amount ing to nearly 200 years of re search (e.g., Fitton, 1824; Web ster, 1826; Topley, 1875; Lake and Thurrell, 1974;
Allen, 1975, 1981; Stew art, 1981a, 1983; Lake and Shephard-Thorn, 1987; Akinlotan, 2015), the el e men tal com po - si tion of these non-ma rine sed i ments is scarcely doc u mented in the lit er a ture. For the first time in the pub lished lit er a ture, the el - e men tal com po si tion of the Lower Cre ta ceous non-ma rine Weald Ba sin is pre sented via XRF and SGR meth ods from the Ashdown and Wadhurst Clay Fms. (Up per Berriasian–Mid dle Valanginian) within the Weald Ba sin in south-east Eng land. The aims of this pa per are to de ter mine the el e men tal com po si tion of the Wealden sed i ments in or der to char ac ter ize the palaeo- and depositional en vi ron ments of the Weald Ba sin and then com pare with pre vi ous in ter pre ta tions made from tra di tional datasets such as field ob ser va tions and palaeontological ex am - i na tions.
GEOLOGICAL SETTING
Wealden is used for the non-ma rine sand stones and mudstones be tween the Purbeck Group and the Lower Greensand of the Weald, the Isle of Wight and Dorset (Fig. 1) in south-east Eng land. The term is also used more widely to re fer to the Lower Cre ta ceous non-ma rine rocks of NW Eu rope (e.g., Hopson et al., 2008; Radley and Allen, 2012). The Wealden sed i ments in Kent and Sus sex are the type area for the non-ma - rine Wealden for ma tions of north-west Eu rope and broadly equates to the Weald Ba sin in south-east Eng land (Radley and Allen, 2012). The Hastings Beds (Up per Berria - sian–Valanginian) in the lower sec tion and the over ly ing Weald Clay (Hauterivian–Lower Aptian) make up the non-ma rine Wealden suc ces sions (Fig. 2) in the Weald Ba sin (e.g., Lake and Shephard-Thorn, 1987; Booth, 2005; Hopson et al., 2008;
Radley and Allen, 2012) and rep re sent ~25 mil lion years of depositional his tory (Stew art, 1981a). The Ashdown, Wadhurst Clay, and Tun bridge Wells Sand for ma tions (see Fig. 2) make up the Hastings Beds (Berriasian–Valanginian). The Ashdown For ma tion ( Up per Berriasian–Lower Valanginian) is made up of fine-grained sand stones, siltstones, and mudstones with mi - nor amounts of shale, mot tled clay, lig nite beds, si der it ic mudstone, and sphaerosiderite nod ules. These lithologies are gen er ally ar ranged in rhyth mic units of cyclothems and di vided by thin peb ble beds (Lake and Young, 1978; Lake and Shephard-Thorn, 1987; Hopson et al., 2008; Radley and Allen, 2012; Akinlotan, 2015). The Wadhurst Clay For ma tion (Lower–Mid dle Valanginian) com prises mainly soft, dark grey shales and mudstones, with mi nor sand stone, con glom er ate, cal car e ous sand, clay iron stone, and lig nite. All these have been changed penecontemporaneously at the sur face to col - oured sed i ments via out crop weath er ing (Lake and Young, 1978; Lake and Shephard-Thorn, 1987; Hopson et al., 2008;
Radley and Allen, 2012; Akinlotan, 2015). In terms of pe trol ogy and min er al ogy of the sed i ments, the sand stones and siltstones in the two for ma tions are mainly fine to me dium-grained quartz arenites with out any feld spar and rock frag ments (Akinlotan, 2015). The quartz grains are gen er ally monocrystalline. A very small amount of feld spar and rock frag ment is, how ever, pres - ent in the Wadhurst Clay Fm. Mus co vite and zir con are pres ent in very small amounts at Had dock’s Cot tages. Iron stains are very com mon fea tures. The mudstones and shales are dom i -
nated by illite and kaolinite with illite-mont mo ril lo nite and ver - mic u lite. Illite-smectite is pres ent in very small quan tity in the Wadhurst Clay Fm. (Akinlotan, 2015). The over all en vi ron ment of de po si tion of the sed i ments within these two for ma tions is flu - vial and non-ma rine. De po si tion oc curred un der both braided and me an der ing river sys tems. The com mon sed i men tary struc tures in the for ma tions are chan nels, cross bed dings, trough and pla nar lami na tions and slump struc tures. Tidal flat con di tions were ev i dent in the Ashdown Fm. while la goonal con di tions were more prom i nent in the Wadhurst Clay Fm.
(Allen, 1975, 1981; Stew art, 1981a, 1983; Lake and Shephard-Thorn, 1987; Akinlotan, 2015).
METHODS
STUDY LOCATIONS
Twenty sam ples com pris ing sev en teen mudstones and shales, and three fine-grained sand stones were sourced from across six lo ca tions from the Ashdown and Wadhurst Clay fms.
(Fig. 3). The study lo ca tions were se lected be cause they are part of the prin ci pal par tial ref er ence sec tions of the Wealden fa cies in the Weald. They rep re sent good spread across the ba sin and pres ent a mix of both coastal and in land sec tions to al low for the re view of spa tial vari abil ity in the com po si tion of the sed i ments.
Sam pling was ra tioned and lim ited to twenty sam ples be cause of the con straints on the num ber of sam ples al lowed by Nat u ral Eng land. This is be cause all the lo ca tions are ei ther Sites of Spe - cial Sci en tific In ter est (SSSI) and/or Geo log i cal Con ser va tion Re view (GCR) sites and are pro tected by law. The sec tion at Rock-a-Nore [TQ 827 095], pres ents sed i ments from the lower part of the Ashdown Fm. at Hastings. This sec tion is dom i nated by siltstones and mudstones with mi nor sand stones. Eight sam - ples, mudstones, and shale were col lected from this lo ca tion.
The out crop at Had dock’s Cot tages [TQ 883 124] is in the up per sec tion of the Ashdown Fm. and pres ents dis tinct sed i men tary cy cles con sist ing of pairs of cross-bed ded sandy and silty beds.
A fine-grained sand stone was col lected from the ac ces si ble part of this sec tion. The out crops at Cliff End [TQ 888 132] fall within the up per most part of the Ashdown Fm. and the low er most sec - tion of the Wadhurst Clay Fm. Cross-bed ded and channelized sand stones, siltstones, and mudstones that are ar ranged rhyth - mi cally dom i nate the sec tion. Two mudstone sam ples were col - lected from the up per part of the Ashdown Fm. while two sam - ples, mudstone and shale were col lected from the lower part of the Wadhurst Clay Fm. The sec tion at Hastingford [TQ 523 259]
in the lower Wadhurst Clay Fm., con sists mainly of cross-bed ded me dium-grained chan nel sands. Two sam ples were col lected from this lo ca tion. The West Hoathly Quarry [TQ 374 328 - TQ 375 329] also in the lower Wadhurst Clay Fm., is dom i nated by mudstone and three sam ples were col lected in this lo ca tion. One sam ple was col lected from the mudstone unit within Northiam Sand stone at Northiam [TQ 829 253] in the up per part of the Wadhurst Clay Fm.
PORTABLE XRF (ENERGY-DISPERSIVE) METHOD
In or der to de ter mine the chem i cal com po si tion of the sed i - ments and ana lyse for a range of im por tant el e ments to an ac - cept able lim its of de tec tion, en ergy-dispersive X-Ray Flu o res - cence (XRF) method was used. All the field sam ples were pul - ver ised and ho mog e nized us ing an ag ate pes tle and mor tar prior to XRF anal y sis. Trace and ma jor el e ment com po si tion was de ter mined us ing an Innov-X Al pha-6500 por ta ble X-ray
spec trom e ter (Olym pus Cor po ra tion, To kyo, Ja pan). This equip ment pro vides mea sure ments of el e men tal con cen tra - tions in parts per mil lion (ppm) for about 20 el e ments in small sam ples. The equip ment pro vides con cen tra tion of each el e - ment in clud ing a stan dard de vi a tion (S.D.) value. Data out put from the anal y sis was pro cessed us ing Microsoft Ex cel. Data pro cess ing in volves cor rect ing for sup pres sion or en hance - ment caused by the other el e ments for ma jor el e ments. Fur ther de tails of the pro ce dures, op er a tions and ap pli ca tions of XRF method have been de scribed (Jenkins, 1974; Jenkins et al., 1982; Lee and McConchie, 1982; Wil liams, 1987).
In or der to en sure the ac cu racy of the con cen tra tion of each el e ment, the stan dard de vi a tion was mul ti plied by three to give the ap pro pri ate de tec tion limit for the el e ment. Where the cal cu lated con cen tra tion is neg a tive, less than or equal to the stan dard de vi a tion value it means the re ported con cen tra - tion is be low the in stru men tal de tec tion limit and this is re - moved from the spread sheet. Some re ported con cen tra tions are greater than the er ror value but they are not up to three times the stan dard de vi a tion val ues and are there fore in dis tin - guish able sta tis ti cally from the de tec tion limit. In or der to en - sure the pre ci sion of the mea sure ment for each el e ment, each sam ple was mea sured twice and the av er age of the two mea - sure ments was taken. The pres ent study in cludes the de ter mi - na tion of Fe, K, Ca, and Ti as ma jor el e ments and Zr, Ba, Co, Mn, Rb, Ni, Zn, Cr, Sr, Cu, Pb, As and Mo as trace el e ments.
El e ments with light atomic mass such as Mg, Al, and Si were not de tected by the por ta ble X-ray spec trom e ter and the ap - plied method but may be pres ent within the sed i ments. To quan tify pos si ble en rich ment of the re ported el e ments, the con cen tra tion data were nor mal ized to avoid di lu tion ef fects in the mea sure ments. Ti nor mal iza tion was car ried out us ing: El - e ment/Ti (sam ple)/El e ment/Ti (shale) for each mea sure ment.
The “av er age shale” val ues of Wedepohl (1971, 1991) were used as ref er ence back ground data. If the re sulted value is >1,
then the given el e ment is en riched, rel a tive to the av er age shale, but value <1 in di cates im pov er ish ment.
SPECTRAL GAMMA-RAY METHOD
To de ter mine the con cen tra tions of K, U and Th which can in turn pro vide use ful in for ma tion about clay min eral types, prov e nance, car bon con tents and grain size within the sed i - ments, spec tral gamma-ray (Gamma Ray Spec tros copy – GRS) log ging was con ducted at Pett Level (Cliff End and Had - dock’s Cot tages) and Rock-a-Nore (Figs. 1 and 3). The sites were se lected for the log ging be cause they have the best-ex - posed sec tions among all the study sites. These sites also pres - ent stratigraphically im por tant and dis tinct fa cies of the lower and up per Ashdown Fm., the bound ary be tween the Ashdown and Wadhurst Clay fms. and the Cliff End Sand stone.
Ninety-one field GRS mea sure ments were made with a Gamma Sur veyor por ta ble spec trom e ter (GF In stru ments, Brno, Czech Re pub lic) with NaI (Tl) scin til la tion de tec tor di am e - ter 3’’, length 3’’ (vol ume 6.3 in3 ). It is equipped with a con trol unit (256 ´ 90 ´ 60 mm) for data en try and dis play of mea sure - ments. The equip ment dis tin guishes the three com po nent de - cay chains of po tas sium, ura nium, and tho rium by the wave - lengths of their char ac ter is tic gamma emis sions. Eight log ging points were se lected, one at Rock-a-Nore and seven at Pett Level, (see Figs. 1 and 3) to cover the dif fer ent fa cies within the two for ma tions. Mea sure ment at each log ging point (in ter val of 20 cm) was per formed with full rock-de tec tor con tact on flat sur - face. The de tec tor was po si tioned per pen dic u lar to the sec tion wall and par al lel to bed ding. The in stru ment con verts quan ti ta - tive val ues (counts per sec onds) in the en ergy win dows to con - cen tra tions of K (%), eU (ppm of ura nium equiv a lent) and eTh (ppm of tho rium equiv a lent), re spec tively. The mea sured con - cen tra tions were trans ferred to spread sheet in Microsoft Ex cel for fur ther anal y ses.
Fig. 3. Stratigraphic positions of the samples used in this study and the summary log of study locations
Each number refers to the number of samples collected at location
RESULTS
XRF ANALYSIS
The geo chem i cal com po si tion of sam ples from the study ar - eas is shown in Ap pen dix 1*. The dis tri bu tion of ma jor (Fig. 4) and trace (Fig. 5) el e ments is pre sented. Com par i son of the el e - men tal suite to the av er age crustal shale val ues (Wedepohl, 1971, 1991) and the re view of Ti-nor mal ized en rich ment fac tors en abled the di vi sion of these el e ments into two cat e go ries (Ta - ble 1): en riched el e ments (Co, Mo, Zr, and As) and non-en - riched el e ments (Pb, Ni, Zn, Cu, Cr, Rb and Mn). The dis tri bu - tion of the en riched el e ments across all the study sites is given in Fig ure 6. The vari a tion of the con cen tra tions with depths of some se lected el e ments across some of the study lo ca tions is shown (Figs. 7 and 8). Al though the av er age con cen tra tions of Pb and Ba in study lo ca tions show that they are not en riched, their con cen tra tions ap pear to be close to the val ues of the av - er age crustal shale (Fig. 9). Co, Mo, Ba, and Pb have their high - est val ues in the sec tion at Rock-a-Nore in the lower Ashdown Fm. Zr has its high est value in the Ashdown Fm. at Had dock’s Cot tages. It then de creases up through the sec tion and north - wards. En riched el e ments As and Ti have their max i mum con - cen tra tions in the West Hoathly Quarry in the lower Wadhurst Clay Fm. Mo has its high est value in the out crop at Northiam (Wadhurst Clay Fm.). Other lo ca tions have sim i lar quan ti ties of
Mo. Fe has its great est amounts at West Hoathly Quarry and Northiam in the Wadhurst Clay Fm.
SPECTRAL GAMMA-RAY ANALYSIS
The sum mary of the spec tral gamma-ray data is pre sented in Ta ble 2 and the strati graphic dis tri bu tion of the ra dio ac tive min er als is pre sented in Fig ure 10. These sed i ments re veal me dium level of ra dio ac tiv ity with the fol low ing mean con cen - tra tions to tal gamma ray: 70.3 cps; K: 0.8%; Th: 9.5 ppm and U:
3.6 ppm. Po tas sium is gen er ally higher in the sand stone com - pared to the mudstone and shale while tho rium and ura nium are gen er ally higher in the mudstone and shale than in the sand stone al though this is not a well-es tab lished trend. Over all, the sand stone fa cies have higher gamma-ray re sponse com - pared to mudstone and shale in the two for ma tions. The coarse-grained sand stones have higher gamma-ray re sponse com pared to finer sands. The lig nite band has higher gamma-ray re sponse than its ad ja cent sand stone. There is a higher gamma-ray re sponse in the lower Ashdown Fm. at Rock-a-Nore than in the up per part of the for ma tion at Cliff End.
This trend is es tab lished in the sand stones and mudstones.
The mudstone in the lower Ashdown Fm. has a higher gamma-ray re sponse com pared to the shale in the lower Wadhurst Clay Fm. The Ashdown sand stones have higher gamma-ray re sponse com pared to the Cliff End Sand stone.
The lower Wadhurst Clay Fm. has a higher re sponse than the up per part of the for ma tion. The Cliff End Sand stone has the low est re sponse to K while the basal sand stone within the Wadhurst Clay Fm. has the high est re sponse. As a whole, the Ashdown Fm. has a higher re sponse to gamma emis sions com pared to the Wadhurst Clay Fm.
DISCUSSION
SOURCE ROCKS
The spec tral gamma-ray data from this study re veals that tho rium is con cen trated at the ex pense of ura nium. Tho rium is gen er ally con cen trated dur ing the for ma tion of kaolinite from the weath er ing of feld spars and mus co vite (Ruffell and Worden, 2000; Schnyder et al., 2006). The con cen tra tion of tho rium dur ing the for ma tion of kaolinite from feld spars sug gest
* Supplementary data associated with this article can be found, in the online version, at doi: 10.7306/gq.1328 Fig. 4. The concentrations of major elements
RN – Rock-a-Nore, HA – Haddock’s Cottages, CE – Cliff End (Pett Level), HC – Hastingford, WH – West Hoathly Quarry, NT – Northiam
Fig. 5. The concentration of trace elements Explanations as in Figure 4
Fig. 6. The concentration of the enriched elements Explanations as in Figure 4
(Wedepohl, 1971, 1991) This study
Ti-EF Av er age shale Av er age N Max i mum Min i mum
En riched
Co 19 269 3 377 93 14.0
Mo 2 6 23 10 3 2.79
Zr 160 365 40 1025 32 2.19
As 10 12 16 37 1 1.19
Non-en riched
Pb 21 19 36 36 9 0.86
Ba 580 480 38 806 26 0.79
Ni 68 45 22 113 15 0.63
Zn 95 55 36 134 7 0.55
Cu 45 24 2 39 8 0.51
Cr 90 46 30 78 21 0.49
Rb 140 69 40 117 4 0.47
Mn 850 220 40 1492 26 0.24
Sr 300 44 40 145 4 0.14
Other ma jor el e ments
Fe 48000 13320 40 38785 549 0.26
K 29900 8131 40 15603 711 0.26
Ca 15700 2807 32 36176 73 0.17
N – num ber of mea sure ments, Ti-EF – Ti-nor mal ized En rich ment Fac tor (>1= en rich - ment, <1=im pov er ish ment)
T a b l e 1 Com par i son of the av er age con cen tra tions (in ppm) of el e ments from this study
to the av er age crustal shale val ues
Fig. 7. Variations of As, Mo and Ni with depths at Rock-a-Nore and Cliff End
that the sed i ments within these two for ma tions have been sourced mainly from gran ites and/or gneiss es from within the source ar eas as sum ing there were pri mary sources. The en - rich ment of Zr also sup ports fel sic sources (e.g., Alvarez and Roser, 2007). Four ma jor pos si ble sources have been tra di tion - ally sug gested as the prov e nance for the sed i ments in the Weald Ba sin: Lon don–Brabant Mas sif (Londinia) in the north and north-east, Armorica in the south-west, Cor nu bia in the west and an other one to wards the north ern (Bo real) Sea (e.g., Kirkaldy, 1947; Allen, 1975, 1989, 1991; Radley and Allen, 2012). The main source of the sed i ments within the Ashdown and Wadhurst Clay fms. was the Lon don–Brabant Mas sif (Londinia) in ad di tion to mi nor in puts from the Armorica and Cornubia (e.g., Allen, 1975; Sla den and Bat ten, 1984;
Akinlotan, 2015). All these sources have gra nitic and/gneissic ma te ri als within them and are ei ther pri mar ily or sec ond arily sourced. The Lon don–Brabant Mas sif con sists of Pre cam brian base ment, strongly folded Lower Pa leo zoic metasedimentary and vol ca nic rocks, De vo nian and Car bon if er ous rocks (Old Red Sand stone). The Armori can Mas sif also con sists of Pre - cam brian core of staurolite-kyan ite-gar net schists, gran ites, New Red Sand stone (Permo-Tri as sic), and Ju ras sic volcanogenic de bris. The Cor nu bia area was par tially cov ered Fig. 8. Variations of Ba and Zr with depths at Rock-a-Nore, Cliff End and West Hoathly Quarry
Fig. 9. Linear correlation between the concentrations of average crustal shale (Wedepohl, 1971, 1991) and those reported in this study based on Table 1 Only the enriched elements and (Pb, Ba) are labelled for emphasis
by out crops of Permo-Tri as sic beds, the New Red Sand stone in clud ing some vol ca nic de bris and Ju ras sic sed i ments (Allen, 1981, 1991; An dre, 1991). The en rich ment of Th and the de ple - tion of Ca also sug gests that these source rocks were sub jected to se vere weath er ing (e.g., McLennan et al., 1983; Alvarez and Roser, 2007). Ev i dence from sand stone pe trog ra phy and clay min er al ogy also sup ports this in ter pre ta tion. The sand stones and siltstones within Ashdown and Wadhurst Clay fms. are mainly quartz arenites and quartzose siltstones and com posed mainly of quartz grains while kaolinite is a dom i nant clay min - eral within the mudstones and makes up about one-third of the clay min er als (Akinlotan, 2015). This in ter pre ta tion agrees with
pre vi ous in ter pre ta tions from tra di tional proxy such as field data (e.g., Kirkaldy, 1947; Allen, 1975, 1989, 1991; Radley and Allen, 2012).
TECTONIC SETTINGS
Bhatia and Crook (1986) used the trace el e ments as sem - blage of clastic sed i ments (sand stones) to de ter mine their tec - tonic set tings. They pre sented that sed i ments sourced from pas sive mar gin are char ac ter ized with high Zr, low Ba, Rb and Sr con cen tra tions. This ap proach can be ap plied to the sed i - ments within the Ashdown and Wadhurst Clay fms. which are
GR [cps] K [%] Th [ppm] U [ppm]
Mean Max. Min. Mean Max. Min. Mean Max. Min. Mean Max. Min.
CE 3 32.1 58.9 17.1 0.23 0.6 0.1 3.5 9.2 0.5 1.6 3.9 0.3
CE 2 82.5 110.3 37.33 1 1.8 0.1 12.5 20 3.8 4.3 7 1.1
CE 1 85.9 234 55.4 1 3.5 0.2 9.8 16.4 4.9 4 6.9 2
HAD 55.8 75.4 38.1 0.3 0.8 0 8.2 11.1 4 3.6 5.4 1
RN 95.2 130.9 47.9 1.5 3.3 0 13.4 20.2 5.8 4.3 6.6 2.4
TOTAL 70.3 234 17.1 0.8 3.5 0 9.5 20.2 0.5 3.6 7 0.3
In strati graphi cal or der: RN – Rock-a-Nore; HAD – Had dock’s Cot tages, CE – Cliff End
T a b l e 2 Sum mary of the field spec tral gamma-ray data
Fig. 10. Distribution of spectral gamma-ray data The locations are in stratigraphic order
also clastics. In the Ashdown and Wadhurst Clay fms. there is a high abun dance of Zr while the con cen tra tions of Ba, Cr, Ni, Rb and Sr are very low com pared to the con cen tra tions of av er age crustal shale. The con cen tra tions of com pat i ble and in com pat i - ble el e ments (Zr, Cr, Ni, Rb and Sr) in vary ing quan ti ties (e.g., McLennan et al., 1983) may sug gest that the sed i ments were sourced from pas sive con ti nen tal mar gin. The com po si tion of sand stones has also been used to de ci pher the tec tonic set - tings in the source ar eas (e.g., Dickinson and Suczek, 1979;
Bhatia, 1983; Dickinson, 1985). Dickinson (1985) dem on - strated that quartzose sands (quartz-rich) with high monocrystalline to polycrystalline grains ra tios and high K-feld - spar to plagioclase feld spar ra tios are de rived from sta ble craton from con ti nen tal in te rior or pas sive plat form.
Petrographic data (see be low) from the Ashdown and Wadhurst Clay fms. (Akinlotan, 2015) also sup port the sta ble craton model. Petrographic de scrip tions of the ma jor sand mem bers within two for ma tions show that the ma jor ity of sand stones and siltstones in these two for ma tions are quartz arenites and quartzose siltstones and ex tremely rich in quartz. The ra tio of monocrystalline quartz grains to that of the polycrystalline is rather high (ap prox i mately 99:1). In ad di tion, only K-feld spar is iden ti fied and no plagioclase feld spar is de scribed (Akinlotan, 2015). The above men tioned geo chem i cal and petrographic data sug gest that the sed i ments from the Ashdown and Wadhurst Clay fms. may have been sourced from a sta ble craton (Dickinson and Suczek, 1979; Bhatia, 1983; Dickinson, 1985; Bhatia and Crook, 1986). The tec tonic sta bil ity of the main source of the sed i ments Lon don–Brabant Mas sif from De - vo nian times has been doc u mented by Rijkers et al. (1993). The pos si bil ity of a sta ble craton model is likely since a sig nif i cant amount of the sed i ments in the Lon don–Brabant Mas sif has been re cy cled from other sources. The in ter pre ta tion on the tec - tonic set ting in the source ar eas is based on lim ited data and is there fore open to fur ther dis cus sion when new data sets be - come avail able.
PALAEOCLIMATE
El e vated ra tio of tho rium to po tas sium or ura nium in clays has been in ter preted to re flect hot and hu mid palaeoclimates.
Tho rium is en riched dur ing weath er ing in hot and hu mid con di - tions with in crease in the pro duc tion of kaolinite as a re sult of mo bi lis ing and leach ing of K and U thereby con cen trat ing tho - rium in the hin ter land soils (Ruffell and Worden, 2000;
Schnyder et al., 2006). In the cur rent study, the con cen tra tion of tho rium is con sid er ably higher than that of ura nium. The mean con cen tra tion of tho rium is 9.5 ppm while that of ura nium is 3.6 ppm. Con sid er ing the higher quan tity of tho rium in re la tion to ura nium and tak ing on the model of Ruffell and Worden (2000) and Schnyder et al. (2006) into con sid er ation, it is in ferred that the palaeoclimatic con di tions at the source ar eas were hu mid and hot. This cli ma tic pat tern sub jected the source rocks to se - vere weath er ing which was also con firmed by the de ple tion of Ca (e.g., McLennan et al., 1983; Alvarez and Roser, 2007).
This palaeoclimatic in ter pre ta tion agrees with pre vi ous in ter pre - ta tions from field ob ser va tions and palaeontological ex am i na - tions (e.g., Kirkaldy, 1947; Allen, 1975, 1989, 1991; Radley and Allen, 2012).
DEPOSITIONAL ENVIRONMENT
The data from the spec tral gamma-ray anal y sis from this study pres ent ev i dence for fresh wa ter en vi ron ments for the Ashdown and Wadhurst Clay fms. Mudstone and shale of ma - rine or i gin are ex pected to dis play higher ra dio ac tiv ity than the ad ja cent sand stone be cause of higher con tents of or ganic mat - ter that is typ i cal of ma rine en vi ron ments (Šimíèek et al., 2012).
In the Ashdown and Wadhurst Clay fms., the sand stone fa cies are more ra dio ac tive than the ad ja cent mudstone and shale and this trend is con sis tent with a fresh wa ter or i gin for these sed i ments (Schnyder et al., 2006; Šimíèek et al., 2012; Šimíèek and Bábek, 2015). Higher gamma-ray counts may be at trib uted to mudstone while low gamma-ray counts may be char ac ter is tic of sand stones and con glom er ates be cause of the ef fect of non-ra dio ac tive quartz, car bon ate ce ment and pore space in the sand fa cies (Šimíèek et al., 2012). Ev i dence from the li thol - ogy ob served in the field and in the bore holes such as sand - stones, siltstones, mudstones, shales, lig nite and the geo - chem i cal com po si tion of the si der it ic iron stone (high en rich - ment of Fe and Mn and the low Mg/Ca ra tio) also sup ports the fresh wa ter and non-ma rine or i gin of these sed i ments (Akinlotan, 2015). These in ter pre ta tion agrees with pre vi ous in - ter pre ta tions based on field data that the sed i ments within the Ashdown and Wadhurst Clay fms. were de pos ited un der pre - dom i nantly non-ma rine, fresh wa ter and flu vial con di tions (Kirkaldy, 1939; Allen, 1975, 1981; Stew art, 1981b, 1983; Lake and Shephard-Thorn, 1987; Hopson et al., 2008; Radley and Allen, 2012).
REDOX CONDITIONS
Trace el e ments such as Mo, U, V and Co are en riched in re - duc ing sed i ments and are highly sen si tive to re dox changes.
These re dox-sen si tive el e ments have been used to dem on - strate anoxic en vi ron ments and for palaeoredox re con struc tion (e.g., Lipinski et al., 2003; Elbaz-Poulichet et al., 2005;
Brumsack, 2006; Jiang et al., 2007; Xu et al., 2012b). These re - dox-sen si tive el e ments Mo, U, V and Co are ex tremely en - riched in the Ashdown and Wadhurst Clay fms. Their en rich - ment strongly sug gests anoxic con di tions at the site of de po si - tion of the sed i ments and re flects the pre vail ing con di tions in the wa ter col umn at the time of de po si tion. In ad di tion to these re dox-sen si tive el e ments, it has been shown that the pre cip i ta - tion of sid er ite is a di rect in di ca tion of re duc ing con di tions in such en vi ron ment (e.g., Huber, 1958; Bahrig, 1989; Browne and Kingston, 1993). Huber (1958) also con firmed that sid er ite is sta ble un der re duc ing con di tions. In the Ashdown and Wadhurst Clay fms., wide spread oc cur rence of si der it ic iron - stones is well doc u mented (Tay lor, 1963; Lake and Thurrell, 1974; Lake and Shephard-Thorn, 1987; Radley and Allen, 2012; Akinlotan, 2015) and their pres ence is a di rect in di ca tion of re duc ing/anoxic con di tions at the time of de po si tion. The anoxic con di tions in the Wadhurst Clay Fm. re flect la goonal con di tions while in the lower Ashdown Fm. it may point to floodplain en vi ron ment. Allen (1989) ob served the de vel op - ment of anoxic la goons within the Wadhurst Clay Fm. in East Sus sex. The quan ti ties of Co, U and Mo re veal that the high est level of anoxic con di tions oc curred in the lower Ashdown Fm.
be fore de creas ing to wards the bound ary with Wadhurst Clay Fm. Anoxic con di tions in creased again in the lower Wadhurst Clay Fm. and then de creased up wards the se quence be fore pick ing up in the up per part of the for ma tion. Based on the avail - able data, it ap pears that the high est level of anoxic con di tions oc curred at the sec tion at Rock-a-Nore (Fig. 1) within the lower Ashdown Fm. In the Wadhurst Clay Fm., anoxic con di tions are ap pear to be at peak at the sec tion at West Hoathly quarry (Fig. 1) fol lowed by the sec tion at the Wadhurst Clay basal beds at Pett Level and Northiam (Fig. 1).
DIAGENESIS
It is well-es tab lished that the Wealden sed i ments have ex - pe ri enced early diagenesis with a mod er ate de gree of diagenetic al ter ation (Allen, 1981; Sla den, 1983; Sla den and Bat ten, 1984; Radley and Allen, 2012; Akinlotan, 2015). This has been dem on strated by the clay min eral as sem blage and the rel a tively shal low depth (1.5–2 km). Clay min eral anal y ses re veal that the illite as sem blage is dom i nated by 1M polymorph which seems to be de tri tal in or i gin and not diagenetic (e.g., Sla - den, 1983, 1987; Akinlotan, 2015). The wide spread oc cur rence of si der it ic iron stones also con firms early diagenesis in the sed i - ments (Lake and Thurrell, 1974; Lake and Shephard-Thorn, 1987; Akinlotan, 2015). The geo ther mal gra di ent be neath the con ti nen tal crust of the United King dom is ~10°Ckm–1 (Rollin, 1995; Westaway et al., 2002). This geo ther mal gra di ent has pro duced in suf fi cient heat (~15–20°C) to drive any ma jor diagenetic al ter ation of the sed i ments or the trans for ma tion of smectite to illite-smectite in these for ma tions. This early diagenesis is un likely to have caused any sig nif i cant changes to the geo chem i cal com po si tion. On the other hand, the de ple tion of Ca sug gests that it was prob a bly leached dur ing this diagenetic pro cess (e.g., McLennan et al., 1983; Alvarez and Roser, 2007).
WIDER IMPLICATIONS
Us ing the el e men tal com po si tion of sed i ments for un der - stand ing the palaeoenvironmental con di tions of sed i men tary bas ins has wider im pli ca tions be yond the Weald Ba sin. This work has shown that the el e men tal com po si tion of terrigenous sed i ments can pro vide use ful in for ma tion about the palaeoenvironmental con di tions of sed i men tary bas ins. This has wider im pli ca tions be yond the Weald Ba sin and the prin ci - ple can be ap plied to other sed i men tary bas ins else where. As shown in this study the geo chem i cal com po si tion of the sed i - ments has pro vided im por tant in for ma tion about the tec tonic set tings of the source ar eas, anoxic and re duc ing con di tions, and palaeoclimatic con di tions in the Weald Ba sin. The in ter pre - ta tions made based on the geo chem i cal com po si tion of sed i - ments in the Weald Ba sin can be made in other sed i men tary bas ins with sim i lar geo chem i cal com po si tion. In the Weald Ba - sin, the in ter pre ta tions of the palaeoenvironments based on the geo chem is try of the sed i ments agree with in ter pre ta tions from
other prox ies. As a re sult, when tra di tional data sets such as field, fos sil and min er al og i cal data are not readily avail able or in - suf fi cient to en able ad e quate in ter pre ta tions, the el e men tal com po si tion of sed i men tary rocks will prove to be ex tremely use ful ei ther as sup ple ment or pri mary data for un der stand ing the palaeoenvironmental con di tions of sed i men tary bas ins. The geo chem i cal meth ods used in this study, XRF and Spec tral GR can also be used in other sed i men tary bas ins and the re sult ing geo chem i cal com po si tions will pro vide use ful in for ma tion about palaeoenvironmental con di tions. The geochemical composition of sediments will provide useful information on tectonic setting, provenance, redox conditions, and weathering conditions of sedimentary basins.
CONCLUSIONS
Geo chem i cal anal y ses re vealed that the Ashdown and Wadhurst Clay fms. are en riched in Co, Mo, Zr, As and de fi cient in Pb, Ba, Ni, Zn, Cu, Cr, Rb and Mn. Re dox sen si tive el e ments such as Fe, Co, Cr, Cu, As and Mo are pres ent in var i ous quan - ti ties. The geo chem i cal as sem blage pro vides use ful in for ma - tion on sed i ment prov e nance and depositional con di tions. The con cen tra tions of Zr, Ba, Cr, Ni, Rb and Sr in vary ing quan ti ties es tab lished that the sed i ments may have been sourced from pas sive con ti nen tal mar gin while the en riched re dox-sen si tive el e ments such as As, Mo and Co con firm the pres ence of anoxic and re duc ing con di tions at the sites of de po si tion.
Anoxic con di tions peaked in the lower Ashdown Fm. and then de creased to wards the bound ary with Wadhurst Clay Fm.
Anoxic con di tions re turned in the lower Wadhurst Clay Fm. and then de creased up wards the se quence. The anal y sis of spec - tral gamma-ray data shows that the sand stone fa cies are more ra dio ac tive than the ad ja cent mudstone and shale, which con - firms their fresh wa ter or i gin. The pri mary sources of the ra dio - ac tive el e ments are likely to be the heavy min er als in the quartz grains and non-ra dio ac tive ce ments while sec ond ary sources are feld spars, micas, and glauconite. The higher quan tity of tho - rium in re la tion to ura nium in di cates hu mid and hot palaeoclimatic con di tions at the source ar eas that fa voured in - tense weath er ing. The geo chem i cal com po si tion of sed i ments has wider im pli ca tions for un der stand ing the depositional con di - tions of sed i men tary basins and they will provide useful information on tectonic setting, provenance, redox conditions, and weathering conditions.
Ac knowl edge ments. This pa per is pro duced from a doc - toral re search con ducted at the Uni ver sity of Brigh ton, United King dom and su per vised by Prof. A. Cundy, Dr. J. Ullyott, and Dr. M. Smith. The re search was partly funded by the Uni ver sity of Brigh ton In ter na tional Re search Schol ar ship and by the School of En vi ron ment and Tech nol ogy. I want to ex press my grat i tude to P. Ly ons for help ing with the run ning of Bench top XRF anal y sis. Many thanks to Dr Bela Raucsik and an anon y - mous re viewer who made con struc tive com ments to im prove the fi nal draft.
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