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Early pleistocene sediments at Great Blakenham, Suffolk, England
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QuaternaD,ScienceReviews,Vol, 15, pp. 413--424, 1996.
i Pergamon PIh S0277-3791(96)00013-3
Copyright© 1996ElsevierScienceLtd. Printed in Great Britain. All rights reserved, 0277-3791/96 $32.00
EARLY PLEISTOCENE SEDIMENTS AT GREAT BLAKENHAM, SUFFOLK, ENGLAND P . L . G I B B A R D , * P. A L L E N , ? M . H . F I E L D ~ a n d D.F. H A L L A M §
*Godwin Institute for Quaternary Research, Department of Geography, University of Cambridge, Downing Place, Cambridge CB2 3EN, U.K. ?Department of Geography, London Guildhall University, Old Castle Street, London El 7NT, U.K. "~.Centre for Quaternary Science, Coventry University, Priory Street, Coventry CV1 5FB, U.K. §School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, U.K. Abstract - - Detailed investigation of a fine sediment sequence, the College Farm Silty Clay Member, that overlies the Creeting Sands (Early Pleistocene) in Suffolk, is presented. The sedimentary sequence is thought to represent a freshwater pool accumulation in a small coastal embayment. Palaeobotanical investigation of the sediment indicates that it accumulated during the late temperate substage of a temperate (interglacial) event. The occurrence of Tsuga pollen, associated with abundant remains of the water fern Azolla tegeliensis indicate that the deposits are of Early Pleistocene age and are correlated with a later part of the Antian-Bramertonian Stage. Correlation with Tiglian TC3 substage in The Netherlands' sequence is most likely. The sediments' normal palaeomagnetic polarity reinforces the biostratigraphical correlation. Copyright © 1996 Elsevier Science Ltd
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diminishes from ca. 15 m to less than 5 m. The western limit of the basin is not clearly defined but borehole records do not show the beds occurring further than 2 km to the west. The wider significance of the Creeting sequence has been discussed by Dixon (1978) and Allen (1984). Zalasiewicz and Mathers (1985) have correlated the Creeting Formation lithostratigraphically with the Chillesford Sands and Clay members of the Norwich Crag Formation that occur in the main Crag basin to the northeast of the area. The Chillesford Sand has been equated with the Early Pleistocene Bramertonian Stage (West and Norton, 1974; Funnell et ak, 1979), and the Chillesford Clays with the subsequent Baventian/PrePastonian a Stage (Zalasiewicz et al., 1991). This correlation is supported by the interpreted sedimentary environment of the Chillesford Sand as a tidal flat deposit (Funnell, 1961; Dixon, 1972; Zalasiewicz and Mathers, 1985), whilst the Chillesford Clay is thought to represent a high intertidal accumulation (Evans, 1965; Zalasiewicz et al., 1991). Biostratigraphical correlation with The Netherlands' succession indicates that these units represent the Tiglian substages TC3 and TC4c respectively (Gibbard et al., 1991). With the exception of one undiagnostic pollen spectrum obtained by D.T. Holyoak (in Allen, 1984), no
INTRODUCTION The substantial quarry complex at Great Blakenham (Fig. 1) lies on the western side of the Gipping Valley in Suffolk, England. Sections here expose an important sequence of Early and Middle Pleistocene sediments over 15 m in thickness. Although it was previously known (Markham, 1972), the major work on this site has been undertaken by Allen (1984) who investigated the sedimentary sequence, identified and interpreted the major aggradational units and proposed a series of formal terms for the units he differentiated. Of these the largest unit was the Early Pleistocene Creeting Formation comprising two members, the Creeting Sands and the College Farm Silty Clay. It is overlain unconformably by sands and gravels of the Kesgrave Group (Fig. 2) (Whiteman and Rose, 1992). According to Allen (1984), the Creeting Formation occurs in basins, kilometres or tens of kilometres in diameter, developed on the Chalk bedrock, the largest basins being around Stradbroke and Eye (Fig. 3) (see also Notcutt, 1978; Bristow, 1983). The Great Blakenham basin is bounded by a rise in the Chalk surface to the north, to over 40 m O.D. between Darmsden and Barking, and similarly to the south at Little Blakenham. Even within the quarry the Chalk surface can be seen rising to the south and the thickness of the Creeting sediments 413
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P.L. Gibbard et al.: Early Pleistocene Sediments at Great Blakenham, England evidence of the age of the Creeting sequence has been hitherto available. This paper reports the palaeobotanical assemblages obtained from the College Farm Silty Clay Member exposures in autumn 1990. Unfortunately, the sediments contained no diatoms or calcareous fossils (see below). However, the results obtained provide an important contribution to the understanding of the depositional environment of the Creeting Formation, to the evolution of vegetation in the Bramertonian Stage and provide the earliest evidence of freshwater sedimentation in the British Pleistocene.
SITE STRATIGRAPHY The Creeting Formation at Great Blakenham comprises up to 11.25 m of Creeting Sand Member overlain by 3.75 m of College Farm Silty Clay Member (Fig. 2). The sediments are contained within a Chalk bedrock depression (Fig. 1) such that the floor in the vicinity of the section is at 36-38 m O.D. but rises to 47.2 m on the south side of the quarry and 3 km to the northwest, at Willisham Hall, reaches 63.4 m. Approximately 6 km to the west, the Chalk surface rises to over 40 m O.D., as at Manor Farm, Elmsett. The Formation thins or is absent over the bedrock highs and locally is not found beyond 2 km west of the quarry. Boreholes indicate that the Great Blakenham basin is separated from the Stradbroke trough to the north and the main coastal Crag basin to the south and east. Locally the Formation is not recorded in boreholes west of Great Blakenham or more than 5 km west of the Gipping Valley in the other Crag basins. The basin form of the Chalk surface and the local westerly limit of the Crag (Fig. 3) are argued to result from Quaternary tectonic activity (Allen, 1984), probably part of regional uplift during the Early Pleistocene (Mathers and Zalasiewicz, 1988). The Creeting Sand (Fig. 2) has a basal lag of nodular flint cobbles and phosphatic nodules which has been attributed to the Red Crag Formation (Allen, 1984). This is immediately overlain by well-sorted, very fine to medium, micaceous sand with occasional matrix-supported, poorly sorted gravelly sand beds (less than 5%) and a small amount of silty clay (up to 15%). The primary sedimentary structures range from ripples, through planar and trough cross-sets (up to 1.3 m thick), to horizontallybedded units (up to 1.6 m thick). Overall, laterally extensive, horizontal beds dominate. Some beds appear structureless. Silty clay occurs in variable amounts, but increasing in frequency upwards, as curled flakes in the structureless and planar cross-sets, as thin seams in simple and wavy types of flaser bedding or as thicker seams within interlayered bedding. Although the planar crosssets appear to give a 'herring-bone' pattern, palaeocurrent measurements (12) show a wide range of readings with a slight concentration between 300-340 ° . Very infrequently the bedding is interrupted by small, cylindrical, U-shaped burrow structures up to 4 cm in vertical extent. The horizontal nature of the bedding, with primary structures typical of the lower and transitional flow
417
regimes, with flaser and interlayered bedding becoming more important upwards, indicate an aggrading environment such as a sand flat changing to a mixed-fiat, with curled flakes of silty clay resulting from desiccation when the surface of the flat was subaerially exposed. The random pattern of the palaeocurrent readings may not be meaningful because of the small number of measurements or it may reflect the fact that the current and wave direction on sand-flats can be highly variable as a consequence of local morphological slope (Reineck and Singh, 1'980) and wind direction (Reineck and Wunderlich, 1969). The CM panicle size characteristics (Passega, 1957) and well-sorted nature of the sand support deposition on a sand-flat. The upper 5-6 m of the sands are significantly coarser than those beneath indicating higher energy levels, possibly associated with shallower water conditions and greater wave activity. The occasional gravelly sands may reflect storm events or powerful flows from nearby rivers. The paucity of biogenic structures, suggesting a lack of faunal activity, may reflect rapid aggradation of the sands. Resting conformably on the Creeting Sands is the College Farm Silty Clay Member, up to 3.75 m thick. The silty clay occurs as a sequence of grey (5Y3/1 to 10YR5/ 1) horizontal laminae and beds up to 1.2 m thick, interbedded with well-sorted sand or as interlayered sand and silty clay laminae forming cosets up to 1 m thick. In places, particularly the lower part, laminated units occur such as those observed during this study, The silty clay is poorly sorted while interbedded sands are well-sorted. Small-scale loading and water escape structures occur relatively frequently. The individual silty clay beds are generally massive, though the upper ones are brecciated in places and oxidised to strong brown (7.5YR5/8). This member is only locally present for less than 100 m in the Great Blakenham quarry. East and west of the sampled section it thins, becoming replaced by flaser and lenticular laminae. Individual beds pass laterally into brecciated and partially contorted sediments, particularly to the north. The whole unit is, in places, glaciotectonically disturbed and overfolded (Fig. 2), The microstratigraphy of the sequence sampled for palaeobotanical investigation is given below: 0-18 cm 18-31 cm
obscured above massive grey fine sand grades into light grey silt massive medium grey silty clay
31-49 cm
finely laminated red fine sand and fine grey sill
49-55 cm
light grey silt with fine sand laminae (<1 mm thick) orange grey fine sand interbedded with light grey silt bands of irregular thickness (<1 cm). light grey silty clay with fine sand laminae (<1 cm thick). massive dark grey silty clay grades upwards into overlying sediment ripple-bedded sand with divided fragmented plant detritus in thin (1 mm thick) drapes dark grey sand with thin grey clay laminae (5 mm thick)
55-68 cm 68-82 cm 82-129 cm 129-139 cm 139-151 cm
418
QuaternaO' Science Reviews: Volume 15
151-158 cm massive grey clay 158-164 cm laminated sand and clay (<5 mm thick), base of sand units shows minor erosion 164-196 cm massive dark grey silty clay with occasional thin laminae of grey silt, and sand band at 93.5-95 cm 196--224 cm light grey silty clay with fine silt laminae (<2 mm thick) >224 cm orange red medium sand with silty clay laminae form interbedded 10 cm thick units The change to clay deposition implies an abrupt reduction in current and wave activity and the appearance of stillwater conditions. Throughout the sequence the rhythmic pattern suggests background fine sediment deposition punctuated by periodic inwash of coarser sediment presumably suggesting current flow or inwash from the nearby land. The presence of plant material, some of which was inwashed or derived from a shallow marginal subenvironment suggests that a stream may have been entering the embayment nearby. Periodically higher energy influxes from the stream or from tidal floods producing bottom currents may account for the coarser sediment strata in the rhythmic structure of the fine sediments. The interlayered bedding and massive silty clay beds are associated with high concentrations of suspended sediment. Such conditions are met in sub-tidal water (Reading, 1978), near the high water mark in estuaries or inshore of an extensive tidal fiat. The presence of organic detritus, the sedimentary structures and the flora (see Palaeobotany) observed suggests the latter is more likely. Two possible explanations seem likely. The fine nature of the sediment, with occasional sand partings, and its horizontal bedding suggest deposition in a shallow basin or an upper mud-flat (cf. Evans, 1965). While the weathering and brecciation of the upper part of the sequence could indicate occasional prolongued subaerial exposure in either case, such structures might reflect lateral transition to mixed flats. If the deposits indicate a change in the depositional environment from sand-flat to a mud-fiat flanked by mixed-flat, they could potentially reflect a local change in the configuration of the flats (Reineck, 1972). The limited lateral extent of the deposits would then represent infill of a channel. Mudflats usually occur in areas sheltered from winddriven waves (Pethick, 1984). The presence of freshwater material in the silty clay (Palaeobotany, below) would indicate that the contemporary shoreline was nearby, in keeping with the higher Chalk bedrock on the south side of the quarry. Nevertheless, the partial or complete closure of the embayment by aggradation of a bar-like beach barrier is also possible. In this case infill of the basin to
contemporary sea level could have been sufficient to isolate a shallow basin that could have been infilled by inwash from the land as well as from the sea at periods of very high water level, e.g. storms or tidal surge. If a basin formed as proposed, it would have become a type of lagoon. The literature on tagoonal sedimentation is summarised in Reineck and Singh (1980) and Kraft and Chrzastowski (1985). Sedimentation in lagoons is strongly determined by sediment supply, both from the sea and from the land. Sedimentary structures and faunal composition is controlled by salinity of the water: abundant bioturbation and rich faunal assemblages occurring in lagoons with high salinity, particularly near sea inlets. By contrast, fauna is very restricted and bioturbation at a minimum in situations where a stream is entering the basin and water is predominantly fresh. On this basis it seems possible that the College Farm Silty Clay sediments could represent a lagoon-like waterbody rather than a mud-flat. The water was probably of very low salinity (although it could have been stratified), their being no evidence of a contemporary fauna (see also Palaeobotany). In this case the sediments would therefore have accumulated in the lagoon-proper subenvironment. If correct this suggests that a barrier or bar partially blocked the embayment and no substantial inlet from the sea occurred. However, given the narrow nature of the Blakenham basin, 3 km in width, and the tidal range expected in a narrow embayment, the development of barrier islands to enclose a lagoon would seem unlikely (Pethick, 1984), unless the basin represents an east-west aligned inlet opening into a lagoon to the east. However, in estuaries with a marked tidal influence, Reineck and Singh (1980) describe the intertidal flats changing from marine to freshwater, as in the Weser estuary. The freshwater flats are characterised by dense reeds typified, in the least saline areas, by Typha angustifolium and Glvceria maxima. Coarse and fine organic matter becomes incorporated into the sediments, particularly in the autumn. Freshwater tidal flats are sandy near the lowwater mark and dominated by fine sediments near the high-water. These tidal flats are also characterised by a low number of faunal species with limited populations, which may imply that the Creeting Sands as well as the College Farm Silty Clay are freshwater but the absence of salinity indicators (e.g. diatoms or ostracods) makes the interpretation difficult. It therefore appears that the Creeting Formation represents the infill of an embayment, and that the change from the Creeting Sands to the College Farm members does not represent a major environmental change, but the evolution of the local embayment environment resulting from sedimentation. There is no indication of sea level change throughout the sequence, but there is evidence of stream inwash during deposition of the silty clays. Presumably this stream was also present during aggradation of the sands and this may have lowered the salinity in the basin sufficiently to reduce the infaunal activity.
P.L. Gibbard et al.: Early Pleistocene Sediments at Great Blakenham, England
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Quaternao' Science Reviews: Volume 15
420 PALAEOBOTANY
Pollen Analysis Sediment samples from the basal 145 cm of the section (i.e. 79-224 cm) were analysed for fossil pollen and spore content and the results are presented below. Pollen preparation followed standard techniques used in the Subdepartment of Quaternary Research, University of Cambridge (West, 1968), including the use of sodium pyrophosphate (Bates et al., 1978). The results of the analyses are presented in the pollen diagram (Fig. 4); the pollen sum used is total land pollen and spores excluding aquatic taxa. Pollen identification conventions follow Andrew (1970) together with some types listed in Birks (1973). The pollen and spores in all samples were generally well-preserved and contained consistent numbers of indeterminable types. A single uniform Pinus-Picea-Alnus-Empetrum pollen assemblage is present. Throughout the pollen of Pinus is dominant with subsidiary quantities of Alnus and Picea. Other tree taxa present are represented by the pollen of Betula and Tsuga, with occasional records of Quercus, Tilia, Carpinus, Abies, Acer and Corylus. The pollen of Empetrum is well-represented in all levels, accompanied by other Ericales at 179 cm. Of the remaining pollen and spore taxa, the Gramineae, Chenopodiaceae and plants of calcareous meadow and or disturbed ground are sporadically present. Of the ferns the spores of Filicales are present throughout, while Osmunda and Equisetum spores also occur in significant numbers. Aquatic plants are poorly represented. These findings closely match those from a previous count from this unit by D.T. Holyoak (in Allen, 1984). Pre-Neogene pollen and spores are present in significant quantities with a slight increase in level, 144 cm, coinciding with the sand unit from which the plant macrofossils were obtained. This relationship probably indicates that inwash continued all the time. but increased in the coarser sediment unit influxes. Such an influx indicates local erosion of soils or exposed sediments in the catchment area. The interpretation of pollen spectra from this sequence may be problematic in view of the factors that influence pollen assemblages preserved in marine and coastal sediments. These include the intermixing of assemblages from wide areas, differential transport, sorting and concentration arising from the differing hydrodynamic characteristics of individual pollen morphology, the proximity of the depositional site to contemporary rivers, the coast or local water currents. These factors are superimposed on other biases such as differential pollen production, differing pollination mechanisms etc. (West, 1980). Despite these factors, the abundance, relative proportions and diversity of taxa, together with the occurrence of delicate plant macrofossils suggest that the site was close to the shoreline. This, reinforced by the sedimentary evidence for accumulation in a pool setting, makes it very probable that the assemblage reflects the contemporary vegetation growing near the depositional site.
The assemblage indicates a well-vegetated landscape with a cover of predominantly coniferous woodland in which Pinus and Picea were the main components. Of particular note is the consistent record of Tsuga pollen indicating local growth of this tree. Alnus and Betula were locally important, possibly in damper areas. The presence of the low pollen producer Abies suggest that it was also possibly growing in the vicinity. The high frequencies of bisaccate types such as Pinus and Picea may be partially a result of some over-representation by differential sorting. Nevertheless, the coherence of the assemblages suggests that this is slight. The low numbers of deciduous tree taxa such as Carpinus or Quercus imply local growth in the hinterland. Acidic heathland was present and supported abundant Empetrum as well as Ericales and Sphagnum. Dry calcareous grassland and disturbed ground also occurred nearby. Whilst the occurrence of high numbers of Chenopodiaceae strongly suggests saltmarsh was also present. The ferns Filicales, Osmunda and Equisetum imply that damp, marshy ground occurred locally, possibly as an understorey beneath the alder woodland.
Plant Macrofossils Known volumes of each sediment sample were disaggregated in water and then wet sieved. Macroscopic plant remains were picked from the resulting residues and identified with the aid of modern reference material. Nomenclature follows Stace (1991), apart from Azolla tegeliensis FlorschiJtz, Cenococcum geophilum Fr., Dulichium arundinaceum (L.) Britt. and Najas minor All. (Table l ). Preservation of macroscopic plant material varied through the section investigated. The most well-preserved and diverse assemblage was recovered from an interbedded fine sand and silt horizon (sample 139-151 cm). In total, there are 28 taxa represented by macrofossils in the samples analysed. The plant macrofossil assemblages are dominated by waterside, damp ground and aquatic taxa. The taxa represented suggest a freshwater environment with little or no saline influence. The plant macrofossils probably originated from a still or low energy waterbody bounded by a reedswamp consisting of Typha cf. latifolia. Other taxa represented that were also likely to have occurred in the reed-swamp habitat include Alisma one species or more spp., Eupatorium cannabinum and Juncus conglomeratus/effusus. The occurrence of a number of taxa with preferences for slow moving or still water conditions (e.g. Callitriche spp., Elatine hexandra, Elatine hydropiper, Lemna spp. and Stratiotes aloides) suggests a low energy environment. Presumably the extinct aquatic fern Azolla tegeliensis also favoured still or slow flowing water. An earlier investigation of the interbedded fine sand and silt bed (139-151 cm: termed sample 105-115 cm, in Field, 1992) yielded six additional taxa (Alnus spp., Betula spp., Dulichium arundinaceum, Rubus section Glandulosus, Apiaceae spp. and Urtica dioica). The
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163-170
Silty Clay
n , n u t l e t ; s, s e e d ; sc, s c e l o t i u m ;
139-151
Blakenham,
from
fr
a
Great
macrolossils
115-139
I. Plant
Azolla tegeliensis F l o r s c h . Callitriche s p . Elatine hexandra ( L a p i e r r e ) Elatine hydropiper L.
Aquatics
Alisma s p ( p ) . Eleocharis s p ( p ) . Eupatorium cannabinum L. Hypericum c f . tetrapterum F r i e s Juncus conglomeratus L.leffusus L . t y p e Ranunculus sceleratus L. Selaginella selaginoides ( L . ) P. Beaut,. Typha c f . latifolia L.
Waterside
(L.) Hill
ground
Sonchus asper
Disturbed
Depth
TABLE
224-230
tO
4~
:7"
Co
Co
co
Co
Co
..
r,,
e~
O"
422
Quaternary Science Reviews: Volume 15
Alnus and Betula remains confirm the pollen records of these taxa and probably originated from trees backing the reedswamp or growing in the reedswamp itself. Dulichium arundinaceum probably occupied the damp margins of the waterbody. Dulichium is described by Godwin (1975) as a circumpolar Tertiary genus which is now extinct in Britain and on the European continent. However, Dulichium arundinaceum has been recorded from a number of Pleistocene sites in Europe. Sample 139-151 cm was collected from the interbedded sand and silt bed that indicates a relatively high energy flow. This suggests that the assemblage recovered from this sample may have been washed into the embayment where deposition occurred by a stream or transported from other parts of the pond or lagoon under high energy conditions. Five of the sediment samples analysed yielded megaspores of Azolla tegeliensis. This discovery is the first record for this taxon in the British Isles and is important for the correlation of the British Early Pleistocene sediments with those in the Netherlands (Field, 1992). This is discussed further below.
Palaeoen vironment The evidence from both the pollen and spore and the plant macrofossil assemblages are consistent and complementary. Together they indicate a waterbody with a fringing marsh, dominated by reedswamp and accompanied by a variety of other marsh and damp ground plants. Water within the basin was predominantly quiet throughout much of the time represented as indicated by floating plants such as Callitriche etc. Nevertheless, occasional rapid influxes from episodic flooding washed material into the basin. This and the occurrence of abundant reworked palynomorphs suggest that a stream may have been entering the basin. An attempt to confirm the water quality by investigating fossil diatom floras by Dr A. Haggart failed because no diatoms were preserved throughout. Behind the reedswamp, birch and alder woodland were well-established, possibly with an understorey of ferns. Beyond, on drier chalk-rich ground grassland with a rich herb flora flourished. Woodland dominated by coniferous trees, including spruce and hemlock, grew in the district. Heathland was common on acid soils and deciduous trees, such as oak and hornbeam, may have been present at some more distant sites or in low numbers in the vicinity.
REGIONAL CORRELATION The pollen assemblages obtained are of late temperate substage character (sensu Turner and West, 1968), being dominated by coniferous tree taxa. The consistent occurrence of Tsuga pollen indicates that the sequence is of Early Pleistocene age (West, 1980; Gibbard et al., 1991; cf. Holyoak in Allen, 1984). The spectra show affinities to assemblages from the Antian or pollen zone Lp 5 of the Ludham sequence (West, 1961) and from the
marine sediments at Easton Bavents, Suffolk (Funnell and West, 1962). There is also some similarity to assemblages of Bramertonian age, including the so-called 'Chillesford pollen assemblage' (Chillesford Sand Member) of West and Norton (1974), Outney Common (West, 1988) and that from the Alnus-Quercus-Carpinus p.a.b, at the Bramerton stratotype (Funnell et al., 1979). However, the assemblages from these sites differ in their higher representation of deciduous tree taxa and low Empetrum. Equally, there are some similarities with assemblages recently obtained from the Chillesford Clay (overlying the Chillesford Sand) in Suffolk by Zalasiewicz et al. (1991). The latter were shown to contain Ericales (including Empetrum) and Gramineae-dominated spectra with subsidiary Pinus, Betula, Alnus and Picea pollen. This assemblage was correlated with the Baventian of the Easton Bavents cliff locality and Ludham Lp 4c (West, 1961; Funnell and West, 1977). The Great Blakenham assemblage again contrasts in its considerably higher frequency of tree taxa. It therefore appears that the assemblage from the College Farm Silty Clay Member (Great Blakenham) is intermediate between the deciduous tree-rich assemblages of the Antian-Bramertonian and the shrub and grassdominated assemblages of the Baventian. One possible age therefore is intermediate between the Bramertonian and the Baventian-type assemblages. Indeed if this is correct, it would appear that the Great Blakenham deposits represent a period between the accumulation of the basal sediments and the uppermost pollen assemblage at Bramerton (Funnell et al., 1979). However, an important criterion may be the occurrence of Tsuga pollen, already mentioned. The latter is particularly common in the Antian marine sediments, often exceeding 25% total pollen. By contrast Bramerton and equivalent sites have yielded little or no Tsuga pollen. At Great Blakenham a maximum of 4% was encountered and this may suggest a position intermediate between the typical Antian and Bramertonian. However, this seems to conflict with the remaining components that are clearly dominated by conifers rather than deciduous trees. It therefore remains difficult to place the assemblage precisely with respect to those known from other Norwich Crag Formation localities. The differences may not solely reflect regional vegetation history, but taphonomy. This could arise from the closer proximity to land in the embayment situation and the freshwater pool-like nature of the Great Blakenham site in contrast to the shallow marine and open mud-flat type environments of the other localities, i.e. further from the higher land areas that may have supported the forest. Indeed the inwash of Tsuga pollen grains from the neighbourhood may explain their occurrence derived from local growth, lnwash of both pollen and plant macrofossils is indicated throughout the sequence represented, as already mentioned. On balance therefore, the College Farm assemblage post-dates that from the Antian-Bramertonian, yet predates that from the Baventian-Pre-Pastonian (sensu Funnell, 1987; Zalasiewicz et al., 1991). It therefore
P.L. Gibbard et al.: Early Pleistocene Sediments at Great Blakenham, England represents a slightly later, late temperate phase of the Antian-Bramertonian temperate event. Regarding regional scale correlation, Gibbard et al. (1991) showed that the East Anglian Pliocene-Early Pleistocene sequence could be reliably equated with the Netherlands' succession. They concluded that the entire Ludhamian to Pastonian succession was equivalent to the complex Tiglian Stage (cf. Zagwijn, 1963); the AntianBramertonian event being equivalent to Tiglian Substage TC3 and the Baventian-Pre-Pastonian a being Tiglian TC4c. The discovery of the water fern Azolla tegeliensis in the College Farm Silty Clay considerably reinforces this correlation since the remains of this plant are found only in sediments of Early to Middle Tiglian age (i.e. the sediments must pre-date substage TC4b; de Jong, 1988) (Field, 1992), unless the remains were reworked. The excellent preservation, abundant numbers of fossils and their complete consistency with the local environment together indicate that these fossils are not reworked from older sediments but are in situ.
PALAEOMAGNETISM Thirty-eight samples were taken from the profile for palaeomagnetic analysis. Plastic cubes, of approximate internal volume 8 cm 3, were pushed into the cleaned sediment face by hand. Cubes were orientated and labelled prior to removal from the sediment. Magnetic remanences were measured using a GM400 cryogenic magnetometer. Twenty samples were subjected to stepwise alternating field (af) demagnetisation, then the remaining samples were treated at selected fields of I0 and 20 mT. The magnetic remanence vectors were extracted using a Linefind program (Kent et al., 1983). One sample was rejected because it had unstable demagnetisation behaviour. Natural remanent magnetisations (NRMs) were all of normal polarity. In all cases, af demagnetisation was effective in removing a significant fraction, or all, of a sample's remanence. No changes in polarity were revealed. Remanence directions extracted by Linefind were clustered, with a mean inclination of +71.3 ° and declination of 6.0 °. During a normal Chron, an axial geocentric dipole field at the latitude of East Anglia is directed towards the north pole and has an inclination of +70 °. However, the timing of remanence acquisition is subject to some uncertainty and will be discussed in detail elsewhere (Hallam, 1995). Van Montfrans (1971) suggested that the Early Pleistocene of northwest Europe might roughly correspond with the Matuyama reversed polarity Chron, but indicated several sites of Tiglian TC age that exhibited normal magnetic polarity. The College Farm Silty Clay may represent, at most, a few hundred years of deposition. Its magnetic remanence, therefore, is a 'snapshot' of the Earth's magnetic field and could have been acquired during a relatively short duration normal polarity subChron such as the Rrunion or 'X'. The longest period of normal polarity in the Early Pleistocene was the Olduvai
423
sub-Chron, but Zagwijn (1985) placed the Tiglian TC3 below the Olduvai (Zagwijn, 1985~ Gibbard et al., 1991). If the remanence of this sediment was acquired during the early part of the Olduvai, it would imply that Tiglian substage TC3 occurred later than previously suggested. The normal magnetic polarity of this sediment, therefore, is not inconsistent with a Tiglian TC3 age, but precise dating is problematic. CONCLUSION Both from the sedimentary evidence and the palaeobotany it appears that the College Farm Silty Clay represents accumulation in a situation marginal to and succeeding a sand flat environment in which the underlying Creeting Sands accumulated, possibly a shallow lagoon-like pool receiving freshets. This prograding sequence probably formed under conditions of stable sea level. The sediment was deposited predominantly in a freshwater body, possibly lacking a substantial outlet to the sea. It was fringed by marsh and reedswamp with alder and birch woodland locally. Beyond on dry ground to the south and west, late temperate coniferous forest with local areas of deciduous trees occurred. Calcareous grassland was also present in the vicinity. On the basis of the palaeobotanical assemblage, the deposits date from the late temperate phase of the AntianBramertonian temperate event. This is shown to be equivalent to Substage TC 3 of the complex Tiglian Stage in the Netherlands by the discovery of the water fern Azolla tegeliensis in the sequence. This correlation confirms previous suggestions by Gibbard et al. (1991). Equally, the apparently normal palaeomagnetic polarity of the sequence does not negate this correlation but precise dating on the basis of the magnetic remanance is problematic. These deposits are the oldest identified freshwater fossiliferous sediments in the British Pleistocene. ACKNOWLEDGEMENTS The sediments were originally sampled during the 1990 INQUA-SEQS meeting excursion with the assistance of Professor R.G. West, Professor J.J. Donner and Dr Th. van Kolfschoten. Assistance during subsequent work was given by Dr B. Maher and Mr S. Boreham. Access to the site was freely given by Blue Circle PLC. Professor West also suggested valuable improvements to the manuscript. We thank all these people.
REFERENCES Allen, P. (1984). Field Guide to the Gipping and Waveney Valleys, Suffolk, May 1992. Quaternary Research Association, Cambridge, 115 pp. Andrew, R. (1970). The Cambridge pollen reference collection. In: Walker, D. and West, R.G. Studies in the Vegetational Histor}' of the British Isles. Cambridge University Press, pp. 225-231.
424
Quaternary Science Reviews: Volume 15
Bates, C.D., Coxon, P. and Gibbard, P.L. (1978). A new method for the preparation of clay-rich sediment samples for palynological investigation. New Phytologist, 81, 459--463. Birks, H.J.B. (1973). Past and Present Vegetation of the Isle of Skye--a Palaeoecological Study. Cambridge University Press, Cambridge. Bristow, C.R. (1983). The stratigraphy and structure of midSuffolk, England. Proceedings of the Geologists' Association, 94, 1-12. Coxon, P. and Coxon, C.E. (1996). A pre-Pliocene or Pliocene landsurface in County Galway, Ireland. In: Widdowson, M. (ed.), Tertiary Palaeosurfaces: process and recognition. Geological Society Special Publication (in press). Dixon, R.G. (1972). A review of the Chillesford Beds. Bulletin of the Ipswich Geological Group, 11, 2-9. Dixon, R.G. (1978). Deposits marginal to the Red Crag basin. Bulletin of the Geological Society of Norfolk, 30, 92-104. Evans, G. (1965). Intertidal flat sediments and their environment of deposition in the Wash. Quarterly Journal of the Geological Society of London, 121, 209-241. Field, M.H. (1992). Azolla tegeliensis Florschiitz from the Early Pleistocene of the British Isles. Geological Magazine, 129, 363-365. Funnell, B.M. (1961). The Palaeogene and Early Pleistocene of Norfolk. Transactions of the Norwich and Norfolk Naturalists Society, 19, 340-364. Funnell, B.M. (1987). Late Pliocene and Early Pleistocene stages of East Anglia and the adjacent North Sea. Quaternary Newsletter, 52, 1-11. Funnell, B.M. and West, R.G. (1962). The early Pleistocene of Easton Bavents, Suffolk. Quarterly Journal of the Geological Society of London, 117, 125-141. Funnell, B.M. and West, R.G. (1977). Preglacial Pleistocene deposits of East Anglia. In: Shotton, F.W. (ed.), British Quaternary Studies: Recent Advances, pp. 247-265. Oxford University Press. Funnell, B.M., Norton, P.E.P., and West, R.G. (1979). The Crag at Bramerton, near Norwich, Norfolk. Philosophical Transactions of the Royal Socie~' of London B, 287, 489-534. Gibbard, P.L., West, R.G., Zagwijn, W.H., Balson, P.S., Burger, A.W., Funnell, B.M., Jeffery, D.H., de Jong, J., van Kolfschoten, T., Lister, A.M., Meijer, T., Norton, P.E.P., Preece, R.C., Rose, J., Stuart, A.J., Whiteman, C.A. and Zalasiewicz, J.A. (1991). Early and early Middle Pleistocene correlations in the southern North Sea basin. Quaternar3' Science Reviews, 10, 23-52. Godwin, H. (1975). Histor3" of the British Flora. Cambridge University Press, Cambridge. Hallam, D.F. (1995). Palaeomagnetic Investigation of Early and Middle Pleistocene Sediments in East Anglia. PhD. thesis. University of East Anglia. Jong, de J. (1988). Climatic variability during the past three million years, as indicated by vegetational evolution in northwest Europe and with emphasis on data from the Netherlands. Philosophical Transactions of the Royal Socie~.' of London, B318, 603--617. Kent, J.T., Briden, J.C., and Mardia, K.V. (1983). Linear and planar structure in ordered multivariate data as applied to progressive demagnetization of palaeomagnetic remanence. Geophysical Journal of the Royal Astronomical Societ3", 75, 593-621. Kraft, J.C. and Chrzastowski, M.J. (1985). Coastal stratigraphic sequences, bT: Davis, R.J. Jr (ed.). Coastal Sedimentara'
Environments, pp. 625-663. Springer-Verlag, Berlin, New York, Heidelberg, Tokyo. Markham, R.A.D. (1972). Notes on some Suffolk Crag localities. Transactions of the Suffolk Naturalists' Socie~.', 15, 520-524. Mathers, S.J. and Zalasiewicz, J.A. (1988). The Red and Norwich Crag formations of southern East Anglia. Proceedings of the Geologists' Association, 99, 261-278. Northcott, G.J. (1978). The concealed Chalk surface of midSuffolk. Transactions of the Suffolk Naturalists' Societ3', 17, 346-356. Passega, R. (1957). Texture as a characteristic of clastic deposition. Geological Society of America Bulletin, 41, 1952-1984. Pethick, J. (1984). An Introduction to Coastal Geomorphology. Edward Arnold, London, 260 pp. Reading, H.G. (ed.) (1978). Sedimentao" Environments and Facies. Blackwell, Oxford. Reineck, H.E. (1972). Tidal fiats. In: Rigby, J.K. and Hamblin, W.K. (eds), Recognition of Ancient Sedimentao' Environments, Special Publication No. 16. Society of Economic Paleontologists and Mineralogists. Reineck, H.E. and Singh, I.B. (1980). Depositional Sedimentary. Environments. Springer-Verlag, Berlin, 551 pp. Reineck, H.E. and Wunderlich, F. (1969). Die Enstehung von Schichten und Schichtbanken im Watt. Senckenbergiana Maritimus, 1, 85-106. Stace, C. (1991). New Flora of the British Isles. Cambridge University Press, Cambridge. Turner, C. and West, R.G. (1968). The subdivision of interglacial periods. Eiszeitalter und Gegenwart, 19, 93-100. van Montfrans, H.M. (1971). Palaeomagnetic dating in the North Sea Basin. Earth and Planetary Science Letters, 11, 226-235. West, R.G. (1961). Vegetational history of the Early Pleistocene of the Royal Society Borehole at Ludham, Norfolk. Proceedings of the Royal Society of London, B155, 437--453. West, R.G. (1968). Pleistocene Geology and Biology. Longmans Green, London. West, R.G. (1988). A pollen diagram from Norwich Crag at Oumey Common, Bungay, Suffolk. New Phytologist, ll0, 603-606. West, R.G. (1980). The Pre-Glacial Pleistocene of the Norfolk and Suffolk Coasts. Cambridge University Press, Cambridge. West, R.G. and Norton, P.E.P. (1974). The Icenian Crag of southeast Suffolk. Philosophical Transactions of the Royal Society of London B, 269, 1-28. Whiteman, C.A. and Rose, J. (1992). Thames river sediments of the British Early and Middle Pleistocene. Quarterna O' Science Reviews, 11, 363-375. Zagwijn, W.H. (1963). Pollen-analytic investigations in the Tiglian of the Netherlands. Mededelingen van de Geologische Stichting. Nieuwe serie No. 16. Zagwijn, W.H. (1985). An outline of the Quaternary stratigraphy of the Netherlands. Geologie en Mijnbouw, 64, 17-24. Zalasiewicz, J.A. and Mathers, S.J. (1985). Lithostratigraphy of the Red and Norwich Crags of the Aldeburgh-Orford area, south-east Suffolk. Geological Magazine, 122, 287-296. Zalasiewicz, J.A., Mathers, S.J., Gibbard, P.L., Peglar, Funnell, S.M., Catt, B.M., Harland, J.A., Long, R. and Austin, Austin (1991). Age and relationships of the Chillesford Clay. (early Pleistocene: Suffolk, England). Philosophical Transactions of the Royal Society of London B, 333, 81-100.
QuaternaD,ScienceReviews,Vol, 15, pp. 413--424, 1996.
i Pergamon PIh S0277-3791(96)00013-3
Copyright© 1996ElsevierScienceLtd. Printed in Great Britain. All rights reserved, 0277-3791/96 $32.00
EARLY PLEISTOCENE SEDIMENTS AT GREAT BLAKENHAM, SUFFOLK, ENGLAND P . L . G I B B A R D , * P. A L L E N , ? M . H . F I E L D ~ a n d D.F. H A L L A M §
*Godwin Institute for Quaternary Research, Department of Geography, University of Cambridge, Downing Place, Cambridge CB2 3EN, U.K. ?Department of Geography, London Guildhall University, Old Castle Street, London El 7NT, U.K. "~.Centre for Quaternary Science, Coventry University, Priory Street, Coventry CV1 5FB, U.K. §School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, U.K. Abstract - - Detailed investigation of a fine sediment sequence, the College Farm Silty Clay Member, that overlies the Creeting Sands (Early Pleistocene) in Suffolk, is presented. The sedimentary sequence is thought to represent a freshwater pool accumulation in a small coastal embayment. Palaeobotanical investigation of the sediment indicates that it accumulated during the late temperate substage of a temperate (interglacial) event. The occurrence of Tsuga pollen, associated with abundant remains of the water fern Azolla tegeliensis indicate that the deposits are of Early Pleistocene age and are correlated with a later part of the Antian-Bramertonian Stage. Correlation with Tiglian TC3 substage in The Netherlands' sequence is most likely. The sediments' normal palaeomagnetic polarity reinforces the biostratigraphical correlation. Copyright © 1996 Elsevier Science Ltd
QSR
diminishes from ca. 15 m to less than 5 m. The western limit of the basin is not clearly defined but borehole records do not show the beds occurring further than 2 km to the west. The wider significance of the Creeting sequence has been discussed by Dixon (1978) and Allen (1984). Zalasiewicz and Mathers (1985) have correlated the Creeting Formation lithostratigraphically with the Chillesford Sands and Clay members of the Norwich Crag Formation that occur in the main Crag basin to the northeast of the area. The Chillesford Sand has been equated with the Early Pleistocene Bramertonian Stage (West and Norton, 1974; Funnell et ak, 1979), and the Chillesford Clays with the subsequent Baventian/PrePastonian a Stage (Zalasiewicz et al., 1991). This correlation is supported by the interpreted sedimentary environment of the Chillesford Sand as a tidal flat deposit (Funnell, 1961; Dixon, 1972; Zalasiewicz and Mathers, 1985), whilst the Chillesford Clay is thought to represent a high intertidal accumulation (Evans, 1965; Zalasiewicz et al., 1991). Biostratigraphical correlation with The Netherlands' succession indicates that these units represent the Tiglian substages TC3 and TC4c respectively (Gibbard et al., 1991). With the exception of one undiagnostic pollen spectrum obtained by D.T. Holyoak (in Allen, 1984), no
INTRODUCTION The substantial quarry complex at Great Blakenham (Fig. 1) lies on the western side of the Gipping Valley in Suffolk, England. Sections here expose an important sequence of Early and Middle Pleistocene sediments over 15 m in thickness. Although it was previously known (Markham, 1972), the major work on this site has been undertaken by Allen (1984) who investigated the sedimentary sequence, identified and interpreted the major aggradational units and proposed a series of formal terms for the units he differentiated. Of these the largest unit was the Early Pleistocene Creeting Formation comprising two members, the Creeting Sands and the College Farm Silty Clay. It is overlain unconformably by sands and gravels of the Kesgrave Group (Fig. 2) (Whiteman and Rose, 1992). According to Allen (1984), the Creeting Formation occurs in basins, kilometres or tens of kilometres in diameter, developed on the Chalk bedrock, the largest basins being around Stradbroke and Eye (Fig. 3) (see also Notcutt, 1978; Bristow, 1983). The Great Blakenham basin is bounded by a rise in the Chalk surface to the north, to over 40 m O.D. between Darmsden and Barking, and similarly to the south at Little Blakenham. Even within the quarry the Chalk surface can be seen rising to the south and the thickness of the Creeting sediments 413
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FIG. 2. Great B l a k e n h a m , section l b (after Allen, 1984).
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Quaternar 3' Science Reviews." Volume 15
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FIG. 3. Distribution of the Creeting Formation, and the location of places referred to in the text.
P.L. Gibbard et al.: Early Pleistocene Sediments at Great Blakenham, England evidence of the age of the Creeting sequence has been hitherto available. This paper reports the palaeobotanical assemblages obtained from the College Farm Silty Clay Member exposures in autumn 1990. Unfortunately, the sediments contained no diatoms or calcareous fossils (see below). However, the results obtained provide an important contribution to the understanding of the depositional environment of the Creeting Formation, to the evolution of vegetation in the Bramertonian Stage and provide the earliest evidence of freshwater sedimentation in the British Pleistocene.
SITE STRATIGRAPHY The Creeting Formation at Great Blakenham comprises up to 11.25 m of Creeting Sand Member overlain by 3.75 m of College Farm Silty Clay Member (Fig. 2). The sediments are contained within a Chalk bedrock depression (Fig. 1) such that the floor in the vicinity of the section is at 36-38 m O.D. but rises to 47.2 m on the south side of the quarry and 3 km to the northwest, at Willisham Hall, reaches 63.4 m. Approximately 6 km to the west, the Chalk surface rises to over 40 m O.D., as at Manor Farm, Elmsett. The Formation thins or is absent over the bedrock highs and locally is not found beyond 2 km west of the quarry. Boreholes indicate that the Great Blakenham basin is separated from the Stradbroke trough to the north and the main coastal Crag basin to the south and east. Locally the Formation is not recorded in boreholes west of Great Blakenham or more than 5 km west of the Gipping Valley in the other Crag basins. The basin form of the Chalk surface and the local westerly limit of the Crag (Fig. 3) are argued to result from Quaternary tectonic activity (Allen, 1984), probably part of regional uplift during the Early Pleistocene (Mathers and Zalasiewicz, 1988). The Creeting Sand (Fig. 2) has a basal lag of nodular flint cobbles and phosphatic nodules which has been attributed to the Red Crag Formation (Allen, 1984). This is immediately overlain by well-sorted, very fine to medium, micaceous sand with occasional matrix-supported, poorly sorted gravelly sand beds (less than 5%) and a small amount of silty clay (up to 15%). The primary sedimentary structures range from ripples, through planar and trough cross-sets (up to 1.3 m thick), to horizontallybedded units (up to 1.6 m thick). Overall, laterally extensive, horizontal beds dominate. Some beds appear structureless. Silty clay occurs in variable amounts, but increasing in frequency upwards, as curled flakes in the structureless and planar cross-sets, as thin seams in simple and wavy types of flaser bedding or as thicker seams within interlayered bedding. Although the planar crosssets appear to give a 'herring-bone' pattern, palaeocurrent measurements (12) show a wide range of readings with a slight concentration between 300-340 ° . Very infrequently the bedding is interrupted by small, cylindrical, U-shaped burrow structures up to 4 cm in vertical extent. The horizontal nature of the bedding, with primary structures typical of the lower and transitional flow
417
regimes, with flaser and interlayered bedding becoming more important upwards, indicate an aggrading environment such as a sand flat changing to a mixed-fiat, with curled flakes of silty clay resulting from desiccation when the surface of the flat was subaerially exposed. The random pattern of the palaeocurrent readings may not be meaningful because of the small number of measurements or it may reflect the fact that the current and wave direction on sand-flats can be highly variable as a consequence of local morphological slope (Reineck and Singh, 1'980) and wind direction (Reineck and Wunderlich, 1969). The CM panicle size characteristics (Passega, 1957) and well-sorted nature of the sand support deposition on a sand-flat. The upper 5-6 m of the sands are significantly coarser than those beneath indicating higher energy levels, possibly associated with shallower water conditions and greater wave activity. The occasional gravelly sands may reflect storm events or powerful flows from nearby rivers. The paucity of biogenic structures, suggesting a lack of faunal activity, may reflect rapid aggradation of the sands. Resting conformably on the Creeting Sands is the College Farm Silty Clay Member, up to 3.75 m thick. The silty clay occurs as a sequence of grey (5Y3/1 to 10YR5/ 1) horizontal laminae and beds up to 1.2 m thick, interbedded with well-sorted sand or as interlayered sand and silty clay laminae forming cosets up to 1 m thick. In places, particularly the lower part, laminated units occur such as those observed during this study, The silty clay is poorly sorted while interbedded sands are well-sorted. Small-scale loading and water escape structures occur relatively frequently. The individual silty clay beds are generally massive, though the upper ones are brecciated in places and oxidised to strong brown (7.5YR5/8). This member is only locally present for less than 100 m in the Great Blakenham quarry. East and west of the sampled section it thins, becoming replaced by flaser and lenticular laminae. Individual beds pass laterally into brecciated and partially contorted sediments, particularly to the north. The whole unit is, in places, glaciotectonically disturbed and overfolded (Fig. 2), The microstratigraphy of the sequence sampled for palaeobotanical investigation is given below: 0-18 cm 18-31 cm
obscured above massive grey fine sand grades into light grey silt massive medium grey silty clay
31-49 cm
finely laminated red fine sand and fine grey sill
49-55 cm
light grey silt with fine sand laminae (<1 mm thick) orange grey fine sand interbedded with light grey silt bands of irregular thickness (<1 cm). light grey silty clay with fine sand laminae (<1 cm thick). massive dark grey silty clay grades upwards into overlying sediment ripple-bedded sand with divided fragmented plant detritus in thin (1 mm thick) drapes dark grey sand with thin grey clay laminae (5 mm thick)
55-68 cm 68-82 cm 82-129 cm 129-139 cm 139-151 cm
418
QuaternaO' Science Reviews: Volume 15
151-158 cm massive grey clay 158-164 cm laminated sand and clay (<5 mm thick), base of sand units shows minor erosion 164-196 cm massive dark grey silty clay with occasional thin laminae of grey silt, and sand band at 93.5-95 cm 196--224 cm light grey silty clay with fine silt laminae (<2 mm thick) >224 cm orange red medium sand with silty clay laminae form interbedded 10 cm thick units The change to clay deposition implies an abrupt reduction in current and wave activity and the appearance of stillwater conditions. Throughout the sequence the rhythmic pattern suggests background fine sediment deposition punctuated by periodic inwash of coarser sediment presumably suggesting current flow or inwash from the nearby land. The presence of plant material, some of which was inwashed or derived from a shallow marginal subenvironment suggests that a stream may have been entering the embayment nearby. Periodically higher energy influxes from the stream or from tidal floods producing bottom currents may account for the coarser sediment strata in the rhythmic structure of the fine sediments. The interlayered bedding and massive silty clay beds are associated with high concentrations of suspended sediment. Such conditions are met in sub-tidal water (Reading, 1978), near the high water mark in estuaries or inshore of an extensive tidal fiat. The presence of organic detritus, the sedimentary structures and the flora (see Palaeobotany) observed suggests the latter is more likely. Two possible explanations seem likely. The fine nature of the sediment, with occasional sand partings, and its horizontal bedding suggest deposition in a shallow basin or an upper mud-flat (cf. Evans, 1965). While the weathering and brecciation of the upper part of the sequence could indicate occasional prolongued subaerial exposure in either case, such structures might reflect lateral transition to mixed flats. If the deposits indicate a change in the depositional environment from sand-flat to a mud-fiat flanked by mixed-flat, they could potentially reflect a local change in the configuration of the flats (Reineck, 1972). The limited lateral extent of the deposits would then represent infill of a channel. Mudflats usually occur in areas sheltered from winddriven waves (Pethick, 1984). The presence of freshwater material in the silty clay (Palaeobotany, below) would indicate that the contemporary shoreline was nearby, in keeping with the higher Chalk bedrock on the south side of the quarry. Nevertheless, the partial or complete closure of the embayment by aggradation of a bar-like beach barrier is also possible. In this case infill of the basin to
contemporary sea level could have been sufficient to isolate a shallow basin that could have been infilled by inwash from the land as well as from the sea at periods of very high water level, e.g. storms or tidal surge. If a basin formed as proposed, it would have become a type of lagoon. The literature on tagoonal sedimentation is summarised in Reineck and Singh (1980) and Kraft and Chrzastowski (1985). Sedimentation in lagoons is strongly determined by sediment supply, both from the sea and from the land. Sedimentary structures and faunal composition is controlled by salinity of the water: abundant bioturbation and rich faunal assemblages occurring in lagoons with high salinity, particularly near sea inlets. By contrast, fauna is very restricted and bioturbation at a minimum in situations where a stream is entering the basin and water is predominantly fresh. On this basis it seems possible that the College Farm Silty Clay sediments could represent a lagoon-like waterbody rather than a mud-flat. The water was probably of very low salinity (although it could have been stratified), their being no evidence of a contemporary fauna (see also Palaeobotany). In this case the sediments would therefore have accumulated in the lagoon-proper subenvironment. If correct this suggests that a barrier or bar partially blocked the embayment and no substantial inlet from the sea occurred. However, given the narrow nature of the Blakenham basin, 3 km in width, and the tidal range expected in a narrow embayment, the development of barrier islands to enclose a lagoon would seem unlikely (Pethick, 1984), unless the basin represents an east-west aligned inlet opening into a lagoon to the east. However, in estuaries with a marked tidal influence, Reineck and Singh (1980) describe the intertidal flats changing from marine to freshwater, as in the Weser estuary. The freshwater flats are characterised by dense reeds typified, in the least saline areas, by Typha angustifolium and Glvceria maxima. Coarse and fine organic matter becomes incorporated into the sediments, particularly in the autumn. Freshwater tidal flats are sandy near the lowwater mark and dominated by fine sediments near the high-water. These tidal flats are also characterised by a low number of faunal species with limited populations, which may imply that the Creeting Sands as well as the College Farm Silty Clay are freshwater but the absence of salinity indicators (e.g. diatoms or ostracods) makes the interpretation difficult. It therefore appears that the Creeting Formation represents the infill of an embayment, and that the change from the Creeting Sands to the College Farm members does not represent a major environmental change, but the evolution of the local embayment environment resulting from sedimentation. There is no indication of sea level change throughout the sequence, but there is evidence of stream inwash during deposition of the silty clays. Presumably this stream was also present during aggradation of the sands and this may have lowered the salinity in the basin sufficiently to reduce the infaunal activity.
P.L. Gibbard et al.: Early Pleistocene Sediments at Great Blakenham, England
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Quaternao' Science Reviews: Volume 15
420 PALAEOBOTANY
Pollen Analysis Sediment samples from the basal 145 cm of the section (i.e. 79-224 cm) were analysed for fossil pollen and spore content and the results are presented below. Pollen preparation followed standard techniques used in the Subdepartment of Quaternary Research, University of Cambridge (West, 1968), including the use of sodium pyrophosphate (Bates et al., 1978). The results of the analyses are presented in the pollen diagram (Fig. 4); the pollen sum used is total land pollen and spores excluding aquatic taxa. Pollen identification conventions follow Andrew (1970) together with some types listed in Birks (1973). The pollen and spores in all samples were generally well-preserved and contained consistent numbers of indeterminable types. A single uniform Pinus-Picea-Alnus-Empetrum pollen assemblage is present. Throughout the pollen of Pinus is dominant with subsidiary quantities of Alnus and Picea. Other tree taxa present are represented by the pollen of Betula and Tsuga, with occasional records of Quercus, Tilia, Carpinus, Abies, Acer and Corylus. The pollen of Empetrum is well-represented in all levels, accompanied by other Ericales at 179 cm. Of the remaining pollen and spore taxa, the Gramineae, Chenopodiaceae and plants of calcareous meadow and or disturbed ground are sporadically present. Of the ferns the spores of Filicales are present throughout, while Osmunda and Equisetum spores also occur in significant numbers. Aquatic plants are poorly represented. These findings closely match those from a previous count from this unit by D.T. Holyoak (in Allen, 1984). Pre-Neogene pollen and spores are present in significant quantities with a slight increase in level, 144 cm, coinciding with the sand unit from which the plant macrofossils were obtained. This relationship probably indicates that inwash continued all the time. but increased in the coarser sediment unit influxes. Such an influx indicates local erosion of soils or exposed sediments in the catchment area. The interpretation of pollen spectra from this sequence may be problematic in view of the factors that influence pollen assemblages preserved in marine and coastal sediments. These include the intermixing of assemblages from wide areas, differential transport, sorting and concentration arising from the differing hydrodynamic characteristics of individual pollen morphology, the proximity of the depositional site to contemporary rivers, the coast or local water currents. These factors are superimposed on other biases such as differential pollen production, differing pollination mechanisms etc. (West, 1980). Despite these factors, the abundance, relative proportions and diversity of taxa, together with the occurrence of delicate plant macrofossils suggest that the site was close to the shoreline. This, reinforced by the sedimentary evidence for accumulation in a pool setting, makes it very probable that the assemblage reflects the contemporary vegetation growing near the depositional site.
The assemblage indicates a well-vegetated landscape with a cover of predominantly coniferous woodland in which Pinus and Picea were the main components. Of particular note is the consistent record of Tsuga pollen indicating local growth of this tree. Alnus and Betula were locally important, possibly in damper areas. The presence of the low pollen producer Abies suggest that it was also possibly growing in the vicinity. The high frequencies of bisaccate types such as Pinus and Picea may be partially a result of some over-representation by differential sorting. Nevertheless, the coherence of the assemblages suggests that this is slight. The low numbers of deciduous tree taxa such as Carpinus or Quercus imply local growth in the hinterland. Acidic heathland was present and supported abundant Empetrum as well as Ericales and Sphagnum. Dry calcareous grassland and disturbed ground also occurred nearby. Whilst the occurrence of high numbers of Chenopodiaceae strongly suggests saltmarsh was also present. The ferns Filicales, Osmunda and Equisetum imply that damp, marshy ground occurred locally, possibly as an understorey beneath the alder woodland.
Plant Macrofossils Known volumes of each sediment sample were disaggregated in water and then wet sieved. Macroscopic plant remains were picked from the resulting residues and identified with the aid of modern reference material. Nomenclature follows Stace (1991), apart from Azolla tegeliensis FlorschiJtz, Cenococcum geophilum Fr., Dulichium arundinaceum (L.) Britt. and Najas minor All. (Table l ). Preservation of macroscopic plant material varied through the section investigated. The most well-preserved and diverse assemblage was recovered from an interbedded fine sand and silt horizon (sample 139-151 cm). In total, there are 28 taxa represented by macrofossils in the samples analysed. The plant macrofossil assemblages are dominated by waterside, damp ground and aquatic taxa. The taxa represented suggest a freshwater environment with little or no saline influence. The plant macrofossils probably originated from a still or low energy waterbody bounded by a reedswamp consisting of Typha cf. latifolia. Other taxa represented that were also likely to have occurred in the reed-swamp habitat include Alisma one species or more spp., Eupatorium cannabinum and Juncus conglomeratus/effusus. The occurrence of a number of taxa with preferences for slow moving or still water conditions (e.g. Callitriche spp., Elatine hexandra, Elatine hydropiper, Lemna spp. and Stratiotes aloides) suggests a low energy environment. Presumably the extinct aquatic fern Azolla tegeliensis also favoured still or slow flowing water. An earlier investigation of the interbedded fine sand and silt bed (139-151 cm: termed sample 105-115 cm, in Field, 1992) yielded six additional taxa (Alnus spp., Betula spp., Dulichium arundinaceum, Rubus section Glandulosus, Apiaceae spp. and Urtica dioica). The
(cm)
and
damp
ground
sp(p).
.
s s cy
Poaceae
Abbrevialions:
Pre-Pleistocene
sp.
a, a c h e n e : af, a t h e n e
structures
Ranunculus s u b g e n u s Urtica dioica L. Viola s p ( p ) . Viola c f . subgenus Melanium
fragments; bi.n, biconvex
Ranunculus
-
s
.
-
13
n u t l e t ; c y , c a r y o p s i s ; fr, f r u i t ; m g , m e g a s p o r e ;
mg
1
-
s
-
-
a
s
l
2
-
3
-
10
1
-
12
I
2
2
1
I
1
1
I
34
4
11
a
-
-
s
sp(p).
-
-
-
sp
bi.n
1
sc
Fr.
--
S
a
--
fr
Carex s p ( p ) . Cenococcum geophilum Chenopodium s p . Hypericum s p . Juncus s p ( p ) . c f . Luzula s p .
Unclassified
Najas minor A l l . Ranunculus s u b g e n u s Batrachium Stratiotes aloides L.
sp.
-
mg
S
-
fr
Lemna
-
mg
--
5
-
a
S
5
-
-
]
--
S
s
s
DC
6
-
af
3
-
I1
1
n
-
.
.
Farm
-
-
-
-
1
1
-
-
1
1
-
-
--
--
--
1
-
2
2
-
-
-
--
-
-
4
-
151-163
Suffolk
the College
7
-
-
-
-
1
-
1
2
-
-
-
--
--
--
-
-
3
1
-
I
3
--
-
-
-
-
9
-
-
-
1
1
3
-
-
4
-
-
-
--
--
--
-
-
15
7
-
-
3
--
1
I
2
-
170-175
-
m
m
175-200
-
220-224
sp, s p i n e . S p ( p ) d e n o t e s o n e o r p o s s i b l y m o r e s p e c i e s .
163-170
Silty Clay
n , n u t l e t ; s, s e e d ; sc, s c e l o t i u m ;
139-151
Blakenham,
from
fr
a
Great
macrolossils
115-139
I. Plant
Azolla tegeliensis F l o r s c h . Callitriche s p . Elatine hexandra ( L a p i e r r e ) Elatine hydropiper L.
Aquatics
Alisma s p ( p ) . Eleocharis s p ( p ) . Eupatorium cannabinum L. Hypericum c f . tetrapterum F r i e s Juncus conglomeratus L.leffusus L . t y p e Ranunculus sceleratus L. Selaginella selaginoides ( L . ) P. Beaut,. Typha c f . latifolia L.
Waterside
(L.) Hill
ground
Sonchus asper
Disturbed
Depth
TABLE
224-230
tO
4~
:7"
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Co
co
Co
Co
..
r,,
e~
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422
Quaternary Science Reviews: Volume 15
Alnus and Betula remains confirm the pollen records of these taxa and probably originated from trees backing the reedswamp or growing in the reedswamp itself. Dulichium arundinaceum probably occupied the damp margins of the waterbody. Dulichium is described by Godwin (1975) as a circumpolar Tertiary genus which is now extinct in Britain and on the European continent. However, Dulichium arundinaceum has been recorded from a number of Pleistocene sites in Europe. Sample 139-151 cm was collected from the interbedded sand and silt bed that indicates a relatively high energy flow. This suggests that the assemblage recovered from this sample may have been washed into the embayment where deposition occurred by a stream or transported from other parts of the pond or lagoon under high energy conditions. Five of the sediment samples analysed yielded megaspores of Azolla tegeliensis. This discovery is the first record for this taxon in the British Isles and is important for the correlation of the British Early Pleistocene sediments with those in the Netherlands (Field, 1992). This is discussed further below.
Palaeoen vironment The evidence from both the pollen and spore and the plant macrofossil assemblages are consistent and complementary. Together they indicate a waterbody with a fringing marsh, dominated by reedswamp and accompanied by a variety of other marsh and damp ground plants. Water within the basin was predominantly quiet throughout much of the time represented as indicated by floating plants such as Callitriche etc. Nevertheless, occasional rapid influxes from episodic flooding washed material into the basin. This and the occurrence of abundant reworked palynomorphs suggest that a stream may have been entering the basin. An attempt to confirm the water quality by investigating fossil diatom floras by Dr A. Haggart failed because no diatoms were preserved throughout. Behind the reedswamp, birch and alder woodland were well-established, possibly with an understorey of ferns. Beyond, on drier chalk-rich ground grassland with a rich herb flora flourished. Woodland dominated by coniferous trees, including spruce and hemlock, grew in the district. Heathland was common on acid soils and deciduous trees, such as oak and hornbeam, may have been present at some more distant sites or in low numbers in the vicinity.
REGIONAL CORRELATION The pollen assemblages obtained are of late temperate substage character (sensu Turner and West, 1968), being dominated by coniferous tree taxa. The consistent occurrence of Tsuga pollen indicates that the sequence is of Early Pleistocene age (West, 1980; Gibbard et al., 1991; cf. Holyoak in Allen, 1984). The spectra show affinities to assemblages from the Antian or pollen zone Lp 5 of the Ludham sequence (West, 1961) and from the
marine sediments at Easton Bavents, Suffolk (Funnell and West, 1962). There is also some similarity to assemblages of Bramertonian age, including the so-called 'Chillesford pollen assemblage' (Chillesford Sand Member) of West and Norton (1974), Outney Common (West, 1988) and that from the Alnus-Quercus-Carpinus p.a.b, at the Bramerton stratotype (Funnell et al., 1979). However, the assemblages from these sites differ in their higher representation of deciduous tree taxa and low Empetrum. Equally, there are some similarities with assemblages recently obtained from the Chillesford Clay (overlying the Chillesford Sand) in Suffolk by Zalasiewicz et al. (1991). The latter were shown to contain Ericales (including Empetrum) and Gramineae-dominated spectra with subsidiary Pinus, Betula, Alnus and Picea pollen. This assemblage was correlated with the Baventian of the Easton Bavents cliff locality and Ludham Lp 4c (West, 1961; Funnell and West, 1977). The Great Blakenham assemblage again contrasts in its considerably higher frequency of tree taxa. It therefore appears that the assemblage from the College Farm Silty Clay Member (Great Blakenham) is intermediate between the deciduous tree-rich assemblages of the Antian-Bramertonian and the shrub and grassdominated assemblages of the Baventian. One possible age therefore is intermediate between the Bramertonian and the Baventian-type assemblages. Indeed if this is correct, it would appear that the Great Blakenham deposits represent a period between the accumulation of the basal sediments and the uppermost pollen assemblage at Bramerton (Funnell et al., 1979). However, an important criterion may be the occurrence of Tsuga pollen, already mentioned. The latter is particularly common in the Antian marine sediments, often exceeding 25% total pollen. By contrast Bramerton and equivalent sites have yielded little or no Tsuga pollen. At Great Blakenham a maximum of 4% was encountered and this may suggest a position intermediate between the typical Antian and Bramertonian. However, this seems to conflict with the remaining components that are clearly dominated by conifers rather than deciduous trees. It therefore remains difficult to place the assemblage precisely with respect to those known from other Norwich Crag Formation localities. The differences may not solely reflect regional vegetation history, but taphonomy. This could arise from the closer proximity to land in the embayment situation and the freshwater pool-like nature of the Great Blakenham site in contrast to the shallow marine and open mud-flat type environments of the other localities, i.e. further from the higher land areas that may have supported the forest. Indeed the inwash of Tsuga pollen grains from the neighbourhood may explain their occurrence derived from local growth, lnwash of both pollen and plant macrofossils is indicated throughout the sequence represented, as already mentioned. On balance therefore, the College Farm assemblage post-dates that from the Antian-Bramertonian, yet predates that from the Baventian-Pre-Pastonian (sensu Funnell, 1987; Zalasiewicz et al., 1991). It therefore
P.L. Gibbard et al.: Early Pleistocene Sediments at Great Blakenham, England represents a slightly later, late temperate phase of the Antian-Bramertonian temperate event. Regarding regional scale correlation, Gibbard et al. (1991) showed that the East Anglian Pliocene-Early Pleistocene sequence could be reliably equated with the Netherlands' succession. They concluded that the entire Ludhamian to Pastonian succession was equivalent to the complex Tiglian Stage (cf. Zagwijn, 1963); the AntianBramertonian event being equivalent to Tiglian Substage TC3 and the Baventian-Pre-Pastonian a being Tiglian TC4c. The discovery of the water fern Azolla tegeliensis in the College Farm Silty Clay considerably reinforces this correlation since the remains of this plant are found only in sediments of Early to Middle Tiglian age (i.e. the sediments must pre-date substage TC4b; de Jong, 1988) (Field, 1992), unless the remains were reworked. The excellent preservation, abundant numbers of fossils and their complete consistency with the local environment together indicate that these fossils are not reworked from older sediments but are in situ.
PALAEOMAGNETISM Thirty-eight samples were taken from the profile for palaeomagnetic analysis. Plastic cubes, of approximate internal volume 8 cm 3, were pushed into the cleaned sediment face by hand. Cubes were orientated and labelled prior to removal from the sediment. Magnetic remanences were measured using a GM400 cryogenic magnetometer. Twenty samples were subjected to stepwise alternating field (af) demagnetisation, then the remaining samples were treated at selected fields of I0 and 20 mT. The magnetic remanence vectors were extracted using a Linefind program (Kent et al., 1983). One sample was rejected because it had unstable demagnetisation behaviour. Natural remanent magnetisations (NRMs) were all of normal polarity. In all cases, af demagnetisation was effective in removing a significant fraction, or all, of a sample's remanence. No changes in polarity were revealed. Remanence directions extracted by Linefind were clustered, with a mean inclination of +71.3 ° and declination of 6.0 °. During a normal Chron, an axial geocentric dipole field at the latitude of East Anglia is directed towards the north pole and has an inclination of +70 °. However, the timing of remanence acquisition is subject to some uncertainty and will be discussed in detail elsewhere (Hallam, 1995). Van Montfrans (1971) suggested that the Early Pleistocene of northwest Europe might roughly correspond with the Matuyama reversed polarity Chron, but indicated several sites of Tiglian TC age that exhibited normal magnetic polarity. The College Farm Silty Clay may represent, at most, a few hundred years of deposition. Its magnetic remanence, therefore, is a 'snapshot' of the Earth's magnetic field and could have been acquired during a relatively short duration normal polarity subChron such as the Rrunion or 'X'. The longest period of normal polarity in the Early Pleistocene was the Olduvai
423
sub-Chron, but Zagwijn (1985) placed the Tiglian TC3 below the Olduvai (Zagwijn, 1985~ Gibbard et al., 1991). If the remanence of this sediment was acquired during the early part of the Olduvai, it would imply that Tiglian substage TC3 occurred later than previously suggested. The normal magnetic polarity of this sediment, therefore, is not inconsistent with a Tiglian TC3 age, but precise dating is problematic. CONCLUSION Both from the sedimentary evidence and the palaeobotany it appears that the College Farm Silty Clay represents accumulation in a situation marginal to and succeeding a sand flat environment in which the underlying Creeting Sands accumulated, possibly a shallow lagoon-like pool receiving freshets. This prograding sequence probably formed under conditions of stable sea level. The sediment was deposited predominantly in a freshwater body, possibly lacking a substantial outlet to the sea. It was fringed by marsh and reedswamp with alder and birch woodland locally. Beyond on dry ground to the south and west, late temperate coniferous forest with local areas of deciduous trees occurred. Calcareous grassland was also present in the vicinity. On the basis of the palaeobotanical assemblage, the deposits date from the late temperate phase of the AntianBramertonian temperate event. This is shown to be equivalent to Substage TC 3 of the complex Tiglian Stage in the Netherlands by the discovery of the water fern Azolla tegeliensis in the sequence. This correlation confirms previous suggestions by Gibbard et al. (1991). Equally, the apparently normal palaeomagnetic polarity of the sequence does not negate this correlation but precise dating on the basis of the magnetic remanance is problematic. These deposits are the oldest identified freshwater fossiliferous sediments in the British Pleistocene. ACKNOWLEDGEMENTS The sediments were originally sampled during the 1990 INQUA-SEQS meeting excursion with the assistance of Professor R.G. West, Professor J.J. Donner and Dr Th. van Kolfschoten. Assistance during subsequent work was given by Dr B. Maher and Mr S. Boreham. Access to the site was freely given by Blue Circle PLC. Professor West also suggested valuable improvements to the manuscript. We thank all these people.
REFERENCES Allen, P. (1984). Field Guide to the Gipping and Waveney Valleys, Suffolk, May 1992. Quaternary Research Association, Cambridge, 115 pp. Andrew, R. (1970). The Cambridge pollen reference collection. In: Walker, D. and West, R.G. Studies in the Vegetational Histor}' of the British Isles. Cambridge University Press, pp. 225-231.
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