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Palaeoenvironmental Reconstruction in the Breydon Formation, Holocene of East Anglia

The foraminiferal and ostracod faunas from two East Anglian, Holocene cores are described. Together they add further environmental evidence to the lithological alternation between peat and alluvial mud sedimentation observed in these sections resulting from local changes in sea-level. Sedimentological and micro-faunal differences between the two sections reflect their relative positions in the estuary with respect to the local palaeogeography. Foraminifera are abundant, the Ostracoda less so, nevertheless a comprehensive environmental interpretation is given and a foraminiferal ecological zonation proposed. Palaeoenvironmental reconstructions are based upon both microfossil groups which complement each other.


INTRODUCTION
Recent investigations into the Holocene sediments of Central Broadland, west of Great Yarmouth, East Anglia, have provided a number of cores for sedimentological and palaeontological analysis. The cores were distributed over a wide area with a view to reconstructing local sea-level and palaeogeographical changes in the, post-glacial, marginal marine deposits of East Anglia. The sedimentary sequences recovered are dominated by estuarine and marine, fine grained, calcareous sediments, mainly clays and muds. In some areas coarser grained sands occur as localised channel fill deposits. Periods of regressive overlap during times of relatively low sea-level resulted in the deposition of peat.
Microfossils particularly the Foraminifera are generally abundant, the Ostracoda less so. An initial palaeoenvironmental reconstruction of these sediments, based upon their microfossil assemblages has been undertaken at the University of East Anglia (Coles, 1977). The present work details ostracod faunal change (I.B.) in two of these cores and combines that information with a foraminiferal ecological zonation based upon a comprehensive study of foraminiferal assemblages from a large numbers of Holocene sequences in this area (M.G.). These results have been used to refine the interpretation of sedimentological changes. The stratigraphical interval studied covers approximately the last six to seven thousand years.
East Anglia possesses a great number of modern, fresh and brackish water environments whose calcareous invertebrate microfauna has not been studied in detail since the last century (Brady & Robertson, 1870). All ostracod species recovered from the Holocene sediments still live in British waters, including the East Anglian region, today. Their preferred ecological niches are, generally, well known (Athersuch et al., 1989) and many of the ostracods have 35 5236" been collected by one of us (I.B.) as living specimens from the present day East Anglian estuaries during the course of this research. By combining this modern ecological information with the faunal composition of each assemblage it has been possible to provide a more detailed record of environmental changes within the sequence than could be afforded through the study of one microfossil group alone. No previous work has appeared on the Ostracoda of the Holocene deposits in this area.

BACKGROUND
The present work is a result of two separate research programmes undertaken at the University of East Anglia. The first, in association with the British Geological Survey, during remapping of the Yarmouth Geological Sheet recovered a number of cores through the Holocene sequence, thus providing a large sample base. The second project concerns the use of ostracod shell chemistry as a means of estimating salinity levels in a modern East Anglian estuary with a view to applying the technique to Holocene faunas, and in so doing, attempting to reconstruct past sea-level changes through the study of fossil shell chemistry. The results of that work, intended to supplement standard micropalaeontological results, appear elsewhere (Boomer, in press).
The Holocene deposits in this area are represented by a 10 to 25 metre thick sequence comprising three peat layers separated by two much thicker clay strata. The transgressive / regressive nature of the sedimentary succession means that the most complete sequences are found in the East of the study area. Agricultural methods have also led to the disturbance and/or removal of the youngest sediments in certain areas. This alternation in sediment type appears to reflect changes in effective sealevel in the area. Previous micropalaeontological and sedimentological investigations into these deposits were undertaken by Alderton (1983), Coles (1977), Coles &Funnell (1981) andFunnel1 (1979). The interpretation of peat and clay alternations is thought to be a complex story involving changes in relative sea-level and development / destruction of spits and barriers across the estuary mouth. The average subsidence rate for the lower Yare Valley is estimated as averaging 1.5 metres every 1000 years since 9000 years B.P. The Holocene evolution of the Suffolk estuaries has recently been studied (Brew, 1990;Brew et al. 1992) and their interpretation compared with the Yare Valley sequence to the north. In their studies of the Holocene sediments underlying the lower Yare Valley and Halvergate Marshes Coles & Funnell (1981) and Funnell (1979)  The Upper and the Lower Peats were not encountered in the present study. The lowermost sediments in both sequences are represented by low intertidal Foraminifera and littoral to sub-littoral, phytal, marine Ostracoda. Thus, the transgressive transition from the 'non-marine' basal peat into 'littoral' conditions is missing. The layer of peat development encountered in both cores (i.e. the middle peat) was thought, by Coles and Funnell (1981), to have formed while the estuary mouth was effectively sealed by a coastal barrier, from about 4500 years ago. The second transgressive phase which, resulted in the deposition of the upper clay, was thought to have been initiated about 2000 years B.P. with its maximum influence at about 1600 years ago. The abrupt cessation of clay deposition at about 1500 years B.P. has been postulated as indicating the southward extension of the Great Yarmouth Spit across the mouth of the Bure-Yare-Waveney estuary system (Brew, 1990).

SAMPLING
The two sections currently under investigation were taken using a "Minuteman" power-driven rotary auger. Cores were retrieved using a block and tackle system, this coring and retrieval method leads to sediment disturbance so that no sedimentary structures were recorded. Sediment samples were taken off the corer in 20 centimetre intervals. Small sub-samples were taken for foraminifera1 analysis (log for MHB1, 5g for SS2) while somewhat larger samples (approximately 75g) were, generally, used for the ostracod studies. All samples were broken down using hydrogen

Explanation of Plate 1
All figured ostracod specimens and comparative Foraminifera specimens deposited at the British Museum (Natural History).   peroxide at room temperature for 24 hours. The resulting sediment was then washed to remove the mud (less than 635m) fraction, the final residue was often very small compared with the original sample size. These small samples, in some cases, yielded diverse ostracod faunas but low numbers of individuals which hampers the recognition of in situ and reworked / transported assemblages (this is particularly true for the upper part of core SS2 where only small samples were available). The palaeoenvironmental interpretation of a given sample is based, therefore, on the total aspect of the ostracod and foraminiferal fauna rather than the occurrence of one or two species unless they appear to be numerically important (i.e. relatively dominant) and in situ. In some cases assemblages were abundant but composed of one or two dominant species. Foraminifera are generally abundant in all samples recovered except where peats were encountered.

Core MHB1.
Core MHBl was drilled on Mautby Marshes (Nat. Grid Ref. TG 4954 1054; Fig 1) and penetrated to a depth of just over 18 metres, twenty seven samples were taken altogether (figure 2). The base of the Breydon Formation was not reached in this core but adjacent boreholes show it to be 21-22 metres below ground level in this area with a 1-2 metre thick oyster bed lying directly on the Pleistocene basement. These sediments are sub-tidal and are believed to have been deposited following a rapid transgression.  figure 3) to a depth of 17.8 metres, twenty seven samples were taken (figure 3). Core SS2 was much less varied in its lithology than MHB1. This is probably due to its palaeogeographical position, being in a more open part of the estuary and, therefore, less likely to have been subject to minor "sea-level" fluctuations than the marginally situated MHB1. The base of the Holocene sequence was not reached in this core and the nature of the basement in this area is not known. Some cores in this area exhibit crude laminations in intertidal flat and sub-tidal flat muds. Table 2. Lithological description of core SS2.

FORAMINIFERAL BIOZONES AND BIOFACIES O F T H E BREYDON FORMATION
In all, some 23 boreholes from the Holocene Breydon Formation of East Anglia have been studied for their foraminiferal content. An environmental zonation scheme has been established where the presence of both individual species and species assemblages are used to determine the relative tidal position of mean sea-level at the time of deposition.

Foraminifera1 Biozones
Introduction. The present scheme is based upon the previous work in this region (Coles, 1977;Coles & Funnell, 1981) and is expanded with new data from the present study. A complete revision of these studies is currently in preparation (Godwin, in prep. a,b) following the approach introduced by Scott & Medioli (1978,1986). Interpretation of the samples does not rely solely on the foraminiferal evidence. Average sediment grain size, plant remains, shell debris and Ostracoda are considered together with the size, condition and probable provenance of the foraminiferal tests with a view to assigning a sample to a given Zone. The Biozones refer to the depth of the environment relative to local, mean sea-level.
Light grey/blue 'soft mud, grades downwards into grey, then dark grey and finally a green grey mud at 1758cm.

Biofacies
Ti Hg / A1 Hg -Lr AtIAb Sub-tidal channels in this zone. Marsh creeks (Zone IIc) and intertidal channels (IIIc), which are largely sub-tidal, display a mixture of saltmarsh and deeper water Foraminifera. Large specimens of A. beccarii var batavus, miliolids and the ostracod C. torosa are diagnostic of these zones. A rare ecomorphotype of A. beccarii var limnetes which has an umbilical boss also occurs in these environments. The sediments here tend to be sandier and the population density may be low.
Intertidal flats (0.4m above to 1.7m below mean sealevel). Zone IIIa (High Intertidal Flat) assemblages are mainly composed of H. germanica and A. beccarii. Small, size-sorted specimens of E. earlandi, E. gerthi, E. magellanicum, E. margaritaceurn and various exotic shelf species constitute a small but consistent faunal element in most assemblages. Forma excavata and selseyensis dominate the ecomorphotypes of E. excavatum in this Zone. Molluscan debris and Phragmites remains are rare in this environment but mineral grains of coal, muscovite mica and framboidal pyrite are generally common in these crudely laminated muds.
Zone IVa (Low Intertidal Flat) is diagnosed by the presence of 4-10% E. oceanensis and/or E. excavatum forma clavata. Assemblages are otherwise similar to zone IIIa except that shelf species are less important but, where they are recorded, tend to be larger than average. Typically, subassemblages of E. gerthi, E. crispum-macellum, E. margaritaceurn and the rare E. excavatum forma magna dominate the transported element of the assemblage and range down into Zone IVc. Ostracoda, where abundant, are dominated by Loxoconcha spp. and/ or Hemicythere villosa.
Sub-tidal flats and channels (more than 1.7m below mean sea-level). Zone IVc assemblages (as defined by Banner & Culver, 1978) constitute a range of test sizes, from juvenile to adult, of the sub-tidal species E. oceanensis, E. excavatum forma clavata and A. beccarii or H. germanica still dominate most populations, and sediments are generally quite shelly. Sub-tidal channels contain the large specimens referred to above in Zones IIc-IIIc and many assemblages consist of up to 100% A. beccarii. H. germanica and miliolids are the most common accessory species in this environment.

Foraminifera1 Biofacies
Four distinct Biofacies have been identified which are much wider ranging zones than the depth controlled Zones referred to above. The biofacies reflect not only depth but also salinity, energy of the environment, substrate type and temperature. This scheme follows the 'associations' of Murray (1991). The Biofacies are named after the dominant or diagnostic species in each assemblage.

Microfossil assemblages from core MHB1.
The sediments at the base of the core (17-27) are characterised by foraminiferal assemblages belonging to the 'Ee' Biofacies and, therefore, to the IVc sub-tidal Biozone (Fig 2). The dominant species is A. beccarii (50-60%) with roughly equal proportions of marine and brackish water morphotypes present. Few tests show evidence of transportation from inner estuarine areas or from the estuary mouth although miliolids become relatively common towards the base of the core. The assemblages suggest medium energy levels and brackish salinity prevailing at this time. Water temperatures may have been lower than at later times suggested by the abundance of E. excavatum forma clavata (Murray, 1991). Further cores in this area suggest that the estuary was of the 'drowned valley' type in Lower Clay times (Godwin, in prep a). The foraminiferal and ostracod biostratigraphy of this core is given in figures 4 and 5, The most abundant and/or environmentally significant taxa are figured in plate 1.
The lowermost samples  are dominated by assemblages of the ostracods H. villosa and L. rhomboidea. These are essentially algal and phytal feeders respectively. The former species decreases markedly at sample 25 with the subsequent samples (24-21) dominated by L. rhomboidea. Another loxoconchid, the marine species P. guttata, is common in the lower part of core SS2 but is only represented by a few specimens in the Lower Clay of core MHB1. Samples 21-20 see a decrease in L. rhomboidea with an increase in the importance of the brackish water species C. torosa and L. castanea. This clearly represents an increased freshwater influence. This pattern continues from sample 19 through to 14 with these two species eventually representing the only Ostracoda at the top of the Lower Clay as diversity decreases.
Samples 16-14 indicate a rapidly shallowing environment. Silting up of the estuary was probably aided by the barring of the estuary mouth by a sand spit (Coles & Funnell, 1981). Samples 15-16 have 6-9.5% E. oceanensis and are interpreted as low intertidal flat, Zone IVa. By sample 14 this component is absent and shallow water species have increased importance suggesting a high intertidal flat Zone IIIa assemblage. This regressive sequence belongs to the 'Ab' Biofacies.
At the level of samples 13-14 there is a major change in the sediment type with shell debris replaced by Phragmites red material. There is a fall in diversity and abundance which is indicative of saltmarsh and marsh creek environments. The assemblages are placed in the marsh creek Zone IIc due to the presence of large specimens of A. beccarii var batavus, typical of channel faunas. It is inferred that the channel has cut into the intertidal flat sediments below. Samples 11-12 are barren peats witnessing the culmination of the regressive phase of deposition. The interval covered by samples 13-9 represents the Middle Peat and no Ostracoda are recorded from these levels. The two samples immediately above the peat, 6 and 8 (sample 7 was not investigated for ostracods) possess specimens of H. rubida, Xestoleberis sp. and L. castanea. There does not appear to be any evidence of a gradual increase in salinity in the ostracod faunas immediately following the peat development. This would support the suggestion that the top of the peat is an erosion layer (Godwin in prep a) marking the sharp onset of a transgressive phase in the Upper Clay. Samples 7-10 belong to the Lower saltmarsh Zones although channel migration has led to the presence of erosive surfaces adding further complexities to this part of the sequence. These samples are assigned to the 'Hg' foraminiferal Biofacies. Sample 10 has 10% E. williamsoni, diagnostic of the Lower saltmarsh Zone IIb. This environment shallowed further in sample 9 with 45% T. injlatu and J. macrescens indicating a IIa level in the 'Ti' Biofacies. The sediments were then cut by a channel (seen in sample 8), with Zone IIc faunas containing 19% A. beccarii var batavus and accessory, transported, shelf material. This feature had silted up by sample 7 represented by Zone IIb faunas in the 'Al' Biofacies.
Samples 1-7 show the transgression regaining momentum and the entire sequence is interpreted as a high intertidal flat Zone IIIa lying in the 'At' Biofacies. Assemblages contain a small percentage of Elphidium species and small, size sorted, shelf taxa. Samples 5-2 are dominated by the brackish water ostracod L. castanea with a few specimens of phytal estuarine Ostracoda as accessory species. The uppermost sample (MHB1-1) contains assemblages of L. castanea, L. lacertosa and C. fisheri the latter two species are tolerant of fluctuating salinity levels and therefore represent a marginal setting within the estuary. The final evidence of silting up is lost in the soil profile. Table 3. Summary of Biofacies Scheme.

Microfossil assemblages from core SS2
The initial transgressive phase (samples 17-27) is similar to the basal part of core MHBl and is interpreted as sub-tidal mudflats (Zone IVc). The 'Ee' Biofacies is not as fully developed as in MHBl and there are higher numbers of exotic Foraminifera. A. beccarii var batavus is the dominant species and suggests higher salinities here than at the more marginally positioned MHBl site. The foraminiferal and ostracod biostratigraphy for this core is given in Figs 6 and 7.
The lowermost samples of this core (27-25) are dominated by two species of Palmoconcha, l? guttata and P. laevata, only a few of the former species and none of the latter were recorded in the other core. These represent sub-littoral marine assemblages. samples 24 through to 16 are dominated by the phytal L. rhomboidea a common shallow sub-littoral species with secondary species such as H. villosa, H. viridis, C. torosa and L. castanea important at certain levels. The latter two species are recorded in many samples throughout the two cores. Although they may constitute a large percentage of some samples, analysis of their population structure would suggest that they are often transported from the shallower water, brackish regions of the estuary.
The regressive phase with abrupt shallowing and low intertidal flat taxa (sample 16) gives way to channel faunas with relatively high numbers of sub-tidal species. These assemblages are still within the 'Ab' Biofacies indicating higher salinity levels persisting. Textularids are common in some of the marsh creek (Zone IIc) material. Sample 11, a thin sand, has a IVc sub-tidal fauna and is thought to represent a storm event. The sequence ends with a Phragmites mud in the 'Ti' Biofacies and IIa lower saltmarsh Zone capped by a much eroded remnant of the Middle Peat. Ostracod assemblages from samples 15 to 9 indicate decreasing salinity conditions and increasing euryhalinity with the predominance of C. torosa and a decrease in diversity. The peat horizon in this borehole (SS2-9) contains Ostracoda. Sample 8 contains five freshwater taxa in addition to the brackish water species.
The succeeding transgressive phase which began the Upper Clay deposition is characterised in this area by a series of channel sands in the 'Ab' Biofacies (samples 8-5).
Here, the presence of the inner estuarine Haplophragmoides spp. indicates a nearby freshwater source. The environment is sub-tidal with typical channel faunas.
By sample 5 the final regression is seen starting with shallow water species becoming increasingly important. The final phase (samples 3-1) marks a return to mud deposition with increasing quantities of Phragrnites debris added to the sediment. The channel here becomes a marsh channel before finally silting up. The five samples succeeding the peat horizon (7-3) are dominated by the two ostracod species most commonly observed in the estuary today (C. torosa and L. elliptica). The two uppermost samples, however, possess few specimens of C. torosa but other species such as Sclerochilus and a number of more marine taxa are recorded. Indeed, samples 5 through to 1 contain a number of instars of sub-littoral, open marine species e.g. Cytheropteron sp., Robertsonites tuberculatus, and Pontocythere elongata. These are interpreted as either reworked specimens from the surrounding estuarine sediments as the estuary silted up and/or the result of storm surges bringing in these small specimens.
The two boreholes described above, although taken within t w o kilometres of each other, show differences in the ostracod assemblages recovered. They both display the same peat / silty-mud alternations reflecting local sea-level and/ o r palaeogeographical changes. Both cores are dominated by brackish water mud dwellers, estuarine phytal species or shallow marine taxa. Core MHBl has yielded moderately rich ostracod assemblages of low diversity, the majority representing in-situ populations. Core SS2, however, has a more diverse fauna. Many samples in the latter core contain species which are represented by only one o r t w o specimens. These clearly represent an allochthonous element. It is assumed, therefore, that SS2 represents an environment where mixing of in situ and transported valves occurs. The faunal dichotomy between the two cores reflects the environmental differences which can occur within short distances in the estuarine setting.

ENVIRONMENTAL INTERPRETATION OF CORES M H B l AND SS2
A complete study of the many boreholes from this area have afforded a detailed reconstruction of the palaeogeographic history of this estuary. The Breydon Formation in the study area occupies a flat bottomed valley with relatively steep sides. MHBl is sited approximately 300 metres due south of the northern margin of the valley which is marked by a palaeo-shoreline which is clearly visible today. Core SS2 is sited south of the River Bure, and towards the centre of the estuary. In the early part of the Lower Clay the MHBl site lay close to the shore. The estuary is interpreted as a drowned river valley, with a sub-tidal channel sweeping close to the shoreline at this point. Immediately to the south the estuary shallowed to low intertidal sand flats before deepening again in the area of core SS2.
At the end of Lower Clay times the main channel had moved further south to the position of the present River Bure. During the regressive phase the SS2 area shallowed first and has a deeper sequence of saltmarsh sediments (mainly channel infills). The MHBl site was transformed into intertidal flats before shallowing into the fresh and brackish water peats of Middle Peat times.
The Middle Peat occupied the whole width of the valley a t this point. The transgression which resulted i n the deposition of the Upper Clay is presumed to have been initiated b y the destruction of a sand barrier which lay across the mouth of the estuary (Coles & Funnell, 1981). In the Upper Clay tidal flats are, once again, found on the northern margin of the valley (MHB1) although at a higher position in the section than seen in SS2. South of the River Bure the sedimentary sequence is dominated b y somewhat deeper situated channel deposits. This is a reverse of the interpretation given for the Lower Clay i n these t w o boreholes. With the formation of a new sand bar from the 10th century onwards, both sites gradually silt up.