2008 geoarchaeological interim report

Transcription

Geoarchaeological study of the Ostriconi
valley
A report based on fieldwork undertaken in 2007-2008 for La
Balagne Landscape Project
Keith Wilkinson BSc PhD PgCert MIFA
Department of Archaeology
University of Winchester
West Hill
Winchester
SO22 4NR
UK
[email protected]
April 2009
1
Abstract
A longitudinal transect of boreholes was drilled along the Ostriconi valley during the
2007 and 2008 field seasons of La Balagne Landscape Project using an Atlas Cobra
petrol-powered pneumatic auger. During the 2008 field season a further lateral
transect of the Ostriconi valley and an east-west transect to the west of the Etang de
Cannuta were also drilled using a mixture of manual and powered auger techniques.
The survey follows previous sampling of the valley and the Etang by Maurice Reille in
the 1980s and his subsequent palynological analysis which suggested that the top
2.5m of stratigraphy that he was able to penetrate using a Russian and Hiller auger
date from the last 3500 14C years. In contrast, the present borehole survey sampled
the entire Holocene succession over much of the valley which totalled 9m at its
deepest point. The results demonstrate that much of the valley west of the Etang de
Cannuta was initially occupied by a marine embayment in which deposition of shellrich sands took place on top of the granite bed rock. Aeolian and alluvial depositional
environments succeeded the marine phase to the west of the Etang de Cannuta and
persisted until the recent past. It is apparent from the borehole stratigraphy that the
channel of the Ostriconi river has migrated across a 100m wide channel belt and
thereby deposited gravels and sands in a variety of locations in this strip. In addition
backswamp and oxbow lake facies (organic muds and peats) were located towards
the base of the alluvial sequence in some boreholesin the central part of the transect.
In the western part of the Ostriconi transect aeolian deposits underlie alluvial and
organic deposits suggesting that dune formation caused the termination of the
marine embayment. The dune systems are still forming and characterise the western
end of the valley and which are also found on the northern slopes. Deposition in the
basin of the Etang de Cannuta was initially of sedge and wood-rich peats which are
most likely to have formed in a freshwater swamp. These sediments are succeeded
by alluvial sands, gravels and floodplain silts/clay, and finally lacustrine sediments.
At present there is no chronology for the events articulated above, while the eastwest borehole transect has not yet been extended far enough to the east to enable
the stratigraphy of the Ostriconi valley to be correlated with that from Etang de
Cannuta. Completing the east-west transect, obtaining high quality cores from the
Etang de Cannuta sequence and providing a high quality 14C chronology for the
sequences are the priorities for the 2009 field season and subsequently.
1
French abstract - Résumé
Un transect longitudinal de carottages a été entrepris le long de la vallée de
l’Ostriconi, au moyen d’un marteau piqueur pneumatique Atlas Cobra, durant les
campagnes de terrains des années 2007 et 2008 du Projet d’Archéologie du
Paysage – La Balagne. Au cours de la saison de prélèvement 2008, un autre
transect perpendiculaire à la vallée de l’Ostriconi, ainsi qu’un second transect
d’orientation Est-Ouest et localisé à l’Ouest de l’étang de Cannuta, ont été réalisés
en utilisant à la fois des outils de carottage mécaniques et manuels. Ces sondages
font suite aux prélèvements effectués par Maurice Reille dans la vallée et à l’étang
de Cannuta, durant les années 80, pour lesquels les analyses palynologiques
suggèrent que la stratigraphie de 2,5 m, recueillie au moyen d’un carottier Russe et
d’une sonde Hiller, couvre les derniers 3500 ans 14C. En revanche, les carottages de
la présente étude échantillonnent l’ensemble de la séquence Holocène et ce dans
une grande partie de la vallée, avec un total de 9 m de sédiment au point le plus
profond. Les résultats démontrent que la plus grande partie de la vallée située à
l’ouest de l’étang de Cannuta, a été initialement occupée par une baie marine, dans
laquelle des sables riches en coquillages se sont déposés sur la roche en place
granitique. Des dépôts éoliens et alluviaux succèdent à cette phase marine à l’Ouest
de l’étang de Cannuta et perdurent jusqu’à nos jours. Il paraît évident, d’après la
stratigraphie des sondages, que le lit de la rivière Ostriconi a migré dans une
ceinture de 100 m de large, et à de ce fait déposé en différents lieux de cette zone
une série de graviers et de sables. En outre, des marais et des méandres morts
(boue et tourbe organiques) se trouvent à la base de la séquence alluviale, dans des
sondages de la partie centrale du transect. Dans la partie ouest du transect de
l’Ostriconi, des dépôts éoliens sont à la base de dépôts alluviaux et organiques,
suggérant que la formation d’une dune a causé l’arrêt de la baie marine. Le système
dunaire est toujours en formation, il caractérise la limite occidentale de la vallée et il
a également été découvert sur les pentes Nord. Le dépôt dans le bassin de l’étang
de Cannuta a été initialement constitué de carex et de tourbe riche en bois, qui sont
susceptibles de s’être formés dans un marais d’eau douce. Ces dépôts ont été
succédés par des sables alluviaux, des graviers, des vases/argiles issues
d’inondations et finalement par des sédiments lacustres.
Actuellement, il n'y a aucune chronologie pour les événements décrits ci-dessus,
alors que le transect de carrottages Est-Ouest n'a pas été encore prolongé assez loin
à l'Est, pour permettre de corréler la stratigraphie de la vallée de l’Ostriconi à celle de
l’étang de Cannuta. Les priorités de la campagne de terrain 2009 et des suivantes
sont de compléter le transect Est-Ouest, d’aquérir des prélèvements de grande
qualité pour l’étang de Cannuta et d’obtenir une chronologie 14C de grande précision
pour l’ensemble des séquences.
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La Balagne Landscape Project: borehole surveys of the Ostriconi valley 2007-2008
Contents
Abstract..................................................................................................................... 1
French abstract - Résumé ......................................................................................... 2
Contents.................................................................................................................... 3
Illustrations................................................................................................................ 3
1
Introduction........................................................................................................ 4
2
The study area and project aims........................................................................ 4
3
Methodology...................................................................................................... 7
4
Results .............................................................................................................. 9
4.1
Ostriconi valley .......................................................................................... 9
4.2
Etang de Cannuta.................................................................................... 13
4
Conclusions..................................................................................................... 14
5
Further work .................................................................................................... 15
6
Acknowledgements ......................................................................................... 15
6
References ...................................................................................................... 16
Illustrations
Figure 1. Location of the Ostriconi study area in North-west Corsica......................... 4
Figure 2. Location of boreholes drilled in 2007-2008 in the Ostriconi valley............... 6
Figure 3. Eijelkamp drilling equipment being used in the Ostriconi valley. ................. 7
Figure 4. A manually-powered Edelmann auger being used to drill the transverse
cross section of the Ostriconi valley................................................................... 8
Figure 5. East-west transect along the Ostriconi valley ............................................. 9
Figure 6. Shallow north-south transect across the Ostriconi valley .......................... 11
Figure 7. A pause in drilling CAN BH 3 at the margins of the current Etang de
Cannuta........................................................................................................... 12
Figure 8. East-west transect across the western margins of the Etang de Cannuta 14
3
1
Introduction
This report presents preliminary results of a geoarchaeological study undertaken of
the stratigraphy of the Ostriconi valley in the north-west of Corsica (Figure 1). The
work was funded by the Department of Archaeology, University of Winchester and
the Collectivité Territoriale de Corse (through the Università di Corsica), and was
carried out as part of La Balagne Landscape Project’s 2007 and 2008 field seasons.
In the text that follows the background to the study is outlined, the field methodology
by which the work was undertaken in discussed, and preliminary conclusions (in
advance of the receipt of chronometric dating and laboratory analytical data) are
drawn. The final section of the report outlines a strategy for completion of the
borehole prospection in La Balagne Landscape Project’s 2009 field season.
Figure 1. Location of the Ostriconi study area in North-west Corsica
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The study area and project aims
The Ostriconi valley is the main access route into La Balagne from the east coast of
Corsica and the interior. It has therefore been an important communication and
transhumance axis in the historic period and also presumably in prehistory. The
valley is occupied by the Ostriconi river which flows north-westwards from a drainage
divide south of the village of Pietralba, about 9.3km north-west of the town of Ponte
Lecchia and is subsequently fed by a series of ephemeral streams emanating from
mountains to both the north and south (Figure 1). For most of its 18.5km stretch the
Ostriconi river flows on the southern side of its valley, and the northern part is formed
4
of a bajada of coalescing alluvial fans originating in mountains to the north. However,
within 5km of the coast the valley widens out, the river adopts a meandering bedform
and there is a 500m wide floodplain fieldwork (Figure 2). The Ostriconi river exits into
the sea via a narrow channel in the extreme southern side of its valley, having been
forced to adopt this route by the extensive aeolian dunes that have developed at the
mouth of the valley and on its northern side. The dunes lie behind (east of) and form
part of one of the few sand beaches in La Balagne. Mountains rise steeply on either
side of the Ostriconi valley and are mainly comprised of Carboniferous granites.
However, metamorphic schists are found as a isolated outcrop to the immediate
south of the Plage d’Ostriconi, while Cretaceous sedimentary rocks are present in the
headwater area.
Fieldwork carried out in July 2007 and 2008 was as part of the second and third full
seasons of LBLP respectively. In the first field seasons boreholes had been drilled
across the Etang de Crovani (Calenzana) and at the Piana di l’Olmu (Galeria) at the
mouth of the Fangu valley (Wilkinson et al. 2007). Both of these areas had previously
been cored by Maurice Reille in the 1980s as part of that authors project to
reconstruct Corsica’s Holocene forest history (Reille 1992). Although Reille’s most
detailed work in La Balagne was on the Piana di l’Olmu, and to a lesser extent the
Etang de Crovani sequences, he also cored at sites in the Ostriconi valley (‘Ostriconi’
and ‘Etang de Cannuta’). Given the significance of the litho- and biostratigraphic data
demonstrated by Reille’s (1992) outline work, the decision was made that the
Ostriconi valley would be the focus of geoarchaeological borehole studies to be
conducted in 2007 and 2008.
The purpose of the present geoarchaeological investigations in the Ostriconi valley
was therefore:
1.
To construct a dated stratigraphic model.
2.
To collect core samples for laboratory sedimentological, palynological, diatom
and chronological studies in order to reconstruction Holocene
palaeoenvironments for the Ostriconi catchment.
3.
To use data obtained by the present study together with those of Reille (1992)
to model palaeoenvironments of the Ostriconi area and thereby too assess
Middle-Late Holocene resource availability in different topographic zones.
4.
To provide a preliminary model of sea level change.
5.
To assess the viability of tephrachronological dating in Balanine coastal
environments.
6.
To compare the stratigraphic, chronological and biostratigraphic results with
similar data from Etang de Crovani and Piana di l’Olmu (Wilkinson et al.
2007).
Of these, Aim 1 is directly addressed by the present report, while the remaining aims
are the subject of ongoing analytical study and future fieldwork and laboratory
analysis.
The exact location of Reille’s Russian auger cores in the Ostriconi valley and the
Etang de Cannuta have not been published, although the lithology columns that
accompany the pollen diagrams enable a rough estimate to be made of their position.
Two and a half metre thick sequences of peat and organic mud strata were sampled
at both sites and palynological slides prepared from sub-samples taken at 10cm
intervals (Reille 1992). Although 14C dates were obtained from neither site, Reille was
able to reconstruct a relative chronology by comparing pollen assemblages from
Ostriconi and Cannuta with those from 14C dated sequences at Piana di l’Olmu,
Etang de Crovani and Saleccia (Reille 1992). According to these data the 2.5m thick
sequences at Ostriconi and Cannuta both date from the last 3500 14C years. Both the
Ostriconi and Cannuta pollen diagrams demonstrate human impact on the
5
surrounding area. There is a rise in olive (Olea sp.) and elm (Ulmus sp.) pollen
between 0.70 and 1.20m at Ostriconi, while a reduction of tree heather (Erica
arborea) coincident with a burning event and the expansion of herbs, and liana (Vitis
sp.) at 2.05m in the Cannuta sequence also appear to be the consequence of human
activity (Reille 1992). Cereals and myrtle (Myrtus sp.) are both present in relatively
high concentrations towards the top of the Cannuta sequence suggesting agricultural
activity and the spread of low maquis to the surrounding uplands.
Figure 2. Location of boreholes drilled in 2007-2008 in the Ostriconi valley
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3
Methodology
Figure 3. Eijelkamp drilling equipment being used in the Ostriconi valley.
A reconnaissance visit was made to the Ostriconi valley in August 2005 during a
planning phase of LBLP. As a result it was decided to drill a series of boreholes along
an east-west longitudinal array between the present coast and the Etang de Cannuta
(Figure 2). The resultant composite cross section would enable facies change along
the Ostriconi to be determined and the major intertidal, alluvial and aeolian units
mapped. However, equipment failure in the 2008 field season meant this strategy
had to be modified and therefore a lateral cross section of the Ostriconi valley was
also drilled (Figure 2).
Eijelkamp augers were used to drill the longitidunal profiles along the Ostriconi valley
(borehole with the prefix ‘OST’) and on the western side of the Etang de Cannuta
(‘CAN’). The Eijelkamp equipment comprises an Atlas Cobra petrol powered
hammer, gouge auger heads, a 53mm diameter core sampler and 1m long extension
rods (Figure 3). A total of 15 boreholes were drilled to the pre-Holocene substrate
using the Eijelkamp system (11 in the mouth of the Ostriconi valley and 4 west of the
Etang de Cannuta) (Figure 5 and Figure 8). Gouge auger heads were used to drill all
boreholes except OST BH 15 which was core sampled, while OST BH 12 and CAN
BH 4 were drilled with both gouge heads and the core sampler. The stratigraphy
7
retained in the gouge auger heads was described in the field using the Troels-Smith
(1955) and Munsell Color (2000) systems, and photographed after each 0.5m or 1m
long drive. Cores were labelled, sealed on site and transported back to the UK for
detailed laboratory study at the end of the season. Following completion of 5
boreholes in the 2008 field season the Atlas Cobra hammer broke and could not be
repaired. As a result hand operated gouge and Edelmann augers were used to
continue the fieldwork (Figure 4). Sediment retained in the manually driven auger
head was described in exactly the same way as described above. However, because
of the limited number of extension rods available, it was only possible to drill to 4.5m
below ground surface with this equipment. The limited penetration afforded by the
hand operated equipment led to the change in strategy outlined above whereby a
shallow lateral cross section of the valley was drilled rather than continuing the
longitudinal cross section (Figure 6).
Figure 4. A manually-powered Edelmann auger being used to drill the transverse
cross section of the Ostriconi valley.
8
4
Results
4.1
Ostriconi valley
Figure 5. East-west transect along the Ostriconi valley
9
The base of the stratigraphic sequence in the Ostriconi valley to which it was
possible to penetrate using the Edelmann drilling equipment comprise rests on
dense, granite-dominated gravels. Such gravels were found in OST BH 14, OST BH
17, CAN BH 2 and CAN BH 4, and are likely to have been deposited by the Ostriconi
river during cold phases of the Late Pleistocene. Optically stimulated luminescence
dating of the youngest fluvial terraces that outcrop adjacent to the floodplain of the
Fangu and Figarella rivers suggest a chronology of > c. 25ky BP (Feathers,
Wilkinson and Llobera unpublished data). These data may provide a terminus post
quem for the Ostriconi gravels outcropping below the present floodplain.
Coarse marine sands containing frequent bivalve mollusc shells overlie the
(presumed) Late Pleistocene gravels in the central part of the Ostriconi study area
around OST BH 12 (not included in Figure 5) and OST BH 14 (Figure 2 and Figure
5). It is likely that these sands extend westwards beneath strata sampled in BH 3-9.
However, boreholes did not penetrate overlying fine sands in these locations (see
below) (Figure 5). The marine sands are likely to have accumulated in an
embayment prior to the formation of barrier dune systems, while the distribution of
the marine sands suggest that the embayment extended eastward from the coast to
a location between OST BH 14 and OST BH 17. Although it is likely that the marine
sands infilling the bay date to the Holocene, it is not possible in the absence of 14C
dates, to suggest when the bay disappeared.
The lowermost part of the sampled sequence in OST BH 9 and boreholes east of that
location consists of well sorted fine sands. The sequences are relatively
homogeneous, while individual sand grains have polished surfaces. These
characteristics are collectively suggestive of deposition by aeolian processes. If this
is the case then these fine sands mark the onset of dune formation, a process that is
still ongoing at the present day at the mouth of the Ostriconi valley and on its
northern flank. Given the nature of deposits overlying the fine sands in OST BH 9
and the stratigraphy above the marine sands in OST BH 14 and locations to the east,
it is highly likely that dune formation played a significant role in the disappearance of
the Ostriconi bay (Figure 5). However, as with the marine sands, the absence of a
14
C chronology means that there is currently no temporal framework in which dune
initiation can be placed.
In the eastern part of the longitudinal cross section, organic sediments conformably
overlie the marine sands (in OST BH 12 and OST BH 14). However, the
interdigitating nature of the contact suggests that marine processes alternated with
marine-marginal (?saltmarsh) deposition before the latter finally predominated. Later
still salt marsh environments are likely to have been succeeded by fresh water marsh
and consequently a wood peat developed (Figure 5). Therefore it is likely that this
part of the sequence represents a facies shift from marine embayment to indertidal
conditions to freshwater fen supporting an arboreal vegetation (wood pieces were
noted in the peat). However, palynological and diatom/foraminiferal analyses are
needed to confirm such a conclusion. It is possible that the facies changes previously
discussed were the result of the sealing of the bay by the dune development noted
above. Further evidence in support of the latter hypothesis is the fact that organic
deposits conformably overlie dune sands in OST BH 9. Such data imply that dune
development led to heightened water tables in lea areas in which organic
sedimentation/peat growth took place. In OST BH 17 organic sediment
unconformably overlies the presumed Late Pleistocene gravels. These latter organic
beds are at the same elevation to those noted in OST BH 12 and BH 14, but may
relate to earlier saltmarsh phases associated with marine sand facies in areas west
of OST BH 17. Clearly all the organic beds discussed above can be relatively easily
dated by AMS 14C measurement and their included botanical micro/macrofossils
10
examined to provide a record of vegetation change. Such a course of action would
enable the hypotheses articulated in the preceding text to be addressed.
Figure 6. Shallow north-south transect across the Ostriconi valley
The organic deposits in the Ostriconi transect are conformably overlain by alluvial
facies. These latter comprise sand and (very occasionally) gravel strata that
accumulated in channels, silts and sands that were deposited on levees,
homogeneous silts and clays that formed on the floodplain and organic strata that
formed in backswamps or cut off meanders (Figure 5 and Figure 6). The fact that
such alluvial deposits also overlie dune sands in OST BH 7 and OST BH 9 (Figure 5)
suggests that fluvial processes succeeded marine depositional environments once
the mouth of the bay had been sealed by dune development. It is extremely difficult
and arguably meaningless to correlate individual alluvial beds longitudinally along the
valley. It is clear that the Ostriconi river had a meandering bedform (as today) during
the formation of the alluvial sequence and that the location of the channel and hence
11
levees, floodplain and backswamp, varied over time. However, the transverse cross
section through the uppermost 4.5m of the alluvial sequence depicted in Figure 6
reveals broad patterns. Firstly it appears that the channel of the Ostriconi river was
initially located in approximately the same position as now or to the south of its
present location. Floodplain or backswamp deposits (the latter in OST BH 18 and
particularly OST BH 24) characterise the lowermost parts of the sequence in all
boreholes in the transverse section except OST BH 26 where they must have
onlapped a fossil dune. Higher in the sequence sand facies tend to be located close
to the present river channel, although sands and gravels were noted as far north as
OST BH 25. Levee deposits were noted in OST BH 18, OST BH 23, OST BH 25, but
are most extensive in OST BH 24. These data suggest that the channel belt of the
Ostriconi river has extended to a maximum of 100m north of the present channel (the
sands and gravels in OST BH 25 are best interpreted as a crevasse splay or a
channel sequence of a short-lived emphemeral tributary stream). The floodplain
extends to the northernmost margins of the Ostriconi valley, albeit that colluvial
sediments interdigitate with floodplain deposits in both OST BH 28 and OST BH 29
(Figure 6).
As previously stated, it is unclear where Maurice Reille drilled his borehole to sample
the Ostriconi valley. However, given that he was only able to penetrate 2.5m below
ground surface he can only have sampled alluvial facies. Assuming that Reille’s
(1992) palynology-derived chronology is correct, the alluvial deposits from the
Ostriconi valley date from at least 3500 14C years BP (i.e. the Bronze Age-present),
suggesting in turn that the presumed intertidal and fen (organic) deposits, the
initiation of dune development and the Ostriconi open marine embayment all date
from the same age bracket or previously.
Figure 7. A pause in drilling CAN BH 3 at the margins of the current Etang de
Cannuta
12
4.2
Etang de Cannuta
The Etang de Cannuta was sampled in a short borehole transect beyond its present
western margin, but also included CAN BH 4 which was drilled through the edge of
the current lake (Figure 2 and
Figure 7). As previously noted the basal deposits at the Etang de Cannuta rest on
fluvial gravels of presumed Late Pleistocene age and which outcrop >7.0m below
present ground surface. They are overlain by a 3m thick sequence of organic muds
and peats. The latter change upwards from reed to wood peats suggesting a seral
change from fen to carr environments (Figure 8). The organic deposits/peats are
therefore likely to have been associated with high water tables, a feature which can
most plausibly be linked with presence of the marine embayment identified in the
Ostriconi boreholes (see Section 4.1). Palynological analysis combined with 14C
dating of the Cannuta organic deposits will enable both this hypothesis to be
addressed and palaeovegetation data to be obtained. Figure 8 depicts sands
interbedded with the organic sediments/peats previously outlined, but sequences
sampled in cores taken from CAN BH 4 (not included on Figure 8) suggest that the
sands are derived from sediment that has collapsed down the borehole.
The organic sequence at Etang de Cannuta is unconformably overlain by deposits
characteristic of high energy fluvial deposition. Normally bedded sands and gravels
were found in all boreholes at this point in the sequence, but are gradually
succeeded in all except CAN BH 3 by fine-grained deposits. The unconformable
nature of the lower contact suggests that the top of the underlying organic sequence
may have been truncated, but even so the gravels mark a dramatic change in
depositional environment. It is probable given the topography of the area that the
coarse sediments were deposited in alluvial fans or fluvial channels emanating from
the Cima a Forca range to the north rather than the Ostriconi river. However, the
nature of the event or process that initiated such a major change in sedimentation is
presently unknown. Hopefully the palynological analysis and 14C dating of the
underlying organic sediment may provide data which will enable this question to be
resolved. Alluvial sediments are succeeded by lacustrine clays in CAN BH 1, CAN
BH 2 and CAN BH 4 at about 2-2.5m below present ground surface. Nevertheless
the presence of occasional beds of sands at higher elevations suggests that alluvial
sediments occasionally prograded into the lake. The coarse nature of the latter
alluvial deposits and the lack of floodplain deposits within the lacustrine succession
suggest that alluvial sediments were transported by an ephemeral stream, perhaps in
the same alluvial fan/channels emerging from the Cima a Forca that deposited the
earlier coarse-grained sediment.
The geographic position of Reille’s (1992) sample point at the Etang de Cannuta is
unknown, but it is highly likely based on his published stratigraphy that he drilled
within the margins of the Etang. As with the Ostriconi sequence that Reille sampled,
palynological data suggest that the uppermost 2.5m of the sequence (i.e. the
lacustrine facies) accumulated within the last 3500 14C years. If this hypothesis is
correct it would suggest that the organic and fluvial deposits revealed by the present
borehole survey in the earlier part to this period or previously.
13
Figure 8. East-west transect across the western margins of the Etang de Cannuta
4
Conclusions
Borehole surveys conducted in the Ostriconi valley in 2007 and 2008 demonstrate a
series of dramatic landscape changes during the Holocene. The valley initially seems
to have lacked a barrier system separating marine and terrestrial environment, and
consequently a marine embayment existed at least 800m east of the present
shoreline. Shell-rich sands accumulated within the bay, while heightened water
tables consequent on the proximity of the sea may have caused the development of
fen and carr environments in the area now occupied by the Etang de Cannuta. Later,
wind blown sand formed dunes at the entrance to the bay, which had the effect of
preventing marine ingress into the valley. As a result the marine embayment is likely
to have disappeared and consequently first saltmarsh and then freshwater swamp
developed behind the dunes. The dunes have continued developing to the present
14
day in the mouth of the Ostriconi valley and on its northern slopes, but elsewhere in
the valley deposition took place in freshwater environments of the Ostriconi river,
while fans and/or fluvial channels originating in the Cima a Forca pulsed coarse
sediment over the area now occupied by the Etang de Cannuta. The Ostriconi river
initially flowed along its present course or to the south, but later migrated up to 100m
north of its present channel. These movements in the channel position, together with
the abandonment of meanders have resulted in a complex sediment stratigraphy
underlying the present ground surface over much of the current cultivated floodplain.
Channel sands and gravels are found within 100m of the current river, levee deposits
and organic sediments within 200m and floodplain deposits across the whole width of
the valley. The latter interdigitate with colluvial sediments within 100m of the northern
valley edge.
5
Further work
While the relative order of landscape development is reasonably well established
absolute chronology for key changes is not, while further work is also required to
determine the cause of some environmental changes and the impact they had on
past human populations. Therefore work in the 2009 field season will focus on the
following:
1. Extending the longitudinal Ostriconi transect to intersect with that drilled west of
Etang de Cannuta thus enabling the two sequences to be correlated.
2. Re-sampling the Etang de Cannuta basal organic sequence in a cased borehole to
prevent sediment collapse contaminating the cores.
3. Obtaining core samples in the vicinity of OST BH 24 to sample organic facies
noted in that borehole.
Subsequently core samples will be:
4. Cleaned, photographed and described in detail.
5. Sub-sampled for sedimentological study and the resultant sub-samples used for
magnetic susceptibility analysis and loss-on-ignition measurement.
6. Sub-sampled for palynological and diatom assessment to reconstruct past
vegetation and salinity status respectively.
7. Sub-sampled for AMS 14C dating to provide a chronological framework for
landscape changes.
6
Acknowledgements
The author would like to thank the following individuals for their help before and
during the geoarchaeological fieldwork in the Ostriconi valley:
Staff
Jay Flaming (University of Washington, Seattle)
Dr Marcos Llobera (University of Washington, Seattle)
Claire Lorrain (University of Winchester)
Dr Nathalie Marini (Università di Corsica)
Dr Silvain Mazet (Università di Corsica)
Prof Michel-Claude Weiss (Università di Corsica)
Myra Wilkinson (University of Winchester)
Students
The following participated in drilling the boreholes:
15
From the University of Winchester: Ben Padbury, Glenn Rose, Kelly Wainwright,
Daniel Wright
From the University of Washington: Keithlyn Alcantara, Jesurun Stockdill, Erica Redd
From the University of Kansas: Sara Robison.
From the University of Georgia: Genevieve Holdridge
From Case Western Reserve University, Cleveland: Anna Wieser
6
References
Jones, A.P., Tucker, M.E. and Hart, J.K. (1999) Guidelines and recommendations. In
A.P. Jones, M.E. Hart and J.K. Hart (eds.) The description and analysis of
Quaternary stratigraphic field sections. Technical Guide 7, Quaternary Research
Association, London, 27-62.
Munsell Color (2000) Munsell soil color charts. Munsell Color, New Windsor (NY).
Reille, M. (1992) New pollen-analytical research in Corsica: the problem of Quercus
ilex L. and Erica arborea L., the origin of Pinus halepensis Miller forests. New
Phytologist 141, 291–307.
Tucker, M.E. (1982) Sedimentary rocks in the field. Wiley, Chichester.
Wilkinson, K.N., Branch, N. and Swindle, G. (2007) Geoarchaeological study of
coastal lagoons at Plana di l’Olmu and Etang de Crovani: A report based on fieldwork
undertaken
in
2006
for
La
Balagne
Landscape
Project.
http://www2.winchester.ac.uk/archaeology/Downloads/BAL06_Geoarchaeology.pdf.
(Accessed 3 February 2009).
16

2008 geoarchaeological interim report

Transcription

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