middle terrace deposits of the tagus river in alpiarça (2001)
TRANSCRIPT
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suggested the possibility of acquiring data on human prehis-
ory, as well as providing an independent way of dating the
deposits. Zbyszewski and the French archaeologist Henri
Breuil studied the lithic industries and related them to culturalevolutionary models then in use in other European regions,
especially France. An initial chronology was proposed for the
different assemblages (Zbyszewski, 1943; Breuil and Zbyszew-
ski, 1942, 1945, 1946), linking archaeological evidence to the
morpho-stratigraphic setting.
In the following decades, intensive and detailed archae-
ological work has been carried out in the Tagus valley, and
some major palaeolithic sites have been excavated (Raposo
et al., 1985a, 1985b, 1993; Raposo, 1995). This has brought
about a thorough revaluation of previous cultural, typolog-
cal, and chronological assessments for the artifacts. Breuil
and Zbyszewskis studies had been based mainly on the
recognition of characteristic artifacts (i.e., coup-de
poing) and on their degree of weathering. Although
Zbyszewskis morpho-stratigraphic interpretations retainedmany inadequacies (Daveau, 1993), virtually no recent in-
vestigations had been carried out concerning the geology of
Tagus fluvial deposits.
This paper concerns the alluvial stratigraphy of the mid-
dle and lower portions of the Q3 middle terrace sediments
which contain important and partially excavated archaeo-
logical sites. The investigations focused on the minor val-
leys of Vale do Forno and Vale de Atela, located south of
the village of Alpiarca and intersecting the middle terrace in
a direction transverse to the Tagus valley axis (Figs. 1 and
2).
FIG. 1. Simplified geologic map of the lower Tagus valley (after Carvalho, 1968; Barbosa and Pena dos Reis, 1989, 1996; Servicos Geologicos de Portugal
992).
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MIDDLE TERRACE DEPOSITS
In the study area, the best outcrops of the alluvium are found
n the extensive gravel, sand, and clay pits, some of which arestill under excavation (Pedreira Hilario and, partially, Pedreira
do Barro), whereas others are now abandoned (Vale do Forno
and Vale de Atela quarries). In addition, several trenches were
dug, thanks to the collaboration of the Municipality of Alpi-
arca, which provided a bulldozer for several days.
Stratigraphic sections were described in the field, and labo-
ratory sedimentological analyses were carried out for several
samples, shown in cross section AA (Fig. 3), in the strati-
graphic log of Figure 8, and on the map of Figure 2.
Two main lithostratigraphic units have been recognized,
from bottom to top, a gravelly unit, defined as the Lower
Gravels (LG) unit, and a sand-and-mud unit designated the
Upper Sands (US) unit, in which sandy channel deposits are
intercalated with fine-grained overbank sediment and a flood-
plain soilsediment complex (Figs. 3 and 4).
Lower Gravels (LG) Unit
The LG unit consists of coarse, heterometric, subangular to
subrounded quartzitic gravels with coarse sandy matrix; other
lithologies, such as micaceous shale, granite, and ferruginous
concretions comprise 10%. The clast diameter ranges be-
tween 0.5 and 30 cm.
LG gravels are organized in tabular bodies 2 to 3 m thick and
show clast imbrication and some planar cross bedding. Some
minor channels with maximum depth of 1 m and width up to
FIG. 2. Map of the study area, comprising the tributary valleys of Vale do Forno and Vale de Atela. Asterisks indicate main paleolithic archaeological sites.
The almost flat bottoms of these minor valleys, cut into the Pleistocene alluvium of the Tagus River, represent the top of Holocene valley fills (Leeuwaarden
and Janssen, 1985); these confined floodplains extend to the Vale do Tejo alluvial plain, together representing one single geomorphic surface.
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15 m were observed. Lenses of coarse sand with pebbles are
common, range from 0.1 to 1 m thick, and mainly are massive
or have planar cross-stratification.
The exposed part of the unit is ca. 12 m thick; although the
ower part of the unit is not visible, evidence from wells drilled
at the bottom of the Vale do Forno show that the thickness of
he unit likely does not exceed 1520 m. The LG unit lies on
he pre-Quaternary bedrock, which consists of the Miocene
acustrine claystone and marls of the argilas de Tomar for-
mation (Servicos Geologicos de Portugal, 1952; Barbosa andPena dos Reis, 1989; Barbosa, 1995). The upper boundary of
he LG unit is erosional and represents an important uncom-
formity traceable throughout the study area.
Upper Sands (US) Unit
In the study area, the US unit reaches a thickness of 20 m.
ts deposits can be subdivided in two main classes, channel
deposits and overbank deposits.
Channel deposits consist of medium to fine poorly sorted
sand, sometimes with a low percentage of fine and medium
gravels, mainly as channel lags. Sands are mainly massive.
Where sedimentary structures are present, planar cross-
bedding and horizontal stratification are predominant; lateral
accretion facies were not seen. Common rounded grayish to
brownish mud balls 2030 cm in diameter appear mainly as
channel lags but also are dispersed in the sandy levels, espe-
cially in the basal portion of the unit. Most samples examined
with a binocular microscope are angular to subangular grains
of hyaline quartz (but also citrine, rose, smoky, and white) with
clear conchoidal fractures, lustrous surfaces, and abundan
impact marks. A small percentage of grains present a tarnished
surface and some are somewhat ferruginized. Other lithologies
including gypsum, muscovite blades, and jasper, are present in
a 510% average. The heavy mineral associations consist
mainly of staurolite, andalusite, zircon, biotite, and muscovite
as abundant grains; tourmaline and sillimanite are less abun-
FIG. 3. Stratigraphic cross section AA.
FIG. 4. Stratigraphic cross section BB. For legend see Figure 3.
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dant or rare. For the 3 and 4 dimensional classes, most
grains are angular to subangular, except for the zircons, which
are euhedral or subrounded. The clayey matrix of the sands can
represent up to 35% by total weight and is kaolinitic-illitic
6.936%) and montmorillonitic and illitic (113%) (Table 1).
Overbank deposits consist mainly of laminated to very thinly
bedded intercalations of fine sand and silt, together with mas-
sive silts; clay deposits are less widespread. Organic-rich back-
swamp deposits and buried soils are embedded within theoverbank series. The clay fraction of the overbank deposits
have an essentially montmorillonitic character, this mineral
varying between 9.3 and 34.3%, associated with kaolinite
9.127%) and illite (9.214.4%) (Table 1). Attapulgite is
present in some clayey strata in small quantities (16%).
In cross sections AA and BB (Figs. 3 and 4), the lower
portion of the US unit consists almost exclusively of sandy
channel deposits. Their thickness ranges between 1 and 6 m
and generally decreases toward the east and southeast, being
dependent on the geometry of the erosive bounding surface
with the LG unit.Directly on top of these channel deposits lies the more
mportant body of overbank deposits, with a thickness of 68
m and a lateral extension of more than 1 km along a north
south transect. Remarkable is the presence of some fairly
well-developed, 1 to 1.5-m-thick, laterally continuous pa-
eosols, vertically stacked in the stratigraphic series and sepa-
rated by 0.5 to 2-m-thick overbank deposits. These soils were
described and sampled in different locations on freshly cut
profiles (sections AL1, AL2, AL3, AL4).
All buried soils have 30- to 70-cm-thick, dark-brown,
organic-rich A horizons, with wavy or irregular abrupt to clear
ower boundaries and mainly have medium to coarse prismatic
and blocky pedogenic structure; upper boundaries with over-
ying overbank deposits are usually abrupt. Underlying B ho-
rizons are relatively thin (20 40 cm), light olive-brown to light
yellowish-brown, with gradual lower boundaries; root traces,
and 1- to 3-mm-diameter tubular pores due to bioturbation are
common. Redoximorphic features such as reddish and grayish
mottles and ironmanganese oxides are generally abundant and
ubiquitous, the latter both in the form of soft and hard concre-
ions (laterally continuous Fe/Mn oxide cemented horizons up
o 23 cm thick are sometimes present in both B and C
horizons) and as coatings in pores and on peds faces; thestructure usually goes from blocky to prismatic, but also a very
coarse columnar structure was observed with 5- to 6-cm-thick
and 15- to 40-cm-long peds. The B horizons often pass grad-
ually downward to pale yellow C horizons through transitional
BC horizons.
Paleosols have clayey to loamy textures. Within each soil,
ateral variations of textures are present, probably related to a
changing granulometry of the parent material. The most sig-
nificant case is represented by the paleosols exposed at AL1,
which are markedly more clayey than those observed else-
where in the study area. Slickensides (surfaces produced by
TABLE
1
PercentageofClaysPresentin
theStudiedSamplesandTheirRespectiveLithologies
Mineral
ALP1,
1A(%)
ALP2
(%)
AL
P3
(%)
ALP4
(%)
ALP4-1
(%)
ALP5
(%)
AL
P7
(%
)
ALP8
(%)
ALP9
(%)
ALP10
(%)
ALP
11
(%)
ALP12
(%)
ALP12A
(%)
ALP13
(%)
AL
P14
(%)
ALP16,
16A(%)
Average
(%)
M
ontmorillonite
36.0
21.0
21.2
9.3
14.7
26.9
28
.4
33.3
27.2
22.1
34.3
21.5
9.8
26.4
13.3
13.9
22.5
I
llite
20.5
11.8
10.5
14.4
9.2
4.9
14
.1
11.1
9.9
7.5
11.8
9.3
4.1
9.2
6.2
11.2
10.3
K
aolinite
13.5
24.2
15.2
13.3
9.1
30.5
16
.4
29.5
32.8
15.8
27.0
15.0
13.0
20.9
14.0
39.4
20.6
Q
uartz
6.7
4.7
6.2
8.2
11.1
4.3
5
.5
3.7
6.7
2.0
10.1
9.4
3.9
9.6
6.1
6.2
I
llite-quartz
23.2
34.3
43.2
48.9
50.7
21.6
34
.3
23.0
8.0
39.9
24.9
37.9
50.7
39.6
53.8
29.3
35.2
T
exture
Sandy
clay
Fineto
medium
sand
Clay
Clay
Clay
Fineto
medium
sand
Clay
Very
fine
sand
Medium
sand
Very
fine
sand
Cla
y
Fine
sand
Fine
sand
Silty
clay
Cla
yey
sa
nd
Medium
to
coarse
sand
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one soil mass sliding past another, due to the swelling of clays
during changes in moisture content) crossing A and B horizons
are very common and well developed in these more-clayey
paleosols.
Above the paleosol sequence lies a 20- to 40-cm-thick layer,
with a lateral extent of1 km, consisting of laminated silts and
muds containing a high percentage of leaves and carbonaceous
matter. These backswamp deposits should correspond to one of
he Q3 gisement de vegetaux described by Dr. Carlos Teix-
eira (Zbyszewski, 1946). He recognized leaves pertaining to
different kinds of Salix and rhizomes of Nymphea, while the
pollen content was apparently restricted to Ericacea and PinusMontenegro de Andrade, 1944). Vegetal macrofossils, mainly
eaves, were observed by one of us (P.M.) in the muddy
nfillings of abandoned channels at the western end of cross
section BB.
In the field it was evident that the thick flood-plain sedimen-
ary body just described is bounded at the top by a flat to
concave-up erosional surface, probably in phase with the dep-
osition of the overlying channel deposits. The geometry of the
different channel units in this upper portion of the US unit is
abular, with thicknesses not exceeding 34 m; the sediments
are mainly sandy, but medium to coarse gravels are locally
present in the uppermost part of the section. Within the channel
deposits 1- to 2-m-thick, tabular or slightly lens-shaped bodies
of laminated to massive silty overbank deposits are common
Some muddy infillings of abandoned shallow channels are also
present.
ARCHAEOLOGICAL SITES AND ALLUVIAL
STRATIGRAPHY
Ten paleolithic archaeological sites found within the US
sedimentary unit in the study area consist of scattered quartzite
artifacts. Among the four sites that have been recently exca-vated (by L.R.), three are particularly significant because the
large number of artifacts allows for a sound cultural diagnosis
of the industries.
VF1. In this site the lithic industry consists of a not very
evolved Acheulian type (Middle Acheulian). Preliminary ty-
pological analysis shows a high percentage of flaked pebbles,
unifacial choppers, and hand axes, commonly of quite rough
manufacture, with many fewer implements on flakes.
VF8. The lithic assemblage of this level is very rich: in a 20
m2 excavation, about 3000 artifacts were collected. It is an
Upper Acheulian industry with many tools on flakes. Common
FIG. 5. Lithic assemblage from the Upper Acheulian VF8 site (Vale do Forno, Alpiarca): (1) Tayac point; (2 to 7) sidescrapers; (8) denticulate; (9 to 11)
borers; (12 to 15) cores; (16) biface; (17) cleaver. All in quartzite, except 1 and 6 (flint) and 5 (quartz).
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bifaces show a good flaking technique (Fig. 5). In Figure 6 a
plan-view perspective of the scattering of the artifacts on the
excavated archaeological paleo-surface is shown. A concentra-
ion along a NESW direction is evident.
VF3. The 338 artifacts from this site, also known as Ml-
haros (Raposo et al., 1985a), have the characteristics of the
so-called Micoquian type (Late Acheulian) (Fig. 7). A predom-
nance of evolved bifaces (lanceolate and Micoquian ones)
with respect to the more archaic shapes (amigdaloides) and
enhanced flaking skills (bilateral and bifacial simmetry, re-
ouched edges, use of elastic percussion instruments, produc-
ion of highly standardized flakes for tool making) contrast
with an apparent lack of specialization in the flake tools (rec-
ognized by Zbyszewski, 1946), almost no use of Levallois
echnique, and common presence of culturally regressive,
.e., simple, worked pebbles.
In the LG unit, a small number of artifacts, commonly worn
out, have been recovered during several years of archaeologicalsurveying. Further investigations have been seriously hindered by
he building of a dam at the end of the Vale do Forno, which led
o the creation of a small reservoir, the water of which covers most
of the LG sequence. Thus, the existence of still-older Lower
Acheulian and Clactono-Abevillian human occupation of the area
Zbyszewski, 1946) has not yet been verified.
Altogether, the paleolithic cultural sequence within the US unit
s well defined, with assemblages from Middle Acheulian to
Micoquian type. This sequence agrees very well with the different
stratigraphic positions of the sites in the alluvial series (Figs. 3 and
4). The more archaic VF1 site occupies the lowermost position,
lying within the US basal channel deposits in an elongated lens of
fine gravel with sandy matrix, a few meters above the US/LG
unconformity. VF8 is incorporated in the overlying soil/sediment
flood-plain sequence, apparently always beneath the major pa-
leosols and swamp deposits that characterize the upper part of the
section. The Milharos VF3 site is within the upper sandy deposits.
It lies outside the cross section of Figures 3 and 4, but corresponds
stratigraphically to the archaeological level of the AL3 (Pedreira
do Barro) section, where a few artifacts were collected (by P.M.)
from a slightly humiferous fine sandy horizon exposed on the
quarry walls.
During this investigation two other sites were found (by P.M.)
the VF8a site and a new site exposed in the AL4 section. On the
basis of the stratigraphic relations shown in Figure 4, site VF8a
appears to be an extension of VF8. A detailed sketch of the
sedimentary context of site VF8a is given by the log of section
AL2 (Fig. 8). Here the artifacts are associated with a silty clay lens
with common angular to subangular granules and pebbles. Thisfew-cm-thick lens lies in silty to very fine sandy overbank depos-
its, at the base of a ca. 0.5-m-thick soil. A somewhat similar
situation was observed in AL4, where flaked quartzite pebbles lie
at the bottom of minor (1 m deep) scours filled with silt.
TL/OSL DATING
In order to assess the age of the VF8 site, an attempt was
made in 1991 by one of us (L.R.) to date the visible sedimen-
tary sequence. In view of (a) the available samples (sandy to
silty sediments), (b) the expected ages, and (c) the international
FIG. 6. Spatial distribution of artifacts in the excavated area of VF8 site (contour line values express the number of retrieved artifacts; grid squares measure
1 m).
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nterdisciplinary cooperation established between the National
Archaeological Museum of Lisbon and the Department of
Scientific Research of the British Museum, it was decided that
he optically stimulated luminescence (OSL) technique, a vari-
ant of the thermoluminescence method (TL), could be success-
fully used (Aitken, 1985).
The most suitable levels were selected, after careful obser-
vation of the local sediments. Dosimeters with cleaned TL
signal were buried at these levels for one year in order to
measure the annual environmental dose rates by gamma radi-
ation and cosmic rays. When retrieved, the dosimeters weresent to the British laboratory, together with sediment samples
collected in their immediate vicinity. The TL signal obtained
during the voyage was controlled by another dosimeter used
only for this purpose. The laboratory measurements, carried
out by Dr. Nick Debenham, are summarized in Table 2.
In general, the results obtained are close to the upper TL/
OSL dating limit for the sediments, and hence they give
mprecise information. On the basis of probability, they allow
ages to be obtained between ca. 150,000 and 100,000 yr B.P.
However, it is not possible on the basis of these TL/OSL dates
o exclude dates beyond that limit.
During this investigation, as new technologies became avail-
able, we again tried to date the same deposits by TL/OSL,
together with VF8a sediments and younger parts of the US unit
exposed in the upper portion of the AL3 section. Six samples
of silt to fine sand were collected by Dr. Debenham. Again, no
precise results were obtained because of the lack of measurable
TL signal in the sediments, due either to their age or to the
absence of sufficient solar exposure before burial.
DISCUSSION
The stratigraphic and sedimentological observations carried
out in the minor valleys of Vale do Forno, Vale dos Tejeiros
and Vale de Atela indicate the existence of two major strati-
graphic units (LG and US) separated by an important uncom-
formity. The alluvial architecture and lithofacies of the LG unit
are markedly different from those of the US unit.
The coarse, heterometric gravels of the LG unit show some
imbrication and have a laterally persistent thickness which
together with the absence of lateral accretion surfaces, indicate
middle or lateral bar deposition in low-sinuosity channels
Overbank deposits are virtually lacking. The characteristics of
FIG. 7. Lithic assemblage from the Late Acheulian, Micoquian, VF3/Milharos site (Vale do Forno, Alpiarca): (1) lanceolate biface; (2 and 3) micoquian
bifaces; (4 and 5) cleavers. All in quartzite.
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hese amalgamated channel deposits suggest deposition by a
coarse bed-load braided river.
Lithofacies in the US unit are more varied than those in the
LG, as both channel and overbank deposits are well preserved.
The essentially planar character of the sandy current structures
and the absence of epsilon cross stratification in the US channel
deposits point to deposition in low-sinuosity channels with
imited lateral accretion. The overall widths of the channel
belts are estimated to range between 2.5 and 4 km. The US
overbank deposits represent vertical flood-plain aggradation
The thick body extending between ca. 19 and 27 m altitude is
the major flood-plain sequence in the study area. The presence
of several fairly well-developed soils suggests that depositional
events alternated with periods of surface stability or that ac-
cretion rates were alternately higher or lower than the rates of
soil formation. This recurrent switching from one condition to
FIG. 8. Log of section AL 2, with VF8a archaeological site.
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another may be related to migration of the river channels both
away and toward this portion of flood plain following avulsive
events upstream, and/or changes in the recurrence intervals of
he largest floods and associated overbank deposition. The
scarcity of pedogenic features in the overbank fines of the
upper portion of the US unit points to higher, more-constant
aggradation rates.
The abundant redoximorphic features in these paleosols
ndicate the presence of a shallow underground water table
with recurrent fluctuations that made B and C soil horizons
alternate between reducing and oxidizing conditions. The well-
developed swell-and-shrink structures (slickensides) of the
clayey soils in AL1, also testify to the existence of wetting and
drying cycles at the time of soil formation.
Available data on US pollen content (Montenegro de An-
drade, 1944) have to be considered with caution, as they are
biased by methodologies that are inadequate compared to mod-
ern standards (Mateus and Queiroz, 1993). The interpretationof US macrofloral assemblages that imply a mild-temperate
climate similar to that of the present day (Zbyszewski, 1946)
should be tested in future investigations. Nevertheless it is
consistent with the observed sedimentological characteristics
of the deposits, such as the high percentages of kaolinite in the
US clays. On the other hand, the extreme hot, moist climate
inferred from the Dal Cin diagram (Dal Cin, 1968) (Fig. 9) for
the LG unit does not strictly relate to climatic conditions at the
time of LG deposition. In fact, the sedimentological character
istics of this unit suggest that it is mainly a product of rede-
position of the Upper Pliocene Serra de Almeirim conglom-
erates (Carvalho, 1968; Barbosa, 1995; Barbosa and Pena dos
Reis, 1996; Azevedo, 1997), the latter being a very coarse, 20-
to 40-m-thick alluvial unit forming the large plateau within
which lie the Tagus valley and its Quaternary terraces (Fig. 1)
The same may apply to at least part of the coarse fraction of the
US unit. It, thus, follows that the diagram in Figure 9 reflects
average PlioceneQuaternary environmental conditions.
A detailed discussion of the geological conditions that led
to significative changes in lithofacies and alluvial architec-
ture at the LG/US boundary is beyond the scope of this
paper. The magnitude of these changes is such that they
most probably reflect the complex response of the Tagusdepositional system to changing external factors, such as
tectonic activity, climate, and sea level. Tectonics in central
Portugal during the last 2 myr are regarded as having
consisted mainly in a general uplift of ca. 200 500 m, with
maximum estimated rates of about 0.25 mm/yr, also result-
TABLE 2
Thermoluminescence Measurements and Dose Rates for the US Unit in Vale do Forno (Site VF8)
Dated levels
(progressive numbers
indicate top to bottom
stratigraphic positions)
Natural
regen. dose
(grays)
Dose Rates
TL age (years)
Alpha
(Gy/103 yr)
Beta
(Gy/103 yr)
External
(Gy/103 yr)
Total
(Gy/103 yr)
VF01 601 56 1.443 3.060 1.498 6.001 127,000 infinite 26,000
VF02 685 140 1.889 3.582 2.018 7.489 119,000 infinite 32,000VF05 639 79 1.512 3.393 1.706 6.611 117,000 infinite 26,000
VF06 624 66 0.894 3.432 1.485 5.811 124,000
FIG. 9. Dal Cin morphoclimatic index (after Dal Cin, 1968).
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ng in the reactivation of pre-Quaternary tectonic structures
Cabral and Ribeiro, 1989, 1990; Ribeiro et al., 1996).
Neotectonics, therefore, may have been active on temporal
and spatial scales broader than those relative to the evolu-
ion of valley fills, such as those preserved in the Alpiarca
middle terrace sedimentary sequence. The deposition of the
LG and US likely took place under contrasting climatoeu-
static conditions, possibly related to glacial/interglacial cy-
cles.
The stratigraphic positions of the three main excavated ar-
chaeological sites (VF1, VF3, VF8) correspond, from bottom
o top, to increasingly recent lithic industries that show typo-
ogical evidence of different cultural stages. The sedimentary
context shows that the Upper Acheulian sites of Vale do Forno
were incorporated in the vertically aggrading US flood-plain
deposits. From reliable excavation data (site VF8), and direct
observation during this investigation (site VF8a, and sectionAL4), the artifacts are concentrated in thin and elongated
enticular sedimentary bodies within the overbank fines. These
enses possibly represent deposition along minor drainage lines
active on the flood-plain surface during and immediately after
flooding, fed by overbank waters. Notably, in all these sites
artifacts have sharp, fresh edges, and small flakes were found
ogether with much larger worked pebbles. This suggests that
he archaeological materials were probably displaced from
heir original position, but that only limited reworking took
place.
The more-ancient Middle Acheulian VF1 site is associated
with basal US coarse channel deposits. In this sedimentary
environment a higher degree of reworking could be expected
nevertheless, here, too, the artifacts generally have well-
preserved, sharp edges, suggesting only limited transport by
tractive flows in fluvial channels.
Recent archaeological revisions of the Vale do Forno lithic
industries, and their regional comparison with other sites where
identical stages of the same Acheulian techno-complex have
been reported and are better dated (Raposo et al., 1993)
suggest a much more recent chronological framework (Late
Riss to Early Wurm of the Alpine chronology, i.e., ca. 150,000
to 70,000 yr B.P.) than that proposed by the traditional inter-
pretation (Late Mindel to Initial Riss of the Alpine chronology
i.e., ca. 400,000 to 300,000 yr B.P.) (Zbyszewski, 1946; Ser-
vicos Geologicos de Portugal, 1952). In the context of this
disagreement, available TL/OSL dating results are of some
interest because, even if imprecise, they support the archaeo-logical arguments for last interglacial/early phases of the last
glacial ages for the VF1, VF8, and VF3 lithic industries and for
the US deposits that contain them (Table 3).
CONCLUSIONS
The ca. 30-m-thick lower alluvial sequence of the middle
terrace in the Vale do Forno, immediately south of the
village of Alpiarca, consists of two sedimentary units. The
lower (LG unit), consists of channel gravel deposits
whereas the upper (US unit) consists of tabular sandy
TABLE 3
Tentative Correlation of Sedimentary, Cultural, and Chronostratigraphic Evidence in the Vale do Forno Sequence
(Alpiarca, Portugal), in Comparison with Previous Interpretations (Zbyszewski, 1946)
Proposed scheme Zbyszewski (1946)
Stratigraphic
unit TL/OSL Age
Lithic
industries
Archaeological
sites
Stratigraphic
unit Lithology Age
Lithic
industries
C. 10 Sand and gravel Riss IIglaciation
Micoquian VF3 C. 8-9 Clayey sands and fine sandy clays
119,000 infinite
32,000 ytr
C.7 Sands, with lenses of fine to coarse
gravels
Riss I glaciation Micoquian
Upper
Acheulian
VF8 C.6 Yellowish clayey sands, greenish
gray or yellowish sandy clays
Mindel/Riss
interglaciation
Upper
Acheulian
117,000 infinite
26,000 yr
C.5 Gray clay with fossil floral remains
124,000 yr Middle
Acheulian
VF1 C.4 Greenish gray clay, with sandy
portions
Middle
Acheulian
C. 3b Gravel lenses, within yellowish
sands in the upper portion
C. 3a Yellowish white sands Lower
Acheulian
? ? C.2 Reddish gravels and sands,
manganese oxides
Mindel
glaciation
Clactono-
Abevillian
C.1 Yellowish sands
UpperSands(US)
LowerG
ravels
(LG
)
Lastinterglaciation(?)
(Isotopicstage5)
Penultimate
glaciatio
n(?)
(IsotopicS
tage6)
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channel deposits and overbank fines. These two units do
not constitute a single, fining-upward depositional sequence,
as previously inferred, but instead they represent two dis-
inct sedimentary episodes, separated by a major disconti-
nuity.
The US unit is particularly important because several ar-
chaeological sites are intercalated in the fluvial deposits, con-
stituting a continuous geoarchaeological record ranging be-
ween Middle Acheulian and Micoquian. Alluvial stratigraphy
s consistent with the different ages of the sites, the only
presently known Middle Acheulian site being embedded in the
owermost part of the US unit, and the Upper Acheulian and
Micoquian sites being located, respectively, in the floodplain
soil/sediment complex and in the overlying intercalations of
channel and overbank deposits (Figs. 3, 4, and 8). Apparently,
only limited reworking of the archaeological materials took
place; tool production by early human inhabitants of the Tagus
valley was contemporary with deposition of the valley fill
represented by the US unit. Artifacts were not eroded from
older sedimentary formations before being incorporated in theUS fluvial sequence.
TL/OSL dates of US sediments support archaeological ar-
guments that this sedimentary unit was deposited between ca.
150,000 and 70,000 yr B.P. Available data suggest that the US
unit formed under temperate climate, and this implies that this
unit may include sediments of last-interglacial age. If this is the
case, it may also be assumed that the LG/US boundary marks
he adaptation of the fluvial depositional system to changing
climate and eustatic conditions at the onset of the last inter-
glaciation, and that the LG unit is related to environmental
conditions of the penultimate glaciation (Table 3). The generalectonic uplift to which central Portugal has been subjected in
Quaternary times was not a major factor controlling the sedi-
mentary dynamics of the Tagus River during deposition of the
LG and US units.
The preservation of archaeological sites within a rather
continuous and complex fluvial sedimentary record makes
Alpiarca an outstanding site in the context of western Iberia
Quaternary geology and prehistoric archaeology. The possibil-
ty that the sedimentary sequence of the Q3 middle terrace may
record environmental changes related to the transition between
oxygen isotopic stages 6 and 5 (Turon, 1984; Chappel and
Shackleton, 1986; Mangerud, 1989) further highlights its im-
portance.
ACKNOWLEDGMENTS
This investigation benefitted from funding by the European Community
Project ERBCHRX-CT94-0597, coordinated in Portugal by CEIPHAR (Cen-
ro Europeu de Investigacao da Pre-Historia do Alto Ribatejo)-Escola Superior
de Tecnologia of Tomar. A grant was provided to P. Mozzi for fieldwork, and
edimentological analysis was carried out in the Geology Center of the Faculty
of Sciences, University of Lisbon. The Municipality of Alpiarca offered
everal facilities during the survey. We thank the editor and two anonymous
referees of this journal for their comments and suggestions, which helped
improve this paper.
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