Middle Terrace Deposits of the Tagus River in Alpiarça (2001)

8/8/2019 Middle Terrace Deposits of the Tagus River in Alpiara (2001)

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2/13

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).

MOZZI ET AL.360


3/13

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.

TERRACE STRATIGRAPHY AND ARCHEOLOGY, PORTUGAL 361


4/13

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.

MOZZI ET AL.362


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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



6/13

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).

MOZZI ET AL.364


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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).



8/13

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.

MOZZI ET AL.366


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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.



10/13

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).

MOZZI ET AL.368


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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)



12/13

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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Middle Terrace Deposits of the Tagus River in Alpiarça (2001)

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