A GIS-Assisted Reconstruction of the Holocene Transgressive

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Journal of Coastal Research,Special Issue No. 65, 2013

A GIS-assisted reconstruction of the Holocene transgressive paleosurface 547

A GIS-assisted reconstruction of the Holocene transgressive

paleosurface of Pederneira lowland (W Portugal)

Vera Lopes, Maria da Conceio Freitas, Csar Andrade, Rui Taborda, Rute Ramos, Maria

Alexandra Oliveira

Centro de Geologia da Universidade deLisboa & Departamento de Geologia,

Faculdade de Cincias da Universidade

de Lisboa, Edifcio C6, 1749-016 [email protected]

[email protected]

[email protected]@fc.ul.pt

IDL-Lattex & Departamento deGeologia, Faculdade de Cincias da

Universidade de Lisboa, Edifcio C6,

1749-016 [email protected]

LNEC-Laboratrio Nacional deEngenharia Civil, Avenida do Brasil 101

1700-066 Lisboa

[email protected]

INTRODUCTIONThe Portuguese hydrographic networks experienced their

maximum re-incision episode in the Last Glacial Maximum, circa18 000 years, when the sea level was about 120 m lower thantoday (Dias et al., 2000). From that time onwards, sea level rose at

different rates, drowning former river valleys and originatingcoastal embayements; after circa 5000 BP (~7000 cal BP), when

sea level rise rate dropped significantly, those features evolved to

estuaries/lagoons, most of them remaining until present, althoughin different stages of infill.

The reconstruction of paleoenvironmental changes recorded in

the sedimentary archive of these transitional systems benefits from

the definition of the paleosurface that accommodated theHolocene flooding and from the characterization of themorphological features of the basins where marine sediments

accumulated. Such a characterization is difficult to obtain insettings where geophysical data or dense borehole networks arenot available. This is the case of the Pederneira lowland (Figure

1), where the application of Transient Electromagnetic Method

(TEM) yielded inconclusive results due to poor contrast insediment resistivity and high conductivity of groundwater (Freitaset al., 2010). Also, the borehole data are scarce and non-uniformly

scattered across the lowland; the available data consist of morethan fifty geotechnical or groundwater exploration logs, added by

three boreholes carried out for paleoenvironmental reconstructionunder the "PaleoNaz" project (Freitas et al., 2010; Moreira et al.,

2010).

STUDY AREAThe Pederneira lowland is located in the Portuguese central

coast, between Nazar and S. Martinho do Porto (Figure 1). It

presents a relatively flat and low surface (less than 10 m abovemslmean sea level), with 16 km2, located downstream of a 420

km2wide drainage basin (Freitas et al., 2010). This surface is atpresent an alluvial plain related with the lower reaches of

Alcobaa (resulting from the confluence of Alca and Baarivers), Meio/S. Vicente and Areia/Coz rivers (Freitas et al.,2010). Alluvial sediments represent the most recent depositional

stage and rest upon lagoonal materials deposited under varyingmarine influence during the Early and Middle Holocene (Freitas et

al., 2010; Moreira et al., 2010) (Figure 2).

ABSTRACT

Lopes, V., Freitas, M.C., Andrade, C., Taborda, R., Ramos, R. and Oliveira, M.A. 2013. A GIS-assisted reconstruction

of the Holocene transgressive paleosurface of Pederneira lowland (W Portugal) In: Conley, D.C., Masselink, G.,

Russell, P.E. and OHare, T.J. (eds.),Proceedings 12thInternational Coastal Symposium(Plymouth, England),Journal

of Coastal Research, Special Issue No. 65, pp. 547-551, ISSN 0749-0208.

The Last Glacial Maximum low stand and coeval re-incision of the Portuguese hydrographic network created the space

and defined the shape of the main morphological features available to accommodate the inundation resulting from the

Holocene transgression and the marine sediments deposited in tune with changing base levels. The reconstruction of the

transgressive paleosurface is relevant in paleoenvironmental studies, namely to establish the paleobasin morphology

and evaluate accommodation volume. In the case of the study area (Pederneira lowland), the geological data set

available to reconstruct that surface consists of borehole logs carried out for geotechnics and groundwater and threecores performed for paleoenvironmental reconstruction. The borehole data scatter non-uniformly across the lowland

providing spatially biased information. The surveying of several Transient Electromagnetic Method (TEM) profilesyielded inconclusive results in separating sediment units below and above the transgressive surface due to poor contrast

in sediment resistivity and high conductivity of groundwater. This study addresses a method for estimating that

paleosurface using the combination of a standard geostatistical interpolation procedure (kriging) with a conceptualgeomorphological model. The geomorphological model comprised the sketching of a paleoriver drainage system in

which the position of the thalwegs was defined departing from present-day slope morphology and depths estimated by

adjusting a mathematical curve describing their longitudinal profile to control points. Final adjustments to the

paleodrainage system were constrained by borehole data and expert judgment. This methodology proved to be effective

in an area where limited objective geological and geophysical data are available.

ADDITIONAL INDEX WORDS: GIS geoprocessing, paleosurface, paleoreconstruction, Holocene.

____________________DOI: 10.2112/SI65-093.1 received 07 December 2012; accepted 06

March 2013. Coastal Education & Research Foundation 2013

www.cerf-jcr.org

www.JCRonline.org
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548 Lopes, et al.

The size and irregular shape of the alluvial plain (Figure 1) are

inherited by tectonic constrains imposed by its location within the

Caldas da Rainha diapiric depression (elongated NNE-SSW) andsecondarily by the ESE-WNW fracture of Fervena (cf. Henriquesand Dinis, 2006, for details). In general, the southern border of the

lowland is steep and well-marked in the morphology by thecontact between intensely folded and tilted Upper Jurassic and

Cretaceous limestone and sandstone beds and horizontal alluvialsediments. These slopes are incised by short and active gullies,yielding sediment to the lowland (Henriques, 1996). The northern

border is in general less abrupt, defined in the contact withJurassic or Pliocene detritic materials or recent dunes (Henriques,1996). The alluvial plain (and paleolagoon) comprises two sectors

(Figure 1): Ponte das Barcas-Valado de Frades (A) with 10 km 2

and major axis (7.6 km long) parallel to the shoreline and Valadode Frades-Maiorga (B) with 7.7 km2, both being connected by anarrow (450 m) rocky gorge. Sector A is separated from thecoastal plain adjacent to the ocean by a second 218 m wide rocky

gorge and presents 5 m (above msl) as maximum elevation. Insector B the surface elevation increases to 5-10 m and the plain is

punctuated by gently sloping hills consisting of outcrops ofPliocene and lower Jurassic (Freitas et al., 2010).

CONCEPTUAL GEOMORPHOLOGICAL

MODELDue to the scarcity of boreholes and their non-uniform spatial

distribution (Figure 3), objective data on location and depth of thebasin paleosurface are inadequate to processing using standardgeostatistical interpolation procedures. In this context,

complementary information to constrain the interpolation was

created through a conceptual geomorphological model.The geomorphological model comprised the sketching of a

paleo drainage system, assuming that: (1) the system was exoreic

and outleting at the western gorge, (2) the incision of each thalwegincreases downstream and (3) it was in equilibrium with the baselevel. In agreement with Leopold et al. (1964), Sparks (1972),

Christofoletti (1980), Morisawa (1985), Scheidegger (1991) andHuggett (2003) a river meeting these conditions, develops a

longitudinal concave profile, with higher slopes towards upstreamand a mild decrease in values towards the base level. Among the

attempts to find a mathematical curve that best applies to thelongitudinal profile of rivers, in this study the description of Green

(1934, in Christofoletti, 1980) was adopted:

)log( xpkay (1)where: y is the elevation of a channel section above sea level, aand k are empirical constants, p is the length of the river and x is

the distance of that section to the mouth.

METHODS AND RESULTS

Paleothalweg LocationA digital terrain model (DTM) of the Pederneira lowland and

Figure 1. Location of study area in the Portuguese coast and in

the Nazar - S. Martinho do Porto coastal section. Black line -limit of present day Pederneira lowland and paleolagoon. (A) -

Ponte das Barcas-Valado de Frades sector; (B) - Valado deFrades-Maiorga sector.

Figure 2. Schematic log of core S2, indicating lithostratigraphicunits, chronological data and facies interpretation.


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associated watershed was created (Figure 4) based on 1: 25 000

topographic maps with equidistance of 10 m. The DTM was

analyzed using the Hydrology tool set available in the SpatialAnalyst Tools of ArcGis (v. 10) to compute the present day

drainage network (Figure 4). Good agreement was found betweenthe computed and existing drainage networks in the watershed

area but results were less satisfactory within the alluvial plain,which was extensively engineered for agriculture purposes. Thecomputed network was automatically hierarchized according tothe Strahler (1952) method, leading to the definition of four main

thalweg lines. The intersection of each line with the limits of the

paleolagoon, yielded four benchmarks (Figure 4) used in laterreconstruction of paleothalwegs.

Location of paleothalweg line (Figure 5A) was determined

using a combination of different methods (1) computation of themid distance separating opposite margins of the paleolagoon(Figure 5A); (2) the surface projection of the in depth intersection

of linear extension of the slopes (Figure 5A). These results wereadjusted where insufficient field data were available or conflictingpaths have been obtained, using objective borehole data and thecomputer generated present day network.

The paleothalweg line is composed by a main segment that crosses

both sectors of the lowland and three tributaries, one located in theeastern sector and two in the western sector roughly aligned withits long axis (Figure 5A).

Assignment of Elevation ValuesAfter the planimetric design of the paleothalweg line, its

elevation was defined. For each segment, all nearby points withknown altimetric information have been used to plot the change ofthe elevation with distance. In all cases, the highest point

coincides with the benchmarks referred above and the lowest pointwith the western gorge with an elevation defined by borehole dataat ca. 37 m below present-day msl. This depth is a conservative

estimate of the Pleisto-Holocene boundary at the gorge area, the

shoreline being located further west. The best fit to equation 1 was

Figure 3. Boreholes location and elevations in the region of Pederneira lowland. Arrow indicates location of borehole S2 (see Figure 2).

Figure 4. Watershed limit; Digital Terrain Model (DTM) with apresent day computed network and bench marks.


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550 Lopes, et al.

obtained by trial and error, using the coefficient of determination(r2) as a measure of goodness of fit (Figure 6). The obtained

equation was used to compute elevation across the paleothalwegline.

Paleosurface ReconstructionA reconstruction of the paleosurface (Figure 7) was achieved in

GIS environment using a standard kriging interpolation (using a,spherical model without nugget effect, maximum and minimum ofeight search neighborhoods with a four 45 sectors and

smoothing). Data used in the interpolation consisted of: (1)

borehole logs; (2) points along the paleothalweg line; (3) pointsalong the alluvial plain limits extracted from the DTM (Figure5B).

The computed paleosurface allow characterizing the basin that

accommodated water and sediments throughout the Holocene. The

available volume was estimated in 2x108 m3. The paleodrainagenetwork reflects structural constrains favoring the development of

a large curve in the eastern sector, with the thalweg leaningagainst the southern border and an asymmetric cross-section(Figure 7). In the western sector incision is much more

pronounced, although produced by shorter tributaries, and thecross section comes out more symmetrical.

CONCLUSIONSThis study presents results of a methodology combining limited

geological (borehole) data with a conceptual geomorphological

model within a GIS environment. This allowed reconstructing the

general morphological patterns of the paleosurface of thePederneira lowland that accommodated sediments deposited in

tune with the Holocene flooding, including marine to transitionaland finally assessing the general characteristics of the paleobasinand coeval drainage system.

This methodology proved to be effective in an area where

limited objective geological and geophysical data are available.

Figure 6Projection of the altimetry versus distance with the bestlogarithmic fit superimposed.

Figure 5. Paleothalweg definition A Mid distances, surface projection of the in depth intersection of linear extension of the slopes andpaleothalweg line. B - Location of the points used in the interpolation of the paleosurface.


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ACKNOWLEDGEMENTThe authors acknowledge support of the FCT project

"PaleoNaz" (PTDC/CTE-GEX/65789/2006).

LITERATURE CITEDChristofoletti, A., 1980. Geomorfologia. 2a. Ed. So Paulo: Edgard

Blucher Lda, 188p.Dias, J. M. A., Boski, T., Rodrigues, A. and Magalhes, F., 2000.

Coastline evolution in Portugal since the Last Glacial Maximum untilpresenta synthesis.Marine Geology, 170, 177-186.

Freitas, M.C., Andrade, C., Ramos, R., Cruces, A. and Henriques, V.,2010. Evoluo Paleoambiental da plancie litoral a sul da Nazar desdeo Tardiglaciar, integrao no modelo de evoluo do litoral ocidentalPortugus.Proceedings, Iberian Coastal Holocene PaleoenvironmentalEvolution, Coastal Hope 2010(Lisbon, Portugal), pp.48-53.

Henriques, M.V., 1996. A Faixa litoral entre a Nazar e Peniche.Unidades geomorfolgicas e dinmica actual dos sistemas litorais.vora, Portugal: Universidade de vora, Ph.D. thesis, 575p.

Henriques, M.V. and Dinis, J. L., 2006. Avaliao do enchimentosedimentar holocnico na plancie aluvial da Nazar (EstremaduraPortuguesa).Actas X Colquio Ibrico de Geografia(vora, Portugal),16p.

Huggett, R. J., 2003. Fundamentals of Geomorphology. London:Routledge Fundamentals of Physical Geography, 386p.

Leopold, B.L., Wolman, G.M. and Miller, P.J., 1964. Fluvial processes ingeomorphology. San Francisco: W. H. Freeman and Company, 522p.

Moreira, S., Freitas, M.C., Arajo, M.F., Cruces, A., Andrade, C., Regala,R. and Lopes, V., 2010. Paleoenvironmental evolution of Pederneiralagoon (Nazar, Portugal) using sedimentological and geochemicalproxies. Proceedings, Iberian Coastal Holocene PaleoenvironmentalEvolution, Coastal Hope 2010(Lisboa, Portugal), pp. 76-77.

Morisawa, M., 1985. Geomorphology texts 7, Rivers.New York: LongmanGroup Ltd., 222p.

Scheidegger, E.A., 1991. Theoretical Geomorphology. Third edition.Berlin. Springer-Verlag, 434p.

Sparks, B. W., 1972. Geomorphology . Second edition. Great Britain:Longman Group Ltd., 530p.

Strahler, A. N., 1952. Hypsometric (area-altitude) analysis of erosion altopography. Bulletin Geological Society of America, 63, 10, 1117-1142.

Figure 7. DTM describing the reconstructed paleosurface and selected cross sections.

A GIS-Assisted Reconstruction of the Holocene Transgressive

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