A GIS-assisted reconstruction of the Holocene transgressive paleosurface of Pederneira lowland (W Portugal)
Vera Lopes1, Maria da Conceição Freitas1, César Andrade1, Rui Taborda2, Rute Ramos1∞, Maria Alexandra Oliveira1
1Centro de Geologia da Universidade de
Lisboa & Departamento de Geologia,
Faculdade de Ciências da Universidade
de Lisboa, Edifício C6, 1749-016 Lisboa
vplopes@fc.ul.pt
cfreitas@fc.ul.pt
candrade@fc.ul.pt
maoliveira@fc.ul.pt
2 IDL-Lattex & Departamento de
Geologia, Faculdade de Ciências da
Universidade de Lisboa, Edifício C6,
1749-016 Lisboa
rtaborda@fc.ul.pt
1∞ LNEC-Laboratório Nacional de
Engenharia Civil, Avenida do Brasil 101
1700-066 Lisboa
rihramos@fc.ul.pt
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 O’Hare, T.J. (eds.), Proceedings 12th International 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 three cores 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) profiles yielded 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 conceptual geomorphological 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.
Keywords: GIS geoprocessing, paleosurface, paleoreconstruction, Holocene
INTRODUCTION
The Portuguese hydrographic networks experienced their maximum re-incision episode in the Last Glacial Maximum, circa 18 000 years, when the sea level was about 120 m lower than today (Dias et al., 2000). From that time onwards, sea level rose at different rates, drowning former river valleys and originating coastal 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, although in 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 the Holocene flooding and from the characterization of the morphological features of the basins where marine sediments accumulated. Such a characterization is difficult to obtain in settings where geophysical data or dense borehole networks are not 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 in sediment resistivity and high conductivity of groundwater (Freitas et al., 2010). Also, the borehole data are scarce and non-uniformly scattered across the lowland; the available data consist of more than fifty geotechnical or groundwater exploration logs, added by three boreholes carried out for paleoenvironmental reconstruction under the "PaleoNaz" project (Freitas et al., 2010; Moreira et al., 2010).
CONCLUSIONS
This 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 the Pederneira lowland that accommodated sediments deposited in tune with the Holocene flooding, including marine to transitional and finally assessing the general characteristics of the paleobasin and coeval drainage system.
This methodology proved to be effective in an area where limited objective geological and geophysical data are available.
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Journal of Coastal Research: Special Issue 65 - International Coastal Symposium Volume 1: 547-551. 2013
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