Shore platforms, notches, and marine caves are common yet distinctive morphological
features along the southern coast of Portugal. The central Algarve rocky coast has provided a
favorable setting in which to understand the processes contributing to shore platform
development. This is because the morphology and characteristics of the features vary along
the coast, as do several factors implicated in their development such as wave climate
(including wave direction and frequency), seabed morphology and depth, bedrock lithology,
karstification and geological structure

Albardeiro, L.

Boski, T.

Moura, Delminda

Veiga-Pires, C.

Sapientia

International Symposium “Sea Level Changes : Records, Processes and Modeling”;Giens (France) – September 25-29, 2006SHORE PLATFORMS IN THE ALGARVE (SOUTH PORTUGAL) ROCKY COAST, ANINHERITANCE OF THE LAST INTERGLACIAL STAGE?MOURA D., VEIGA-PIRES C., ALBARDEIRO L., BOSKI T.Centro de Investigação Marinha e Ambiental (CIMA)- Universidade do Algarve, FCMA, Campus de Gambelas,8000-139, Faro (dmoura@ualg.pt)Shore platforms, notches, and marine caves are common yet distinctive morphologicalfeatures along the southern coast of Portugal. The central Algarve rocky coast has provided afavorable setting in which to understand the processes contributing to shore platformdevelopment. This is because the morphology and characteristics of the features vary alongthe coast, as do several factors implicated in their development such as wave climate(including wave direction and frequency), seabed morphology and depth, bedrock lithology,karstification and geological structure.Although shore platforms are a common morphological feature on rocky coasts, thefirst-order factors that determine their genesis remain contentious. This constitutes a challengeto ongoing investigations on shore platforms and, in particular, to studies concerning theinterpretation of raised platforms. Raised platforms (shore platforms higher than the presentmean sea level) have been frequently interpreted as inherited morphological features ofmarine abrasion during the Pleistocene highstands or as the result of emerging coasts [1; 2; 3;4; 5]. Raised platforms may thus represent former sea level highstands and/or coastal uplift, orthey may be formed by sub-aerial weathering. Determining the genesis of such platforms is achallenge in areas where, as in the Algarve coast, several raised platforms occur but containno marine deposits, thereby making it difficult to investigate paleo-shorelines.Large-scale morphologic features characteristic of the study area include: i) step cliffswith at least two different raised shore platforms; ii) merging cliffs; iii) zeta bays; and iv)pocket beaches. The measured wave climate in the area shows that waves approach from theWSW for 90 % of the year on average, and from the ESE during the remaining 10 % [6].Wave height ranges from 0.30 m to 1.8 m, with rare exceptional heights of more than 3.7 m.Such high waves are associated with storms from the SW, during which waves attain anaverage 2-3 m height with a period of 7-8 s [7]. The tidal regime is semidiurnal and mesotidalwith tides ranging from 2.70 to 1.36 m during neap tides and from 3.82 to 0.64 m duringspring tides. The mean rate of relative sea level rise is 1.5 ± 0.2 mm/yr according to data fromthe Lagos tide gauge’s series registered between 1908 and 1987 [8].In order to correlate the shore platforms’ dimensions and morphologies withheterogeneities in bioerosion/bioprotection, karstification, and joint patterns, these aspectswere described by field observation. In the study area two lithofacies were individualizedbased on the rock’s compressive strength which increases with increasing CaCO3 content [9]:a) fine calcarenite with large fossil shells and strength values from 1 to 3 MPa, and b) shellylimestone which strength range from 5 to 15 MPa. Accordingly, and considering that rock’sresistance and hardness are positively correlated, CaCO3 content is used in the present workas a proxy of rock strength. Several pools in the shore platforms’ surfaces remain filled by seawater during low tides. In order to evaluate salt saturation during exposure to evaporationconditions, and then the possibility of salt precipitation, salinity in the water pools wasmeasured with a multiparametric sound. The width, length, and height of each platform weremeasured using a Real Time Kinematic Differential GPS with a 1cm precision.155In the Algarve south coast, the occurrence of shore platforms is closely tied toexposure to the prevailing wave direction. Wave attack is therefore the first order factor onsculpting shore platforms, whereas platform dimensions are influenced by the degree of waveexposure and by the lithology and structural trend of geological layers. Smaller-scalemorphological variations are caused by chemical weathering, bioerosion, and bioprotection.Shore platforms lying above the present tidal range are inherited from older sea-levelhighstands. These platforms are currently undergoing differential subaerial weathering insympathy with local lithologic variations. Karstic features and joints contribute to theroughness and discontinuity of the platforms’ surface. They can promote both reduced orincreased platform’s width by means of water percolation easiness to attain some locals at thecliff-platform junction. Wide platforms occur when platform is scraped into the contactbetween calcarenite and softer sediments, and when sea water washes the surface severaltimes a year. Narrow platforms are due to rock fall or granular disintegration both processesfavored by karst and joint discontinuities.AcknowledgementsThe research was supported by COCHAL Project (POCTI/34162/CTA/00), funded by the PortugueseFoundation of Science and Technology, FEDER and OE.References[1] Bradley, W.C., Griggs, C.B., 1976. Form, genesis and deformation of central California wave-cut platforms.Geological Society of American Bulletin 87 (3), 433-449.[2] Kelsey, H.M., Bockheim, J.G., 1994. Coastal landscape evolution as a function of eustasy and surface upliftrate, Cascadia margin, southern Oregon. Geological Society of American Bulletin 106 (6), 840-854.[3] Trenhaile, A.S., 2001a. Modelling the effect of late Quaternary interglacial sea levels on wave-cut platforms.Marine Geology 172, 205-223.[4] Trenhaile, A.S., 2001b. Modeling the Quaternary evolution of shore platforms and erosional continentalshelves. Earth Surf. Process. Landforms 26, 1103-1128.[5] Chao, R.B., Casais, M.C., Cortizas, A.M., Alberti, A.P., Trenhaile, A.S., 2003. Evolution and inheritance of arock coast: Western Galicia, Northwestern Spain. Earth Surface Processes and Landforms 28, 757-775.[6] Costa, M., 1994. Agitação marítima na costa portuguesa. Anais do Instituto Hidrográfico, (Lisboa) 13, 35-40.[7] Pires, H.N.O., 1989. O clima de Portugal. Alguns aspectos do clima de agitação marítima de interesse para anavegação na costa de Portugal. Instituto Nacional de Meteorologia e Geofísica (INMG), Lisboa, 34 pp.[8] Dias, J.M.A., Taborda, R., 1992. Tidal gauge data in deducing secular trends of relative sea level and crustalmovements in Portugal. Journal of Coastal Research 8 (3), 655-659.[9] Marques, F.M.S. da F., 1997. As arribas do litoral do Algarve: dinâmica, processos e mecanismos. Ph.D.Thesis, Universidade de Lisboa, Portugal, 549 pp.156SEA LEVEL CHANGES : Records, Processes, and ModelingVARIATIONS DU NIVEAU MARIN :Enregistrements, Processus et ModélisationGIENS, France25-29 Septembre, 2006Convenors  -  OrganisateursG. CAMOIN, CNRS-CEREGE, Aix-en-Provence, FranceA. DROXLER, Rice University, Houston, USAC. FULTHORPE, University of Texas, USAK. MILLER, Rutgers University, USA1

Shore Platforms in the Algarve (South Portugal) Rocky Coast, an Inheritance of the Last Interglacial Stage?

Universidade do Algarve

International Symposium “Sea Level Changes : Records, Processes and Modeling”;
Giens (France) – September 25-29, 2006
SHORE PLATFORMS IN THE ALGARVE (SOUTH PORTUGAL) ROCKY COAST, AN
INHERITANCE OF THE LAST INTERGLACIAL STAGE?
MOURA D., VEIGA-PIRES C., ALBARDEIRO L., BOSKI T.
Centro de Investigação Marinha e Ambiental (CIMA)- Universidade do Algarve, FCMA, Campus de Gambelas,
8000-139, Faro (dmoura@ualg.pt)
Shore platforms, notches, and marine caves are common yet distinctive morphological
features along the southern coast of Portugal. The central Algarve rocky coast has provided a
favorable setting in which to understand the processes contributing to shore platform
development. This is because the morphology and characteristics of the features vary along
the coast, as do several factors implicated in their development such as wave climate
(including wave direction and frequency), seabed morphology and depth, bedrock lithology,
karstification and geological structure.
Although shore platforms are a common morphological feature on rocky coasts, the
first-order factors that determine their genesis remain contentious. This constitutes a challenge
to ongoing investigations on shore platforms and, in particular, to studies concerning the
interpretation of raised platforms. Raised platforms (shore platforms higher than the present
mean sea level) have been frequently interpreted as inherited morphological features of
marine abrasion during the Pleistocene highstands or as the result of emerging coasts [1; 2; 3;
4; 5]. Raised platforms may thus represent former sea level highstands and/or coastal uplift, or
they may be formed by sub-aerial weathering. Determining the genesis of such platforms is a
challenge in areas where, as in the Algarve coast, several raised platforms occur but contain
no marine deposits, thereby making it difficult to investigate paleo-shorelines.
Large-scale morphologic features characteristic of the study area include: i) step cliffs
with at least two different raised shore platforms; ii) merging cliffs; iii) zeta bays; and iv)
pocket beaches. The measured wave climate in the area shows that waves approach from the
WSW for 90 % of the year on average, and from the ESE during the remaining 10 % [6].
Wave height ranges from 0.30 m to 1.8 m, with rare exceptional heights of more than 3.7 m.
Such high waves are associated with storms from the SW, during which waves attain an
average 2-3 m height with a period of 7-8 s [7]. The tidal regime is semidiurnal and mesotidal
with tides ranging from 2.70 to 1.36 m during neap tides and from 3.82 to 0.64 m during
spring tides. The mean rate of relative sea level rise is 1.5 ± 0.2 mm/yr according to data from
the Lagos tide gauge’s series registered between 1908 and 1987 [8].
In order to correlate the shore platforms’ dimensions and morphologies with
heterogeneities in bioerosion/bioprotection, karstification, and joint patterns, these aspects
were described by field observation. In the study area two lithofacies were individualized
based on the rock’s compressive strength which increases with increasing CaCO3 content [9]:
a) fine calcarenite with large fossil shells and strength values from 1 to 3 MPa, and b) shelly
limestone which strength range from 5 to 15 MPa. Accordingly, and considering that rock’s
resistance and hardness are positively correlated, CaCO3 content is used in the present work
as a proxy of rock strength. Several pools in the shore platforms’ surfaces remain filled by sea
water during low tides. In order to evaluate salt saturation during exposure to evaporation
conditions, and then the possibility of salt precipitation, salinity in the water pools was
measured with a multiparametric sound. The width, length, and height of each platform were
measured using a Real Time Kinematic Differential GPS with a 1cm precision.
155
In the Algarve south coast, the occurrence of shore platforms is closely tied to
exposure to the prevailing wave direction. Wave attack is therefore the first order factor on
sculpting shore platforms, whereas platform dimensions are influenced by the degree of wave
exposure and by the lithology and structural trend of geological layers. Smaller-scale
morphological variations are caused by chemical weathering, bioerosion, and bioprotection.
Shore platforms lying above the present tidal range are inherited from older sea-level
highstands. These platforms are currently undergoing differential subaerial weathering in
sympathy with local lithologic variations. Karstic features and joints contribute to the
roughness and discontinuity of the platforms’ surface. They can promote both reduced or
increased platform’s width by means of water percolation easiness to attain some locals at the
cliff-platform junction. Wide platforms occur when platform is scraped into the contact
between calcarenite and softer sediments, and when sea water washes the surface several
times a year. Narrow platforms are due to rock fall or granular disintegration both processes
favored by karst and joint discontinuities.
Acknowledgements
The research was supported by COCHAL Project (POCTI/34162/CTA/00), funded by the Portuguese
Foundation of Science and Technology, FEDER and OE.
References
[1] Bradley, W.C., Griggs, C.B., 1976. Form, genesis and deformation of central California wave-cut platforms.
Geological Society of American Bulletin 87 (3), 433-449.
[2] Kelsey, H.M., Bockheim, J.G., 1994. Coastal landscape evolution as a function of eustasy and surface uplift
rate, Cascadia margin, southern Oregon. Geological Society of American Bulletin 106 (6), 840-854.
[3] Trenhaile, A.S., 2001a. Modelling the effect of late Quaternary interglacial sea levels on wave-cut platforms.
Marine Geology 172, 205-223.
[4] Trenhaile, A.S., 2001b. Modeling the Quaternary evolution of shore platforms and erosional continental
shelves. Earth Surf. Process. Landforms 26, 1103-1128.
[5] Chao, R.B., Casais, M.C., Cortizas, A.M., Alberti, A.P., Trenhaile, A.S., 2003. Evolution and inheritance of a
rock coast: Western Galicia, Northwestern Spain. Earth Surface Processes and Landforms 28, 757-775.
[6] Costa, M., 1994. Agitação marítima na costa portuguesa. Anais do Instituto Hidrográfico, (Lisboa) 13, 35-40.
[7] Pires, H.N.O., 1989. O clima de Portugal. Alguns aspectos do clima de agitação marítima de interesse para a
navegação na costa de Portugal. Instituto Nacional de Meteorologia e Geofísica (INMG), Lisboa, 34 pp.
[8] Dias, J.M.A., Taborda, R., 1992. Tidal gauge data in deducing secular trends of relative sea level and crustal
movements in Portugal. Journal of Coastal Research 8 (3), 655-659.
[9] Marques, F.M.S. da F., 1997. As arribas do litoral do Algarve: dinâmica, processos e mecanismos. Ph.D.
Thesis, Universidade de Lisboa, Portugal, 549 pp.
156
SEA LEVEL CHANGES : 
Records, Processes, and Modeling
VARIATIONS DU NIVEAU MARIN :
Enregistrements, Processus et Modélisation
GIENS, France
25-29 Septembre, 2006
Convenors  -  Organisateurs
G. CAMOIN, CNRS-CEREGE, Aix-en-Provence, France
A. DROXLER, Rice University, Houston, USA
C. FULTHORPE, University of Texas, USA
K. MILLER, Rutgers University, USA
1


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Shore Platforms in the Algarve (South Portugal) Rocky Coast, an Inheritance of the Last Interglacial Stage?

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