Holocenic infill of the Caldas da Rainha tectonic valley. Anthropological impacts and the fast evolution of three coastal lagoons.
by
Dinis, Jorge
Earth Sciences Department, Coimbra University, Largo Marquês de Pombal, 3000-272 Coimbra, Portugal
Coauthors: Costa, Pedro

Introduction Historically, the different uses of the land by humans reflected its organisation as a society keeping an anthropocentric view of Nature and with short-term economic logic. To justify environmental conservation and the respect for natural phenomena as key issues on land use policy, the recent natural or human driven rapid and/or catastrophic changes must be studied and disseminated among policy makers and the public. As in the presented case study, the social memory can be quite short, and even deep changes affecting the economic and social structures can be unknown from the huge majority of the inhabitants of the region. Regional Geology The Caldas da Rainha structure is a main fault (from Pombal to Santa Cruz) with transtensional movements essentially during the Late Jurassic/Early Cretaceous rifting and acting as an inverse fault during the Late Miocene Betic transpression. Both phases included strong diapiric movements affecting the Meso-Cenozoic cover, with the crest of a basement block interacting with the mobile Upper Triassic/Lower Jurassic evaporites, marls and mudstones (Dagorda Formation) that acted as a detachment level. The main morphological effect is the so-called Caldas da Rainha diapir: the erosion of the dome of the elongated diapir created a valley limited by the uplifted borders between Nazaré and Óbidos, mainly Upper Jurassic limestone and siliciclastics (Fig. 1). The Foz do Arelho fault crosses the Caldas da Rainha structure between the coast and Óbidos, and SW of the intersection a gentle syncline probably corresponds to the salt withdrawal to the adjacent diapir. The Óbidos lagoon location is controlled by the coupled influences of the diapir wall uplift and salt withdrawal subsidence; the intersection of the aforementioned structures also provides a connection between the axis of the lagoon and the main diapiric valley. Fig. 1: A: Location of the Caldas da Rainha region. B: Main geomorphologic features, referred locations and synthetic geology of the Caldas da Rainha region. The Flandrian transgression According to Dias et al. (2000), after the Last Glacial Maximum (about 18 kyr BP) the sea-level rose from about -130 to -140 m to the present sea level, reached circa 3500 yr BP. Due to the speed of the Flandrian rise, the deposition did not compensate the coastline retreat and the resulting transgressive contour should be very irregular, with large estuaries drowning the valleys and several islands; the western wall of the Caldas da Rainha diapir separated the ocean from S. Martinho do Porto and Pederneira lagoons and controlled the Óbidos lagoon inlet (Fig. 2). In those very shallow marine to brackish lagoons, several fishing and commercial harbours existed until four centuries ago, some of them with an important role in the Portuguese Discoveries saga. Fig. 2: Reconstruction of early Holocene landscape (lagoons and coast line) against the present distribution of lagoons and the main dune field (north of Pederneira). Based in topography (Army Cartographic Services, 1/25 000) and geology (Portuguese Geological Survey, 1/50 000). Main feeders of clastic sediments indicated (rivers, wind and waves). See also legend of Fig. 1. Regional history and environmental changes Narration by the historian Plini (1st Century AD) of the Roman conquest of Óbidos throughout a naval landing indicates that the Óbidos lagoon was still wide. Significant continental erosion by those times was inferred by the progradational infill of several incised valleys (Hoffman, 1989), probably reflecting the general deforestation within the Roman Empire (Brown, 1995). Later, with the barbarian invasion in the 5th century AD (Vandals, Sueves and Visigoths), the fights between Visigoths and Byzantines, and the conquest of Iberia in AD 711 by the Muslims (Maghribins led by Tarik), in the drainage area of the main rivers feeding the central and northern coast of Portugal (Douro, Mondego, Minho and Vouga) the population was severely reduced and, consequently, many cultivated lands were regained by the natural vegetation (Desprat et al., 2003). The so-called reconquista (recapturing) of Iberia from the Moors progressed southward from the Asturias region. Once the fertile lands of northern Portugal reached the political stability under Christian rule, during the 10th century, the Minho and Douro basins increased the nourishment of the coast. The Mondego basin was a frontier between the northern Christian kingdoms (initially the Kingdom of León and later the Kingdom of Portugal) and the Muslim states during several centuries (10th to 12th AD). When the first King of Portugal (Afonso Henriques) defeated the Muslim armies in several battles in central Portugal (from Coimbra to Lisbon), a period of expansion started: demographic grow, new settlements, and fields again cultivated, mainly reclaimed from forests and swamp areas (Coelho, 1997). During the paroxysm of the Medieval Climatic Optimum the better condition to humans (hotter and moister) seems to have played an important role in deforestation, as all over Europe between AD 1050 and 1250 (Brown, 1995). In the Portuguese coast it was reflected by an intense clastic input, accumulated by longshore drift south of the mouth of the main rivers: Douro - the spit that created the large Aveiro lagoon started in the 10th century AD, but greatly increased in the 12th (Abecassis, 1955). The Mondego produced a large dune field prograding to the hinterland. Trying to prevent the sand invasion, several campaigns of pine forestation were carried out by the late 12th and the 13th centuries, with some success. However, it is noteworthy that this warm period was possibly coupled with a slight rise in sea level, producing a tendency to deposition in proximal areas (estuaries and coastal lagoons). In the studied region, King Afonso Henriques donated the vast domains of the Cistercian Abbey of Alcobaça in 1153 (currently a UNESCO Cultural World Heritage), where the agriculture replaced large areas of forest, and the consequent erosion contributed to the silting up of the nearby lagoons. The 13th century was a turning point toward cooler conditions. In the first half of the 14th century, the Black Death (plague), particularly in AD 1348, and several internal wars produces a crisis in the agricultural landscape (Coelho, 1997; Desprat et al., 2003). This crisis, coupled with a small sea level rise, was responsible by a decrease in sediment supply to the coast and delayed the progradational process in the Portuguese shore (Dias et al., 2000). By this century, the commercial harbours were located up to 6 km inland from the actual coast in Alfeizerão, Valado, Arelho, etc., locations with notorious salt and naval industries. However, in the region the expansion of agriculture with destruction of the natural vegetal cover encouraged the soil erosion from mid 14th to mid 15th centuries, accelerating the infill of the focused lagoon (Henriques, 1992). Until the 19th centuries increased deforestation and agricultural land use, coeval with the dry and cold conditions of the Little Ice Age, drastically reduced the lagoonal area and led to the intense supply of terrigenous material to all the Portuguese coast. This late effect is expressed by the fast grow of sandy barriers, in particular those of the Aveiro lagoon (Abecassis, 1955) but also in the Óbidos and Pederneira (Freitas et al., 1992), and by the translation toward the coast of estuarine harbours like those of the Mondego due to silting (Cunha & Dinis, 1995). One of the main reasons to the increased erosion in the catchment area of the main rivers was the enormous number of large trees cut to face the exponential increase of vessels built during the Portuguese overseas discoveries and trade (mainly 15th to 17th centuries). The surplus of sand in the coast was in part used by the progradation of the beaches over the ocean, and in part was carried by the prevailing winds to the interior, widening the dune fields, like the one north of Nazaré. Only in the late 18th century new campaigns of pine forestation of the dunes western stripe, finished in 1909, could gradually reduce the intense transit of aeolian sand (André & Rebelo, 1992). The sedimentary infill origin and pathway The Holocene infilling of the studied lagoons has different provenances: Fluvial: the fluvial tributaries in the area are short and with relatively small flows, but with significant transport proficiency due to the steep profiles from the surrounding diapir borders. Erosion of friable fluviatile deposits, mainly Upper Jurassic, Lower Cretaceous and Plio-Pleistocene, was the main source. They are concentrated in the axis of the main watercourses reaching the three lagoons (Arnóia, Real, Salir e Alcoa), incised in the Pliocene during the Würm regression. Marine to brackish: The area is affected by a mesotidal regime, with the most frequent range near 2.4 m, a minimum of 0.9 m and a maximum of 3.6 m. The time for flood is shorter than ebb, and so the flow is faster and has greater transport capability during flood, promoting the silting (Freitas et al., 1992). Nowadays, the tide carries sand only about 1 km upstream of the lagoon mouth, but in the past, in particular before the historic close of the inlet by the sand barrier, the influence was inevitably much greater. The marine to brackish nature of an important part of the Holocene deposits is proved by the fossil content, including bivalves, diatoms and calcareous nanoplankton (Zbyszewski & Almeida, 1960; França & Zbyszewski, 1963; Freitas, 1989). The conglomerates with shells at the base of the (presumed) Holocene (França & Zbyszewski, 1963) can be interpreted as deposits of small fan-deltas reworking the fluvial deposits, followed by some intercalation of organic and lignitic muds that can correspond an early Holocene low level. The prevailing W-NW waves (azimuth 305º; Carvalho & Barceló, 1966) generate a dominant southward longshore drift. However, almost all of the bedload is derived to the deep offshore by the Nazaré canyon; this erosive feature is several tens of meters deep near the Nazaré promontory (Vanney & Mougenot, 1990). Accordingly, the sand in the considered coastal reach has a short travel after eroded from the Upper Jurassic and Lower Cretaceous sea cliffs, presenting a morphoscopy transitional from fluvial (Henriques, 1992) to beach (Freitas, 1989). Aeolian: Dune fields, most of them stabilised by vegetation, occurs south of the Óbidos and around the S. Martinho lagoons. But the most outstanding is the southern part of the above referred large dune field – it extends for about 70 km, starting from the Mondego estuary, and has a mean width of near 5 km (approximately 350 km2). The southward progradation ceased due to human forestation, but benefiting from the morphological constrain created by several (at least three) doleritic remnant hills; in particular the S. Bartolomeu hill rises 150 m above the nearby alluvial plain: dunes at that narrow pass are almost 70 m high. Therefore, the silting of the Pederneira lagoon probably had a main contribution from aeolian transport, as suggested by the clear aeolian calibration and morphoscopy of the superficial level of the fields surrounding the Alcoa river. We believe that this extremely flat surface corresponds to the fixation of the wind blown sand by the alluvial plain phreatic level. Biogenic: It includes essentially calcareous bivalves, abundant in several levels, and lignitic muds, concentrated in the lower beds of the (presumed) Holocene. Chemical: In the southernmost part of the valley, karstic springs created calcareous tuffs, with an extension of about 10 km2. Eneolithic ceramics (circa 3000 to 2000 yr BC) were found in the deposits (Zbyszewski et al., 1960). Conclusion The studied-case shows drastic environmental changes both natural and anthropogenic. The changes were extremely fast in a geological perspective, but, overall, the change in morphology, sedimentary systems dynamics and distribution and ecology overstepped the adaptational capability of the regional society, creating or contributing to social crisis. The human activities in the areas surrounding the three lagoons interfered directly in its evolution. However, we must emphasise the “tele-implications” (i.e. delayed or distant) of alterations (natural or human) performed in the drainage basin of the rivers feeding the northwestern coast of Portugal, as an expression of the Continuum naturale concept. Even if it is impossible to revert such impacts, they should be included in future projects as costs and for prevention. Moreover, the knowledge of this chain of processes can contribute to establish a clear cultural shift to the coming generations towards a better interaction between human society and Earth's natural systems. 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Date received: November 18, 2003