Abstract
LOUSÃ AND ARGANIL BASINS. GEOMORPHOLOGICAL AND SEDIMENTOLOGICAL STUDY OF THE HERCYNIAN BASEMENT AND ITS COVER, EAST OF COIMBRA
The present memoir sums up the results of investigations fulfilled by PIERRE BIROT and ORLANDO RIBEIRO thence a, half a century forth and taken again from 1971 onwards under the direction of SUZANNE DAVEAU, who is the redactor of this memoir. After an introduction of the region and its problems, the first volume refers to the sediments remaining in the Lousã and Arganil basins. The second one refers to the regional evolution of landformes and led to a general synthesis of the achieved results. Besides the figures and maps enclosed in the text, each volume contains a photo album and a serie of maps. The bibliography can be found at the end of the second volume.
VOLUME I
THE SEDIMENTARY BASIN
PART ONE, consisting of 3 chapters, is a general introduction to the problems of the region (fig. 1).
Chapter I refers to the history of the investigations fulfilled since the middle of the XIXth century on the «Buçaco Sandstones», giving emphasis to the report of four geologists: CARLOS RIBEIRO, VASCONCELOS PEREIRA CABRAL, PAUL CHOFFAT and NERY DELGADO. We owe them the demarcation between a lower sandstone serie, dated through its Cretaceous or Lower Tertiary flora, and a heterometric higher serie which, they thought, might have glaciar origin. Their accurate observations gave some interessant clues on the deposit conditions and subsequent tectonic deformations, showing the importance of these sediments for a tectonic and geomorphologic explanation of the region.
After a long interruption, geomorphologists, sedimentologists and paleontologists reiniciated the research. About 1970 the new knowledge, in relation to 19th century one, consisted the probable demarcation between four sedimentary series based on physionomic, sedimentologic and mineralogic evidences, and the Ludian age of a fauna found in the second of these series.
Chapter II refers to the basins structural position and tries the up-to-date of the fastly evolving studies dedicated to the large morpho-structural units where the region is fitted (fig. 2 to 6 and tables I and II). The basins are confined by the great North-South transcurrent fault which separates hercynian Ossa-Morena and Center-Iberic belts; resting on a shale and greywacke complex they are crossed by the roots of two long quartzitic synclines and bordered, on the East, by granitic batholiths. Late hercinian faults pattern has been deformed by posterior tectonic movements.
The analysis of paleogeographic meaning of Trias study by PALAIN (fig. S) shows the facies important changes in a piedmont sedimentary serie and the activity, almost permanent though uneven, of Nazaré fault during the Trias. The Jurassic is characterized by the openning of a North-South sinking trench and by a tectonic steadiness permitting the levelling evolution of the hercynian basement itself. From Callovian henceforth restricted transgression took place on the western edge of Iberian Meseta, then transformed in the oceanic margin of North Atlantic; the most part of sediments reveals, however, continental facies, Which difficult study is in due course (table III).
West of Coimbra, Dogger limestones were cutted by a planation surface developped in biostasic environment. This was covered by clastic layers with inserted limeatones of Cenomanian transgression (fig. 4 to 6). While the lower aeries are new well known, the study of the superior ones comes to a fast development phase.
Chapter III describes and locates in a broad way the main morphological units of the region under study. The Lousã and Arganil basin is one of the inumerous und small tectonic basins, which marks the Atlantic slope of the Iberian Peninsula.
Forming the lower extremity of large Mondego platform, their relief is divided in two parts, separated by Penedos de Gois-Buçaco quartzitic ridge. The problems that come about are referred in a preliminary way, with the aim of turning easier the understanding of the following chapter dedicated to sediments. The pattern of relief and drainage and the chief landscape features of the two raised blocks which frame the basins are also mentioned: the Cordilheira Central on South-East and the Marginal Ridge on West.
PART TWO, composed by 8 chapters, is concerned to the study of sediments remaining in the Lousã and Arganil basins.
Chapter IV deals with methodological problems raised during the study of this very faulty region, with sediments whose outcrops are numerous but separated; the continental facies, quite unsteady spatially and with scarse fossils were, besides, submitted to deep weathering which could change deeply their characteristics.
The study methods, as well as its development during the investigation, are next introduced.
Finally, the explanatory technic used in the present report is described: first (Chapters V to VII), the establishment of regional types of vertical sequences of sedimentary facies from evidences from the best exposures (fig. 7), then the general featuring, relative chronology and paleogeographic meaning of the main sedimentary series which was possible to distinguish (Chapter VIII to XI).
Chapter V introduces sediments of the occidental part of the basins (Maps B and C, figures 8-12, tables III and IV). The exposures good quality allows the distinction of four sedimentary series: the whitish sandstone of Buçaco at the basis, over which lay the reddish sands of Buçaqueiro, then the greyisch clays of Fábrica do Pisco and, at the top, heterometric pebbles. It is proved that the weathered mineralogic facies with kaolinite, considered for a long time typical of Buçaco sandstones, comes from a sediment originally deposited as a feldapatic sandstone, characterized by montmorillonite. The features of the erosional surface on which the Buçaqueiro sands are laying is analised subsequently and described, finally, the particularities of the sedimentary basin meridional end, sequeezed between the Cordilheira Central and the Marginal Ridge at the passage of the important Nazare-Lousa fault.
Chapter VI concerns to sedimentary layers observed in the hills which turn irregular the central part of the basins (Maps D and E, fig. 13-18). The lower slopes of the sedimentary hill of Sacões show clear exposures revealing the accumulation over the sandatones, first of the arcosic sandstones arith montmorillonite (locally with attapulgite), then the thick clastic layers where the fluvial and the fanglomerate raña facies alternate. Santa Quiteria slopes enlarge the previous evidences revealing that large lateral changes of facies occur frequently. By means of the sedimentary cover of the long quartzitic ridge of Penedos de Gois-Buçaco, the original thickness of Bugaco sandstone can be appraised at a minimum of 150 m; it enables also to state the silicification and tectonic mouvements problems, likewise the posterior erosion which affected this ridge. The occidental branch of the incline, although of lower altitude at the present, is completely free from Buçaco sandstone, when the oriental branch, though higher and massif, is still largely covered by it.
Chapter VII gathers the evidences of the oriental and septentrional parts of the basins where the sedimentary exposures are less continuous (Maps F and G, fig. 19-28, tables V and VI). Arganil basin arcoses are covered by the Travanca piedmont accumulation, consisting of broken shale boulders in caotical arrangement. In Coja basin is located the important exposure of Fábrica da Carriça which arcose is dated from the Ludian by TELLES ANTUNES. The Mondego-Alva interfluve carries sedimentary patches of variable thickness. While it is possible to rebuilt local features of pre-Buçaco surface at Sanguinheda, at other places the facies, besides quite different, were often deeply weathered.
Santa Eufémia quartzitic ridge became levelled and covered by sediments, before a longitudinal compression had replaced a light relief. The characteristics of these sediments lead to a discussion concerning the eastern stretching of Bugaco sandstone and put an hypothesis of regional evolution. Characteristics of the sediments of Mortigua basin are summarised according to previous studies, complemented with a drilling report.
Chapter VIII-XI presents the evidences already mentioned, now at the scale of the whole basins and according to a, chronological order. The maps H to L sum up the mineralogic features and map M shows a general interpretation of the sediments.
Chapter VIII describes the facies in the Buçaco sandstone serie (fig. 24). Two facies are predominant: the pure quartz sandstone with kaolinite and the feldspathic sandstone with montmorillonite. The former results from the second one by weathering. The deposit seems to have occurred essentially as sands, carried by low velocity rivers, in a chemical environment less active, that the feldspath grains overlast to a transportation by tens of kilometres. The discussion concerning the problems raised by the other facies comes next. The silicification which might have occurred several times is always in a close spatial correlation with the quartzitic ridges. A few greyish clay levels supplied pollens. The facies with smoothed pebbles seem to resuit from now and then washing of those retained for a long time in the upward areas. The angular breccias closed to quartzitic ridges show that a dynamic fragmentation worked, at the same time with the deposition of sandstones. Although very changing from place to place the facies remain on the whole the same in all the basins extent, including next to the faults which limit them.
Chapter IX discusses the Buçaco sandstone paleogeographic environment (fig. 25 and 26). Its sedimentation area run to East, at least up to the granitic exposures, not being limited neither by Nazare-Lousa fault in South-East, nor by the important meridian fault in West. The sandstones covered smooth shale plains, abruptly dominated by the quartzitic residual ridges, not being yet decided whether its culminant levelling is pre- or inter-Buçaco. The problem concerning Penedos de Gois-Bugaco ridge crossing by the rivers which deposited the sandstones, is not 'yet cleared up.
The age determination of the sandstones is based on the stratigraphic, position regarding the two deposits with Debeya lusitanica (Sula, and Vale de Madeira) and the two layers of dark clay with pollens (Vila Flor and Carvalhais). Probably sedimentation occurred since the Cenomanien, lasting to the Cretaceous utmost limit. The mouvement of Nazaré fault might have its origin from the ocean in lower Cenomanien, and spread progressively landward along the Senonien, causing a long erosive phase of previous deposited sandstones.
Chapter X refers to both sedimentary series, the Buçaqueiro sands in West and the Coja arkoses in East. The Buçaqueiro reddish sands are an improverished sediment, next to pure quartz, predominating almost entirely the kaolinite. The exposures are mainly in West of the basins and seems to result from the erosion of a reddish soil, developed on the Buçaco sands. The place of Via Longa (fig. 27) shows that Nazarene fault moved smoothly before the sands have been deposited, followed by a stronger mouvement.
On the contrary, the Coja arkoses facies is yet quite close to its granitic and complementary shale sources; deposited in a restricted environment, it is featured by the montmorillonite predominance and locally by attapulgite, calcareous concretions and weathered granitic pebbles. Ludian dated, it is posterior to the paroxysmic phase of the Pyrenean orogeny.
Table IV sums up the possible regional evolution from the Cretaceous to the Oligocene. After the long phase of weathering and erosion of the Bugaco sandstone, a perfect planation surface might be formed where uranium loaded waters run. The uplift of the western part of the basins is responsible for the Coja arkose deposition in the eastern part (fig. 28). The present Cordilheira Central horst would only take shape afterwards.
Chapter XI refers to the sediments correspondent to the Cordilheira Central uplift (fig. 28 and 29). It discusses first the problem concerning the Coja arkose passage to those sediments which clearly derive from the erosion of a shale horst. It appears to have existed a period of transition during which sediments from several sources converged towards the eastern part of the basins; a first thrust up of the Cordilheira Central occurred precociously, eventually from the Oligocene on. The great variety of the facies turn interpretation hard. The shale fluviatile series are predominant at the bottom and the quartzitic raña topward; however, numerous recurrences of facies can be seen. It is in the central part of the basins, from Sacoes to Coja, that fanglomerate piedmont deposits are moat abundant, yet patches of them do exist not only in West but also to North. It is introduced an actualization of opinions concerning the piedmont fanglomerates and particularly the raña facies. Several generations have to be determined, yet this problem will be discussed later, along with the landforms analysis which becomes, from the individualization of the Cordilheira Central horst on, the best of the guides.
VOLUME II
LANDFORMS EVOLUTION
The second volume, which contains Parts 3 to 5 of this memoir, is dedicated to the geomorphclogical evolution from the uplift of the western part of Cordilheira Central on. Part three is composed by three chapters; after the presentation of different methodological problems it deals with the shale mountains, which dominate the basins on its south-eastern rim. Part fourth examines, in five chapters, the evolution of the basins and of its occidental rim. Last, Part five seeks the looking over of the achieved results and of the yet not cleared up problems.
PART THREE
Chapter XII shows investigation methods and its fit to two quite different environments (shale uplift blocks and basins still partly filled by complex sediments). Geometric forms analysis is fruitful in a region, where landforms are built up, in an almost schematic way, by planed surfaces, bounded by vigorous slopes. Several graphic techniques consist its support: hypsometry analysis (Maps N-P), planed surfaces cartography and projected profiles. This analysis is easier on uplift blocks; basins sediments show up complementary information but of sometimes difficult interpretation. The recognition of the role of tectonic mouvements, occured during the landforms evolution, is always difficult. It stands mainly on river system changings and on some vigourous gullies and slopes, being local facts always logically inserted in the general landforms pattern, as depicted by remotesensing recordings.
Chapter XIII describes the top ridges and higher benches of the shale ranges (fig. 30-34). These are built up by two great tilted blocks, Serra do Açor, drained westward by Rio Ceira which runs into the basins at Góis, and Serra da Lousã which dominates them by its abrupter scarp, as it slops gently southward to Rio Zêzere. Quartzitic pinnacles dominated only the shale ridges by a few ten metres. These are narrow, but longitudinally very uniform, testimonies of an erosion surface that proceed from the Eocene on. Its good preservation is explained by the relative permeability of alternate shale and greywackes almost vertical layers. River Simonte regimen, the springs that pear out by the slopes and the irrigated small fields in the ravine bottoms, all are the obvious proof of significant underground flow.
Higher benches are relics of large mature valleys which pattern is quite similar to the pre-sent ones. They were the origin of the thick fluviatile piedmont sediments.
Chapter XIV seeks the difficult reconstruction of the most recent stages of the uplift of the mountainous blocks (fig. 35-39). It stands on the evidences achieved on its southern slope in Rio Zêzere basin. The pattern of planed forms that are located both sides of Cebola fault, which confines northward Zêzere graben, shows that the main mouvement of this fault is more recent than the shaping of the mature valleys. The quartzitic ridge Fajão-Moradal profile (fig. 31) is made of blocks deeply tilted but still little touched by erosion. Some captures suggest a less perfect tectonic steadiness in Zêzere basin than in the uplift northern blocks.
The interference, in Gris region, of piedmont blocks formations with faults and levelled benches, shows that sedimentation did happened there in a marginal subsident trench; then occurred a compressional stage (overthrust of Portela.), after which local levelling of ending stage has slightly marked the mountain rim.
The forms of the valley bottoms brings up new learnings. The incised meanders of Rio Zêzere proceed with lateral evolution, upstream of the up-lifting blocks, while they are entrenching vertically on these, fastening along the joint. Large blocks are separated by a serie of regularly framed WNW-ERE faults, on which small North-South fault-line valleys of vigorous incised forms articulate.
PART FOURTH
Chapter XV describes the intricate landforms pattern of the basins and Marginal Ridge (Maps N-P and fig. 40-41). Graphic analysis of maximum, minimum and relative altitude allows the distinction of spaces mainly shaped by tectonic uplift or headward river erosion and the appraising of local relief energy. Stepped planed surfaces cartography is a difficult undertaking, now that they are imperfect and local forms, afterwards tilted. The culminant planed surfaces, among them those of Góis region already spoken on previous chapter, establish a rather dissimilar set, developed as much on softy rocks as on hard ones. Those of the Marginal Ridge losted any testimony of sedimentar cover which shows that they are more recent than a first phase of uplift and erosion; they were, later on, largely desiccated and carved.
Chapter XVI studies the pattern and evolution of the river systems (fig. 42-48). Several drainage styles exist from East to West. Networks are made by fitted segments, epigenic gorges, junction knots and large wind-gaps. Longitudinal profile of main rivers shows nickpoints placed at unsimilar altitudes, which seems, therefore, to be in close relation with sea-level fluctuations. The altitude pattern of the terraces and benches is much more complex in Ceira valley than in Rio Alva one. Lower Alva shaped successively two large valleys of soft longitudinal profile, at 200 m and 150 m, before entrenching its meanders into shale basement. The complexity of landforms pattern in the Ceira valley shows the intricacies of its history.
Chapter XVII deals with the extention and drainage of Serra da Vila erosion level (fig. 49-54). Inlaid in the highest levels of the brains, it extends broadly on its occidental part. Its altitude is about 300m on Lousã, basin rims where it extends mainly in soft rocks but it develops also on shalom and limestones. Analysing the benches which dominate both the valleys and large wind-gaps some aspects of the correlative river system can be rebuilt in a broad way: the higher Rio Ceira ran to Rio Alva; the superior Dueça flowed till Lousã region; a drainage of meridian orientation existed in the occidental part of the basins and, at last, Mondego and Alva might have found their way out to the Ocean somewhere North of Luso. The Marginal Ridge, next to be uplifted, was then planed on its southern part and incised North by large flat bottomed valleys, which outline shapes the subsequent entrenched gorges.
Chapter XVIII examines the correlative sediments of Serra da Vila level, as well as the tiltings which this underwent (fig. 55-59). While the greater extension of the level is free of sedimentary cover some remnants of it, of difficult interpretation, can be found, bordering Santa Quitéria hill. Another set is situated at North of Miranda do Corvo, on the top of Marginal Ridge and adjoining Buçaqueiro sedimentary hill.
A set of great importance is the one placed on West of Serra do Buçaco. A raña facies deposit seems to have reached the old coastline preserved on Cha da Mata, in a present altitude of 240 m, being its pebbles rounded by the waves. This complex which seems older than Serra da Vila surface, might be correlative of the preceding entrenchment. Other coastlines of lower altitudes (about 170-2IW m) can be observed at South of Luso. They would be a part of Serra da Vila level, also tilted at the same time. Southwest of Coimbra, the Morouços clastic Complex, which is about to study, might bring, in future, important elements of interpretation. A relative subsidence tilted Serra da Vila level, as well at West (limestone plateaus) as at East (confluences region, upstream of Penedos de Gois-Buçaco quartzitic ridge), while the higher raised regions are the northern Marginal Ridge and Serra da Lousã.
Chapter XIX is dedicated to Chã do Freixo level, where the present river system did organized (fig. 60-67). It is a set of leveled forms, of different shapes, stepped in Lousã basin, from 200 to 170 m. The terraces made of large quartzitic blocks which frame Rio Ceira, the large wind-gap which connects Miranda do Corvo and Lousã, the Almalaguez platform and the perched plateau of Vila Nova de Poiares allow to reconstruct the evolution which led to Ceira and Dueça system building up. Later on the opening of lower Mondego gorge interruped its confluence with Rio Ceira. This forms serie seems to be bound downstream to the higher terraces about Coimbra; a relative tectonic steadiness made its formation possible.
Chapter XX speaks, as the following one, of several aspects of recent geomorphologic evolution which, as a rule, are stretching out landforms contrasts, through vigorous valleys entrenchment and small and isolated flat bottom basins opening (fig. 68-71). These, where the Little towns located, line up along two principal directions: along the NNE-SSW Penacova-Verin transcurrent-fault and at Cordilheira Central piedmont. Several types of structural position and shaping are shown by four examples: Ribeira de Coja valley, Miranda do Corvo and Lousã basins, Semide valley. Its bottom and low terraces were shaped during Quaternary cold periods which originated much frest-shattered blocks.
The evolution of Lousã basin and principally that of Miranda suggest that some tectonic instability is going on.
Chapter XXI is dedicated to another recent aspect of landforms evolution: the active linear entrenchments which trigger locally the reactivation of slopes evolution (fig. 72-76). The meanders are landforms of rapid entrenchment, fitted to the structure details. The little valleys with vigourous, slope forms and meridian orientation and the dissymetric ones located between Serra do Buçaco and Mortagua basin are the result of the interference of erosion, structure and recent tectonics.
Longitudinal profile of lower Mondego, as of its terraces and other forms of relative stability which follow its gorge through the basins, permit to fit the recent regional shaping in relation to the last eustatic oscilations.
While many slopes are covered with inherited coluvium mantle, the most ancient of which are dated from Serra da Vila level, present forms of evolution (landslides and gullying, often associated) are acting locally. They result of the interference of many factors, still little known. The learning of climatic holocenic evolution is going to be improved by the palynological studies on course. Two aspects of human activities had important consequences on the landforms evolution: mineral exploitation of Alva alluvium, since the Megalithic period, and the destruction of the forest which rhythm has yet to be determined.
PART FITH, consisting of two Chapters, presents, the sum up of the evidences locally achieved and, then, the general problems of the geomorphological evolution of the Atlantic front of the Iberian Peninsula.
Chapter XXII collects and ordains the conclusions about the featuring of present landforms. Sinthesis is initiated by the recconstitution of the evolution of the three hill chains that divide the basins; they are, from East to West, from the simplest to the most complex one: Santa Eugenia quartzitic swell, Penedos de Gois-Buçaco quartzitic crest and the Marginal Ridge. The collection of all the partial results will allow to set up the chronic of the regional evolution, from Eocene planation surface on. It will be divided into 28 more or less complex episodes. As a simple characterization and ordering of theses, it do not attempt to any absolute datation nor estimation of relative duration. It is a willingly not very elaborated introduction to the results of the local geomorphological analysis.
Chapter XXIII tries, on the contrary, to connect this local chronic with the general stratigraphic, scale and to place it in the spatial context of the Atlantic front of the Iberian Peninsula. A brief historical account of the investigations that were made about this one and of the difficulties then experienced about the separation and datation of the stepped planation levels that fashion it, shows that each of the regional areas did have a peculiar tectonical and erosional history.
On the contrary the remote sensing techniques concerning the continent show that these areas are connected by long faults that, bind them together in an interdependant way. The investigation techniques dedicated to the submarine forms of the continental margin allow to date the main tectonic sedimentary and erosional episodes that marked it.
The plio-quaternary evolution is described, on one hand by means of comparisons with that of Bierzo basin (Galicia) and of Baixo Alentejo shoreland ones and, on other hand according to the stepping and weathering stages of the alluvial and colluvial deposits of the basins.
Table VIII sums up the final proposal of explanation suggested by the present investigation.