GEOSSITIO das Aguas Quentes
(Ladeira dos Envendos)
POINT 1 - Coordinates
39°36'37.0“N 7°51'36.5”W
The Neoproterozoic Padrão-Silveira Unit is generally characterized by alternating dark phyllites, often laminated, and sets of metagrauwacke beds, sometimes coarse, and microconglomerates. These latter strata, amalgamated and with a thickness that can reach 4-5 m, consist of coarse to very coarse sediments, progressively decreasing in grain size to finer materials at the top of the beds. They consist of isolated clasts of quartz, lidite, volcani c rock fragments, feldspars, and dark-colored phosphate-rich clayey intraclasts (?), varying in size from 0.1 mm to 3-4 mm, within a gray silty-clay matrix. The bases of these benches are highly erosive, sometimes showing load figures that develop in a “flame” shape. The top is often eroded by overlapping layers or with fine lithological decantation terms. The finer metagrauvaque benches have a more homogeneous grain size and are sometimes laminated.
POINT 2 - Coordinates
39°36'33.1“N 7°51'37.2”W
Armorican Quartzite Formation, Lower Ordovician. It is generally characterized by beds of metagrauwacke, usually amalgamated and several tens of meters thick, relative to the strata of metasiltstone and metapelite.
The Armorican Quartzite Formation consists, from bottom to top, of conglomerates and arkosic sandstones, occasionally bioturbated (Serra do Carvalhal Member). This is overlaid by a set of quartzite beds varying in thickness between 1.5 m and 0.3 m (Ribeira da Cabroeira Member). Above the previous quartzite beds are greenish-gray silt-sand and silt strata, ranging in thickness from centimeters to meters, interspersed with gray quartzites and pelites (Vale da Mua Member). This member is approximately 80 m thick. The Armorican Quartzite Formation ends with the appearance of a set of metric to centimeter-thick quartzite benches, often with an abundance of ichnofossils, namely Cruziana and, more sparsely, Skolithos, reaching a thickness of about 70-80 m (Ribeira da Cabroeira Member).
The Armorican Quartzite Formation shows a more marked lateral variation in lithofacies on the southern flank of the Amêndoa-Carvoeiro synform. Its thickness and lithofacies, particularly on the southern edge of the larger structure, show great diversity due to its own paleogeographic characteristics, probably related to tectonic movements that produce different rates of subsidence, near the contact zone between the ZCI and ZOM.
On the northern edge, there is greater homogeneity, both in thickness (80-100 m) and in the lithofacies of this unit.
The Armorican Quartzite Formation is characterized by abundant ichnofossils, mainly of the Skolithos type (Skolithos linearis Haldemann, Skolithos dufrenoyi Rouault, and Skolithos isp. identified in the Carvalhal and Galega mountains and in Pico do Ar, Robalo, and Cruziana (Cruziana goldfussi Rouault, Cruziana furcifera D'Orbigny, Cruziana rugosa D'Orbigny and Cruziana isp. in Amêndoa, Gargantada and Capela, and Cruziana bagnolensis Moriére, Cruziana cf. cordieri Rouault and Cruziana beirensis Delgado, respectively, in Gargantada, Serra da Galega and Amêndoa). The Skolithos ichnofacies show low diversity and occur preferentially in the lower part of this formation and more occasionally in some quartzite benches scattered throughout its upper part. The ichnogenera of the Cruziana association are highly diverse and are found predominantly in the upper part of this unit, particularly in the Vale da Mua Member.
The globally transgressive sequence of the Armorican Quartzite Formation is generally considered to have been deposited in a very extensive and shallow marine coastal platform environment, where the action of tides, waves, coastal currents, and storms predominate, with very local deltaic fan deposition possible.
POINT 3 - Coordinates
39°36'30.8“N 7°51'40.7”W
Brejo Fundeiro Formation, from the Middle Ordovician.
This unit, approximately 150 to 200 m thick, consists generally of a pelitic sequence, rich in fossils, interbedded at the base with thin quartzite bars and siltstones. Within this unit, a lower assembly was identified, approximately 50-65 m thick, which begins with a ferruginous lenticular microconglomeratic horizon a few centimeters thick and can be observed in Zimbreira and Vale. This darker-colored level contains clasts of various types (sandstones, phosphate pelites, etc.), lingulid fragments, and heavy minerals (monazite, zircon, and tourmaline). This horizon is locally interbedded in the silt-pelitic lithologies that occur in the first meters of the lower member of the Brejo Fundeiro Formation.
This horizon is overlain by a 100-120 m thick sequence consisting of dark pelites interbedded, at times, with siltstones and, more occasionally, with centimeter-thick lenticular strata formed by intraformational conglomerates (Fonte da Amêndoa Member, MFA). The pelitic benches typically show small-scale parallel lamination and, at the base of the layers, sometimes flame structures; the siltstones are massive or slightly laminated. The intraformational conglomerate strata, varying in thickness from 3-5 cm to 50-60 cm, have a silty-clayey matrix, sometimes ferruginous. Within this matrix, there are silt-like clasts, more or less altered, subrounded to rounded and highly spherical, with channeled geometry and imbricated, as is the case in the Zimbreira region. These conglomerates appear to result from the erosion of benches located immediately below them by the possible action of large waves (storms).
The transition from this formation to the next unit (Monte da Sombadeira Formation) occurs through a rapid transition from pelites, interbedded with siltstones and fine sandstone benches, to micaceous quartz-sandstone benches.
This formation is very rich in fossils, particularly benthic fauna (trilobites, brachiopods, and bivalves). However, there are also pelagic or epipelagic fossils, namely graptolites in smaller quantities, which allow for more precise stratigraphic determinations. The characteristic lithotypes and fauna found indicate that the lithofacies of this formation originated in open platform (offshore) conditions, where the topographic gradient was probably quite low, possibly less than 0.1°.
POINT 4 - Coordinates
39°36'25.0“N 7°51'47.0”W
Boundary between the Brejo Fundeiro Formation (to the right) and the Monte de Sombadeira Formation (to the left).
The Monte de Sombadeira Formation, from the Middle Ordovician, is characterized by a succession of regressive sequences, consisting of quartzites and sandstones interbedded with silt-sandstones, highly micaceous, with few fossils. Each sequence begins with alternating thin to medium beds of pelites and siltstones, often phyllitic, and finely laminated sandstones; on top of these are deposited thin to medium beds of quartzites, sometimes thick, occasionally interbedded with beds of micaceous pelites. Near the top of this formation, there are sets of sandy-quartzite beds 0.5-2 m thick, sometimes interbedded with volcanic rocks with spheroidal alteration 1-2 m thick, recognized in the Estiveira Valley (Corga) and south of the village of Casalinho, on the southern flank of the Amêndoa-Carvoeiro synform. The acidic volcanic intercalations are only a few meters long and disappear laterally between arenitic benches.
The formation consists of sandy lithofacies with tempestitic characteristics, which are relatively homogeneous throughout the entire region under study. However, the thickness can vary markedly, from a few meters to several tens of meters. This unit is characterized by sparse fauna and low diversity of trilobites, brachiopods, and mollusks.
In the silty-pelitic lithofacies, particularly in the bioturbated upper part, several types of ichnofossils occur, namely Planolites, Palaeophycus, Arthraria, Didymaulichnus, and Taenidium.
Its upper limit is marked by the last quartz-arenitic bench, on which a ferruginous conglomeratic level with characteristics similar to the horizon described above sometimes develops, from which pelitic lithologies of the subsequent unit occur.
POINT 5 - Coordinates
39°36'24.0“N 7°51'48.0”W
Boundary between the Monte de Sombadeira Formation (to the right) and the Fonte da Horta Formation (to the left).
The Fonte da Horta Formation, from the Middle Ordovician, corresponds to a pelitic succession consisting of intercalated fossiliferous pellets, often siltstones, more rarely thin to medium beds of dark quartz-arenites and intraformational conglomeratic horizons, generally centimeter thick. Another characteristic feature of this unit is the presence of siltstone intraclasts, which are more or less spherical in shape and vary in size (<10 cm). These occur individually in the pelitic beds or form lenticular strata a few centimeters thick.
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This formation is characterized by macrofauna dominated by bivalves and ostracods, with trilobites, brachiopods, gastropods, mollusks, echinoderms, and rare graptolites also occurring. Ichnofossils of the genera Tomuculum and Palaeophycus frequently appear in this unit, preferably pyritized, and are usually associated with homogeneous pelitic strata.
The upper limit of this unit corresponds to the base of the first quartzite bar belonging to the Cabril Member of the Ribeira do Casalinho Formation. Sometimes, in contact with the base of the previous quartzite bench, there is a level of oolitic iron with phosphates and fragments of siltstones and pelites.
POINT 6 - Coordinates
39°36'22.0“N 7°51'51.0”W
Boundary between the Fonte da Horta Formation (to the right) and the Ribeira do Casalinho Formation (to the left).
The Ribeira do Casalinho Formation, from the Middle Ordovician, consists, in its lower part, of benches of impure quartzites, usually laminated and/or with hummocky cross-stratification, interspersed with dark-colored silt-pelitic alternations (Cabril Member). On top of the eroded top of the last layer of impure quartzite, there is a thin, lenticular conglomerate stratum. This conglomerate is characterized by being clast-supported and having a silty-sandy matrix pigmented by iron oxides, taking on the appearance of breccia in areas of lesser thickness. The subrounded to subangular clasts, measuring a few millimeters in size, are composed of quartzite, quartz, sandstone, siltstone, pelite, and phosphates, with no fossil fragments having been discovered. When forming the conglomeratic breccia, iron oolites and black phosphate clasts can be observed in a heavily altered ferruginous sand matrix.
Above the previous conglomeratic horizon, there are black pelites and siltstones occasionally intercalated with intraclasts or intraclast horizons of silt-pelitic composition with a thickness of a few centimeters (Carregueira Member). Some of these intraclasts contain bryozoans (Casalinho stream) and disseminated pyrite, sometimes aggregated, forming small nodules. This member is overlain by the “Favaçal Layers,” which form the base of the Cabeço do Peão Formation.
POINT 7 - Coordinates
39°36'21.0“N 7°51'55.0”W
Boundary between the Ribeira do Casalinho Formation (to the right) and the Cabeço do Peão Formation (to the left), of the Lower Caradoc.
At this location, it can be observed that the oolitic iron horizon (“Camadas de Favaçal”) at the base of the Upper Ordovician Cabeço do Peão Formation lies on top of 9 m of ferruginous quartzite benches included in the Cabril Member of the Ribeira do Casalinho Formation. At the top of this member, remains of eroded pelites from the Carregueira Member of the Ribeira do Casalinho Formation are still visible on the last quartzite bench. Thus, there is an erosive discontinuity, covered by an oolitic iron horizon consisting of chamosite ooids and remobilized material that includes phosphate clasts and nodules with microfossils (chitinous organisms) from the underlying units. This composition shows a period of intense erosion, particularly of the units underlying the Cabeço do Peão Formation. This erosive episode is responsible for the disappearance of the entirety (about 24 m) of the lithologies that constitute the Carregueira Member and, on a macroscopic scale, highlights a clear cartographic discordance between the Ribeira do Casalinho Formation and the Cabeço do Peão Formation, which can be observed, particularly on the northern flank of the larger Amêndoa-Carvoeiro structure.
The strong erosion of the top of the Ribeira do Casalinho Formation, the variability in thickness of the members that constitute the Cabeço do Peão Formation, within the Amêndoa-Carvoeiro synform, and the presence of centimetric levels of stratiform subvolcanic rocks, with a thickness of decimeters, intercalated in the first strata of the Cabeço do Peão Formation suggest that there may have been a regional-scale tectonic conditioning of crustal distension that occurred during this period of time at the edge of the Gondwana platform. The maximum amplitude of this event would eventually occur in the SW of Sardinia and would be associated with the Sardinian unconformity s. s., a consequence of the movement of the Sardinian-Turkish uplift.
POINT 8 - Coordinates
39°36'19.2“N 7°52'00.2”W
Boundary between the Cabeço do Peão Formation (to the right) and the Ribeira da Laje Formation (to the left).
The Ribeira de Laje Formation, from the Upper Ordovician, is characterized by a regressive sequence, which begins with a conglomeratic horizon on which micaceous pebbles are intercalated with bioturbated sandstones and impure quartzites, which progressively increase in frequency and thickness towards the top of the succession. The conglomeratic horizon is characterized by black pelitic clasts and more or less circular phosphate nodules, measuring a few millimeters in size, scattered within a sandstone matrix. At the top of the succession, amalgamated beds of impure quartzites predominate, normally massive, sometimes with laminated structures with a thickness of about 5 to 10 m.
The formation is characterized by a scarcity of fossil content. However, associated with the strongly bioturbated siltstone and sandstone beds, there is a sparse fauna of poorly preserved brachiopod fragments with evidence of remobilization. Numerous ichnofossils are also observed, namely Bifungites, Chondrites, Palaeophycus, and Skolithos. These ichnofossils are usually associated with lithofacies that have been remobilized by organisms that partially or almost completely destroyed the original sedimentary fabric of these deposits.
Their upper limit corresponds to an erosive surface marked by a centimetric horizon of ferruginous conglomeratic breccia, sometimes with fossil fragments (brachiopods and bryozoans) and altered carbonate clasts (?), pelites, and sandstones, which occurs on the last set of quartzite benches.