GEOSSITIO of Bando Maior (Castelo do Mação)
POINT 1 - Coordinates
39°36'43.8“N 7°58'13.6”W
Bando dos Santos Viewpoint
Decimetric to pluridecimetric benches (about 30 m) of white quartzites with lenticular geometry, generally massive to laminated, from the Bando Member of the Bando dos Santos Formation.
In terms of landscape, to the southeast one can see the S. Mamede mountain range (Castelo de Vide), the quartzite ridge of Ródão (Portas do Ródão) and to the northeast the Gardunha mountain range and, further in the distance, on days with high visibility, the Estrela mountain range.
This site is interesting for explaining the depositional environment of the regressive quartzite sequence of the Bando dos Santos Formation, from the Lower Devonian. The quartzite unit is organized into sequences with thinner layers at the base and thicker layers at the top, in addition to containing numerous fossil tracks and sedimentary figures.
This quartzite complex can be interpreted as channeled deposits associated with high-energy coastal environments that developed near the coastline. On the other hand, the occasional presence of millimeter-thick horizons composed of red pelitic rocks between the sandstone strata suggests that they may reflect a drop in sea level and, eventually, deposition in an oxidizing environment.
At the base of the succession, thin silty-sandy and sandy layers can be observed, which disappear almost completely at the top, where thick layers of quartzite dominate.
The bioturbated sandstone strata in the middle member of the succession, which are white in color and fine to medium in grain size, are characterized by strongly disturbed benches, usually twisted and interrupted due to the activity of organisms. The bases of these benches often feature numerous disordered, low-relief load structures and flame-shaped structures.
Associated with these benches, fossil tracks in the form of vertical tubes (Skolithos and Monocraterion) and horizontal, sometimes sinuous tracks (Planolites and Arthrophycus?) have been recognized.
POINT 2 - Coordinates
39°37'11.9“N 7°59'02.6”W
Fold Field
Folds with an orientation axis close to north-south and a plane sloping slightly to the west. The succession recognized here is dominated by bioturbated sandstones and white quartzites, Member of Chão do Brejo of the Bando dos Santos Formation, from the Lower Devonian, with lenticular geometry, as well as sedimentary structures, namely cross-laminations, channels, and erosion grooves. The quartzite strata with cross-lamination can be interpreted as resulting from the action of storm waves or deposition in an area subject to the action of permanent waves.
This formation corresponds to a quartzite succession, a regressive detrital sequence, characterized by a lower member (Corga Member), consisting of millimeter to centimeter alternations of siltite and pelite strata. This lithological assemblage gradually transitions, towards the top, into benches of bioturbated sandstones and massive, laminated quartzites, which are white in color (Chão do Brejo Member). The top constitutes the upper member (Bando Member) marked by the presence of about 30 m of decimetric to pluridecimetric benches of white quartzites with lenticular geometry, generally massive to laminated.
The beds of pelites and siltstones, ranging from centimeter to millimeter in size and with flat bases and tops, are massive or have very fine lamination and are usually fissile. Sometimes, centimeter-thick beds of impure quartzites with flat geometry are intercalated in the silt-pelitic sediments. The quartzite strata, which are fine-grained and light in color, have hummocky stratification and are sometimes laminated.
The bioturbated sandstone assemblages, which are white and fine to medium-grained, are characterized by heavily disturbed beds, usually twisted and interrupted due to the activity of organisms. The bases of these benches often feature numerous disordered, low-relief load structures and “flaming” structures. Associated with these benches, fossil tracks in the form of vertical tubes (Skolithos and Monocraterion) and horizontal, sometimes sinuous tracks (Pianolites and Arthrophycus?) have been recognized.
The white quartzite benches, with fine to medium grain size and lenticular geometry, are positioned at the top of the sequences. They are characterized by strata with parallel lamination and low-angle cross-stratification, occurring more occasionally in the form of massive strata or sub-flat laminations. Sometimes, millimetric to centimetric silt-pelitic levels are observed between the completely amalgamated quartzite benches and, at other times, very fine ferruginous horizons. These benches generally have a channeled geometry, forming channels that can reach several tens of meters.
This formation is marked by the presence of brachiopods, arthropods, gastropods, bryozoans, bivalves, orthoceras, and an abundance of ichnofossils. Fossil tracks occur throughout the unit, and other faunal elements are concentrated in ferruginous sandy horizons, centimeter thick.
However, isolated macrofossils also occur more occasionally.
The identified ichnofossils correspond to Skolithos, Planolites, Monocraterion, and, more rarely, Cruziana (Chão do Brejo and south of the Santos valley), and are associated with bioturbated quartzite or quartz-sandstone lithofacies. Skolithos and Monocraterion aspects consist of circular vertical tubes with a diameter varying from 0.5 to 2 cm and usually occur in groups of individuals on top of the benches.
POINT 3 - Coordinates
39°36'46.9“N 7°59'54.4”W
Castelo do Mação
Grey to black laminated schists from the Silurian period, Castelo Formation, sometimes carbonaceous, with a thickness of about 40 to 50 m. The schist layers are very thin and gray, so they disintegrate very easily.
Scattered throughout these lithologies are abundant phosphates, iron, and fossil sulfides, namely graptolites, orthoceras, and mollusks (Cardiola interrupta).
Clay phosphate nodules, also fossiliferous, are still common. The nodules vary in shape from almost spherical to ellipsoidal, with dimensions up to 20-30 cm in diameter, and consist of several concentric layers filled with various species of nautiloids (orthocones), preserved in the form of “straws or cigars.”
The fossils are positioned both vertically and horizontally, with dimensions of 2 to 3 cm wide and 4 to 5 cm long.
The outer layers of the nodules are more siliceous and oxidized than the inner layers, where soft gray siltstones predominate, consisting of phosphates and some organic matter.
The Castelo Formation consists of alternating layers of siltstones, often greenish, and gray pelites, sometimes laminated, interspersed with bars of impure quartz-sandstones or sets of quartz-sandstone bars, which tend to progressively decrease in frequency and thickness, or even disappear, towards the top of the sequence.
The quartz sandstone benches are usually finely graded, often with hummocky cross-stratification at the top and, more sparsely, oscillation ripples. The thickness of these benches can reach one meter at the base and only a few centimeters near the top of the succession. Many of these benches have a lenticular development and clear erosive bases. Numerous fossil tracks occur in this unit, which have not been studied, but no fossil organisms have been found.
Observation of the quartz sandstones under a petrographic microscope indicates that they are composed of subangular quartz clasts with corrosion grooves, rare feldspars, green biotites, chlorites, and clayey detrital particles within a quartz-sericite matrix. Heavy minerals also occur accessorily, namely zircon, rutile, sphene, tourmaline, and opaque minerals. The greenish silt-pelitic rocks are micaceous phyllosilicates with abundant quartz, green biotite, and chlorite.
The formation is characterized by the almost complete absence of macrofauna, particularly in lithologies consisting of alternating silt-pelitic and quartz-arenitic beds, without associated black sticks, which are indicative of more energetic environments. Fossil tracks are often abundant in this lithofacies. The upper limit of this unit corresponds to a sudden and rapid transition to the phyllites of the following formation, marked by a level of a few centimeters of iron with very small oolites.
The bioturbated sandstone strata in the middle member of the succession, which are white in color and fine to medium in grain size, are characterized by strongly disturbed benches, usually twisted and interrupted due to the activity of organisms. The bases of these benches often feature numerous disordered, low-relief load structures and flame-shaped structures.
Associated with these benches, fossil tracks in the form of vertical tubes (Skolithos and Monocraterion) and horizontal, sometimes sinuous tracks (Planolites and Arthrophycus?) have been recognized.