Stratigraphical-sedimentological investgations of new and old outcrops allowed to better reconstruct the post-Variscan Upper Pennsylvanian to Middle Triassic succession in SW Sardinia. It starts with the Upper Pennsylvanian limnic San Giorgio Fm (Lower Rotliegend), formed by alternations of coarse- to mediumgrained siliciclastics. Intercalations of carbonates and volcanic rocks are present. The unit deposed in a narrow warm-wet alluvial-to-lacustrine collapse basin. The Early – Middle (?) Permian red bed Guardia Pisano Fm (Upper Rotliegend) follows: pelites, sandstones, and rare conglomerates are represented. Volcanic rocks are rare. The unit was deposed in a warm-arid alluvial environment of mid-to low-energy. The Guardia Pisano Fm is followed by the red bed Upper Permian – Lower Triassic red bed Rio Is Corras Fm (Sardinian Buntsandstein Group?). It is formed by conglomerates and calcretes with rare sandstones, pelites, and evaporites. This unit deposed in a warm(hot?)-arid fan-delta to lake environment of variable energy. Stratigraphic type-sections have been described for all the mentioned units. Scattered red beds of the Guardia Pisano Fm or the Rio Is Corras Fm are often unconformably posed over the Variscan Cambrian carbonates reliefs. Examples are known in all the SW Sardinia. At Guardia Pisano the limnic fine deposits of the upper San Giorgio Fm are covered through a weak unconformity by the Guardia Pisano Fm red beds pointing to a smooth climatic and environmental transition. The new outcrops found in the S.Giorgio basin area (Iglesias) represent the missing link between the formerly described stratigraphic units. Here the Rio Is Corras Fm conglomerates are conformably posed over red bed alternations of pelites, sandstones, conglomerates, carbonates, and evaporites representing the upper prosecution of the Guardia Pisano Fm: they mark a gradual passage from a low-energy playa to a fan delta lake environment. Preliminary petrographic analysis of the S. Giorgio sandstones classify them as litharenites and sublitharenites. Calclithites have been found. The Guardia Pisano sandstones are litharenites and minor subarkoses. The Rio Is Corras Fm rare sandstones are all calclithites. Compositional and textural maturity trends in times suggest repeated cycles of reworking of the landscape under a (warm-hot?) wet to dry climate. The almost undisturbed superposition of the various units on each other, the growing areal extent in times of the outcropping units area from the Rio S. Giorgio Fm to the Rio Is Corras Fm, the unconformable setting of the younger units also on the basement with the growing distance to the hypothesized main depocenter (S. Giorgio area?), suggest a progressive widening of this depositional continental basin in times, starting as a narrow basin in Late Pennsylvanian times and merging in the end with the wider extensional basins related to the W-Tethys rifting. Subsurface data by drills suggest at least 350 m of thickness.
The Upper Pennsylvanian to Middle Triassic continental succession in SW Sardinia: new key sections, stratigraphic-sedimentological refinements, and petro-sedimentary notes
Costamagna Luca Giacomo
2021-01-01
Abstract
Stratigraphical-sedimentological investgations of new and old outcrops allowed to better reconstruct the post-Variscan Upper Pennsylvanian to Middle Triassic succession in SW Sardinia. It starts with the Upper Pennsylvanian limnic San Giorgio Fm (Lower Rotliegend), formed by alternations of coarse- to mediumgrained siliciclastics. Intercalations of carbonates and volcanic rocks are present. The unit deposed in a narrow warm-wet alluvial-to-lacustrine collapse basin. The Early – Middle (?) Permian red bed Guardia Pisano Fm (Upper Rotliegend) follows: pelites, sandstones, and rare conglomerates are represented. Volcanic rocks are rare. The unit was deposed in a warm-arid alluvial environment of mid-to low-energy. The Guardia Pisano Fm is followed by the red bed Upper Permian – Lower Triassic red bed Rio Is Corras Fm (Sardinian Buntsandstein Group?). It is formed by conglomerates and calcretes with rare sandstones, pelites, and evaporites. This unit deposed in a warm(hot?)-arid fan-delta to lake environment of variable energy. Stratigraphic type-sections have been described for all the mentioned units. Scattered red beds of the Guardia Pisano Fm or the Rio Is Corras Fm are often unconformably posed over the Variscan Cambrian carbonates reliefs. Examples are known in all the SW Sardinia. At Guardia Pisano the limnic fine deposits of the upper San Giorgio Fm are covered through a weak unconformity by the Guardia Pisano Fm red beds pointing to a smooth climatic and environmental transition. The new outcrops found in the S.Giorgio basin area (Iglesias) represent the missing link between the formerly described stratigraphic units. Here the Rio Is Corras Fm conglomerates are conformably posed over red bed alternations of pelites, sandstones, conglomerates, carbonates, and evaporites representing the upper prosecution of the Guardia Pisano Fm: they mark a gradual passage from a low-energy playa to a fan delta lake environment. Preliminary petrographic analysis of the S. Giorgio sandstones classify them as litharenites and sublitharenites. Calclithites have been found. The Guardia Pisano sandstones are litharenites and minor subarkoses. The Rio Is Corras Fm rare sandstones are all calclithites. Compositional and textural maturity trends in times suggest repeated cycles of reworking of the landscape under a (warm-hot?) wet to dry climate. The almost undisturbed superposition of the various units on each other, the growing areal extent in times of the outcropping units area from the Rio S. Giorgio Fm to the Rio Is Corras Fm, the unconformable setting of the younger units also on the basement with the growing distance to the hypothesized main depocenter (S. Giorgio area?), suggest a progressive widening of this depositional continental basin in times, starting as a narrow basin in Late Pennsylvanian times and merging in the end with the wider extensional basins related to the W-Tethys rifting. Subsurface data by drills suggest at least 350 m of thickness.
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