Palynology of the Triassic - Jurassic of the Silves Group in Portugal, its implications on a changing world

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Palynology and palynofacies studies in the lowermost Jurassic of the Lusitanian Basin (Pereiros Formation of the Silves Group), Portugal: evidence of the first transgressive episode

Publication . Vilas Boas, Margarida; Pereira, Zélia; Cirilli, Simonetta; Duarte, Luís Vítor; Sêco, Sérgio Luís Rodrigues; Fernandes, Paulo

The Silves Group of the Lusitanian Basin in Portugal represents the initial infill of the continental rifting basins that formed during the breakup of northern Pangaea regions. Evaporites, especially halite, mark the transition from continental to marine settings and the beginning of the deposition in passive margin basins. This work presents the results of the palynostratigraphic and palynofacies analysis of two partial sections from the Pereiros Formation at the top of the Silves Group. The two sections are composed of sandstones, mudstones and dolostones interpreted as deposited in fluvial and lacustrine settings without apparent marine influence. The palynological content is diverse and wellpreserved, dating both sections to the early Hettangian (Lower Jurassic), indicated by the presence of spores Ischyosporites variegatus, Kraeuselisporites reissingeri, Porcellispora longdonensis and the pollen grains Perinopollenites elatoides and Pinuspollenites minimus. The palynological content of one of the sections (Lamas I) is noticeable by microforaminifera linings, suggesting evidence for a hitherto marine incursion at this age in the Silves Group stratigraphy. The beds that yielded the microforaminifera linings are interpreted as having been deposited in an estuarine-type setting, created by the first and short-lived marine transgressive event in the Lusitanian Basin during the early Hettangian.

2023Journal article

Open Access

The age of the first pulse of continental rifting associated with the breakup of Pangea in Southwest Iberia: new palynological evidence

Publication . Vilas Boas, Margarida; Paterson, Niall W.; Pereira, Zélia; Fernandes, Paulo; Cirilli, Simonetta

In this work, we report the first palynological age for the base strata of the Silves Sandstones of the Silves Group in the Algarve Basin, located in Southern Portugal. The group is the oldest sedimentary unit of the Algarve Basin and was deposited unconformably over late Pennsylvanian turbidites of the Mira Formation, which were folded and faulted during the Variscan Orogeny. The Silves Group comprises a detrital red bed succession, representing the earliest phase of sedimentation associated with the initial rifting of Pangaea. Macrofossils are rare, occurring predominantly in the top layers of this group, and do not accurately constrain the age of the entire group's deposition. From an outcrop exposed in the central Algarve, a grey mudstone bed positioned 2.5 m above the Variscan unconformity plane yielded palynomorphs that date the beginning of sedimentation in this basin to the early Carnian age (Late Triassic). The moderately well preserved and low-diversity palynological association comprises Aulisporites astigmosus, Enzonalasporites densus, Ovalipollis pseudoalatus, Samaropollenites speciosus, Tulesporites briscoensis and Vallasporites ignacii, among others, and is indicative of an early Carnian age.

2022Journal article

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New insights on the upper triassic silves group in Algarve Basin, Portugal: palynological, paleophytogeography and paleoclimatology advances

Publication . Vilas Boas, Margarida; Pereira, Zélia; Cirilli, Simonetta; Fernandes, Paulo

This paper presents the results of palynostratigraphic studies in the Silves Group in the Algarve Basin, Portugal. From bottom to top comprises the Silves Sandstones, the Silves Marl-Carbonate Evaporitic Complex, and the Volcano-Sedimentary Series. This study aims to detail the age of the Silves Group, bracketing the Triassic-Jurassic transition, using palynology. For this purpose, 250 samples were collected from 14 main sections. Previous results from a section above the Variscan unconformity, enabled to date the base of the Silves Sandstones and the onset of the Mesozoic sedimentary cycle in the Algarve Basin to lower Carnian. In this work, the top of the Silves Sandstones, containing Camerosporites secatus, Enzonalasporites vigens, Granuloperculatipollis rudis, Lagenella martinii, Patinasporites densus, Samaropollenites speciosus, and Vallasporites ignacii, is dated to the upper Carnian. The base of the Silves Marl-Carbonate Evaporitic Complex, consisting of Alisporites sp., Araucariacites australis, Classopollis meyerianus, Classopollis torosus, Paracirculina quadruplicis and Triadispora sp., indicates an upper Carnian age. The presence of Alisporites diaphanus, Araucariacites australis, Cerebropollenites macroverrucosus, Classopollis meyerianus, Classopollis torosus, Perinopollenites elatoides, Calamospora mesozoica, and Kraeuselisporites reissingeri allows to date the top of the Silves Marl-Carbonate Evaporitic Complex as upper Rhaetian-lower Hettangian. This study allows to date the Silves Group in the Algarve Basin from the lower Carnian to lower Hettangian (Triassic-Jurassic boundary) for the first time. The Carnian microflora provides new insights of the Onslow Microflora in the Western Tethys.

2024-10Journal article

Open Access