Constraining the age of the first pulse of continental rifting associated with the breakup of Pangea in Southwest Iberia

<p>The Algarve Basin is a Mesozoic sedimentary basin located in southern Portugal. The basin was initiated by rifting associated with the opening of the North and Central Atlantic Ocean during the initial breakup of Pangea. Sedimentation commenced with continental red beds, which unconformably overlie folded and faulted late Carboniferous strata. The red bed succession (Silves Sandstones) consists mainly of sandstones and conglomerates at the base, overlain by variegated mudstones interbedded with siltstones and dolomites (Silves Mudstones, Siltstones and Dolomites). The sandstones were deposited in alluvial environments, and the mudstones in alluvial to shallow lacustrine environments. Upper Triassic (Carnian to Norian) macrofossils are scarce in the red bed succession, occurring predominantly in the upper beds of the succession above the Silves Sandstones, and do not accurately constrain the age of the beginning of the Algarve Basin.</p><p>A palynological study of a new road cut outcrop of Silves Sandstones, located in central Algarve, was undertaken in order to ascertain its age. A 3 m thick bed of grey siltstones located ca. 2.5 m above the unconformity yielded age-diagnostic palynomorphs, which date the onset of sedimentation in the basin. Samples from the latter bed yielded a moderately well preserved, low diversity palynomorph assemblage, which is dominated by <em>Aulisporites astigmosus, Converrucosisporites</em> sp. and <em>Tulesporites briscoensis</em>. Other taxa present in the assemblage include <em>Alisporites</em> sp., <em>Calamospora</em> sp., <em>Cycadopites</em> sp., <em>Deltoidospora</em> sp., <em>Ovalipollis</em> cf. <em>ovalis</em>, <em>Triadispora</em> sp., and <em>Vallasporites ignacii</em>.</p><p>The dominance of <em>A. astigmosus</em> together with <em>V. ignacii</em> is indicative of an early Carnian age based on comparison with independently dated sections described elsewhere in Europe. This new dating evidence thus constrains the beginning of sedimentation in the Algarve Basin to the earliest Late Triassic. The co-occurrence of <em>T. briscoensis</em> and <em>A. astigmosus</em> suggests a mixing of palynofloral elements typical of North American and central European Carnian assemblages respectively, which is consistent with the intermediate position of Portugal between those regions. The dominance of phytoclasts and the absence of marine palynomorphs confirms a continental depositional environment as also suggested by sedimentary lithofacies.</p><p><strong>Acknowledgements</strong></p><p>The authors would like to acknowledge the financial support of the Portuguese Foundation of Science and Technology (FCT) with the scholarship with the reference SFRH/BD/144125/2019 and would also like to acknowledge the financial support of the FCT to CIMA through UIDP/00350/2020.</p>

Detrital zircon provenance of Triassic sandstone of the Algarve Basin (SW Iberia): evidence of Gondwanan- and Laurussian-type sources of sediment

Detrital zircon populations from six samples of upper Triassic sandstone (Algarve Basin) were analysed, yielding mostly Precambrian ages. zircon age populations of the Triassic sandstone sampled from the western and central sectors of the basin are distinct, suggesting local recycling and/or lateral changes in their sources. Our findings and the available detrital zircon ages from the Palaeozoic terranes of SW Iberia, Nova Scotia and NW Morocco were jointly examined using the Kolmogorov–Smirnov test and multidimensional scaling diagrams. The obtained results enable direct discrimination of competing Laurussian-type and Gondwanan-type sediment sources, involving recycling and mixing relationships. The detrital zircon populations of the Algarve Triassic sandstone are very different from those of the lower–upper Carboniferous Mértola and Mira formations (South Portuguese Zone), upper Devonian – lower Carboniferous Horta da Torre, Represa and Santa Iria formations (Pulo do Lobo Zone), and the late Carboniferous Santa Susana and early Permian Viar basins, which are ruled out as potential sources. The detrital zircon populations of Triassic sandstone from the central sector and those from the Ossa–Morena Zone Ediacaran–Cambrian siliciclastic rocks, upper Devonian – Carboniferous Ronquillo, Tercenas, Phyllite-Quartzite and Brejeira formations (South Portuguese Zone), and Frasnian siliciclastic rocks of the Pulo do Lobo Zone are not statistically distinguishable. Thus, sedimentation in the central sector was influenced by Gondwanan- and Laurussian-type putative sources exposed in SW Iberia, in contrast to the western sector, where Meguma Terrane and Sehoul Block Cambrian siliciclastic rocks allegedly constituted the main (Laurussian-type) sources. These findings provide insights into the denudation of distinctive source terranes distributed along the late Palaeozoic suture zone that juxtaposed the Laurussian and Gondwanan margins.