The Early OrdovicianAguada de la Perdiz Formation of northern Chile is one of the oldest unmetamorphosed sedimentary units in Chile. Graptolites indicate a late Floian to early Dapingian age of the formation at the Aguada de la Perdiz type locality and small nearby outcrops in Chile. Correlative outcrops on the Argentinian side occur at Huaitiquina, Filo Pircas, Sierra de Guayaos and Lever Mucar. Graptolites are associated with scarce occurrences of brachiopods and conodonts in some outcrops. In the Argentinian Puna the correlative units were assigned to the Coquena Formation. All outcrops consist mainly of volcaniclastic turbidite and ash-rich flow deposits with intercalations of bimodal lavas, volcanic breccias and reworked felsic tuffs. The lithological assemblage reflects deposition in a marine volcaniclastic apron located on the eastern flank of the Famatinian magmatic arc. We propose to collectively group all respective outcrops in the Aguada de la Perdiz Formation thus respecting precedence of its first definition by Garc & iacute;a et al. (1962) because of the common characteristics of the formation on both sides of the international border. New U-Pb detrital zircon ages of a sample (n=124) from the Huaitiquina locality range between 3,530 and 550 Ma and reflect a common polycyclic provenance from older Amazonian sources. The youngest U-Pb zircon age corresponds to the Ediacaran and predates the biostratigraphically defined depositional age by ca. 80 Myr. Thus, the synsedimentary Famatinian felsic volcanism, otherwise common in coeval units, is not reflected in the detrital zircon age record at Huaitiquina. The absence of Famatinian ages may indicate that sediment delivery from the Famatinian arc line source bypassed this site and that erosion of the arc had locally dissected the volcanic edifices and had progressed to access the pre-Pampean Neoproterozoic arc basement.
This study presents a comprehensive characterization of the Lower Pennsylvanian Caleta Loa Metamorphic Complex (CLMC), located in the Coastal Cordillera of northern Chile at around 21 degrees 30 ' S. Through integrated petrographic, geochemical, thermodynamic, and geochronological analyses, the research aims to constrain the pressure-temperature (P-T) conditions of migmatite formation, refine the protolith identity, and assess the regional geodynamic context of the complex. Thermodynamic modelling using P-T pseudosections and mineral composition reveals that the CLMC migmatites formed under upper amphibolite facies conditions, with peak estimates of approximately 3.3-3.4 kbar and 650 degrees C. These results are consistent with a high geothermal gradient and suggest a crustal depth of similar to 13 km during peak metamorphism. U-Pb detrital zircon age patterns indicate a protolith sedimentation age no older than ca. 350 Ma (Early Mississippian), consistent with a genetic correlation with the nearby El Toco Formation, a low-grade metasedimentary unit in the forearc of the SW Gondwana margin. Structural data show evidence of compressional deformation, including top-to-the-west shear indicators and westward verging folds, which are interpreted as syn-anatectic features. These observations, combined with the metamorphic and geochronological data, point to a tectono-thermal event in the SW Gondwana forearc during the Early Pennsylvanian (ca. 318-316 Ma), associated with the so-called 'Toco Orogeny' and potentially linked to contemporaneous inland magmatic activity. The existence of this event supports a scenario of renewed subduction and/or the end of a magmatic lull along the proto-Pacific Gondwana margin. The current juxtaposition of the CLMC and the El Toco Formation on either side of a major fault suggests significant uplift and exhumation of the former prior to the Late Triassic.
This study reevaluates the chronostratigraphic extent and depositional conditions of the President Beaches Formation at the Byers Peninsula, Livingston Island, South Shetland Islands (Antarctica). Our aim is to constrain its age and improve its regional correlation using new palynological evidence. Dinoflagellate cyst assemblages in four productive samples enabled the recognition of the mid-late Valanginian Senoniasphaera tabulata Interval Zone, identified by the presence of its index-species and the coeval occurrence of characteristic dinoflagellate cyst taxa such as Canningia reticulata, Cerbia tabulata, Kaiwaradinium scrutillinum, Spiniferites sp., Oligosphaeridium complex, Circulodinium vermiculatum, and Heslertonia heslertonensis. These data extend the scope of the President Beaches Formation, shifting its upper boundary compared to earlier findings. Low-energy neritic conditions are indicated for the investigated succession, characterized by normal marine nutrient availability and relatively oligotrophic water masses. An elevated continental/marine particle ratio (predominance of saccate pollen grains among sporomorphs) further suggests an offshore setting with significant terrestrial input. Palynofacies data support a mid-to-distal shelf depositional environment, consistent with previous lithofacies evidence. The diversity of sporomorphs indicates the hinterland flora primarily comprised pteridophyte-spore generating plants and coniferous woodlands, with nearly equal amounts of Araucariacites and Podocarpidites. A warm temperate climate with high humidity is also proposed for the mid to late Valanginian in this area.
This study presents a comprehensive characterization of the Lower Pennsylvanian Caleta Loa Metamorphic Complex (CLMC), located in the Coastal Cordillera of northern Chile at around 21°30′ S. Through integrated petrographic, geochemical, thermodynamic, and geochronological analyses, the research aims to constrain the pressure-temperature (P-T) conditions of migmatite formation, refine the protolith identity, and assess the regional geodynamic context of the complex. Thermodynamic modelling using P-T pseudosections and mineral composition reveals that the CLMC migmatites formed under upper amphibolite facies conditions, with peak estimates of approximately 3.3-3.4 kbar and 650 °C. These results are consistent with a high geothermal gradient and suggest a crustal depth of ~13 km during peak metamorphism. U-Pb detrital zircon age patterns indicate a protolith sedimentation age no older than ca. 350 Ma (Early Mississippian), consistent with a genetic correlation with the nearby El Toco Formation, a low-grade metasedimentary unit in the forearc of the SW Gondwana margin. Structural data show evidence of compressional deformation, including top-to-the-west shear indicators and westward verging folds, which are interpreted as syn-anatectic features. These observations, combined with the metamorphic and geochronological data, point to a tectono-thermal event in the SW Gondwana forearc during the Early Pennsylvanian (ca. 318-316 Ma), associated with the so-called ‘Toco Orogeny’ and potentially linked to contemporaneous inland magmatic activity. The existence of this event supports a scenario of renewed subduction and/or the end of a magmatic lull along the proto-Pacific Gondwana margin. The current juxtaposition of the CLMC and the El Toco Formation on either side of a major fault suggests significant uplift and exhumation of the former prior to the Late Triassic.
The polycotylids, mostly short-necked plesiosaurs, were a clade of marine reptiles with an almost cosmopolitan geographical distribution in the Cretaceous Period. In austral continents, like South America, Antarctica and Australia, the presence of this group has remained relatively poorly documented, where apart from some rare specimens, most records are limited to fragmentary material. Reports of this group from South America are infrequent, particularly from Chile. Therefore, a critical review of the fossil record of Upper Cretaceous polycotylids from Chile is presented here. Previous putative reports of the group from the Quiriquina Formation (upper Maastrichtian) of the Arauco Basin (central Chile) were exclusively supported by misidentified axial elements belonging to elasmosaurid plesiosaurs, and are thus dismissed here. On the contrary, the presence of Polycotylidae in upper Campanian-lower Maastrichtian levels of the Dorotea Formation, in the Magallanes Basin (southern Chile), is confirmed. The pattern of taxonomic diversity, including polycotylids together with abundant, mostly non-aristonectine elasmosaurs in uppermost Cretaceous rocks of the Magallanes Basin, is similar to that observed in coeval localities of Argentinean Patagonia and Antarctica, but different from that observed in the Quiriquina Formation and the upper levels of the Dorotea Formation, where aristonectine elasmosaurids dominate the plesiosaur fauna. Polycotylids of equivalent age have also been described in other regions of the Weddellian Province, such as New Zealand. Polycotylidae seem to disappear from the fossil record during the late Maastrichtian in high southern latitudes of former Gondwana territories.
