New Insights on the Impact of Tidal Currents on a Low-gradient, Semi-enclosed, Epicontinental Basin—the Curtis Formation, East-central Utah, USA

Based on a methodic sedimentological analysis, the Late Jurassic (Oxfordian) Curtis Formation unravels the intricate facies variability which occurs in a tide-dominated, fluvially starved, low-gradient, semi-enclosed epicontinental basin. This unit crops out in east-central Utah, between the eolian deposits of the underlying Middle Jurassic (Callovian) Entrada Sandstone, from which it is separated by the J-3 unconformity, and the conformable overlying supratidal Summerville Formation of Oxfordian age. A high-resolution sedimentary analysis of the succession led to the recognition of eight facies associations (FA) with six sub-facies associations. Based on the specific three-dimensional arrangement of these eight facies associations, it is proposed to separate the Curtis Formation into three sub-units: the lower, middle and upper Curtis. The J-3 unconformity defines the base of the lower Curtis, which consists of upper shoreface to beach deposits (FA 2), mud-domi­nated (FA 3a) and sand-dominated heterolithic subtidal flat (FA 3b), sand-rich sub- to supratidal flat (FA 4a) and correlative tidal channel infill (FA 4c). It is capped by the middle Curtis, which coincides with the sub- to intertidal channel-dune-flat complex of FA 5, and its lower boundary corresponds to a transgressive surface of regional extent, identified as the Major Transgressive Surface (MTS). This surface suggests a potential correlation between the middle and the upper Curtis and the neighboring Todilto Member of the Wanakah Forma­tion or Todilto Formation. The upper Curtis consists of the heterolithic upper sub- to intertidal flat (FA 6) and coastal dry eolian dunes belonging to the Moab Member of the Curtis Formation (FA 7), and it conformably overlies the middle Curtis. The spatial distribution of these sub-units supports the distinction of three different sectors across the study area: sector 1 in the north, sector 2 in the south-southwest, and sector 3 in the east. In sector 1, the Curtis Formation is represented by its three sub-units, whereas sector 2 is dominated by the middle and upper Curtis, and sector 3 encompasses the extent of the Moab Member of the Curtis Formation. This study also highlights the composite nature of the J-3 unconformity, which was impacted by various processes occurring before the Curtis Formation was deposited, as well as during the development of the lower and middle Curtis. Local collapse features within the lower and middle Curtis are linked to sand fluid over­pressure within a remobilized sandy substratum, potentially triggered by seismic activity. Furthermore, the occurrence of a sub-regional angular relationship between the middle Curtis and substratum implies that the area of study was impacted by a regional deformational event during the Late Jurassic, before the deposition of the middle Curtis. The spatial distribution of these sub-units supports the distinction of three different sectors across the study area: sector 1 in the north, sector 2 in the south-southwest, and sector 3 in the east. In sector 1, the Curtis For­mation is represented by its three sub-units, whereas sector 2 is dominated by the middle and upper Curtis, and sector 3 encompasses the extent of the Moab Member of the Curtis Formation. This study also highlights the composite nature of the J-3 unconformity, which was impacted by various processes occurring before the Curtis Formation was deposited, as well as during the development of the lower and middle Curtis. Local collapse features within the lower and middle Curtis are linked to sand fluid over­pressure within a remobilized sandy substratum, potentially triggered by seismic activity. Furthermore, the occurrence of a sub-regional angular relationship between the middle Curtis and substratum implies that the area of study was impacted by a regional deformational event during the Late Jurassic, before the deposition of the middle Curtis.

Stages of Late Palaeozoic deformation and intrusive activity in the western part of the North Patagonian Massif (southern Argentina) and their geotectonic implications

Analyses of structures in the western part of the North Patagonian Massif (southern Argentina) suggest a polyphase evolution, accompanied by continuous intrusive activity. The first two deformations (D1, D2) and metamorphism affected the upper Palaeozoic, partly possibly older Cushamen Formation clastic succession and different intrusive rocks. A second group of intrusions, emplaced after the second deformational episode (D2), in many places contain angular xenoliths of the foliated country rocks, indicating high intrusive levels with brittle fracturing of the crust. Deformation of these magmatic rocks presumably began during (the final stage of) cooling and continued under solid-state conditions. It probably coincided with the third deformational event (D3) in the country rocks. Based on published U–Pb zircon ages of deformed granitoids, the D2-deformation and younger event along with the regional metamorphism are likely to be Permian in age. An onset of the deformational and magmatic history during Carboniferous times, however, cannot be excluded. The estimated ~W–E to NE–SW compression during the D2-deformation, also affecting the first group of intrusive rocks, can be related to subduction beneath the western Patagonia margin or an advanced stage of collisional tectonics within extra-Andean Patagonia. The younger ~N–S to NE–SW compression might have been an effect of oblique subduction in the west and/or continuing collision-related deformation. As a cause for its deviating orientation, younger block rotations during strike-slip faulting cannot be excluded. The previous D2-event presumably also had an effect on compression at the northern Patagonia margin that was interpreted as result of Patagonia's late Palaeozoic collision with the southwestern Gondwana margin. With the recently proposed Carboniferous subduction and collision south of the North Patagonian Massif, the entire scenario might suggest that Patagonia consists of two different pieces that were amalgamated with southwestern Gondwana during Late Palaeozoic times.

Precise U–Pb zircon ages from Alderney, Channel Islands: growing evidence for discrete Neoproterozoic magmatic episodes in northern Cadomia

The northernmost exposures of rocks formed during the Late Neoproterozoic Cadomian orogeny in the Channel Islands–northern France region occur on Alderney. The island mainly comprises foliated quartz diorite, once considered to be 2 Ga, pre-Cadomian basement, and an undeformed basic to intermediate plutonic complex. A precise age of 610±2 Ma, based on U–Pb analyses of single and small groups of zircons, for the foliated Fort Tourgis quartz diorite demonstrates that the oldest rocks were emplaced and deformed during a Cadomian magmatic event. The age is virtually identical to ages from similar, foliated syntectonic quartz diorite bodies on the islands of Guernsey and Sark and at La Hague (north Normandy), indicating that this magmatic and deformational event was regional in extent. Discordant zircon xenocrysts define an upper intercept age of c. 2 Ga indicating the presence of Palaeoproterozoic basement at depth. Single zircons from the undeformed Bibette Head granodiorite give a precise U–Pb age of 572±1 Ma. This age is closely similar to that for the emplacement of the Northern Igneous Complex of Guernsey. The emerging data indicate that Cadomian magmatism in the northern Channel Islands region was not a protracted continuum, but occurred during two distinct, short-lived events separated by c. 30–40 my.

Development of inverted metamorphic gradient in the internal domain of the Taconian belt, Gaspé Peninsula

The Mont Logan Nappe is part of the Taconian internal domain of the Quebec Appalachians, and is entirely made up of synrift to passive margin elastics and volcanics of the Shickshock Group. Rocks of the Mont Logan Nappe were affected by both Taconian and Acadian deformations but regional prograde metamorphism is Taconian and limited to the D1 deformational event. Thermobarometry and mineral assemblages indicate that the metasedimentary and metavolcanic rocks of the Mont Logan Nappe have recorded peak temperatures in the range 610–700 °C under pressures of approximately 600–700 MPa, and that prograde metamorphism was accompanied by the development of an inverted metamorphic gradient of −40 to −75 °C/km. The preferred interpretation of the cause of the inverted gradient is dissipative heating accompanying deformation along an intracontinental synmetamorphic thrust fault located at the top of the inverted metamorphic sequence.

O SUPERGRUPO RIO DAS VELHAS DA FAIXA MATEUS LEME-PITANGUI - PARTE MERIDIONAL DO CRÁTON DO SÃO FRANCISCO, MG – E SEU SISTEMA DE ALTERAÇÃO HIDROTERMAL

The studied area is located at the southern São Francisco Craton, between the northwest comer of the Quadrilátero Ferrífero and the Upper Proterozoic Bambui basin. The region comprises Archaean granite-gneisses and two supracrustal sequences: a) The Archaean Rio das Velhas Supergroup, which consists of a thick sequence of a greenstones associated with metamorphic felsic volcanites, volcano-clastic and sedimentary rocks; and b) the overlying Early Proterozoic Minas Supergroup with its widespread banded iron formations.Two major deformational events are present. The oldest Dn event can be detected only in the Rio das Velhas rocks and produced large folds with mainly E-W trending axis. The latter Dn+1 of Transamazonian age (±2,0 Ga) is a progressive deformational event, that has also affected the Minas Supergroup. It produces NW-SE to NE-SW folds associated to conspicuous planar and linear structures. This event envolved to a NW-vergent thurst system with the development of a strong stretch lineation. This event also produced strike-slip faults interpreted as tear-faults parallel to the direction of the thrusts.This paper deals with the large hydrothermal alteration which has affected the lower volcanoclastics rocks of the Rio das Velhas Supergroup. This alteration has been produced by a solfataric hydrothermal system in mesothermal P-T conditions. It produced a high aluminium mineralogical assemblage of corundum, kyanite (minor andaluzite) and muscovite. This mineralogy is latter retrograde altered to diaspore, pyrophyllite, kaolinite. Minor amounts of tourmaline, rutile, chloritoid are also present. A hydrothermal zoning with three major subdivisions is proposed as follows: high aluminium zone, sericitic (muscovitic) zone and propilitic zone. Some cristallochemical aspects of the mineralogical phases are discussed, and also the origin of the protolite and the relationship between alteration and deformatiom. The hydrothermal system has evolved under pre to syn tectonic conditions during the Archean Dn tectonism.For this event, a model of collisional tectonics is also proposed for the area, as the Rio das Velhas Supergroup represents a former continental back-arc basin.

Syntectonic minor intrusions or synemplacement deformation?

The concept of dating deformation using "syntectonic" minor intrusions involves flawed reasoning. At best, the time of a deformational event can only be bracketed, using pre- and postdeformation minor intrusions.