minerals Article Sedimentology, Provenance and Radiometric Dating of the Silante Formation: Implications for the Cenozoic Evolution of the Western Andes of Ecuador Cristian Vallejo 1,2,* , Santiago Almagor 1, Christian Romero 3 , Jose L. Herrera 4,5, Vanessa Escobar 1, Richard A. Spikings 6, Wilfried Winkler 7 and Pieter Vermeesch 4 1 Departamento de Geología, Facultad de Ingeniería en Geología y Petróleos, Escuela Politécnica Nacional, Quito 170517, Ecuador; [email protected] (S.A.); [email protected] (V.E.) 2 Geostrat SA. Jorge Drom N39-188 y Jose Arizaga, Quito 170507, Ecuador 3 Instituto de Investigación Geológico Energético (IIGE), Quito 170518, Ecuador; [email protected] 4 Department of Earth Sciences, University College London, Gower Street, London WC1E 6BT, UK; [email protected] (J.L.H.); [email protected] (P.V.) 5 GIDAC, Instituto de Investigaciones, Escuela Superior Politécnica de Chimborazo, Riobamba 060155, Ecuador 6 Department of Earth Sciences, University of Geneva, CH-1205 Geneva, Switzerland; [email protected] 7 Department of Earth Sciences, ETH-Zürich, CH-8092 Zürich, Switzerland; [email protected] * Correspondence: [email protected]; Tel.: +593-998726188 Received: 31 August 2020; Accepted: 10 October 2020; Published: 21 October 2020 Abstract: The Silante Formation is a thick series of continental deposits, exposed along a trench-parallel distance of approximately 300 km within the Western Cordillera of Ecuador. The origin, tectonic setting, age and stratigraphic relationships are poorly known, although these are key to understand the Cenozoic evolution of the Ecuadorian Andes. We present new sedimentological, stratigraphic, petrographic, radiometric and provenance data from the Silante Formation and underlying rocks. The detailed stratigraphic analysis shows that the Silante Formation unconformably overlies Paleocene submarine fan deposits of the Pilalo Formation, which was coeval with submarine tholeiitic volcanism. The lithofacies of the Silante Formation suggest that the sediments were deposited in a debris flow dominated alluvial fan. Provenance analysis including heavy mineral assemblages and detrital zircon U-Pb ages indicate that sediments of the Silante Formation were derived from the erosion of a continental, calc-alkaline volcanic arc, pointing to the Oligocene to Miocene San Juan de Lachas volcanic arc. Thermochronological data and regional correlations suggest that deposition of the Silante Formation was coeval with regional rock and surface uplift of the Andean margin that deposited alluvial fans in intermontane and back-arc domains. Keywords: provenance analysis; Western Andes; Miocene; Silante Formation; Ecuador 1. Introduction The Western Andes of Ecuador located between 1◦ N to 3◦ S along the active margin of South America (Figure1) are composed of allochthonous oceanic blocks, which accreted in the late Cretaceous period [1,2]. The events of accretion are partly considered to influence orogenic processes, and the chemical composition of arc volcanism in the Northern Andes [3,4]. Previous studies of the Western Cordillera of Ecuador improved our understanding of the tectonic and stratigraphic evolution of this accretionary complex [5–11]. However, the Cenozoic sedimentary and tectonic evolution of the Western Andes of Ecuador remains poorly understood. Minerals 2020, 10, 929; doi:10.3390/min10100929 www.mdpi.com/journal/minerals Minerals 2020, 10, x FOR PEER REVIEW 2 of 30 evolution of this accretionary complex [5–11]. However, the Cenozoic sedimentary and tectonic evolution of the Western Andes of Ecuador remains poorly understood. The Paleocene to Miocene period is particularly significant because several ore deposits formed over a large latitudinal range (0° to 3°30′ S) within the Western Cordillera during the Cenozoic (42–6 Ma; [12,13]). Thus, a reconstruction of the areal distribution and temporal framework of magmatism andMinerals its 2020volcanic, 10, 929 products will improve our understanding of the spatial distribution of mineral2 of 31 deposits. FigureFigure 1. Lithotectonic mapmap ofof Ecuador Ecuador (modified (modified from from Luzieux Luzieux et al.et [al.1]) [1]) and and location location of the of studythe study area. area. The Paleocene to Miocene period is particularly significant because several ore deposits formed overThe a large Silante latitudinal Formation range is a ~2000-m-thick (0◦ to 3◦300 S) series within of red the beds Western with Cordillera intercalated during conglomerates the Cenozoic and sandstones(42–6 Ma; [rich12,13 in]). volcanic Thus, a material reconstruction [14]. It ofoccupies the areal a large distribution area within and temporalthe Western framework Cordillera, of extendingmagmatism from and 1° its North volcanic to 1° products South (Figure will improve 2). The ourSilante understanding Formation is of intruded the spatial by distributionmafic (diorite of andmineral andesite) deposits. dikes and sills [7,15]. Several authors have proposed different stratigraphic and geodynamicThe Silante interpretations Formation for is a the ~2000-m-thick origin of thes seriese thick of continental red beds with deposits intercalated [5–7,11–18]; conglomerates however, theand stratigraphic sandstones rich relationships in volcanic of material the Silante [14]. ItFo occupiesrmation aremain large areaunsolved, within mainly the Western because Cordillera, of the discontinuousextending from rock 1◦ North exposures, to 1◦ South and the (Figure absence2). The of a Silante coherent Formation chronost isratigraphic intruded by framework. mafic (diorite and andesite)This dikesstudy andpresents sills [ 7new,15]. sedimentological, Several authors have strati proposedgraphic, distructuralfferent stratigraphic and geochronological and geodynamic data frominterpretations several exposures for the origin of the of Silante these thick Formation continental and depositsthe underlying [5–7,11– rocks.18]; however, These data the stratigraphicare used to determinerelationships the ofhistory the Silante of sedimentation Formation remainand the unsolved,tectonic regime. mainly Radiometric because of dating the discontinuous and provenance rock studiesexposures, including and the heavy absence minerals of a coherent and single chronostratigraphic grain geochemistry framework. are used to constrain the age, mineralogicalThis study composition presents new and sedimentological, approximate location stratigraphic, of the sediment structural source and geochronological regions, which facilitates data from tectonicseveral exposuresreconstructions of the Silante and stratigraphic Formation and correla the underlyingtions [2]. rocks.This study These dataaddresses are used the to geological determine evolutionthe history of of a sedimentationsegment of the andWestern the tectonic Cordillera regime. of Ecuador Radiometric and includes dating a and detailed provenance description studies of rockincluding outcrops heavy exposed minerals along and east single to west grain road geochemistry sections that are traverse used to constrainthe Western the Cordillera age, mineralogical (Figure composition and approximate location of the sediment source regions, which facilitates tectonic reconstructions and stratigraphic correlations [2]. This study addresses the geological evolution of a segment of the Western Cordillera of Ecuador and includes a detailed description of rock outcrops exposed along east to west road sections that traverse the Western Cordillera (Figure2). These traverses are the Nono–Tandayapa, Calacalí–Nanegalito, Quito–Santo Domingo, Quito–Chiriboga and Otavalo–Selva Alegre road sections. Minerals 2020, 10, x FOR PEER REVIEW 3 of 30 2).Minerals These2020 traverses, 10, 929 are the Nono–Tandayapa, Calacalí–Nanegalito, Quito–Santo Domingo, Quito–3 of 31 Chiriboga and Otavalo–Selva Alegre road sections. Figure 2. Geological map of the Western Cordillera from 1° North to 1° South, showing the main Figure 2. Geological map of the Western Cordillera from 1◦ North to 1◦ South, showing the main geologicalgeological units (modified(modified after after [18 [18])]) and and radiometric radiometri agesc ages from from the literaturethe literatur [10,e11 [10,11,16,17],16,17] and this and study. this study. 2. Regional Geology 2. RegionalEcuador Geology can be subdivided into five morphotectonic regions (Figure1). (1) The coastal forearc is composedEcuador of maficcan be oceanic subdivided crust [into1,3, 19five] and morphotectonic is covered by regions Paleogene (Figure to Neogene 1). (1) The forearc coastal deposits forearc [2]. is(2) composed The Western of mafic Cordillera oceanic (Figure crust2 [1,3,19]) consists and of is mafic covered and intermediateby Paleogene extrusive to Neogene and forearc intrusive deposits rocks [2].that (2) are The tectonically Western Cordillera juxtaposed (Figure with sedimentary 2) consists of rocks mafic of and Late intermediate Cretaceous extrusive to Miocene and age intrusive [2,9,11]. rocks(3) The that Interandean are tectonically Valley juxtaposed or Interandean with Depressionsedimentary is rocks located of betweenLate Cretaceous the Western to Miocene and Eastern age [2,9,11].cordilleras (3) The and Interandean is covered by Valley thick or Quaternary Interandean volcanic Depression deposits is located [20], whichbetween are the underlain Western byand a Easterncrystalline cordilleras basement and composed is covered of by metamorphic thick Quaternary and maficvolcanic rocks