Typology of detrital zircon as a key to unravelling provenance in rift siliciclastic sequences (Permo-Carboniferous of Spiti, N India)

AbstractDetrital zircon populations from Carboniferous to Permian sandstones from the Lozar Section of Spiti, northern India, were analyzed with the typology method in order to obtain complementary information on the source areas of the sediments. Zircon grains were subdivided into several groups and subgroups, according to degree of abrasion and morphological features.First appearance of detrital zircons with distinct typologic signature within successive stratigraphic intervals provided useful data about the tectono-magmatic evolution of the northern Indian margin during Late Paleozoic rifting of Gondwana and initial opening of Neotethys. The base of the studied sequence (Lower Carboniferous Thabo Fm.) is characterized by a largely cratonic provenance, seemingly from the Indian Shield to the South. In the Upper Carboniferous Chichong Fm., first occurrence of typical zircons from anatectic granites and increasing abundance of granitoid detritus suggest rapid uplift and unroofing of anatectic rocks of pro..

discovery of upper cretaceous neo tethyan trench deposits in south tibet luogangcuo formation

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FIRST REPORT OF CLARAIA (BIVALVIA) IN THE SERVINO FORMATION (LOWER TRIASSIC) OF THE WESTERN OROBIC ALPS, ITALY

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STRATIGRAPHY OF THE EARLY CRETACEOUS GIUMAL GROUP (ZANSKAR RANGE, NORTHERN INDIA)

Detailed stratigraphic work in the course of four geologic expeditions has allowed refinement of the Early Cretaceous stratigraphy of the Zanskar Range. The Giumal Sandstone, which is comprised between the Late Jurassic to earliest Cretaceous? Spiti Shale and the mid-Cretaceous Chikkim and Fatu La pelagic limestones, is formally elevated to Group rank and subdivided into two new formations, both capped by laterally continuous condensed sections here considered as formal stratigraphic horizons. The Takh Formation, overlying the Spiti Shale stratigraphically in the Zumlung Unit and commonly with tectonic contact in the Zangla  Unit, is in turn subdivided into two parts. The lower part (62 to 86 m thick in the Zumlung Unit) is characterized by very fine to fine-grained subarkoses and medium to very coarse-grained quartzarenites derived from the rejuvenated Indian continental block. In the upper part (103 to 117 m thick), deeper-water dark pelites locally yielding ill-preserved faunas probably Aptian in age become more important. The formation is capped by a glauconitic marker horizon (Labar La Arenite), testifying to a major starved transgressive stage possibly close to the Aptian/Albian boundary. The Pingdon La Formation (214 to 240 m thick in the southwestern part of the Zangla Unit and only 92 to 130 m thick in the Nerak area and Zumlung Unit) is characterized by the sudden appearance of up to medium-grained volcanic detritus. Decametric intervals of up to very coarse-grained quartzo-feldspathic sandstones characterize the middle part of the formation in proximal areas, whereas more monotonous distal sections contain only up to lower fine-grained volcanic arenites. In the upper part, volcanic arenite layers are interbedded with glaucony-rich or bioclastic sediments yielding sporadic foraminifers of Late Albian age. The formation is capped by a condensed horizon (Nerak Glauco-phosphorite) of Late Albian age (R. subticinensis Subzone), which in distal areas is overlain by latest Albian multicolored pelagic limestones of the Fatu La Formation. In the western part of the Zangla Unit, another condensed horizon rich in phosphates, glaucony and bioclasts (Oma Chu Glauco-phosphorite), reaching up to the Late Cenomanian (W. archaeocretacea Zone), is followed by Early Turonian grey pelagic limestones of the Chikkim Formation. Final drowning of the Zanskar shelf occurred through successive episodes of starvation and deepening, related to a complex interplay of geodynamic, eustatic and paleoceanographic processes

THE PERMIAN KULING GROUP (SPITI, LAHAUL AND ZANSKAR; NW HIMALAYA): SEDIMENTARY EVOLUTION DURING RIFT/DRIFT TRANSITION AND INITIAL OPENING OF NEO-TETHYS

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provenance and drainage system of the early cretaceous volcanic detritus in the himalaya as constrained by detrital zircon geochronology

Abstract The age range of the major intra-plate volcanic event that affected the northern Indian margin in the Early Cretaceous is here defined precisely by detrital zircon geochronology. U–Pb ages of Early Cretaceous detrital zircons found in the Cretaceous to the Paleocene sandstones cluster mainly between 142 Ma and 123 Ma in the northern Tethys Himalayan unit, and between 140 Ma and 116 Ma in the southern Tethys Himalayan unit. The youngest and oldest detrital zircons within this group indicate that volcanism in the source areas started in the latest Jurassic and ended by the early Albian. Stratigraphic data indicate that volcaniclastic sedimentation began significantly earlier in southern Tibet (Tithonian) than in Nepal (Valanginian), and considerably later in Spiti and Zanskar (Aptian/Albian) to the west. This apparent westward migration of magmatism was explained with progressive westward propagation of extensional/transtensional tectonic activity and development of fractures cutting deeply across the Indian continental margin crust. However, detrital zircon geochronology provides no indication of heterochroneity in magmatic activity in the source areas from east to west, and thus lends little support to such a scenario. Westward migration of volcaniclastic sedimentation may thus reflect instead the westward progradation of major drainage systems supplying volcanic detritus sourced from the same volcanic centers in the east. Development of multiple radial drainage away from the domal surface uplift associated with magmatic upwelling, as observed for most large igneous provinces around the world, may also explain why U–Pb ages of detrital zircons tend to cluster around 133–132 Ma (the age of the Comei igneous province) in Tethys Himalayan units, but around 118–117 Ma (the age of the Rajmahal igneous province) in Lesser Himalayan units

IL CARNICO DI LIERNA (COMO): STRATIGRAFIA E PALEOGEOGRAFIA

The Carnian sedimentary succession of the Lierna area, more than 4OO m thick and belonging to the Coltignone tectonic unit (Grigne Group), has considerable paleogeographic significance, for it documents the western termination of the deltalagoon-carbonate platform depositional system of the Lombard Prealps. The Carnian succession, directly overlying basinal (Perledo-Varenna Formation) to shallow-marine (Lierna Formation) carbonates of Ladinian age, is characterized by deltaic terrigenous deposits (Val Sabbia Sandstone), progressively replaced by lagoonal (Gorno Formation) to peritidal (Breno Formation) limestones. In the studied stratigraphic sections these three lithofacies alternate vertically at decametric scale, and are each characterized by different types of cyclothems at metric scale. High-frequency "punctuated aggradational cycles" (PAC) are best recognized at the top of the Val Sabbia clastic body, where the final transition to the Gorno lagoon is marked by arenite layers containing ferruginous ooids. Grains of this type have nor been previously reported from Southalpine arenites. The petrographic composition of the Val Sabbia Sandstone points to provenance from an active volcanic chain, but abundance of quartzose detritus and occurrence of perthitic K-feldspar and sedimentary rock fragments suggest extensive erosion of the Hercynian basement and Upper Carboniferous to Ladinian sedimentary cover. These peculiar characteristics confirm the existence of a small "Lierna lobe" separated from the deltaic systems of central Lombardy, and fed by a basement- high located southwest of Como lake

PALEOZOIC TO EARLY MESOZOIC STRATIGRAPHY AND SEDIMENTARY EVOLUTION OF CENTRAL DOLPO (NEPAL HIMALAYA)

The stratigraphic succession of central Dolpo (Tarap-Atali area) begins with thick calcsilicate marbles of inferred Cambrol-Ordovician age, injected by pegmatitic dykes. The strongly deformed mid-Paleozoic sequence comprises fossiliferous marls, dolomitic quartzarenites and dolomites. The latter are unconformably followed by earliest Late Devonian biocalcarenites, capped by a maior ironstone layer. The overlying offshore black pelites contain quanzose sandstones and bioclastic limestones at several intervals, indicating deposition in shelfal environments at latest Devonian times. The Carboniferous is poorly represented in the surveyed area. A major disconformity, cutting deep into Early? Carboniferous fenestellid-rich biocalcarenites at Tarap, is overlain by white quartzarenites interbedded with dark pelites, sharply followed by a fossiliferous marker horizon yielding a large brachiopod fauna of mid-Permian age (Costiferina arenites); the overlying fossiliferous shelfal pelites are unconformably followed by coarsening-upward quartzarenite sequences deposited in estuarine environments (Thini Chu Formation). In the Late Permian, another disconformity is overlain by conglomeratic to glauconitic arenites and shelfal pelites (Kuling Formation). The Triassic Tamba Kurkur Formation, with dolomitic subarkoses ar the base, consists of two condensed pelagic carbonate horizons of Dienerian and Smithian age, separated by dark pelites. Marly limestones and marls were deposited from the Spathian through the Carnian (Mukut Formation), and are overlain by the thick Tarap Shale. A thin basal condensed bed is followed by dark pelites yielding Early Norian ammonoids, by thick siltstones with phosphatic nodules and next by dark shales and calcareous siltstones with large Zoophycos-type burrows. The upper part of the unit contains up to fine-grained quartzo-feldspathic sandstones, nodular marly limestones and ironstone horizons ("upper assemblage"). The Late Triassic shallowing- upward succession is capped by dolomitic and oolitic quartzarenites with spectacular herringbone structures (Quartzite Series). The largely Lower Jurassic shallow-water Kioto Limestone, still containing metric intervals of up to medium-grained hybrid quartzarenites in the lower part, is followed by Middle Jurassic lumachelles (Laptal Formation). Younger terms of the succession are not exposed in Dolpo

ANATOMY OF A SEMIARID COASTAL SYSTEM: THE UPPER CARNIAN OF LOMBARDY (ITALY)

The mixed terrigenous-carbonate-evaporitic  S.Giovanni Bianco  Formation  and dolomitic Campolungo  Tongue  (upper part of the Breno Formation),  generally 2OO  to  3OO  m thick, are assigned  to the Late Carnian.  They  respectively  overlie lagoonal limestones  (Gorno  Fm.)  and peritidal  carbonates  (Annunciata Member of the Breno Fm.), and underlie  intraformational  breccias  and recrystallized  limestones  (Castro Fm.). Recognition of an unconformity,  ascribed  to a relative  fall of sea-level (sequence  boundary), allowed  us to subdivide the Upper  Carnian  succession  into two parts. ln the lower part (SGBL),  six lithosomes  were recognized. Red  to green  alluvial  clastics in the south-east  and south-west  pass northward  to mixed terrigenous-carbonate  coastal sediments  and finally to dolostones  deposited  in carbonate tidal flats. In the proximal sections  of the Brescia  Prealps,  renewed  north-westward  progradation  of alluvial redbeds  with  intercalated  calclithite conglomerates  points to a stage of teconic uplift. A distinct  increase  in quartz, representing a regional  petrographic  marker  followed  all  across Lombardy, indicates  deepening  of erosion  into the metamorphic wallrocks  of the volcanic  belt. A major hiatus  at the top of the SGB1  is best documented  in the northern  Presolana  area by a silcrete crust directly overlying  the Julian Annunciata Member of the Breno Formation.  In the Brembana  Valley area, the discontinuity occurs within  a greenish  siliciclastic  coastal  plain  succession, and may be traced at the top of a marker  interval of interbedded  reddish siltstones  and sandstones. The upper part (SGB2) consists  of four lithosomes. Greenish sandstones  and siltstones,  accumulating  in coastal  plains in  the south-west,  passed  northward  to mudrocks and dolostones.  In the southernmost  Camonica Valley  area,  mudrocks  are locally interbedded  with  calcarenites  containing  bored or pedogenized  lithoclasts ripped from the underlying sequence and varied  bioclasts,  restifiying to relatively  open shallow-marine conditions  during transgression. Next, thick gypsum  accumulated in coasral  salinas  barred  by locally oolitic  platform carbonates to the north. Rare sandstone lenses  occurring  in the Brembana Valley at the top of the unit conrain  exclusive rhyolitic  detritus,  indicating either  a terminal phase of explosive  volcanism  or erosion of older  felsic volcanic products

TRIASSIC STRATIGRAPHY AND SEDIMENTARY EVOLUTION OF THE ANNAPURNA TETHYS HIMALAYA (MANANG AREA, CENTRAL NEPAL)

After initial opening of Neotethys in the Permian, thermal subsidence and deepening continued in the Triassic. In the Scythian, three pelagic nodular carbonate intervals of Early Griesbachian to Early Dienerian, Early to mid-Smithian and latest Smithian to earliest Aegean age are interbedded with shelfal shales (Tamba Kurkur Formation). Ammonoid-and conodont-rich condensed carbonates were deposited on the outer shelf, with maximum depths around 150+200 m reached during transgressive stages. The Anisian to lowermost Norian shelfal succession mostly consists of marly limestones and marls (Mukut Formation). Thick calcareous siltstones rapidly accumulated in the Carnian, testifying to a new stage of tectonic extension affecting the Tethys Himalayan passive margin. The thick Tarap Formation of largely mid-Norian age points to strong continuing subsidence. Interbedded siltstones and subarkosic sandstones ("lower member") are locally overlain by coral-bearing patch reefs ("middle member"). Chamosite-bearing hybrid arenites deposited at transgressive stages characterize the "upper member". The Triassic succession is capped by subarkoses and quartzarenites, interbedded with dolomitic to bioclastic hybrid sandstones and silty limestones ("Quartzite Series"), in turn overlain by the Kioto Limestone