Sandstone Petrography Research Articles

Provenance, tectonic setting and source area-paleoweathering of sandstones of the Bahariya Formation in the Bahariya Oasis, Egypt: An implication to paleoclimate and paleogeography of the southern Neo-Tethys region during Early Cenomanian

The Lower Cenomanian Bahariya Formation forms the most prolific hydrocarbon reservoir in the Western Desert of Egypt, and represents a good example to illustrate the paleoclimatic conditions and paleogeographical evolution in the southern Neo-Tethys region during Early Cenomanian. It is also particularly important for its diverse vertebrate fauna including dipnoans, chelonians, crocodyliforms, squamates, plesiosaurs, decapod crustaceans and dinosaurs, which reflect sustained regional biostratigraphical and paleoenvironmental significance. In this study, a multidisciplinary approach of fieldwork, petrography and bulk-rock geochemistry of sandstones of the Bahariya Formation has been performed to constrain their provenance, tectonic setting, source area-paleoweathering and paleoclimatic conditions within the Neo-Tethyan realm. Petrographically, the studied sandstones are mainly of quartz-arenite type with subordinate sub-arkose and sublithic ones. Quartz types, trace and rare earth element concentrations and ratios indicate the sandstones were sourced from felsic granitic rocks with a rare contribution of felsic gneisses. The modal analysis reflects that the sandstones are mainly of cratonic interior and quartzose recycled tectonic provenance. The chondrite-normalized REE distribution pattern of the Bahariya sandstones are characterized by enriched LREE and flat HREE similar to those of UCC with negative Eu/Eu* anomalies. Major and trace elements of the sandstones suggest their deposition on a Neo-Tethys passive continental margin related to the Early Cretaceous rifting. The weathering indices (CIA, CIW, PIA and ICV) and the Al2O3-(CaO + Na2O)-K2O diagram suggest moderate to severe chemical weathering in the source area under semi-humid to humid climatic conditions. In conclusions, the Bahariya sandstones could be sourced by a fluvial reworking of moderately- to intensively-weathered Precambrian granites and gneisses of the surrounding Gebel Uweinat Massive (at the southwest of the study area), and were deposited in a low-lying basin (Bahariya area) of the Neo-Tethys passive continental margin. In a broad interest, results of this study could contribute in a reconstruction of the paleoclimatic conditions and paleogeographical evolution of the southern Neo-Tethys region during Early Cenomanian.

Petrography, diagenesis and geochemistry of the Cambro-Ordovician Khabour sandstones, western Iraq: Implications for reservoir quality and the impact of the Hirnantian glaciation

Deeply buried (2.3–3.5 km) sandstones of the Cambro-Ordovician Khabour Formation are important hydrocarbon reservoirs in western Iraq. An integrated approach incorporating petrography, scanning electron microscope (SEM) and X-ray diffraction (XRD) was applied to investigate diagenesis and diagenetic minerals and their impacts on reservoir quality. Major and trace element distribution using X-Ray Fluorescence (XRF) analysis also was carried out to identify sandstones deposition during or after the Hirnantian glaciation. The sandstones of the Khabour Formation are dominated by sublitharenites and subarkoses. Several diagenetic processes have affected the sandstones of the Khabour Formation. These include early diagenetic porosity occluding events, such as common formation of illite and sericite coatings, along with rare quartz overgrowths, and calcite cementation. This was followed by secondary porosity forming events which include calcite dissolution, leaching of feldspar, alteration of micas and fracturing. The pore systems are dominated by secondary pores, vuggy and fractured porosity, intergranular and intragranular pores and poorly-connected macropores. A decrease in heavy minerals in sandstones from the upper part of the Khabour Formation and an associated decrease in both Zr and Hf concentration, which are typically associated with zircon, supported by a decrease in mica content and low API gamma ray unit values are considered to be a signature of sandstones affected by the Hirnantian glaciation.

Hidden Middle Devonian Magmatism of North-Eastern Siberia: Age Constraints from Detrital Zircon U–Pb Data

We present new data on the tectonic evolution of north-eastern Siberia using an integrated provenance analysis based on U–Pb detrital zircon dating and sandstone petrography of Devonian sedimentary strata. Our petrographic data suggest that Upper Devonian sandstones of north-eastern Siberia were derived from a local provenance, supported by the widespread distribution of ca. 1900–2000 Ma magmatic events in the basement of the neighboring Ust’-Lena and Olenek uplifts. Devonian detrital zircon age distributions of the Devonian sandstones are similar to ages of Middle Paleozoic magmatic rocks of Yakutsk-Vilyui large igneous province (LIP). Therefore, we suggest that the studied sandstones were derived from proximally-located uplifted blocks composed of Proterozoic–Devonian rocks and Middle–Late Devonian volcanics. Moreover, the abundance of Middle–Late Devonian zircons is suggestive of a wider distribution of coeval magmatism across north-eastern Siberia than previously supposed. We propose that widespread Devonian magmatism associated with the Yakutsk-Vilyui LIP also occurred to the east of our study area and is now buried beneath thick Carboniferous–Jurassic sedimentary rocks of the eastern Siberian passive margin, subsequently deformed into the Late Jurassic–Cretaceous Verkhoyansk fold-and-thrust belt. We also propose that the major pulse of the Yakutsk-Vilyui LIP occurred in north-eastern Siberia during the Middle Devonian at ca. 390 Ma, some 15 million years earlier than within the Vilyui rift basin in eastern Siberia (ca. 375 Ma).

Petrography and geochemistry of the Lower Carboniferous Abu Durba sandstones, West Central Sinai, Egypt

Petrographically, the Lower Carboniferous Abu Durba sandstones are classified as quartz arenites reflecting their high maturity and have an average framework composition of Q96F2L2. The sandstones are rich in SiO2, Zr, Sr, and Ba and depleted in most other major and trace elements, compared with the average upper continental crust. The positive correlation between Al2O3 and most trace elements signifies that trace elements are associated with clay minerals rather than with source rocks. Provenance discrimination diagrams, major elemental ratios (Al2O3/TiO2 and K2O/Na2O), and SiO2, Cr, Ni, V, and Zr contents in sandstones reveal that the sandstones are mainly derived from quartzose recycled sedimentary rocks with subordinate felsic plutonic sources. The chemical index of alteration (avg. 88.21) and Rb/Sr ratio (> 0.8) values for the Abu Durba Sandstone indicate an intensive degree of weathering either of the original source or during transport before deposition, and may reflect intense recycling in the source area. Also, low index of compositional variability values and high SiO2/Al2O3 ratios indicate high mineralogical maturity of Abu Durba Sandstone. The discriminant function-based multidimensional diagrams reveal that the Abu Durba Sandstone was deposited in a rift basin, which is consistent with the general geology of West Central Sinai during the Lower Carboniferous. The older sandstones of Naqus (Lower Cambrian) and Abu Thora (Lower Carboniferous) formations in addition to the Precambrian basement rocks are probably the sources of the Abu Durba Sandstone.

Role of the Down-Bending Plate as a Detrital Source in Convergent Systems Revealed by U–Pb Dating of Zircon Grains: Insights from the Southern Andes and Western Italian Alps

In convergent zones, several parts of the geodynamic system (e.g., continental margins, back-arc regions) can be deformed, uplifted, and eroded through time, each of them potentially delivering clastic sediments to neighboring basins. Tectonically driven events are mostly recorded in syntectonic clastic systems accumulated into different kinds of basins: trench, fore-arc, and back-arc basins in subduction zones and foredeep, thrust-top, and episutural basins in collisional settings. The most widely used tools for provenance analysis of synorogenic sediments and for unraveling the tectonic evolution of convergent zones are sandstone petrography and U–Pb dating of detrital zircon. In this paper, we present a comparison of previously published data discussing how these techniques are used to constrain provenance reconstructions and contribute to a better understanding of the tectonic evolution of (i) the Cretaceous transition from extensional to compressional regimes in the back-arc region of the southern Andean system; and (ii) the involvement of the passive European continental margin in the Western Alps subduction system during impending Alpine collision. In both cases, sediments delivered from the down-bending continental block are significantly involved. Our findings highlight its role as a detrital source, which is generally underestimated or even ignored in current tectonic models.

UPPER CRETACEOUS AND PALEOCENE IN ZANSKAR RANGE (NW Himalaya)

A detailed Upper Cretaceous/Paleocene stratigraphic section was measured from the Spanboth Chu Valley, High Himalaya Nappes. Placement Of the Cretaceous—Tertiary boundary was refined, but some uncertitudes still remain because of poor exposure in this portion of the sequence. The Maastrichtian is characterized by floral and faunal assemblages consisting of Cymopolia, Ornphalocyclus, Siderolites and Odon togryphaea. The Lower Paleocene seems to be missing, whilst the Middle to Upper Paleocene is characterized by Rotalia, Daviesina, Lockhartia and Furcoporella assemblages. Sandstone petrography and carbonate microfacies were analyzed for environmental interpretation. Moreover, a reconsideration of the recent published data on the Upper Cretaceous/ Paleocene from the Zanskar mountains allows us to delineate a tentative depositional history of this part of the Indian continental margin during the considered time.

Late Paleozoic detrital history of eastern Gondwanaland: petrofacies and detrital geochronology of Permo-Carboniferous intracratonic sequences of the northwest Bengal Basin

ABSTRACTPermo-Carboniferous Gondwanan sequences have been reported from several isolated basins of Peninsular India. These siliciclastic sequences were preserved in several intracratonic basins in northwest Bangladesh. Sandstone petrography, heavy-mineral assemblages, mineral chemistry, and 40Ar/39Ar geochronology of sediment cores were used in this study to decipher the provenance history of Gondwanan sediments at two localities (Khalashpir and Barapukuria). Petrographic studies suggest that these sequences are mostly immature and poorly sorted arkosic sandstones (Khalashpir-Qt60F27L13, Barapukuria-Qt52F31L17), with compositions ranging from quartzarenite to litharenite. Among lithic fragments, sedimentary types are abundant. Heavy minerals are volumetrically rare and of low diversity in sediments of northwest Bangladesh. Garnet geochemistry indicates that metamorphic grades in the source terranes were of the amphibolite to granulite facies. Laser 40Ar/39Ar ages for single crystals of detrital muscovite from the deepest drilled Gondwanan sequences yielded the broadest age range, with a dominant mode at circa 515 Ma and lesser clusters of ages at circa 550, 570, and 600 Ma. The other two shallower samples are dominated by ages with similar single modes at circa 495–500 Ma. The oldest muscovite crystals may have been derived from the adjacent Indian craton and/or the Meghalayan craton. Younger muscovite crystals may have been contributed from the Pinjarra Orogen, formed during episodes of Neoproterozoic to early Paleozoic collision among India, Antarctica, and Australia.

Permian–Triassic magmatism in response to Palaeotethys subduction and pre-Late Triassic arrival of northeast Gondwana-derived continental fragments at the southern Eurasian margin: Detrital zircon evidence from Triassic sandstones of Central Iran

The closure of Palaeotethys that led to the collision of the Cimmerian blocks with the southern Eurasian margin causing the Eo-Cimmerian orogeny during the Early Mesozoic is still controversially discussed. The Triassic Nakhlak Group in Central Iran is a key sedimentary succession for better understanding the closure of Palaeotethys and the Eo-Cimmerian orogeny in the Middle East. The Nakhlak Group is composed of the Alam (Olenekian to Middle Anisian), Baqoroq (?Upper Anisian to Middle Ladinian) and Ashin (Upper Ladinian to ?Carnian) formations, which consist mainly of volcaniclastic sandstones, mixed siliciclastic conglomerates, and marine carbonates. Here we present for the first time detrital zircon UPb ages from the Nakhlak Group to unravel its provenance and constrain its palaeotectonic position within the Palaeotethyan realm. Most detrital zircons from the Nakhlak Group are euhedral and subhedral with Permian–Triassic ages (ca. 280–240 Ma) suggesting sediment supply from Permian–Triassic magmatic rocks of the Silk Road Arc. Minor zircon populations show pre-Permian Palaeozoic ages, with age peaks at ca. 320 Ma and 480 Ma, which are probably derived from the basement on which the magmatic arc developed. Neoproterozoic–latest Mesoproterozoic (ca. 550–1100 Ma) and Palaeoproterozoic (ca. 1800–2200 Ma) zircon grains are anhedral (rounded). The latter are prominent in the upper Baqoroq Formation (Middle Ladinian) suggesting recycling of older sedimentary rocks. Sandstone petrography points toward an additional metamorphic provenance for this formation. This short-lived provenance change can be explained by tectonic uplift in the source area that led to erosion of metamorphosed rocks with a northeast Gondwanan affinity. It clearly indicates that northeast Gondwana-derived continental fragments likely belonging to the Cimmerian blocks already arrived at the southern Eurasian margin in pre-Late Triassic time. Current palaeotectonic models of the closure of Palaeotethys and the Eo-Cimmerian orogeny in the Middle East during the Triassic may need to be revised.

Petrofacies of Eocene sand injectites of the Tumey Giant Injection Complex, California (USA)

The forearc succession of Great Valley Group in Central California provides some of the best examples of giant sand injection complexes in the world and is therefore considered valuable analogues for injectite systems in the subsurface. Several sand injection complexes are well described in the outcrop and subsurface, however the petrographic characteristics of injectites are still poorly documented. In this paper, we present the results of an integrated study of field observations, quantitative and qualitative sandstone petrography, provenance, and petrofacies analysis of the Tumey Giant Injection Complex (TGIC) in order to understand its lithostratigraphy and petrological evolution, and its impacts on reservoir petrofacies characteristics and fluid migration. The TGIC intrudes into a 450 m thick deep-water succession of slope mudrocks and sandy channel-fills of the Kreyenhagen Shale (Eocene), forming an interconnected network of sandstone sills, dykes and injection breccias. The complex generated a horizontal and vertical plumbing system for fluid migration, connecting isolated sandy channel-fills among low-permeability mudrocks. The primary detrital composition, diagenetic products, microtextures, and provenance signatures allowed for the definition and discrimination of depositional and intrusive petrofacies and their genetic relations. Petrofacies associations confirm that the gypsum-cemented feldspathic litharenites from the Kreyenhagen Shale channel-fills are the only source for the injection complex. Eodiagenetic compaction and extensive gypsum cementation reduced the primary porosity of the complex, while telodiagenetic dissolution of autigenic constituents formed pervasive secondary porosity. The underlying calcite-cemented arkosic sandstones of the Lodo and Domengine formations acted as barriers for fluid flow, aiding lateral fluid migration and overpressure buildup within the overlying Kreyenhagen channel-fills. Intense grain microfracturing occurred during sand remobilization and injection along with erosion of the host mudrocks. This study has significance for the generation of reliable stratigraphic and petrological models for sand injection complex genesis and evolution that consequently can help the understanding and exploration of injectite complexes elsewhere.

Petrography and Geochemistry of Cenozoic Sandstones in the Dunhua Basin, Northeast China: Implications for Provenance, Source Weathering, and Tectonic Setting

Petrographic, mineralogical, and geochemical analyses were conducted on Cenozoic sandstones from the Dunhua Basin, which located in Dunhua-mishan fault zone of northeast China, to investigate the provenance of the sediments, as well as the weathering intensity and tectonic setting of the source region. Petrographic data indicate that average quartz-feldspar-lithic (Q-F-L) proportions in the sandstones are Q = 68%, F = 16%, and L = 16%, the lithic fraction mainly contains volcanic clasts. The chemical index of alteration (CIA) varies from 59 to 69 (average 63), while the index of chemical variability (ICV) ranges from 0.68 to 0.91 (average 0.77), and the average Th/U ratio is 3.2. Chondrite-normalized REE distributions show LREEs enriched relative to HREEs, and a prominent negative Eu anomaly. These data indicating that the sandstones are compositionally immature and a weak degree of weathering in the source region. The petrographic and mineralogical characteristics, combine with Zr/Sc–Th/Sc, Hf–La/Th and Co/Th–La/Sc discrimination diagrams reveal that these sandstones are derived from the surrounding felsic volcanic and intrusive rocks of Late Triassic-Early Jurassic, which exposed to the southeast or northwest of the basin. Multidimensional tectonic discrimination diagrams, including geochemical data and Dickinson triangular charts, indicate that the sandstones were derived from recycled orogenic in an active continental margin setting.

Provenance of the Hangu Formation, Lesser Himalaya, Pakistan: Insight from the detrital zircon U-Pb dating and spinel geochemistry

Clastic rich Hangu Formation of the Early Paleocene exposed at the apex of Hazara-Kashmir syntaxis, is significant in providing evidence of the regional Mesozoic–Cenozoic unconformity and nature of detritus supplied. The current study focuses on U-Pb age dating of the detrital zircons along with spinel geochemistry to interpret tectonic implications on provenance. The Hangu Formation is composed of coarse grained sandstone, arenaceous limestone, carbonaceous shale and laterite around the Paras region and rests unconformably over Mesozoic Samana Suk Formation. The probability density plots of the detrital zircons of the Hangu Formation exhibit strong similarity to the Tethyan Himalayan detrital record. The younger Mesozoic detrital zircons exhibit resemblance to Tethyan Himalayan as well as ophiolitic detrital suites, but are very few compared to their coeval Stumpata, Jidula and Denggeng/Sangdanlin formations. The quantitative comparison reflects strong resemblance to the Tethyan Himalaya, whereas, the ophiolitic component relationship is quite poor. Comparison of spinel geochemical data with Northwestern ophiolites (Chilas Ophiolites) and Tethyan Himalaya exhibits mixed source regime of the Hangu sediments. However, the combined U-Pb dating, sandstone petrography and spinel geochemical data suggest that the detritus of the Hangu Formation is mainly derived from the Tethyan Himalayan source (Indian source). The absence of ophiolitic signature in petrography and U-Pb geochronology may suggest that the northern ophiolites were not exposed to provide detritus to the Early Paleocene Hangu Formation. This may also be interpreted that the ophiolites were emplaced during the Late Cretaceous on the northern Indian margin but uplifted and eroded later with the final India–Eurasia collision, which is clearly recorded in various studies all along the Indian margin. This fact also intimates the ophiolite emplacement as a possible cause of the regional Mesozoic–Cenozoic unconformity.

Sandstone petrography and provenance of the Chubut Group (Cretaceous) in the Cañadón Matasiete (Golfo San Jorge Basin, central Patagonia): Implications for basin evolution and alluvial organization

Sandstone petrographic data have been used to investigate the provenance of the Matasiete, Pozo D-129, and Castillo formations (Chubut Group) in the Cañadón Matasiete, San Bernardo Fold Belt at Golfo San Jorge Basin, Argentina. Available petrographical data provides evidence of several distinctive volcanic source areas, with minor contributions of metamorphic, plutonic, or sedimentary components. Paleo-volcanic contributions from basic-intermediate (lathwork and microlithic textures) and acidic compositions (felsitic, seriate, granular, and vitric textures) were mainly derived from the Middle-Upper Jurassic rocks of the Lonco Trapial Formation. Pyroclastic components (glass shards, pumice fragments, and eutaxitic textures) derived from coeval volcanic activity in the Andes Cordillera, generated either gradual or abrupt changes in the source of the lithic fragments along the study succession. The observed trends in detrital lithic provenance can be integrated with previous sedimentological and architectural studies of the Matasiete and Castillo formations, revealing that narrow-sheet geometries of the channel fill characterize stratigraphic intervals dominated by pyroclastic components, whereas stratigraphic intervals that contain a low content of pyroclastic materials in the channel fill show ribbon-shaped geometries. Sandstone petrography in volcaniclastic basins can be used to complement other indicators of the evolution of the fluvial systems, providing reliable information about the nature of the changes in the sediment supply within the drainage catchment.

Towards a Southern European Tethyan Palaeomargin provenance signature: sandstone detrital modes and detrital zircon U–Pb age distribution of the Upper Cretaceous–Paleocene Monte Bignone Sandstones (Ligurian Alps, NW Italy)

Constraining the source terranes of Alpine siliciclastic flysch sequences is crucial for building a clearer picture of the palaeogeography and geodynamic evolution of the Western Tethys in the framework of impending continental collision. This paper presents an integrated study that involves palaeocurrent dispersal analysis, sandstone petrography and detrital zircon geochronology of the Upper Cretaceous–Paleocene Monte Bignone Sandstones, a siliciclastic turbidite system deposited during the pre-collisional evolution of the Ligurian Alps. Palaeocurrent analysis illustrates an overall eastward transport of the proximal sediments in the present configuration. Considering the ca. 45°–50° counter-clockwise rotation of the Tertiary Piedmont Basin and of the Corsica–Sardinia block in the late Paleogene, this indicates the derivation of the sediments from the northern margin of the Piedmont–Ligurian Ocean. Sandstone petrography records a stratigraphic evolution from quartzose sandstones towards lithic and then to lithic sub-arkosic composition. This trend is interpreted to reflect the gradual unroofing of the provenance terrane. The lithotypes of the recycled sedimentary rock fragments and the up-section increase in dolostone and carbonate clast proportions suggest the erosion of the sedimentary cover of the southern European palaeo-margin. New geochronological data (U–Pb detrital zircon ages) correspond to the pre-Alpine stages of crustal growth recorded in the Variscan Maures-Tanneron Massif, and therewith confirm the derivation of the sediments from the passive palaeo-European margin. This conclusion highlights the importance of the lower plate in providing the source of coarse-clastic deep-water successions during pre-collisional convergent steps. Results from this multi-proxy provenance analysis contribute to better defining the detrital signatures associated to the continental micro-fragments that constituted the palaeo-European plate as it supplied deep-sea siliciclastic sediments into the Piedmont–Ligurian Ocean prior to continental collision.

Petrography and Geochemistry of the Upper Triassic Sandstones from the Western Ordos Basin, NW China: Provenance and Tectonic Implications

Petrographic and geochemical characteristics of the Upper Triassic sandstones in the western Ordos Basin were studied to provide insight into weathering characteristics, provenance, and tectonic implications. Petrographic features show that the sandstones are characterized by low‐medium compositional maturity and textural maturity. The CIA and CIW values reveal weak and moderate weathering history in the source area. The geochemical characteristics together with palaeocurrent data show that the northwestern sediments were mainly derived from the Alxa Block with a typical recycled nature, while the provenance of the western and southwestern sediments were mainly from the Qinling‐Qilian Orogenic Belt. The tectonic setting discrimination diagrams signify that the parent rocks of sandstones in the western and southern Ordos Basin were mainly developed from continental island arc, which is closely related to the evolution of the Qinling‐Qilian Orogenic Belt. However, the sandstones in the northwestern Ordos Basin show complex features, which may be resulted from a typical recycling process. Overall evidence from petrography, geochemistry and sedimentology, together with previous researches suggest the Kongtongshan and Helanshan areas were the southwestern and northwestern boundary of the Ordos Basin, respectively, and there was no clear boundary between the Hexi Corridor Belt and Ordos Basin, where a large, uniform sediment dispersal system developed during the Late Triassic.

Provenance of Eocene–Oligocene sediments in the San Jacinto Fold Belt: Paleogeographic and geodynamic implications for the northern Andes and the southern Caribbean

AbstractMesozoic and Cenozoic strata of the San Jacinto Fold Belt (Colombian Caribbean) provide insights about sedimentary environments and paleogeographic evolution in the transition between the northern Andes and the South Caribbean deformed belt. We report new provenance (conventional sandstone petrography, heavy mineral analysis, and detrital zircon U-Pb geochronology and typology) and micropaleontologic data (palynology, calcareous nannofossils, and foraminifera) in samples collected from the lower Eocene (San Cayetano Formation) and upper Eocene–Oligocene (Toluviejo and Ciénaga de Oro Formations) rocks in boreholes drilled by the Colombian Agencia Nacional de Hidrocarburos as well as from recently exposed Oligocene outcrops from the Ciénaga de Oro Formation. Sandstone petrography shows modal variations, with high feldspar content in the lower Eocene rocks and high quartz content in the Oligocene deposits. This shift in compositional maturity may be due to climatic variations, tectonic activity, and/or changes in source areas. Heavy mineral analyses indicate variations that suggest sources primarily related to felsic igneous and/or low-grade metamorphic and mafic and ultramafic rocks. Zircon U-Pb geochronology displays age populations mainly in the Late Cretaceous, Late Jurassic, Permian–Triassic, and Precambrian (ca. 900–1500 Ma). In addition, zircon typology analyses indicate that the igneous zircons came primarily from monzogranites and granodiorites. Finally, the micropaleontologic and sedimentary data sets indicate that the sediments were deposited in tropical coastal and shallow marine environments. The sediments were transported by short rivers from the crystalline massifs of the Lower Magdalena Valley and the northern Central Cordillera basements, while distal transport of sediments may have occurred along longer rivers, which brought sediments from southern regions located between the Central and Western Cordilleras.

Provenance of Upper Pennsylvanian siliciclastic‐carbonate deposits from the Monte Alegre and Itaituba formations, North Brazil: An integrated study of sandstone petrography, heavy mineral analysis and garnet geochemistry

The Monte Alegre and Itaituba formations record the transition from a fluvial‐aeolian to a carbonate platform system during the Pennsylvanian on the southern margin of the Amazonas Basin, North Brazil. Sandstones of these formations were analysed for their petrofacies, provenance, tectonic configuration and diagenetic history to determine their origin. The Monte Alegre sandstones are fine‐ to medium‐grained, well‐sorted, bimodal quartz‐arenites with a normal grain‐supported framework of straight and concavo‐convex contacts. They present 97% of ZTR (zircon‐tourmaline‐rutile) heavy minerals. The Itaituba sandstones are fine‐ to very fine‐grained, well‐sorted, sub‐arkoses with an open to normal framework supported by carbonate cement. The heavy minerals are represented by zircon, tourmaline, rutile, kyanite and garnet, with almandine and pyrope as the main types derived from high‐grade amphibolite‐ to‐ granulite‐facies metasediments, in addition to intermediate‐acidic igneous rocks. The petrofacies analysis revealed that the sandstones of these formations belong to the tectonic regime of the continental block, Interior Craton. The Amazonas Basin basement on the southern edge consists of intrusive suites and cratonic rocks of the Tapajós and Iriri‐Xingu provinces, which are represented by the Cuiú‐Cuiú Complex and the Iriri Group. These groups of rocks were the main sources of sandstones of these formations. This conclusion indicates that these rocks were uplifted, exposed and eroded during and/or after the Middle Carboniferous. The imprints of early and deep burial diagenesis of these sandstones, including different stages of compaction, cementation and heavy minerals elimination, produced differences in sandstones porosity and heavy minerals assembly.

A multidisciplinary approach to sediment provenance analysis of the late Silurian–Devonian Lower Old Red Sandstone succession, northern Midland Valley Basin, Scotland

Sediment provenance analysis is important in reconstructing chronological and spatial relationships between source area erosion/exhumation and sediment deposition in adjacent basins. Here we provide new provenance data from conglomerate clast populations, sandstone petrography, heavy mineral assemblages and U–Pb detrital zircon geochronology from the 9 km-thick, Siluro-Devonian Lower Old Red Sandstone (LORS), northern Midland Valley Basin (MVB), Scotland. The MVB developed in the foreland to the Caledonian Orogeny and comprises a mixed succession of conglomerate and pebbly sandstone interbedded with volcanic lithologies. Analysis of 554 samples (137 petrographical thin sections across 22 formations/members; 390 conglomerate clast recordings across 7 locations; 17 heavy mineral and 10 detrital zircon samples) from the entire succession shows a proximal, consistent source or sources located to the east or NE. These results indicate that sediment derived directly from areas of Scandian uplift of Norwegian lithologies is unlikely to have made a significant contribution to the LORS of the northern MVB and the provenance instead lies more proximally in both the neighbouring Dalradian Supergroup metasedimentary rocks and other contemporaneous lithologies. The coarse-grained nature of the sedimentary rocks suggests consistent and continued uplift of these lithologies throughout deposition of the LORS. Integration of these provenance techniques allows robust interpretation of basin fill from a tectonically rejuvenating source in the Caledonian foreland. Supplementary material: MVB LORS palaeocurrent data; List of sample locations is available at https://doi.org/10.6084/m9.figshare.c.4661966

Climatic control on stacking and connectivity of fluvial successions: Upper Cretaceous Bajo Barreal Formation of the Golfo San Jorge basin, Patagonia

Abstract Climate impact on alluvial organization owing to its control on water availability and sediment delivery within the catchment, but temporal changes in stacking patterns are often interpreted to reflect changes in subsidence and base level. To test for evidence of climatic control on the stacking pattern, we study an outcrop succession with two styles of stacking within the Upper Cretaceous Bajo Barreal Formation in the Cerro Ballena anticline, Golfo San Jorge Basin, Argentina. The 385 m thick and 2.5 km wide exposure has layer-cake geometry, lacking either large-scale erosional surfaces, fluvial terraces, or evident paleosols, dismissing either local tectonic activity or base-levels shifts. Rooted in a paleohydrological study recognizing upward increasing in both channel width and flow depth of formative rivers, we use spectral gamma-ray logs, x-ray diffraction in mudstones, and sandstone petrography to understand the controls on the stacking pattern. At the base, Section A consists of small-scale, isolated channels fills in a siliciclastic floodplain with sand:mud ratio of ~1:6, whereas the overlying Section B has a sand-mud ratio of ~1:3, with larger-scale channels and greater inter-connectivity within a volcaniclastic floodplain. Upward reduction in K percentage through Section A parallel with increasing kaolinite content, and mudstone samples from Section B contains a higher proportion of kaolinite than Section A samples, evidencing an upward increase in paleo-weathering in humid conditions. Detrital components of Section A indicate several volcanic sources (e.g., basic-intermediate components derived from the Middle Jurassic Bahia Laura Group and acidic components sourced from the Andes Cordillera), whereas Section B exclusively contains acidic clasts derived from the Andes Cordillera. Simultaneous changes in detrital constituents, suspended load type, and increasing scale of the rivers in Section B occur coeval with increases in channel inter-connectivity, here related to the increase of river discharge and sediment supply in a humid climate, favoring more frequent avulsions or higher channel migration rates in a relatively flat geomorphic scenario. The study demonstrates, combining independent lines of evidence, that climate change can impact the stacking and connectivity of potential sandstone reservoirs.

Petrography and geochemistry of the Lower Silurian sandstones from the Angzanggou Formation in the North Qilian Belt, China: implications for provenance, weathering and tectonic setting

AbstractThe North Qilian Orogenic Belt is on the northeastern margin of the Tibetan Plateau. It is connected with the Alxa Block in the north and the Tethyan orogenic assemblage in the south. The Lower Silurian Angzanggou Formation, deposited in the northern area of the North Qilian Orogenic Belt, is the most important for revealing the architecture and orogenic processes of the North Qilian Belt. Provenance analysis of the Angzanggou Formation can reveal not only the tectonic evolution of the central orogenic belt of China but also Palaeozoic Asia plate reconstructions. Petrographic analysis indicated that the compositional and textural maturity of the sandstones was low. The detrital composition of the Angzanggou Formation samples consists of quartz (8–14 %), feldspar (6–29 %) and lithic fragments (56–86 %). The sandstones could be classified as litharenites or feldspathic litharenites. The detrital modal composition suggests that these sandstones were probably deposited in a fore-arc basin. The element ratios and some discrimination diagrams based on geochemistry indicate that felsic and intermediate rocks were the main source rocks. The SiO2/Al2O3 ratio, the index of chemical variability and the Th/Sc versus Zr/Sc discrimination diagram suggest that the compositional maturity and degree of recycling were moderate to low. The index of alteration (CIA) and the A–CN–K diagram indicate the intensity of weathering was moderate. The discrimination diagrams based on major and trace elements and petrographic discrimination diagrams imply that the Angzanggou Formation rocks were derived from a continental island arc, and a sedimentary cover probably overlaid the volcanic arc. Therefore, we infer that during Early Silurian time the North Qilian Belt sediments accumulated in a fore-arc basin.

Detrital zircon geochronology along a structural transect across the Kahiltna assemblage in the western Alaska Range: Implications for emplacement of the Alexander-Wrangellia-Peninsular terrane against North America

AbstractThe Kahiltna assemblage in the western Alaska Range consists of deformed Upper Jurassic and Cretaceous clastic strata that lie between the Alexander-Wrangellia-Peninsular terrane to the south and the Farewell and other pericratonic terranes to the north. Differences in detrital zircon populations and sandstone petrography allow geographic separation of the strata into two different successions, each consisting of multiple units, or petrofacies, with distinct provenance and lithologic characteristics. The northwestern succession was largely derived from older, inboard pericratonic terranes and correlates along strike to the southwest with the Kuskokwim Group. The southeastern succession is characterized by volcanic and plutonic rock detritus derived from Late Jurassic igneous rocks of the Alexander-Wrangellia-Peninsular terrane and mid- to Late Cretaceous arc-related igneous rocks and is part of a longer belt to the southwest and northeast, here named the Koksetna-Clearwater belt. The two successions remained separate depositional systems until the Late Cretaceous, when the northwestern succession overlapped the southeastern succession at ca. 81 Ma. They were deformed together ca. 80 Ma by southeast-verging fold-and-thrust–style deformation interpreted to represent final accretion of the Alexander-Wrangellia-Peninsular terrane along the southern Alaska margin. We interpret the tectonic evolution of the Kahiltna successions as a progression from forearc sedimentation and accretion in a south-facing continental magmatic arc to arrival and partial underthrusting of the back-arc flank of an active, south-facing island-arc system (Alexander-Wrangellia-Peninsular terrane). A modern analogue is the ongoing collision and partial underthrusting of the Izu-Bonin-Marianas island arc beneath the Japan Trench–Nankai Trough on the east side of central Japan.