Craton Interior Research Articles

Deconvolving the pre-Himalayan Indian margin – Tales of crustal growth and destruction

The metamorphic core of the Himalaya is composed of Indian cratonic rocks with two distinct crustal affinities that are defined by radiogenic isotopic geochemistry and detrital zircon age spectra. One is derived predominantly from the Paleoproterozoic and Archean rocks of the Indian cratonic interior and is either represented as metamorphosed sedimentary rocks of the Lesser Himalayan Sequence (LHS) or as slices of the distal cratonic margin. The other is the Greater Himalayan Sequence (GHS) whose provenance is less clear and has an enigmatic affinity. Here we present new detrital zircon Hf analyses from LHS and GHS samples spanning over 1000 km along the orogen that respectively show a striking similarity in age spectra and Hf isotope ratios. Within the GHS, the zircon age populations at 2800–2500 Ma, 1800 Ma, 1000 Ma and 500 Ma can be ascribed to various Gondwanan source regions; however, a pervasive and dominant Tonianage population (∼860–800 Ma) with a variably enriched radiogenic Hf isotope signature (εHf = 10 to −20) has not been identified from Gondwana or peripheral accreted terranes. We suggest this detrital zircon age population was derived from a crustal province that was subsequently removed by tectonic erosion. Substantial geologic evidence exists from previous studies across the Himalaya supporting the Cambro-Ordovician Kurgiakh Orogeny. We propose the tectonic removal of Tonian lithosphere occurred prior to or during this Cambro-Ordovician episode of orogenesis in a similar scenario as is seen in the modern Andean and Indonesian orogenies, wherein tectonic processes have removed significant portions of the continental lithosphere in a relatively short amount of time. This model described herein of the pre-Himalayan northern margin of Greater India highlights the paucity of the geologic record associated with the growth of continental crust. Although the continental crust is the archive of Earth history, it is vital to recognize the ways in which preservation bias and destruction of continental crust informs geologic models.

Седиментология и стратиграфическая корреляция вендских отложений на юго-западе Сибирской платформы: выдающийся вклад внешнего источника кластического материала в образование осадочных систем

The study focuses on the sedimentation and stratigraphy of a large Vendian (Ediacaran) sedimentary basin in the southwestern Siberian craton, with implications for the origin of its units and subunits. Sediments are been correlated along the craton edge and inward into the craton interior using stratigraphic sequences, including glacial and post-glacial deposits. Clastic complexes in the Vendian: upper Chapa, Taseeva, Oselok, and Baikal groups composed of fluvial deposits are of special importance for correlations. The structure of the continental Aysa Fm. of the Oselok Group in the Sayany area has been characterized in detail, based on universal classification of lithofacies and their sequences, and correlated with sediments in the craton interior. There are two Ediacaran subunits: a continental-marine complex below and a continental complex above. The continental-marine subunit in the subsided craton margins was deposited mainly upon postglacial plains (sandurs) and marginal sea shelves, with contributions from intracratonic clastic sources. The deposits of the continental subunit belong to centripetal fluvial systems directed from distant craton-margin mountain upland toward the epicontinental sea in the central Siberian craton. The continental subunit is composed of large sections of red and variegated fluvial deposits from a foreland basin which was accumulated mainly in foredeeps in the over-compensation setting and partly reached the slopes of forebulges. Big deep sand rivers with large channel bars were formed at early and late during orogenic activity, while perennial anastomosing and braided mud-sand rivers with levees originate during passive stages of orogeny. The largest progradation of fluvial systems onto the Siberian craton is marked by channel deposits of the lower members from the Nemchanka, Moshakovka, Aysa, Khuzhir and Kachergat fms., as well as by the Bokhan sandstone member of the Chora Fm.

Testing Patterns of Association between Brachiopod Bioclast Deposits and Stratigraphic Discontinuities: A Case Study in Middle–Upper Devonian Carbonate-Dominated Settings of North America

AbstractNondepositional hiatuses tend to favor the dense accumulation of skeletal material because they are associated with minimal sedimentary dilution of bioclast input. However, bioclast durability or production must be sufficient to counterbalance the rigors of delayed burial and especially the repeated events of seafloor exposure and reworking that bioclasts can experience during sedimentary bypass or starvation associated with the formation of hiatuses. Detailed field investigations of skeletal concentrations in Middle–Upper Devonian brachiopod-rich carbonate stratigraphic records from the continental margin of Nevada and the cratonic interior of Iowa reveal a surprising lack of abundant skeletal material mantling major stratigraphic discontinuities. Rather, the distribution of densely packed brachiopod deposits tends to be determined by physical processes that operate within the timescale of deposition of a single lithofacies, producing skeletal concentrations that are mostly associated with beddin...

Petrogenesis of the NE Gondwanan uppermost Ediacaran-Lower Cretaceous siliciclastic sequence of Jordan: Provenance, tectonic, and climatic implications

Detrital framework modes of the NE Gondwanan uppermost Ediacaran-Lower Cretaceous siliciclastic sequence of Jordan are determined employing the routine polarized light microscope. The lower part of this sequence constitutes a segment of the vast lower Paleozoic siliciclastic sheet flanking the northern Gondwana margin that was deposited over a regional unconformity truncating the outskirts of the East African orogen in the aftermath of the Neoproterozoic amalgamation of Gondwana. The research aims to evaluate the factors governing the detrital light mineral composition of this sandstone. The provenance terranes of the Arabian craton controlled by plate tectonics appear to be the primary factor in most of the formations, which could be either directly inferred employing Dickinson‘s compositional triangles or implied utilizing the petrographic data achieved and the available tectonic and geological data. The Arabian-Nubian Shield constitutes invariably the craton interior or the transitional provenance terrane within the NE Gondwana continental block that consistently supplied sandy detritus through northward-flowing braided rivers to all the lower Paleozoic formations. On the other hand, the Lower Cretaceous Series received siliciclastic debris, through braided-meandering rivers having same northward dispersal direction, additionally from the lower Paleozoic and lower-middle Mesozoic platform strata in the Arabian Craton. The formations making about 50% of the siliciclastic sequence represent a success for Dickinson‘s plate tectonics-provenance approach in attributing the detrital framework components primarily to the plate tectonic setting of the provenance terranes. However, even under this success, the varying effects of the other secondary sedimentological and paleoclimatological factors are important and could be crucial. The inapplicability of this approach to infer the appropriate provenance terranes of the remaining formations could be ascribed either to the special influence of local intracratonic syn-rift rhyolitic extrusions, where their plate tectonic setting is not represented by the standard plate tectonics-provenance diagrams, or to the rather unusual effect of the Late Ordovician glacial event.

SEDIMENTOLOGY AND PETROGRAPHY ATTRIBUTES OF OUTCROPPING SEDIMENTS OF COASTAL PLAIN SANDS FROM ABUDU AND ITS ENVIRONS, SOUTHERN NIGERIA

Relying on field and laboratory evidences, outcropping sediments at Abudu and its environs were studied for their sedimentological and petrographic attributes. Based on field and sedimentological evidences, two lithofacies, sandstone and claystone were identified in the study area. The claystone facies include two (2) sub-lithofacies of silty clay and sandy clay; while the sand facies include medium to coarse-grained friable sands and, fine to medium-grained sands. Particle size analysis shows sorting values ranging from 0.94 to 2.10 indicating moderately to poorly sorted; skewness values of -0.13 to 0.2 indicating near symmetrical to positively skewed and kurtosis values in the range of 0.77 to 1.34 suggestive of platykurtosis to leptokurtosis. The petrographic analysis reveals the major framework minerals to include quartz (76.9 to 97.5%), rock fragment (0 to 12.9%) and feldspar (0 to 15.4%) and mineral maturity index (MMI) range from 3.8 to 39.0. Poor to moderate porosity was inferred for the sediments. Using field evidences and scatter plots of Skewness versus sorting and mean versus sorting, a fluvial depositional setting was interpreted for the sediments. The sediments range from sub-mature to super-mature; subarkosic to quartz arenites and sediments have undergone intense chemical weathering and/or erosion as evident in the predominance of quartz over feldspar. A craton interior source with contributions from recycled orogen is suggested for the sediments of the study area.

Ediacaran forebulge grabens of the southern São Francisco basin, SE Brazil: Craton interior dynamics during West Gondwana assembly

The Ediacaran Bambuí Group corresponds to the main sedimentary succession exposed in the intracratonic São Fransico basin, E Brazil. Comprising an unconformity-bounded 1st-order sequence, it has been widely interpreted as the record of a marine foreland system developed on the São Francisco plate during the Brasiliano/PanAfrican West Gondwana assembly. Nevertheless, the lack of well characterized foreland system depocenters and the paucity of precise geochronological data have hampered the understanding of its basinal evolution. Based on new subsurface data (2D seismic and well), aerogeophysical information, field work and literature compilation, we present the first tectono-stratigraphic analysis of a series of kilometer-long subsurface grabens preserved in the southern São Francisco basin and discuss their significance in the context of the Bambuí foreland system. Occupying mostly the crest of the Sete Lagoas basement high, they formed through the extensional reactivation of ancient cratonic structures and evolved partially contemporaneously to the deposition of two mixed carbonate-siliciclastic, transgressive–regressive 2nd-order sequences of the basal to middle Bambuí Group. These troughs developed in consequence of successive pulses of forebulge uplift and migration of the Sete Lagoas basement high, apparently in the late Ediacaran. Together with an important post-glacial eustatic rise that affected lower latitudes, this forebulge dynamics and its distinctive structures controlled the tectono-stratigraphic architecture and dispersal of the studied Ediacaran strata. The recognition of this foreland depocenter and its tectono-stratigraphic evolution reveals important aspects of one of the most expressive orogenic basins developed during the West Gondwana assembly. Along with available paleotectonic reconstructions, it may contribute to explain the apparent mismatch between the evolution of the southern Brasília belt and the Ediacaran basin. Furthermore, flexural subsidence interference patterns associated with the evolution of the neighboring Brasiliano orogenic systems seem to have played an important role on the evolution of the Sete Lagoas forebulge, as well as on the development of the entire Bambuí foreland system. Our study offers elements to comprehend the mechanisms operating on stress and sedimentary partitioning within foreland systems, as well as to better evaluate the potential for Precambrian mineral and unconventional hydrocarbon resources of the São Francisco basin and elsewhere.

Integrated Petrographic, Mineralogical, and Geochemical Study of the Upper Kaimur Group of Rocks, Son Valley, India: Implications for Provenance, Source Area Weathering and Tectonic Setting

ABSTRACT The upper Kaimur Group (UKG) of the Vindhyan Supergroup in central India, primarily consists of three rock types-Dhandraul sandstone, Scarp sandstone and Bijaigarh shale. The present study aims to reconstruct the parent rock assemblages, their tectonic provenance, mineralogy, weathering intensity, hydraulic sorting and depositional tectonic setting. Samples from the UKG rocks representing the Dhandraul sandstone, Scarp sandstone and Bijaigarh shale were studied using a combination of petrographic, mineralogical, and geochemical techniques. Texturally, medium to coarse grained UKG sandstones are mature and moderate to well sorted. Deficiency of feldspars in these sandstones indicates that the rocks are extensively recycled from distant sources. Their average modal composition for Scarp (avg. Qt99F0.2L0.8) and Dhandraul (avg. Qt99 F0.1L0.8) sandstones, classifies them as quartz arenite to sub-litharenite types, which is consistent with geochemical study. Major element concentrations revealed that sandstones have high SiO2, K2O > Na2O, and low Fe2O3, which are supported by the modal data. On the other hand, sandstone samples are enriched in most trace elements such as Ce, Sr, V, Sc and Zr and depleted in U and Th. The CIA values (43.17-76.48) of the UKG rocks indicate low to moderate weathering, either of the original source or during transport before deposition, which may have related to low-relief and humid climatic conditions in the source area. Further, petrographic and geochemical interpretations indicate that they are derived from craton interior to quartzose recycled sedimentary rocks and deposited in a passive continental margin. Therefore, granitic and low grade metamorphic rocks of Mahakoshal Group and Chotanagpur granite-gneiss, situated on the southern and south-eastern side of the Vindhyan basin are suggested as possible provenance for the UKG rocks.

Petrography and geochemistry of sandstone–mudstone from Barakar Formation (early Permian), Raniganj Basin, India: Implications for provenance, weathering and marine depositional conditions during Lower Gondwana sedimentation

Post‐glacial Permian coal‐bearing sedimentary successions are traditionally interpreted as fluvially deposited within the Gondwanaland continental set‐up throughout the globe. Recent attempts to reinterpret such successions in terms of marine flooding events, mainly based on sedimentological and ichnological attributes, raised doubt on the existing palaeogeographic model of the Late Palaeozoic Gondwanaland. Reassessing these sedimentary successions using other proxies, like petrographic and geochemical properties, may provide more reliable clues to improve the present understanding.In this paper, petrographic and geochemical analysis of sandstones, sandstone–mudstone heteroliths, and mudstones of the early Permian Barakar Formation of Lower Gondwana Supergroup, Raniganj coal Basin, India, is presented to understand the provenance, palaeoweathering pattern and palaeodepositional conditions. Petrographically, the sandstones are arkosic to sub‐arkosic in nature, with abundant unstable components and heavy minerals. Clay minerals in the matrix and the mudstones are dominated by kaolinite, illite‐smectite with authigenic glauconite, indicating a marine diagenetic realm. Mineralogical assemblage attests to sedimentation in craton interior passive margin conditions. Geochemically, the lithounits show large variations in major element and trace element (including rare earth element) concentrations. Chemical index of alteration and A–CN–K ternary plot indicate moderate to strong chemical weathering, leading to deposition of compositionally immature sediments close to their source without much recycling. Ratios of major elements (e.g., MgO, K2O, Fe2O3, Al2O3, and SiO2) signify a continental–marine transitional depositional environment developed in a stable passive margin setting under gradually warming up climatic conditions. Trace element ratios manifest an estuarine depositional setting during Barakar sedimentation, comparable with modern major river mouth estuaries.Thus, this paper provides unequivocal evidence of significant marine influence during sedimentation in the Lower Gondwana basins in India, particularly during the Permian time, and signifies the importance of re‐examining the so called “continental fluvial” coal‐bearing deposits as considerably marine‐influenced. The interpretations provide important clues in understanding and reconstructing the Permian Lower Gondwana palaeogeography in eastern peninsular India.

Kinematics and timing of continental block deformation from margins to interiors

AbstractDuring the deformation of continental blocks, the magnitude of tectonic stress generally decreases with increasing distance from the margin of the block. However, the timing and kinematics of stress transmission from the margins to the interiors of continents are poorly resolved, even though this information is critical to our understanding of the dynamics of continental deformation. Here, we present a case study of Mesozoic deformation of the North China Craton (NCC). Field investigations of Mesozoic thrust faults and folds, granitic intrusions and dykes, combined with zircon SHRIMP and LA–ICP–MS dating and muscovite 40Ar/39Ar plateau ages, reveal the age of the NE–SW‐trending tectonic belts as ~180–155 Ma, where the deformation of the craton margin occurred 10–20 Ma earlier than that of the craton's interior. Although the kinematics of deformation are similar for the interior and the margin of the NCC, strain decreases with increasing distance from the margin. Notably, the bulk of the strain in the cratonic interior was focused in zones of pre‐existing weakness. Overall, we determined that the NCC deformed under conditions of uniaxial compression, a conclusion that is compatible with simple rheological models, and that the stress magnitude attained in the cratonic interior was much less than that along its margin.

Provenance of coastal dune sands along Red Sea, Egypt

Texture, mineralogy, and major and trace element geochemistry of 26 coastal dune sand samples were studied to determine the provenance and tectonic environment of two dune fields close to the beaches of Safaga (SF) and Quseir (QS) at the Egyptian Red Sea coast. Onshore winds generate fine, moderate, moderately-well to well-sorted, coarse-skewed to near-symmetrical dune sands with mesokurtic distributions. Winds pick up and transport grains from nearby beach sands and alluvial deposits into a wide Red Sea coastal plain at the border of the beach. The mineralogical (Qt–Ft–Lt) and geochemical composition of the sands, indicate that SF and QS coastal dune sands are mature and influenced by quartz-rich sands. The average CIA values in SF and QS coastal dune sands are low relative to the range of the PAAS, suggesting an arid climate and a low intensity of chemical weathering. The SF and QS coastal dune sand samples are plotted in the recycled orogen and partly in craton interior fields suggesting recycled older sedimentary and partly metamorphic-plutonic sources. The high content of quartz with shell debris and carbonates in coastal dune sands support the recycled sedimentary beach and alluvial sand sources. The dominance of heavy minerals like amphiboles (hornblende) and biotite in the coastal dune sands also supports the effect of metamorphic-plutonic source rocks. The new tectonic discriminant-function diagrams suggest that the coastal dune sands were deposited in a passive margin of a synrift basin. The results provide a good evidence for the extension in the Red Sea rift system during Oligocene-post Pliocene, which is consistent with the general geology of Egypt.

Investigating the Paleozoic–Mesozoic low-temperature thermal history of the southwestern Canadian Arctic: insights from (U–Th)/He thermochronology

The Arctic Amerasia Basin, located between the Canadian margin and Alaska, formed by purported Jurassic–Cretaceous rifting related to the rotation of the Arctic Alaska – Chukotka microcontinent from northern Laurentia. Rifting may have been accompanied by rift shoulder uplift and cooling that is recorded in low-temperature thermochronometers. Furthermore, the southwestern Canadian Arctic has a widespread Devonian–Cretaceous unconformity with a poorly understood burial-unroofing history. We evaluate new zircon (U–Th)/He thermochronology (ZHe) and organic maturity (vitrinite reflectance (VRo)) data from Neoproterozoic strata of the Amundsen Basin, Cambrian strata of the Arctic Platform, and Devonian strata of the Franklinian Basin to help resolve the sedimentary thickness deposited and eroded during the time represented by the regional unconformity. ZHe and VRo models identify the thermal maximum occurring between the late Paleozoic – Mesozoic interval. Proximal to the rifted Canadian margin, models estimate 3.7–4.5 km of deposition between the Devonian–Cretaceous, in marked contrast to <1 km towards the craton. Jurassic–Cretaceous exhumation is estimated at 2.3–3.5 km and is more uniform across the region. Although the magnitude of burial and erosion can be resolved by modelling, the timing of these events cannot be elucidated with confidence. The thermochronology models can be satisfied by either (1) late Paleozoic – early Mesozoic burial with a thermal maximum prior to Jurassic rifting, followed by cooling; or (2) Late Devonian maximum burial, with gradual unroofing until Cretaceous sedimentation. Although continued deposition into the Mesozoic towards the craton interior seems unlikely, it remains possible that there was continued deposition proximal to the rifted Canadian margin.

Fluvial transport of impact evidence from cratonic interior to passive margin: Vredefort-derived shocked zircon on the Atlantic coast of South Africak

Meteorite impacts produce shocked minerals in target rocks that record diagnostic high-pressure deformation microstructures unique to hypervelocity processes. When impact craters erode, detrital shocked minerals can be transported by fluvial processes, as has been demonstrated through studies of modern alluvium at some of the largest known impact structures. However, the ultimate fate of distally transported detrital shocked minerals in fluvial systems is not well understood and is an important parameter for constraining the location of a source crater. In South Africa, detrital shocked minerals from the 2020 Ma Vredefort impact structure have been documented in the Vaal River basin, downriver from the structure. Here, we report results of an extensive microstructural survey of detrital zircon from the Orange River basin and the Atlantic coast of South Africa to search for the presence of far-traveled Vredefort-derived detrital shocked zircon grains in different modern sedimentary environments. Three shocked grains were found out of 11 168 grains surveyed (0.03%) by scanning electron microscopy, including two in beach sand on the Atlantic coast and one from a sandbar 15 km upstream from the mouth of the Orange River. Shock-produced {112} twins documented by electron backscatter diffraction in each of the three grains confirm their impact provenance, and U-Pb ages from 3130 to 3040 Ma are consistent with derivation from bedrock at the Vredefort impact structure. These results demonstrate the transport of Vredefort-derived shocked zircon to the coast via the Vaal-Orange river system, which requires 1940 km of fluvial transport from their point source on the Kaapvaal craton to the Atlantic coast passive margin. These results further demonstrate that shocked zircon grains can be detected in detrital populations at abundances

Petrology and geochemistry of clastic sedimentary rocks as evidences for provenance of the Late Palaeozoic Madzaringwe Formation, Tshipise-Pafuri Basin, South Africa

Abstract Petrography and geochemistry (major, trace and rare earth elements) of clastic rocks from the Late Palaeozoic Madzaringwe Formation, in the Tshipise-Pafuri Basin, Northern South Africa, have been investigated to understand their provenance. Sandstone petrography and detrital modes indicates that the Late Palaeozoic succession was derived from craton interior and recycled orogen provenance. Sandstones in the Madzaringwe Formation are sub-arkosic to sub-litharenite. The sediments may represent a recycled to craton interior provenance. The geochemical data of major elements show that sandstone and shales have the same source. The study of paleoweathering conditions based on modal composition, chemical index of alteration (CIA) and A-CN-K (Al2O3-CaO+Na2O-K2O) relationships indicate that probably chemical weathering in the source area and recycling processes have been more important in shale and sandstone rocks. The relatively high CIA values (70–90%) indicates moderate to high weathering conditions of the samples and the paleoclimate of the source area was warm. K2O/Na2O versus SiO2 and Na2O-CaO-K2O tectonic setting discrimination plots, suggest a passive continental margin. In the study of trace elements, triangular Th-Sc-Zr/10 and La-Th-Sc plots both suggest a passive margin setting of the basin. Petrographic and geochemical results of the samples suggest uplifted basement source areas dominated by sedimentary rocks and/or granite-gneiss rocks. The source rocks might have been the recycled pre-Soutpansberg Karoo Supergroup rocks and the metasedimentary rocks of the Soutpansberg Group. Other source rocks may have been the pre-Beit-Bridge basement rocks (granites and gneisses).

Plates or plumes in the origin of kimberlites: U/Pb perovskite and Sr-Nd-Hf-Os-C-O isotope constraints from the Superior craton (Canada)

Neoproterozoic kimberlite, ultramafic lamprophyre, and carbonatite magmatic activity was widespread across the Canadian-Greenland Shield. Models to explain the preponderance of this deeply-derived CO2-rich magmatism between 680–540Ma range from impingement of multiple mantle plumes to rifting activity linked to the breakout of the Laurentian plate from the Rodinia supercontinent configuration. We add to the debate about the origin of kimberlite magmas and evaluate possible mantle sources of the 655Ma ‘diamond-rich’ Renard (new SIMS U/Pb perovskite ages) and 629Ma ‘barren’ Wemindji kimberlites on the eastern Superior craton in Quebec, Canada.Our Sr-Nd-Hf and carbon isotope data (87Sr/86Sri=0.70241–0.70442; εNdi=+0.2 to +4.8; εHfi=+0.3 to +6.5; δ13C=−5.6 to −3.9‰) suggest a common and moderately depleted convecting upper mantle source region for both the Renard and Wemindji kimberlites, which occur 400 km apart in the interior of the Superior craton. In contrast, the low Os isotope ratios (187Os/188Osi=0.11078–0.12620; γOsi=−13.7 to −1.6) and unfractionated chondritic relative HSE abundances (Os, Ir, Ru, Pt, Pd, Re) indicate significant involvement of ancient refractory cratonic mantle material in kimberlite magma formation. Our model calculations suggest that for both the diamond-rich Renard and the barren Wemindji kimberlite magmas up to 30% of the Os was derived from refractory cratonic peridotites. This material might have been assimilated by originally more CO2-rich carbonated silicate melts derived from the asthenosphere. We also show that the geochemical and Sr-Nd-Hf-Os isotopic compositions of the Renard and Wemindji kimberlites do not require significant input from melts derived from olivine-poor cratonic mantle lithologies such as MARID-type veins and pyroxenites/eclogites. This contrasts with the petrogenesis of deeply-derived volatile-rich potassic magmas found along the peripheries of cratons (e.g., ultramafic lamprophyres, kamafugites, and olivine lamproites), a setting where abundant non-peridotitic components have been added to the lithospheric mantle over the course of continent evolution.Provided that CO2-rich melts, such as proto-kimberlites, occur near the solidus of volatile-fluxed peridotites, no excess mantle heat is required in their formation. This important but often overlooked constraint, together with the observation that there exist no spatial or temporal relationships between the Superior craton kimberlites and Large Igneous Provinces during the Late Neoproterozoic, suggests that kimberlite magmatic activity was tectonically controlled. In our preferred model, ubiquitous CO2-rich proto-kimberlite melts form during volatile-controlled redox melting processes at ambient mantle temperatures in a thermal boundary layer directly beneath thick cratonic lithosphere. The success rate of ‘evolving’ hybrid kimberlite magmas reaching Earth’s surface increases when tensile stresses propagate into the >200km thick keels of continental lithosphere. These conditions are frequently met during fast and changing plate motions associated with the assembly and breakup of supercontinents.

Carboniferous sediment dispersal in the Appalachian–Ouachita juncture: Provenance of selected late Mississippian sandstones in the Black Warrior Basin, Mississippi, United States

The Black Warrior Basin is one of several Carboniferous foreland basins along the Appalachian–Ouachita fold-thrust belt in the southeastern United States. Sediment dispersal within the Black Warrior Basin has been a long-debated topic because of a complex tectonic history and the potential interaction between the Appalachian and Ouachita orogenic belts, as well as far field sediment sources. Three dispersal patterns have been proposed, including dispersal routes from the craton, dispersal via the Appalachian foreland, and dispersal from the arc side of the Ouachita suture, but sediment dispersal in the Black Warrior Basin remains inconclusive. In this study, sandstone modal analysis and U–Pb detrital zircon geochronology are used to document the provenance and potential dispersal patterns for selected Mississippian sandstone units in the Black Warrior Basin, Missouri, USA. Results show that the majority of the Lewis, Evans, Sanders, and Carter sandstones are sublitharenite to mature quartzarenite and fall within the Cratonic Interior field on Q–F–L diagrams. U–Pb detrital zircon analyses of the Lewis, Sanders, and Carter sandstones show that there are four distinctive age clusters, including a prominent Paleozoic age cluster (~350–500Ma), a broad Grenville age cluster (~900–1350Ma), and two minor age clusters of the Granite–Rhyolite (~1360–1600Ma) and the Yavapai–Mazatzal (~1600–1800Ma) provinces. All Mississippian sandstones have similar age distributions except for the Lewis sandstone, which lacks zircon grains from the Superior province (>~2500Ma). Based on the compositional maturity, similarity of age distributions, and changes of relative abundance among different age groups, we conclude that the Late Mississippian sandstone units analyzed during this study were derived from the Laurussian craton and the northern part of the Appalachian foreland through a major axial drainage that occupied the Mississippi Valley Graben.

MIDDLE TO UPPER DEVONIAN SKELETAL CONCENTRATIONS FROM CARBONATE-DOMINATED SETTINGS OF NORTH AMERICA: INVESTIGATING THE EFFECTS OF BIOCLAST INPUT AND BURIAL RATES ACROSS MULTIPLE TEMPORAL AND SPATIAL SCALES

Skeletal concentrations, defined here as deposits ≥ 10% by volume invertebrate bioclasts (> 2 mm), are very common targets of paleobiological investigations. The complex interactions among biological, taphonomic, and physical environmental processes influence the type and quality of information that can be drawn from these paleobiological repositories. This study examines the relative roles of bioclast input and burial on the formation of Middle–Upper Devonian skeletal concentrations from tropical carbonate-dominated settings. Based on original field observations of skeletal concentrations and host lithologies, skeletal concentrations are compared (1) between a thermally subsiding passive margin (Nevada) and a relatively stable cratonic interior (Iowa) and (2) across a range of comparable subtidal depositional environments. The majority of skeletal concentrations are thin (≤ 30 cm), monotypic, have simple internal stratigraphy, and exhibit moderate to high degrees of skeletal fragmentation and disarticulation. Compared to the subsiding margin of Nevada, the cratonic interior of Iowa preserves a higher proportion of polytaxic skeletal concentrations with complex internal stratigraphy and higher taphonomic damage, consistent with control by delayed or slow burial. From shallow to deep subtidal environments, stratigraphic thickness and internal complexity decrease and fragmentation increases, consistent with strong control of these attributes by the frequency of physical and biogenic reworking. While low bioclast production rates may exert a dominant control on the failure to accumulate thick, dense skeletal concentrations in these deposits, taphonomic and physical environmental processes, such as low net sediment accumulation, sediment winnowing and starvation, and bioturbation can explain much of the variation in Middle–Upper Devonian skeletal concentrations.

A comparative study between the depositional environment and provenance of the Hussainiyat (Lower Jurassic) and the Rutbah (Upper Cretaceous) Formations, Western Desert of Iraq

This work deals with the sedimentology of the Hussainiyat Formation (Lower Jurassic) and Rutbah Formation (Upper Cretaceous) in the Western Desert of Iraq with special emphasis on their depositional environments and nature of their source rocks. These formations are particularly chosen since they belong to different episodes, which are before (Hussainiyat Formation) and after (Rutbah Formation) the late Kimmerian deformation phase. Both units comprise supermature sandstone and contain a fairly similar suite of heavy detrital accessories (almost ultrastable minerals only). Such nature of the clastics impairs the identification in the usual methods, commonly carried out by sedimentary petrologists, although petrographic textures of quartz sand grains and the polycrystalline quartzitic pebbles are found useful. Five sections were described from both formations, and a total of 58 sandstone samples were collected, in addition to 153 gravel samples. The Hussainiyat Formation represents the classic model for high-sinuosity (meandering) river system, and the Rutbah Formation represents deposition in low-sinuosity (braided) river system resembling Platt-type model. Paleocurrent analysis of the Hussainiyat sandstones showed a southeasterly flow, and the Rutbah sandstone showed a northeasterly flow. Provenance analysis of the Hussainiyat and Rutbah Formations showed that they have different sources. Petrographic analysis of the quartz sand grains and the heavy minerals indicates derivation from sedimentary rock and plutonic or medium- to high-rank metamorphic rocks, but there is a marked difference between the percentages of the constituents in each formation. The textural features encountered in the majority of the studied pebbles characterize low-grade metamorphic rocks, and the statistical procedure (ANOVA) used shows that they have been derived from two different sources. Petrofacies studies show that the tectonic setting of the provenance area of both sandstones is craton interior of the continental block.

Sedimentary provenance and weathering processes in the 1.1 Ga Midcontinental Rift of the Keweenaw Peninsula, Michigan, USA

Fluvial, alluvial, lacustrine, and debris flow sedimentary rocks from the Keweenaw Peninsula (Michigan, USA) are geochemically characterized to examine weathering processes and provenance. These sedimentary rocks result from the erosion, weathering, and redistribution of effusive and plutonic igneous lithologies during periods of volcanic quiescence associated with the failed 1.1Ga North American Midcontinental Rift. Three units comprising the sedimentary/igneous-mixed Portage Lake Volcanics, coarseness and source-varied Copper Harbor Conglomerate, and the quartz-rich post-rift fill unit of the Jacobsville Sandstone were studied over a geographically-wide and lithologically-varied range of localities across the Keweenaw Peninsula, Michigan. Provenance indicators such as Ti/Zr and Ti/Al ratios and La-Th-Sc ternary diagrams reveal that the Portage Lake Volcanic conglomerates are near-source, and that the Copper Harbor Conglomerate is primarily derived from felsic uplifted basement and recycled orogen lithologies to the east. The Jacobsville Sandstone is a combination of pulses of sources from the top to the bottom of the stratigraphic section: (1) felsic arkosic arenites sourced from the erosion and uplift of basement orogenic volcanic and plutonic rocks (top), (2) felsic arkosic-subarkosic arenites sourced from the erosion of uplifted basement, recycled orogeny and reworking of primary sedimentary and metasedimentary rocks (middle), and (3) quartz arenites eroded from the cratonic interior (highly mature). This work can consequently aid in structuring a stratigraphic nomenclature for the Jacobsville Sandstone. Both petrographic data and weathering indices such as the chemical index of alteration (CIA) and the paleosol hydrolysis metric (Al/Ca+Na+Mg+K) indicate low degrees of weathering during syn-rift deposition at 1.1Ga, but increases with evolution of the rift in post-rift units toward the Late Mesoproterozoic/Early Neoproterozoic. Values for the CIA throughout the Mesoproterozoic from this and other localities have the same trend to that of proposed atmospheric CO2 concentrations derived from paleosol data, demonstrating that characterizing the weathering trends of floodplain deposits can be a useful addition to deciphering past paleoclimatic conditions in the Precambrian geologic record.

Depositional Dynamics In A Mixed Carbonate–Siliciclastic System: Middle–Upper Cambrian Abrigo Formation, Southeastern Arizona, U.S.A.

Abstract The Middle and Late Cambrian mixed carbonate–siliciclastic Abrigo Formation of southeastern Arizona was deposited during the Sauk transgression in the craton interior landward of the passive margin of Laurentia. It overlies shallow-marine sandstone of the Bolsa Quartzite, which mantled the Precambrian land surface. The Abrigo Formation consists of ten distinct rock types that comprise fifteen lithofacies, which are grouped into eight facies associations. The strata represent an array of shallow-marine environments that were dominated by wave and storm activity. The interpreted paleoenvironments include lower offshore, upper offshore, offshore transition, and lower, middle, and upper shoreface. These environments migrated laterally as a function of relative sea-level changes along with input of siliciclastic sediment and its effect on carbonate deposition. Stratigraphic distribution of facies indicates that there were two separate carbonate factories: one in the nearshore immediately seaward of the siliciclastic shoreface and the other in the distal offshore area. Correlation across 170 km of the study area suggests that these lithofacies were deposited in six temporally distinct phases involving two transgressive–highstand systems tract couplets during the Bolaspidela through Crepicephalus biozones, a falling-stage systems tract in the Aphelaspis Biozone, and a lowstand systems tract in the Elvinia Biozone following the Sauk II–Sauk III hiatus. In general, the mixed carbonate–siliciclastic depositional environment of the Abrigo Formation shows that fine-grained siliciclastic facies dominate the transgressive systems tract. By contrast, carbonate sedimentation was dominant mostly during the middle to late phase of the highstand. The upper part of the highstand systems tract records progradation of the sandy shoreface. Nevertheless, the ratio between siliciclastic and carbonate sediment in various bathymetric zones differs from previously described inner-detrital-belt examples of Cambrian age. In the Abrigo Formation, some bioclastic grainstone in shoreface deposits contains siliciclastic sand, indicating that the two were deposited together until sand was dominant. However, carbonate production and siliciclastic mud sedimentation were for the most part mutually exclusive, suggesting that the shallow-water carbonate factory during the Middle and Late Cambrian was vulnerable to poisoning from clay or nutrient input. Consequently, carbonate sedimentation in the offshore transition, located between sand-dominated shoreface and mixed carbonate–siliciclastic offshore facies, reflects the bypassing of siliciclastic mud.

The Ordovician Exposed: Contributions from the 12th International Symposium on the Ordovician System

For nearly two centuries some of the world's most notable geologists and paleontologists have intensively studied the Ordovician rocks of North America. The "layer cake" sedimentary strata deposited in the vast epicontinental seas of the interior and the flanking folded and thrusted mountain belts provided both complex problems and fertile ground for new theories and methods. The problem of correlating between regions in the large cratonic interior and between disparate biofacies gave rise to exciting new studies in sequence, event, quantitative, and chemostratigraphy. Distinctive and traceable marker beds, geochemically fingerprinted K-bentonites, and carbon isotope excursions allow for correlation with unprecedented precision, which leads to new questions (and answers) about Ordovician climate and oceanography, global carbon cycling, and the timing of tectonic events. Recent innovations in using geochemistry to study the ancient environment has greatly increased our understanding of Ordovician climate cyclicity and its effects on Earth's biota. Advances in geochronology have given the geologic community amazingly precise dates on Ordovician K-bentonites, increasing the precision and usefulness of our most fundamental tool the geologic time scale. In short, it is an exciting time in Ordovician research and the highlights of this work were presented at the 12th International Symposium on the Ordovician System. The conference included fifty-nine talks and 18 posters presentations given by over 80 delegates representing 14 different countries. Its overall theme was Integrated Stratigraphy, and as noted above comprises the latest research in sedimentary geochemistry, geochronology, sequence stratigraphy, event stratigraphy, and quantitative stratigraphic methods. These studies provide the stratigraphic framework and paleoenvironmental context for examining the evolution, diversification, and extinction of Ordovician life. ISOS 2015 also offered 4 field trips - 3 to the southern, central, and northern Appalachian Mountains, and one to eastern and central Oklahoma - to examine classic exposures of Ordovician rocks and the only Ordovician GSSP in the United States. This theme issue includes nine contributed papers that are an expansion on the presentations of 12th International Symposium on the Ordovician System. We would like to thank the presenters, field trip leaders, and manuscript reviewers, all of whom are essential to a successful symposium.