Precambrian Basins Research Articles

Alluvial Sedimentary Records in Indian Precambrian Basins: Implications Toward Unique Precambrian Sedimentary Environment?

“Present is the key to the past”—in this all-inclusive uniformitarians’ geological panacea, the Precambrian continental sedimentation system stands out as an odd candidate. The distinctive nature of Precambrian alluvial sedimentation, not analogous to their Phanerozoic counterparts, is highlighted in literature in the absence of rooted vegetation, soil, and an overall greenhouse atmospheric condition. Documentation from Precambrian alluvial deposits around the globe suggests a sedimentation pattern dominated by scree cones, alluvial fans, and braid-plains under the combined effects of aggressive weathering and absence of vegetation; ephemeral braid-plains convincingly outweigh the other alluvial systems in the rock record. It is also surmised that Precambrian fluvial systems, unlike their Phanerozoic-modern analogs, maintained unequivocal ephemeral character across different climatic zones with very less chance of assuming meandering channel planform in absence of buffering effects of rooted plant and adequate mud. Local ponding of muddy sediments, though visualized in some rare cases, was mostly connected with the generation of local paleoslope and unusual riverine hydrology. Recent studies, however, raised a question about the actual paucity of mud, documented meander channel planforms from Precambrian fluvial records, and drew a parallel between the character of the Pre-Silurian alluvial plain on the surface of Earth and meander channel planforms on the Martian surface, interpreted from satellite imagery data. From the collation of data on alluvial deposits from a number of Indian Precambrian basins, augmented by new data from the Bayana basin, a Paleoproterozoic basin in the North Delhi fold belt, the present study attempts to document nuances and intricacies in Precambrian alluvial sedimentation motif that include facies architecture and paleohydrology. From an extensive, though not exhaustive, database, the study intends to claim the establishment of a meander channel-form in Precambrian river systems at a local scale in areas of low discharge and availability of higher silt and mud.

The Pre-Grenvillian assembly of the southeastern Laurentian margin through the U–Pb–Hf detrital zircon record of Mesoproterozoic supracrustal sequences (Central Grenville Province, Quebec, Canada)

AbstractThe detrital zircon perspective on the pre-collisional crustal evolution of the Grenville Province remains poorly explored. In this study, we conducted in situ laser ablation U–Pb–Hf isotopic microanalysis on detrital zircon grains from three pre-orogenic (>1 Ga) supracrustal sequences that crop out in the Central Grenville Province (Lac Saint-Jean region, QC, CA). Detrital zircon grains from vestiges of these sequences record three dominant age peaks at c. 1.46 Ga, 1.62 Ga, 1.85 Ga, and a subordinate peak at 2.7 Ga. The 1.46 Ga and 1.62 Ga age peaks are recorded in detrital zircon grains from a quartzite associated with a metavolcanic sequence (i.e. Montauban Group) with a maximum depositional age of c. 1.44 Ga. In contrast, the c.1.85 Ga age peak is observed from recycled zircon grains in metasediments with maximum depositional ages between 1.2 and 1.3 Ga. The suprachondritic Hf isotope composition in detrital zircon grains of the 1.46 Ga and 1.62 Ga age populations records juvenile crustal growth during peri-Laurentian accretionary orogenesis related to the Pinwarian (1.4–1.5 Ga) and Mazatzalian–Labradorian (1.6–1.7 Ga) events. The detrital zircon grains associated with Penokean–Makkovikian (1.8–1.9 Ga) source rocks record reworking of c. 2.7 Ga continental crust derived from a near-chondritic mantle reservoir. Overall, crust-forming and basement reworking events associated with accretionary orogenesis in southeastern Laurentia are retained in the detrital zircon load of Precambrian basins even after the terminal Grenvillian collision and assembly of Rodinia.

Paleogeography of the Late Precambrian basin in the south of the Siberian Platform

This work is largely a development of the ideas and continuation of the research of S. V. Bogdanova. For many years, she worked at the Department of Lithology of Gubkin University, engaged in research of Precambrian complexes, actively attracting young scientists. In her works, the main regularities of the spatial distribution of Precambrian complexes on ancient platforms were formulated.It is established that a system of folded structures existed on the western periphery of the Siberian Platform in the Late Precambrian already to the border 860 Ma. Some of them had a later age of 640 Ma. On the southern and eastern parts of the frame of the Siberian Platform in the Neoproterozoic, different-aged volcanic arcs that arose during the Neoproterozoic (the Late Riphean and Vendian) are reconstructed. Orogenic complexes from the west and southeast separated the platform sea basin from the Paleoasiatic Ocean and served as sediment provenance for the emerging foreland basin. The sediment provenance of sedimentary material for the Neoproterozoic terrigenous deposits of the sedimentary cover of the Siberian platform were located both inside the platform and on its periphery. The sediment provenances differed in the composition and age of the rocks composing them, and the glacial factor played a significant role in the transfer of sedimentary material. A fairly sharp differentiation of the composition of the sediment provenance rocks determined a wide variety of sedimentary material.Based on the results of U-Pb isotope dating of zircons, the age of the Taseeva Group in the south of the Siberian platform is determined, which cannot be older than 612 ± 6 million years. It is established that the formation of terrigenous deposits of Neoproterozoic productive complexes in the south of the Siberian Platform was significantly influenced by different age sources located both within the platform and on the territories surrounding it. They were composed of magmatic and metamorphic rocks with an age of 2800 up to 600 million years.

Geochemical Proxies of Endo- and Exospheric Processes in Sedimentary Sequences and the Upper Precambrian Section in the South Urals

The article characterizes some geochemical proxies of endo- and exospheric processes in sedimentary sequences and analyzes their average values in Upper Precambrian clayey rocks in the South Urals. It is shown that a gradual decrease of the Th/Scaver value in Lower Riphean (Mesoproterozoic) ~1750–1300 Ma ago rocks correlates to a certain extent with the assembly and disintegration of the Columbia/Nuna supercontinent. The assembly of Rodinia corresponds to two peaks (Biryan Member and Min’yar Formation) of relatively high Th/Scaver values. However, they are separated by rocks with Th/Scaver values comparable with those typical of the fine-grained clastic rocks of the Mashak plume episode (~1380 Ma). Variations of Th/Craver in the Lower–Middle Riphean clayey rocks also correlate quite well with the assembly of Columbia/Nuna, whereas the Upper Riphean–Vendian (Neoproterozoic) interval does not show any significant variation in Th/Craver values. Hence, the assembly and disintegration of Rodinia and the associated changes in rock composition in the provenance did not have any notable effect on the fine-grained aluminosilicate rocks of the Riphean stratotype. The Middle–Upper Riphean sequences were also formed at relatively constant values of TDM and eNd(T). Hence, the Grenvillian events did not make significant contribution to the formation of rocks of the Yurmatau and Karatau groups. Average values of indicators of the redox parameters of water in the Late Precambrian basins, which existed in the eastern part (in modern coordinates) of the East European Platform, indicate that their sedimentary infilling was deposited predominantly under oxidizing or similar conditions. This conclusion contrasts markedly with the notion of the prevalence of ferruginous and euxinic environments in the World Ocean during the Late Precambrian. Comparison of variations in the mean EFNi and EFZn values in clayey rocks of the South Urals, as well as other paleoproductivity proxies, does not reveal any definite relationship of their excursions with the Late Precambrian global and subglobal, endo- and exospheric events. In the Early–Middle Riphean succession, the P2O5 values, slightly higher than in PAAS, correspond approximately to the period of Columbia/Nuna supercontinent existence. Lower values are typical for the clayey rocks formed during its disintegration, although the volume of mafic igneous rocks (main sources of phosphorus) in the provenances at that time should be larger. During the existence of Rodinia, the Upper Riphean clayey rocks were characterized by P2O5aver values both higher and lower than the PAAS. The same is true for its disintegration period. These facts suggest that the global and subglobal, endo- and exospheric processes did not significantly affect the formation of the Riphean–Vendian sedimentary sequences in the South Urals. More important here is the role of local factors.

Sequence stratigraphy of the ca. 1730 Ma Wollogorang Formation, McArthur Basin, Australia

Abstract The ca. 1730 Ma Wollogorang Formation is a mixed carbonate-siliciclastic unit in the southern McArthur Basin of northern Australia. It is an exploration target for base metals and may be a petroleum source unit and unconventional reservoir. Facies analysis indicates that the Wollogorang Formation records deposition in supratidal to offshore environments. Lateral consistency in facies, thickness, and stratigraphic architecture suggests deposition on an east-west trending (present coordinates) epeiric platform. According to recent tectonic models for northern Australia, this epeiric platform likely deepened to the present-day east. The Wollogorang Formation comprises one second-order and five nested third-order sequences. Despite the unavailability of biostratigraphy and biofacies, and the general poor time control in Precambrian basins, we are able correlate third-order (i.e., our lowest rank) sequences over tens to hundreds of kilometers using high-resolution facies and petrophysical logs. As the distinction between stratigraphic sequences (related to shoreline shifts) and sedimentological bedsets (not related to shoreline shifts) becomes more difficult with decreasing hierarchical level, sedimentological, stratigraphic, paleontological, and mineralogical (including diagenetic) criteria were recently proposed to distinguish these cycles. Our study suggests that sedimentological and stratigraphic criteria are the most useful in Precambrian sequence stratigraphy. In contrast, paleontological criteria are irrelevant in Precambrian successions. Mineralogical (including diagenetic) criteria have not been tested in this study. In addition to further study these criteria, we suggest testing geochemical criteria, such as the enrichment of redox-sensitive trace elements and minor carbonate carbon isotope variation, to distinguish stratigraphic sequences from sedimentological bedsets.

Pyroxene 40Ar/39Ar Dating of Basalt and Applications to Large Igneous Provinces and Precambrian Stratigraphic Correlations

AbstractCorrelations within and between Precambrian basins are heavily reliant on precise dating of volcanic units (i.e., tuff beds and lava flows) in the absence of biostratigraphy. However, felsic tuffs and lavas are rare or absent in many basins, and direct age determinations of Precambrian basaltic lavas have proven to be challenging. In this paper, we report the first successful application of 40Ar/39Ar dating to pyroxene from a Neoproterozoic basalt unit, the Keene Basalt in the Officer Basin of central Australia. 40Ar/39Ar analyses of igneous pyroxene crystals yielded an age of 752 ± 4 Ma (mean squared weighted deviation = 0.69, p = 72%), which is underpinned by 40Ar/39Ar plagioclase age (753.04 ± 0.84 Ma) from the basalt. This age is significant because the Keene Basalt is one of the very few extrusive igneous rocks identified within the Neoproterozoic successions of central Australia and is potentially an important time marker for correlating the Neoproterozoic stratigraphy within, and beyond, the central Australian basins. Our geochronological and geochemical data show that the Keene Basalt, which is characterized by enriched elemental and Nd‐Pb isotopic signatures, is strikingly similar to, and coeval with, the 755 ± 3 Ma Mundine Well Dolerite in northwestern Australia. Here we suggest that both are part of the same large igneous province (~6.5 × 105 km2) related to breakup of the supercontinent Rodinia. This study demonstrates the potential of pyroxene 40Ar/39Ar geochronology to date ancient flood basalts and to provide pivotal time constraints for stratigraphic correlations of Precambrian basins.

Sequence stratigraphy of the mixed wave-tidal-dominated Mesoproterozoic sedimentary succession in Chapada Diamantina Basin, Espinhaço supergroup– Ne/Brazil

Although the application of the sequence stratigraphy concepts in the Phanerozoic basins is well constrained, few studies are available on the Proterozoic successions to better understand their basin infill using modern stratigraphic concepts. This work aims to provide a well-constrained application of sequence stratigraphic concepts in Proterozoic sedimentary succession, taking as a case study the well-preserved Mesoproterozoic shallow marine Morro do Chapéu Formation. This records the accumulation in a sag-phase of the intracontinental Espinhaço basin developed into Congo-São Francisco Paleoplate. Two third-order depositional sequences have been identified, separated by a regional subaerial unconformity. The lower third-order sequence includes the succession of lowstand, transgressive, and highstand systems tracts (LST, TST, and HST), whereas the upper third-order sequence only preserves LST and TST. The sedimentary record is composed of alluvial fan, wave-reworked fan-delta, unconfined sheetfloods, subtidal deposits and wavy-dominated shallow marine sediments. The relative sea level variation was the main control of sedimentation during the sag phase on the Upper Espinhaço Megasequence. This study presents an exceptional example of stratigraphic evolution during Proterozoic, increasing the knowledge about sedimentation patterns, that should be compared with other Precambrian basins formed in similar tectonic settings.

U-Pb geochronology of monazite in Precambrian tuffs reveals depositional and metamorphic histories

Uranium-lead geochronology of zircon in felsic tuffs plays a key role in the development of chronostratigraphic frameworks for sedimentary successions. This is particularly true for Precambrian basins, where biostratigraphy is of limited use for age determination or stratigraphic correlation. Although zircon is widely used to date tuffs, it can yield null results due to the presence of xenocrystic populations or isotopic resetting. Other potential U-Pb chronometers, such as monazite, which also crystallize from felsic magmas, are largely unreported from felsic tuff layers. Consequently, there has been a lack of studies to evaluate the potential of monazite for U-Pb isotope geochronology of tuffs. We present results of in situ U-Pb ion microprobe dating of monazite that co-exists with zircon in felsic tuffs from two Precambrian basins: the Earaheedy Basin (Frere Formation) in Western Australia and the Bushveld Basin (Lephala Formation) in southern Africa. We show that 207Pb/206Pb ages from monazite in felsic tuffs from the Frere Formation (1891 ± 9 Ma) and the Lephala Formation (2307 ± 9 Ma) are identical to zircon ages from the respective tuffs. Also, monazite in the Bushveld tuff preserves additional information, including a chronological fingerprint of the volcanic source (2.4–2.5 Ga xenocrysts) and the timing of low-grade metamorphism (2.16 Ga overgrowths). The ca. 2.16 Ga low-grade metamorphism identified in the Bushveld Basin is reminiscent of that widely recorded in the Pilbara Craton, Western Australian, lending support to arguments for a connection between the Kaapvaal and Pilbara cratons. Igneous and low-grade metamorphic monazites are readily distinguished based on texture and chemistry. Our study demonstrates that monazite can be used to accurately date felsic tuffs, offering a powerful tool for constraining depositional and metamorphic histories of Precambrian sedimentary basins.

Precambrian olistoliths masquerading as sills from Death Valley, California

Olistolith production and magmatism are processes commonly associated with extensional tectonic settings, such as rift basins. We present a cautionary exemplar from one such Precambrian basin, in which we reinterpret metabasite bodies, previously documented as sills, to be olistoliths. We nevertheless demonstrate that, on the basis of field observation alone, the previous but erroneous sill interpretation is parsimonious. Indeed, it is only by using isotopic age and compositional analysis that the true identities of these metabasite olistoliths are revealed. We present new data from metabasites and metasedimentary strata of the Kingston Peak Formation (Cryogenian) and Crystal Spring Formation (Mesoproterozoic) of Death Valley, USA. These include field observations, U–Pb apatite ages, U–Pb zircon ages (detrital and igneous) and whole-rock geochemistry. These data also provide a new maximum age for the base of the Pahrump Group and suggest that the Crystal Spring Diabase was more tholeiitic than previously thought. Similar sill/olistolith misinterpretations may have occurred elsewhere, potentially producing erroneous age and tectonic-setting interpretations of surrounding strata. This is particularly relevant in Precambrian rocks, where fossil age constraints are rare. This is illustrated herein using a potential example from the Neoproterozoic literature of the Lufilian belt, Africa. We caution others against Precambrian olistoliths masquerading as sills. Supplementary material: Details of a meta-igneous boulder from P12 of the Silurian Hills, LA-ICP-MS and whole-rock geochemistry methods and standards, and U–Pb apatite and zircon isotopic data, including standards and selected cathodoluminescence images, are available at https://doi.org/10.6084/m9.figshare.c.3990639

Book reviews. Precambrian basins of India – Stratigraphic and tectonic context

Book reviews. Precambrian basins of India – Stratigraphic and tectonic context

Zircon U–Pb age constraints on the Paleoproterozoic sedimentary basement of the Ediacaran Porongos Group, Sul-Riograndense Shield, southern Brazil

Thick quartzites record significant information on cratonic environments during long geological periods. The capacity to resist weathering and deformation turn the quartzite covers especially useful in the provenance studies of Precambrian basins. Provenance of 194 detrital zircon grains from two samples of thick quartzite cover on the Paleoproterozoic Encantadas Complex displays mostly Paleoproterozoic (95%) and minor Archean (5%) sources. The results indicate that sediments were derived from the La Plata Craton with the maximum depositional age at 2.03 Ga possibly up to 1.7 Ga. In comparison, the adjacent Porongos Group has provenance data of 61 detrital zircon grains indicating mostly Mesoproterozoic (69%), subordinately Paleoproterozoic (26%) and minor Archean ages (5%). Considering previous published data, the Porongos Group is Ediacaran in age and probably chronocorrelated with sedimentary basins from the Tandilia Belt (Argentina). Therefore, the quartzite cover and the Porongos Group require distinct evolution in time and in tectonic environment.

Chapter 1 Precambrian basins of India: stratigraphic and tectonic context

The Indian shield represents a vast repository of the Precambrian geological record. The Precambrian sedimentary basins were developed on four major Archaean nuclei (Dharwar, Bastar, Singhbhum and Aravalli–Bundelkhand; Fig. 1.1) and are comparable to those of Australia, South Africa, Canada and Brazil in scale and importance for global studies of Precambrian crustal evolution. Some of the Indian basinal successions, as elsewhere, contain valuable economic resources. Notwithstanding their global relevance, many aspects of the Indian Precambrian basins and even entire basins are not well known to international readers. Although specialist papers, a book (Naqvi & Rogers 1987) and special publications (e.g. Mazumder & Saha 2012) have been written by several authors on specific aspects of the Precambrian geology of India, a comprehensive account of the time–space distribution of the Precambrian supracrustal successions is lacking; in particular, the interrelation between tectonics and sedimentation, and the similarities and dissimilarities of their evolutionary histories with other comparable successions in other parts of the world are yet to be addressed in any detail. The purpose of the present memoir is to fill in this knowledge gap. In addition, the memoir will assess the potential mineral resources of Indian Proterozoic basins and integrate piecemeal information to provide a comprehensive picture of Indian cratonic evolution during the Precambrian. Fig. 1.1. Sketch map showing four Archaean nuclei in India. Note Proterozoic sedimentary basins (dark grey patches) and the large Deccan basaltic province. SGT, Southern Granulite Terrain; NFB, Nallamalai fold belt; CITZ, Central Indian Tectonic Zone; PG, Pranhita Godavari Valley; Ab, Abujhmar Basin; Am, Ampani Basin; Bh, Bhima Basin; Ch, Chattisgarh Basin; Idv, Indravati Basin; Kal, Kaladgi Basin; Suk, Sukma Basin; Ban, Bangalore; Cd, Cuddapah; Hyd, Hyderabad; K, Kurnool; Kh, Khariar; Nel, Nellore; Ong, Ongole; Vij, Vijaywada (after Saha & Mazumder 2012). The content of the memoir has been divided …

Chapter 23 A brief synthesis of Indian Precambrian basins: classification and genesis of basin-fills

Abstract The geology, inferred evolution and classification according to widely accepted schemes of 22 basins from the Indian Precambrian record on the Arravali–Bundelkhand, Singhbhum, Bastar and Dharwar cratons are discussed in this volume. Although their classification is biased owing to all depositories having continental lithospheric substrates, most of the basins reflect divergent plate motion and thus lithospheric stretching and cooling. Convergent plate motion and concomitant lithospheric flexure owing to loading are postulated for the Kurnool Basin and the Eastern Dharwar Craton supracrustals. Transcurrent plate motion is interpreted for the Bhima and Kaladgi–Badami basins. The Cuddapah Basin suggests a complex polyhistory influenced by both mantle circulation/dynamic topography and loading-related flexure of the lithosphere. Mantle circulation and dynamic topography may have played a role in the evolution of the Dhalbhum and Dalma–Chandil basins. Examination of possible time trends indicates that pre- c. 2.0 Ga basins were mostly continental rifts, followed by some intracratonic basins and lesser rift-sag and back-arc basins; post- c. 2.0 Ga basins exhibit a much larger range of basin types. While this Memoir offers a broad sample of the application of plate-tectonic principles to the Precambrian basins of the large Indian shield, it also underscores that Phanerozoic-style plate tectonics and basin evolution histories are widely identified within the Precambrian sedimentary rock record. Analogously, the case studies in this book support the essential similarity between the features observed within the Precambrian basins of India and the norms that describe Phanerozoic successions in terms of sequence stratigraphic architecture. The Indian Precambrian basin-fill record shows that all types of sequence, systems tract and sequence stratigraphic surface that are known from the Phanerozoic record also occur within Precambrian successions. Differences between the stratigraphic architecture of Precambrian and Phanerozoic basin-fill successions can be ascribed to variable rates and intensities of the controls on accommodation and sediment supply, the changes inherent in the evolution of the hydrosphere–atmosphere system and related physical processes, and the evolution of the biosphere system and associated biogenic processes.

About this title ‐ Precambrian Basins of India: Stratigraphic and Tectonic Context

This Memoir provides a comprehensive review of the Precambrian basins of the four Archaean nuclei of India (Dharwar, Bastar, Singhbhum and Aravalli-Bundelkhand), encompassing descriptions of the time–space distribution of sedimentary–volcanic successions, the interrelationship between tectonics and sedimentation, and basin histories. Studies of 22 basins within the framework of an international basin classification scheme deepen an understanding of the basin architecture especially for cratonic basins. Most Indian sedimentary successions formed as cratonic to extensional-margin rift and thermal-sag basins, some reflecting mantle plume movement, subcrustal heating or far-field stress. This Memoir shows that Phanerozoic plate-tectonic and sequence stratigraphic principles can be applied to the Precambrian basins of large Archaean provinces. The differences between the stratigraphic architecture of the Indian Precambrian and examples of Phanerozoic basin-fill successions elsewhere are ascribed to variable rates and intensities of the controls on accommodation and sediment supply, and changes inherent in the evolution of the hydrosphere–atmosphere and biosphere systems.

Chapter 4 Lithostratigraphic, geochronological and depositional framework of the Precambrian basins of the Aravalli Mountains and adjoining areas, Rajasthan, India

Abstract The Aravalli Mountains and neighbouring areas have well-preserved records of a protracted history of development of Precambrian basins, which spans about 2500 myr of the Earth's history. The oldest depositional basins are the greenstone belts that occur within the Archaean gneiss–granite terrain. Geochronological data indicate evolution of one such greenstone belt from 3300 to 2850 Ma. The younger Proterozoic cover successions include the Aravalli Supergroup, Delhi Supergroup and Sirohi Group, which evolved during three successive orogenic cycles spanning between c. 2200 and c. 1850 Ma, c. 1700 and c. 1450 Ma, and c. 1000 and c. 850 Ma, respectively. Each of these lithostratigraphic units evolved in separate basins having distinctive tectonic, metallogenic and evolutionary histories. The inversion of the youngest ‘orogenic’ basin marks the final cratonization of the Precambrian Aravalli crust at around 850 Ma. The succeeding depositional episodes include formation of ephemeral basins during the early phase of the plume-related Malani Group from 780 to 730 Ma, and the formation of the Marwar Supergroup deposited in stable platformal basins marks the terminal phase of the Precambrian crust-building history in the region.

Assessing the syngeneity and indigeneity of hydrocarbons in the ∼1.4 Ga Velkerri Formation, McArthur Basin, using slice experiments

Many applications involving analysis of the hydrocarbons in petroleum systems rely upon the assumption that there has been no contamination of the sample. Precambrian basins are particularly susceptible to hydrocarbon overprinting from contamination due to both age and commonly a low total organic carbon (TOC) content. In this study three cores (McManus-1, Walton-2 and Shea-1) from the deep marine 1.46Ga old Velkerri Formation, McArthur Basin, were subjected to slice experiments and analysis using gas chromatography–mass spectrometry (GC–MS). Internal slices had consistent hydrocarbon signals, providing strong evidence that the organic matter (OM) in the inner slices was indigenous. In all three cores, both the aliphatic and aromatic hydrocarbon signatures indicated that the exterior of the core was exposed to contamination from both the drilling and sawing processes. This contamination overprinted, and had the potential to completely obscure, the indigenous hydrocarbon signal, highlighting the importance of the removal of the exterior portion of the core prior to analysis, even for organic-rich rocks. Aromatic ratios indicated all cores were within the oil window and that McManus-1 had the highest maturity and Walton-2 the lowest. Walton-2 was the only core to contain biomarkers above the limit of detection (LOD). Evidence of high bacterial input to the OM included the presence of hopanes, bicyclic sesquiterpanes and monomethyl alkanes. Steranes were absent or below the LOD in all cores, suggesting a very low or zero contribution from eukaryotes to this deep water, low O2 depositional environment.

Unincised fluvial and tide-dominated estuarine systems from the Mesoproterozoic Lower Tombador Formation, Chapada Diamantina basin, Brazil

The Mesoproterozoic Lower Tombador Formation is formed of shallow braided fluvial, unconfined to poorly-channelized ephemeral sheetfloods, sand-rich floodplain, tide-dominated estuarine, and shallow marine sediments. Lowstand braided fluvial deposits are characterized by a high degree of channel amalgamation interbedded with ephemeral, intermediate sheetflood sandstones. Sand-rich floodplain sediments consist of intervals formed by distal sheetflood deposits interbedded with thin layers of eolian sandstones. Tide-dominated estuarine successions are formed of tide-influenced sand-bed braided fluvial, tidal channel, tidal sand flat and tidal bars. Shallow marine intervals are composed of heterolithic strata and tidal sand bars. Seismic scale cliffs photomosaics calibrated with vertical sections indicate high lateral continuity of sheet-like depositional geometry for fluvial–estuarine successions. These geometric characteristics associated with no evidence of incised-valley features nor significant fluvial scouring suggest that the Lower Tombador Formation registers deposition of unincised fluvial and tide-dominated systems. Such a scenario is a natural response of the interplay between sedimentation and fluctuations of relative sea level on the gentle margins of a sag basin. This case study indicates that fluvial–estuarine successions exhibit the same facies distributions, irrespective of being related to unincised or incised-valley systems. Moreover, this case study can serve as a starting point to better understand the patterns of sedimentation for Precambrian basins formed in similar tectonic settings.

Crustal thickness map of the Chinese mainland from teleseismic receiver functions

Abstract We present a new crustal thickness map of the Chinese mainland and adjacent areas based on a compilation of previously published data. More than 1900 estimates of crustal thickness from receiver function studies were collected and the weighted averages were used to develop a contour map of crustal thickness beneath the Chinese mainland. The combined dataset provided good coverage, especially over eastern and Central China, and our analysis revealed crustal thickness variations of the Chinese mainland with considerably more detail than in previous models. Our crustal thickness map shows a large variation, from a thickness of less than 30 km in the Mesozoic basin of eastern China to over 80 km in the Tibetan plateau. The thickest crust was found in central Tibet, while the thinnest continental crust was observed beneath eastern China. Normal to slightly thick crust (38–51 km) was found for the stable Precambrian basins and orogenic belts. Comparison between our model and other models reveals that the models share similar large-scale and smaller-scale features over large portions of eastern China where the data coverage is good; however, large differences (up to ± 25 km) in crustal thickness occur in areas with complex crustal structure or poor data coverage. We also evaluated the quality of previous models by comparing them with our newly compiled point observations of crustal thickness.

New multi-dimensional diagrams for tectonic discrimination of siliciclastic sediments and their application to Precambrian basins

Siliciclastic sediments owe their origin mainly to land sources. New discriminant-function-based major-element diagrams for the tectonic discrimination of siliciclastic sediments from three main tectonic settings: island or continental arc, continental rift, and collision, have been constructed for the tectonic discrimination of high-silica [(SiO2)adj=63%–95%] and low-silica rocks [(SiO2)adj=35%–63%], where (SiO2)adj refers to the SiO2 value obtained after volatile-free adjustment of the ten major-elements to 100wt.%. These diagrams are based on worldwide examples of Neogene–Quaternary siliciclastic sediments from known tectonic settings, loge-ratio transformation of ten major-elements with SiO2 as the common denominator, and linear discriminant analysis of the loge-transformed ratio data. The success rates of these diagrams as judged from the original data varied from 84.5% to 93.6%. These diagrams were successfully tested on Neogene to Quaternary rocks not included in the original database. These discriminant diagrams were also successfully applied on older high-silica and low-silica sandstones and shales of Paleoarchean–Ediacaran age. Finally, these diagrams were shown to be useful against chemical changes related to analytical errors, weathering, recycling and post-depositional processes.

Basin analysis in polymetamorphic terranes: An example from east Antarctica

U–Pb and Lu–Hf isotopic signatures of detrital and metamorphic zircon in granulite facies paragneiss from Kemp and MacRobertson lands, east Antarctica, distinguish two extensive Precambrian basins: (1) an Archaean basin deposited between c. 2570Ma and c. 2500–2430Ma, which extended across Kemp Land contiguous with equivalent paragneiss in the adjacent Napier Complex in Enderby Land; and (2) a late Mesoproterozoic basin deposited after c. 1110–1080Ma in MacRobertson Land that extends into eastern Kemp Land, which also incorporates rocks currently exposed in the Eastern Ghats Province of India and the Prydz Bay region of east Antarctica. An unconformity that separates the Archaean and Mesoproterozoic sedimentary packages is near the Stillwell Hills. An eastward increase in the proportion of fragmentary detrital zircon in the younger package reflects proximity to late Mesoproterozoic volcanic sources. The Mesoproterozoic basin likely developed along an Andean-type margin to the Indo-Antarctic craton, possibly in a back-arc setting during the assembly of Rodinia. The widespread dissolution of zircon at c. 990–940Ma during Rayner orogenesis in Kemp and MacRobertson lands largely precludes a traditional basin analysis via detrital geochronology, but the Hf isotopic ratio of metamorphic zircon precipitated following anatexis provides a homogenised signature, equivalent to a bulk-rock model age, which can be used as a stratigraphic proxy. This approach captures “hidden” provenance information to enable lithostratigraphic correlation in complex high-grade terranes.