Continental Sediments Research Articles

Multiproxy Paleosol Evidence for Jurassic Paleoclimate Fluctuations in the Sichuan Basin, SW China

AbstractPaleoclimate reconstruction of continental environments has been hampered by the limited evidence. A thick sequence of Jurassic continental deposits in the Sichuan Basin, Southwest China yields abundant paleosols that may offer valuable insights regarding Jurassic climate scenarios. A succession of 169 paleosols belonging to Protosols, Calcisols, and Argillisols from 23 detailed stratigraphic sections was recognized and characterized macro‐ and micromorphologically and assessed for mineral and geochemical compositions. Quantitative paleoclimate reconstructions using bulk geochemical proxies, the depth to and the stable oxygen isotopic composition of paleosol carbonates indicated a predominant alternation of semiarid and arid cool/warm‐temperate climatic conditions punctuated by several episodes of subhumid and humid climates that generally prevailed in the Sichuan Basin during the Jurassic. The estimated paleoatmospheric CO2 concentrations (pCO2) from calcic paleosols yielded a low range of ∼104 ± 58 to ∼610 ± 152 ppmv during the Middle Jurassic. The terrestrial paleotemperature changes in the Sichuan Basin coincided with the pCO2 variations, which probably resulted from global geological events (e.g., volcanic activities, magmatic and oceanic events, and the ephemeral caps development) in the Middle Jurassic. Jurassic climatic fluctuations in the basin were likely attributed to true polar wander due to global plate motion, megamonsoon effect linked to global and regional paleogeography, and regional paleotopography.

A Record of Phylactolaemata from Late Cenozoic Continental Deposits of Northern Eurasia and North America

A Record of Phylactolaemata from Late Cenozoic Continental Deposits of Northern Eurasia and North America

Interconnected multi-layer aquifer with evaporitic fossil waters in Chott-El-Gharbi endorheic basin (Western high plateaus, Algeria): Hydrochemistry, environmental and strontium isotopes

Wetland are of paramount importance for the entire province of Nâama as well as for the entire Algerian Western High Plateaus. One of them, Chott El Gharbi, is a major but poorly-known hydrogeological unit shared between Algeria (80%) and Morocco (20%). Chott El-Gharbi is a multi-layered aquifer system whose most productive layers are the Middle Jurassic (Bajocian-Bathonian dolomite and limestone) and the Cenozoic continental sediments (lacustrine intercalation of clays and limestones, marls and sands). Groundwaters exhibit depleted δ18O (−8.98 to −5.53‰) and δ2H (−69.4 to −48.0‰) isotope ratios linked to an evaporative process. Groundwaters are old, with 14C activities from 2 pmC to 58 pmC and residence times of 23 ky to 9 ky. These old recharges occurred during a more humid and colder epoch. 87Sr/86Sr ratios, coupled with major element abundances, determine that Gharbi waters resulted from the mixing of waters having been in contact with the Triassic evaporite-rich sequence (0.70810, rich in sulfates) and waters in contact with Lago Mare (late Miocene) sequence (0.70875, less rich in sulfates). Water with Triassic signature is present in the Jurassic aquifer (Bathonian-Bajocian) while water with Lago Mare or mixed signature is present in the Miocene, locally along faults in the Jurassic aquifer. 14C ages indicate that the mixing process occurred at c. 10,000 years along faults probably during an active tectonic period. Waters with modern signature are only known outside the endorheic Gharbi basin in shallow Jurassic aquifers. The Gharbi aquifer system is separated from the large Chergui aquifer system to the east.

Experimental Constraints on the Fate of Subducted Sedimentary Nitrogen in the Reduced Mantle

AbstractNitrogen is considered to be transported from Earth′s surface to the top of the lower mantle through subduction. However, little is known on the transportation and fate of subducted nitrogen to the Earth′s interior during slab‐mantle interactions. In this study, the stability of subducted sedimentary nitrogen in the reduced mantle was investigated to 35 GPa and 1600 K by laser‐heated diamond anvil cell experiments and first‐principles calculations. Our results showed that subducted nitrogen‐bearing silicates and fluids could not coexist with the metallic iron or iron‐rich alloys, and reacted with them to form different products at high pressure‐temperature conditions. Combining our results with previous data, we re‐determined the relative stability of iron‐light element binary compounds to 35 GPa and 1600 K to be Fe‐O > Fe‐N > Fe‐S > Fe‐C. This stability sequence contributes to explaining the observation that iron nitrides are trapped as inclusions in sulfur‐depleted lower‐mantle diamonds and are absent in sulfur‐rich ones. The recycling efficiency of subducted sedimentary nitrogen is strongly related to the availability of the metallic iron of the reduced mantle. Hydration of the metallic iron limits the storage of nitrogen in it and contributes to recycling nitrogen to Earth′s surface. Therefore, unlike subducted continental sediments, subducted marine sediments are unlikely to transport a large amount of surficial nitrogen to the metallic iron of the reduced mantle in which nitrogen could reside over long geologic periods.

Provenance of the first terrigenous sediments in the western Sichuan Basin during the Late Triassic: Implications for basin evolution from marine to continental

The Late Triassic was a key period for the evolution of the western Sichuan Basin from marine to continental sedimentation. However, the provenance of the earliest terrigenous sediments during this period remains debated, hindering our understanding of the tectonic events that ruled the evolution of the basin at that time. Herein, samples of fine sandstone from the Upper Triassic Ma'antang and Xiaotangzi formations in the western Sichuan Basin were collected for petrology, heavy mineral analysis, bulk rock geochemistry, and detrital U–Pb dating. In addition, corresponding data from potential source areas were collected for comparison. The sedimentological, geochronological, and geochemical characteristics of terrigenous sediments suggest that the clastic materials were mainly sourced from the Qinling orogenic belt and Yangtze Craton (including the northern and western margin). The Longmen Shan thrust belt likely provided clastics since the Early Norian. Siliciclastic deposits of the Late Triassic sedimentary succession of the western Sichuan Basin (Ma'antang and Xiaotangzi formations) yielded young zircon U–Pb ages of 214–245 Ma, suggesting that these zircons were likely sourced from the magmatic activities in the South Qinling orogenic belt or Yidun Island Arc. Combined with previous research, this study predates the transformation of the western Sichuan Basin from marine to continental sedimentation in the Late Carnian/Early Norian period.

African sandstorms, blood rain and continental mineral delivery to the Canary Islands

The origin of volcanism in the Canary Islands has been a matter of controversy for decades. Discussions have hinged on whether the Canaries owe their origin to seafloor fractures associated with the Atlas Mountain range or to an underlying plume or hotspot of uprising hot material from the deep mantle. The debate has recently concluded, however, following the discovery of nannofossils preserved in the products of the 2011–2012 submarine eruption at El Hierro, which constrain the age and growth history of the westernmost island of the archipelago and so cement a clear East to West age progression within the archipelago. Light‐coloured, quartz‐bearing pumice‐like ‘floating rocks’ (xeno‐pumice) were found on the sea surface during the first days of the 2011 El Hierro eruption and proved to be fragments of pre‐island, sedimentary strata that were picked up by ascending magma. Upper Cretaceous to Pliocene calcareous nannofossils such as coccolithophores were retrieved from the xeno‐pumice fragments, and these marine micro‐organism biostratigraphical markers now provide crucial evidence that island growth at El Hierro commenced in the Pliocene. Here we discuss how these essentially continental (quartz‐bearing) sediments on the African continental shelf derive from dominantly wind‐blown Sahara dust and marine (re)‐deposition and describe present‐day aeolian processes that are in operation in the region. We investigate the mineralogy of Sahara dust that is currently deposited in the Canary Islands and discuss source areas and intra‐transport fractionation of mineral dust during trans‐Atlantic transport. Finally, we explore how present‐day dust deposition can be used as analogue to explain the deposition of pre‐island continental material in the East‐Atlantic Ocean basin beneath the Canary archipelago and we show how the dust‐derived sedimentary deposits can be utilized as geological tool in the Canary Islands.

The Tethyan and Tyrrhenian margin record of the Central Apennines: a guide with insights from stratigraphy, tectonics, and hydrogeology

This guide is a 5-day geotraverse across the Central Apennines, which expose the inherited Meso-Cenozoic sedimentary successions and tectonic architecture of the Apennine Tethyan passive margin. This structure was progressively involved into orogenic and post-orogenic tectonics, related to the Tyrrhenian back-arc dynamics. To show the different margin dynamics, we present: 1) sedimentologic and stratigraphic aspects, 2) relations with Neogene compressive and Quaternary extensional structures and 3) a hydrogeological case-study. The trip starts north of Rome, where Quaternary deposits at the transition from marine to continental deposition, including travertines, record the evolution of the Tyrrhenian passive margin. The excursion then moves to the Mesozoic platform margins of the Latium-Abruzzi and Apulia carbonate platforms, where perfectly preserved Cretaceous to Neogene successions occur. The Maiella mountain and Gran Sasso range testify for carbonate production changes, as recorded in facies associations and isotope geochemistry records, related to global climate shifts and regional tectonics. A focus on the hydrogeologic dynamics of the Gran Sasso aquifer is also offered in relation to the current climate change. This guide is the result of a workshop of GeoSed, the Italian Association for Sedimentary Geology, and it also aims at providing a scientific base for geo-touristic explorers.

Evidence for the early Toarcian Carbon Isotope Excursion (T-CIE) from the shallow marine siliciclastic red beds of Arabia

The Toarcian Oceanic Anoxic Event (T-OAE) and its corresponding Carbon Isotope Excursion (CIE) have been reported widely across the Tethyan region and globally. In Arabia, and based on ammonite dating, the time window of the T-OAE coincided with the deposition of the reddish siliciclastic unit of the Marrat Formation. However, no evidence of the T-OAE/CIE was ever reported from Arabia because these red beds were previously interpreted as continental deposits. Recently, these red beds have been recognized as shallow marine deposits which opened an opportunity to assess the occurrence and expression of T-OAE–CIE in Arabia. In this study, a multiproxy geochemical characterization was performed on the Toarcian Marrat Formation to infer the chemistry of the paleowater column and identify intervals of possible T-OAE/CIE in Arabia. While the low concentrations of redox-sensitive elements (Mo, U, V, Cr) may indicate a shallow oxic marine settings, the coupled negative δ13Corganic excursion and apparent increase in the chemical weathering suggests that the deposition of Marrat red beds coincided with the development of T-CIE and possibly time-equivalent to the T-OAE globally. The origin of reddening is interpreted to have occurred during the middle Marrat deposition due to the stabilization of unstable hydrous iron oxides to hematite under oxic marine conditions. The proposed model further indicates the possible development of source rocks in the deep, anoxic environment counterpart where the T-OAE may be expressed. Since our study documents the first record of the T-CIE and discuss the origin of shallow marine siliciclastic red beds in the Arabian Plate, this will have significant implications for the overall understanding of the T-CIE globally and for hydrocarbon exploration through realizations of potential new source rocks associated with the OAEs in the Toarcian and other time intervals.

Alluvial fans in the latest phase of the Messinian Salinity Crisis: An example from the Chaabet Et Tabbela Formation, northern Tunisia, Central Mediterranean

Alluvial fan deposits record important information about paleoclimate, tectonic setting, and paleogeography that can be deciphered with detailed sedimentological analysis. The late Messinian Chaabet Et Tabbela Formation, in the western part of the Kechabta Basin in northern Tunisia, shows excellent examples of alluvial fans deposited during the latest phase of the Messinian Salinity Crisis, the Lago Mare event. This exposure provides the opportunity to investigate terrestrial depositional conditions during this period. The Chaabet Et Tabbela Formation was investigated through sedimentological logging and detailed facies analysis at eight locations. Sixteen facies types were identified and interpreted along with variation in clast compositions. These facies were further grouped into four facies associations: (1) proximal alluvial fan, dominated by gravity-driven processes, (2) medial alluvial fan, which mainly comprises a mix of debris flow and high-sediment load streamflow deposits, (3) distal alluvial fan, which consists of laterally continuous sheetflood and subordinate channels deposits, and (4) playa and floodplain, which are made of red mudstones with calcareous paleosols. Clast composition of these fan deposits comprises Triassic, Jurassic, and Cretaceous bedrock suggesting that sediment was sourced from the surrounding Maghrebian mountain chain. Two laterally adjacent alluvial fans were identified along the N-S Messeftine Fault. These fans have prograded during the Late Messinian, with an estimated radius of ca 2 km.The evolution of these alluvial fans is related to a complex interplay of allogenic processes. The uplift related to intra-Messinian tectonic event and climate variation contributed to the formation of these fans during the late Messinian. Although, these deposits are relatively limited in extension, similar continental deposits form synchronously in different marginal Mediterranean basins during the Lago Mare stage, suggesting that the alluvial fans of the Chaabet Et Tabbela Formation should be placed in a context of regionally change climactic and paleogeographic conditions.

Late Hercynian tectonic evolution of the Jebilet Massif (Western Meseta, Morocco) based on tectono-sedimentary analyses of related Permian continental deposits

Late Hercynian tectonic evolution of the Jebilet Massif (Western Meseta, Morocco) based on tectono-sedimentary analyses of related Permian continental deposits

Extremely High Primary Productivity and Organic Matter Enrichment in Lacustrine Sediments: Late Triassic Chang 73 Sub-Member, Western Ordos Basin

The 7th member of the Late Triassic Yanchang Formation is the most important Mesozoic source rocks and shale oil reservoir in the Ordos Basin. The hydrocarbon potential and organic matter enrichment mechanism of this organic-rich member are questioned because of the high heterogeneity of continental sediments. In particular, the lack of drilling in the Chang 73 sub-member of the western Ordos Basin (central Tianhuan Depression) hinders this understanding and the progress of shale oil exploration in this region. In this study, 87 samples were collected from the well DP1 in the Tianhuan Depression, western Ordos Basin, and analyzed for sedimentology and geochemistry to unravel the paleoenvironment, paleo-productivity, and hydrocarbon potential of the Chang 73 sub-member. The results show that the Chang 73 shale is characterized by ultra-high organic matter abundance (up to 10.60%), type II kerogen, and good hydrocarbon generation potential. An intermediate chemical weathering intensity suggests that this set of organic-rich shale was deposited in a warm and humid paleoclimate setting. Extreme organic matter enrichment in study area may be attributed to ultra-high productivity (hypereutrophic lake), superimposed terrigenous organic matter input, and transgression, rather than volcanism and anoxic bottom water. This model is different from those of Chang 73 interval with higher organic matter content in the central and southern parts of the Ordos Basin. Therefore, this study proposes that the Chang 73 shale in the central Tianhuan Depression is not a good target layer for shale oil exploration and that the favorable area may be located in an undisturbed deep lake with higher organic matter abundance (e.g., southwestern Ordos Basin).

Late Carboniferous palaeodepositional changes recorded by inorganic proxies and REE data from the coal-bearing strata: An example on the Czech part of the Upper Silesian Coal basin (USCB)

In the Czech Republic, coal-bearing siliciclastic sediments have been deposited during the Serpukhovian and Bashkirian (Carboniferous). Until now, no attention has been paid to inorganic geochemical assessment of the coals and associated non-coal rocks from the mixed shallow-marine to continental sediments (Ostrava Formation), and continental non-marine settings (Karviná Formation). Samples were collected from a 750 m deep coal exploration borehole at the ČSM Mine. The bulk parameters, total organic carbon TOC, total inorganic carbon TIC, total sulphur TS, major elements, trace elements, and REEs were measured on these samples, and their mineral associations have been investigated using microscopy combined with the principal component analysis (PCA). Common redox proxies V/Cr, U/Th, Ni/Co, Mo/U, and the ratio S/TOC have been tested on the samples to investigate their usefulness for studying anoxia. Research concludes that redox proxies such as U/Th, Ni/Co and V/Cr have been strongly influenced by the clastic input and carbonates, which it hinders for them to be reliable indicators of anoxia. On the basis of Eu anomaly and REEs distribution, the primary source of detrital elements comes from the parent rock, being governed more by physical than redox processes.

A new ornithopod dinosaur, Transylvanosaurus platycephalus gen. et sp. nov. (Dinosauria: Ornithischia), from the Upper Cretaceous of the Haţeg Basin, Romania

ABSTRACT Rhabdodontid dinosaurs were a group of medium-sized iguanodontian ornithopods from the Late Cretaceous of Europe. The uppermost Cretaceous continental deposits from the Haţeg Basin of western Romania yielded a very rich assemblage of vertebrates including abundant rhabdodontid remains, which have been exclusively referred to the genus Zalmoxes thus far. Here we describe a new rhabdodontid dinosaur, Transylvanosaurus platycephalus gen. et sp. nov., from the uppermost Cretaceous of the Haţeg Basin. The holotype of the new taxon was discovered in early–late Maastrichtian strata near Pui in the eastern part of the basin and comprises the articulated basicranium and both frontals. Transylvanosaurus differs from all previously reported rhabdodontids in having particularly wide and crested frontals, elongated and straight paroccipital processes that make only a gentle lateral curve and project mostly posterolaterally, prominent and massive prootic processes that extend mainly anterolaterally and ventrally, wide and crest-like basal tubera that meet the long axis of the braincase at a very flat angle, widely splayed basipterygoid processes that extend mainly ventrolaterally and slightly anteriorly, as well as a well-developed notch on the lateral side of the basicranium that is continuous, straight, and inclined anteroventrally. Phylogenetic analyses employing two different datasets consistently recovered the new taxon within the Rhabdodontidae, at the base of the iguanodontian radiation. Based on the morphological comparisons presented herein, we propose a particularly close relationship between Transylvanosaurus and Rhabdodon from southern France, which in turn provides evidence for a more complex biogeographic history of the Rhabdodontidae than previously thought.

The effect of rock lithology on the background concentrations of trace elements in alluvial soils: Implications for environmental regulation

To assess the extent of soil contamination with a potentially toxic element, it is important to determine the background concentration of that element in the soil. In this study we decided to investigate the relationship between trace element concentrations in soils and the lithology of their parent rocks. We collected 85 topsoils (0–20 cm) samples from the Petorca, La Ligua, Aconcagua, and Casablanca River basins (Valparaiso region, central Chile). For soil sampling, we chose 11 tributary watersheds free of anthropogenic influence. The areas that contributed to the formation of each alluvial soil were delineated and the parent lithologies were acquired from 1:250,000 geological maps. The results show that the total concentrations of arsenic (As), copper (Cu), lead (Pb), and zinc (Zn) in soil vary considerably among the four river basins studied. Soil concentrations of all elements under study are positively correlated with the presence of andesitic volcanic rocks. Furthermore, soils formed from marine sediments and felsic volcanic rocks had the highest concentrations of Pb and As. The carbonate-bearing marine sediments may have adsorbed As and Pb released by weathering of felsic volcanic rocks found upstream of the study area. In contrast, lithologies dominated by continental sedimentary and plutonic rocks showed negative correlations with the concentrations of As, Cu, Pb, and Zn in the soil. Thus, the differences in soil element concentrations in the watersheds under study were due to the different rock types present in the area. We argue that absolute “one-limit-fits-all” background trace element values may be inadequate to assess the extent of soil contamination. Therefore, it is important to determine background concentrations of soil elements in each river basin in the anthropogenically impacted areas. In contrast, erroneous conclusions can be drawn when background concentrations are determined in one region and then applied in another region that is lithologically different.

Isolated Mesoeucrocodylian tooth crown from the lower Cretaceous cerro La Isla pterosaur site in the Atacama region, northern Chile

We describe the first crocodyliform fossil found in continental deposits from the Lower Cretaceous of the Quebrada Monardes Formation on Cerro La Isla, Atacama region, northern Chile. The material consists of a single, poorly preserved tooth crown (SGO.PV 1160), whose enamel surfaces bear tightly-packed apicobasal ridges. Our comparisons of SGO.PV 1160 with the dental morphology of possible taxa that it may belong to suggest a hitherto unknown pholidosaurid, or, more likely, notosuchian. Therefore, the tooth crown is here provisionally identified as Mesoeucrocodylia indet., pending future discoveries of more complete material. This find increases the known biodiversity in the Lower Cretaceous deposits of Cerro La Isla, as well as the importance of the fossil site, since pholidosaurids have not been reported from Chile, and if it proves to be notosuchian, it may represent the earliest known taxon of this group from South America, and the second earliest in the world.

Linking Archaean climate change with gold metallogeny

First large-scale concentration of gold to ore grade in Earth’s crust took place at around 2.9 Ga in quartz-pebble conglomerates, well before endogenous deposits, such as porphyry and epithermal systems or orogenic-type deposits, started to play a significant role from ca. 2.75 Ga onwards. The conglomerate-hosted gold placers, typified by those in the Mesoarchaeaen Witwatersrand Basin in South Africa, are thought to have been sourced from the leaching of background levels of gold in the Archaean continental crust, promoted by deep chemical weathering under a reducing acidic atmosphere. Gold dissolved in meteoric waters was trapped by possibly acidophile microbes, fossil remnants of which are preserved as kerogen layers in the 2.9 Ga lower Central Rand Group of the Witwatersrand Supergroup. Mechanical reworking of the delicate microbial mat-bound gold led to rich gold placers from 2.9 Ga onwards. Well-endowed gold placers older than 2.9 Ga are conspicuously missing, although suitable siliciclastic host rocks, even containing relics of former microbes, are known from continental sediment sequences as old as 3.22 Ga. A review of climate-sensitive rock types and geochemical data on the extent of chemical weathering reveals that the period from 2.96–2.91 Ga was cold, with repeated glaciations as evidenced by glaciogenic diamicite, whereas warmer and (or) wetter climates reigned from 2.90 to 2.78 Ga. In concert with the temporal distribution of placer gold accumulation, the conclusion is reached that chemical weathering rate, dictated by climate, was a key determining factor in the exogenous binding of Au into sedimentary deposits.

Progress and prospects of carbon dioxide capture, EOR-utilization and storage industrialization

Carbon dioxide capture, EOR-utilization and storage (CCUS-EOR) are the most practical and feasible large-scale carbon reduction technologies, and also the key technologies to greatly improve the recovery of low-permeability oil fields. This paper sorts out the main course of CCUS-EOR technological development abroad and its industrialization progress. The progress of CCUS-EOR technological research and field tests in China are summarized, the development status, problems and challenges of the entire industry chain of CO2 capture, transportation, oil displacement, and storage are analyzed. The results show a huge potential of the large-scale application of CCUS-EOR in China in terms of carbon emission reduction and oil production increase. At present, CCUS-EOR in China is in a critical stage of development, from field pilot tests to industrialization. Aiming at the feature of continental sedimentary oil and gas reservoirs in China, and giving full play to the advantages of the abundant reserves for CO2 flooding, huge underground storage space, surface infrastructure, and wide distribution of wellbore injection channels, by cooperating with carbon emission enterprises, critical technological research and demonstration project construction should be accelerated, including the capture of low-concentration CO2 at low-cost and on large-scale, supercritical CO2 long-distance transportation, greatly enhancing oil recovery and storage rate, and CO2 large-scale and safe storage. CCUS-EOR theoretical and technical standard system should be constructed for the whole industrial chain to support and promote the industrial scale application, leading the rapid and profitable development of CCUS-EOR emerging industrial chain with innovation.

Palaeosedimentary Environment and Formation Mechanism of High-Quality Xujiahe Source Rocks, Sichuan Basin, South China

AbstractTriassic Xujiahe source rocks, the main gas source of shallow tight gas, are the most typical continental coal-bearing source rocks in the Sichuan Basin, South China. However, the organic matter enrichment section cannot be identified easily, leading to limited progress in the exploration of coal-bearing tight gas. This paper reveals the main controlling factors of the organic matter enrichment, reconstructs the evolution process of the Xujiahe palaeosedimentary environment, proposes a dynamic enrichment mechanism of the organic matter, and determines the organic matter enrichment section of the high-quality coal-bearing source rocks by geochemical characteristics of the source rocks, major elements, and trace elements. The results show that the Xujiahe sedimentary environment can be divided into a fluctuating stage of transitional sedimentation, stable stage of transitional sedimentation, fluctuating stage of continental sedimentation, and stable stage of continental sedimentation. The Xujiahe source rocks were featured with high-quality coal-bearing source rocks with high total organic carbon and maturity and good parent material in the stable stage of transitional sedimentation and fluctuating stage of continental sedimentation, in which the water was connected with the Palaeo-Tethys Ocean with abundant terrestrial organisms. The water was shallow in the fluctuating stage of transitional sedimentation with a low sedimentation rate, leading to poor organic matter enrichment. The Palaeo-Tethys Ocean withdrew westward from the Yangtze plate in the late period of the fluctuating stage of continental sedimentation, leading to the absence of algae and dinosteranes and a decrease in biological productivity in the stable stage of continental sedimentation. Therefore, high terrestrial inputs and biological productivity and high sedimentation rate were conducive to the organic matter preservation in the coal-bearing source rocks.

Rare preservation of Triassic pedorelicts with biogenic traces from a hot semi-arid upland palaeoenvironment at Portishead, SW England

Pedorelicts occur sporadically in coastal exposures of Middle to Late Triassic continental sediments, at Portishead, SW England. The clasts include aeolian and bedrock constituents, calcrete textures, vesicles and other pedogenic features reflecting a hot semi-arid palaeoenvironment. Interpreted as being derived from an upland soil, they are classified in palaeosol terms as Calcic Protosol. In the most significant exposure, cryptic tubular trace fossils in branching networks occur within these pedorelict clasts in a localised fluvial deposit. This extremely rare palaeoecological archive offers a glimpse into continental upland life in a Pangaean desert landscape. It is documented for its own merit, to support continuing studies of its palaeobiology and to prompt investigation for comparable deposits.The pedorelicts occur within the Mercia Mudstone Marginal Facies (MMMF), close to its basal unconformity with underlying Paleozoic strata, at the margin of the Somerset Basin. The soil from which they are derived is interpreted to have developed in an upland regolith over Tournaisian bedrock of interbedded limestone and siltstone. These weathered and, with aeolian dust containing calcite and iron minerals and siliciclastic sand, formed a structured soil. This was aided by intermittent light and moderate rainfall that promoted mainly vadose pedogenic calcretization with displacive calcite crystallisation and siliciclastic grain breakage. In places, the soil was vesicular, as an ‘Av’ horizon, and in others, it was bioturbated with the development of calcite-lined tubules and unlined tunnels. At intervals of perhaps 104 to 105 years, catastrophic deluges eroded the regolith and transported clasts downslope towards the basin to form onlapping coarse clastic beds typical of the MMMF. The soil structure disintegrated but fragments that became pedorelicts were segregated in a fault-controlled palaeo-valley, possibly partly as a debris-flow, to be deposited as localised conglomeratic lenses with fluvial sand. After burial by further sedimentation, this deposit underwent diagenetic calcite cementation and baryte mineralisation before its present exposure by coastal erosion.

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.