Sedimentary Geology最新文献

Organic-rich shale in the Cretaceous succession of the large non-marine Songliao Basin in northeastern China, especially the Qingshankou Formation of the Turonian and Coniacian stages, provides a unique record for studying the role of global climate control on organic matter accumulation. Organic enrichment in the Qingshankou Formation along the Southeastern Uplift of the basin has been widely reported and is implicated in the formation of algal blooming and a saline anoxic water environment. However, studies on the Qingshankou Formation in the Changling Sag are relatively few, which leaves a significant gap to understand the controlling factors of organic enrichment at a basin scale. In this study, the sedimentary environment of the first member of the Upper Cretaceous Qingshankou Formation (K₂qn¹) is reconstructed from petrological and geochemical data to discuss the sources and preservation mechanisms of lacustrine shale organic matter in the Changling Sag (southern Songliao Basin). The K₂qn¹ is subdivided into three stratigraphic units: Sq1 phase, Sq2 phase, and Sq3 phase. Biomarker (abundance of tricyclic terpane and regular steranes) and petrographical (maceral composition) data indicate that the organic matter in K₂qn¹ was mainly from terrigenous plants, with some algal input. Inorganic minerals (major and trace elements) and petrological (framboidal pyrite size) data suggest a humid climate and saline anoxic water environment at the Changling Sag during the most organic-rich Sq1 phase. During the Sq2 and Sq3 phases, a relative low lake level and semi-arid climatic conditions likely prevailed. Moreover, these results imply that apart from a saline anoxic water environment, the persistence of continentally derived organic matter in lacustrine shale is mainly controlled by clay minerals. Accordingly, this study proposed a new enrichment model for lacustrine shale organic matter. The new model complements existing lacustrine shale sedimentary models for sources of organic matter and emphasizes the role of clay minerals in preserving organic matter.

Paleocene deposits of the Subbetic Zone (southern Spain) provide outstanding evidence of the influence of sea mountains on deep marine currents. This part of the Betic Cordillera External Zones corresponds to the distal and deepest area of the original basin, where hemipelagic sedimentation prevailed during most of the Turonian-early Lutetian interval. This sedimentation is recorded by the so-called Capas Rojas and Quipar-Jorquera formations, units up to 250 m and 425 m thick, respectively, predominantly consisting of marls and marl/limestone alternations. These units draped and smoothed an irregular submarine topography of fault-bounded Mesozoic carbonate blocks. Some of these blocks became uplifted and subaerially exposed after a mid-Danian tectonic episode, transforming the Subbetic Zone in an archipelago during the late Danian-early Selandian interval. The emerged blocks were colonized by Microcodium-producing terrestrial plants, Microcodium consisting of aggregates of submilimetric monocrystalline calcite grains. Massive resedimentation of these grains into depressed zones of the archipelago resulted in discrete accumulations up to 100 m thick but of comparatively modest extent (≤150 km²) of calcarenites rich in Microcodium remains. The study of one of these calcarenite units, the Olivares Formation, demonstrates that most of its constituent Microcodium remains were brought to the deep sea by turbidity currents, but were subsequently reworked by oscillatory and unidirectional bottom-currents. The analysis of the Capas Rojas Formation in its type section and surrounding areas, where Microcodium-rich calcarenites are absent, demonstrates that the Danian-Selandian succession is riddled with hiatuses, which resulted in a drastic thickness reduction of the interval. Clearly, the rugged sea floor topography resulting from the mid-Danian tectonic event enhanced the sedimentary transport capacity of bottom-currents that, in addition to piling-up Microcodium-rich calcarenites in restricted zones, disturbed the hemipelagic sedimentation elsewhere in the Subbetic Zone. From late Thanetian times onwards the background hemipelagic sedimentation typical of the Capas Rojas progressively resumed throughout the Subbetic Zone, recording the gradual abatement of the sea floor relief by protracted erosion and/or subsidence.

Turbidites have been recognized as significant reservoir units in both marine and lacustrine basins. This study was conducted on the Eocene Dongying Sag (Bohai Bay Basin, East China) using sediment cores, well logs, seismic data, and petrography to investigate the diagenesis and reservoir quality of lacustrine turbidites controlled by climate-driven base-level changes. The early forced regression within a third-order sequence stratigraphy can be divided into rising and falling stages of fourth-order base-level changes. Throughout these stages, extrabasinal turbidites were triggered by river floods and comprised the main channel, distributary channel, lobe, and lobe fringe. Intrabasinal turbidites only occurred during the falling stage, which were caused by sediment failures and corresponded to tongue deposits. Compared to extrabasinal turbidites, intrabasinal turbidites possess enhanced anti-compaction capabilities, reduced clay matrix content, and higher carbonate cement content. Climate-driven base-level changes dominated the diagenesis of lacustrine turbidites. From the rising to falling stage, the climate transitioned from humid to arid conditions. This caused a decline in the formation of clay-sized sediments that were subsequently transported into deep-water environments, thereby enhancing mechanical compaction of extrabasinal turbidites. Stacking patterns of channel complexes changed from vertical aggradation to lateral migration, reducing the occurrence of carbonate cementation near channel amalgamation and sandstone–mudstone interfaces. Therefore, these processes controlled by base-level changes led to a decrease in carbonate cement content and mechanical compaction, simultaneously promoting dissolution in extrabasinal turbidites during burial. Additionally, falling base-level facilitated rapid progradation of clinothems and delta collapse, resulting in the development of limited-scale tongues characterized by lower sorting and clay matrix content inherited from the delta front. Tongues, despite their enhanced resistance to compaction, were surrounded by thick-layer deep-water mudstones, contributing to strong carbonate cementation during burial. This study confirms that extrabasinal turbidites during the falling base-level possessed better reservoir quality, thereby providing guidance for the exploration of gravity flow sandstone reservoirs in lacustrine basins.

Exquisitely exposed Cambrian carbonates with exceptional thickness in North China offer a unique opportunity for studying the biota evolution and Earth's history. Here we characterize Cambrian oncoids by integrating petrological, geochemical, and isotopic analyses in order to unravel their genesis, assess the role of detrital fractions, and reconstruct the paleoenvironment of these ancient strata. The presence of nanosphere, microbial fossils, and relics of extracellular polymeric substances enables us to infer an important role of the microbial consortium in the origin of oncoids. These interpretations are also reinforced by fluorescence microscopy, ultraviolet excitation, and Raman spectral signature. Likewise, the depleted values of δ¹³C isotopes (−1.11 to −0.46 ‰) strongly support a significant input of microbial relics in the formation of oncoids. The geochemical modeling of trace and rare earth elements (REEs) advocates for oxygenated conditions in surface waters in North China during the Cambrian. Geochemical and isotopic data also reveal that oncoids are influenced by siliciclastic detrital contamination and stress the negligible role of diagenetic processes. These results provide evidence that oncoids can represent a reliable proxy of palaeoceanographic changes in the early history of Earth.

The near-field glacial record of the late Paleozoic Ice Age in Gondwanan basins is widely used in global paleogeographic and paleoclimatic models. Nevertheless, several late Paleozoic glacial successions still lack a detailed depositional history or reassessment of their genesis under modern glacial sedimentology concepts. One such example is the Aquidauana Formation in the western Paraná Basin, Brazil, which corresponds to about 65 % of the total outcrop area of the late Paleozoic glacial strata (Itararé Group) within the basin. Yet, the stratigraphic architecture and depositional environments are full of uncertainties as the strata are very poorly exposed. This study brings new insights on the depositional history and stratigraphic architecture for the upper half of the Aquidauana Formation by describing a 290-m-thick section cropping out along a recently developed highway in the Mato Grosso do Sul state. A subaqueous slope and glaciated outer shelf depositional setting is interpreted for the upper half of the Aquidauana Formation, which differs considerably from the glacioterrestrial and postglacial arid context traditionally proposed. More specifically, deposition chiefly took place under ice-free conditions on a subaqueous slope within an erosionally-confined channel–levee complex through sediment density flows and subaqueous landslides. Episodic glacial influence on sedimentation is recorded within the uppermost portion of the section by glacially-modified and iceberg-rafted debris as well as a deglacial trace fossil assemblage within proglacial deposits. Sediment transport toward the southeast into the basin indicates a glacial source placed westward of the Paraná Basin, reinforcing the hypothesis of ice-spreading centers located westward of the basin. The outcomes of this study also emphasize the relevance of subaqueous gravity-driven resedimentation in emplacing the late Paleozoic glacial record.

In a Mediterranean climate, sands in drainages of the composite Cordilleran Orogen in the Sierra Nevada, Klamath Mountains, Cascade Mountains and Coast Ranges of Southern Oregon and Northern California are derived from multiple orogenic source domains as varied as oceanic crust, continental crust and magmatic arc. Quantitative petrographic data shows the carbonate-poor sands commonly contain <25 % quartz; those with greater quartz abundance include admixed detritus from quartzose sedimentary rocks or quartz-rich Sierran alluvium. Geochemistry specifically identifies much of the detritus as mafic to intermediate composition with mg* values of 45 to 55, Th/Sc <0.3 and Cr/Th > 20 with Cr/V > 1, Rb/Ni < 1 and Y/Ni < 1. Within the Klamath area streams, in ultramafic-rich source areas, sands are >50 % ultramafic with mg* > 60. Significant quantities (5–45 %) of ultramafic + mafic grains with high abundances of MgO, Cr and Ni are present in downstream sands. In other cases, downstream, with mixing of various less mafic detritus, ultramafic grains are <5 %, mg* < 50, with some high Cr due to detrital chromite. To the south, sands from several major sources contribute to the Sacramento River. From Coast Ranges streams significant schist-rich siliceous and ophiolite sand are added to volcanic-rich, basalt-andesite (Cascades arc source) upstream Sacramento River sands. Feldspathic sand is contributed from uplifted dissected arc basement Sierra Nevada and Klamath Mountains streams. Along the river course, in a reactive landscape, there is variable mixing of sands from different sources due to river flow levels at normal and flood stages and by erosional reworking of river basin sand deposits.

A high-amplitude global sea level rose rapidly during the early Toarcian (Early Jurassic), making the transgression deposits potential stratigraphic correlation markers of marine and terrestrial successions. Green clay minerals (i.e., glauconite and chamosite), carbonate concretions, and geochemical database associated with the early Toarcian global transgression are archived in the Lower Jurassic Da'anzhai Member (the Early Toarcian age) in the Sichuan Basin of China, a mega lake along the Tethys Ocean. The fine-grained glauconites with high Al and low Si contents suggested that they were transported from offshore to the lake by marine incursion and were oxidized during the transport processes. The green authigenic chamosite cement and redeposited chamosite grains with high Mg/Fe and low Al/Si ratios were diagenetically transformed from berthierine formed in the brackish water caused by marine incursion. The chemically impure siderite concretions with low ⁸⁷Sr/⁸⁶Sr ratios and high δ¹³C values suggested that the lake water and sediment pore waters were mixed with seawater. Coeval calcareous concretions have low ⁸⁷Sr/⁸⁶Sr ratios and high δ¹³C values, indicating the mixing of lake water with seawater. Combining the stratigraphic low ⁸⁷Sr/⁸⁶Sr values, we suggested that there was a marine incursion into the Sichuan Basin. According to the organic carbon isotopic stratigraphy correlation, the marine incursion occurred during the onset of T-OAE. The rapid and high-amplitude global sea-level rise may have caused this marine incursion.

Rhizoliths in the Ironshore Formation on the Cayman Islands, up to 12 cm in diameter and 1 m long, developed in skeletal grainstones that accumulated on sand flats above high tide level. Internally, these well-preserved rhizoliths are characterized by well-defined concentric zones, centered around the central root opening, that reflect radial variations in calcite–aragonite content, decreases in micrite content, increases in porosity, and radial variations in trace element (e.g., Ca, Ni, S, Si, and Sr) concentrations. The stable isotopes, δ¹³C and δ¹⁸O, which vary in accord with the internal zones are more variable than the δ¹³C and δ¹⁸O of the host sediments. For the rhizoliths there is a high correlation between the δ¹³C and δ¹⁸O values and the calcite content. The contrast between the rhizoliths and their host sediments is largely a reflection of the more intense diagenetic processes that were largely mediated by the microbial populations that were concentrated in the rhizosphere that developed around each plant root. Microbial activity, which mediated micritization of the skeletal sands and micrite precipitation in the intergranular pores, was largely responsible for development of the internal zonation in the rhizoliths. Use of the δ¹³C values to determine if the rhizoliths developed in association with C₃ or C₄ vegetation failed because the δ¹³C values correlate with the calcite content that developed largely as diagenetic products.

The sedimentology of upper flow regime bedforms represents an important research topic at the present. Deposits interpreted as those of supercritical flows are widely recognized in modern fan systems, but their recovery is challenging. Most of the sedimentological information has come from channel thalwegs but supercritical bedforms are also frequently downslope from the channel mouths. Such an environment has been identified in the Paola basin, where erosive and depositional cyclic steps have been imaged and identified in a sandy submarine lobe of the Amantea Fan. High-resolution sub-bottom profiles provide insight into the bedform internal architecture and their relationships with a frontally-confining ridge. For the first time, supercritical bedforms in a submarine lobe have been interpreted in two distinct positions: in the scour of an erosional cyclic step and in the stoss side of a depositional cyclic step. Coarse to medium-grained massive sand with flame structures, indicating rapid sediment fall-out and frequently associated with the occurrence of hydraulic jumps, has been identified in the scour and at the toe of the ridge. The latter represents an example of topographically induced hydraulic jumps driven by a frontal confinement. Top-cut-out medium to fine sands with tractive structures have been interpreted as the deposits related to the stoss side of a cyclic step or small-scale antidune superimposed on the cyclic step surface. The presented data broaden the understanding of the range of processes that are driven by the interaction between turbidity currents and seafloor topography and the dip of the slope. The recognition that topography influences the density structure and the degree of criticality of the flow and, consequently, the morphodynamics and facies of the relative deposits may help to explain sediment distribution and improve depositional models of fan lobes in confined settings.

Understanding the factors that control the temporal and spatial evolution of the Lower Triassic Buntsandstein Group, eastern France, is important not just for palaeoenvironmental reconstruction but also because it is an important reservoir for the lithium-rich geothermal brines in the Upper Rhine Graben region. The interval of interest in this study, the Lower Grès Vosgien Formation (LGV), is made up of c. 200 m of clastic deposits of mixed fluvial and aeolian origin. Given the proximity between the outcrops and the areas of mineral and heat exploration, this region offers a unique opportunity for quantitative analysis and correlation between outcrops and reservoirs. The LGV is the thickest Formation in the Buntsandstein Group. However, hitherto, no detailed architectural analysis has been published to reveal the controlling factors of its depositional elements. In this study, high-resolution facies analysis was applied to thirteen outcrops, and a core, to quantify fluvial and aeolian depositional architectures, and to comprehend palaeoenvironmental conditions during their deposition. Two facies associations were identified. The fluvial channel facies association, composed mainly of sandstones deposited in poorly confined, occasionally ephemeral channels, corresponds to 93 % of the total thickness. The analysis of >800 sets of strata characterised by lower-, transitional-, and upper-flow regime sedimentary structures, distributed within different architectural elements, enabled the distinction of temporal and spatial variabilities, and an estimate of fluvial palaeohydraulic characteristics. The wind- and water-laid facies association, which accounts for 7 % of the total thickness, records an architecture controlled by a fluctuating water-table level, and ephemeral floods. Results revealed that aeolian dunes and sand sheets accumulated in distinct temporal moments. The vertical trends, recorded in the core, indicate an upward increase in the frequency of intercalations between the two facies associations, and a decrease in the occurrences of thicker cross-bedded sandstone sets of fluvial origin, which, associated with the overall retrogradational stacking pattern of the LGV, is interpreted as the preserved record of a large distributive fluvial system (DFS). The occurrence of laterally extensive aeolian deposits, overlying fluvial deposits, is often associated by authors with periods of relatively more arid conditions at a regional scale. However, evidence indicates channel belt avulsion as an alternative interpretation for the establishment of the two facies associations. The findings enhance the knowledge about the depositional controlling factors in braided fluvial and aeolian systems, and improve predictive models that account for reservoir heterogeneity in accumulated successions of this type.