Depositional Record最新文献

Microbialites have long been utilised by geologists as palaeoenvironmental indicators, despite outstanding questions regarding their formation and preservation in the rock record. Here, we leverage cathodoluminescence (CL) microscopy, a technique commonly used to investigate carbonate formation and diagenetic alteration, to better understand the textural characteristics, formation mechanisms and diagenetic histories of microbialites. We compare CL features to gain insight into palaeoredox conditions and alteration histories for a suite of six microbialite samples spanning from the Proterozoic to modern, finding a strong degree of similarity amongst samples regardless of age or depositional environment. CL reveals that microbialites typically have complex microfabrics that include other accessory minerals and grains, all of which provide insight into their unique formation and palaeoredox histories. We find that the modern microbialite sample showed the greatest difference in CL characteristics compared to the other microbialite samples, most probably because of its aragonitic composition and incomplete lithification. In contrast, the ancient microbialite samples preserve a distinct and most probably primary, mottled luminescence texture despite spanning more than 500 Myr; this mottled texture may typify ancient microbialite fabrics that formed in shallow water settings. We also distinguish a variety of CL characteristics that support previously proposed formation and/or diagenetic histories in these samples. Lastly, we use energy-dispersive X-ray spectroscopy to compositionally identify rare grains observed with CL, highlighting the utility of CL as a possible screening tool for both geological and non-geological components within samples. Our analyses demonstrate the power of using classic CL techniques to answer modern questions in microbialite research.

Palaeogene depositional systems in the South Pyrenean foreland were influenced by eustatic sea level changes, compressive and salt tectonics, as well as biotic and environmental changes during and after the Palaeocene–Eocene Thermal Maximum (PETM). Decoupling these factors requires careful sedimentary and stratigraphic analysis. This study combines outcrop observations and microfacies analysis to derive a depositional model and to evaluate the relative roles of eustasy, tectonics and skeletal biota during deposition of the lower Eocene Alveolina Limestone, which immediately post-dates the PETM. The studied succession is preserved in the footwall of the Montsec thrust. Diagnostic skeletal grains show a systematic upward change from coastal (miliolid foraminifera, charophytes) and inner ramp (miliolid and alveolinid foraminifera), through tidal bars or dunes (alveolinid and nummulitid foraminifera), to middle ramp (bryozoans, echinoderms, encrusters—mainly acervulinid foraminifera and coralline red algae) environments, a deepening succession recording the global early Eocene transgression. A condensation interval rich in red algae, iron and glauconite grains and cement marks the maximum flooding and passage to the overlying tidally-influenced Baronia Formation sandstones. The fossil assemblage is consistent with expansion of foraminifera at the expense of corals in the aftermath of the PETM. Lower accommodation space and higher detrital input in the footwall of the Montsec thrust caused stratigraphic thinning and interbedding of carbonate debrites and sandstones. This suggests that the Montsec tectonic structure was at least partially emergent already during the earliest Eocene. Uplift of the Montsec tectonic structure, which was probably related to salt movements and compressive tectonics, and the early Eocene transgression facilitated a detrital provenance shift from a southern provenance in the Palaeocene to north/northeasterly Pyrenean sources. The large tidal bedforms in the Alveolina Limestone, deposited by currents amplified in a narrow strait, may provide evidence for the development of an Atlantic-Mediterranean seaway.

The Dachstein in Austria is the second largest karst massif in the Alps. It is dominated by Upper Triassic limestone and 290 km of cave passages have been documented. For this study, five major cave systems on the northern edge of the Dachstein were investigated: Hirlatzhöhle, Mammuthöhle, Rieseneishöhle, Schönberghöhle and Günter-Stummer-Höhle. Thirty-five sediment profiles or exposures between 860 m a.s.l. and 1945 m a.s.l. were analysed with respect to stratigraphy, grain size, mineral composition and carbon content. Our study distinguishes the following main types of non-autochthonous sediments. The stratigraphically deepest sediment in the highest—and therefore oldest—cave (Ruin Level, c. 1900 m a.s.l.) is a dark clay with laterite-derived minerals, suggesting formation before c. 15 Ma, consistent with the previously proposed Eocene–Oligocene age. At intermediate elevations (Giant Cave Level, c. 1350 m a.s.l.), a poorly sorted channel facies forms the initial allochthonous sediment. In Hirlatzhöhle to the west, it is reddish-brown and dominated by para-autochthonous radiolarite components, while in the eastern caves, it is greenish (chlorite) and contains Augensteine sensu lato from metamorphosed rocks of the Niedere Tauern. Previous burial age dating of the quartz suggests an emplacement in the caves 4–6 Ma ago. In caves at this elevation, a dark brown fine-grained sediment of slack-water facies follows, which is probably a palaeosol derived from the allochthonous crystalline sediment cover. Above, rounded limestone gravel is at least partly attributed to glacial meltwater. At the top, and almost ubiquitous, is a bright clayey silt. It is carbonate rich and has a varve-like layering. This sediment is attributed to Pleistocene meltwaters and glacial backflooding. While it is normally a few tens of centimetres thick in the Giant Cave Level, it can reach a few metres in the Berger Level (c. 950 m a.s.l.) below. Only there is it partly overlain by Holocene carbonate sands.

Poorly preserved belemnite samples are often disregarded in palaeoclimatology and palaeoceanography research within the Eastern Pontides (NE Türkiye), which is part of the Alp-Himalayan Belt that experienced extensive orogenic and magmatic activity. Late Jurassic–Early Cretaceous carbonates are widely exposed in the Eastern Pontides, having undergone a complex diagenetic history since deposition. Belemnite rostra samples from the uppermost part of the carbonate sequence (Aptian–Albian strata) exhibit diagenetic overprints. These poorly preserved belemnite samples can serve as excellent archives for basin early diagenetic evolution. However, the diagenetic evolution of these poorly preserved belemnite samples has not been fully studied within the context of the diagenetic evolution of the basin. Thus, this study presents a comprehensive dataset including isotope analyses (Sr/C/O) and trace element, including REE chemistry of these belemnite samples to elucidate the basin's early diagenetic history. The studied samples are commonly dark grey and show visible signs of diagenetic processes, including dissolution, silicification and pyrite mineralisation. These samples show high Fe (1259–5176 ppm; ave. 2859 ppm) and Mn (77–387 ppm; ave. 232 ppm), low Sr/Mn (0.63–12.33; ave. 3.47) and low Sr/Rb (42–568, ave. 200) values, consistent with a diagenetic alteration. Interestingly, carbon and strontium isotopes of belemnite samples correlate well with those from well-preserved belemnite rostra from the Aptian–Albian strata. They exhibit a diagnostic sea water REE_N signature, including mildly enriched Gd/Gd* ratios of 1.04–1.48 (ave. 1.24) and high La/La* of 1.54–5.50 (ave. 2.51) and Ce/Ce* of 0.46–0.60 (ave. 0.51). However, slight LREE enrichment is observed, with La/LuN ratios ranging from 0.81 to 1.74 (average: 1.21) and La/YbN ratios from 0.9 to 1.79 (average: 1.22). A slight positive Eu anomaly is also present, with Eu/Eu* ratios between 1.15 and 1.55 (average 1.36). Y/Ho ratios range from 35.00 to 45.00 (average 39.84), and Sm/Yb ratios from 1.70 to 4.05 (average 2.42). These geochemical characteristics, along with δ¹⁸O values that deviate from typical marine carbonate signatures, suggest a subsequent low-temperature hydrothermal alteration, potentially triggered by hydrothermal activity associated with an Early Cretaceous magmatic event. This finding underscores the significance of analysing REEs in poorly preserved belemnite rostra, offering valuable insights into early diagenetic pathways, dissolution/precipitation processes and water–rock interaction.

The cover of living reef-building corals has declined due to ocean warming, acidification, pollution and disease outbreaks. This decline endangers future reef development relying on acroporid corals, which are highly sensitive to environmental changes. Given these threats, there is a need for accretion models that do not rely on species dominance. To assess the potential of non-acroporid framework and identify the key variables controlling their accretion, we reconstructed the internal structure and Holocene development of Bermuda patch reefs, lacking acroporids. Four rotary drill cores and two vibrocores recovered sequences composed of five facies in decreasing abundance: a framework of massive, stress-tolerant corals, unconsolidated skeletal sand and gravel, coralline-algal bindstone, Halimeda floatstone and peat. Radiocarbon dating of corals and peat shows that patch reefs developed in two stages. From 7 to 4 kyr B.P., during a sustained sea-level rise, the coral framework facies accreted vertically, while the unconsolidated facies filled interstitial sand pockets in intermediate water depths. From 4 kyr B.P. onwards, during a reduced rate of sea-level rise, vertical accretion has continued with a decline in coral diversity. Accretion was primarily controlled by massive coral framework construction and variation in environmental energy, turbidity and bioerosion, which collectively shaped the size, diversity and preservation of the framework. This led to a suppressed, steady vertical accretion rate of 2.18 ± 0.66 m/kyr on average. Although lower than that of acroporid reefs in the western Atlantic, the accretion rate did not decline throughout the Holocene, even as sea-level rise slowed over the last 4 ka. The suppressed accretion rate resulted in submerged keep-up reefs thriving in several metres of water during the mid-late Holocene. This growth mode highlights the potential of coral reefs to develop under different controls beyond coral species dominance, a mode that could be applied to current and future coral reef development.

Carbonate clumped isotopes are a powerful tool for paleoclimate reconstruction due to the ability to reconstruct past changes in both temperature and precipitation-evaporation balance. Here we test the utility of this method on last millennium carbonate lagoonal sediments from Kiritimati, a coral atoll where modern climate variability is driven by interannual changes in the El Niño-Southern Oscillation. We find last millennium lagoonal temperatures from clumped isotopes are cooler than anticipated compared to modern measurements and other paleoclimate reconstructions. This discrepancy is probably due to sediments containing a mixture of high-magnesium calcite derived from primary precipitates and benthic foraminifera and aragonite derived from warm water corals. We employed an inverse mixing model to minimise the impact of vital effects related to coral growth on clumped isotope compositions and found an increasing difference between modelled and measured T(Δ₄₇) values through time. This potentially indicates that the composition of lagoon water became increasingly unique from the coral carbonate formation waters through the last millennium. This study highlights the necessity of detailed understanding of carbonate mineralogy, sedimentology and provenance in interpreting clumped isotope temperature reconstructions.

This study represents a detailed analysis of a stratigraphic section of Oxfordian (Late Jurassic) age with the aim to reconstruct the facies pattern and sequence-stratigraphic evolution, followed by a discussion of the dominant controls on shallow marine carbonate platform sedimentation in the Lower Saxony Basin (LSB). During the Oxfordian, the LSB was covered by a shallow epicontinental sea, in which a thick succession of marine limestones and marls was deposited. The stratiform Oxfordian carbonate bodies hold significant economic potential due to their ability to form hydrocarbon reservoirs or targets for future geothermal exploration. The scarcity of open-marine biostratigraphic marker fossils and the presence of numerous sedimentary gaps pose significant challenges for establishing reliable correlations with Oxfordian deposits globally. This study focuses on the Oxfordian Korallenoolith Formation, which is well exposed in the Bisperode section located ~40 km SW of Hannover. The ~126 m thick section was described bed-by-bed during fieldwork and complemented by 73 petrographic thin sections that were analysed for carbonate microfacies. Based on differences in grain and mud composition and primary sedimentary fabrics, an integrated log of the Bisperode quarry was constructed. Seven facies types (FTs) and 13 microfacies types (MFs) are characterised upon which a reef-bearing carbonate ramp model was built. Following the magnitude of the transgressive/regressive trends concept, three low-order sequences are recognised, which are composed of eight higher order sequences. The low-order sequences correlate well to the established sequence-stratigraphic model and the higher order sequences point to a possible control of their stacking pattern by Milankovitch long eccentricity cycles. This study is important because it helps to better understand the link between the stacking pattern of Oxfordian strata in the LSB and allocyclic processes. It thus provides a foundation for basinwide and inter-basin sequence-stratigraphic correlation, allowing future global chemostratigraphic comparisons and providing valuable insights into the synchronicity of geological events.

The Great Blue Hole is a prominent flooded karst sinkhole, located in the lagoon of Lighthouse Reef atoll off the coast of Belize. Short cores recovered from varved bottom sediments have been used in previous studies as climate and cyclone archives covering as much as the past ca 1.7 ka BP. A new 30 m long sediment core, encompassing the entire Holocene and the latest Pleistocene, allows a reconstruction of the development of this geoscientifically significant site in the light of the postglacial and Holocene sea-level rise. The sedimentary succession in the core is tripartite. The lowermost sedimentary unit A (30.0–28.6 m) comprises grey-brown to black organic-rich carbonate sediments, which contain freshwater snails (Pyrgophorus), tropical forest pollen (Myrtaceae) and a low-diversity dinocyst assemblage. The intermediate unit B (28.6–24.65 m) is a dark greyish-green to greyish-brown, weakly laminated carbonate silt, which comprises marine fossils, that is, euryhaline foraminifera (Elphidium) and Halimeda (codiacean algal) platelets. Unit B contains abundant red mangrove (Rhizophora) pollen and a dinocyst assemblage indicating high productivity in surface waters. The uppermost unit C (24.65–0 m) is an annually layered buff and light green carbonate silt with abundant marine fossils. A total of 574 intercalated, coarser-grained and lighter-coloured event (storm) beds usually rich in Halimeda and coral fragments were identified in unit C. Fully marine conditions, including trophic seasonality, are also indicated by the dinocyst spectrum. The pollen spectrum derives from a variety of trees (largely pine, oak, podocarp), shrubs and herbs. The sedimentary succession represents the transition from an initial terrestrial cenote phase on a vegetated limestone island (unit A: 12.5–7.2 ka BP), via a subsequent restricted marine phase on an initially flooded carbonate platform with mangrove swamps (unit B: 7.2–5.7 ka BP), to a fully marine phase in an open, well-circulated atoll lagoon (unit C: 5.7–0 ka BP).

The Palaeogene Kumugeliemu Group in the Kuqa Depression of the Tarim Basin of western China is characterised by a thick and massive sequence of evaporites, with a maximum thickness of up to 1000 m. The formation mechanism of such thick evaporite deposits remains unclear. In this study, sedimentological and geochemical research methods were employed to investigate the sedimentary characteristics and sources of halite. Sedimentary fabrics in halite, mudstone, sandstone, gypsum and carbonate indicate that halite was formed in a shallow water environment. Results of X-ray diffraction of halite and mudstone show that there is no tachyhydrite in halite and there is no pyrite in mudstone, which differs in the characteristics of deep-water salt and hydrothermal salt, respectively, indicating that halite was not formed in a deep-water environment. Rare earth elements indicate that sea water was the main source material for salt deposition. The colour of mudstone, Sr/Cu ratio and Rb/Sr ratio indicate that palaeoclimate was characterised by dry and hot conditions. The distribution characteristics of lithologic succession and the thickness of halite in Mbr 2 of the Kumugleiemu Group indicate that the halite was formed in restricted environments. According to above evidence, a ‘multistage marine transgression’ salt formation model was established, which suggests that, in a restricted paleogeographic environment, multiple transgressions brought sea water that, under dry and hot climate conditions, underwent repeated evaporation and concentration to form thick evaporite deposits.

Saline-alkaline lakes are common in tectonically active, semi-arid regions, resulting from the interplay between tectonic, hydrothermal, surface processes and climate. This study investigates their contribution to the evolution of the saline-alkaline succession in the intramontane Ibar Basin (Southern Serbia). The succession is investigated using detailed sedimentological analysis, in situ U–Pb geochronology of carbonates acquired by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), major and trace element geochemistry and fluid inclusion analysis. Dominantly clastic sedimentation is represented by alluvial fan, flood plain, delta and marginal and profundal lake facies associations. Microbialite and littoral to sublittoral oil shales formed in the areas protected from clastic input. The stratigraphic succession shows a transition from alluvia to a hydrologically open and closed lacustrine environment. U–Pb geochronology of microbialite of a close lake phase (~17 Ma) suggests basin evolution during the Early to Middle Miocene age. The hydrologically closed lake phase is marked by borate-rich facies, which comprise mainly colemanite overgrown by secondary ulexite within profundal lake facies associations. The textural features suggest that colemanite initially precipitated at or below the sediment/water interface. The subsequent growth, as observed from the primary fluid inclusions in colemanite and calcite, indicates precipitation from the evolving bittern brine under evaporitic, redox conditions, which during diagenesis reached temperatures of 200–220°C. High Ca²⁺/Na⁺ ratios in the brine favour colemanite precipitation. Gradual Ca²⁺ depletion and clay breakdown (dewatering) lead to ulexite and borax precipitates. The results imply that B, Ca and Na were mobilised and transported into the basin by hydrothermal fluids that previously leached bedrock and/or by streams carrying products of the volcanic rocks' weathering. However, brine saturation and syndepositional precipitation were initiated by an arid climate through evaporation, while further growth was driven by reflux of fluids during diagenesis. This study highlights the importance of the tectonics of the collisional orogens and associated processes, arid climate and basin hydrological regime on the deposition of B, Ca-rich deposits in saline-alkaline lakes.