Depositional Record最新文献

Euendolithic microorganisms, capable of bioerosion in carbonate substrates, play an important role in modern marine ecosystems and have a fossil record extending into deep time. Understanding the factors driving microboring behaviour is essential for interpreting their ecological impact and reconstructing ancient environmental conditions. In this study, we conducted field incubation experiments across multiple sites at Little Ambergris Cay in the Turks and Caicos Islands, examining microboring density in abiotic optical calcite and aragonite under varying conditions of light, subaerial exposure, current energy, substrate mineralogy and trace metal content. We observed sinuous tunnels within 1 week of incubation in transparent calcite, with longer deployment times (2.5–5 months) resulting in meaningful increases in boring density. We also documented boring activity in dark conditions, suggesting potential for enhanced mineral dissolution at night when geochemical conditions are more optimal. Trace metal analysis of our experimental substrates revealed Fe/Ca and Mn/Ca ratios exceeding western Atlantic sea water estimates by 1–3 orders, with calcites more enriched in Mn than aragonites, offering preliminary support for the novel hypothesis that dissolution of CaCO₃ minerals might be a useful source of trace metals for euendoliths. Sea water chemistry varied across sites, particularly between restricted interior and open platform sites. A comparison of boring densities suggests that trace metal abundance, mineralogy, local sea water CaCO₃ mineral saturation state (Ω) and subaerial exposure (e.g. intertidal vs. shallow subtidal) may all influence microboring. These findings offer new perspectives on the euendolithic lifestyle, showing how substrate selection and temporal partitioning of dissolution activity balance metabolic costs with environmental constraints. They also enhance our ability to interpret the fossil record and bioerosion dynamics under changing conditions.

A 300 m thick section at Nini Hill in the proximal foredeep of the Western Canada Foreland Basin is dominated by shallow-marine mudstone that spans the Cenomanian–Turonian boundary (CTB). The section preserves a 185 m thick record of OAE2, characterised by an ornate positive excursion in the organic carbon-isotope profile. Osmium-isotopes show the characteristic shift to unradiogenic ratios 21 m below the onset of OAE2. Carbon-isotope events (CIE) at Nini Hill are correlated with both the Chalk reference section at Eastbourne, UK and the well-dated SH#1 core in Utah, the latter permitting correlation to other sections in the southern USA. However, only the ~450 m thick, deep-water CTB section in the Saku Formation, Japan, appears to match the CIE detail at Nini Hill. High-resolution correlation utilising CIEs allows, for the first time, sea-level changes, mapped in the poorly fossiliferous strata of Western Canada, to be correlated with coeval events in the USA and Europe. The globally-recognised sub-plenus unconformity that underlies OAE2 in many passive-margin sections spanning the North Atlantic region is correlative with up to six high-frequency sequences preserved in the highly expanded foredeep. Various studies have inferred sea-level change of 10–40 m for this event, suggesting that thermo- and aquifer-eustasy may have been supplemented by glacio-eustasy. Other sea-level changes of ~10–30 m recognised in Canada correlate with coeval events in the USA and Europe. Lower-amplitude sea-level cycles of ~5–10 m, form a persistent signal throughout the Canadian CTB interval. Strata thin dramatically from foredeep to forebulge due to condensation and lap out, hiatuses being represented by cryptic mud-on-mud disconformities. Without knowledge of physical stratigraphy, interpretation of carbon- and osmium-isotope profiles in attenuated successions is prone to misinterpretation. Osmium data show that the influence of a large igneous province diminished markedly northward within the Western Interior Seaway.

Mixed arenites are widespread in the Quaternary record of the peri-Mediterranean region where they accumulated in a variety of marginal-marine depositional environments. In this study, Early Pleistocene mixed arenites and their associated conglomerates and sandstones were studied in detail near the town of Cataforio, in Calabria, southern Italy. Sedimentological logging, facies and palaeocurrent analyses and characterisation of trace and body fossils were combined with the interpretation of drone-derived panoramic photographs and orthomosaics. Three major units, with the upper two separated by a major erosional surface, and a total of seven sedimentary facies were recognised. The basal Unit 0 is found near the fault bounding the basin to the east and consists of at least several decametres of conglomerates and sandstones (Facies S1). Unit 1 is up to 45-m thick and consists of fossiliferous mixed arenites (Facies M1–4), whereas Unit 2 is at least 30-m thick and mainly consists of conglomerates and sandstones (Facies S2–3). The mixed arenites of Unit 1 show single cross-beds up to several metres thick and with a predominant dip direction to the west; these are intercalated with plane-parallel-bedded facies. The cross-bedded deposits also show complex architectures up to 6-m thick and hundreds of metres long interpreted as compound dunes. They are inferred to have formed in a strait environment, although this probably had more complex geometries and dynamics than previously suggested by other authors and other alternative hypotheses were also considered in this study. The siliciclastic-dominated deposits of Unit 0 and Unit 2 were interpreted to have formed at the base of scarps and in submarine canyons, respectively. This study suggests that the inferred palaeo-strait was not necessarily a wider and similarly oriented expression of the modern Strait of Messina. We advocate that straits and similar systems show a high level of spatio-temporal complexity that should be investigated in detail.

Chile's west coast is frequently struck by megathrust earthquakes and tsunamis, as illustrated by the CE 2010 Maule (M_w 8.8) and CE 1960 Valdivia (M_w 9.5) events. Despite numerous palaeoseismic and palaeotsunami studies, uncertainties remain regarding the rupture extent and tsunamigenic potential of M_w 8–9 earthquakes. This study examines the sedimentary record of Laguna Gemela West, a coastal lake at 5–6 m a.s.l. and of 17.5 m depth. It is separated from the Pacific by a 400 m long channel bordered by northward-propagating dunes and controlled by Pleistocene sandstones forming a knickpoint in the channel profile. Multiple sedimentary proxies (e.g. grain-size, X-CT, XRF scanning) identified five distinct sand-enriched layers, interpreted as tsunami deposits. Age-depth modelling (based on 137Cs and 14C) allowed linking these deposits to the CE 2010, 1960, 1837, 1737 and 1575 megathrust earthquakes. While historical records confirm significant tsunamis in CE 2010, 1960, 1837 and 1575, no reports exist for a CE 1737 tsunami. However, a potential tsunami deposit and evidence for subsidence were found at the nearby Chaihuín site, albeit with large dating uncertainty (CE 1600–1820). The more precise age for a sand layer at Laguna Gemela West (CE 1672–1746) supports the occurrence of a local tsunami in CE 1737. Additionally, deposits linked to the CE 1837 and 2010 events suggest tsunamis can impact sites >100 km adjacent to megathrust ruptures. A second pulse in the uppermost sand layer may reflect the CE 2011 Japan tsunami, which reached a similar height (~1.6 m a.s.l.) in the nearest tide gauge as the CE 2010 tsunami. Unlike coastal plain sites, which often require coseismic subsidence for deposit preservation, coastal lakes can capture a more complete tsunami history. This study highlights their complementary role in palaeotsunami research, providing insights in local, regional and transoceanic tsunami events.

Volcanogenic detritus in sandstones not only conveys important provenance information but also significantly impacts diagenetic processes. This study specifically focuses on such understudied effects by carrying out grain size, petrographic, fluid inclusion, scanning electron microscopy, cathodoluminescence imaging, physical property and geochemistry of the second member of the Shanxi Formation (P₁s²) and the first member of the Lower Shihezi Formation (P₂h¹) from the Xinzhao area of the north Ordos Basin. These two members represent potential tight-sandstone gas reservoirs containing different volcanic materials derived from the Central Asian Orogenic Belt. Petrographic and geochemical analyses testify the deposition in an intracontinental basin close to the continental arc of the orogenic belt and reveal the closure of the Palaeo-Asian Ocean after the early Middle Permian. Magmatic crystallisation ages indicate that the P₁s² and P₂h¹ sandstones correspond to a magmatic lull phase testified by low pyroclastic production and to a period of intense magmatic activity evidenced from enhanced sediment supply and river overload, respectively. Both members experienced post-depositional compaction, cementation, dissolution and replacement and remain in the mesodiagenetic stage. The P₁s² sandstone suffered from only weak compaction and dissolution because of the abundance of more rigid and insoluble felsic fragments, whereas the P₂h¹ sandstone experienced strong compaction and dissolution owing to the higher content of ductile and metastable mafic debris, and it shows cementation by clean kaolinite, grain-coating chlorite and pore-filling analcite. Because these cements are soluble, the P₂h¹ sandstones display a high total porosity and permeability despite extensive compaction. The presence of mafic volcanogenic materials may thus favour the formation of superior-quality, litho-quartzose sandstone reservoirs. This contributes to understanding deeply the diagenetic processes in volcanoclast-rich sandstones of the North Ordos Basin. The knowledge gained in this study may serve as a paradigm for analogous tight sandstone reservoirs forming at intracontinental basins near active margins globally.

The Arcadia Formation beneath Long Key, Florida Keys consists of a composite sequence with three third-order sequences based on a regional correlation of gamma-ray logs and cores. Deposition of the composite sequence was a result of a second-order transgression on the underlying Suwannee Limestone, following an ~8 my hiatus. Combined, the three Arcadia third-order sequences consist of a basal set of 10 fining-up cycles (41 m thick), overlain by 22 coarsening-up cycles (99 m thick). Both cycle types show mid-ramp facies (~40–50 m depth) composed of molluscs, benthic foraminifera, bryozoan and red-algal bioclastics. The mineralogy of the fining-up cycles consists of alternations of a basal limestone and upper dolomite (75%–100%). The overlying, coarsening-up cycles are predominantly limestone with only minor dolomite. The 10 basal cycles are pervasively dolomitised with enhanced permeability. The dolomitised intervals not only exhibit the highest permeability (mean = 3309 mD) but also contain an occasional, thin, low-permeability (mean = 26 mD) cycle cap. Within the cycle, dolomite usually has an order-of-magnitude greater permeability than the limestone (mean = 214 mD). Dolomitisation of the upper part of the fining-upward cycle produces a permeability inversion relative to the original finer-grained (lower permeability) sediments. The intra-cycle transition from limestone to dolomite suggests early dolomitisation, perhaps even syndepositional or prior to deposition of the subsequent cycle. The results of this study are consistent with the proposal by Goldhammer et al. that ‘stratigraphy of early diagenesis’ is operational, where marine diagenesis is favoured during deposition in the transgressive systems tract. The Miocene example described herein provides an example for the formation of limestone–dolomite cycles and may represent preferential dolomitisation during sea-level transgression.

Phanerozoic patch reefs are extensively studied because they are abundant and are potential reservoirs for valuable fluids. In contrast, geologic studies of modern reefs have largely focused on platform margins, leaving patch reefs comparatively neglected. What conditions favour patch-reef development? What is their potential as reservoirs or as coral refugia under deteriorating environmental conditions? Where other environmental conditions are favourable, the availability of lithified Holocene sediment or Pleistocene rock as substrates for coral colonization appears to control patch-reef distribution in The Bahamas. A census of patch reefs from satellite imagery of the eastern Great Bahama Bank (GBB) tallies 23 reefs/km², for a total of 70,000 reefs over 3,000 km². Higher resolution images of favorable areas tally up to 80 reefs/km². Patch reefs accumulate sediment typically 3 m, and exceptionally 10 m, above the surrounding sea floor and amalgamate into complexes up to 2150 m long, illustrating their potential to form reservoirs. Patch-reef corals have adapted to conditions of elevated suspended sediment, salinity and temperature, suggesting they could provide refuge for corals as platform-margin reefs are decimated by climate change. Data on the distribution, abundance, morphology and health of patch reefs on the GBB may stimulate further research on modern patch reefs.

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.