Depositional Record最新文献

Sequential analyses of δ¹³C, δ¹⁸O and Δ₄₇ values of calcite and dolomite deposited in millimetre-sized cavities are reported from the Ronaldsway Member packstones, Isle of Man. The Ronaldsway brachiopods have δ¹³C values of ca +2.3‰ and δ¹⁸O values of ca −7.2‰; carbon is like predicted Carboniferous values, while oxygen values are more negative. The brachiopods show preserved microstructure but have marginal alteration and a streaky cathodoluminescence pattern. Crinoid ossicles have δ¹³C values of ca +2.3‰ and one with a δ¹⁸O value of ca −3.1‰, compatible with Carboniferous marine precipitates; three samples have δ¹⁸O values of ca −6.5‰ and are ¹⁸O-depleted. Calcite stages 1 and 2 have δ¹³C values ca +3.2‰ and δ¹⁸O values ca −2.5‰, compatible with Carboniferous sea water. Stage 1 and 2 have non-luminescent to orange CL zones. Stage 1 and early stage 2 contain red luminescent dolomite micro crystals generated during Mg calcite stabilisation. The Δ₄₇ values for stage 1 and 2 cements indicate temperatures of 86 and 105°C that occurred after the stabilisation of Mg calcite. Stage 3–8 zoned cements preserve their original growth surfaces and their δ¹³C and δ¹⁸O values suggest precipitation during burial and exhumation. The Δ₄₇ values of the brachiopods and crinoids indicate temperatures between 85 and 140°C indicating they were either recrystallised at high temperatures or affected by solid state reordering. To evaluate these alternatives two quantitative models, water–rock reaction and reordering models are performed. The allochems and cements are progressively altered by porewater towards the fluid-buffered behaviour. The quantitative evaluation of calcite and dolomite solid-state reordering suggests the elevated clumped isotopic temperatures are produced by interaction with hydrothermal fluids. This study improves understanding by applying previously untried techniques; further Δ₄₇ data and quantifying elemental variations would help further interpretation but the poorly documented post-depositional history is a drawback.

An enigmatic transition from the storm-dominated, offshore to lower shoreface deposits of the Redwater Shale Member (Sundance Formation) to the overlying mixed tidal and aeolian Windy Hill Sandstone (Morrison Formation) in the Oxfordian of the North American Western Interior has long been a source of intrigue. Previously proposed drivers include the progradation of a large, tide-dominated delta onto a storm-dominated shelf, a complete reorganisation of the basin's hydrodynamics and climate, or the development of a regional unconformity (termed the J-5). In south-eastern Wyoming, the Redwater Shale is characterised as an offshore to distal shoreface deposit with glauconitic siltstones and sandstones punctuated by coquinoid and sandy tempestites and hosting a Cruziana Ichnofacies. The Windy Hill Sandstone, a time-transgressive, sand-rich, intertidal succession with classic Pteraichnus and stressed Skolithos Ichnofacies, sharply overlies the Redwater Shale and records an abrupt basinward shift in facies that accompanied at least tens of metres of sea-level fall. New, detailed sedimentological, ichnological and architectural data collected across this transition in the study area provide fresh insights into the depositional history of these units and demonstrates the existence locally of a composite J-5 unconformity. The unconformity developed as tectonically driven, prograding shoreline trajectories of the Redwater Shale gave way to degrading trajectories of the Windy Hill Sandstone, leading to a forced regression and formation of a regressive surface of marine erosion. The sharp juxtaposition of intertidal flat facies (Pteraichnus Ichnofacies) directly upon offshore to lower shoreface deposits (Cruziana Ichnofacies) is the key to recognising the unconformity and proves the value of the previously underutilised ichnological data.

In natural deltaic settings, mixed hydrodynamic forcings and sediment properties are known to influence the preserved delta deposits. One process that has not received much attention yet is syn-sedimentary compaction of clastic sediment on millennial-scale delta evolution. To study how compaction interacts with delta morphodynamics and preserved sediment, a modelling approach is proposed. A 1D grain-size dependent compaction model was implemented into Delft3D-FLOW, which provides an opportunity to understand the underexplored connection between grain sizes supplied to the deltas and sediment compaction. The compaction model allows deposited sediment to decrease in volume due to the accumulation of newly deposited sediments above or the elapsed time. Differences in morphological trends are presented for scenarios defined by the composition of sediment supply (mud rich and sand rich) and the maximum allowed compaction rate in the model (0–10 mm year⁻¹). The resultant deposits are classified into sub-environments: delta top, delta front and pro delta. The delta top geometry (e.g. area increase, rugosity and aspect ratio), sediment distribution alongshore and across sub-environments, and delta top accommodation (e.g. volume reduction and average water depth) are compared. The modelling results show that compaction of the underlying delta front and pro delta deposits increases the average water depth at the delta top, driving morphological variability observed in the mud-rich and sand-rich deltas. The morphological changes are more prominent in the mud-rich deltas, which experience larger compaction-induced volume reduction for the same scenario. Moreover, higher compaction rates further increase the delta top accommodation, resulting in more deposition and evenly distributed sediment at the delta top. This leads to a less significant area increase and a wider delta top with a smoother coastline. The presented modelling results bridge the knowledge gap on the influence of syn-sedimentary compaction on long-term delta morphodynamics and preserved sediment. These findings can be applied to unravel the controlling processes in ancient delta deposits and predict the evolution of modern systems under changing climates.

The dynamic inter-relationships between marine and freshwater carbonate depositional environments are illustrated in the Sian Ka'an Wetlands, a 5 280 km² complex of groundwater-fed freshwater marshes, lakes and brackish coastal lagoons in the South-East Yucatán Peninsula (Mexico). The Yucatán Platform was subaerially emergent and extensively karstified during the last glacial maximum at 18, 000 yr bp. The Late Holocene transgression has caused progressive reflooding of the continental margin, backstepping of the MesoAmerican Reef and encroachment of coastal environments into the platform interior as rising groundwaters flood an interconnected cave and sinkhole system and feed seasonal marshes above. The Sian Ka'an Wetlands form a vast palustrine carbonate factory which is directly juxtaposed and dynamically linked with the marine carbonate factory to seaward. Continuing sea-level rise has caused synchronous landward migration of marginal marine and freshwater environments as beach barriers were breached and palustrine sloughs flooded to form marginal marine seagrass lagoons. The Rio Hondo Fault conditions fluid inflow while the sub-environments of the Sian Ka'an Wetlands reflect tectonic controls on microtopography and hydroperiod. Modern analogues for the Sian Ka'an Wetlands include the Florida Everglades, formed during transgression of the Florida Platform, and relict marsh environments preserved on leeward shores of Andros, Abaco and other Bahama islands. A wide range of ancient examples deposited in coastal and continental interior settings similarly reflect seasonal aquifer rise in response to marine transgression and/or onlap of late-stage basin fill onto a karstified pediment. Freshwater palustrine carbonate factories on carbonate platforms are transient deposystems, controlled by subtle water depth, climate, vegetation and hydrological factors while being critically sensitive to sea-level changes and tectonics. The preservation potential of palustrine carbonates may be relatively low in coastal settings due to erosion or shallow marine overprinting, while greater further inland where marine flooding is rarer and in tectonically subsident continental interior basins where accommodation space is continuously created.

The Niobrara Formation of north-east Colorado, USA, has anomalously negative δ¹⁸O values compared to all other Cretaceous chalks. These unique δ¹⁸O values have been attributed to elevated heat flow and/or freshening of the Cretaceous Western Interior Seaway. This work utilises clumped isotopes of calcite (Δ₄₇), peak burial temperatures estimated from pyrolysis data, and strontium and neodymium isotopes of carbonate to re-evaluate the origin of the calcite's ¹⁸O-depletion. Peak temperatures indicate lateral variability in geothermal gradients of ca 20°C/km at the tens of kilometre scale, and corroborate prior studies proposing locally elevated palaeotemperatures. Greater insight is provided by numerical models of calcite recrystallisation and oxygen isotope evolution that are constrained by measured Δ₄₇-derived temperatures, calcite δ¹⁸O values and inferences from the ⁸⁷Sr/⁸⁶Sr and εNd values. The models indicate that (1) sea water in the seaway had normal marine δ¹⁸O values of −1 (VSMOW) except on the eastern margin of the basin where some freshwater dilution yielded −2 to −3‰ (VSMOW) water, and (2) the main driver of the anonymously negative calcite δ¹⁸O values was a semi-open hydrologic system that provided a few percent by pore volume of meteoric groundwater derived from post-Laramide recharge into the basin. Minor contributions were a Laramide-aged heat pulse related to the underlying Colorado Mineral Belt, the thermal insulating effects of now eroded coals, and a small flux of compaction-driven Cretaceous sea water evolved by smectite dehydration. However, those three factors alone were insufficient drivers of the calcites' ¹⁸O depletion. High burial temperatures are interpreted to have caused clumped isotope reordering in at least one well, but those temperatures cannot yield the observed calcite δ¹⁸O values. The study illustrates the unique attributes of the Niobrara's diagenetic system that results in its anomalous δ¹⁸O values, and reaffirms the value of clumped isotopes in unravelling the diagenetic history of chalk systems.

Cenozoic limestones from Hawaii and Enewetak were studied to characterise diagenesis in deep sea water. Hawaii samples were from subsea outcrops of drowned Pleistocene reefs 150–1,505 m deep (maximum age 550–600 ka). Most samples had early fibrous aragonite and high-magnesium calcite cements precipitated in shallow sea water. Partial dissolution of aragonite (including coral) and high-magnesium calcite were significant at 412 m and increased to 1,505 m. Crusts of ‘stubby’ sparry calcite cement (2–8 mol.% MgCO₃; ‘lower Mg calcite’) precipitated on early aragonite and high-magnesium calcite cements at 473–1,358 m. Dissolution of aragonite and high-magnesium calcite was incomplete. Aragonite and high-magnesium calcite were not neomorphosed to low-magnesium calcite ( <5 mol.% MgCO₃). Enewetak well samples came from 3 to 1,400 m (Holocene to Upper Eocene). Lower Miocene to Upper Eocene carbonates at 380–1,380 m near the atoll margin showed pervasive dissolution of aragonite and conversion of high-magnesium calcite fossils to low-magnesium calcite. Their lower-Mg calcite cements (380–820 m; mainly radiaxial) were associated with aragonite dissolution. The lower-Mg calcite cements and bulk limestones below 500 m had geochemistry indicating precipitation or stabilisation in sea water at 10–27°C. Data indicate Enewetak dolomitisation (1,250–1,320 m) in cold sea water during burial >1,000 m. Coralline algae showed little petrographic alteration, but Mg decreased downward from 15 to 1.5 mol.% MgCO₃. In both areas, aragonite dissolution, alteration of high-magnesium calcite, and precipitation of lower-Mg calcite cements occurred in deep sea water (>300 m) undersaturated for aragonite, but supersaturated for low-magnesium calcite. Original high-magnesium calcite was partially dissolved in Hawaii samples, but converted to low-magnesium calcite in deep Enewetak cores, possibly due to gradual deepening at Enewetak. Dolomitisation and low-magnesium calcite dissolution occurred below the calcite saturation depth (approximately 1,000 m) in Enewetak, but not deep Hawaii samples, possibly because dolomitisation is slower. Temporal variations in carbonate saturation, especially related to pCO₂, are interpreted as the main control on mineralogy during marine diagenesis now and in many ancient oceans.

The Ediacaran–Cambrian Radiation marks the widespread appearance of metazoans and calcareous biomineralised hard parts. These innovations occurred during an interval of dynamic changes in marine redox and sea water chemistry. Here, changing carbonate mineralogy, Mg/Ca ratios and rare earth element concentrations including the relative abundance of cerium (Ce anomaly: Ce_anom) are documented to track sea water oxygen levels, in well-preserved early marine cements from shallow marine reefs from Cambrian Stages 2–4 (ca 525–512 Ma). First, integrating the mineralogical data with published records, several shifts in dominant carbonate mineralogy are inferred: ‘dolomite-aragonite seas’ in the late Ediacaran; ‘aragonite/high-Mg calcite seas’ in Cambrian Stage 2; a temporary shift to a ‘calcite sea’ during early Cambrian Stage 3; an ‘aragonite sea’ between late Cambrian Stage 3 and late Cambrian Stage 4, then a gradual shift from mixed ‘aragonite–calcite seas’ during the middle and upper Cambrian towards a ‘calcite sea’ by the early Ordovician. Second, based on measured mMg/Ca in early marine cements, calculated sea water mMg/Ca at 15 and 35°C ranges from 1.2 to 0.8 in Cambrian Stage 2, 0.7–0.4 in Stage 3 and 1.4–0.9 in Stage 4 respectively. Finally, analysed Ce_anom data combined with existing Ce_anom data suggest potentially three phases of global oxic expansion. First, a long-lived phase of progressive oxygenation during the late Ediacaran to Fortunian (ca 550–540 Ma; average Ce_anom from 0.99 to 0.41), and possibly two shorter phases during early Cambrian Stage 3 (ca 519 Ma; average Ce_anom from 0.91 to 0.40) and Stage 4 (ca 512 Ma; average Ce_anom from 1.02 to 0.49), bounded by intervals of more dominant anoxia. Summarising, these data demonstrate that early marine cements offer an underused and high-resolution archive of shallow marine redox and sea water chemistry through this critical transition in Earth's evolution.

The Derbyshire Platform is a Mississippian aged flat-topped, steep sided platform that forms the westernmost expression of the Derbyshire-East Midlands Platform. On the south-east platform margin, 60 km² of Visean limestone has been dolomitised, forming two distinct bodies. One of these bodies forms along a major NW–SE trending basement fault and smaller, associated, N–S trending faults and fractures. This study uses outcrop, petrographic and geochemical analysis to better constrain the timing and mechanism for this fault-controlled dolomitisation. Field relationships demonstrate dolomitisation was multi-phase and initiated after the main phase of matrix pore-occluding calcite cementation on the Derbyshire Platform and terminated prior to the main phase of mineralisation. Fluids are interpreted to have fluxed from adjacent basins, primarily along strike-slip crustal faults that were reactivated during basin inversion at the onset of the Variscan Orogeny. Fluid supply was episodic and progressively confined to fractures as matrix porosity became occluded. The study demonstrates the complex interplay between basin kinematics, host rock permeability and timing of fluid supply through seismic valving along faults that connect the carbonate platform to basin compartments. This ultimately controlled the position of dolomite geobodies along faults and provides a record of fluid flow during the transition from thermal subsidence to post-rift basin inversion. The findings have implications for the exploration of both minerals and hydrocarbon within dolomitised host rocks and can inform studies of fluid transfer and reaction on carbonate platforms within the burial realm.

X-ray fluorescence, grain-size and oxygen and carbon stable isotope measurements of a 33 m long piston core, recovered from the Pen Duick drift located at the foot of the prominent Pen Duick Escarpment (Atlantic Moroccan margin), are combined to decipher past oceanographic conditions. The data indicate that, similar to the northern Gulf of Cádiz, the Azores Front exerts a major control on the palaeoclimatology of the region. Contrasting the northern Gulf of Cádiz, where Mediterranean Outflow Water is the main water mass at similar water depths, the palaeoceanography of the studied area is mostly influenced by the amount of Antarctic Intermediate Water advected from the south. The density contrast between the Antarctic Intermediate Water and the overlying North Atlantic Central Water determined the strength of the prevailing internal tides and corresponding high current speeds, which drastically impacted the sedimentary record. The most notable impact is the presence of a 7.8 kyr condensed section (30.5–22.7 ka bp). The formation of the Pen Duick sediment drift was not just controlled by the strength of the bottom currents and the intensity of the internal tides, but also by the amount of (aeolian) sediment supplied to the region. Although variable, drift-growth phases seem to mainly occur during colder periods of the last glacial, that is Heinrich and Dansgaard-Oeschger events during Marine Isotope Stage 3 and late Marine Isotope Stage 2. These periods, characterised by increased aeolian dust supply and higher bottom currents, coincide with a phase of prolific cold-water coral growth and enhanced coral mound formation as recorded in numerous cores obtained from the southern Gulf of Cádiz. This implies that both records (on and off mound cores) are pivotal to provide the complete picture of the palaeoclimatic and palaeoceanographic conditions in the region.

End-member modelling of bulk grain-size distributions allows the unravelling of natural and anthropogenic depositional processes in salt marshes and quantification of their respective contribution to marsh accretion. The sedimentology of two marshes is presented: (1) a sheltered back-barrier marsh; and (2) an exposed, reinstated foreland marsh. Sedimentological data are supplemented by an age model based on lead-210 decay and caesium-137, as well as geochemical data. End-member modelling of grain-size data shows that marsh growth in back-barrier settings is primarily controlled by the settling of fines from suspension during marsh inundation. In addition, nearby active dunes deliver aeolian sediment (up to 77% of the total sediment accretion), potentially enhancing the capability of salt marshes to adapt to sea-level rise. Growth of exposed marshes, by contrast, primarily results from high-energy inundation and is attributed to two sediment-transport processes. On the seaward edge of the marsh, sedimentation is dominated by coarser-grained traction load, whereas further inland, settling of fine-grained suspension load prevails. In addition, a third, coarse-grained sediment sub-population is interpreted to derive from anthropogenic land-reclamation measures, that is material from drainage channels relocated onto the marsh surface. This process contributed up to 34% to the total marsh accretion and terminated synchronously with the end of land reclamation measures. Data suggest that natural sediment supply to marshes alone is sufficient to outpace contemporary sea-level rise in the study area. This underlines the resilience potential of salt marshes in times of rising sea levels. The comparison of grain-size sub-populations with observed climate variability implies that even managed marshes allow for the extraction of environmental signals if natural and anthropogenic sedimentary processes are determined and their relative contribution to bulk sediment composition is quantified. Data series based solely on bulk sediments, however, seem to be of limited use because it is difficult to exclude bias of natural signals by anthropogenic measures.