Depositional Record最新文献

Magnesian Limestone zoned dolomite (red and non-luminescent zones) now partly dedolomitized), calcite (yellow luminescence), blue luminescent albite has non-luminescent overgrowths) Mansfield Quarry, Nottinghamshire. Photo credit: J.A.D. Dickson.

Unconsolidated artificial ground is an ever-increasing feature on the Earth's surface but it poses various challenges such as pollutant release and ground instability. The process of lithification could be an important factor in changing the properties of artificial ground and ameliorating these challenges. In this study, a lithified deposit of a furnace slag associated with a former iron and steel works in Scotland was analysed to determine the mechanisms and drivers of lithification. Scanning electron microscope analysis showed that Ca leached from around the edges of clasts of slag through reaction of the chemically unstable slag with water from an adjacent water body. Dissolution of Ca (and OH⁻) from the slag caused the water in contact with the slag to become hyperalkaline, facilitating ingassing and hydroxylation of CO₂ from the atmosphere (fingerprinted through carbon isotope analysis). Reaction of the dissolved Ca and CO₂ led to precipitation of calcite. Scanning electron microscope analysis showed the calcite is distributed between slag clasts, forming rims around clasts and cementing clasts together into a solid rock-like mass. Understanding the mechanisms and drivers of lithification in artificial ground will be important, given its widespread nature particularly in urban areas where artificial ground is the substrate of most development.

Sketch diagram illustrating the various hydrodynamic processes occurring in coastal to shelf environments, with numbers referring to articles from this issue.

Mixed siliciclastic and carbonate sediments are common in the stratigraphic record, but fully homogenised mixes are not. Many occurrences of mixed sediment sequences are dominated by end-members with stacking of ‘nearly pure’ lithfacies (e.g. cyclothems containing alternating sandstone, limestone and coal units). The Plio-Pleistocene sediments within south-west Florida provide insights into the occurrence of fully homogenised siliciclastic/carbonate deposits. In all defined environments from lagoon to supratidal to inner tidal to beach to offshore to coral reef, quartz sand coexists with carbonates. Perhaps the key feature that allowed full homogenisation of the sediments within all facies and subfacies was the relatively shallow water (<10 m), which facilitated mixing during low-order eustatic sea-level events and storms. However, four factors contributed to the full homogenisation of the sediment types without termination or inhibition of carbonate organism growth. These factors are (1) the shallow water allowing wave-driven sediment transport (all environments within the wave orbital depth), (2) close proximity and perhaps irregular nature of the depositional environment boundaries, (3) low influx rate of quartz sand via longshore transport, and (4) the lack of significant terrigenous mud transport into the system. Mixing processes at the large-scale included movement of sediments from one depositional environment to another during storms, mixing along facies boundaries, and in situ mixing within autochthonous and parautochthonous mollusc death assemblages. At the smaller scale, mixing occurred by bioturbation and diagenetic dissolution of carbonate skeletal grains during minor high sea-level stands.

When compared to their temperate coastal counterparts, sediments deposited and preserved along arid aeolian to shallow-marine margins remain relatively poorly understood, particularly at the scale of lithofacies units and architectural elements. These systems often record evidence for relative sea-level change within sedimentary basins. This work focusses on the Entrada–Curtis–Summerville formations that crop out in central eastern Utah, USA, and provides a detailed analysis of the aeolian Moab Member of the Curtis Formation (informally known as the Moab Tongue) that was impacted by cycles of marine transgressions and regression in the late Jurassic. This study utilises photogrammetry, sedimentary logging and sequence-stratigraphical analysis techniques. Results indicate that four short-lived transgressive-regressive cycles are preserved within the Moab Member, followed by a broad regressive event recorded at the transition between the Curtis and Summerville formations. These cycles relate to changes in the relative sea level of the Sundance Sea and the deflation and expansion of the neighbouring aeolian dune field. During periods of normal regression, marine sediments displayed evidence of tidal and wave action, whereas the continental domain was characterised by growth of the aeolian system. However, when regression occurred within optimal physiographic conditions such as a restricted, semi-enclosed basin, and at sufficient magnitude to outpace erg expansion, this acted to shut-down bedform development and preservation. A rapid restriction of aeolian sediment availability and the inability of the dune field to recover resulted in the formation of deflationary sandsheets, arid coastal plain strata and contemporaneous shallow-marine deposits that are starved of wind-sourced sediments. This study highlights how a rapidly developing high-magnitude regression can lead to the overall retraction of the erg. Deciphering the evolution and sequence stratigraphical relationships of arid aeolian to shallow marine margins is important in both understanding environmental interactions and improving the characterisation of reservoir rocks deposited in these settings.

Multi-scale transgressive–regressive cycles from the mid-Jurassic were recognised in the Central Lusitanian Basin, Portugal. These cycles allow the depositional evolution of the basin to be better understood and aid in the construction of stratigraphic sequences composed of three hierarchies. The stacking pattern of high-frequency transgressive–regressive sequences forms larger clusters that define medium-frequency transgressive–regressive sequences. Likewise, the stacking pattern of medium-frequency transgressive–regressive sequences generates two Bathonian–early Callovian low-frequency transgressive–regressive sequences. Integration of several methods supported the interpretation of facies associations representing clastic deposition in offshore to shoreface environments and carbonate sediments in outer to inner ramp settings. New data from calcareous nannofossils and dinoflagellate assemblages constrained the interval's Bathonian–early Callovian age, thus unveiling the Middle–Upper Jurassic disconformity and filling the Middle Jurassic stratigraphic record gap in the Central Lusitanian Basin. This study may be helpful for similar successions in Tethyan domains and comparable depositional settings elsewhere.

A detailed sedimentological analysis of the Palaeocene Lockhart Limestone has been conducted to evaluate the depositional environment, diagenetic processes and hydrocarbon potential of the eastern margin of the Upper Indus Basin. From bottom to top, there are three microfacies recorded. The lower microfacies, composed of fine-grained micrite and some diagenetic dolomite, reflect the low energy and calm palaeo-current in the shallower section (1–2 m) of the inner shelf close to shore. The middle microfacies contain algae that suggest 5–15 m of water depth, especially along the inner-middle shelf, but fractured and mixed bioclasts in micrite material indicate calm to moderately active water close to the wave base. Progressing from the lower microfacies to the middle microfacies, a gradual shift from orthochem to allochem components is observed. The top microfacies is dominated by massive benthic microfossils, indicating moderate energy-water conditions with normal salinity. However, the presence of limestone intraclasts surrounded by microspar, miliolids and nummulites at the top indicates a high-energy environment with increasing salinity and water depths from 20 to 130 m. These findings show that the Lockhart Limestone was deposited in a shallow shelf environment, spanning the inner-mid shelf. Diagenetic processes observed include micritisation, cementation, dissolution, replacement, physical and chemical compaction, and fracture filling by calcite cement. The Lockhart Limestone represents a deepening upward sequence deposited below the shelf margin system tract and highstand systems tract in a regressive environment that could reflect good reservoir characteristics, has the potential to serve as an excellent hydrocarbon reservoir rock, and could be a primary target for future hydrocarbon exploration.

Exceptionally well-preserved tufas located west of Calama, Atacama Desert, Chile, designated Santa Juana tufas, record episodic wetter conditions, relative to today, over the past 500,000 years. Globally, tufa architecture and depositional details are poorly understood as most described tufas have been degraded by weathering and erosion. In the hyperarid Atacama, post-depositional alteration is negligible, therefore, the exceptional preservation of Santa Juana tufas documented in this study provides new information about tufa facies and their complex interactions. Santa Juana facies include microbial stromatolites, phytoherms, cascadestone, flowstone and porous limestone. Phytoherms, consisting of former plant stems coated with calcite, developed in channels, within pools, and along spring discharge aprons. Cascadestone, representing former waterfalls, preserves microbial filaments and delicate V-shaped calcite crystals. Flowstone lines shallow subvertical to subhorizontal channels, representing sites of rapidly sluicing water flow. Porous limestone, containing sparse calcite and/or gypsum and anhydrite cement crystals, represents detrital accumulations. Stable isotope results, coupled with U/Th ages, show that by the Quaternary, relative to the Neogene, groundwater was less supercharged with volcanogenic CO₂ so degassing was moderated. The δ¹⁸O ratios from Miocene–Pliocene palustrine and lacustrine freshwater carbonates that underlie Santa Juana tufas indicate significant evaporation, but the tufa δ¹⁸O signal indicates a less evaporative trend due to shorter atmosphere exposure time. Biological fractionation in δ¹³C is largely masked by the region's volcanogenic carbon footprint, although tufa petrography shows well-preserved microbial filaments and laminations. The range of tufa ages in this study shows that there were wetter time periods within the drainage basin headwater area in the Quaternary, but that by the late Pleistocene to early Holocene, aridity to hyperaridity became established. The lack of diagenesis or alteration within the Santa Juana tufas indicates that there has been minimal rainfall since their deposition.