Thick Sedimentary Sequence Research Articles

West Siberian sedimentary basin. Crustal subsidence caused by rock contraction in its lower part due to prograde metamorphism

The history of the crustal subsidence in the Mesozoic and Cenozoic in the West Siberian Basin – the largest sedimentary basin in the world – is considered. Most researchers associate its formation with post-rift subsidence of the crust, which followed an episode of strong lithospheric stretching about 250 million years ago near to the Permian to the Triassic transition. A characteristic feature of post-rift subsidence is a decrease in its rate in time. During the Mesozoic-Cenozoic history, in Western Siberia the rate of crustal subsidence should have slow down by an order of magnitude. However, the analysis of long (700–900 km) seismic profiles in the north of Western Siberia and in the Southern Kara Sea shows that since the beginning of the Mesozoic in these regions, on average, there has been an acceleration of the crustal subsidence. Under such circumstances, lithospheric stretching in them could be responsible for only a small part of the total subsidence of the crust of 6–7 km. In Western Siberia, the Earth’s crust is close to the isostatic equilibrium. Then, in the absence of strong stretching, the accumulation of thick sedimentary sequences in the basin could only have been caused by rock contraction in the lower crust due to prograde metamorphic reactions. To obtain the above results, we used, for the first time, some simple methods to analyze the structure of the sedimentary cover in the West Siberian Basin. Detailed seismic profiles have been published for many other deep basins on all the continents. The methods of their interpretation implemented in this paper can be easily applied to determine the role of lithospheric stretching in the formation of deep sedimentary basins on the global scale.

Impacts of groundwater flow on borehole heat exchangers: Lessons learned from Estonia

The production of geothermal energy relies on subsurface properties. In low-enthalpy geothermal regimes, thermal energy extracted with a borehole heat exchanger (BHE) is the subsurface heat conducted from the rock and grout to the BHE fluid. Estonian geology provides a good opportunity to investigate how groundwater flow impacts BHE heat production. The hundreds of metres thick sedimentary rock sequence holds four main aquifers. We modelled single BHEs with lengths of 400 m, 500 m and 1000 m and a BHE field of ten 400-m wells at three sites across Estonia. Then, we parametrized different hydraulic conditions to compare how the groundwater flow velocity impacts the thermal energy yield of the systems.Our results indicate that if groundwater flow increases above 0.03 m/day, it increases the total energy yield from 3 % to 20 %. Thermogeological parameters, such as subsurface temperature and thermal conductivity, affect the thermal energy yield of the wells more than the modelled groundwater flow. Our study provides an estimate of the thermal energy yield from different BHE types and the detailed sensitivity analysis. We conclude that geothermal energy is a significant renewable energy resource for Estonia and areas with similar geology and climate.

Trilobites from the upper middle Cambrian (Miaolingian, Guzhangian) of the Sierra Los Ajos, northern Sonora, Mexico: Biostratigraphic and paleogeographic considerations

A Cambrian sedimentary sequence with a predominant lithology of limestone, sandy limestone, and quartzite pertaining to the Bolsa and Abrigo formations is exposed in the Sierra Los Ajos to the east of Cananea city (northeastern Sonora, Mexico). Two fossil assemblages from Guzhangian layers of the Abrigo Formation are described herein. The trilobites identified comprise Tricrepicephalus texanus (Shumard), Crepicephalus sp., Coosia ariston (Walcott), Coosia sp., Cedaria cf. eurycheilos Palmer, Coosella sp., Llanoaspis modesta Lochman, and Llanoaspis? sp., representing the Cedaria and Crepicephalus zones (Miaolingian, Guzhangian). The Cambrian rocks of northeastern Sonora (Sierra Los Ajos, El Tule, and Mesteñas hills), southern Arizona, and Texas, were deposited as part of a thick sedimentary sequence on the southwestern margin of Laurentia, associated with an inner shelf marine environment. The distribution of the trilobites identified confirms the existence of a widespread Cambrian faunal province that includes the southeastern United States of America and northwestern Mexico.

Rapid barrier estuary infill in a geologically-constrained setting: Aireys Inlet/Painkalac Creek, Victoria, Australia

The Holocene infill of an intermittently open-closed barrier estuary (Painkalac Creek/Aireys Inlet) was examined on the southern margins of Australia, through aerial LiDAR, coring, sedimentology, radiocarbon dating and pollen analysis. It is found that almost all the accommodation space within the estuary has been occupied, with a thick sequence (>9 m) of sediments being deposited rapidly soon after flooding by the sea. The rate of sedimentation was 4.8 mm/year which closely matches the rate of sea level rise (4.5 mm/year) during the 1200 year period that the estuary infilled, from around 8000–6800 years ago. The evidence of estuarine deposition above the elevation of the modern intertidal zone provides significant further evidence for the presence of the mid-Holocene highstand sea level in this relatively unstudied part of the Australian coast. The pattern of infill departs from the traditional barrier evolution models. The first phase of infill was likely tide dominated with subtidal sand shoals being found landward of a subtidal bedrock sill. Once a beach-barrier sequence formed, the estuary infilled in a more traditional manner, with lateral progradation of a flood tide delta and central mud basin infill. The barrier estuary of Painkalac Creek therefore has undergone a complex evolutionary history, characterised by a change in process dominance from classic tidal to wave-dominated form. Infill occurred in a keep-up (or fast catch-up) mode, with geological control occurring in regard to the presence of an intertidal bedrock sill at the mouth of the estuary.

On the occurrence of kalsilite in melilite-bearing ultrapotassic lavas from the Roman Province (Vulsini Mts., central Italy)

The Montefiascone Volcanic Complex belongs to the Vulsini Mts. volcanic district, one of the several leucite-bearing ultrapotassic Pleistocene volcanoes of the Roman Province (Central Italy). It is characterized by abundant pyroclastic successions and minor lava flows emplaced during a caldera formation phase occurred ∼0.3–0.2 Ma. The volcanic products span from leucite-basanites and leucite-tephrites to rarer leucitites and melilitites, occasionally associated with very rare kalsilite melilitolite ejecta. Here we report for the first time the presence of primary kalsilite in two lava flows in Forcinella and Feltricci localities, identifying a kamafugitic component among the Montefiascone volcanics.Detailed petrographic, SEM and EPMA studies on ten samples from the two localities revealed, in addition to kalsilite, variable amounts of clinopyroxene, melilite, leucite and nepheline plus rarer olivine coupled with carbonate-bearing segregation pockets in the groundmass. Geochemical and isotopic data confirm the exotic nature of the analysed rocks, showing ultrabasic/basic (SiO2 = 42.15–45.94 wt%) and ultrapotassic (K2O = 6.17–8.48 wt%; K2O/Na2O = 5.3–8.7) compositions, as well as strongly radiogenic 87Sr/86Sr (0.71038–0.711068), poorly radiogenic 143Nd/144Nd (0.512057.512091) and uniform middle radiogenic 206Pb/204Pb (18.76–18.78). Boron isotopes (δ11B ranges from 7.36 to 8.82‰) almost overlaps the typical MORB-OIB range values.These geochemical features recall those of the nearby kamafugite rocks from the Intra-Apennine Province. Major oxide and trace element modelling support fractionation links among the Italian kamafugite types. The overall geochemical and isotopic signatures speak for a common strongly subduction-metasomatized and orthopyroxene-free lithospheric mantle source. As already proposed for some nearby kamafugite centres in central Italy, limestone assimilation during magma ascent is a viable option to explain the mineralogical and whole-rock composition of Montefiascone lavas (i.e., the presence of carbonate plagues in the lava groundmass). This is particularly relevant considering the thick sedimentary sequence pierced by the limited volume of the upwelling magma. On the other hand, many other key features (such as the general lack of crustal xenoliths, the overall incompatible element fractionation and the radiogenic Sr isotopic ratios) clearly require also derivation from subduction-modified mantle sources.

THE SIGNIFICANCE OF THE PLEISTOCENE AQUATIC FAUNA OF MOLLUSKS FOR THE REPRODUCTION OF SEDIMENTATION CONDITIONS IN WESTERN UKRAINE

The territory of Western Ukraine is characterized by the development of a wide range of genetic types of continental formations. The Carpathian Mountain Country (properly the Carpathians, East Carpathian Foreland, and Transcarpathia) and the highly fragmented parts of the Volhynia and Podolia Uplands are characterized by a special diversity. The time range of their formation is vast – from the Early Pleistocene to the present. The complexity of the structure of the Quaternary sequence gives rise to discussions about the origin of some components of anthropogenic accumulations. Despite the fact that most of the challenging issues have already been resolved, we find debatable information in the works of some researchers of the 20th and even the 21st centuries about the aqueous origin of the thick sequences of anthropogenic sediments covering the terraces of the Dniester and Dnieper basins. The researchers use many methods to solve the issue of the deposit genesis. Some of them are based on the study of paleontological (including paleomalacological) remains, which are often found in both aqueous (alluvial, lake, swamp) and aerial (primarily loess) formations. Malacofauna has been used since the second half of the 19th century to determine the age, conditions of formation, and origin of the deposits. To establish the origin of sediments we have used the mollusk fauna in this work, which can be often found in the formations of the studied territory. The interpretation of the fauna is debatable in the sections where several genetic types of formations are exposed, primarily alluvial and eolian–deluvial loess. This type of combination of deposits is found in outcrops on the numerous terraces of the Dniester and the terraces of its East Carpathian Foreland’s and Podolia tributaries. The results of the research obtained by us using paleomalacological studies correlate well with data obtained using other methods. Such methods include actual field observations, as well as lithological, palaeopedological, paleontological (study of fossil mammals, fish, spores, and pollen), archaeological, and other methods. The results presented in this work made it possible to establish the genesis of some components of polygenetic complex strata, as well as testify to the effectiveness of the applied research method. Keywords: anthropogen; malacofauna; alluvium; loess; paleogeography; Podolia; East Carpathian Foreland.

Changes in Late Pleistocene Dust Activity in the Southern Tibetan Plateau in Response to Orbital Precession and Mountain Glaciers

AbstractThe Tibetan Plateau (TP) serves not only as the “water tower” of Asia but also as an important source in the global atmospheric dust cycle. While our knowledge of modern dust activity and its impacts and interactions with climate change in the TP has greatly advanced in the past decades, the emission, transport, and deposition of dust on the geological time scale remains unclear. This study analyzed a 7.6‐m thick sedimentary sequence consisting of loess and sand from the Yarlung Tsangpo River (YTR) valley in the southern TP. The sequence chronology was established using nineteen K‐feldspar post‐infrared infrared stimulated luminescence (pIRIR) ages, which ranged from 47.11 ± 1.95 to 116.65 ± 5.55 ka in a general stratigraphical order. The dust sedimentation rate and sorting coefficient of grain size were used to reflect dust activity and near‐surface wind, respectively. The results indicated that dust activity in the southern TP is mainly regulated by the near‐surface wind intensity and follows the variation pattern of precession, although the waxing and waning of mountain glaciers also affect the amplitude of dust activity. This pattern is not consistent with the Greenland dust record, which follows the variation pattern of obliquity. Therefore, dust accumulation in the southern TP is concluded to be primarily controlled by the South Asian winter monsoon (SAWM) forced by precession, whereas dust accumulation in Greenland is closely related to the intensity of the high‐level westerlies forced by obliquity.

Chronology of late Holocene sediments related to the Qicheng ruins in central China

The Qicheng city wall is located in Puyang, Henan Province, China, and it is an important archaeological site from the Neolithic period in the North China Plain region. A trench across the northwest corner of the city wall revealed an 11 m thick sediment sequence, closely associated with human activities, which provides valuable records for the dating and environmental evolution of the site. In this study, we have applied field geological investigation, grain size analysis, optically stimulated luminescence (OSL) dating, and radiocarbon (14C) dating to reveal the history and changes in the site. The ancient city wall was built on the sedimentary sequence (U1) of the high terrace of the Ancient Yellow River. The upper 11 m thick sedimentary sequence (U2) recorded early moat sedimentation (U2L) and was followed by three cycles of paleosol-flooding deposits (U2U). The quartz OSL signals of the U2 silts are extremely bright, indicating high sensitivity. Partial bleaching was observed as well. The quartz OSL single-grain SAR dating results are consistent with that of charcoal 14C dating and are closer to the ‘true’ burial ages of the sediments than those obtained with multiple grains. By using a Bayesian statistical model, a chronological framework for the U2 sedimentary sequence was established. The earliest construction date of the Qicheng city wall is dating to 2100–1300 BC, during the late Longshan period, which is consistent with archaeological evidence (Li et al., 2015). The Qicheng city was not significantly affected by paleo-flooding prior to 870–960 AD, but experiences frequent flooding thereafter.

Depositional system and plant ecosystem responses to long-term low tempo volcanism, the Interbasaltic Formation, Antrim Lava Group

Abstract The Antrim Lava Group of NE Ireland comprises a volcanic sequence dominated by basaltic lava flows. Including subsidiary sedimentary interlayers and some evolved lavas and intrusions, the overall sequence reaches a cumulative thickness of ∼800 m. The tempo of eruption of the Antrim Lava Group is poorly constrained but can be evaluated via weathering patterns and environmental reconstructions derived from lava-flow interbeds. In this contribution, we present palynology from a newly identified and well-developed 2.0–2.5 m thick sedimentary sequence (interbed) at Ross's Quarry, Ballycastle, Co. Antrim, that helps elucidate the contemporary development of environments in a setting subject to periodic basaltic volcanism. The interbed is subdivided into geologically distinct subunits of cross-bedded and parallel-bedded sandstones and sandy siltstones, all rich in visible organic remains such as rootlets and fragments of wood and bark. A total of 19 samples was collected from the sequence and subsequently analysed for palynological content. The palynomorph data point toward a diversity of inputs ranging from estuaries, chalky soils, dry soils, swamps, lakes, floodplains, sand bars, wet soils, established bogs and fenlands. In contrast to current understanding, the palynological data and their inferred environments collectively reveal the presence of flora that favour a temperate climate rather than the subtropical climate that has previously been inferred from the lateritic interbeds of the Antrim Lava Group. By combining the Ross's Quarry observations with palynological data from other quarry sites and boreholes in Antrim, we provide new insights into the climate, weathering systems and eruptive history of the Antrim Lava Group.

Joint interpretation and modeling of potential field data for mapping groundwater potential zones around Debrecen area, Eastern Hungary

The Debrecen area, as part of the Great Hungarian Plain (GHP), is associated with a multi-aquifer system that is overly exploited to fulfill the development plans. This research aims to jointly interpret and model gravity and magnetic data to map the subsurface geology and structures that govern groundwater occurrence. Various potential field techniques, including spectral analysis, anomaly derivatives, analytical signal, and Euler deconvolution were employed to map the distribution and depth of the buried geological structures. The combination of the potential field techniques enabled the construction of a detailed lineament map, providing valuable insights into the distribution of the subsurface structural features. It was indicated that the main structural trend is NW–SE and NE–SW, that coincides with the main structural trends in Hungary. Subsequently, a lineament density map is derived, indicating that the eastern, central, and northwestern parts of the area form the most promising zones for groundwater prospection. The joint inversion of gravity and magnetic data has further enhanced the understanding of subsurface geology. The depth to the basement rock varied between 1.18 and 2.2 km. The highest depth to the basement meets with thick sedimentary sequences bounded by normal faults forming graben and horst structures. Moreover, the distribution of these sediments is investigated using lithological logs indicating the thickness of the main hydrostratigraphic units in the Debrecen area. These units include Nagyalföld Aquifer, Algyő Aquitard, Endrőd Aquitard, and Miocene Badenian Aquifer units, which mainly consist of sand, silt, marl, and gravel. The recent study demonstrated the effectiveness of the joint interpretation in enhancing the knowledge of lithology and geological structures. However, a detailed geophysical survey is recommended to characterize the hydrostratigraphic units in the Debrecen area.

AMS 14C DATING AND STABLE ISOTOPE ANALYSIS ON AN 8-KYR OYSTER SHELL FROM TAIPEI BASIN: SEA LEVEL AND SST CHANGES

ABSTRACT Seven accelerator mass spectrometry radiocarbon (AMS 14C) dates (7260±106∼7607±95 BP averaged 7444±103 BP) on a giant oyster shell, collected from an ancient shore of the Taipei Basin, are similar to the LSC (liquid scintillation counting) 14C age (7260±46 BP) of a grass sample inside the shell. The calibrated 14C ages of the C. gigas by Marine20 are 7490±240∼7805±230 cal BP (average 7660±96 cal BP), generally agreed with the calibrated LSC 14C ages of the grass and the oyster shell. Combined with other 14C ages of shoreline samples in the Taipei Basin, it is evident that sea level rose from 8600 to 7600 cal BP and reached a stand higher than modern sea level. During this marine transgression, the sedimentation rate along the shoreline was very high because 14C dating was not able to detect age differences for 4–5 m thick sediment sequences. Sixty-nine analyses of δ18O and δ13C from the oldest part of the shell exhibit clear seasonal cycles, with a 4-year period of growth in the 5.5-cm section. According to the δ18O values, the ancient oyster grew in a warmer-than-present shoreline environment, suggesting that the current absence of the giant oyster in Taiwan is not due to warming conditions.

Spatial and temporal variability in Holocene trough-fill sediments, King Haakon Trough System, sub-Antarctic South Georgia

The climate in the South Atlantic sector of the sub-Antarctic, and therefore on and around the island of South Georgia, is dependent on the Southern Hemisphere Westerlies (SHW) and the Antarctic Circumpolar Current (ACC). The SHW and the ACC, in turn, are strongly controlled by climate variability in the Southern Hemisphere. Accordingly, thick sediment sequences in the troughs across South Georgia's continental shelf serve as valuable archives for past climate variations in the Southern Ocean. Since Holocene climate fluctuations led to only minimal oscillations in glacier margin positions within the fjords, the entire shelf was exposed to dynamic ocean currents since at least 10 ka BP. Its depositional systems are therefore a suitable target for the reconstruction of Holocene dynamics of both the SHW and the ACC. Sub-bottom profiler data and radiocarbon ages from four gravity cores from the south-western South Georgia continental shelf provide evidence for a complex interplay between island run-off and ocean currents intruding into a unique cross-shelf trough system during the last ∼10 ka. The data reveal several prominent changes in sediment and Holocene climate dynamics, the most significant occurring between 8 and 7.7 cal ka BP and between 2.6 and 2.2 cal ka BP. Both of these time periods represent transitions from warmer to cooler and windier conditions in South Georgia and the Southern Hemisphere. Our record from the King Haakon Trough System is the first highly resolved Holocene archive from the marine realm on the south-western South Georgia continental shelf and suggests several large-scale Southern Hemisphere climate changes during the mid-to late Holocene.

The Luna structure, India: A probable impact crater formed by an iron bolide

The Luna structure of India has been rumored to be an impact crater for more than a decade without any convincing evidence. This structure (1.5–1.8 km) is prominently visible in the low-lying Banni Plains of the tectonically active Kutch Basin as a circular morphological feature with a less-prominent rim. Luna area is strewn with melt-like rocks having high specific gravity and displaying wide range of magnetic properties. It contains minerals like wüstite, kirschsteinite, ulvöspinel, hercynite, and fayalite. The whole rock analysis denotes PGE enrichment, with notably higher average concentrations of Ru (19.02 ppb), Rh (5.68 ppb), Pd (8.64 ppb), Os (6.03 ppb), Ir (10.63 ppb) and Pt (18.31 ppb). The target is not exposed at Luna, owing to the overlying thick sequence of Quaternary sediments. The mineralogical and geochemical signatures points to an impact into a target, which is rich in clay with elevated calcium and silica (sand/silt) content. Geochemical data suggests an iron or stony-iron meteorite as the potential projectile at Luna. The silt layer containing plant remnants, underlying the strewn layer, yielded a radiocarbon age of 6905 years, making Luna the biggest crater to result from an iron bolide within the last 10,000 years.

Magnetic surveying as a proxy for defining cyclicity in thick sedimentary fillings: Application to the Cretaceous Cameros Basin (N Spain)

This work proposes the application of magnetic surveying to the study of cyclicity in sedimentary basins. This application is favoured by the particular bed attitude/topography relationships found in the eastern sector of the Cretaceous Cameros Basin, N Spain. Both the measured total magnetic field (after diurnal correction) and the vertical magnetic gradient, provide a profile (14 cm separation between measurements, in average) of the short-wavelength magnetic anomalies associated with the sedimentary sequence. These anomalies depend on the magnetic susceptibility (dominant for low Koenigsberger ratios, a common scenario in sedimentary rocks), and the magnetic remanence (whose orientation is fairly constant in remagnetised basins) of the different sedimentary units. The comparison with direct measurements of magnetic susceptibility (in the field and in the laboratory) provided a successful double-check. The cyclostratigraphic analysis of magnetic profiles of this 6 km-thick series indicates the existence of cycles of 405 kyr (long eccentricity) and 100–125 kyr (short eccentricity), as well as other lower and higher frequency cycles. Cycles with similar, but slightly different, periodicities are the result of different sedimentation rates (mean of ∼0.2 mm/yr, with values ranging between ∼0.1 and ∼0.36 mm/yr). These results provide and indirect method for dating the basin fill, between the Tithonian–Berriasian boundary and the basalmost Albian (∼143.2–∼112.6 Ma), and impose new constraints for basin evolution. The results obtained open a promising field for quick and reliable determination of magnetic properties and cyclicity in thick sedimentary sequences.

New data on the palynostratigraphy of the Mikulinian (Eemian) deposits in the lower reaches of the Pyoza River (northeastern White Sea region)

The palynological method was used to study a 455 cm thick marine sediment sequence, which overlies the Moscovian moraine in the Bychye-2 section on the Pyoza River (Arkhangel’sk region). The pollen records obtained allowed us to make a palynozonal subdivision of the section and to trace the plant cover transformations that occurred in the northeastern White Sea region during the Boreal transgression. When comparing the established local palynozones with the regional subdivisions of the reference sections Bychye-1, located 5 km upstream on the Pyoza River from the Bychye-2 site, and Peski on the Karelian Isthmus, we also employed the methods of correlation and event palynostratigraphy. It has been found that the marine sediments in the Bychye-2 section accumulated from the end of the Moscovian glaciation (~ 131 ka BP) until ~ 119,5 ka BP. The seven local palynozones established in the Bychye-2 section indicate a consistent change in climatic conditions and vegetational feedbacks. The main trend is the rapid displacement of Arctic-type shrub-herbaceous communities by pine-birch forests at the very beginning of the Mikulinian (Eemian) interglacial and the subsequent rise in the proportion of dark coniferous elements in the forests due to the growing humidity of the climate in the course of the transgression. A significant warming of the climate led to the emergence of thermophilic deciduous plants (oak, elm, hazel, and possibly even hornbeam) in the forests of the region. The predominance of pine-birch forests and the disappearance of all thermo- and hygrophilic elements of the vegetation followed the deterioration of climatic conditions during the final stage of the Mikulinian interglacial.

Modal Quantitative Logs for the Objective Recording and Analysis of Very Thick Sedimentary Sequences

The quantitative logs method is designed for objective facies analysis of thick sedimentary successions. This method enables the analysis of whole successions instead of selected intervals arbitrarily considered as representative and results in a database suitable for further analysis, e.g., statistical, quantitative stratigraphy, or facies modelling. The logging procedure involves the following steps: (i) each of the logged sedimentary features is classified according to a standard, e.g., grain-size follows the phi scale or Wentworth classes are applied for bed thickness; (ii) in the course of logging, the whole succession is subdivided into intervals, each of which is characterised by a predominant (i.e., modal) class of the considered feature; (iii) such modal class characterises a part of the section that has a specific thickness and is called a ‘modally homogeneous interval’ (MHI); and (iv) the lower and upper boundary of each MHI are defined by a change in the modal class. The thickness of all MHIs characterised by the same class interval is then added. The grand total of such component sub-totals for all class intervals of the logged feature equals the log thickness and is the basis for the calculation of frequency distribution, which characterises the succession from the viewpoint of this feature. Each bar of the resulting histogram represents the percentage of the whole section composed of the strata among which the given class occurs as the modal (i.e., predominant) feature. The method of modal quantitative logs is illustrated here with an example of statistical analysis of selected sections of the Krosno Beds (Oligocene) turbidite succession from the Western Outer Carpathians of Poland.

Geothermal groundwater study and its energetic potential assessment in Abu Gharadig Basin, North Western Desert, Egypt

The Abu Gharadig Basin (AGB) is one of Egypt's main oil-producing basins, characterized by a range of tectonic structures generated by intense extension and the deposition of a thick sedimentary sequence, making it an ideal location for geothermal energy research. In this study, seven groundwater samples were collected from wells drilled in the AGB to conduct hydrochemical and isotopic analyses. The results indicate that the region's groundwater is dominated by Na+ cations and Cl− anions, which originate from precipitation and runoff. The application of cation geothermometers reveals that all the groundwater samples are in partial equilibrium with the host rock and may also be from mixed sources. The temperature of the deep geothermal reservoir can be estimated using cation geothermometers. The calculated reservoir temperature range deduced from Na–K–Ca geothermometry is 127–184 °C, with an average of 157 °C. The quantities of geothermal power stored in the vicinity of the AGB low-enthalpy geothermal field over the next 25 and 50 years were estimated using stochastic Monte Carlo simulations. For a site with a surface area of 1 km2, the geothermal power potential is 14.3 MWe for a 25-year lifespan and 7.16 MWe for a 50-year lifespan.

Accretion and subduction mass transfer processes: Zircon SHRIMP and geochemical insights from the Carboniferous Western Series, Central Chile

ABSTRACT In the region of Pichilemu (Central Chile), the exhumed roots of the Carboniferous paleo-accretionary wedge developed during subduction of an oceanic realm underneath the western Gondwana margin are exposed. We focus on the areas of Infiernillo beach and Punta de Lobos: the former representing a well-characterized transitional blueschist-facies coherent stack of metabasitic lithologies interlayered at the centimetre- to metre-scale within metasedimentary rocks and glaucophanites, whereas the latter corresponds to a massive, up to hundreds of metres thick, strained metapillow lavas edifice surrounded (not interlayered) by metasedimentary rocks. We provide new field, geochemical, Raman thermometry and zircon SHRIMP U-Th-Pb geochronology on the lithologies forming the Infiernillo-Punta de Lobos complex aiming at characterizing the sedimentary sources and the extent of mechanical mixing. Field observations suggest that the Infiernillo-Punta de Lobos complex can be considered as part of the same successions, subducted and basally accreted almost coherently at high pressure-low temperature (HP-LT) conditions. The geochemistry signal in metasedimentary rocks and maximum deposition ages point to a marked forearc source for all the metasedimentary and glaucophanite lithologies, whereas massive metapillow lavas and greenschists might represent volcanic products and oceanic roughnesses erupted near the trench when the oceanic material approached the Gondwana margin at c. 329 Ma. It is suggested that the interlayering now observed in the field is mostly the consequence of sedimentary and/or volcanic processes close to the trench environment, with minor tectonic reworking of the pre-subduction structures (e.g. lithological contacts). Our results suggest that the Punta de Lobos edifice is closely related to the subduction of large volumes of continent-derived, trench-filling sediments towards HP-LT conditions. Thus, in line with previous experimental and numerical simulations, we emphasize the potential role of oceanic roughness as stress perturbations that could contribute to the subduction of thick sedimentary sequences, and discuss their implications for seismogenesis.

Geological and geotechnical characterisation of soft Holocene marine sediments: A case study from the north Irish Sea

The seabed of the Western Irish Sea Mud Belt (north Irish Sea) comprises a thick sequence of fine-grained Holocene marine sediments that overlay complex glacial deposits. The Holocene sequence preserves a record of environmental change in the area. Part of this area is earmarked for a number of offshore wind farm developments. The area poses a number of geotechnical challenges to such developments, including soft marine silts and clays as well as shallow gas. This paper characterises the soil conditions in the area in the context of offshore renewable energy development using various geophysical techniques, geotechnical data and shallow (3 m) sediment samples obtained across multiple research surveys between 2009 and 2020. The purpose is to assess the use of such techniques, including multichannel sparker seismic and underwater multichannel analysis of surface waves (UMASW), in characterising these kinds of sediments. It is also intended to build on previous research and provide a reference dataset and case study for further work in developing offshore wind in the area. UMASW and in-situ cone penetration testing with pore pressure measurement (CPTu) results showed good agreement in characterising these Holocene sediments, with multichannel sparker delineating shallow gas from surrounding sediments. Core profiles show distinct lithological variation in the upper 3 m, which can be related to CPTu parameters, but is not resolvable by UMASW profiles. Paleoenvironmental reconstruction for this upper 3 m suggests changes in depositional setting over the past ca. 7000 years, which could influence sedimentary structure. These parameters have implications for cable installation design.

Geological framework of the concealed East Tennant area constrained by SHRIMP U–Pb zircon geochronology: Implications for Paleoproterozoic stratigraphic correlations in northern Australia

The fundamental geological framework of the concealed Paleoproterozoic East Tennant area of northern Australia is very poorly understood, despite its relatively thin veneer of Phanerozoic cover and its position along-strike from significant Au–Cu–Bi mineralisation of the Tennant Creek mining district within the outcropping Warramunga Province. We present 18 new U–Pb dates, obtained via Sensitive High Resolution Ion Micro Probe (SHRIMP), constraining the geological evolution of predominantly Paleoproterozoic metasedimentary and igneous rocks intersected by 10 stratigraphic holes drilled in the East Tennant area. The oldest rocks identified in the East Tennant area are two metasedimentary units with maximum depositional ages of ca. 1970 Ma and ca. 1895 Ma respectively, plus ca. 1870 Ma metagranitic gneiss. These units, which are unknown in the nearby Murphy Province and outcropping Warramunga Province, underlie widespread metasedimentary rocks of the Alroy Formation, which yield maximum depositional ages of 1873–1864 Ma. While parts of this unit appear to be correlative with the ca. 1860 Ma Warramunga Formation of the Warramunga Province, our data suggest that the bulk of the Alroy Formation in the East Tennant area is slightly older, reflecting widespread sedimentation at ca. 1870 Ma. Throughout the East Tennant area, the Alroy Formation was intruded by voluminous 1854–1845 Ma granites, contemporaneous with similar felsic magmatism in the outcropping Warramunga Province (Tennant Creek Supersuite) and Murphy Province (Nicholson Granite Complex). In contrast with the outcropping Warramunga Province, supracrustal rocks equivalent to the 1845–1810 Ma Ooradidgee Group are rare in the East Tennant area. Detrital zircon data from younger sedimentary successions corroborate seismic evidence that at least some of the thick sedimentary sequences intersected along the southern margin of the recently defined Brunette Downs rift corridor are possible age equivalents of the ca. 1670–1600 Ma Isa Superbasin. Our new results strengthen ca. 1870–1860 Ma stratigraphic and ca. 1850 Ma tectono-magmatic affinities between the East Tennant area, the Murphy Province, and the mineralised Warramunga Province around Tennant Creek, with important implications for mineral prospectivity of the East Tennant area.