Paleozoic Formations Research Articles

Geothermal energy potential of sedimentary formations in the Athabasca region, northeast Alberta, Canada

The Athabasca region, located in the northeast of Alberta, Canada, hosts many ongoing projects of bitumen extraction from oil sands and Devonian carbonate and siliciclastic reservoirs, which require a vast amount of thermal energy. Geothermal energy as a green renewable source of heat can help to reduce the amount of fossil fuels used to provide the required thermal energy for these projects and consequently decrease the greenhouse gas emission. To assess the geothermal development potential in this region, an integrated regional-scale 3D model was constructed with geologic and geophysical data (approximately 7000 formation tops and approximately 800 km seismic 2D profiles). Incorporation of 2D seismic profiles that filled in the gaps between sparse geologic tops particularly for deeper formations adds to structural details of the modeled formations. The temperature and porosity fields were simulated using the sequential Gaussian simulation approach within the modeled sedimentary formations. Based on spatial distribution, thickness, formation porosity and permeability analysis, five Paleozoic formations of Keg River, Waterways, Cooking Lake, Leduc, and Grosmont were identified as potential aquifers for geothermal development. These aquifers have enough coverage and thickness in the area and possess a high amount of thermal energy content. Because the sedimentary basin in the Athabasca region is quite shallow (less than 1400 m), these aquifers are all recognized as low enthalpy geothermal reservoirs with maximum of 40°C temperature and hence direct heating applications are not feasible. Use of industrial-scale heat-pump technologies that have long been used in northern Europe with high coefficients of performance would be recommended for heat extraction from these reservoirs.

Structural paragenesises of the flysch complexes in the central part of the Uralian Foredeep

The structural features of the Upper Paleozoic flysch formations in the central part of the Cis-Uralian Foredeep were studied. Three types of structural paragenesises typical of thrust zones were distinguished. Among them are west vergent inclined folds and overfolds that pass into recumbent folds near a fault plane, structured tectonic melange, local thrusts, schistosity zones, and flat slipping planes. The various structural elements extend from northwest to northeast, due to the structural heterogeneity of the allochthon, approximately parallel to the strike of the Karantrav thrust.

Early Paleozoic oceanic inliers and reconstruction of accretionary tectonics in the Middle Gobi region, Mongolia: Evidence from SHRIMP zircon U-Pb dating and geochemistry

The ophiolites that occur as inliers among the Late Paleozoic formations in the Middle Gobi area are crucial for understanding the tectonic evolution of South Mongolia. In this paper, we conducted detailed studies on the Namdain hundy ophiolite to provide some constraints on the Early Paleozoic evolution of the Middle Gobi region in Mongolia. The ophiolite mainly consists of ultramafic rocks (carbonatation), plagiogranite, metagabbro, basalt and chert. The metagabbro and plagiogranite from Namdain hundy ophiolite yielded SHRIMP zircon U-Pb ages of 528±7Ma and 519±5Ma, respectively. Though most of the volcanic rocks of this ophiolite show supra-subduction zone (SSZ) affinity, samples with OIB and N-MORB geochemical features were also identified, indicating genesis in a forearc setting. The granodiorite intruding into the Namdain hundy ophiolite yielded a SHRIMP zircon U-Pb age of 491±3Ma, which constrained the upper age limit of ophiolite emplacement. This granodiorite shows adakitic geochemical affinity, attesting to the existence of Cambrian paleo - subduction in South Mongolia. Based on the available data so far, we suggest the Middle Gobi area comprises of the Manlay accretion complex, the island arc and the Biluutiin ovoo back-arc basin. The spatial configuration of these three tectonic belts suggests that the polarity of the paleo-ocean subduction was from south to north in the Early Paleozoic, forming a trench-arc-basin system south of the Central Mongolia microcontinent.

Structural parageneses of flysch complexes in the central part of the Uralian Foreland basin

In this work we reported the structural characteristics of the late Paleozoic flysch formation in the central part of the Uralian foreland basin. Three types of structural associations specific for thrust zones are established there. These include: 1) asymmetrical to recumbent west-vergent folds; 2) re-structured tectonic melange; 3) local thrusts, zone of schystosity, flat tectonic mirrors. Generally structural elements strike approximately parallel to the regional Karantrav thrust; orientation of striking ranges from the northwest to the northeast, accordingly to tectonic heterogeneity of the allochtone.

Assessing the trace element content in water samples from Badovci Lake (Kosovo) using inductively coupled plasma-mass spectrometry analysis

The water quality of a lake at any point reflects several major influences, including the lithology of the basin, atmospheric and climatic conditions, and anthropogenic inputs. Surface water may contain heavy metals and organic pollutants as undesirable substances that are dangerous to human health and also may cause chronic diseases and various cancers. Inductively coupled plasma-mass spectrometry (ICP-MS) was used to determine concentrations of 67 trace elements in water samples of Badovci Lake, which is an important source of drinking water in Kosovo. This area is known to be under the influence of a Pb–Zn ore field Hajvalia–Badovci–Kishnica and from improperly discharged sewage of surrounding villages, so the main objective of the current paper was to assess the impact of anthropogenic and natural activities on the water environment. Statistical methods were applied to determine possible anomalous values (extremes and outliers) for 10 ecotoxic elements (Ni, Cu, Zn, Mn, As, Pb, Fe, Cd, U, and Cr). The use of boxplot method showed that only one outlier of Cd2+ (5.91 μg dm−3) and one extreme value of Zn2+ (78.9 μgdm−3) are present. It is suggested that elevated amounts of Pb, Cd, and Zn are related to geological origin, as it is a rich Pb–Zn polymetallic mining area, belonging to the ore field Hajvalia–Badovci–Kishnica, which is important for the economic development of Kosovo. At this location source, rocks (serpentinite, schist, phyllite, dacite, andesite, and listwaenite) originate within a wide stratigraphic range, from the Paleozoic formations up to the Tertiary ones. Using Croatian standards, according to which data were evaluated, it was concluded that the water of Badovci Lake is rather polluted with respect to some metals, especially with Cd and Pb, for which it belongs to III, IV, or even V category.

Yerdi sand dunes (Erfoud area, southeastern of Morocco): color, composition, sand’s provenance, and transport pathways

This study aims to prove sand dunes’ color difference across Ziz riversides at Yerdi and then to investigate sand’s origin and transport pathways. We have opted for remote sensing and Geographic Information System tools as a methodology, in addition to laboratory analyses, that included mineralogy, grain size, and shape investigation of samples collected from studied dunes. The results reveal the following: (i) variation in grain size and shape of the studied sand. A high concentration of quartz and a low amount of carbonates and iron oxide were recorded in the studied dunes. Heavy mineral analyses have shown a similar mineralogical concentration in both samples, yet sand in the left side appears to be richer in hematite and goethite than the right one. (ii) Band ratios 6:4 and 5:7 of Landsat TM allowed the mineralogical distinction of mafic minerals, carbonates, and quartz, while Landsat 3:1 band ratio and Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) 2:1 band ratio were the most suitable for iron oxide distinction. The results confirm the dominance of quartz in both sides. However, iron oxide III (Fe3+) concentration is much important in the left side, which explains the difference in sand color. (iii) The studied dunes are potentially supplied from Quaternary, Cretaceous, and Paleozoic formations. The abundance of ferruginous conglomerate among potential formations supplying dunes and the abundance of hematite and goethite in sand from the left bank more than the right explain the red color marking these dunes. While, dunes’ morphodynamic explains the pal color observed in dunes located in the right bank.

Geochemical comparison between the gas in bulk fluid inclusions and reservoir gas produced in Paleozoic formation, Ordos Basin, China

Taking the reservoir sandstones of natural gas pools in the Upper Paleozoic and the calcite veins in Ordovician carbonate collected in the Ordos Basin, China, as samples, a unique method for geochemical analysis of gases captured in fluid inclusions (FIs) is presented in this study. Geochemical comparison between FI gases and reservoir gases in this basin is conducted. The results show that the gases from FIs developed in Upper Paleozoic reservoir sandstones and from the corresponding reservoirs share similar geochemistry in hydrocarbon gaseous compositions, although there is a high concentration of non-hydrocarbon gases (e.g. CO2) in FIs. The occurrence of FIs in sandstones and the value of the carbon isotopic ratio of carbon dioxide (δ13CCO2>−8‰) suggest that non-hydrocarbon gas in FIs originated from primary FIs formed during hosted mineral (quartz) crystallization from magma. Similar geochemistry in hydrocarbon gases from FIs and from reservoirs indicates that the gases pooled in Upper Paleozoic sandstones represent the original state charging from the source rocks and thus experienced a weak or no secondary alteration. Dominating hydrocarbon gaseous components, such as wells Lt1 and Yu9, in FI gases at a secondary formation in calcite veins of Ordovician carbonate imply that the FIs are related to oil and gas charging. The gases originated from the Lower Paleozoic source rock is characterized by δ13C1<−38‰, although there is a great difference in geochemistry of gases from FIs of different samples and from FIs and reservoirs. The heavier δ13C2 value of reservoir gas in the Lower Paleozoic reservoir compared with that of FI gases might be attributed to thermochemical sulfate reduction (TSR) after oil and gas charging into the reservoir.

MINERALOGY AND SHALE GAS POTENTIAL OF LOWER SILURIAN ORGANIC-RICH SHALE AT THE SOUTHEASTERN MARGIN OF SICHUAN BASIN, SOUTH CHINA

Exploration for unconventional gas resources from Paleozoic formations in the Sichuan Basin in South China is just getting started. Large, potential gas reservoirs are presumed to exist in the southern and south- eastern Sichuan Basin where Silurian marine organic-rich shale occurs. This paper provides geochemical and mineralogical data and other relevant information on potentially economic black shale formations of the Lower Silurian. The results show that in the shale, mineral components are dominated by clay minerals and quartz with minor amounts of plagioclase, potash feldspar, calcite and pyrite. The brittle mineral content ranges from 38 to 73% by weight (average content 56.2 wt%). In general, the Lower Silurian organic-rich shale of the Longmaxi Formation in the Sichuan Basin is highly mature, with the vitrinite reflectance between 1.6 and 3%. The shale has a high content of organic carbon, with an average of 1.8%, is tens of meters thick and of large areal extent. It can be regarded as a potential gas shale target.

Development techniques of multi-layer tight gas reservoirs in mining rights overlapping blocks: A case study of the Shenmu gas field, Ordos Basin, NW China

Abstract The difficulties of making joint development of coal and natural gas were examined and the technical countermeasures were given through studying the case of the Shenmu gas field of the Ordos Basin, where the mining rights of coal and natural gas overlap completely. Based on the study of the main controlling factors of the distribution of favorable areas in the Upper and Lower Paleozoic formations, technical measures for different areas were determined considering well type, well pattern, efficient drilling and production, and ground gathering technology. The results show: The mining rights overlapping area of coal and natural gas can be developed jointly. The effective reservoirs of the multi-layer tight gas reservoirs can be divided horizontally into the superposition area for multi-boundaries sand body, multi-layers sand body and isolation sand body in the Upper Paleozoic formations, and the superposition area for effective reservoirs in the Upper and Lower Paleozoic formations. A cluster well group which includes nine wells is the optimal well pattern. Different reservoir area should be developed by different well type and patterns. The construction and development period of gas reservoirs will be shortened by the application of cluster well group, optimized technologies of drilling and production, and ground supporting facilities. Practice shows that the three dimensional development of the multi-layer tight gas reservoirs is also realized in the Upper and Lower Paleozoic when the joint development of coal and natural gas is done in the mining rights overlapping areas.

Stratigraphy and fossil plants from the Cutler Formation (Late Paleozoic) and their paleoclimatic implications, eastern Paradox Basin, Colorado

Exposures of the late Paleozoic Cutler Formation, near the town of Gateway, Colorado, have traditionally been interpreted as the product of alluvial-fan deposition along the western flank of the Uncompahgre uplift and within the easternmost portion of the Paradox Basin. The Paradox Basin formed between the western margin of the Uncompahgre uplift, a segment of the Ancestral Rocky Mountains, and the western paleoshoreline of the North American portion of Pangea. This part of Pangea is commonly thought to have experienced semi-arid to arid conditions and warm temperatures during the Pennsylvanian and Permian. We present stratigraphic and fossil plant evidence in this paper to support prior interpretations that the Cutler near Gateway, Colorado, was deposited by alluvial fans that hosted localized wetland areas. Our findings are consistent with the results of prior studies that have suggested the climate in the area was warm, semi-arid, and ice-free at the time the plants described in this paper were living. &#x0D; Plant fossils collected from the Cutler Formation came from two sites in The Palisade Wilderness Study Area (managed by the U.S. Department of the Interior, Bureau of Land Management) of western Colorado. The stratigraphic sections at the sites were composed mostly of pebble to cobble conglomerate and sandstone, but the fossil plants were mainly preserved in fine-grained intervals (fine-grained sandstone to siltstone). The preservation of plant fossils in the proximal Cutler Formation is remarkable because the surrounding sections consist mostly of conglomerate and sandstone interpreted as fluvial and debris-flow deposits. The fine-grained strata containing the plant horizons must have been deposited in a wet and protected setting, possibly a spring-fed abandoned channel on the alluvial fan. The plants and their surrounding sediment must have been rapidly buried in order to allow for long-term preservation of the fossils. It seems likely that vegetation was abundant in and adjacent to low-lying wet areas on the fan’s surface, based on the abundance of plant fossils found at the two sites. The fossil plant assemblage includes Calamites, Walchia, and Pecopteris. The flora are interpreted to have lived near the apex of the alluvial-fan system. These fossils suggest that warm and at least seasonally and locally wet conditions existed in the area during the time that the plants were growing. More arid conditions during the late Paleozoic are suggested by the characteristics of some of the time-equivalent and near time-equivalent rocks exposed to the west of the study area in the central Paradox Basin.

Late Paleozoic final closure of the Paleo-Asian Ocean in the eastern part of the Xing-Meng Orogenic Belt: Constrains from Carboniferous–Permian (meta-) sedimentary strata and (meta-) igneous rocks

Zircon U–Pb dating and whole-rock geochemical data for Carboniferous–Permian (meta-) sedimentary sequences, igneous rocks, and Precambrian amphibolite in the eastern Xing-Meng Orogenic Belt (XMOB) were used to constrain the final stage evolution and the position of closure of the Paleo-Asian Ocean (PAO). Detrital zircons from Late Paleozoic strata range in age from Archean to Late Paleozoic, with major age groups at ~1.0–0.7Ga and ~0.5–0.25Ga in the northern part and at ~2.7–2.5Ga, ~2.1–1.8Ga, and ~0.5–0.25Ga in the southern part of the XMOB. Striking changes in zircon age distribution patterns indicate inputs to these strata were separated by the Paleo-Asian Ocean (PAO) in the Late Paleozoic. The PAO closed along the Solonker–Linxi suture. The Late Paleozoic formation ages (~346Ma, ~303Ma, and ~269Ma) of the igneous rocks with arc-like geochemical features south to the PAO, together with previously published data on the regional igneous rocks and ophiolites, indicate double-side subduction of the PAO in the Late Paleozoic.

Active sinkholes: A geomorphological impact of the Pajares Tunnels (Cantabrian Range, NW Spain)

Two parallel base tunnels (Pajares Tunnels) were built from 2005 to 2009 through the Cantabrian Range (NW Spain), crossing an alternation of Paleozoic formations (shale, sandstone, quartzite and limestone) characterized by a complex geological structure. A section of the tunnels was built 450m depth below Alcedo Valley (León, N Spain). Some evidence of collapse and swallow holes have been appearing from 2007 to present at the bottom of the valley. Although the stream was channeled in 2009 to control water infiltration, the process could not be avoided, constituting a good example of geomorphological impact caused by a base tunnel. The management of hydrogeological, geomorphological and climatological information using a GIS allowed mapping the affected area and estimating the mean water volume of infiltration into the sinkholes, and the runoff decrease in the Alcedo Stream after the drilling of the tunnel. Precipitation data series (1970–2000) and four spatial variables (outcrops, shallow deposits, slope and vegetation) were used to create a rainfall–runoff model. Presently, geomorphological evidence includes 4 main sinkholes (8–12m long), 13 minor hollows, 7 swallow holes and a 120m2 area with subsidence evidence, which developed over Quaternary deposits covering karstified limestone bedrock. These active swallow holes capture the surficial runoff of the Alcedo Stream throughout the year. Because of that, the upper reach of the stream is isolated from the rest of the fluvial network. The sudden development and active growth of cover-collapse sinkholes is consistent with 1) the drop of the water table by tunnel drainage after excavation, 2) the increase in percolation from surficial runoff and 3) the internal erosion of the overlying Quaternary sediments by suffosion processes. The estimated mean water volume of infiltration into the sinkholes is close to 308,903m3yr−1, and the Alcedo Stream runoff in the natural base level has decreased by 35% throughout the year after the tunnel perforation. At present, the process is active and it is expected to progress in the future.

Stratigraphy and fossil plants from the Cutler Formation (Late Paleozoic) and their paleoclimatic implications, eastern Paradox Basin, Colorado

Exposures of the late Paleozoic Cutler Formation, near the town of Gateway, Colorado, have traditionally been interpreted as the product of alluvial-fan deposition along the western flank of the Uncompahgre uplift and within the easternmost portion of the Paradox Basin. The Paradox Basin formed between the western margin of the Uncompahgre uplift, a segment of the Ancestral Rocky Mountains, and the western paleoshoreline of the North American portion of Pangea. This part of Pangea is commonly thought to have experienced semi-arid to arid conditions and warm temperatures during the Pennsylvanian and Permian. We present stratigraphic and fossil plant evidence in this paper to support prior interpretations that the Cutler near Gateway, Colorado, was deposited by alluvial fans that hosted localized wetland areas. Our findings are consistent with the results of prior studies that have suggested the climate in the area was warm, semi-arid, and ice-free at the time the plants described in this paper were living. Plant fossils collected from the Cutler Formation came from two sites in The Palisade Wilderness Study Area (managed by the U.S. Department of the Interior, Bureau of Land Management) of western Colorado. The stratigraphic sections at the sites were composed mostly of pebble to cobble conglomerate and sandstone, but the fossil plants were mainly preserved in fine-grained intervals (fine-grained sandstone to siltstone). The preservation of plant fossils in the proximal Cutler Formation is remarkable because the surrounding sections consist mostly of conglomerate and sandstone interpreted as fluvial and debris-flow deposits. The fine-grained strata containing the plant horizons must have been deposited in a wet and protected setting, possibly a spring-fed abandoned channel on the alluvial fan. The plants and their surrounding sediment must have been rapidly buried in order to allow for long-term preservation of the fossils. It seems likely that vegetation was abundant in and adjacent to low-lying wet areas on the fan’s surface, based on the abundance of plant fossils found at the two sites. The fossil plant assemblage includes Calamites, Walchia, and Pecopteris. The flora are interpreted to have lived near the apex of the alluvial-fan system. These fossils suggest that warm and at least seasonally and locally wet conditions existed in the area during the time that the plants were growing. More arid conditions during the late Paleozoic are suggested by the characteristics of some of the time-equivalent and near time-equivalent rocks exposed to the west of the study area in the central Paradox Basin.

Early Paleozoic shale properties and gas potential evaluation in Xiuwu Basin, western Lower Yangtze Platform

From 2012, China has produced significant amount of shale gas from Wufeng–Longmaxi Shale of Late Ordovician–Early Silurian and Qiongzhusi Shale of Early Cambrian in the Sichuan Basin and its southern and eastern surrounding area, Upper–Middle Yangtze Platform. The Wufeng–Longmaxi Shale and Qiongzhusi Shale, covering the most area of the Yangtze Platform, have become the primary target formations for shale gas exploration in the South China. The current studies mainly focused on Sichuan Basin and its surrounding areas in Upper–Middle Yangtze Platform with a few studies in the eastern Lower Yangtze Platform. Therefore, we implemented this research, on the one hand, to evaluate the shale gas potential of Early Paleozoic formations in Xiuwu Basin in the western Lower Yangtze Platform, and on the other hand, to contribute to the comprehensive studies of shale gas exploration in the entire Yangtze Platform. Mineral composition by X–ray diffraction (XRD), organic matter features by total organic carbon (TOC) analysis, Rock–Eval pyrolysis, and vitrine reflection (Ro), pore structure by mercury intrusion and low–temperature/low–pressure N2 physisorption, and sorption capability by methane isothermal adsorption were implemented to estimate the geological characteristics of organic shale in Xiuwu Basin. The results indicated that the heterogeneity of Early Cambrian shale is serious, but in general, its fracability is strong, mesopores and micropores are well-developed, and the capacity of gas generation and adsorption are strong. These analyses indicate the Early Cambrian shale in Xiuwu Basin to be a promising play of shale gas development.

Palynostratigraphical zonation of Palaeozoic sediments in southern Tunisia (the well HBR-1)

Based on palynological index species and other significant taxa, subsurface Paleozoic formation can be differentiated based on biostratigraphy. Five palynozones are recognized. One biozone is present in the Sidi Toui Formation, and indicates a middle ± upper Cambrian age. Four biozones are identified in the Ordovician, Silurian, and Permian. Permian and Triassic sediments unconformably overlie a subcrop of different Paleozoic units. Thus, a major unconformity has been identified in the south of Tunisia, which may be related to the Hercynian orogeny.

Full-azimuth, high-density, 3D point-source/point-receiver seismic survey for shale gas exploration in a loess plateau: a case study from the Ordos basin, China

The Ordos Basin, located in the central part of China, contains abundant oil and gas shale resources. This study took place in an area southwest of Yan’an City, Shaanxi province, and its main exploration targets were the Mesozoic Yanchang Formation, with a maximum depth of around 1500 m, and the Upper Paleozoic Benxi Formation, with a maximum depth of around 3550 m. The tectonic structure at the target levels is almost flat, with dips of only around one degree. The objective of the survey was to identify ‘sweet spots’ in a heterogeneous gas shale reservoir beneath thick deposits of loess, which is sediment formed by the accumulation of wind-blown silt. Variations in the topography, thickness, and seismic velocity of the loess were identified as key factors that had led to inadequate imaging results from previous 2D exploration surveys in the area (Yao and Li, 2004). The new 3D survey, acquired by CNPC Sichuan Geophysical Company (SCGC) during 2013 using the WesternGeco UniQ integrated point-receiver land seismic system, delivered imaging results that improved the characterization of the gas shale. Several features of the project area presented challenges for seismic exploration, many of which are typical of a loess plateau. The area is covered by unconsolidated loess with severe variations in thickness and velocity that can result in serious statics problems. The loess also causes dramatic signal absorption, leading to poor signal-to-noise ratio and potentially contributing to low resolution in the seismic data. As shown in Figure 1, surface conditions include rapid variations in topography that further complicate near-surface statics correction and can be the cause of various types of interference and noise. Survey logistics needed to account for heavily forested areas, hills and gulches. Also, more than 500 oil wells were pumping and some rigs were drilling during survey acquisition, representing additional sources of noise (Figure 2).

Modelling CO2-brine-rock interactions in the Upper Paleozoic formations of Ordos Basin used for CO2 sequestration

Subsurface immobilization and conversion of CO2 into solid mineral phases in deep siliciclastic saline formations containing silicate minerals, commonly known as “mineral trapping”, is gaining research attention as a significant option to reduce CO2 emissions in the atmosphere. Although mineral trapping of CO2 is a long-term process, a combination of short-term results from both laboratory experiments and numerical simulations can lead to some general understanding of the required long-term CO2 sequestration mechanisms. This is a 100 year preliminary batch simulation study of four sandstone samples, under CO2 saturated water at 75 °C from the Upper Permian formations in the Ordos Basin, using the TOUGHREACT/ECO2N module to simulate the CO2-brine-rock interaction processes in deep siliciclastic multilayered saline aquifers. The samples approximately correspond to the four target saline formations selected by the Shenhua Group for a CO2 sequestration field demonstration project in the Ordos Basin, PR China. Preliminary simulation results show that the initial salinity of formation brine plays a significant role in determining the amount of CO2 that will be sequestered by solubility or mineral trapping in a deep saline aquifer. Minimal differences between experimental results and numerical calculation occur in low salinity waters, and significantly larger differences in high salinity waters, which is still under the maximum acceptable difference between experimental and computed data (10 %). The upper Liujiagou formation, with the highest level of salinity (ca. 88.7 g/L TDS) and lowest level of CO2 solubility, offers the highest mineral trapping capacity, with a maximum carbonate mineral storage of ca. 0.7 kg/m3 of bulk rock over a 100 year period. Regardless of the initial acidity or alkalinity of the aquifer brine, injection of CO2 will inflict a sudden drop in pH of the brine to acidity levels in a range of 3.0–4.6. The subsequent amount of dissolved and precipitated minerals, arising from the CO2-brine-rock interaction, is site specific and mainly dependent on initial aquifer mineralogy and brine composition.

Detrital zircon U–Pb geochronology and tectonic implications of the Paleozoic sequences in western South Korea

The Middle Paleozoic metasedimentary sequences of the western Gyeonggi Massif (Taean Formation and Wolhyeonri Complex), Imjingang Belt, and southwestern Okcheon Belt, provide clues to the successive tectonic events that occurred during the evolution of the Korean Peninsula. In this study, we present SHRIMP U–Pb ages of detrital zircon grains from the metasandstones of the Taean Formation and Yeoncheon Group of the Imjingang Belt and from quartzites in three Paleozoic formations of the southwestern Okcheon Belt. The zircon grains from all of the samples yielded similar U–Pb age spectra of Paleoarchean/Neoarchean to Middle Paleozoic, with a dominant age population within the Paleozoic. The maximum ages of deposition estimated from the youngest dominant concordant detrital zircons were 431–457Ma for the western Gyeonggi Massif and Imjingang Belt, and 380–355Ma for the southwestern Okcheon Belt. Permian metamorphic zircon overgrowths correlated with subduction prior to the Triassic collision were also found. Middle Paleozoic metamorphic zircon overgrowths that can be compared to those of the Yunkai–Wuishan Orogenic Belt of the Cathaysia Block were indentified in the Wolhyeonri Complex. The dominant zircon populations of this study and the available data for the Korean Peninsula compare well with those from the Yangtze Block, Cathaysia Block, and Qilian–Qinling Orogenic Belt, which were located at the northeastern margin of eastern Gondwana during the Early Paleozoic.

Aqueous geochemistry of the Thermopolis hydrothermal system, southern Bighorn Basin, Wyoming, U.S.A.

The Thermopolis hydrothermal system is located in the southern portion of the Bighorn Basin, in and around the town of Thermopolis, in northwest Wyoming. It is the largest hydrothermal system in Wyoming outside of Yellowstone National Park. The system includes hot springs, travertine deposits, and thermal wells. Published models for the hydrothermal system propose the Owl Creek Mountains as the recharge zone, simple conductive heating at depth, and resurfacing of thermal waters up the Thermopolis Anticline. The geochemistry of the thermal waters of three active hot springs—Big Spring, White Sulphur Spring, and Teepee Fountain—is similar in composition and characteristic of carbonate or carbonate-bearing siliciclastic aquifers. Previous studies of the Thermopolis hydrothermal system postulate that the thermal waters are a mixture of waters from Paleozoic formations. Major element geochemical analyses available for waters from these formations are not of sufficient quality to determine whether the thermal waters are a mixture of the Paleozoic aquifers. In the time frame of this study (one year), the geochemistry of all three springs was constant through all four seasons, spanning spring snowmelt and recharge as well as late-summer and fall dryness. This relationship is consistent with a deep source not influenced by shallow, local hydrogeology. Anomalies are evident in the historic data set for the geochemistry of Big Spring. We speculate that anomalies occurring between 1906 and 1926 suggest mixing of source waters of Big Spring with waters from a siliciclastic formation, and that anomalies occurring between 1926 and 1933 suggest mixing with waters from a formation containing gypsum or anhydrite. Decreased concentrations measured in our study—relative to concentrations measured between 1933 and 1976—may reflect mixing of thermal waters with more dilute waters. Current data are not sufficient to rigorously test these suggestions, and events of sufficient scale taking place in these timeframes have not been identified.

Impacts of Pleistocene glacial loading on abnormal pore‐water pressure in the eastern Michigan Basin

AbstractThe hydromechanical effects of Pleistocene glacial loading on the Michigan Basin are assessed using numerical analysis based on coupled stress and pore‐water pressure. The two‐dimensional model domain included the Basin cross section and extended 10 km into the Precambrian. In the analysis, we considered the effects of the number of glacial loading cycles, the presence and connectedness of a deep Cambrian aquifer, the direction of glacial advance, the effect of a wet versus dry glacier/soil interface, topographic effects, density‐driven flow effects, and lithosphere flexure on the development of anomalous pressures. The modeling results were compared with data collected from highly instrumented wells completed in the eastern margin of the Basin. The present‐day results define regions of significant underpressure in the upper Ordovician and lower Silurian formations characterized by very low hydraulic conductivity and regions of overpressure where hydraulic conductivity is higher. To achieve the degree of underpressure observed in the instrumented wells using the model, a specific loading cycle applied over 100 000 years was repeated four times. As the number of loading cycles increased, the Paleozoic formations reached a state where the underpressures remain constant. Our results also illustrate the difference in poroelastic modeling between the application of mechanical loads on the land surface and the application of an equivalent hydraulic head, where the latter developed overpressures rather than the observed underpressures. The modeling also shows that the overpressures observed in the Cambrian formations are most likely to be the result of density‐driven flow. Finally, the simulations show that the effects of lithosphere flexure in the hydromechanical models results in the development of lateral stresses that generate overpressures rather than underpressures in the southern half of the domain. As there are no suitable observation points, these results remain unconfirmed, and further study is warranted.