Unconformable Contact Research Articles

Engineering geomorphology of coastal landslides at Limestone Downs, North Island, New Zealand

Coastal landslides are widespread geomorphological features on New Zealand's North Island, representing a key coastal erosion process. Often, New Zealand's coastal landsliding is rainfall-induced, with young, weak, permeable sediments, failing along a shear surface coincident with an unconformable contact with underlying older, sedimentary rock. An example is the Plio-Pleistocene Awhitu Group sands in the west of the North Island, which in recent years have failed via extremely rapid flows, with little warning, leading to fatalities at built-up coastlines. Understanding the nature of the hazard posed by coastal landslides is often precluded by the heavily vegetated nature of the coast, and site access. We present a case study of two coastal landslides at Limestone Downs, on the Waikato coast, ∼70 km south of Auckland, using field investigation, laboratory testing, remote sensing and modeling. The northern landslide appears to be a relict, low-angle bedding-plane failure in sedimentary rock, possibly between the Miocene Waikawau Sandstone and the underlying Te Kuiti Group. In contrast, the adjacent southern landslide appears active, failing by retrogressive upslope extension, and is limited to Plio-Pleistocene Awhitu Group sediments. While both landslides may have been initiated by marine erosion, triggering from near-field earthquakes and tsunami effects cannot be ruled out.

El Paso Formation: A key unit for the correlation of the Carboniferous glaciation in the Calingasta-Uspallata basin of western Argentina

The Late Paleozoic successions exposed along the western flank of the Precordillera at the latitude of Barreal in western Argentina are characterized by mid-Carboniferous glacial marine and glacially-influenced sediments (San Eduardo Group sensu Mésigos, 1953), followed by shallow marine, sand-rich deposits (Barreal Group).The El Paso Formation, exposed in the homonymous brachyanticline, is here subdivided into six facies associations (FAs), in ascending stratigraphic order: Boulder Conglomerate Complex FA, Sandstone Complex FA, Mudstones with Gastropods FA, Diamictite Complex FA, and Mudstones and Fine Sandstones FA. This analysis focuses on climatically sensitive facies related to the late Paleozoic Ice Age (LPIA), evidence of which is recorded in the adjacent Hoyada Verde Formation. The Boulder Conglomerate Complex FA includes boulders with striations on polished surfaces, indicative of glacially derived material. It grades up to sandstones with tangential cross-bedding and wave ripples, indicative of deposition as delta-scale sand-prone subaqueous clinoforms along a slope. The Diamictite Complex FA includes ice-rafted detritus (IRD) in two of its facies (dropstone mudstones and thin-bedded diamictites). The postglacial flooding, identified in various basins of western Argentina, is also present in the uppermost interval of the El Paso Formation (Mudstones and Fine Sandstones FA). It includes mudstones and rippled sandstones with abundant invertebrate fauna mainly brachiopods, bivalves, and gastropods, accompanied by conulariids, nautiloids, ostracods and corals, associated with palynomorphs. This study demonstrates through several lines of evidence that the upper interval of the El Paso and the Hoyada Verde Formations are time equivalent and represent key localities for the mid-Carboniferous (Serpukhovian-Early Bashkirian) event of the LPIA in the Calingasta Uspallata basin. Furthermore, unlike in the Hoyada Verde Formation, where the Carboniferous/pre-Carboniferous ("Pre-Tillítico") unconformity contact is concealed, the El Paso basal coarse fill rests on basement. The lower interval of the El Paso Formation exhibits similar facies characteristics and fining-upwards stacking pattern to the lower interval of the Majaditas Formation exposed further south.

Sensitivity, robustness, and reproducibility of U-shaped delamination test for evaluation of candidate ultra-high molecular weight polyethylene materials for joint replacements.

The delamination of ultra-high molecular weight polyethylene (UHMWPE) in artificial joints is a major cause limiting the long-term clinical results of arthroplasty. However, the conventional test method using simple reciprocation to evaluate the delamination resistance of UHMWPE materials has insufficient detection sensitivity. To reproduce delamination, the unconformity contact must be maintained throughout the test so that the maximum stress is generated below the surface. Therefore, a test method that applies a U-shaped motion comprising two long-linear and one short linear sliding motion was developed. The sensitivity, robustness, and reproducibility of the U-shaped delamination test were investigated and compared with the traditional test method. The traditional test method could reproduce delamination only in materials that had degraded considerably, whereas the U-shaped delamination test could reproduce delamination in a wide range of materials, demonstrating its superior sensitivity. Additionally, using a higher load helped accelerate the test without affecting the test results. The optimal length of the short linear sliding motion was confirmed to be 1 mm. Finally, the inter-laboratory reproducibility of the U-shaped delamination test was confirmed using the round-robin test. The U-shaped delamination test demonstrates high sensitivity, robustness, and reproducibility and contributes to the selection and development of UHMWPE materials and artificial joints with a lower risk of delamination.

A tale of two end members: Tidal deposits in a semi‐arid, low subsidence, open coastal setting versus a high runoff, high subsidence, restricted environment

AbstractJurassic and Palaeocene tidal deposits of the epeiric Western Interior Sea in Wyoming, USA, differ significantly due to their contrasting climates and tectonic, geographic and depositional settings. Tidally generated, cross‐bedded sandstone bodies contained by incisions are common to both settings and can potentially be uncritically attributed to marine flooding of fluvial systems. Key differences in lithology, ichnology and relationships with surrounding sediment reveal fundamentally different depositional settings, however. The Jurassic system occupied a low accommodation, semi‐arid environment, with geographically open shorelines as relative sea‐level fell, creating an unconformable contact with the underlying, storm wave‐dominated shelf and shoreface deposits. Siliciclastic, intertidal flats formed adjacent to coastal aeolian and fluvial environments during brief turnarounds from the degradational (forced regressive) to aggradational and retrogradational (transgressive) system tract. Basinward of these environments, metre to decimetre‐scale cross‐bedded, bioclastic, subtidal compound dunes and tidal inlet complexes accumulated in areas of minimal clastic flux and within incisions created by submarine tidal currents. By contrast, the Palaeocene tidal systems formed in a high accommodation, subtropical setting, as rising sea levels forced the fluvial to marine transition zone landward and flooded coastal swamps, forming geographically irregular, back‐barrier complexes and tidally influenced fluvial systems. High volumes of siliciclastics, terrigenous organic material and freshwater were delivered by the rivers and created physicochemical stress on the marine embayments. Sandy tidal flat deposits accumulated in lagoons and interdistributary bays, but unlike the Jurassic examples, they do not mark a turnaround from the falling stage to the transgressive system tract. The potential preservation window for tidalites is significantly greater vertically in the aggradational to retrogradational setting than in the degradational system due to greater accommodation. The preservation window is vertically smaller, but areally greater in the Jurassic, forced regressive system because of basinward enhancement of tidal currents driven by complex palaeobathymetry caused by tectonic activity of local pre‐Laramide uplifts.

Episodic sediment accumulation linked to global change in the endorheic Qaidam Basin of the Tibetan Plateau revealed by feldspar luminescence dating

Given its high accumulation rate in sediments, the endorheic basin in arid regions plays a crucial role in reconstructing paleo-environment. However, it would not provide high-resolution climatic records if significant wind-eroded depositional hiatuses were inevitable within a severe wind environment. Unconformable contacts have been found during field survey in the top part of the stratigraphy in the Qaidam Basin, a prototypical endorheic basin in central Asia. To elucidate whether the sediment is continuous, we employed luminescence dating methods of feldspar pIR200IR250 and quartz OSL to date sediments, including lacustrine sediments and salt crusts, from the Qarhan Playa that is the Quaternary depositional center of the Qaidam Basin. Various experiment tests, including preheat plateau, dose recovery, and residual dose, were conducted. The dating results show that the feldspar pIR200IR250 can provide dependable ages with negligible residual doses. The pIR200IR250 dating ages within the last glacial cycle reveal several environmental shifts. A Qarhan paleolake in Marine Isotope Stage (MIS) 5 was demonstrated by the cluster ages of ∼140–80 ka of lacustrine sediments. The salt crust ages of 136 ± 10 ka and 97 ± 6 ka indicate that, before completely disappeared, the paleolake with shallow dish-shape depression had been dried up more than once, illustrating “shallow——dry——shallow” cycles. The playa in the eastern Qaidam Basin evolved mainly in warm periods, such as MIS 5 and MIS 1, supported by the salt crust ages of MIS 5 (136 ± 10 ka and 97 ± 6 ka) and MIS 1 (10 ± 1 ka and 0.6 ± 0.1 ka). Depositional hiatus in the last glacial cycle were identified by unconformable contacts in three sections between the lacustrine strata and their overlying playa strata, with corresponding age gaps of 158 ± 20–97 ± 6 ka, 131 ± 10–10 ± 1 ka, and 84 ± 7–0.6 ± 0.1 ka, respectively. In all the three sections the sediments for the whole of the last glacial period is missing, eroded by wind. This process should have occurred periodically during the glacial-interglacial cycles of the orbital global climatic changes. The results also show that hiatuses are common for endorheic basins on the Earth with age gaps of up to ∼100 ka, suggesting that robust chronology should be established before any climatic correlation due to severe wind erosion leading to episodic sediment accumulation.

Lacustrine evidence reveals spatially and temporally distinct Holocene ruptures on the Sawtooth Fault, Central Idaho, USA

ABSTRACTThe Sawtooth Fault in central Idaho, USA, is a range‐bounding normal fault with a Late Pleistocene–Holocene scarp near the up‐valley ends of several range‐front lakes. Cores from Redfish Lake, which spans the fault, exhibit evidence of catastrophic sediment re‐mobilization in two sequences consisting variably of intraclastic mud‐clast conglomerate, massive homogenite, and graded silt and clay. Event strata of the younger sequence are in unconformable contact with Pleistocene sediments in two cores on the northern lake‐basin slope, suggesting transport from the down‐valley portion of the lake toward the basin center. A silt cap associated with the event strata is preserved at all coring sites. The event sequence, distributed lake‐wide, dates to ca. 4300 cal a bp, while additional event strata in two Redfish Lake cores pre‐date the 7627 cal a bp Mazama ash. We propose seismic triggering of the sediment disruptions. Asynchronous event stratigraphy in Pettit Lake, 15 km south of Redfish Lake, suggests distinct ruptures along two portions of the Sawtooth Fault. The sedimentology and geochemistry of remobilized sediments in both lakes indicates that sources included Late Pleistocene and early Holocene lake‐floor sediments and suggests failures that cut deeply into the sedimentary sequence. Cores collected from Stanley Lake (17 km northwest of Redfish Lake) following the nearby 2020 M6.5 Stanley earthquake exhibit a thin, sandy event horizon associated with documented delta collapse. Subaerial slope failures associated with the 2020 event left no widespread disturbance stratigraphy in Redfish Lake. We conclude that the large‐magnitude lake‐floor sediment remobilization events probably required rupture of fault sections proximal to Redfish and Pettit lakes.

Project-scale prospectivity analysis for unconformity-related uranium deposits in the Badami Group of the western Kaladgi basin, India

Developing successful exploration strategies for finding new ore deposits is a major challenge for exploration geologists, particularly, for concealed deposits in terranes with thick sediment cover. Proterozoic unconformity-related uranium deposits occur at, below, or above, unconformable contacts between Archean and Paleoproterozoic crystalline basement and siliciclastic sediments of middle or later Proterozoic age, often at locations where fault systems cut across either one or both of the geologic units that lie above and below the unconformity. The present study implements project-scale prospectivity analyses to identify drilling targets in the Badami Group of the western Kaladgi basin, India. Topographic profiles; field observations of breccia and lineaments; surface and sub-surface sampling; petrography and geochemistry; drill-core lithologs; and interpreted aeromagnetic anomalies were used to build a generalized local geological model of unconformity-related uranium deposits in the study area. Based on the geological model and a generalised mineral systems model, hydrothermal alteration, proximity to lineaments, proximity to unconformity surface, and anomalous uranium content on the surface and in the groundwater were identified as key project-scale targeting criteria. Seven high-priority drilling targets were mapped based on expert driven prospectivity analysis constrained by a GIS-based fuzzy prospectivity modelling. Out of these, four target areas coincide with the targets identified by the Atomic Minerals Directorate (AMD) of India based on their independent investigation. The three new targets from this study are recommended as targets for detailed exploration and drilling. In addition to this, about eight additional low priority targets were identified for follow-up exploration activities. This paper demonstrates that the prospectivity mapping methods can be applied at the project scale. The strategy demonstrated in this study can be used for exploration targeting in similar basins like the western Kaladgi basin that are structurally less disturbed.

Uranium Mineralization in the MacInnis Lake Area, Nonacho Basin, Northwest Territories: Potential Linkages to Metasomatic Iron Alkali-Calcic Systems

The intracratonic Paleoproterozoic Nonacho Basin, deposited on the western margin of the Rae craton, contains historic polymetallic (i.e., U, Cu, Fe, Pb, Zn, Ag) occurrences spatially associated with its unconformable contact with underlying crystalline basement rocks and regionally occurring faults. This study presents the paragenesis, mineral chemistry and geochemistry of uranium mineralized rocks and minerals of the MacInnis Lake sub-basin of the Nonacho Basin, to evaluate the style and relative timing of uranium mineralization. Mineralization is restricted to regionally occurring deformation zones, and post-dates widely spread and pervasive albitization and more local Ba-rich K-feldspar alteration of host rocks. Uranium mineralized rocks show elevated concentration of Cu, Ag and Au relative to variably altered host rocks. Microscopic and compositionally heterogeneous altered uraninite occurs (i) as overgrowths on partially dissolved Cu-sulphides with magnetite in chlorite ± quartz, calcite veins, and (ii) with minor uranophane in hematite-sericite-chlorite ± quartz breccia and stockwork. Both uraninite types are Th poor (<0.09 wt.% ThO2) and variably rich in SO4 (up to 2.26 wt.%), suggesting a low-temperature hydrothermal origin in a relatively oxidized environment. Rare-earth element (+Y) concentrations in type-i uraninite are high, up to 9.5 wt.% Σ(REE+Y)2O3 with CeN/YN values > 1, similar to REE compositions of uraninite in metasomatic iron and alkali-calcic systems (MIAC), including low-temperature hematite-type IOCG-deposits (e.g., Olympic Dam, Gawler Craton, Australia) and albitite-hosted uranium deposits (e.g., Southern Breccia, Great Bear Magmatic Zone, Canada, and Gunnar Deposit, Beaverlodge District, Canada). Both uraninite types are variably rich in Ba (up to 3 wt.% BaO), a geochemical marker for MIAC systems, provided by the dissolution of earlier secondary Ba-rich K-feldspar. Chemical U-Th-Pb dating yields resetting ages of <875 ± 35 Ma for type-ii uraninite-uranophane, younger than strike-slip movement along regional structures of the basin that are spatially associated with the uranium occurrences. We suggest that MacInnis Lake uranium occurrences formed from oxidized hydrothermal fluids along previously altered (albitized, potassically altered) regional-scale faults. Uranium minerals precipitated on earlier Fe-rich sulfides (chalcopyrite, bornite), which acted as a redox trap for mineralization, in low-temperature (~310–330 °C, based on Al-in-chlorite thermometry) breccias and stockwork zones, late in a metasomatic iron and alkali-calcic alteration system.

Bedrock Geochemistry and Alteration History of the Clay‐Bearing Glen Torridon Region of Gale Crater, Mars

AbstractGlen Torridon is a topographic trough located on the slope of Aeolis Mons, Gale crater, Mars. It corresponds to what was previously referred to as the “clay‐bearing unit,” due to the relatively strong spectral signatures of clay minerals (mainly ferric smectites) detected from orbit. Starting in January 2019, the Curiosity rover explored Glen Torridon for more than 700 sols (Martian days). The objectives of this campaign included acquiring a detailed understanding of the geologic context in which the clay minerals were formed, and determining the intensity of aqueous alteration experienced by the sediments. Here, we present the major‐element geochemistry of the bedrock as analyzed by the ChemCam instrument. Our results reveal that the two main types of bedrock exposures identified in the lower part of Glen Torridon are associated with distinct chemical compositions (K‐rich and Mg‐rich), for which we are able to propose mineralogical interpretations. Moreover, the topmost stratigraphic member exposed in the region displays a stronger diagenetic overprint, especially at two locations close to the unconformable contact with the overlying Stimson formation, where the bedrock composition significantly deviates from the rest of Glen Torridon. Overall, the values of the Chemical Index of Alteration determined with ChemCam are elevated by Martian standards, suggesting the formation of clay minerals through open‐system weathering. However, there is no indication that the alteration was stronger than in some terrains previously visited by Curiosity, which in turn implies that the enhanced orbital signatures are mostly controlled by non‐compositional factors.

Age and causes of the Yellow River dissecting the Zoige Basin in the eastern Tibetan Plateau, China

Through extensive field investigation in the Zoige Basin of the eastern Tibetan Plateau, a typical sedimentary profile was found on the front scarp of the second terrace of Maqu reach of the Yellow River at the outlet of the Zoige Basin. This profile contains the stratigraphic sequence of paleolake facies and paleo-riverbed floodplain facies. Field observation and the analysis of particle size characteristics and optically stimulated luminescence dating showed the following results: (1) The profile includes a pale blue-grey paleolake deposit, which formed in a deep-water anoxic environment. This deposit was overlain by a mottled pebble layer with pale yellow-orange lenticular sand, which represents typical paleo-riverbed floodplain deposits, indicating a strong hydrodynamic process. The typical unconformity contact relationship between the two deposits shows that the profile recorded the sedimentary information of the transformation of the Zoige paleolake environment into the Yellow River paleo-river environment, which provides powerful sedimentary evidence for the Yellow River dissecting the Zoige paleolake. (2) The optically stimulated luminescence dating results showed the Zoige paleolake was dissected by the Yellow River at 37 ka BP, resulting in the discharge of lake water. At 35 ka BP, the lake water gradually became shallow and disappeared, and the Yellow River subsequently connected the water system of the Zoige Basin. (3) The East Kunlun fault induced strong tectonic movement in the late Pleistocene, and the Tibetan Plateau had a warm and humid climate at 37 ka BP. Under these conditions, the headward erosion of the Yellow River was aggravated in the faulted wide valley grassland between the Anyemaqen mountains and Xiqing mountains, and the Zoige paleolake was dissected by the Yellow River from west to east in the Maqu bottleneck reach. The results of this study have important scientific significance for in-depth understanding of the evolution of river-lake water systems and the formation of the Yellow River source water system in the eastern Tibetan Plateau.

Biostratigraphy of the Late Cretaceous-Early Paleocene Succession in Selected Wells, Jambur Oil field, Kirkuk, Northern Iraq

The Late Cretaceous-Early Paleocene Shiranish and Aliji formations have been studied in three selected wells in Jambur Oil Field (Ja-50, Ja-53, and Ja-67) in Kirkuk, Northeastern Iraq. This study included lithostratigraphy and biostratigraphy. The Late Campanian-Maastrichtian Shiranish Formation consist mainly of thin marly and chalky limestone beds overlain by thin marl beds, with some beds of marly limestone representing an outer shelf basinal environment, the unconformable contact with the above Middle Paleocene-Early Eocene Aliji Formation contain layers of limestone with marly limestone and chalky limestone which represents an outer shelf basinal environment. Five Biozones in the Shiranish Formation were determined which are: 1.Globotruncanita calcarata Range Zone (part). 2.Globotruncanella havanensis Partial Range Zone. 3.Globotruncana aegyptiaca Interval Zone. 4.Gansserina gansseri Interval Zone (Part). 5.Abathomphalus mayaroensis Range Zone. While in Aliji Formation there were two biozones determined which are: 1.Guembelitria Cretacea Partial-Range Zone. 2.Parvularugoglobigerina eugubina Total-Range Zone.

Lithostratigraphy and structural framework of the Ediacaran volcano-plutonic complex of the Imiter Ag–Hg mining district (Jbel Saghro, Anti-Atlas, Morocco)

New lithostratigraphical and structural data provide a better assessment of the still controversial metallogenetic context of the world-class Ag–Hg Imiter deposit, located in the north-eastern part of the Saghro massif (Anti-Atlas, Morocco). Three Ediacaran volcano-plutonic cycles separated by two major angular unconformities are identified: (1) early plutonic bodies (Imider and Igoudrane) intruding the Middle Neoproterozoic (NP2) of the Saghro Group and overlain by, (2) the monoclinal Imiter Sequence (lower part of Late Neoproterozoic, NP3) emplaced in basinal conditions and consisting from bottom to top of (i) a lower volcano-sedimentary unit (LVSU) with conglomerates, tufaceous siltstones and sandstones, and interbedded andesites, (ii) the >500 m thick compound Imiter Ignimbrite, (iii) an upper volcano-sedimentary unit (UVSU), and (iv) upper andesites, unconformably overlain by (3) an upper silicic sequence (upper NP3) emplaced in continental conditions and made of rhyolitic lava flows and ignimbrites currently dated at ca. 550 Ma. The structural analysis shows that the Imiter sequence was deposited under a N–S extensional regime, then slightly tilted under a NE-SW extension. At the end of the lower NP3 period and prior to an emersion and erosional time lapse, the Imiter sequence was slightly deformed by a NW-SE transpressional regime under which the Taouzzakt and Bou Teglimt plutons (ca. 570 Ma) were emplaced. The upper NP3 rhyolitic volcanism was early controlled by the opening of two major N–S and NW-SE crustal fault systems and further evolved with the emplacement of N–S dykes and NW-SE extensional normal faulting in response to a NE-SW transtensive regime. Two post-Precambrian kinematic regimes have been identified from faults crosscutting the unconformable contact between the NP3 and the overlying Middle Cambrian formations. The first regime is expressed by ENE to E sinistral normal faults, the second one by an echelon NE dextral strike-slip faults. The so-called Imiter fault is a polyphase E-W trending fault complex that is a first-order control on the Ag ore location. Our data show that the main, vertical motion of this fault that lowers the northern side by 300–400 m offset might have been progressive during deposition of the Imiter Sequence, and is sealed by the erosional surface between the lower NP3 and upper NP3 periods. Our data further assign a lower NP3 age for the transpressional regime, responsible for the mineralized Ag veins according to previous microstructural studies at the mine scale. These lithostratigraphic and structural results are likely to be relevant to the Saghro massif as a whole, and thus may help assessing the metallogenic context of the numerous ore deposits found in this area, and guide further ore explorations.

Structural Characteristics and their Significance for Hydrocarbon Accumulation in the Northern Slope of the Central Sichuan Paleo‐uplift

AbstractBased on 2D and 3D seismic data, the latest drilling data and field outcrop data of the northern slope of the Central Sichuan paleo‐uplift, the structural analysis method is used to analyze unconformity development characteristics and fault characteristics during the key structural transformation period, discussing the influence of the structural characteristics on the hydrocarbon accumulation of deep carbonate rocks. The results show that: (1) The two key unconformities of the Tongwan and Caledonian periods were primarily developed in deep carbonate rocks. Firstly, Tongwan's unconformities are characterized by regional disconformities between the second and third members of the Dengying Formation, the top formation of the Sinian and the lower Cambrian, strips of which zigzag through the north and south sides of the study area. Secondly, the Caledonian unconformity is characterized by a regional unconformable contact between the lower Permian and the ower Paleozoic strata. From NE to SW, the age of the strata, which were subject to erosion, changes from new to old, the denudation distribution showing as a nose‐shaped structure which inclines towards the ENE. (2) Boundary fault and transtensional strike‐slip faults developed in the Sinian to Paleozoic strata. In profile, there are three types of structural styles: steep and erect, flower structures, ‘Y’ and reversed ‘Y’ type faults. In plane view, the Sinian developed extensional boundary faults extending in an almost NS direction, strike‐slip faults developing and extending linearly in approximately EW, WNW and NE strikes in the Cambrian, with characteristically more in the south and less in the north. (3) The faults in the northern slope show obvious zonal deformations in transverse view as well as significant stages and stratified activity in a longitudinal direction. Among them, the activity of faults in the Sinian was the strongest, followed by the activity in the Cambrian period, the activity intensity of faults in the Permian period being the weakest. This fault activity can be divided into four periods: Sinian, Cambrian‐Permian, the early Indosinian period and the late Indosinian‐Himalayan period, the transtensional strike‐slip faults being the products of oblique extensions of pre‐existing weak zones in the Xingkai and Emei taphrogenesis, with a particular inheritance in the main faults. (4) Combined with hydrocarbon accumulation factors, it is considered that the epigenetic karstification of the Tongwan and Caledonian unconformities in the northern slope controlled the formation and distribution of carbonate karst reservoirs over a large area, also acting as a good pathway for oil and gas migration. The extensional faults developed at the margin of the NS trending rift, controlling the sag‐platform sedimentary pattern in the Dengying Formation of the Sinian. Strike‐slip faults in NE, WNW and ENE directions may control the micro‐geomorphological pattern inside the platform and intensify the differential distribution of grain beach facies. The multi‐stage hereditary activity of strike‐slip faults not only improved the porosity and permeability of the reservoirs, but also acted as the main channel of oil and gas migration, providing favorable conditions for the development of the current multi‐layer gas‐bearing scenario in the northern slope of the Central Sichuan Basin.

Dynamic Bayesian Network for Predicting Tunnel-Collapse Risk in the Case of Incomplete Data

Collapse is one of the most dangerous aspects of drilling–blasting construction in highway tunnels. To accurately control tunnel-collapse risk, a multistate dynamic Bayesian network (DBN) evaluation method for highway tunnel collapse based on parameter learning was proposed. First, by analyzing the risk mechanism of tunnel construction, the initial BN model was established based on the causal relationship between risk factors and construction risk in hydrogeological conditions, construction technology, and construction management. Next, the construction process was discretized into finite time slices. In consideration of the fuzzy uncertainty of nodes, node polymorphism was introduced to construct a multistate DBN. Then, 50 typical tunnel-collapse cases were taken as sample data, and the conditional probability distribution of initial BN was derived using parameter learning based on the expectation-maximization (EM) algorithm. Using DBN reasoning and sensitivity analysis, the dynamic risk probability and the dominant factors of tunnel collapse were predicted. Finally, the DBN model was fed back with the measured cumulative values and velocity of the crown settlement, which updated the dynamic risk probability assessment results. In analyzing the collapse probability of Jinzhupa tunnel passing through the angular unconformity contact zone as an example, the results demonstrated that dynamic risk assessment results combined with monitoring data could better reflect the reality of construction contingencies, providing real-time risk management guidance.

Effects of Regional Differences in Shale Floor Interval on the Petrophysical Properties and Shale Gas Prospects of the Overmature Niutitang Shale, Middle-Upper Yangtz Block

The lower Cambrian Niutitang/Qiongzhusi shale gas in the Middle-Upper Yangtz Block had been regarded as a very promising unconventional natural gas resource due to its high total organic carbon, great thickness, and large areal distribution. However, no commercial shale gas fields have yet been reported. From the northwest to the southeast there are considerable differences in the sedimentary environments, lithology, and erosive nature of the underlying interval (the floor interval) of the Niutitang shale. However, systematic research on whether and how these regional differences influence shale petrophysical properties and shale gas preservation in the Niutitang shale is lacking. A comparison of Niutitang shale reservoirs as influenced by different sedimentary and tectonic backgrounds is necessary. Samples were selected from both the overmature Niutitang shales and the floor interval. These samples cover the late Ediacaran and early Cambrian, with sedimentary environments varying from carbonate platform and carbonate platform marginal zone facies to continental shelf/slope. Previously published data on the lower Cambrian samples from Kaiyang (carbonate platform), Youyang (carbonate platform marginal zone) and Cen’gong (continental shelf/slope) sections were integrated and compared. The results indicate that the petrophysical properties of the floor interval can affect not only the preservation conditions (sealing capacity) of the shale gas, but also the petrophysical properties (pore volume, porosity, specific surface area and permeability) and methane content of the Niutitang shale. From the carbonate platform face to the continental shelf/slope the sealing capacity of the floor interval gradually improves because the latter gradually passes from high permeability dolostone (the Dengying Formation) to low permeability dense chert (the Liuchapo Formation). In addition, in contrast with several unconformities that occur in the carbonate platform face in the northern Guizhou depression, no unconformity contact occurs between the Niutitang shale and the floor interval on the continental shelf/slope developed in eastern Chongqing Province and northwestern Hunan Province. Such regional differences in floor interval could lead to significant differences in hydrocarbon expulsion behaviour and the development of organic pores within the Niutitang shale. Therefore, shale gas prospects in the Niutitang shales deposited on the continental shelf/slope should be significantly better than those of shales deposited on the carbonate platform face.

Study on Hydrogeochemical Characteristics of Deeply Buried Mining Area in Inner Mongolia-Shaanxi Province, China

Abstract This paper adopted organic components, environmental isotopes, and organic components to study water quality comprehensively, in order to distinguish the differences of hydrogeochemical characteristics between coal measure strata and aquifers on the roof of deeply buried mining areas in Inner Mongolia-Shaanxi, China. The results show that the deeply buried mining area in Inner Mongolia-Shaanxi belongs to Mu Us Desert, and the surface is covered by aeolian sand, with excellent precipitation infiltration capacity. Salinity of surface water and Quaternary water<500 mg/L, the main cation is Ca2+, and the main anion is HCO3-, which belongs to HCO3-Ca·Mg type water. The Cretaceous Zhidan Formation is in unconformable contact with the Quaternary and constitutes a unified water-bearing complex on the whole, which makes Zhidan Formation have a better supply water source, and its inorganic water quality characteristics are close to the Quaternary water. The deep aquifer is affected by the Anding Formation relative impermeable layer and has weak recharge runoff condition. The salinity of Jurassic water is generally >3500 mg/L due to long-term water-rock action. The main cation is Na+, and the main anion is SO42-, which belongs to SO4-Na type water. According to the analysis of hydrochemical characteristics, there is no direct hydraulic connection between Luohe Formation and Zhiluo Formation. The characteristics of environmental isotopes show that the rainwater, surface water, and Quaternary water in the study area belong to the modern groundwater, while Zhidan Formation water is between the modern groundwater and the ancient water. The values of δD and δ18O in the deep Zhiluo Formation and Yan’an Formation are low, and the groundwater falls below the rainwater line of Ordos Basin with a deep circulation depth. Before mining, the groundwater is in a stagnant state with good closed conditions. Qualitative and quantitative signatures of various organic components in water are on the basis of organic geochemistry and hydrogeology, in order to facilitate the study of the quantity, composition, and distribution patterns of organic substances in groundwater and their role in geological, geochemical, and other processes. By testing the organic matter of dissolved organic matter (DOM) in each aquifer, the organic water chemistry characteristics of each aquifer in the Taokutu well field can be established. The content of dissolved organic matter (TOC and UV254) in groundwater decreases gradually with the increase of aquifer depth. Fluorescence peaks in Area I and Area III mainly appeared in surface water and Quaternary water, and DOM sources were more abundant in surface water. The fluorescence peak in Area I also appears in the water of Zhidan Formation, Zhiluo Formation, and Yan’an Formation, and the fluorescence peak between Area I and Area II is a symbol. The fluorescence peak intensity of Cretaceous → Zhiluo Formation in Area V area has an increasing trend, indicating that there are humus-like DOM from other sources in the deep Zhiluo Formation aquifer. In general, the comprehensive analysis of hydrochemical characteristics by various means can well distinguish the differences of hydrogeochemical characteristics among aquifers, which provides a scientific basis for the rapid and accurate discrimination of water situation and disaster in coal mines and the safe production.

MICROFACIES STUDIES OF THE JURASSIC TAKATU FORMATION, WESTERN SULAIMAN FOLD-THRUST BELT, PAKISTAN

Takatu Formation is well exposed in the western Sulaiman Fold- Thrust Belt, Pakistan. It has an unconformable upper contact with Sembar Formation which is marked by oxidized surface and lower contact with Triassic Wulgai Formation. The Takatu Formation contains a wide variety of limestones, which are fine to coarse grained, palatal, lumpy, skeletal, micritic, nodular, oolitic, and intraclastic. The Takatu Formation is mainly carbonate-litho package with minor siliciclastic content interbedded as shale and marls. The petrography of limestone allowed the differentiation and demonstrated of four major and sub microfacies types. These includes, Mudstone, Wackestone, Packstone and Grainstone microfacies, which are further sub-divided into five microfacies such as, Bioclastic wackstone, Calcispheric packstone, Peloidal packstone, Ooidal grainstone, and Lithoclastic grainstone. These microfacies were compared with standard microfacies and standard zones for their possible depositional environments. On the bases of our studies, it is interpreted that Takatu Formation was deposited in diverse environment ranging from the marginal shallow shelf, upper slope and in deeper parts of the shelf.

Development characteristics and petroleum geological significance of Permian pyroclastic flow volcanic rocks in Western Sichuan Basin, SW China

By examining field outcrops, drilling cores and seismic data, it is concluded that the Middle and Late Permian “Emeishan basalts” in Western Sichuan Basin were developed in two large eruption cycles, and the two sets of igneous rocks are in unconformable contact. The lower cycle is dominated by overflow volcanic rocks; while the upper cycle made up of pyroclastic flow volcanic breccia and pyroclastic lava is typical explosive facies accumulation. With high-quality micro-dissolution pores and ultra-fine dissolution pores, the upper cycle is a set of high-quality porous reservoir. Based on strong heterogeneity and great differences of pyroclastic flow subfacies from surrounding rocks in lithology and physical properties, the volcanic facies and volcanic edifices in Western Sichuan were effectively predicted and characterized by using seismic attribute analysis method and instantaneous amplitude and instantaneous frequency coherence analysis. The pyroclastic flow volcanic rocks are widely distributed in the Jianyang area. Centering around wells YT1, TF2 and TF8, the volcanic rocks in Jianyang area had 3 edifice groups and an area of about 500 km2 which is the most favorable area for oil and gas exploration in volcanic rocks.

Geology, zircon U–Pb dating and εHf data for the Julie greenstone belt and associated rocks in NW Ghana: Implications for Birimian-to-Tarkwaian correlation and crustal evolution

The Julie greenstone belt of NW Ghana is characterized by a predominately volcanic succession of basalt with minor rhyolite and a mainly sedimentary succession of siltstone, shale and greywacke. The first unit is correlated with the Sefwi Group, and the second with the Kumasi Group of the Birimian Supergroup. The volcano-sedimentary succession is separated by a sheared unconformable contact from clastic sedimentary rocks, including sandstones and conglomerates, correlated with the Tarkwaian Group of Ghana. The supracrustal rocks are intruded by various granitoids. Two major deformational events have been identified. A regional N–S shortening event (D1) affected rocks of the Birimian Supergroup and associated granitoids. A later NW-SE shortening event (D2) affected both the Birimian and Tarkwaian units. Rocks have undergone regional greenschist to amphibolite facies metamorphism. Peak metamorphism took place during D2.Several samples of igneous and sedimentary rocks were subjected to U–Pb zircon dating and Lu–Hf isotope analyses. All zircons provided Paleoproterozoic ages in the range of 2112 ± 5 to 2337 ± 11 Ma, which is consistent with existing zircon ages from volcanic and plutonic rocks in various parts of Ghana, and other Birimian and Tarkwaian terrains within the West African Craton. Rhyolite of the Sefwi Group has an age of 2182 ± 3 Ma and marks a period of explosive felsic volcanism. Detrital zircon ages from greywacke of the Kumasi Group yield a maximum depositional age of 2158 ± 4 Ma. This age compares well to a maximum depositional age of 2154 ± 2 Ma defined in southern Ghana. Zircon ages from Tarkwaian sandstones indicate a maximum depositional age of 2135 ± 5 Ma, which is in accord with similar depositional ages for the Tarkwaian in SW Ghana and Houndé in Burkina-Faso. Thus the Tarkwaian rocks from the Julie greenstone belt area are correlative to those in SW Ghana and elsewhere. Detrital zircon ages point to a derivation from erosion of felsic volcanic and plutonic rocks within the Paleoproterozoic terrain. Detrital zircon ages of Birimian and Tarkwaian rocks show a ∼70% overlap. This suggest that the Tarkwaian detritus may have been derived from reworking of older Birimian rocks. Alternatively, both groups sampled a similar source but were deposited at different periods. Dated granitoids include a 2209 ± 5 Ma phase and a post-Tarkwaian intrusion of 2112 ± 5 Ma in age. The older granitoid is in sheared contact with basalt, and both share similar D1 structures.The ϵHf values for zircon grains from analyzed rocks are in the range of −1.78 to +6.37, in agreement with most Hf isotopic signatures currently available from the Paleoproterozoic Baoulé-Mossi Domain, and indicate crust components with apparent mantle derivation ages averaging c. 2300 Ma and ranging from c. 2100 to c. 2500 Ma.

The distribution of theropod-dominated ichnofaunas in the Moab Megatracksite area, Utah: implications for Late Jurassic palaeobiology along an arid coast

ABSTRACT The Late Jurassic, ‘single surface’ Moab Megatracksite between Green River, Utah and the Colorado-Utah State line is associated with the sharp, colour-enhanced, inter-formational contact between white, reworked dune sandstones from the aeolian Moab Member of the Curtis Formation and red, sabkha facies siltstones and sandstones of the upper tongue of the Summerville Formation. Previous studies described theropod (Megalosauripus and Therangospodus) dominated assemblages from ~25 sites in the megatracksite core area around Arches National Park, and briefly interpreted the transgressive mechanisms of the Sundance sea which helped create and preserve the megatracksite. Since the 1990s ~20 newly studied tracksites reveal a cumulative total of ~4,300 tracks and three sites are now designated as, federally managed, interpretative tourist destinations. Similar track assemblages, collectively constituting Megalosauripus-Therangospodus ichnocoenoses have now also been found at lower stratigraphical levels associated with J-3 Unconformity contact zones. These new discoveries relate to stratigraphic studies associated with short-term changes in Sundance Sea level affecting coastal dune facies (Slick Rock and Moab members) across low gradient, inter- and sub-tidal sand flats, and sabkha-like deposits under arid conditions. The multiple (>50) theropod dominated ichnofaunas are integral to palaeobiological understanding of an area from which no body fossils are known.