Central Uplift Research Articles

Distribution, origin, and evolution of overpressure in the Shahejie Formation of northern Dongpu Depression, Bohai Bay Basin, China

The distribution, origin, and evolution of overpressure in the petroliferous basin are important problems that need to be addressed for oil and gas exploration. The distribution and origin of overpressure in the Shahejie Formation in the northern Dongpu Depression are analyzed based on geological studies, logs, and pressure data. The contribution ratios of different overpressure origins are quantified, and the evolutionary stages of overpressures of different origins are further divided. The results show that the formation pressure coefficients of Shahejie Formation in the Dongpu Depression are mainly within the range of 0.9–1.5. The overpressures are mainly distributed in the Sha-3 and Sha-4 Members of the Haitongji sag, the Central uplift belt and the Qianliyuan sag. From the sag to its surrounding area, the formation pressure coefficient gradually decreases. The high deposition rate and strong hydrocarbon generation are the main causes of overpressure formation in the Shahejie Formation in the Dongpu Depression. Based on the stress–strain characteristics of different origin overpressures and the log response parameters, two models, acoustic travel time-effective vertical stress and electrical resistivity-effective vertical stress, are established to identify and quantify the different origin overpressures. The calculation results for the area from the Haitongji sag and Qianliyuan sag to the Central uplift belt show that the main cause of overpressure gradually changes from both disequilibrium compaction and hydrocarbon generation to disequilibrium compaction as the main factor, with the contribution of disequilibrium compaction to overpressure in the Central uplift belt at about 87%. The Sha-3 Members of the Shahejie Formation in the Haitongji sag and the Qianliyuan sag are more strongly affected by hydrocarbon generation, which accounts for 42% and 47.5% of overpressure origin, respectively. There are five stages in the evolution of overpressure in the Shahejie Formation in the northern Dongpu Depression: normal compaction (before 35 Ma), mixed pressurization (35-27 Ma), uplift and pressure release (27-17 Ma), disequilibrium compaction (17-11 Ma), and secondary mixed pressurization (12 Ma-present).

In situ U–Pb analysis of shocked zircon from the Charlevoix impact structure, Québec, Canada

AbstractLaser ablation inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) U–Pb geochronology of shocked zircon grains in a vesicular‐fluidal impact melt rock from the ≥54 km Charlevoix impact structure, Québec, Canada, suggests an Ordovician to Silurian age of 450 ± 20 Ma for the impact. This age is anchored by concordant U–Pb results of ~450 Ma for a U‐rich, cryptocrystalline zircon grain in the melt rock, interpreted as a recrystallized metamict zircon crystal; the U–Th–Pb system of the metamict grain was seemingly chronometrically reset by the Charlevoix impact, but withstood later tectonometamorphic events. The new zircon age for Charlevoix is in agreement with a stratigraphically constrained Late Ordovician maximum age of ~453 Ma and corroborates earlier suggestions that the impact occurred most likely in the Ordovician, and not ~100 Myr later, as indicated by previous K/Ar and 40Ar/39Ar geochronologic results. The latter may reflect postimpact thermal overprint of impactites during the Salinian (Late Silurian to Early Devonian) and/or Acadian (Late Devonian) orogenies. U–Pb geochronology of zircon crystals in anorthosite exposed in the central uplift of the impact structure yielded a Grenvillian crystallization age of 1062 ± 11 Ma. The preferred Ordovician age for the Charlevoix impact structure, which is partially overthrusted by the Appalachian front, suggests the impact occurred during a phase of Taconian tectonism and an episode of enhanced asteroid bombardment of the Earth. Our results, moreover, demonstrate that (recrystallized) metamict zircon grains may be of particular interest in impact geochronology.

Microstructural dynamics of central uplifts: Reidite offset by zircon twins at the Woodleigh impact structure, Australia: COMMENT

Microstructural dynamics of central uplifts: Reidite offset by zircon twins at the Woodleigh impact structure, Australia: COMMENT

Microstructural dynamics of central uplifts: Reidite offset by zircon twins at the Woodleigh impact structure, Australia: REPLY

Microstructural dynamics of central uplifts: Reidite offset by zircon twins at the Woodleigh impact structure, Australia: REPLY

Subsidence and exhumation of the Mesozoic Qiangtang Basin: Implications for the growth of the Tibetan plateau

AbstractThe subsidence and exhumation histories of the Qiangtang Basin and their contributions to the early evolution of the Tibetan plateau are vigorously debated. This paper reconstructs the subsidence history of the Mesozoic Qiangtang Basin with 11 selected composite stratigraphic sections and constrains the first stage of cooling using apatite fission track data. Facies analysis, biostratigraphy, palaeo‐environment interpretation and palaeo‐water depth estimation are integrated to create 11 composite sections through the basin. Backstripped subsidence calculations combined with previous work on sediment provenance and timing of deformation show that the evolution of the Mesozoic Qiangtang Basin can be divided into two stages. From Late Triassic to Early Jurassic times, the North Qiangtang was a retro‐foreland basin. In contrast, the South Qiangtang was a collisional pro‐foreland basin. During Middle Jurassic‐Early Cretaceous times, the North Qiangtang is interpreted as a hinterland basin between the Jinsha orogen and the Central Uplift; the South Qiangtang was controlled by subduction of Meso‐Tethyan Ocean lithosphere and associated dynamic topography combined with loading from the Central Uplift. Detrital apatite fission track ages from Mesozoic sandstones concentrate in late Early to Late Cretaceous (120.9–84.1 Ma) and Paleocene–Eocene (65.4–40.1 Ma). Thermal history modelling results record Early Cretaceous rapid cooling; the termination of subsidence and onset of exhumation of the Mesozoic Qiangtang Basin suggest that the accumulation of crustal thickening in central Tibet probably initiated during Late Jurassic–Early Cretaceous times (150–130 Ma), involving underthrusting of both the Lhasa and Songpan–Ganze terranes beneath the Qiangtang terrane or the collision of Amdo terrane.

A Possible Second Large Subglacial Impact Crater in Northwest Greenland

AbstractFollowing the discovery of the Hiawatha impact crater beneath the northwest margin of the Greenland Ice Sheet, we explored satellite and aerogeophysical data in search of additional such craters. Here we report the discovery of a possible second subglacial impact crater that is 36.5‐km wide and 183 km southeast of the Hiawatha impact crater. Although buried by 2 km of ice, the structure's rim induces a conspicuously circular surface expression, it possesses a central uplift, and it causes a negative gravity anomaly. The existence of two closely spaced and similarly sized complex craters raises the possibility that they formed during related impact events. However, the second structure's morphology is shallower, its overlying ice is conformal and older, and such an event can be explained by chance. We conclude that the identified structure is very likely an impact crater, but it is unlikely to be a twin of the Hiawatha impact crater.

Formation of Complex Craters in Layered Targets With Material Anisotropy

AbstractMeteorite impacts often occur in layered targets, where the strength of the target varies as a function of depth, but this complexity is often not represented in numerical impact simulations because of the high computational cost of resolving thin layers. To address this limitation, we developed a method to approximate the effect of multiple thin weak layers within a sedimentary sequence using a single material layer to represent the entire sequence. Our approach, implemented in the iSALE (impact‐Simplified Arbitrary Lagrangian Eulerian) shock physics code, combines an anisotropic yield criterion with a cell‐based method to track the orientation of layers. To demonstrate the efficacy of the method and constrain parameters of the anisotropic strength model required to replicate the effects of thin, weak layers, we compare results of simulations of an ~20‐ to 25‐km diameter complex crater on Earth using the new method to those from simulations that explicitly resolve multiple thin weak layers. We show that our approach allows for a reduction in computational cost, negating the need for an increase in spatial resolution to resolve thin layers in the target, while replicating crater formation and final morphology from the high‐resolution models. In keeping with field observations, we also find that anisotropic layers may be responsible for a lack of central uplift expression observed at many craters formed in targets with thick sedimentary layers (e.g., the Haughton and Ries impact structures).

Sedimentary successions and onset of the Mesozoic Qiangtang rift basin (northern Tibet), Southwest China: Insights on the Paleo- and Meso-Tethys evolution

Abstract The Mesozoic Qiangtang Basin is investigated to reconstruct the tectonic evolution of the Paleo- and Meso-Tethys Oceans. We present a detailed sedimentary facies analysis of the Nadi Kangri and Quemoco formations and the systematic zircon U Pb ages of the volcanoclastic rocks in the Nadi Kangri Formation to understand the onset and nature of the Mesozoic Qiangtang Basin. The Nadi Kangri Formation, consisting of volcanic-volcanoclastic facies associated with alluvial-diluvial facies association, which represents continental volcanic succession. The Quemoco Formation, consisting of braided fluvial, restricted platform and tidal flat facies associations, represents continental siliciclastic and marine carbonate successions. The deepening-upward transgressive sequences reconstructed from the two formations are indicative of a rift basin. Zircon U Pb ages of the volcanoclastic rocks in the bottom, middle, and upper sections of the Nadi Kangri Formation in the Fanghu area are 221.7 ± 1.3 Ma, 217.0 ± 1.5 Ma, and 207.1 ± 1.3 Ma, respectively. Combined with the previous zircon ages of the Nadi Kangri Formation, we interpreted that 221.7–220.4 Ma is the initial age for the onset of the Mesozoic rift basin. An evolution pattern for the Tethys Ocean and Mesozoic Qiangtang Basin has been proposed based on the magmatism, metamorphism, sedimentary sequence, and deformation data obtained from the Kunlun (KLT), Hoh Xil–Bayan Har (HBT), North Qiangtang (NQS), Central Uplift Belt (CUB), South Qiangtang (SQS), and North Lhasa subterranes/terranes (NLS) during the Early Triassic–Middle Jurassic. Based on the new data, we interpret a double-sided subduction and subsequently a collision of the Paleo-Tethys Ocean beneath the KLT to north and NQS to south, probably occurring at ca. 252-237 Ma and 237-227 Ma, respectively. This interpretation is consistent with the Late Triassic residual sea (ocean) model that attributed to the origin of the Hoh Xil–Bayan Har Basin. The QT and NLS experienced a strong extension between ca. 225 and 201 Ma owing to the rapid expansion of the Bangong Lake–Nujiang Ocean (Meso-Tethys Ocean), which is consistent with the massive volcanic eruption and deposition of volcanic sediments in the QT and NLS. In addition, the QT and NLS experienced a stabilized subsidence during 201–166 Ma, due to the existence of the mature Bangong Lake–Nujiang Ocean.

Подбор состава вытесняющего газа для условий нефтяного месторождения Центрально-Хорейверского поднятия

Подбор состава вытесняющего газа для условий нефтяного месторождения Центрально-Хорейверского поднятия

Reconstructing the basin thermal history with clumped isotope

<p indent=0mm>Due to the lack of effective paleo-geothermometers, it has often been difficult to reconstruct the thermal history of marine carbonate strata, resulting in unreliable conclusions. As emerging paleo-geothermometers, the clumped isotope of carbonate rocks could directly provide the temperature information without relying on the assumed isotopes of paleo-fluids, which has shown excellent potential in the study of the thermal history of carbonate reservoirs. To exclude the interference from late diagenesis, this study examined multiple carbonate samples under minor diagenetic alteration, which were selected from several wells in the Tarim Basin and Sichuan Basin to examine the clumped isotope (measured as Δ₄₇ values). Considering the temperature range and acid fractionation factors of various clumped isotope formulae, the Deflieses formula was selected to calculate the temperature of clumped isotope. By analyzing the experimental results, the meaning of Δ₄₇ temperature in the naturally evolved limestones and dolomites, the reordering rules of ¹³C–¹⁸O solid-state bond in calcite clumped isotope, and the effects of thermal history on clumped isotope in different lithologies have been comprehensively discussed herein. The Shun-Ka area located in the center of the Tarim Basin has been subjected to simple tectonic movements and continuous subsidence, because of which the strata of this area are currently experiencing the highest burial depth and temperature, which provide ideal conditions for studying the influence of temperature on Δ₄₇ values. The measured Δ₄₇ values of 17 samples from the Shun-Ka area were in the range 0.443‰–0.634‰, and the calculated temperatures of the clumped isotope were in the range 49.9–201.7°C. The Δ₄₇ temperatures of these samples were higher than their diagenetic temperatures, indicating that Δ₄₇ was affected by the burial process with the reordering of solid-state ¹³C–¹⁸O bonds. Based on the relationship between the Δ₄₇ temperature and borehole temperature in the Shun-Ka uplift of the Tarim Basin, it was suggested that the blocking temperature of ¹³C–¹⁸O solid-state bond reordering in the naturally evolved calcites would not be higher than 120°C and the equilibrium temperature would not be lower than 160°C, which was consistent with the foreign laboratory results. Furthermore, to compare the effect of clumped isotope in different lithologies, another 11 dolomite samples were selected from the central uplift of the Sichuan Basin for analyzing the clumped isotope. The measured Δ₄₇ values of dolomites from the Sichuan Basin were in the range 0.423‰–0.537‰, and the calculated temperatures of the clumped isotope were in the range 105.7–233.5°C. Similarly, it is possible that the warming effect associated with increased burial depth resulted in ¹³C–¹⁸O solid-state bond reordering and a higher temperature, which was the result of both diagenesis and burial heating. To investigate the effects of thermal history on clumped isotope, the first-order approximation model was used to simulate the clumped isotope values based on the thermal history reported in previous research. The initial diagenesis temperature was set to 20°C, and it was assumed that the Δ₄₇ temperatures of samples were only affected by diagenesis temperature and thermal history. Results revealed that the Δ₄₇ temperature calculated using the first-order approximation model of limestone samples from the Yingshan Formation of the Tarim Basin exhibited good agreement with the measured temperature, but the calculated Δ₄₇ temperature in the dolomites considerably differed from the actual measured temperature. This conflict could have occurred due to various thermodynamic properties (such as frequency factors and activation energy) and the complex diagenesis process between limestones and dolomites. Therefore, the first-order approximation model could be used more accurately to describe the effect of the basins thermal history on ¹³C–¹⁸O solid-state reordering in calcites. However, further research should be conducted on dolomites to build an inversion model focusing on the thermal history of the basin and clumped isotope.

Mesozoic–Cenozoic denudation and thermal history in the Central Uplift of the South Yellow Sea basin and the implications for hydrocarbon systems: Constraints from the CSDP-2 borehole

Abstract This study focuses on the Mesozoic–Cenozoic tectono-thermal events in the Central Uplift of the South Yellow Sea Basin (SYSB) based on the CSDP-2 borehole. CSDP-2 is the first borehole to penetrate the integrated Paleozoic–Mesozoic sedimentary strata and recovered approximately 2736 m of core. The drilling results confirm a significant stratigraphic hiatus, i.e., Indosinian unconformity, between the Neogene–Quaternary and the underlying Lower Triassic. The eroded thickness estimated from sonic log and vitrinite reflectance data range from ∼1200 to 1400 m. Multiple paleo-thermometer data from the CSDP-2 cores, including vitrinite reflectance, apatite/zircon fission-track (AFT/ZFT) and fluid inclusions, were used to reconstruct the Mesozoic–Cenozoic thermal history in the study area. There was a continuous heating process during the early Mesozoic. The peak paleo-temperature may have reached up to 160 °C (base Permian) during the Late Jurassic, with the highest paleo-geothermal gradient of approximately 58.8 °C·km−1. The tectono-thermal evolution was dominated by the Mesozoic intracontinental collision between the Yangtze Craton and the Sino-Korean platform. The heating process was mainly caused by rapid burial on the passive continental margin and igneous activities. The paleo-temperature significantly decreased following the Late Jurassic. Two rapid cooling processes ca. 100–65 Ma and ca. 35 Ma are suggested by the AFT modeling results. Cooling history indicates that the tectonic uplift and denudation likely occurred from the Early Cretaceous to the Miocene. The reconstruction results of thermal evolution confirm that the Mesozoic–Paleozoic source rocks are mature or even highly mature, and hydrocarbon generation, migration and charge undoubtedly occurred in the Central Uplift, showing substantial hydrocarbon potential. Moreover, the reconstruction of the tectono-thermal history provides important constraints for regional tectonic studies.

Integrated geological-geophysical characterizations of deeply buried fractured-vuggy carbonate reservoirs in Ordovician strata, Tarim Basin

Abstract The Central Uplift region of the Tarim Basin contains a series of thick Ordovician carbonates that form fractured-vuggy hydrocarbon-rich reservoirs along an unconformity. These fractured-vuggy reservoirs are difficult to characterize using traditional well logging and seismic interpretation methods because these carbonates feature complex and heterogeneous lithologies and variations in scale, geometry and spatial distribution. In this paper, a series of deeply buried fractured-vuggy carbonate reservoirs are characterized on micro-, macro- and mega-scales. These characterizations are based on previous studies detailing the carbonate sedimentation, diagenesis, and tectonic processes in the study area and abundant geological and geophysical data, including core samples, thin sections, and well logging and seismic datasets, which have been obtained and upscaled using the “choosing-classification-evaluation- generalizing-upscaling” (CCEGU) method. In this area, different facies can be identified at different scales: (1) Detailed core and thin section observations performed based on the Dunham classification demonstrate that 7 lithofacies and 19 rock types are present at the micro-scale (0.01 m); (2) logging data and assorted reservoir characteristics indicate that these 19 different rock types consist of 9 different electro facies at the macro-scale (0.1 m–1 m), and the presence of electro facies in individual wells can be recognized; and (3) these 9 different electro facies comprise 6 distinct seismic facies at the mega-scale (10 m), and can be used to delineate the spatial outlines of potential reservoirs and evaluate their properties. The targets of particular interest in the study area are a series of partially dissolved dolomites located near large faults and a series of vuggy soluble limestones located near NE-trending strike-slip faults. This study combines geological and geophysical response data from fractured-vuggy carbonate reservoirs to achieve seamless connections among micro-scale, macro-scale and mega-scale observations to effectively characterize a series of strongly heterogeneous fractured-vuggy reservoirs. This method can also be applied to characterize other deeply buried paleokarst reservoirs in the Tarim Basin, as well as other strongly heterogeneous reservoirs.

Deriving Morphometric Parameters and the Simple‐to‐Complex Transition Diameter From a High‐Resolution, Global Database of Fresh Lunar Impact Craters (D ≥ ~ 3 km)

AbstractWe compiled a high‐resolution, global lunar impact crater database composed of 5,505 pristine craters ≥ ~3 km. This database contains detailed morphometric data, and their trends are examined with best‐fit power‐laws. We compared several different functions for simple, transitional, and complex craters to report one best‐fit representation. We integrated transitional craters into these fits as an independent crater class. Transitional craters are in a transitional state, between simple and complex craters. They are devoid of a central uplift and terraces but show wall slumping and a flat floor. In regard to depth‐diameter relationships, simple craters exhibit similar scaling all over the Moon, whereas larger craters are different in highland and mare regions. In the present work, we sought the best representation of the simple‐to‐complex transition diameter. The intersection of simple power‐law fits for simple and complex crater populations is shown to be a poor representation of the transition diameter. We used a misclassification method for finding transition diameters. This process reveals a transition from simple to transitional craters at diameters of ~17 km (highland) and ~14 km (mare). Transitional morphology is replaced by complex morphology at diameters of ~ 28 km (highland) and ~ 24 km (mare). The lower transition diameters of mare craters are attributed primarily to the layered mare basalts, which enables an earlier onset of crater modification. We also analyzed the relationship between aspect ratio and depth‐diameter ratio of simple craters: Simple craters with ε ≥ 1.1 are significantly shallower in depth/diameter plots than craters with ε &lt; 1.1.

Microstructural dynamics of central uplifts: Reidite offset by zircon twins at the Woodleigh impact structure, Australia

Research Article| September 27, 2018 Microstructural dynamics of central uplifts: Reidite offset by zircon twins at the Woodleigh impact structure, Australia Morgan A. Cox; Morgan A. Cox * 1Space Science and Technology Centre (SSTC), School of Earth and Planetary Science, Curtin University, Perth, Western Australia 6102, Australia *E-mail: morgan.cox@student.curtin.edu.au Search for other works by this author on: GSW Google Scholar Aaron J. Cavosie; Aaron J. Cavosie 1Space Science and Technology Centre (SSTC), School of Earth and Planetary Science, Curtin University, Perth, Western Australia 6102, Australia Search for other works by this author on: GSW Google Scholar Phil A. Bland; Phil A. Bland 1Space Science and Technology Centre (SSTC), School of Earth and Planetary Science, Curtin University, Perth, Western Australia 6102, Australia Search for other works by this author on: GSW Google Scholar Katarina Miljković; Katarina Miljković 1Space Science and Technology Centre (SSTC), School of Earth and Planetary Science, Curtin University, Perth, Western Australia 6102, Australia Search for other works by this author on: GSW Google Scholar Michael T.D. Wingate Michael T.D. Wingate 2Geological Survey of Western Australia, Department of Mines, Industry Regulation and Safety, Perth, Western Australia 6004, Australia Search for other works by this author on: GSW Google Scholar Geology (2018) 46 (11): 983–986. https://doi.org/10.1130/G45127.1 Article history received: 09 May 2018 rev-recd: 10 Sep 2018 accepted: 11 Sep 2018 first online: 27 Sep 2018 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Morgan A. Cox, Aaron J. Cavosie, Phil A. Bland, Katarina Miljković, Michael T.D. Wingate; Microstructural dynamics of central uplifts: Reidite offset by zircon twins at the Woodleigh impact structure, Australia. Geology 2018;; 46 (11): 983–986. doi: https://doi.org/10.1130/G45127.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Impact cratering is a dynamic process that is violent and fast. Quantifying processes that accommodate deformation at different scales during central uplift formation in complex impact structures is therefore a challenging task. The ability to correlate mineral deformation at the microscale with macroscale processes provides a critical link in helping to constrain extreme crustal behavior during meteorite impact. Here we describe the first high-pressure-phase–calibrated chronology of shock progression in zircon from a central uplift. We report both shock twins and reidite, the high-pressure ZrSiO4 polymorph, in zircon from shocked granitic gneiss drilled from the center of the >60-km-diameter Woodleigh impact structure in Western Australia. The key observation is that in zircon grains that contain reidite, which forms at >30 GPa during the crater compression stage, the reidite domains are systematically offset by later-formed shock deformation twins (∼20 GPa) along extensional planar microstructures. The {112} twins are interpreted to record crustal extension and uplift caused by the rarefaction wave during crater excavation. These results provide the first physical evidence that relates the formation sequence of both a high-pressure phase and a diagnostic shock microstructure in zircon to different cratering stages with unique stress regimes that are predicted by theoretical and numerical models. These microstructural observations thus provide new insight into central uplift formation, one of the least-understood processes during complex impact crater formation, which can produce many kilometers of vertically uplifted bedrock in seconds. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

The central pit and dome at Cerealia Facula bright deposit and floor deposits in Occator crater, Ceres: Morphology, comparisons and formation

The prominent bright deposit Cerealia Facula, Ceres, coincides with the central depression (or central pit) of the recently formed 92 km-wide complex crater Occator. The central pit is 9–10 km wide and up to 1 km deep and is partially filled with a 700 m-high 2 km-wide dome. The upper surface of the central dome is densely fractured but the flanks are not, indicating that uplift of the dome surface occurred after the bright deposit was emplaced, and primarily through uplift of the surface from below by laccolithic intrusion or volume expansion. The pit is rimless except for two prominent massifs that flank it to the east and west. This pit-dome morphology bears a strong resemblance in morphology, topography, and dimensions to central pit craters observed on Ganymede and Callisto, but is essentially absent on the similar sized ice-rich moons of Uranus, Saturn, and Pluto. The lack of pit craters on midsize icy moons and the possible lack of central pit craters at the cold poles of Ceres suggest that temperature is a significant controlling factor in central pit formation, beyond the canonical inverse-gravity scaling of complex crater transitions. While renewed modeling will benefit from these new constraints, the mapping of central pits on Ceres, including Occator, suggests that either the rheologically layered target model or more likely the melted central uplift model may be the most probable explanation for central pit formation on Ceres (lack of similar high-resolution imaging on the Galilean icy satellites precludes definitive evaluation of models there). Large volumes of lobate floor-fill material at least 600 m thick in some places cover large areas of the southern and eastern floor of Occator, and small outcrops of this unit are found perched high on closed topographic depressions on the Occator terrace zone and other higher areas of the crater floor. The swirling and knobby surface textures of these lobate deposits are essentially indistinguishable from those on impact melt sheets on large lunar craters such as Tycho. These observations indicate that this unit is most likely an impact melt sheet formed from melted water ice admixed with unmelted particulate or dissolved phyllosilicates, salts and/or carbonates, and large quantities of fragmented debris. Melt of ice phases within Ceres crust is also supported by independent numerical models of impact at Occator and would provide a ready source of fluids to promote hydrothermal deposition of the observed bright Na-carbonates on the surface of the central pit at Occator, if the deposit formed within a million years or so of impact. The central dome finds analogs in salt domes on Earth and pingos (frost-mounds) on Earth and Mars, which form by water and solute migration and freeze expansion. Later freezing of a central reservoir of volatiles beneath the central pit at Occator or mobilization of ice or other phases during the cooling and refreezing process could explain the uplift of the central dome. Finally, no changes were detected in the Cerealia and Vinalia Facula bright deposits during the course of the prime Dawn mapping mission, placing additional constraints on their formation time and limits on ongoing activity.

Hydrogeomorphologic architecture of epikarst reservoirs in the Middle-Lower Ordovician, Tazhong Uplift, Tarim Basin, China

Abstract Paleokarst vugs and fissures in the Middle-Lower Ordovician from the Tazhong Uplift have been a productive unit in the Tarim Basin for more than two decades. A new approach was applied to delineate the characteristics of these reservoirs by integration of 3D seismic data with thin sections, cores, and imaging logs, as well as related geochemical analysis. Reconstructed hydrogeomorphologic architecture was defined to illustrate the origin of the paleokarst reservoirs in this study. Fissures and fissure-vugs of epikarst reservoirs are the predominant pore type in the Central Uplift in the Tazhong area, which pervasively distributed as planes and steps in shallow depth of 0–30 m below the regional unconformity (T74). The favorable position for reservoir development was intimately related to fissures and faults in NE and NWW directions. Surface topography of the Middle-Lower Ordovician was flat initially, with karren as the most widespread landform category. Hence, surface water tended to be sheet flow and concentrated in places where karrens and cracks developed well, and thus the infiltration strength and exhumation capacity of surface water suffered from extensive constrains due to limited hydraulic gradient, which resulted in immature drainage system and incomplete karst sequence. Therefore, the hydrogeomorphologic architecture in the Middle-Lower Ordovician was likely lack of surface drainages, and a fissure-seepage karst reservoir development model was established to interpret the influence of hydrogeomorpholoic architecture on paleokarst reservoirs distribution. This study confirms that paleokarst fissure-vug system developed in karst initiation period can still be potential petroleum-bearing target in deeply buried subsurface.

Shock metamorphic history of &gt;4 Ga Apollo 14 and 15 zircons

AbstractDuring impact events, zircons develop a wide range of shock metamorphic features that depend on the pressure and temperature conditions experienced by the zircon. These conditions vary with original distance from impact center and whether the zircon grains are incorporated into ejecta or remain within the target crust. We have employed the range of shock metamorphic features preserved in &gt;4 Ga lunar zircons separated from Apollo 14 and 15 breccias and soils in order to gain insights into the impact shock histories of these areas of the Moon. We report microstructural characteristics of 31 zircons analyzed using electron beam methods including electron backscatter pattern (EBSP) and diffraction (EBSD). The major results of this survey are as follows. (1) The abundance of curviplanar features hosting secondary impact melt inclusions suggests that most of the zircons have experienced shock pressures between 3 and 20 GPa; (2) the scarcity of recrystallization or decomposition textures and the absence of the high‐pressure polymorph, reidite, suggests that few grains have been shocked to over 40 GPa or heated above 1000 °C in ejecta settings; (3) one grain exhibits narrow, arc‐shaped bands of twinned zircon, which map out as spherical shells, and represent a novel shock microstructure. Overall, most of the Apollo 14 and 15 zircons exhibit shock features similar to those of terrestrial zircon grains originating from continental crust below large (~200 km) impact craters (e.g., Vredefort impact basin), suggesting derivation from central uplifts or uplifted rims of large basins or craters on the Moon and not high‐temperature and ‐pressure ejecta deposits.

Impactites of the Mistastin Lake impact structure: Insights into impact ejecta emplacement

AbstractThe Mistastin Lake impact structure is an intermediate‐size (~28 km apparent crater diameter), complex crater formed ~36 Myr. The original crater has been differentially eroded; however, a subdued terraced rim and distinct central uplift are still observed and impactites are well exposed in three dimensions. The inner portion of the structure is covered by Mistastin Lake and the surrounding area is locally covered by soil/glacial deposits and vegetation. The crystalline target rocks of the Mistastin Lake region are dominated by anorthosite, granodiorite, and quartz monzonite. Previous studies of the Mistastin Lake impactites have primarily focussed on the impact melt rocks. This study further evaluates the entire suite of impactite rocks in terms of their location within the crater structure and emplacement mechanisms. Locally, allochthonous impactite units including impact melt and various types of breccias are distributed around the lake in the terraced rim and are interpreted as proximal ejecta deposits. A multistage model for the origin and emplacement of impact melt rocks and the formation of impact ejecta is proposed for the Mistastin Lake impact structure based on a synthesis of the field and petrographic observations. This model involves the generation of a continuous ballistic ejecta blanket during the excavation stage, followed by the emplacement of melt‐rich, ground‐hugging flows during the terminal stages of crater excavation and the modification stage of crater formation. Impact melt‐bearing breccias—also termed “suevite” at other sites—are present in several distinct settings within the Mistastin Lake structure and likely have more than one formation mechanism.

Gravity and magnetic field characteristics and hydrocarbon prospects of the Tobago Basin

ABSTRACTThe Tobago Basin, which is located offshore northern Venezuela with a southern margin close to Trinidad and Tobago, has an area of approximately 59,600 km2. The Tobago Basin has relatively favourable hydrocarbon prospects, and to date, exploration work has mainly concentrated on small areas of the southwestern portion of the basin. To conduct a comprehensive study of the structural framework of the basin and the characteristics of the basement in order to identify prospective zones for hydrocarbon exploration, shipborne‐measured and satellite‐measured gravity data, shipborne‐measured magnetic data, and aeromagnetic survey data were analysed. A regularisation filtering method was used to separate and obtain regional and residual gravity and magnetic anomalies. Directional gradients of gravity and magnetic anomalies and the total horizontal gradient and vertical second derivative of gravity anomalies were employed to extract information about fault structures. Regression analysis methods were used to determine the basement depth. The geological significance of the gravity and magnetic fields was examined, the structural framework of the basin was assessed, the basement depth was estimated, and favourable hydrocarbon exploration prospects within the basin were identified. The results show that the Tobago Basin contains complex structures consisting mainly of two groups of faults trending in northeasterly and northwesterly directions and that the major northeasterly trending faults control the main structural configuration and depositional system within the basin. The basement of the Tobago Basin has deep rises and falls. It can be divided into the following four secondary tectonic units: the western sub‐basin, the central uplift area, the southern sub‐basin, and the northeastern sub‐basin. The central uplift area and northeastern sub‐basin are most likely to have developed hydrocarbon accumulations and should be targeted for further exploration.

Combined tectonic and paleogeographic controls on the genesis of bauxite in the Early Carboniferous to Permian Central Yangtze Island

Two bauxite horizons, with a total reserve of 1.29 Bt, occur in the Lower Carboniferous and Lower Permian in the central South China Block. Different from other intracontinental bauxites in China, which are genetically linked to the orogenesis along the continental margin, these bauxites were generated in the absence of marginal orogenesis. The genesis of these bauxites were studied based on tectonic, paleogeographic, and paleoclimatic controls. The orebody in Huofengchong (HFC) region is hosted within the Lower Carboniferous, which overlies upon the Cambrian dolomite unconformably. The orebody in the regions of Yanfengba (YFB) and Baiyantou (BYT) occurs in Lower Permian, which overlies upon the Silurian shale or Devonian dolomite unconformably. The mineral assemblages in the Carboniferous HFC bauxite consist of diaspore, illite, anatase, and in Permian YFB and BYT bauxite, the minerals are diaspore, anatase, illite, clinochlore, and kaolinite. The data from the U-Pb isotope measurements of detrital zircons from these bauxites provide two main ages: (1) a maximum U-Pb age peak of ca. 800 Ma from the Carboniferous HFC and (2) a maximum age peak of ca. 1100 Ma for the Permian BYT and YFB. The ca. 800 Ma detrital zircons from the Carboniferous bauxite are dominating in the Paleozoic sedimentary units, which likely derive from the erosion of the Mid- to Neo-proterozoic felsic volcanic and intrusive rocks (Panxi-Hannan arc) occurring in the western margin of the South China Block. By contrast, the zircons with ages of ca. 1100 Ma are prevalent in the Paleozoic sediments from the center of the block. The above results indicate that the Lower Carboniferous and Lower Permian bauxites are sourced from the sedimentary rocks in the western and central parts of the South China Block, respectively. Despite the difference in the provenance, these two episodes of deposits were both deposited in a tidal flat-restricted platform attached to an EW-trending Yangtze island, occupies the western to central parts of the South China block. This indicates that the shift from a marginal to central uplift within the paleo-island from Early Carboniferous to Early Permian has induced the transition in the provenance of the source material. In addition, the weathered materials from the two provenances both accumulated in the coastal region receiving further bauxitization. The bauxitization was boosted by the persistent preferable tepid and humid paleoclimate, defined by the equatorial position of South China block and the drop of global temperature during the metallogenic period.