Unconformable Contact Research Articles

Surface radioactive anomalies at Srisailam sub basin, Cuddapah basin, India

Surface radioactive survey has been carried with portable gamma ray spectrometer (PGRS) about the 45 km2 around the Srisailam sub basin, the surface radioactive anomalies were found at Chennakesavula gutta in the Srisailam sub basin of Cuddapah basin in the age of meso-neo proterozoic, India. The integrated geochemical and radiometric surveys were conducted as part of research work in the Srisailam basin, it has resulted the locating of surface radioactive anomalies near the Chennakesavula gutta. Secondary uranium minerals are observed along with the fracture planes in the Archean basement granite in the proximity of unconformity contact of Srisailam quartzite. The granite underlies the Srisailam formation and forms the source for possible unconformity related uranium mineralisation in the Srisailam sub-basin evidence by the significant radioactive anomalies. The radioactivity recorded by PGRS reflected uraniferous nature of the radioactivity (Average total counts = 195 ppm; U = 202 ppm; Th = 22 ppm; %K = 4.1, n = 15).

Role of magmatic and fluid concentrating in formation of platinum mineralization in the Lower Zone and Platreef as follows from composition of phlogopite, cumulus silicates, and sulfide melt, the northern limb of Bushveld Complex

As follows from the results of new geological exploration, the thick sequence of primitive cumulates of the Lower Zone underlies Platreef in separate magmatic depressions of the northern limb of the Bushveld Complex. In the Turfspruit area, the Lower Zone is separated from the overlying Platreef by an interval of contact metasedimentary rocks with sills of fine-grained norite and plagioclase orthopyroxenite of the Marginal Zone. The magmatic stratigraphy of Platreef in Turfspruit and its western plunge, where it is slightly contaminated with sedimentary rocks, is close to the section of Critical Zone in the Bushveld. The Lower Zone and Platreef contain sulfide mineralization enriched in PGE in particular units-reefs-throughout the section. The main reef can be correlated with the Merensky Reef. The objective of this study was to estimate the relationships between fluid and magmatic PGE concentrating in the reefs localized in the upper part of Platreef near its unconformable contact with the Main Zone. It is shown that Ni partitioning between cumulus olivine and orthopyroxene, on the one hand, and coexisting sulfide liquid, on the other, indicates their equilibrium crystallization in the Lower Zone and Platreef at close redox conditions. The composition of sulfide liquid was recalculated as sulfide tenor on the basis of bulk Ni, Cu, and S concentrations in rock. In contrast, Ni partitioning between phlogopite and sulfide melt does not provide evidence in favor of their equilibrium crystallization, although Mg # of phlogopite from the intercumulus assemblage is correlated with Mg # of cumulus mineral throughout the section. The results of phlogopite microprobing throughout the section show that the highest F and Cl contents are characteristic of highly evolved rocks, including both the PGE-bearing plagioclase pyroxenite from reefs and barren norite from the Marginal Zone. We have arrived at the conclusion that enrichment in volatile components is the attendant rather than necessary condition of PGE mineralization formation in reefs, whereas the partitioning of metals between silicate and sulfide melts is the main ore-controlling factor.

Locating New Uranium Occurrence by Integrated Weighted Analysis in Kaladgi Basin, Karnataka

Interpretation of airborne geophysical data especially gamma ray spectrometric data is important for mineral exploration, specifically radioactive elements and to identify uranium targets. Exploration was carried out in MesoNeoproterozoic Kaladgi basin for five decades to target different types of uranium mineralisation. The first sizable sub-surface uranium mineralisation was intercepted at Deshnur within Badami arenites close to unconformity contact along the southern margin of the basin (Shobhita et al., 2011). Heliborne geophysical surveys were carried out in western part of the basin from Kadgaon to Ramdurg to acquire Time Domain Electromagnetic, Magnetic and Radiometric data. The radiometric data was collected using Exploranium GR820 airborne (256-channel recording) spectrometer system mounted in a helicopter with four 4.2L downward looking crystals and one 4.2L upward looking crystal for a total effective volume of 16.8L downwards and 4.2L upwards. The raw radiometric data was processed for corrections (background, radon, stripping, height attenuation etc) and counts were converted to ground concentration of %K, eU ppm, and eTh ppm, and their grid maps were prepared (Sridhar et al, 2012) . Kaladgi basin is an irregular E-W trending area in the northern part of Dharwar craton. In the study area, Kaladgi basin sediments are exposed as unconformably overlying the basement crystallines in the south and are covered by Deccan traps in the north. The crystalline basement of the Kaladgi Supergroup is represented by NNW trending Meso to Neoarchaean peninsular gneiss, greenstone belts and Closepet granite. The sedimentary sequence in the Kaladgi Supergroup has been divided along an angular unconformity surface into two groups: the lower Bagalkot Group, and the overlying Badami Group. The sediments of Bagalkot Group comprises of arenaceous, argillaceous and carbonate sequences which are folded into a series of anticlines and synclines. The sediments of Badami Group are undeformed and rest unconformably over the Archaean basement rocks as well as the Bagalkot Group. The Badami Group comprises of horizontal to gently dipping beds of arenite, shale and limestones (Jayaprakash et al., 1987). This study aims at identifying uranium potential zones by integrated analysis of thematic layer interpreted and derived from airborne radiometric and magnetic data, satellite data along with available ground geochemical data in western part of Kaladgi basin. Integrated weighted analysis of spatial datasets which included airborne radiometric data (eU, eTh & % K conc.), litho-structural map, hydrogeochemical U conc., and geomorphological data pertaining to study area, was attempted. The weightage analysis was done in GIS environment where different spatial dataset were brought on to a single platform and were analyzed by integration (Fig.1). The processed radiometric database was taken up for quantitative analysis to identify radiometric uranium anomalies. The database was clipped by using exposed and inferred lithological contacts of Badami Group to separate out data over sediments of Badami Group only. The analysis was carried out in two methods - 1 st the whole block and 2 nd Badami Group exclusive. Statistical analysis was applied to the airborne spectrometric data for separation of uranium anomalies from background. The statistics of the databases for uranium, thorium, ratio of uranium to thorium and uranium squared by thorium were calculated. The anomalies were extracted by applying cutoff threshold values for uranium, uranium by thorium and uranium squared by thorium channels in both methods. These anomalies were plotted spatially and they were classified as cluster anomalies and point anomalies depending upon their nature. Weightages were assigned based on nature of anomalies and cluster anomalies were given more weightage as they are more significant compared to point anomalies. The anomalies extracted from the

Meso-Neoproterozoic isotope stratigraphy on carbonates platforms in the Brasilia Belt of Brazil

Carbonate platforms were present worldwide during the Proterozoic Eon, and variations in their C and Sr isotope ratios are commonly used as a correlation tool particularly through the Neoproterozoic, when rapid secular change in marine C and Sr isotope values permit distinction between Cryogenian glacial events. In central Brazil, the late Mesoproterozoic and Neoproterozoic eras are represented by a thick succession of sedimentary rocks that were deposited, and later deformed, along the eastern margin of the São Francisco Craton. These strata are divided into the three major groups: (1) the Paranoá Group, which consists of a succession of sandstone, siltstone, rhythmite, and selected intervals of carbonate, (2) the Macaúbas Group, which consists of a glacial diamictite (Jequitaí Formation), and (3) the Bambuí Group with an important carbonate-bearing succession that includes characteristic “cap carbonate” facies in its lower strata. Carbonate facies of Paranoá and Bambuí Groups typically occur in unconformable contact, and when the diamictite of Jequitaí Formation is absent, it can be difficult to determine the stratigraphic position of these lithological similar groups. Furthermore, uncertainties in the age of the Bambuí Group has lead to several distinct interpretations regarding the age of the Jequitaí glacial diamictites.We investigated the C, O, and Sr isotopes and chemical composition of carbonate rocks in five measured sections, including both pre- and post-glaciation carbonate successions. The δ13C (‰ pdb) values in the upper Paranoá Group occur in a narrow interval between +0.6 and +3.6, whereas the post-glacial Bambuí Group begins with substantially negative values (as low as −5.7‰) in cap dolomite facies and rises to values up to +11 permil in limestone of the upper Sete Lagoas Formation. Similarly, carbonate rocks of the Paranoá and Bambuí groups are distinct in terms of their 87Sr/86Sr ratio. Generally non-radiogenic ratios between 0.7056 and 0.7068 are recorded in the upper Paranoá Group, and ratios between 0.7074 and 0.7080, occurring within the Bambuí Group.The stratigraphic pattern of the C and Sr ratios indicates distinctive isotopic characteristics for these two carbonate successions. The isotopic data for the cabonates in the Paranoá Group are consistent with a sedimentation age in upper Mesoproterozoic or lowermost Neoproterozoic, preceding the first Cryogenian glaciation. Carbonate facies and the isotopic data for the lower Bambuí Group suggest a relationship with the second Cryogenian glaciation.

Clastic rock geochemistry of Punagarh basin, trans-Aravalli region, NW Indian shield: implications for paleoweathering, provenance, and tectonic setting

The trans-Aravalli region of the northwestern Indian shield is characterized by the presence of Meso–Neoproterozoic linear sedimentary Punagarh basin comprising shales and sandstones and mafic volcanic flows in the upper formations. It is constituted by four formations (Sojat, Bambolai, Khamal, and Sowania), among which the Sojat Formation, in unconformable contact with overlying formations, is significantly older. Major and trace element compositions of sandstones and shales of this basin have been investigated to infer the nature of provenance, weathering processes, and tectonic setting. Provenance analysis using trace elements and petrography suggests that this basin initially received detritus from Archean Banded Gneissic Complex (BGC) and later Delhi arc of the Aravalli craton. The shale and sandstone constituting the lower part of the Punagarh basin, namely, Sojat Formation, are distinctive in age, field, and geochemical characters in comparison to succeeding sedimentary units comprising the upper formations of the Punagarh basin. The significantly different geochemical characteristics of Sojat Formation (i.e., highest SiO2/Al2O3, K2O/Na2O, Th/U, and Cr/Th ratios coupled with lowest Zr content, least fractionated light-group rare-earth element (LREE), more fractionated heavy-group rare-earth element (HREE), and largest Eu* anomaly) suggest a change in provenance with sediments in the lower formation being derived from distal source and those of the upper formations from proximal source. Based on REE patterns and Eu/Eu* values, the clastic sediments of Sojat Formation can be modeled by a mixture of 0.24:0.42:0.34 (granite/tonalite–trondhjemite–granodiorite/mafic enclaves). On the other hand, the upper formations are best modeled with a Delhi arc in the proportion 0.25:0.40:0.35 (tholeiites/calc-alkalines/felsites). Weathering indices like chemical index of alteration (CIA), plagioclase index of alteration (PIA), A–CN–K plot, and trace element ratios suggest that the source areas experienced low to high degree of chemical weathering under warm and humid conditions. The shift of sediment source from distal in the beginning to local during later period suggests sedimentation in a tectonically active basin. The depositional history, tectonic setting, and present position of the Punagarh basin provide inputs about the role of assembly and breaking of supercontinents in the buildup of Greater India during the Proterozoic period.

The Flin Flon paleosol revisited

A ca. 1.85 Ga paleosol developed on seafloor-altered mafic rocks is preserved beneath continental sandstones and conglomerates of the Missi Group near Flin Flon, Manitoba. Previous work has focused on a locality where the paleosol is developed on pillowed greenstone (chlorite + albite + epidote + calcite + quartz + Fe–Ti oxides ± mica). This area has been revisited and a new (∼5 m) profile was excavated and sampled at a very high density for geochemical (major element; ∼60 samples) and mineralogical (petrography, X-ray diffraction) investigation. The high sampling density has revealed a relatively sharp immobile-element offset (e.g., Al/Ti) in the profile. This could reflect weathering across two separate parent rocks or preferential addition of allochthonous material into the top of the profile. Regardless, comparison with a wide suite of volcanic rocks in the stratigraphy suggests that the original concentration of Fe and probably also Ca and Mg in the greenstone on either side of the boundary was similar, permitting quantitative treatment of major-element trends. The profile shows pronounced upward loss of Ca, Mg, Mn2+, and Fe2+and a corresponding increase in Al, Ti, and total Fe. Physical weathering features, such as corestones, are well preserved and show progressive alteration upwards in the profile in parallel with the chemical trends. Deep in the profile, Na and Ca are lower than the greenstone due to the weathering of albite and epidote, whereas Mg, Fe, and Mn were largely retained in chlorite and secondary dolomite. Upwards in the profile, the loss of chlorite and dolomite and the increase of illite – mixed-layer clays and white mica (probably 2:1 layer or mixed-layer clays originally) and hematite are accompanied by a transition from green to maroon colour. Above a well-defined depth in the profile, carbonate is virtually absent, and the paleosol is highly depleted in Ca. This transition in the paleosol coincides with the immobile-element offset. The significance of this coincidence remains to be explored. Reworking of regolith (e.g., rip-up clasts) occurred during deposition of the Missi sediments, and burial and subsequent metamorphism resulted in the upward enrichment of K and Na and the associated conversion of pedogenic clays to illite and micas (muscovite and paragonite). Crosscutting quartz and epidote veins and tourmaline formation near the unconformity contact implies minor post-burial fluid movement. Despite these complexities, there is consistent retention of Fe through formation of pedogenic Fe oxides, attested by the near-constant ∑Fe/Al ratio in the profile. There is very little evidence for subsequent Fe reduction by hydrothermal or organic fluids. Instead, trends in the Fe speciation are controlled predominantly by the modal variation of Fe–Ti oxides in the greenstone and spheroidal weathering textures in the paleosol.

A new geological map of the western Junggar, north Xinjiang (NW China): Implications for Paleoenvironmental reconstruction

Recently, new geochronological data and geological observations including the discovery of garnet amphibolite, Ordovician micro-fossils, unconformable contact between the ophiolitic melange related flysch and the Devonian to Early Carboniferous terrestrial volcano-sedimentary strata provide critical evidences for Paleo-environmental reconstruction in the western Junggar (Xinjiang, China). Two different geological layers could be clearly identified: the Early Paleozoic metamorphic terrane mainly consists of ophiolitic melange and flysch, and the Late Paleozoic volcano-sedimentary units lying on the ophiolitic melange and flysch. Both of these layers were intruded by granitic to dioritic magma during the Late Carboniferous to Early Permian period. Based on these data, a new geological map for the western Junggar has been compiled, which is essential not only for better understanding of the geological evolution but also for exploration of mineral resources.

A Rodinian suture in western India: New insights on India-Madagascar correlations

We report detailed evidence for a new paleo-suture zone (the Kumta suture) on the western margin of southern India. The c. 15-km-wide, westward dipping suture zone contains garnet-biotite, fuchsite-haematite, chlorite-quartz, quartz-phengite schists, biotite augen gneiss, marble and amphibolite. The isochemical phase diagram estimations and the high-Si phengite composition of quartz-phengite schist suggest a near-peak condition of c. 18kbar at c. 550°C, followed by near-isothermal decompression. The detrital SHRIMP U–Pb zircon ages from quartz-phengite schist give four age populations ranging from 3280 to 2993Ma. Phengite from quartz-phengite schist and biotite from garnet-biotite schist have K–Ar metamorphic ages of ca. 1326 and ca. 1385Ma respectively. Electron microprobe-CHIME ages of in situ zircons in quartz-phengite schist (ca. 3750Ma and ca. 1697Ma) are consistent with the above results. The Bondla ultramafic-gabbro complex in the west of the Kumta suture compositionally represents an arc with K–Ar biotite ages from gabbro in the range 1644–1536Ma. On the eastern side of the suture are weakly deformed and unmetamorphosed shallow westward-dipping sedimentary rocks of the Sirsi shelf, which has the following upward stratigraphy: pebbly quartzite/sandstone, turbidite, magnetite iron formation, and limestone; farther east the lower lying quartzite has an unconformable contact with ca. 2571Ma quartzo-feldspathic gneisses of the Dharwar block with a ca. 1733Ma biotite cooling age. To the west of the suture is a c. 60-km-wide Karwar block mainly consisting of tonalite-trondhjemite-granodiorite (TTG) and amphibolite. The TTGs have U–Pb zircon magmatic ages of ca. 3200Ma with a rare inherited core age of ca. 3601Ma. The K–Ar biotite cooling age from the TTGs (1746Ma and 1796Ma) and amphibolite (ca. 1697Ma) represents late-stage uplift. Integration of geological, structural and geochronological data from western India and eastern Madagascar suggest diachronous ocean closure during the amalgamation of Rodinia; in the north at around ca. 1380Ma, and a progression toward the south until ca. 750Ma. Satellite imagery based regional structural lineaments suggests that the Betsimisaraka suture continues into western India as the Kumta suture and possibly farther south toward a suture in the Coorg area, representing in total a c. 1000km long Rodinian suture.

The Early Hercynian paleo-karstification in the Block 12 of Tahe oilfield, northern Tarim Basin, China

It is indicated in this paper that the karstification occurred in Block 12 of Tahe oilfield during Early Hercynian stage on the basis of identification features obtained through the latest research on seismology, petrology, and geochemistry. Distinguishable markers of the paleo-karstification mainly include: the lost strata and unconformable contact occurring between Lower–Middle Ordovician carbonates and Lower Carboniferous Bachu Formation, the occurrence of collapse breccias and fillings in subsurface stream, the evident difference between cave-fill calcite and matrix carbonate in the Sr/Ba ratio, and the log response for cavernous formations. The paleo-geomorphology of Early Hercynian karstification can be divided into four units: karst highland, I- and II-level karst slope, and karst basin in Tahe area, and the Block 12 was basically located in II-level karst slope. Three karst cycles developed in the Block 12 according to Early Hercynian tectonism characterized by multiphase pulse-type uplift movement and three sets of cavernous formation formed. II- and III-level cavernous formations mainly developed in Block 12. The Early Hercynian karstification is distributed on the north area of the pinch-out line of Upper Ordovician Sangtamu Formation in the plane, and is vertically developed in the Lower–Middle Ordovician carbonates. The karstification is essentially controlled by the paleo-geomorphology and fault. Fissures and caves formed by the karstification are the keys to affecting reservoir quality. The evolution model of Early Hercynian karstification has been established on the basis of the synthesis of the above-mentioned research results. It can be divided into three stages: rapidly uplifting and denudation stage, three episodes of uplifting and karstification, and subsiding and reworking stage.

Early Precambrian tectonothermal events of the North China Craton: Constraints from in situ detrital zircon U-Pb, Hf and O isotopic compositions in Tietonggou Formation

Clastic sediments and sedimentary rocks are widely used for understanding the formation and evolution of the continental crust. The Tietonggou Formation outcrops in the Xiaoqinling region at the southern margin of the North China Craton (NCC) and has unconformable contacts with the underlying Taihua Complex and overlying Paleoproterozoic Xiong’er Group. It mainly consists of quartzite and its protoliths are mature terrigenous clastic rocks. On the basis of the ages of the youngest detrital zircons from the quartzites and the ages of the Xiong’er Group, the depositional ages of the protoliths of the Tietonggou Formation were well constrained to 1.91–1.80 Ga. The U-Pb isotopic data of detrital zircons from the Formation show a major age peak at ∼2.1 Ga, which is consistent with 2.2–2.0 Ga magmatism in the Trans-North China Orogen of the NCC. Taking into account the texctural and compositional maturity of the Tietonggou Formation quartzite, the ∼2.1 Ga lithologic units in the Trans-North China Orogen are interpreted as the major source of the Tietonggou Formation. The majority of these ∼2.1 Ga detrital zircons mostly have high δ 18O values (>6.5‰) and negative ɛ Hf(t) values (−7.8–0.0), with corresponding Hf model ages significantly older than their crystallization ages, indicating that these zircons formed from the partial melting of ancient continental crust. The majority of the 2.8–2.7 Ga and ∼2.5 Ga detrital zircons from the Tietonggou Formation had positive ɛ Hf(t) values, and mantle-like δ 18O values, suggesting that the NCC has experienced two stages of significant crustal growth in the Neoarchaean at 2.7 and 2.5 Ga, respectively. The Hf isotopic data of detrital zircons from Paleoproterozoic metasedimentary rocks in the Trans-North China Orogen varied mainly toward the reduction of the radiogenic Hf isotope and gradually show a similar trend of the isotope trajectories of crustal evolution. This reveals that the NCC probably has not developed a long-lived subduction to complete the final assembly of the NCC. Alternatively, these maybe imply that the tectonic setting of the NCC substantially changed at ∼2.1 Ga, the reduction of the radiogenic Hf isotope could be attributed to the rollback of the subducting slab.

Geological Milieu of the Bijawar Basin based on interpretation of geophysical data; Central India

SummaryIntegrated interpretation through physical properties of rocks provides better approach towards understanding of geological set-up. High resolution heliborne time domain electromagnetic, magnetic and radiometric dataset acquired over Palaeo Proterozoic Bijawar Basin has been interpreted to enhance the geological understanding. The process involved knowledge driven analysis through correlation with geological units that outcrop and data driven analysis for extrapolation of information. Radiometric data in conjunction with spectral remote sensing data was used for generation of surface geology map. Spatial distribution of rock magnetization properties defines the subsurface extension of litho-structural elements and magnetic basement configuration. Electromagnetic data aided demarcation of resistive basement topography and various conductive layers. Radio-elemental distribution refines the unconformable contact shared by Bijawar Group with overlying and underlying group of rocks and also outlined the formational boundaries of arenaceous-argillaceouscarbonate-volcano-sedimentary sequence. Subsurface extent of basic volcanics is much more than its surface manifestation and appears to extend well below the overlying Vindhyan Supergroup of rocks. The unconformable contact of Bijawar Group with underlying basement and overlying Vindhyan Supergroup is marked by distinct change in conductivity parameter and has been outlined in depth to resistive basement topography. Fault system and its manifestation over different litho-units indicate multiple tectonic episodes.

Geological evolution of the Iraqi Mesopotamia Foredeep, inner platform and near surroundings of the Arabian Plate

Geological evolution of the Iraqi Mesopotamia Foredeep, inner platform and near surroundings of the Arabian Plate

Post-kinematic lithospheric delamination of the Wuyi–Yunkai orogen in South China: Evidence from ca. 435 Ma high-Mg basalts

Understanding the processes responsible for the intra-plate early Paleozoic Wuyi–Yunkai orogeny (>460Ma to 420–415Ma) in the South China Block (SCB) is important for deducing the interactions of this block with other continents at that time, as well as the tectonic evolution of East Asia. One salient feature of the orogen is that despite the wide occurrence of syn- to late-orogenic (440Ma to 420–415Ma) granites in the orogen, neither syn- to late-orogenic volcanic rocks nor mafic rocks of any type have been reported. Such mafic rocks could shed clues about any mantle–crust interaction during such a major orogeny, thus help to understand the dynamics of the orogenic event. We present here, for the first time, geochronological, isotopic and geochemical data for a mafic-intermediate volcanic succession in northern Guangdong, near the edge of the metamorphic core of the orogen. The volcanic rocks unconformably overlie strongly deformed Cambro-Ordovician strata, but are in low-angle unconformable contact with overlying post-orogenic mid-Devonian strata. LA-ICP-MS and SHRIMP U–Pb dating of zircons from two andesitic and dacitic samples gives a consistent crystallization age of ca. 435Ma, younger than the 460–445Ma peak metamorphism of the orogeny but synchronous with the widespread late-orogenic (ca. 440–415Ma) granitic intrusions. Nine least crustally-contaminated basaltic samples are characterized by high MgO (12.3–19.2wt.%), Ni (214–715ppm) and Cr (724–1107ppm), but low TiO2 (0.6–0.8wt.%), Al2O3 (10.2–12.8wt.%) and Fe2O3T (total Fe as Fe2O3) (8.7–11.4wt.%) contents. The basalts also exhibit low Nb/La ratios (0.4–0.8) and constant εNd(t) values (−8.0 to −8.4) with variable SiO2 (44.8–51.5wt.%) contents, suggesting a likely sub-continental lithospheric mantle origin. These high-magnesian basalts have chemical compositions similar to their primary magma, which was estimated using geochemical modeling at SiO2≈50wt.%, MgO≈14wt.% and FeOT≈9wt.%. The estimated potential temperature for the melts is >1300°C, much higher than that of a normal sub-continental lithosphere. This implies that the magma was likely generated from partial melting of lithospheric peridotite heated by hot upwelling asthenosphere. The high-magnesian andesites are interpreted as the products of differentiation and AFC processes from the same basaltic magma source, as supported by their negative zircon εHf(t) values (−21.7 to −6.3) and high zircon δ18O values (7.3–9.0‰). Overall, we interpret that this post-kinematic basalts, plus andesites and dacites as differentiates, resulted from a late-orogenic lithospheric delamination which led to an orogenic collapse, melting of the SCLM, and widespread late-orogenic granitic intrusions in the orogen.

Allostratigraphy of Punung Paleoreef based on Lithofacies Distributions, Jlubang Area, Pacitan Region-East Java

DOI: 10.17014/ijog.v7i2.140 Lithologically, Punung Formation as a paleoreef comprises coral boundstone rhodolith, algal grainstone, algal packstone, algal wackestone, algal floatstone, and algal rudstone. It is dominated by red algae and had formed a fringing reef in a warmly shallow marine environment. They built seven phases of paleoreef complex. Each paleoreef complex has been bounded by a local unconformity that is characterized by caliche. The Jaten Formation has becomes the base of the Punung paleoreef which build up by an angular unconformity contact on uppermost part. It consists of tuffaceous wacky sandstone with silicate cement. The formation as the reef base indicates two factors. The external factor because of the decrease of a volcanic activity and the internal one was caused by the depositional environment of the Jaten Formation becoming shallower. The subsurface runoff systems in many caves (like: Jaran cave and others) have the same southward direction to the dipping direction of algal grainstone lithofacies of Punung Formation. The vertical caves are formed by a jointing system.

A drill hole query algorithm for extracting lithostratigraphic contacts in support of 3D geologic modelling in crystalline basement

The identification and extraction of lithostratigraphic contacts in crystalline basement for constraining 3D geologic models is commonly hampered by the sparseness of diagnostic lithostratigraphic features and the limited availability of geophysical well log data. This paper presents a query algorithm that, instead of using geophysical well log measurements, extracts lithostratigraphic contacts by exploiting diagnostic patterns of lithology-encoded intervals, recurrent in adjacent drill holes. The query algorithm allows defining gaps in the pattern to search across unconformable, intrusive and tectonic contacts and allows combining multiple search patterns in a single query to account for lateral lithofacies variations. The performance of the query algorithm has been tested in the Precambrian Flin Flon greenstone belt (Canada) by evaluating the agreement between queried and logged lithostratigraphic contacts in 52 lithostratigraphic reference drill holes. Results show that the automated extraction of the unconformable and partly tectonized contact between metavolcanic rocks and its metasedimentary cover was relatively unambiguous and matched all the contacts previously established by visual inspection of drill core. The 100% match was nevertheless paired with 23% false positives due to mafic and felsic sills emplaced in sandstone and conglomerate, which overlap in composition and thickness with extrusive volcanic rocks. The automated extraction of the contact between a mine horizon, defined by laterally complex volcanic and volcaniclastic lithofacies variations and overlying basalt flows, matched the visually logged contacts for 83% with 27% false positives. The query algorithm supplements geological interpretation when patterns in drilled lithostratigraphic successions, suspected to be diagnostic for lithostratigraphic contacts, need to be extracted from large drill hole datasets in a systematic and time-efficient manner. The application of the query algorithm is particularly relevant for 3D modelling projects in Precambrian shield areas where unique lithostratigraphic markers are difficult to establish or are completely lacking.

Late Quaternary tectonics of the Pearl River Delta, SE China: Evidence from Xilingang

The Pearl River Delta in SE China is commonly believed as originating from continuous subsidence of the basement fault-(bounded) blocks initiated at the about 60 ka B.P. A section recently exposed at Xilingang in the hinterland of the Pearl River Delta, however, reveals that the fault-block movement is likely periodic or paroxysmal. Quaternary system of the section can be divided into three parts named as layer A, B and C from the bottom upwards. Both layer A and B are alluvial while layer C is aeolian. OSL dating of sediments proves that layer A is older than 43 ka, layer B in between 43-23ka and layer C younger than 23ka. The unconformable contact with a dip angle larger than 30° between layer A and B suggests a paroxysmal tilting and subsidence of basement fault-blocks after the formation of layer A. The upper interface of layer B is about 20 meters high above the sea level while that in the delta plain used to be buried at 20 meters under the S.L., implying a relatively rapid uplifting of the Xilingang hill after formation of layer B. Three layers, i.e. layer A, B and C are all cut off by a NNE trending normal fault with a fault-throw of 53cm, suggesting that the third paroxysmal movement of fault-blocks in the Pearl River Delta were occurred in recent epoch.

Polyphase Neoproterozoic orogenesis within the East Africa–Antarctica Orogenic Belt in central and northern Madagascar

Abstract Our recent geological survey of the basement of central and northern Madagascar allowed us to re-evaluate the evolution of this part of the East Africa–Antarctica Orogen (EAAO). Five crustal domains are recognized, characterized by distinctive lithologies and histories of sedimentation, magmatism, deformation and metamorphism, and separated by tectonic and/or unconformable contacts. Four consist largely of Archaean metamorphic rocks (Antongil, Masora and Antananarivo Cratons, Tsaratanana Complex). The fifth (Bemarivo Belt) comprises Proterozoic meta-igneous rocks. The older rocks were intruded by plutonic suites at c. 1000 Ma, 820–760 Ma, 630–595 Ma and 560–520 Ma. The evolution of the four Archaean domains and their boundaries remains contentious, with two end-member interpretations evaluated: (1) all five crustal domains are separate tectonic elements, juxtaposed along Neoproterozoic sutures and (2) the four Archaean domains are segments of an older Archaean craton, which was sutured against the Bemarivo Belt in the Neoproterozoic. Rodinia fragmented during the early Neoproterozoic with intracratonic rifts that sometimes developed into oceanic basins. Subsequent Mid-Neoproterozoic collision of smaller cratonic blocks was followed by renewed extension and magmatism. The global ‘Terminal Pan-African’ event (560–490 Ma) finally stitched together the Mid-Neoproterozoic cratons to form Gondwana.

High‐resolution seismic images of potentially seismogenic structures beneath the northwest Canterbury Plains, New Zealand

The transpressional boundary between the Australian and Pacific plates in the central South Island of New Zealand comprises the Alpine Fault and a broad region of distributed strain concentrated in the Southern Alps but encompassing regions further to the east, including the northwest Canterbury Plains. Low to moderate levels of seismicity (e.g., 2 > M 5 events since 1974 and 2 > M 4.0 in 2009) and Holocene sediments offset or disrupted along rare exposed active fault segments are evidence for ongoing tectonism in the northwest plains, the surface topography of which is remarkably flat and even. Because the geology underlying the late Quaternary alluvial fan deposits that carpet most of the plains is not established, the detailed tectonic evolution of this region and the potential for larger earthquakes is only poorly understood. To address these issues, we have processed and interpreted high‐resolution (2.5 m subsurface sampling interval) seismic data acquired along lines strategically located relative to extensive rock exposures to the north, west, and southwest and rare exposures to the east. Geological information provided by these rock exposures offer important constraints on the interpretation of the seismic data. The processed seismic reflection sections image a variably thick layer of generally undisturbed younger (i.e., < 24 ka) Quaternary alluvial sediments unconformably overlying an older (>59 ka) Quaternary sedimentary sequence that shows evidence of moderate faulting and folding during and subsequent to deposition. These Quaternary units are in unconformable contact with Late Cretaceous–Tertiary interbedded sedimentary and volcanic rocks that are highly faulted, folded, and tilted. The lowest imaged unit is largely reflection‐free Permian ‐ Triassic basement rocks. Quaternary‐age deformation has affected all the rocks underlying the younger alluvial sediments, and there is evidence for ongoing deformation. Eight primary and numerous secondary faults as well as a major anticlinal fold are revealed on the seismic sections. Folded sedimentary and volcanic units are observed in the hanging walls and footwalls of most faults. Five of the primary faults represent plausible extensions of mapped faults, three of which are active. The major anticlinal fold is the probable continuation of known active structure. A magnitude 7.1 earthquake occurred on 4 September 2010 near the southeastern edge of our study area. This predominantly right‐lateral strike‐slip event and numerous aftershocks (ten with magnitudes ≥5 within one week of the main event) highlight the primary message of our paper: that the generally flat and topographically featureless Canterbury Plains is underlain by a network of active faults that have the potential to generate significant earthquakes.

EXTENDED DISTRIBUTION OF NATAL GROUP WITHIN SOUTHERN KWAZULU-NATAL, SOUTH AFRICA: IMPLICATIONS FOR SEDIMENT SOURCES AND BASIN STRUCTURE

The southern margin of the basin that contains the Ordovician (~490 Ma) Natal Group has long been defined by the “Dweshula palaeo-topographic high” located near Port Shepstone in southern KwaZulu-Natal, South Africa. Six recently discovered outcrops of Natal Group rocks occurring up to 30 km south of the Dweshula palaeo-topographic high have prompted a re-evaluation of the basin morphology. These exposures consist of maroon-coloured arenaceous and argillaceous sediments attaining a maximum thickness of ~46 m, with a mean of 15 m, which are correlated with the Eshowe Member of the Natal Group. The unconformable contact with the overlying Devonian Msikaba Formation is preserved in four outcrops and provides additional evidence that the Msikaba Formation is younger than the Natal Group. The Natal Group lithologies are dominated by laterally variable arkosic arenite, quartz wacke and siltstone, which appear to have been deposited as proximal channel and overbank sediments in a fluvial, braided alluvial plain environment. The extreme mineralogical and textural immaturity of the lithologies suggests a proximal derivation from the underlying Mesoproterozoic granitoids and gneisses of the Natal Metamorphic Province (NMP). Localized granitic-boulder conglomerate and reworked sediment drapes of granitic origin, also derived from NMP granitoids, are preserved directly beneath the unconformable contact with the Msikaba Formation. These localised units that separate the two successions represent deposition of NMP clasts by flood flow during the ~100 Myr erosional hiatus that separates the fluviatile Natal Group from the shallow marine Msikaba Formation.

Contact relationships of Tertiary volcanic units from the Andes of central Chile (33°S): a reinterpretation based on 40Ar/39Ar dating.

Se dan a conocer ocho edades 40Ar/39Ar de rocas de dos sectores de la Cordillera de los Andes cerca de los 33°S donde se presentan unidades volcanicas separadas por discordancias angulares. Estos sectores corresponden a la Cuesta de Chacabuco en la continuacion norte del Valle Central y al Cerro Las Ollas en la parte oeste de la Cordillera de los Andes. En Cuesta de Chacabuco se reconocio la unidad Chacabuco, formada principalmente por basaltos, andesitas basalticas y rocas sedimentarias clasticas continentales. Esta unidad esta cubierta, con discordancia angular, por flujos piroclasticos de la unidad Algarrobo, un complejo volcanico erosionado. Una edad plateau 40Ar/39Ar (roca total, basalto) de 28,8 ± 0,3 Ma fue interpretada como la mejor estimacion de la edad del volcanismo para la unidad Chacabuco, mientras que las edades plateau obtenidas sobre anfibolas de una lava de andesita basaltica y de un dique andesitico para la unidad Algarrobo, fueron 19,6 ± 0,3 y 18,6±0,4 Ma, respectivamente. Se detecto asi un hiatus del orden de 8,6 Ma entre estas dos unidades. El Cerro Las Ollas ha sido un lugar clasico en la discusion acerca de la naturaleza del contacto entre unidades que han sido atribuidas a las formaciones Abanico y Farellones el que ha sido interpretado como discordancia de plegamiento y erosion o como contacto tectonico. Las nuevas edades obtenidas sobre biotita en tobas muestran una continuidad temporal para rocas situadas bajo y sobre el plano de contacto, evidenciando la pertenencia de estas tobas a un mismo evento volcanico. La mejor estimacion de edad para este evento es la edad media ponderada de 20,1 ± 0,1 Ma. En consecuencia, la relacion discordante observada en el cerro Las Ollas puede ser interpretada como intraformacional.