Regional Angular Unconformity Research Articles

Role of Faults in Hydrocarbon Leakage in the Hammerfest Basin, SW Barents Sea: Insights from Seismic Data and Numerical Modelling

Hydrocarbon prospectivity in the Greater Barents Sea remains enigmatic as gas discoveries have dominated over oil in the past three decades. Numerous hydrocarbon-related fluid flow anomalies in the area indicate leakage and redistribution of petroleum in the subsurface. Many questions remain unanswered regarding the geological driving factors for leakage from the reservoirs and the response of deep petroleum reservoirs to the Cenozoic exhumation and the Pliocene-Pleistocene glaciations. Based on 2D and 3D seismic data interpretation, we constructed a basin-scale regional 3D petroleum systems model for the Hammerfest Basin (1 km × 1 km grid spacing). A higher resolution model (200 m × 200 m grid spacing) for the Snøhvit and Albatross fields was then nested in the regional model to further our understanding of the subsurface development over geological time. We tested the sensitivity of the modeled petroleum leakage by including and varying fault properties as a function of burial and erosion, namely fault capillary entry pressures and permeability during glacial cycles. In this study, we find that the greatest mass lost from the Jurassic reservoirs occurs during ice unloading, which accounts for a 60%–80% reduction of initial accumulated mass in the reservoirs. Subsequent leakage events show a stepwise decrease of 7%–25% of the remaining mass from the reservoirs. The latest episode of hydrocarbon leakage occurred following the Last Glacial Maximum (LGM) when differential loading of Quaternary strata resulted in reservoir tilt and spill. The first modeled hydrocarbon leakage event coincides with a major fluid venting episode at the time of a major Upper Regional angular Unconformity (URU, ~0.8 Ma), evidenced by an abundance of pockmarks at this stratigraphic interval. Our modelling results show that leakage along the faults bounding the reservoir is the dominant mechanism for hydrocarbon leakage and is in agreement with observed shallow gas leakage indicators of gas chimneys, pockmarks and fluid escape pipes. We propose a conceptual model where leaked thermogenic gases from the reservoir were also locked in gas hydrate deposits beneath the base of the glacier during glaciations of the Hammerfest Basin and decomposed rapidly during subsequent deglaciation, forming pockmarks and fluid escape pipes. This is the first study to our knowledge to integrate petroleum systems modelling with seismic mapping of hydrocarbon leakage indicators for a holistic numerical model of the subsurface geology, thus closing the gap between the seismic mapping of fluid flow events and the geological history of the area.

Neoproterozoic Jiangnan Orogeny in southeast Guizhou, South China: evidence from U–Pb ages for detrital zircons from the Sibao Group and Xiajiang Group

The Jiangnan Orogeny generated regional angular unconformities between the Xiajiang Group and the underlying Sibao Group in the western Jiangnan Orogen along the southeastern margin of the Yangtze Block in southeast Guizhou, South China. Laser ablation – inductively coupled plasma – mass spectrometry (LA–ICP–MS) U–Pb zircon dating of two samples of the Motianling granitic pluton yielded U–Pb zircon ages of 826.2 ± 3.4 and 825.5 ± 6.1 Ma, with an average age of 825.6 ± 3.0 Ma, which is considered the minimum depositional age of the Sibao Group. The U–Pb ages of the youngest detrital zircon grains from the Sibao Group and the Xiajiang Group yielded average ages of 834.9 ± 3.8 and 794.6 ± 4.2 Ma, respectively. The depositional age of the Sibao Group can be constrained at 825–835 Ma, and deposition of the Xiajiang Group did not begin before ca. 800 Ma. These results suggest that the Jiangnan Orogeny, which led to the assembly of the Yangtze and Cathaysia blocks, ended at 795–835 Ma on the western segment of the Jiangnan Orogen. The detrital zircon distribution spectrums of the Sibao and Xiajiang groups suggest a provenance from Neoproterozoic basement sedimentary sequences along with a mixture of local Neoproterozoic subduction-related felsic granitoids, distant plutons from the western Yangtze Block and eastern Jiangnan Orogen, and recycled materials from the interior of the Yangtze Block. By comparing the basin evolution histories and magmatic and metamorphic events along the continental margins of the Rodinia supercontinent, it is proposed that the South China Block might have been located at the periphery, adjacent to North India and East Antarctica, rather than in the interior of Rodinia in Neoproterozoic time.

Continental shelf morphology and stratigraphy offshore San Onofre, California: The interplay between rates of eustatic change and sediment supply

Abstract New high-resolution CHIRP seismic data acquired offshore San Onofre, southern California reveal that shelf sediment distribution and thickness are primarily controlled by eustatic sea level rise and sediment supply. Throughout the majority of the study region, a prominent abrasion platform and associated shoreline cutoff are observed in the subsurface from ~ 72 to 53 m below present sea level. These erosional features appear to have formed between Melt Water Pulse 1A and Melt Water Pulse 1B, when the rate of sea-level rise was lower. There are three distinct sedimentary units mapped above a regional angular unconformity interpreted to be the Holocene transgressive surface in the seismic data. Unit I, the deepest unit, is interpreted as a lag deposit that infills a topographic low associated with an abrasion platform. Unit I thins seaward by downlap and pinches out landward against the shoreline cutoff. Unit II is a mid-shelf lag deposit formed from shallower eroded material and thins seaward by downlap and landward by onlap. The youngest, Unit III, is interpreted to represent modern sediment deposition. Faults in the study area do not appear to offset the transgressive surface. The Newport Inglewood/Rose Canyon fault system is active in other regions to the south (e.g., La Jolla) where it offsets the transgressive surface and creates seafloor relief. Several shoals observed along the transgressive surface could record minor deformation due to fault activity in the study area. Nevertheless, our preferred interpretation is that the shoals are regions more resistant to erosion during marine transgression. The Cristianitos fault zone also causes a shoaling of the transgressive surface. This may be from resistant antecedent topography due to an early phase of compression on the fault. The Cristianitos fault zone was previously defined as a down-to-the-north normal fault, but the folding and faulting architecture imaged in the CHIRP data are more consistent with a strike-slip fault with a down-to-the-northwest dip-slip component. A third area of shoaling is observed off of San Mateo and San Onofre creeks. This shoaling has a constructional component and could be a relict delta or beach structure.

An overview of the Cretaceous stratigraphy and facies development of the Yazd Block, western Central Iran

The Cretaceous successions of the Yazd Block, the western of three structural blocks of the Central-East Iranian Microcontinent (CEIM), have been studied using an integrated approach of litho-, bio- and sequence stratigraphy associated with litho-, bio- and microfacies analyses. The Cretaceous System of that area is in excess of 5km thick and a generalized relative sea-level curve can be inferred from the facies and thickness development. This curve can be subdivided into two transgressive–regressive megacycles (TRMs), separated by a major tectonic unconformity in the Upper Turonian. TRM 1 comprises the Early Cretaceous to Middle Turonian, TRM 2 the Coniacian to Maastrichtian. TRM 1 starts with up to 1500-m-thick conglomerates and sandstones covering Palaeozoic–Triassic basement rocks, metasediments, or Upper Jurassic–lowermost Cretaceous granites. The basal tectonic unconformity, related to the Late Cimmerian event (Jurassic–Cretaceous boundary interval), shows a pronounced palaeo-relief that is levelled by the basal siliciclastic formations. Sparse biostratigraphic data from calcareous intercalations in the upper part of these strata indicate a Hauterivian to Barremian age. The Aptian facies development is marked by the onlap of thick-bedded, micritic carbonates with abundant orbitolinid foraminifera and rudists representing a large-scale shallow-marine carbonate platform system that fringed the Yazd Block in the north and west. These platforms are up to 1000m thick and drowned during the middle to Late Aptian, followed by up to 1500-m-thick basinal marly sediments of Late Aptian to mid-Late Albian ages, representing the maximum relative sea-level during TRM 1. During the latest Albian–Middle Turonian, a gradual shallowing is indicated by progradation of shallow-water skeletal limestones separated by marl tongues, representing a carbonate ramp system. Strata of TRM 2 overlie older units along a regional angular unconformity and indicate tectonic stability and lowered subsidence rates as shown by widespread uniform Coniacian–Maastrichtian facies and thickness development. Above a thick basal conglomerate, the Coniacian–Campanian facies is characterized by shallow-water limestones of a large-scale epeiric platform. Relative sea-level peaked in the Late Campanian, followed by Maastrichtian infilling. The Cretaceous succession is truncated along a tectonic unconformity at the base of the Palaeocene. The major tectonic unconformities recognized at the base of the succession (Late Cimmerian Event), in the Late Turonian, and in the Cretaceous–Palaeogene boundary interval are evidence of significant tectonic activity of the Yazd Block. Their formation is probably related to plate tectonic processes in response to the opening and closure of ocean basins surrounding the CEIM.

Two-phase subduction and subsequent collision defines the Paleotethyan tectonics of the southeastern Tibetan Plateau: Evidence from zircon U-Pb dating, geochemistry, and structural geology of the Sanjiang orogenic belt, southwest China

A curved belt consisting of terranes or blocks and associated late Paleozoic to early Mesozoic sutures are features of the southeastern part of the Tibetan Plateau. The southeastern Tibetan Plateau Paleotethyan belt is divided into a northwestern segment (central Tibetan Plateau) and a southern segment (the Sanjiang orogenic belt). The Paleotethyan tectonic history of this belt remains a subject of considerable debate. A major controversy concerns whether or not it evolved as a single genetic element. This paper provides new geochronological, geochemical, and structural data to constrain the Paleotethyan tectonics of this belt. A detailed field study across three sections of the Sanjiang orogenic belt has identified two volcaniclastic successions separated by a regional angular unconformity in the northwestern segment, and one continuous volcaniclastic succession in the southern segment. New zircon U-Pb ages reveal that: (1) the lower succession of the northwestern segment was produced by three volcanic phases at ca. 272 Ma, 253–247 Ma, and 240–235 Ma; (2) the upper succession of the northwestern segment formed during a shorter period along the Yushu–Yidun arc from ca. 227 to 211 Ma; and (3) the single volcaniclastic succession of the southern segment was produced continuously from ca. 260 to 210 Ma. Whole-rock geochemical data including trace elements show that the late Paleozoic to early Mesozoic magmatic activity has a continental arc–like affinity. The spatial distribution and field relationships of the magmatic rocks define two continuous volcanic arcs in the southeastern Tibetan Plateau, which correspond to two phases of arc-like magmatism. The Late Triassic Yushu–Yidun arc belt is exposed along the northern half of the northwestern segment to the south of the western Jinsha–Garze–Litang suture. Southward subduction along the western Jinsha–Garze–Litang suture is likely to have generated the Yushu–Yidun arc. The second arc (Jomda–Weixi–Yunxian) was diachronous and is continuous through the southeastern Tibetan Plateau. The northwestern part of the Jomda–Weixi–Yunxian arc belt is overlain by the Yushu–Yidun arc from the north. From northwest to southeast, the cessation of magmatism in the Jomda–Weixi–Yunxian arc varies from the Middle Triassic (ca. 235 Ma) to the Late Triassic (ca. 210 Ma), whereas the onset of magmatism appears to have been in the Early Permian throughout the arc. Zircon Lu-Hf isotopic data suggest that large degrees of melting of a depleted mantle source produced the earliest magmas in each segment of the Jomda–Weixi–Yunxian arc. The similarity in zircon ages of the ophiolitic units and their distribution imply that the Longmu Co–Shuanghu suture is a continuation of the Changning–Menglian suture. The spatial relationship between the Jomda–Weixi–Yunxian arc and the Longmu Co–Shuanghu–Changning–Menglian suture suggests that diachronous eastward subduction of part of the Paleotethys along the suture produced the Jomda–Weixi–Yunxian arc belt. Widespread folds of the continental arc–volcaniclastics with orogen-parallel hinges reflect compression perpendicular to the orogen, subsequent to arc magmatism. As such, the Paleotethyan tectonics of the southeastern Tibetan Plateau are characterized by two-phase subduction and subsequent two-phase continent-continent collision.

Origin of the ca. 90 Ma magnesia-rich volcanic rocks in SE Nyima, central Tibet: Products of lithospheric delamination beneath the Lhasa-Qiangtang collision zone

Bulk-rock major and trace element, Sr–Nd–Hf isotope, zircon U–Pb age, and zircon Hf isotopic data of the Late Cretaceous Zhuogapu volcanic rocks in the northern Lhasa subterrane provide a new insight into tectonic processes following the collision of the terrane with the Qiangtang zone. SHRIMP zircon U–Pb dating reveals that the Zhuogapu volcanic rocks crystallized at ca. 91Ma, postdating the development of a regional angular unconformity between the Upper Cretaceous and the underlying strata in the Lhasa–Qiangtang collision zone. Compared to the Andean arc-type andesites and dacites, the Zhuogapu volcanic rocks are characterized by higher MgO of 2.78–5.86wt.% and Mg# of 54–64 for andesites and MgO of 2.30–2.61wt.% and Mg# of 55–58 for dacites. Eight andesite samples have whole-rock (87Sr/86Sr)i of 0.7054–0.7065, εNd(t) of −3.2 to −1.7, and εHf(t) of +3.8–+6.4, similar to those of the three dacite samples with (87Sr/86Sr)i=0.7056–0.7060, εNd(t) of −2.7 to −2.2, and εHf(t) of +5.6–+7.0. Thirteen analyses from a dacite sample give positive zircon εHf(t) of +5.6 to +8.7. These signatures indicate that the Zhuogapu Mg-rich andesites were most likely derived from partial melting of a delaminated mafic lower crust (including the lowermost crust straddling the northern and central Lhasa subterranes) that led to the generation of the Zhuogapu primary melts with adakitic signatures and small negative εNd(t). Such melts subsequently experienced interaction of melt-asthenospheric mantle peridotite followed by the modification of highly fractionated magmas in shallow crustal magma chamber. Hornblende-controlled fractionation results in the change of geochemical composition from Mg-rich andesitic to Mg-rich dacitic magmas. Field observations, together with geochronological and geochemical data, indicate that the Zhuogapu Mg-rich volcanic rocks and coeval magmatism in the northern Lhasa subterrane may be the result of thickened lithospheric delamination following the final Lhasa–Qiangtang amalgamation.

U–Pb LA-ICP-MS detrital zircon ages of the São João del Rei and Carandaí basins: New evidence of intermittent Proterozoic rifting in the São Francisco paleocontinent

The Tiradentes Formation (São João del Rei basin) is a shallow marine quartzite succession resting on the Paleoproterozoic crust of the southern part of the São Francisco Craton. A regional angular unconformity separates the Tiradentes strata from the younger Carandaí, Barroso and Prados Formations (Carandaí basin). The Carandaí Formation is constituted of fault-bounded diamictites containing lithoclasts derived from basement rocks and the Tiradentes Formation. The Barroso Formation is a sag basin limestone succession, whereas the Prados Formation is a pelitic succession that fills a NE–SW-trending aulacogen. Weak deformation and metamorphism (600–590Ma) overprinted the São João del Rei and Carandaí sequences due to the Brasiliano Orogen and, likewise, the overlying Andrelândia Megasequence (<950Ma). Detailed field geology and U–Pb LA-ICP-MS detrital zircon ages from the Tiradentes quartzites and the Carandaí diamictites indicate that these units are associated with two separate Mesoproterozoic rifting episodes. The youngest zircon grains extracted from the basal and top Tiradentes quartzites (1536±33Ma and 1540±45Ma) estimate the maximum age of the initial extension and sedimentation. The timing of the second extensional phase is marked by the youngest zircon age for the Carandaí diamictites (1379±3Ma). These rifting episodes allow a tectonic relationship with two basin cycles of the Espinhaço intracratonic rift system, as suggested by geologic correlations and geochronology. The Prados pelites represent renewed rifting genetically related to the Andrelândia Megasequence; one sheared quartzite that is probably related to the lower unit of the Andrelândia sequence contains zircon grains as young as 1145±79Ma. This maximal depositional age also allows a tectonic correlation with the ultimate extensional episode of the Espinhaço system, according to geologic and geochronologic evidence. From a tectonic view point, the studied rift sequences are related with the evolution of the Espinhaço intracratonic system, allowing a better understanding of intermittent extensional and basin tectonics in the São Francisco/Congo paleocontinent at Mesoproterozoic times.

Tectonic Setting of the Cu-Ni Sulfide-Bearing Mafic-Ultramafic Complexes in Northern Jilin Province, NE China

There are series of Cu-Ni sulfide-bearing mafic-ultramafic intrusions widespread in north JilinProvince,Northeastern China. The intrusions formed in Xing’an-Mongolian Orogenic Belt near to the northeastern margin of North China Craton. The complexes were formed in almost same period according to the zircon U-Pb dating reported recently, which means that the complexes were formed in same tectonic period and belong to one tectonic magmatic event. The rock assemblages are different from the ophiolite type and Yidun type in orogenic belt. The mafic-ultramafic complexes formed in the range from 217 Ma to 232 Ma coeval with A-type granites in the area, which formed bimodal igneous rock assemblage. According to the regional angular unconformities, there were existed the orogenies of Caledonian, Hercynian, Early Indosinian, Late Indosinian and Yanshanian. The Early Indosinian coeval with orogenic I-type granites and sanukitie that suggesting the lithosphere thickening in the extrusion tectonic setting of orogenic processes, however the Late Indosinian coeval with bimodal igneous rock assemblage that suggesting the lithosphere thinning in the extension tectonic setting of post-orogenic processes in the Xing’an-Mongolian Orogenic Belt. Chemical composition of the mafic-ultramafic rocks has the characteristics of high-Mg and low-K tholeiites related with inter-continental post-orogenic tectonic setting. The trace elements indicate their formed in conditions of continental extension belt or initial rift and has the characteristics of revolution from oceanic island arc, volcanic arc of continental margin to continental extended belt. The low initial Sr isotopic ratios and positive εNd(t) values suggest that the initial magma of the complexes come from the parting melting of depleted lithospheric mantle. The depleted ithospheric mantle was new formed supported by zircon Hf isotope in Hongqiling complex. The depleted lithospheric mantle may be caused by the asthenosheric mantle upwelling and underplating in the tectonic setting of extension during the Late Indosinian post-orogenic processes.

Neoproterozoic tectonic evolution of the Jebel Saghro and Bou Azzer—El Graara inliers, eastern and central Anti-Atlas, Morocco

New mapping, geochemistry, and 17 U–Pb SHRIMP zircon ages from rocks of the Sirwa, Bou Azzer–El Graara, and Jebel Saghro inliers constrain the Neoproterozoic evolution of the eastern Anti-Atlas during Pan-African orogenesis. In the Sirwa inlier, Tonian quartzite from the pre Pan-African passive margin deposits of the Mimount Formation contains detrital zircon derived entirely from the West African Craton (WAC), with most grains yielding Eburnean Paleoproterozoic ages of about 2050Ma. Cryogenian Pan-African orogenic activity (PA1) from about 760 to 660Ma included northward-dipping subduction to produce a volcanic arc, followed by ophiolite obduction onto the WAC. In the Bou Azzer–El Graara inlier, calc-alkaline granodiorite and quartz diorite, dated at 650–646Ma, are syn- to post-tectonic with respect to the second period of Pan-African orogenesis (PA2), arc-continent accretion, and related greenschist facies metamorphism. Slab break-off and lithospheric delimination may have provided the source for the supra-subduction calc-alkaline plutons. At about 646Ma, quartz diorite intruded the Tiddiline formation placing an upper limit on molassic deposition. Widespread Ediacaran high-K calc-alkaline to shoshonitic plutonism and volcanism during the final stage of Pan-African orogenesis (PA3) occurred in a setting related to either modification of the margin of the WAC or formation of a continental volcanic arc above a short-lived southward-dipping subduction zone. In the Saghro inlier, eight plutonic rocks yield ages ranging from about 588 to 556Ma. Sampled plutonic rocks previously considered to be Cryogenian yielded Ediacaran ages. Peraluminous rhyolitic volcanic rocks in the lower part of the Ouarzazate Supergroup, including ash-flow tuffs of the Oued Dar’a caldera, yield ages between about 574 and 571Ma. The Oued Dar’a caldera developed in a pull-apart graben produced by a left-step in a northeast-trending, left-lateral strike-slip fault zone, and much of the lower Ouarzazate Supergroup volcanic rocks in the area are probably related to caldera out-flow facies and collapse. Late stage PA3 intrusive rocks include the Bouskour–Sidi Flah and Timijt rhyolitic dike swarms at about 563Ma, the voluminous pink Isk-n-Alla granite (559±5Ma), and volumetrically minor gabbro of Tagmout (556±5Ma). Rhyolite flows from the upper part of the Ouarzazate Supergroup, above a regional angular unconformity, yielded ages of 558±4 and 556±4Ma. The youngest ages place an upper limit on block faulting and weak folding during latest Pan-African tectonic activity (PA3), coincident with the departure of the Cadomian crustal fragment from the northern margin of the WAC.

The Palu-Uluova Strike-Slip Basin in the East Anatolian Fault System, Turkey: Its Transition from the Palaeotectonic to Neotectonic Stage

The East Anatolian fault system (EAFS) is the 80-km-wide, 700-km-long, NE-trending sinistral strike-slip fault system forming a seismically very active intracontinental transfom fault boundary. It is located between Karlıova County in the northeast and Karataş-Samandağ counties in the southwest, and forms the southeastern boundary of the Anatolian platelet. The Palu-Uluova basin is one of several strike-slip basins located along the EAFS. It is surrounded by several push-ups such as the Karaömerdağı, Mastardağı and Askerdağı push-ups caused by the complexities peculiar to strike-slip faulting. The Palu-Uluova basin consists of three sub-sections: two are NE-trending strike-slip sub-basins, the Uluova and the Palu-Kumyazı sub-basins, while the third is a ramp basin, the E-W-trending Yolüstü basin which links the earlier two sub-basins. The Palu-Uluova basin is characterized and shaped by a 130-m-thick neotectonic basin infill (Palu Formation) and a series of bounding strike-slip fault zones such as the Sivrice, Adıyaman, Uluova, Elazığ, Pertek and Yolüstü fault zones. The Palu Formation is an undeformed fluvio-lacustrine sedimentary sequence. The youngest palaeotectonic rock-stratigraphic unit is the Upper Miocene-Lower Pliocene Çaybağı Formation, deposited in a ramp type of intermontane basin bounded and controlled by the reverse faults. The Çaybağı Formation is intensely deformed (steeply tilted, folded and thrust to reverse-faulted) on a regional (mappable) scale. The compressional deformation pattern of the Çaybağı Formation is truncated, sealed and overlain with angular unconformity by the nearly horizontal undeformed Plio-Quaternary Palu Formation. This regional angular unconformity reflects: (a) a series of pre-Late Pliocene regional tectonic inversions (e.g., type of the tectonic regime, style of deformation and nature of magmatic activity), and (b) the timing of the major transition from the folding and thrust to reverse faulting-dominated palaeotectonic period into the strike-slip faulting-dominated neotectonic period is Late Pliocene.

Sedimentary and palaeoenvironmental evolution of the Junggar Basin, Xinjiang, Northwest China

This review paper summarizes the sedimentary and palaeoenvironmental evolution of the Junggar Basin in Northwest China largely based on hardly accessible Chinese language papers, and complemented by own field observations and a critical survey of key sediment cores from petroleum wells. We have combined this information and updated existing lithofacies and isopach maps for characteristic time slices of basin evolution and palaeoenvironmental change. The Junggar Basin was initiated during the late stage of collisional tectonics in the southern Central Asian Orogenic Belt (Altaids) since the Early Permian. According to studies in surrounding mountain chains and geophysical surveys, the basement consists of a collage of oceanic basins, intraoceanic island arcs, and microcontinents of Precambrian to Palaeozoic age. The basin fill is subdivided into three tectonically controlled stratigraphic sequences which are separated by two regional angular unconformities. The first cycle in the Permian and Triassic is characterized by an Early Permian extensional strike-slip and a Late Permian to Triassic compressional foreland setting. After an Early Permian marine regression, persistent nonmarine fluvio-lacustrine conditions were established containing probably the thickest organic-rich mudstone interval in the world, which act as major source rocks of the basin. Starting with four depocenters, the basin was unified during the Triassic. The preserved total maximum thickness of this cycle is about 8,500 m in the southern depocenter. During the second intracontinental depression cycle, subsidence slowed down and the depocenter migrated towards the basin center reaching a maximum thickness of 6,000 m. The palaeoenvironment was dominated by a large oscillating freshwater lake receiving changing quantities of clastic sediments from the surrounding mountain ranges and forming alluvial fans, braid plains, and deltas partly containing coal seams of economic interest. Sedimentary facies, pollen, and palaeobotanical plant fossils show an overall aridization trend and a shrinking lake cover. During the Neogene cycle, the depocenter migrated back to the south and the former asymmetric foreland basin was reactivated due to thrusting and rapid uplift of the Tian Shan. The maximum thickness of these molasse-type deposits exceeds 5,000 m. Despite its strong potential, there is still a lack of high resolution bio- and cyclostratigraphy, sequence stratigraphy, and palaeoclimate studies in the Junggar Basin to elucidate local versus regional palaeo-environmental patterns and to better constrain far-distance tectonic forcing.

Early uplift and orogenic deformation in the Neuquén Basin: Constraints on the Andean uplift from U–Pb and Hf isotopic data of detrital zircons

Provenance of detrital sediments of the Neuquén Basin in Central Argentina was investigated using U–Pb and Lu–Hf isotopic composition of zircon grains in order to evaluate the timing of uplift of the southern Andes at these latitudes (36°–39°S). Samples of fluvial synorogenic deposits of the Candeleros Formation, at the base of the Neuquén Group (Upper Cretaceous), as well as from older deposits of the Rayoso and Agrio Formations (Lower Cretaceous) were investigated. A regional angular unconformity separates the Upper from the Lower Cretaceous units and is generally attributed to the onset of the foreland basin stage. The evident differences between these suites (above and below the unconformity) are clearly shown by their distinct provenance patterns, which correspond to different bedrock sources and patterns of dispersal. Detrital zircons of the Rayoso and Agrio Formations beneath the unconformity indicate provenance from the eastern foreland basement, presently exposed along the Sierras Pampeanas of central Argentina. Detrital zircon ages correspond to the Permian and Triassic Choiyoi province, the Ordovician Famatinian orogen, the Lower Cambrian–Neoproterozoic Pampean orogen, the Grenvillian age Mesoproterozoic basement and some other older sources. In contrast, detrital zircons above the unconformity, from the Candeleros Formation, have striking different prominent peaks. The pattern indicates direct derivation from the Early Cretaceous magmatic arc, probably from plutons exhumed during the Late Cretaceous deformation along the western Chilean slope of the Andean cordillera. The contrasting dispersal patterns are interpreted as a result of the uplift of the southern Central Andes at these latitudes. The youngest detrital zircons constrain the timing of uplift as being younger than 99 Ma, supporting the assumed Cenomanian age for the synorogenic deposits. This age is further constrained by fission-track data from zircons from an ash-tuff layer of the lowest parts of the Neuquén Group, which yielded ages of about 88 Ma. The Hf isotopic data indicate derivation from a juvenile mantle for the Grenvillian age zircons, which corroborates the previously assumed island arc setting. The intermediate Hf isotopic values for the Pampean age zircons indicate a transition to the extensive recycling of the continental crust for the oldest Paleoproterozoic zircons with negative values that characterize the foreland basement.

Cenozoic stratigraphic development in the north Chilean forearc: Implications for basin development and uplift history of the Central Andean margin

Analysis of the Cenozoic stratigraphic development of the forearc of northern Chile between 18°S and 23°30′S, allows constraints to be placed on the timing and nature of basin formation and the uplift history of the Central Andes. Chronostratigraphic charts have been constructed from 20 lithostratigraphic sections distributed throughout the forearc. Sections were taken from the Longitudinal Valley, Central Depression, Calama Basin, Salar de Atacama, Precordillera and the western flank of the Western Cordillera. Correlation and timing of events is largely based on the presence of dated volcanic horizons in all the studied sections. Three chronostratigraphic units are defined based upon the presence of regional unconformities. Deposition of the Late Eocene to Early Miocene chronostratigraphic unit (38–19 Ma) commenced across an irregular unconformity surface between ∼ 38 and 30 Ma with alluvial fan and fluvial sediments derived from the east interbedded with rhyolitic ignimbrites. Aggradation after 25 Ma resulted in development of a large broad basin over much of northern Chile that expanded eastwards through onlap onto basement. Deposition terminated around 19 Ma with the development of an angular unconformity over much, but not all of the study area. During deposition of the Early to Late Miocene chronostratigraphic unit (18–10 Ma) emergent volcanic source areas to the east provided catchments for large fluvial systems that drained westwards into endorheic ephemeral lacustrine basins. Fold growth affected sedimentation restricting accommodation space to small intra-thrust basins in the Precordillera and localised disruption and unconformity development in the Longitudinal Valley. The Late Miocene to present day chronostratigraphic unit (10–0 Ma) followed the development of a regional angular unconformity at 10 Ma. Sedimentation was restricted to a series of thrust-bounded endorheic basins in both the Central Depression and the Precordillera sourced from the east with volcanic activity limited to the periodic eruption of extensive ignimbrite sheets. Alluvial fan, fluvial and lacustrine sedimentation dominated within the endorheic basins from ∼ 8 to 3 Ma. After development of a regional unconformity at 3 Ma a change to isolated evaporite sub-basins took place in the Central Depression with small lacustrine basins developed along the flank of the Western Cordillera. The scale and grain size recorded in the sedimentary systems indicates that a substantial source area was located in the present day area of the Western Cordillera by 30 Ma and that this has persisted to the present day. This area also shed material eastwards into the Altiplano. The presence of such a topographic feature by 30 Ma suggests that a significant proportion of Andean uplift had occurred prior to the Late Miocene. This important uplift phase should be incorporated into any model of Andean uplift. The evidence from the basin-fill succession suggests that sediments accumulated in a basin developed in front of a broad monocline between 38 and 19 Ma and that a transition to a thrust-bounded foreland style basin took place after the development of the unconformity at 19 Ma.

Zircon U–Pb dating and in-situ Hf isotopic analysis of Permian peraluminous granite in the Lhasa terrane, southern Tibet: Implications for Permian collisional orogeny and paleogeography

The Lhasa terrane has long been interpreted as a simple tectonic block rifted from Gondwana during the late Paleozoic and then drifted northward before finally amalgamating with the Qiangtang terrane during the Early Cretaceous. In this paper we document Permian peraluminous granites near Pikang in the southern margin of the central Lhasa terrane, close to the recently documented Songdo eclogite of Permian age. Zircon SHRIMP and LA-ICPMS U–Pb dating for a Pikang granite sample gives an identical crystallization age of about 263 Ma and a wide age range of inherited zircons (283–2141 Ma). In situ Hf isotopic analyses for 20 zircons of 263 Ma yielded ε Hf( t) values of − 4.5 to + 1.9. The Pikang granites have high A/CNK values (≥ 1.08) and high normative corundum (1.3–2.0%), indicative of peraluminous S-type granite. They are characterized by moderately negative Eu anomalies (Eu/Eu⁎ = 0.48–0.61), and strongly negative Ba, Nb, Sr, P and Ti anomalies. The granites have high ε Nd( t) values (− 6.4 to − 6.0) and low initial 87Sr/ 86Sr ratios (0.7082–0.7096) relative to melts derived from mature continental crust. These rocks are interpreted to have been generated by mixing between mantle melts and their induced melting of mature crustal materials. We interpret the Pikang peraluminous granite magmatism, the regional angular unconformity between the Middle and the Upper Permian and the eclogite of the same metamorphic age (∼ 262 Ma) from the same geotectonic location to represent different products of a common event in time and space. We tentatively term this common event as syncollisional orogeny, i.e., “the Permian Gangdese Orogeny”. We further hypothesize that the orogeny may be genetically associated with the collision between the Lhasa terrane and the northern margin of Australia, following the closure of the Paleo-Tethyan Ocean south of the Lhasa terrane.

Constraints on the late Mesozoic regional angular unconformity in West Liaoning–North Hebei by LA-ICP-MS dating

The angular unconformity between the Tuchengzi Formation and the Zhangjiakou Formation in the Chengde area is commonly regarded as the direct evidence of boundary between Jurassic and Cretaceous systems, and as the evidence of tectonic transformation interfaces in Mesozoic Era in the West Liaoning-North Hebei. Moreover, the lower boundary age of the Zhangjiakou Formation in the Chengde was to be thought about 135 Myr in the past. Based on the detailed study and systematic sampling in Yixian-Beipiao, Lingyuan, Chengde and Luanping areas in the field, the authors obtained the following isotopic ages by means of LA-ICP-MS dating: In the Lingyuan area, the Zhangjiakou Formation and the Yixian Formation are in angular unconformable contact; the age range of the Zhangjiakou Formation volcanic rock is from 133 to 130 Myr (seven samples), and that of the Yixian Formation volcanic rock is from 126 to 121 Myr (eleven samples). In the Luotuoshan of Chengde, the Tuchengzi Formation and the Zhangjiakou Formation are in angular unconformable contact; the age at the lower Zhangjiakou Formation is about 124 Myr (one sample), which reflects that the age range of the lower Zhangjiakou Formation is from 129 to 124 Myr combined with the previous study. In the Luanping basin, there is the largest sediment gap in upper Mesozoic, whose lower boundary age is confined in 131 to 130 Myr. In the Yixian-Beipiao area, the second Member of the Yixian Formation, making up more than 3/4 thickness of the Yixian Formation, was formed in 126 to 119 Myr, and the age of the Yixian Formation at the bottom is about 132 Myr. Moreover, the upper boundary age of the Tuchengzi Formation is about 136 Myr in the West Liaoning-North Hebei region. Based on the above isotopic data combined with the geological characteristics in the field in the West Liaoning--North Hebei region, it reflects that there was a widespread tectonic event in the period of 136 to 126 Myr (mainly in 130–126 Myr) in the West Liaoning-North Hebei, and it formed the late Mesozoic regional angular unconformity. Moreover, the lower boundary ages of the late Mesozoic regional angular unconformity in different areas are different, and the tectonics and the creature assemblage are different in the strata at the top and bottom of the regional angular unconformity. It implies that the late Mesozoic regional angular unconformity is a regional interface of tectonic transformation in Mesozoic Era in the West Liaoning-North Hebei.

The transition from passive to active margin sedimentation in the Cantabrian Mountains, Northern Spain: Devonian or Carboniferous?

In the Cantabrian Mountains of northern Spain, a Cambrian through Pennsylvanian sedimentary succession is preserved that was deformed during the Hercynian orogenic cycle. Within this succession, Silurian and Devonian strata indicate sedimentation on a passive margin. Pennsylvanian deposits are the product of syndeformational uplift and erosion of the newly forming orogen and were deposited in a rapidly filling foreland basin in front of the orogenic belt. The intervening Mississippian succession is strongly condensed and consists of coarse-grained sandstones mantling a regional angular unconformity, crinoidal limestones followed by phosphatic black shales, nodular limestones and siliceous shales. This succession shows abundant erosional surfaces and mass-flow deposits and it is here interpreted as the initial starved foreland basin fill. If this interpretation is correct, the foreland basin system had already developed during the Late Devonian, much earlier than hitherto assumed. Its initiation can be correlated to processes in the internal zones of the Iberian Hercynian orogen where during the Late Devonian there is the change from subduction of oceanic lithosphere to intended continental subduction, its blocking and the subsequent collisional-type deformation. As a direct, long-distance effect of these changes in the plate-tectonic regime, the Cantabrian Hercynian foreland basin was created.

A correlation of Aileron Province stratigraphy in central Australia

In the Palaeoproterozoic Aileron Province of central Australia, the ages of detrital zircons in sedimentary rocks help to establish the superposition of major basin packages, and the correlation with similar sediment packages elsewhere in northern Australia. The earliest known sedimentary sequence across the west and north Aileron Province, the Lander Package, contains detritus from a terrane dominated by 1880–1840 Ma crystalline rocks and was intruded by 1820–1800 Ma magmatism. The Killi Killi Formation of the Tanami Region and the Ooradidgee Group of the Tennant Region have similar provenance and depositional timing. Deposition of the Ongeva Package of metamorphosed sedimentary and volcanic rocks in the east Arunta Region was coeval with the 1810–1800 Ma Stafford Event, the Ware Group of the Tanami Region, and the Hatches Creek Group of the Tennant Region. A previously unrecognised sandstone sequence with maximum age ca. 1805 Ma underlies the regional angular unconformity below the Reynolds Range Group and the first thermal event seen to affect the overlying Reynolds Package was the ca. 1590 Ma Chewings Event, 200 Myr later. This permits a range of correlation possibilities for the Reynolds Package and its basal unconformity. The detrital zircon age basis for these correlations is preserved in rocks that were metamorphosed to granulite facies and offers a means of identifying the protoliths of high-grade metamorphic units in a complex polymetamorphic terrane.

Geochronology on Cenozoic volcanic rocks of a profile Linzhou Basin, Tibet, China and their geological implications

Geochronology on Cenozoic volcanic rocks of a profile Linzhou Basin, Tibet, China and their geological implications

New significant advances of regional geological survey in the blank regions of Qinghai-Xizang Plateau

Himalayan Journal of Sciences Vol.2(4) Special Issue 2004 pp.136-7

A probable late Neoproterozoic age for the Kennedy Channel and Ella Bay formations, northeastern Ellesmere Island and its implications for passive margin history of the Canadian Arctic

The Kennedy Channel and Ella Bay formations are the two oldest stratigraphic units exposed in the Franklinian margin sedimentary sequence in the Canadian Arctic Islands. An Early Cambrian age had previously been accepted by the occurrence of trilobites and small shelly fossils in the type section of the Kennedy Channel Formation. Reinvestigation of the area around the type section shows that several large strike-slip faults cut the succession and that the olenelloid trilobites are from an infaulted slice of a younger unit, the Lower Cambrian Kane Basin Formation. Thus, there is no unambiguous paleontological evidence for the age of either the Kennedy Channel or Ella Bay formations. However, the abundance of stromatolites, absence of trace fossils, and separation from overlying Lower Cambrian clastics by a regional angular unconformity indicate a probable late Neoproterozoic age for these two formations. The Ella Bay Formation likely correlates with the Portfjeld Formation in North Greenland, the Spiral Creek Formation in East Greenland, and the Risky Formation of the Mackenzie Mountains in northwestern Canada. The passive margin that existed in northern Laurentia during the early Paleozoic was, therefore, established in the late Neoproterozoic, and the onset of rifting must have preceded this, rather than occurring in the Early Cambrian as some authors have suggested.