Major Unconformity Research Articles

Facies models for rocky shorelines and their application to transgressed basement highs in the North Sea

Rocky shorelines form where basement highs are eroded and flooded during marine transgressive events. Although the Mesozoic North Sea rift generated numerous platform margins and rotated fault blocks that acted as basement highs, rocky shoreline deposits have not previously been reported in this region. Rocky shoreline deposits are usually represented in the rock record by thin conglomerates overlying major unconformities. They are typically characterized by their ichnological aspects rather than by their depositional facies. This study used the sedimentological aspects of modern and Miocene rocky shorelines from Spain and Austria to create facies models, which were then applied to the recognition of rocky shorelines in the Mesozoic of the Central North Sea. Our results demonstrate that structureless, clast-supported, poorly to moderately sorted conglomerate–breccia deposits are associated with competent basement lithologies, which produce hard, resistant coastal cliffs around previously overlooked volcanic centres in the subsurface of the North Sea. The basement lithologies in most of the Central North Sea favoured the formation of softer coastal cliffs, with less resistant lithologies that did not generate or preserve gravel-sized particles. These softer coastal cliffs were mostly characterized by low-angle, unconformity-bounded sandstones and fine-grained deposits, precluding the preservation and recognition of Mesozoic rocky shores in much of the North Sea's stratigraphic record.

New insights into the structural and stratigraphic evolution of the Malay Basin using 3D seismic data: Implications for regional carbon capture and storage potential

AbstractThe Malay Basin is a mature hydrocarbon province currently being re‐assessed for CO2 storage. Selecting an appropriate storage site requires a comprehensive understanding of the structural and stratigraphic history of the basin. However, previous studies have been limited to observations from either regional 2D seismic lines or individual 3D seismic volumes. In this study, we access and utilise a basin‐wide (ca. 36,000 km2) 3D seismic and well database to describe the structural and stratigraphic features of the basin, particularly those within the uppermost ca. 4 km (Oligocene to Recent) and gain new insights into the basin's evolution. E–W transtensional rift basins first developed due to sinistral shear across an NW‐SE strike‐slip zone. The NW‐SE basin morphology seen today was generated during the late Oligocene–early Miocene, during which time dextral motion across marginal hinge zones created en‐echelon antithetic, extensional faults and pull‐apart basins, especially well preserved along the western margin of the basin. Collisional forces to the southeast during the early to middle Miocene resulted in the shallowing of the basin, intermittent connection to the South China Sea and a cyclic depositional pattern. Around 8 Ma (late Miocene), a significant uplift of the basin resulted in a major unconformity with up to 4.2 km of erosion and exhumation in the southeast. In the centre and northwest of the basin, the inversion of deeper E–W rifts resulted in the folding of Miocene sequences and the formation of large anticlines parallel to the rift‐bounding faults. The Pliocene to Pleistocene history is more tectonically quiescent, but some extensional faulting continued to affect the northwest part of the basin. Larger glacio‐eustatic sea‐level fluctuations during this time resulted in major changes in sedimentation and erosion on the Sunda Shelf, including the formation of a middle‐Pliocene unconformity. These structural events have created a variety of hydrocarbon traps across the basin of different ages, including transpressional anticlines, rollover anticlines and tilted fault blocks. Each of these has discrete and distinct trap elements with important implications for their CO2 storage potential.

Miocene sequences and depocentres in the Roer Valley Rift System

AbstractThe Miocene sequence in the Roer Valley Rift System consists of alternating open‐to‐shallow marine, coastal and fluvio‐deltaic deposits. In this study, well logs, bio‐chronostratigraphy and seismostratigraphy are used to characterize major units and their bounding unconformities and to infer sediment dispersal patterns. Three major unconformities occur in the sequence: the early, middle and late Miocene unconformities (EMU, MMU and LMU). The EMU formed due to tectonic motions related to the Savian phase. After formation of the EMU, a broad depocentre developed in the south‐eastern part of the Roer Valley Graben (RVG). Sediment accumulation increased during this period and peaked in the middle Langhian, after which it diminished again to a low level during the late Serravallian. The decrease in sediment accumulation coincided with a period of tectonic subsidence along the major bounding fault zones (i.e. the Peel Boundary Fault System, the Feldbiss Fault System and the Veldhoven Fault System). The resulting transgression caused sediment starvation in the central RVG. Subsequently, global sea‐level fall during the early Tortonian caused large‐scale erosion, and formation of incised valleys on the highs adjacent to the RVG (Peel Block and Campine Block), as well as the south‐eastern RVG, forming the MMU. However, sedimentation continued during this period in the central part of the RVG where no erosional hiatus developed. From the Tortonian onwards, accumulation rates increased again. The depocentre shifted towards the north‐west and clinoforms developed in the RVG. During the latest Miocene, the depocentre was concentrated along the south‐western margin of the RVG. Meanwhile, the depositional environment of the entire RVRS gradually shallowed as the LMU was formed.

Development of major unconformities in the forearc regions: A signal of west Myanmar−Asia assemblage before the late Paleocene

It has long been debated whether the India-Eurasia collision was a single-stage event that began 60-55 million years ago, or whether it was a two-stage process that involved a collision between India and the Trans-Tethyan Arc before the early Paleocene, and the collision of India with Eurasia during the middle Eocene. Here, we report a late Paleogene angular unconformity (ca. 40-28 Ma) in western Myanmar. This angular unconformity developed around the same time as the Assam unconformity (NE India) but is younger than those found in northern Myanmar. Development of these unconformities indicates that an oblique convergence margin in western Myanmar formed before the middle Eocene, with a major dextral strike-slip fault (proto-Sagaing/Shan Scarp Fault) in the backarc. We interpret this oblique convergence margin to be partial continental collision between the West Myanmar Terrane (WMT) and NE India. In backarc regions, syn-rift successions of the Shwebo sub-basin have formed as a consequence of transtensional tectonics along the proto-Sagaing/Shan Scarp Fault since at least the late Paleocene. The syn-rift successions consist of Asian-derived materials that were not identified in the forearc because of the Wuntho-Popa Arc served as a geographical barrier. The presence of the unconformities and tectonic configuration of the Myanmar backarc sub-basins are inconsistent with the scenario inferred from paleomagnetic data, in which the WMT was part of an intra-oceanic arc at near-equatorial latitudes before the late Oligocene. Instead, we propose that the WMT has been part of continental SE Asia since at least the Paleocene (ca. 60-58 Ma). We reconsider the paleomagnetic data and suggest that the Mawgyi Arc, rather than the WMT, is the oceanic fragment that rifted from the northern Gondwana margin during the Late Jurassic. The Mawgyi Arc collided with continental SE Asia (WMT) during the Late Cretaceous, and then with India during the early Eocene (ca. 51-49 Ma). Our results support the collision between India and Eurasia is a multistage event.

Testing alternative tectonic models for the Permian-Pleistocene tectonic development of the Kyrenia Range, N Cyprus: Implications for E Mediterranean Tethyan palaeogeography

Three published alternative tectonic models of the Permian-Pleistocene development of the Kyrenia Range, N Cyprus are tested, supported by new field, geochemical and micropalaeontological evidence: 1. The Kyrenia Range represents the northern continental margin of the S Neotethys, close to its present relative position. The range initiated as a Permian-Cretaceous rift/passive margin, switching to a N-facing active margin during Late Cretaceous-Neogene; 2. The Range was located along the N African continental margin until the Neogene when northward subduction transferred it to the southern margin of the Eurasian plate; 3. The Range is a far-travelled allochthon that was emplaced to near its present position, probably during the Eocene.In the light of regional comparisons, especially with southern Turkey, the combined evidence mainly supports tectonic model 1. Sedimentary and palaeontological data show that the restored stratigraphy of the Kyrenia Range indicates Late Permian initial rifting and Early-Middle Triassic advanced rifting, followed by Jurassic-Early Cretaceous passive margin subsidence. Small exposures of ophiolite-related melange located between the Mesozoic carbonate platform and the overlying latest Cretaceous-Palaeogene deep-water volcanic-sedimentary succession include evidence of HP/LT metamorphism, pointing to Late Cretaceous subduction. MORB/boninites, diabase-gabbro and extensive harzburgitic serpentinite originated as a SSZ ophiolite, together with a possible high-grade metamorphic sole (garnet amphibolite) and an accretionary prism (E-MORB/OIB; metachert). Microfossil evidence indicates exhumation of the melange and the underlying platform prior to Late Maastrichtian. A mass-transport complex formed within a compression-related foredeep during the Middle Eocene. Associated southward thrusting and folding culminated in emergence and subaerial erosion, generating a major unconformity, that was followed by subaerial and then marine deltaic deposits (Late Eocene-Oligocene). Following major Late Miocene southward thrusting, uplift of the Kyrenia Range took place during the Pleistocene, related to collision of the leading edge of the North African plate (Eratosthenes Seamount) with the Eurasian (Anatolian) plate.

Stratigraphic evidence for a major unconformity within the Ediacaran System

The subdivision and correlation of the Ediacaran System within the geologic timescale is desirable, but challenging. The Ediacaran Doushantuo Formation (∼635–550 Ma) in South China is one of the most important sedimentary successions for studying the co-evolution of Earth's environment and life at that time. However, its stratigraphic continuity is confusing and deserves scrutiny. In this study we evaluate stratigraphic continuity of the Doushantuo Formation from a key section deposited in an alleged deep-water “basinal” setting. Using litho-, chemo-, and chrono-stratigraphic data anchored by two new CA-ID-TIMS U-Pb zircon dates of 629.7 Ma and 556.4 Ma from two tuff layers, we reveal that this section contains a cryptic subaerial unconformity with a >44 Myr gap in the sedimentary record. The unconformity occurs immediately prior to the EN3/Shuram δ13C excursion in the mid-Ediacaran, and can be traced regionally and may be correlative to other mid-Ediacaran unconformities globally. We suggest that its development is related to glacio-eustatic sea-level changes during the mid-Ediacaran. This finding provides a critical stratigraphic boundary for future subdivision and correlation of the Ediacaran System in South China, and we advocate for a reconsideration of the completeness of the Ediacaran stratigraphic record in this area.

MAPPING RADIOACTIVITY IN CONTINENTAL TERMINAL 3 (CT3) FORMATIONS: THE CASE OF THE THREE SISTERS, NIAMEY REGION (EASTERN EDGE OF THE WEST AFRICAN CRATON, ILLUMMEDEN BASIN)

The Continental Terminal 3 (Ct3) formations, which rest in major unconformity on the Birimian basement of the West African Craton, correspond to the last filling deposits of the Iullemmeden basin in the Niamey region (West, Niger). The general objective of this study is to produce a surface radiometric map of the Niamey Three Sisters Hills area. The methodology used included field and laboratory work, with radiometric data processed using Surfer software. Radioactivity values in the Three Sisters area ranged from 5 to 25 shocks per second (C/s). High radioactivity values (15 to 25 c/s) are recorded in the upper terms, corresponding to the breastplates. Low values of radioactivity (5 to 15 c/s) are measured in the basal layers, corresponding to clays and alluvium. This radioactivity is thought to be due to trace amounts of uranium in these outcrops.

Sedimentary evolution and distribution of benthic fauna of an Aptian protected bay (Oliete Subbasin, Spain)

Detailed facies and sequence stratigraphic analysis of an Aptian shallow-marine succession illustrates the evolution of sedimentary environments and their benthic faunal associations in a marginal bay developed in northeastern Iberia (Maestrazgo Basin, Oliete Subbasin). In this bay, marine circulation was hampered by wide stationary shoals located in the low-subsidence transitional areas between subbasins. The Aptian sedimentary record of this bay (Josa and Oliete formations) is composed of four transgressive–regressive (T–R) sequences bounded by major unconformities generated during relative sea-level falls. The Josa Fm (formally defined herein) encompasses an asymmetrical T–R cycle (sequence J). It comprises transgressive prodelta clays with episodic orbitolinid blooms, grading upwards to regressive freshwater-influenced littoral and delta-front sandstones and sandy limestones which yield diverse bivalve communities dominated by trigoniids. The Oliete Fm encompasses three T–R cycles (sequences O1–O3), mostly deposited in a protected lagoon. The transgressive hemicycle of each sequence is dominated by marls containing low-diversity faunal associations composed of ostracods and mm-sized gastropods and bivalves, indicating stressed (probably dysoxic) conditions in the deepest parts of the lagoon, related to a lack of water circulation. The regressive hemicycle of sequences O1 and O2 is characterised by low-energy intertidal to shallow subtidal deposits including sponge spiculites, mud-dominated accumulations of articulated bivalves and gastropods with low-diversity microfaunal associations composed mainly of Choffatella and few other textulariids, and marginal oyster-serpulid patches. The uppermost part of sequence O2 is characterised by the development of siliciclastic tide-influenced coastal plains. Sequence O3 consists of massive intertidal to shallow subtidal glauconite accumulations related to the transgressive reworking of siliciclastic deposits. Foraminiferal associations in sequence O3 show an increase in diversity (including miliolids, lageniids and diverse textulariids), which was probably fostered by a more stable connection with open seas, caused by tectonic activity in the bounding shoal areas. The analysis of these four sequences represents a comprehensive and detailed characterisation of shallow-to-marginal marine benthic communities integrated in a sequence stratigraphic framework.

Multi-phase subsidence history of the Sarawak continental margin and its regional significance

Subsidence analysis of Sarawak Basin using stratigraphic data from a selection of exploration wells revealed a multi-phase history of crustal extension (rifting), subsidence and uplift. A relatively rapid subsidence during the early rift phase from Eocene to Oligocene (ca. 37–28 Ma) was followed by a gradual decrease in subsidence rate as the extended lithosphere underwent post-rift thermal relaxation (ca. 28–22 Ma). A second phase of extension during the Early Miocene (ca. 22–17 Ma) resulted in an increase in subsidence rate, which coincided with a major episode of compressional deformation, uplift and localised erosion. This deformation event culminated in a major unconformity dated ~16 Ma, known as the Middle Miocene Unconformity (MMU), which is recognised throughout the Bunguran Trough and North Luconia regions of Sarawak Basin as a major stratigraphic hiatus spanning the Early to Middle Miocene. Since the Late Miocene, there had been an increase in the subsidence rate, probably due to progradation of the Sarawak shelf to its present-day configuration. The complex subsidence history of Sarawak Basin is similar to those reported from other parts of the South China Sea margin. The subsidence histories indicate a common, underlying tectonic factor which is probably related to rifting and sea-floor spreading in the southwestern prong of the South China Sea oceanic basin.

A Deformed Wedge‐Top Basin Inverted During the Collapse of the Variscan Belt: The Permo‐Carboniferous Lorraine Basin (NE France)

AbstractA new structural model is presented for the Permo‐Carboniferous Lorraine Basin (NE France), a major intramountain basin that developed during the latest stages of the Variscan orogeny (ca. 315–270 Ma). Digitalized well logs and reprocessed seismic data were used to decipher the kinematic evolution of this basin located along the Rhenohercynian orogenic suture zone. The basin was initiated during the late collision stage (Early to Middle Pennsylvanian) in a wedge‐top position upon the Saxothuringian retro‐wedge. The syn‐orogenic sequence is delimited to the north by the major SE‐verging Metz Thrust, which is part of the backthrust system that propagated during Middle Pennsylvanian (Late Westphalian). Seismic data provide evidence of negative tectonic inversion, allowing the formation of syn‐rift depocenters (Late Pennsylvanian‐Early Permian) above the former anticlines. Erosion of these anticlines results in a major unconformity marking the onset of post‐orogenic collapse. The late Early Permian shortening (Saalian phase) is suggested to reactivate former thrusts and normal faults, thus generating late uplift of the basin. The post‐orogenic phase is complex and diachronous at basin scale, and both compression and extension can be recorded in the same area over a short period (<10 Myr). The Late Carboniferous negative tectonic inversion along the Rhenohercynian suture zone is proposed to result from the lithospheric delamination of the Variscan orogenic roots. The associated upwelling of asthenospheric material is recorded by intense magmatic activity, and can be, in turn, considered as the main trigger for the subsequent thermal subsidence of the Mesozoic Paris Basin.

Canadian Arctic–Beaufort Sea Rifted Margin Tectono-Sedimentary Element, SE Canada Basin

Abstract The Canadian Arctic–Beaufort Sea Rifted Margin (CARM) Tectono-Sedimentary Element (TSE) is located on the continental shelf and slope that lie to the west of the Canadian Arctic Archipelago and to the north of the Mackenzie Delta. The TSE comprises the rift succession deposited on the eastern and southern margins of the Amerasia Basin, coincident with the opening of this basin. The TSE strata range in age from the latest Triassic (Rhaetian) to Early Cretaceous (Albian). A major unconformity marks the base of the TSE, with underlying rocks consisting of moderately to highly deformed Proterozoic and Lower Paleozoic rocks that are regarded as basement. The TSE is overlain by the Canadian Arctic Prograded Margin (CAPM) TSE, with the boundary being a significant unconformity in landward areas. Well data are limited to the Beaufort–Mackenzie area, and reflection and refraction seismic data indicate that the succession is up to 4 km thick. The TSE is divided into four structural domains, with deformation increasing to the north. The Beaufort–Mackenzie Domain is dominated by extensional structures, with later contractional structures present in its western portion. The Southern Domain is extensional and characterized by normal faults with tilted fault blocks. The structure of the Central Domain is similar to that of the Southern Domain but may include broad folds formed during the Paleogene Eurekan Orogeny. The succession in the Northern Domain is likely to be strongly folded and cut by thrust faults of the Eurekan Orogeny. Cretaceous extrusive and intrusive basic rocks, related to magmatism in the northern Amerasia Basin, are present in both the Central and Northern domains. Petroleum source rocks, of both lacustrine and marine origin, may be present in the Jurassic portion of the succession and marine shales in the Lower Cretaceous succession. The potential for structural and stratigraphic traps in widespread sandstone units of alluvial fan to marine slope origin is high. The remote location of the TSE, however, makes it likely that it will not be a target for petroleum exploration in the foreseeable future.

U–Pb zircon age and chronology of the Torfufell central volcano: implications for timing of rift relocation in North Iceland

The Late-Miocene Torfufell central volcano (ToCV), situated between the now extinct Snæfellsnes-Húnaflói rift zone and the presently active rift in North Iceland, provides an excellent opportunity to recreate the construction history of a volcanic edifice. We present new U–Pb zircon ages from six silicic units of the ToCV. The results range from 7.15 ± 0.12 to 6.76 ± 0.02 Ma, taken here to represent a ~ 400 kyr time-span for silicic activity at the volcano. Before that, the central volcano had produced basaltic lavas for 600–800 kyr, implying that it was active for ~ 1–1.2 Myr. A stratigraphically documented ~ 1 Myr hiatus above the volcano is contemporaneous with, but shorter than, a major unconformity in the Flateyjarskagi peninsula, considered to result from a major rift relocation in North Iceland. The new U–Pb ages show that silicic volcanism at the ToCV took place 1–2 Myr earlier than assumed previously and nearly synchronously with the rift relocation. As the age progression of the ToCV and the neighboring 5–6 Ma Tinná central volcano conflicts with the generally established geotectonic framework of central N-Iceland, we propose that these two volcanoes were formed at a leaky transform zone that developed to accommodate the rift relocation, with the ToCV formed at its junction with the embryonic rift zone, thus marking the initiation of the presently active rift in North Iceland. Since then, the two volcanoes have drifted away from the rift system due to plate spreading and migration of the plate boundary relative to the Iceland mantle plume.

The Reticulinella?-Mangashtia foraminiferal association: Characterisation of the (upper?) middle-?lower upper Turonian interval in the Sarvak Formation of SW Iran and its bearing upon Upper Cretaceous Arabian Plate sequence stratigraphy

During the mid-Cretaceous the Arabian Plate underwent a major tectonically-driven stratigraphic reorganization, with the development of a major regional unconformity, often ascribed to the middle Turonian. However, evidence for the age calibration of this unconformity (e.g., from biostratigraphy) is limited. A previously undescribed assemblage of larger benthic foraminifera has been discovered in a section at Khormuj in the Coastal Fars region of the Iranian Zagros. This section lies in the uppermost Sarvak Formation, directly beneath the major unconformity surface. Dominant components of the assemblage are Mangashtia viennoti Henson, and Reticulinella? kaeveri Cherchi, Radoičić and Schroeder. Their presence places the uppermost Sarvak Formation at this locality in the age range (upper?) middle – ?lower upper Turonian, and in the context of evidence from other localities in the region it is most likely upper middle Turonian. This provides new constraint on the timing of the unconformity and stratigraphic organisation, and the tectonic events leading to its creation (e.g., forebulge creation related to ophiolite obduction). The presence of R.? kaeveri is reported for the first time from outside of the Mediterranean region and indicates that this taxon is useful for biostratigraphic calibration. As a result, biozonation schemes for the upper Sarvak Formation can be updated.

Tecono-stratigraphy of Paleogene Zhu-3 depression of the Pearl River Mouth Basin, South China Sea: Implications for syn-rift architecture in multiphase rifts

In multi-phase rift basins, the links between fault activities and sequence stratigraphic architectures are still poorly understood compared to single-rift basins. The purpose of this work is to give implications for tectono-stratigraphic signatures in multiphase rifts based on seismic data from the Zhu-3 Depression in the Pearl River Mouth Basin, China. Stratigraphically, three composite sequences (CS1-3) defined by major unconformities in the Paleogene successions of the Zhu-3 Depression are interpreted, with each one subdivided into two or three third-order sequences. Regional unconformities and basin configurations indicate two episodic rift phases, termed rift phase 1 (formed CS1), and rift phase 2 (formed CS2) followed by a lithospheric breakup phase (formed CS3). From rift phase 1 to rift phase 2, the basin evolved from small-sized, independent half-grabens into relatively large-scale grabens and finally a dish-like basin configuration. Fault displacement and stratigraphic stacking patterns revealed three distinctive structural stages of rift phase 1: rift initiation (WC-SQ1), rift development (WC-SQ2), and rift termination (WC-SQ3) stages. However, an immediate decrease in fault activity was recorded in the rift phase 2. Immediate strain localization under rotated extension fields during episodic rift phases led to variabilities in tectono-stratigraphic architectures. In turn, this transition may likely contribute to interpreting the depositional pattern from balanced infill during rift phase 1 to an overfilled pattern during rift phase 2 as well as the breakup stage in response to possible enhanced sediment supply. This work enhanced our understanding of additional hydrocarbon evaluations in the Zhu-3 Depression, South China Sea.

Last Interglacial–Glacial sequence of palaeosols and calcareous slope deposits at Verteuil (Charente, southwest France)

ABSTRACTThe 12 m thick sequence of calcareous slope deposits and palaeosols exposed in a quarry at Verteuil (southwest France) has been studied and dated by optically stimulated luminescence. Emphasis was given to the identification and characterisation of palaeosols through geochemistry and micromorphology. The main clastic units, separated by major unconformities, were dated to MIS 6, MIS 5–4 and MIS 3–2. Stratified deposits (‘grèzes litées’), consisting of alternating diamictic and matrix‐free clastic beds and attributed to stacked stone‐banked solifluction lobes, developed mainly after ca. 40 ka, i.e. during late MIS 3 and MIS 2. MIS 3 is recorded as a palaeosol complex including two humic A horizons overlying a chromic cambisol associated with burrows of ground squirrels, while MIS 5 corresponds to a reworked rubified chromic cambisol. Overall, this sequence shows similarities with the clastic deposits found in regional cave porches and rock shelters, strongly suggesting that anthropogenic activities did not significantly modify the climate‐driven sedimentary and soil‐forming processes at the Palaeolithic sites.

Chapter 2. Sequence stratigraphic concepts and methodologies

Abstract This chapter reviews sequence stratigraphic concepts and methodologies and presents an approach that is most applicable to the North Sea Jurassic, based on the concept of genetic sequence stratigraphy. The concept of depositional sequences, comprising rock units bounded by unconformities, has been developed from the late nineteenth century up to the present day. Many different studies have been carried out on North Sea Jurassic sequence stratigraphy, from the early 1980s to the present day and involving a range of different approaches. Many authors have adopted the J sequence approach that was first published in the early 1990s; however, a number of alternative North Sea Jurassic sequence schemes have also been described. A close relationship existed between tectonics and sequence boundary development, particularly during the Middle–Late Jurassic in the North Sea region. Several of the major unconformities that are known to be of regional extent can be directly related to significant tectonic phases. Other sequence boundaries, for which a tectonic control is not evident, for example, particularly in the Early Jurassic, were potentially driven by glacio-eustatic cycles, which may have been controlled by orbital forcing cycles.

Major unconformity between two juxtaposed sedimentary belts of the Lesser Himalaya, as revealed from the U–Pb zircon geochronology

Major unconformity between two juxtaposed sedimentary belts of the Lesser Himalaya, as revealed from the U–Pb zircon geochronology

Genesis of island dolostones during rapid island subsidence: Example from the Xisha (Paracel) islands, South China Sea

AbstractMiocene carbonate successions found on many isolated oceanic islands throughout south‐east Asia commonly include unconformity capped ‘island dolostones’ units. On Shidao, located in the Xisha Islands in the South China Sea, well XK‐1 (1268.2 m deep) penetrated a thick succession of carbonates before being terminated in the Jurassic metamorphic basement. The lower part of the succession is composed of the Sanya Formation (Early Miocene) and the overlying Meishan Formation (Middle Miocene), ca 700 m thick, that are formed of limestones and dolostones. An unconformity divides the Sanya Formation into a Lower Member formed largely of limestones, and an Upper Member formed of dolostones (DI‐8). DI‐8 is separated by a major unconformity into lower (DI‐8L) and upper (DI‐8U) units, that both include numerous subaerial unconformities. The overlying Meishan Formation is a limestone succession that includes three unconformity‐capped dolostones intervals (DI‐5, DI‐6 and DI‐7). Strata below the numerous exposure surfaces throughout the Sanya and Meishan formations are highlighted by Fe‐staining, biomoulds, cavities, caves and solution breccia development. The weakly fabric‐retentive dolostones in DI‐5 to DI‐8 are formed of dolomite crystals that have a dirty core encased by a clear rim, with δ18O values of +0.3 to +4.7‰ (average +2.0‰, n = 50), δ13C values of +2.2 to +3.2‰ (average +2.5‰, n = 50), Fe concentrations of 44 to 442 ppm (average 149 ppm, n = 50), Mn concentrations of 5 to 197 ppm (average 27 ppm, n = 50), Sr concentrations of 169 to 257 ppm (average 218 ppm, n = 50) and 87Sr/86Sr values of 0.70847 to 0.70900 (average 0.70879, n = 32). In all cases, dolomitization post‐dated the subaerial exposure. The 87Sr/86Sr values suggest that the dolostones in DI‐8L formed during the late Early Miocene, whereas the dolostones in DI‐8U, DI‐7, DI‐6 and DI‐5 probably formed simultaneously during the Late Miocene even though these units are stratigraphically hundreds of metres apart and were located well below sea‐level. Dolomitization was preferentially focused in those strata with high porosities and permeabilities that allowed easy circulation of the seawater through the rock. This model suggests that dolomitization was not restricted to strata that were at or close to sea‐level. Although dolomitization may have been related to the new oceanographic circulation regime established during that time, the exact properties of the seawater that promoted dolomitization are open to debate. This model of dolomitization may be applicable to many of the ‘island dolostones’ that are found in the oceans throughout the world.

Tectono-stratigraphic framework and evolution of East Junggar Basin, Central Asia

The late Paleozoic development of the East Junggar Basin and adjoining mountain belts was critical to the construction of the larger Central Asian Orogenic System, yet the structural framework of basement rocks and overlying strata of the basin and their spatio-temporal relationship with adjacent thrust-bounded ranges remain poorly constrained. In this study, we conducted reflection seismology and compiled existing geologic observations of the East Junggar Basin and adjacent Bogda Shan to the south and Kelameili Shan to the north to construct cross sections depicting the tectono-stratigraphic framework and evolution of the region. Our work shows that Carboniferous–Permian strata overlying basement rocks in the East Junggar Basin occur in structural uplifts and depressions and feature at least five major unconformities. The structure of the East Junggar Basin is characterized by: (1) horsts and grabens involving basement rocks and overlying Carboniferous–Jurassic strata; (2) series of stratigraphic depressions marked by Jurassic–Cretaceous strata; and (3) a south-dipping monocline in uppermost Neogene–Quaternary strata. Mesozoic–Cenozoic strata are absent along the northern margin of the East Junggar Basin at the Kelameili Shan. Strata are generally older from the Kelameili Shan to the northern East Junggar Basin where Devonian–Carboniferous strata are thrust southward over younger strata. In contrast, thicker sequences of Cretaceous–Quaternary strata are present in the southern portion of the East Junggar Basin. Strata of the East Junggar Basin were deposited in a foreland depression formed in the footwall of a leading, north-directed thrust of the Bogda Shan. The overall tectono-stratigraphic framework of the East Junggar Basin is a product of tectonic activity varying in time and space along the northern and southern basin-mountain interfaces since the late Paleozoic.

Canadian Arctic Prograded Margin Tectono-Sedimentary Element, SE Canada Basin

Abstract The Canadian Arctic Prograded Margin Tectono-Stratigraphic Element (TSE) is located on the continental shelf and slope, which lie to the NW of the Canadian Arctic Archipelago. The TSE comprises a post-rift succession deposited on the eastern margin of the Amerasia Basin, and the strata range in age from Late Cretaceous to Pleistocene. Over much of the TSE, a major unconformity marks the base of the succession, and the underlying strata vary from Jurassic to Lower Cretaceous strata of the Canadian Arctic Rift Margin TSE to older Upper Paleozoic–Triassic strata of the Sverdrup Basin Composite Tectono-Sedimentary Element. Sparse reflection and refraction seismic data indicate that the succession can be more than 10 km thick. The TSE is divided into three structural domains with deformation increasing to the NE. The Southern Domain is extensional and is characterized by listric growth faults with rollover anticlines and tilted fault blocks. The pre-Oligocene portion of the Central Domain is deformed by broad folds with extensional faults in the younger strata. The pre-Oligocene succession in the Northern Domain is likely to be strongly folded and cut by thrust faults of the Eurekan Orogeny with extrusive and intrusive igneous rocks occurring in the Upper Cretaceous strata. Petroleum source rocks, as well as abundant reservoir and seal strata, occur throughout the TSE, indicating good potential for the presence of petroleum resources. The remote and environmentally sensitive location of the TSE, however, makes it likely that it will never be a target for petroleum exploration.