Development Of Unconformity Research Articles

Astronomical timescale across the middle Permian–Early Triassic unconformities in the northwestern Junggar Basin: Implications for the origin of the unconformities

The middle Permian–Early Triassic terrestrial successions in the Junggar Basin record an important tectonic transformation stage characterized by the development of unconformities. These unconformities control billion-ton-scale hydrocarbon reservoirs in the northwestern Junggar Basin; however, their age, duration and underlying causes remain uncertain in the absence of an accurate Permian‒Triassic timescale. This study performed a comprehensive cyclostratigraphic analysis utilizing natural gamma ray data from 14 wells to determine the age and duration of the unconformities spanning from the middle Permian Lower Wuerhe (P2w) Formation to the lower Triassic Baikouquan (T1b) Formation while also revealing their potential origins. Our results suggest that astronomically forced climate changes were recorded in the western Junggar Basin during the Permian‒Triassic period. By applying astronomical tuning through sedimentation rates across each well, 14 time-domain series were generated. Using two previously reported maximum sedimentary ages from detrital zircon U–Pb dating, a 38.2-million-year long astronomical timescale was constructed, encompassing the period from the middle Permian Xiazijie Formation to the lower Triassic Baikouquan Formation. This analysis constrains the duration of unconformities between the P2w and T1b formations to ∼7–23 Myr. By integrating the duration, order, and implications of these unconformities with previously reported evidence, we conclude that the unconformity between the middle Permian and upper Permian was driven primarily by tectonic inversion resulting from intensified regional tectonic activity. In contrast, the unconformity between the upper Permian and Lower Triassic likely arose from uplift and erosion induced by compressive stresses associated with tectonic activity, which contributed to the formation of unconformities and stratigraphic discontinuities. This work provides a crucial reference for further studies aimed at integrating detailed sedimentological analyses with high-resolution geochronological data to further elucidate the timing and effects of these tectonic events on sedimentary facies and basin development.

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.

Using the tectophase conceptual model to assess late Triassic–Early Jurassic far-field tectonism across the South-central Barents Sea shelf

The Upper Triassic–Lower Jurassic succession of the Barents Sea Shelf (BSS) represents one of Europe’s most prolific and strategic petroleum systems. This succession reflects various depositional environments and tectonostratigraphic events. Even though these strata are considered largely well-understood, connections with far-field stresses triggered by regional tectonics remain a subject of investigation. This study presents new interpretations that focus on relationships between the stratigraphic succession across the south-central BSS and Triassic–Jurassic Novaya Zemlya compressional tectonics. By applying the “tectophase model,” developed in the Appalachian Basin, to analyze this succession, the presence of foreland-basin depozones and associated far-field processes related to compressional tectonics in an adjacent orogen are suggested. This model addresses unconformity development, lithostratigraphic succession, and reactivation of structures. Use of this model suggests far-field tectonostratigraphic responses during two episodes of Novaya Zemlya tectonism, reflected in the coeval BSS stratigraphy. Overall, this tectonostratigraphic study aligns with other research suggesting a Late Triassic inception for Novaya Zemlya compressional tectonism, which influenced larger parts of the BSS through extensive clastic sedimentation, far-field structural reactivation, and flexural responses to deformational loading triggered by tectonics.

Translation, collision and vertical‐axis rotation in the Organyà and Montsec minibasins (South‐Central Pyrenees, Spain)

AbstractThis paper presents a sequentially restored cross‐section of the Organyà and Montsec minibasins based on geological mapping, new field observations and available borehole data. The main objective was to describe the geometry and evolution of both basins in terms of salt tectonics and minibasin mobility. To this end, a comprehensive palaeomagnetic database has been used to constrain vertical‐axis rotations potentially related to minibasin translation and pivoting. The Organyà minibasin constitutes an asymmetric depocentre formed during the Upper Jurassic‐Lower Cretaceous by translation above a southerly inclined salt layer. Salt evacuation and minibasin touchdown induced salt accumulation on the northern side of the basin that culminated in the development of the major Santa Fe unconformity during the late Albian—early Cenomanian. Indicative of salt quiescence is the following isopachous Cenomanian to lower Santonian sequence Salt tectonics resumed during the late Santonian—Palaeocene, with the Montsec minibasin downbuilding coinciding with the onset of Pyrenean convergence. Changes of the base‐salt topography reflects regional‐scale geodynamic processes. The acceleration of crustal thinning in the North Pyrenean zone during the late Albian‐early Cenomanian favoured uplift in the Axial Zone, increasing slope and triggering salt mobilization in the Southern Pyrenees. Likewise, the onset of contraction renewed the downslope gliding of the Organyà and Montsec minbasins, and supports the idea that the early stages of basin inversion were governed by gravity tectonics. The kinematic reconstruction suggests that the more that 30° counterclockwise vertical axis rotation records pivoting during the suprasalt translation of the Organyà minibasin rather than solely the Iberian microplate rotation.

Further Development of the Lower Cretaceous Clinoform Model of the North of West Siberia Based on the Sequence Stratigraphy Principles: New Possibilities of Stratification

The clinoform concept of the Lower Cretaceous of Western Siberia was and remains the scientific basis for predicting its oil and gas potential. At the same time, new challenges and new technological opportunities require its further development. The basis for this can be a modern model independent methodology of sequence stratigraphy, the essence of which is to divide the stratigraphic section into a succession of systems tracts by mapping of all sequence stratigraphic surfaces. At the same time, the choice of the type of surfaces limiting the sequence is carried out already at the second stage on the basis of the expression of the surfaces in the stratigraphic section or the preferences of the researcher. The wide development of unconformities in the Western Siberian Lower Cretaceous clinoform complex makes it possible to subdivide clinoforms into four systemic tracts: highstand HST, falling-stages FSST, lowstand LST, transgressive TST. The subdivision is carried out by mapping sequence stratigraphic surfaces based on the analysis of stratal stacking patterns. According to the authors, the boundaries of the sequences in this case should be the tops of the TST – the downlap surfaces (DLS), traditionally interpreted as maximum flooding surfaces (MFS). This ensures the consistency of the proposed approach with the clinoform concept. The named clinoforms of Western Siberia can be considered as genetic sequences of the III order. However, the position of facies in their systems tracts is far from always predictable. For further detailing of facies models, clinoforms – III order sequences should be subdivided into clinoforms – IV order sequences, and subdivide them into systems tracts. But the latter are not always mapped by modern seismic method. Special conditions are necessary for this. Therefore, it is proposed at first to subdivide the III order sequences into systems tracts. Then – to map their thicknesses, and to allocate of their depocenters. Further, within the depocenters (under suitable seismogeological conditions), to define IV-order clinoforms – sequences and their constituent system tracts as a basis for mapping reservoirs and hydrocarbons traps.

Tectonic and stratigraphic evolution of the late Paleozoic Darwin Basin, eastern California, USA, and implications for the onset of subduction along the southwestern Cordilleran margin of Laurentia

The Darwin Basin, eastern California, USA, represents a key sedimentary record of tectonics along the southwestern margin of Laurentia for the late Pennsylvanian and Cisuralian (Early Permian). The basin formed when the subsidence rate of the southwestern margin of the Bird Spring Shelf abruptly increased, resulting in the deposition of deep-water facies in areas that had been shelf or littoral environments since the Neoproterozoic. We present new sedimentologic, stratigraphic, conodont biostratigraphic, and subsidence analyses of Darwin Basin strata that document the Pennsylvanian−Permian evolution of the Darwin Basin. We argue that the Darwin Basin evolved in two phases: (1) a Pennsylvanian to mid-Cisuralian foreland basin phase that co-occurred with transpressional deformation of the southwestern margin of Laurentia and was characterized by unconformity development, deposition of coarse-grained carbonate slope-apron facies, syndepositional deformation, and the development of complex basin-floor bathymetry; and (2) a mid-Cisuralian to Guadalupian (middle Permian) phase that preceded and co-occurred with the onset of arc magmatism outboard of the Darwin Basin and is characterized by rapid, uniform subsidence of the entire basin, burial of paleobathymetric highs, and the end of syndepositional deformation. We propose that the transition between these phases may represent a local response to the initiation of the North American Cordilleran subduction zone to the west of the Darwin Basin. We argue that the onset of rapid mid-Cisuralian subsidence documented within the Darwin Basin provides an estimated date of subduction zone initiation that is consistent with previous estimates of the timing of this event, the arc magmatic record, and Cenozoic subduction zone analogs.

Migration of paleo-uplift in southwestern Tarim Basin and its implications for orogenesis and reservoir development, NW China

Based on well horizon calibration, the typical seismic profiles in southwestern Tarim Basin were interpreted systematically, regional geological sections were established, and the regional denudation thickness of each tectonic period was restored. On this basis, the plane morphology maps of ancient structures of the Cambrian pre-salt dolomites in different periods were compiled, and the spatial distribution, development, evolution and migration of paleo-uplift in the late Early Paleozoic were analyzed. In the late Early Paleozoic, there existed a unified regional paleo-uplift widely distributed in southwestern Tarim Basin, which is called the southwestern Tarim plaeo-uplift. The “Tarim SW paleo-uplift” and “Hetian paleo-uplift” proposed in previous literatures are not independent, but the result of the spatio-temporal migration and evolution of the southwestern Tarim paleo-uplift identified in this paper. The southwestern Tarim paleo-uplift emerged at the end of Middle Ordovician, and took its initial shape with increased amplitude in the Late Ordovician. During the Silurian, the southwestern Tarim paleo-uplift rose steadily and expanded rapidly to the east, incorporating Pishan—Hetian and other areas, with the structural high locating in the Pishan—Hetian area. During the Devonian, the southwestern Tarim paleo-uplift began to shrink gradually, to a limited range in the Pishan—Hetian area in the southern part of the basin. During the Carboniferous, the southwestern Tarim paleo-uplift became an underwater uplift, that is, the paleo-uplift gradually died out. The southwestern Tarim paleo-uplift belongs to the forebulge of the southwestern Tarim foreland basin in the late Early Paleozoic, and its formation and evolution are related to the early Paleozoic orogeny of the West Kunlun orogenic belt in the south of the Tarim Basin. The migration of the southwestern Tarim paleo-uplift from the northwestern part of the southwestern Tarim Basin to the Pishan—Hetian area indicates the early Paleozoic orogenic process of the West Kunlun orogenic belt, which started in the western section at the end of Middle Ordovician and extended from west to east in a “scissor” style. The migration and evolution of the southwestern Tarim paleo-uplift controlled the development of unconformities at the end of Middle Ordovician, the end of Late Ordovician, and the end of Middle Devonian, and the spatial distribution of dissolved fracture-cave reservoirs in weathered crust below the unconformities in the southwest of Tarim Basin. The migration of the structural high of the southwestern Tarim paleo-uplift also played an important role in controlling the development of dissolved fracture-cave reservoirs in weathered crust.

Depositional and diagenetic controls on sandstone compactional fabrics in an Andean broken foreland basin, the Miocene Vinchina formation, northwestern Argentina

The very thick (up to 6000 m thick) Miocene Vinchina Formation was deposited between 16 and 8 Ma ago and later covered by more than 2500 m of Pliocene-Pleistocene sediments in a broken foreland basin. The very thick sedimentary pile accumulated in a medium to low-gradient basin must have suffered intense mechanical compaction during burial.Seventy thin sections of unaltered sandstones collected in three sections along the depositional strike of the Vinchina Formation at the Sierra de Los Colorados, La Rioja Province, northwestern Argentina, were analyzed under the microscope for determining compactional paths and controls.The role of compaction in sandstone diagenesis was firstly assessed using the change in intergranular volume (IGV), which is the sum of all cement and remaining primary porosity. It is clear from the IGV analysis that most of the studied sandstones lose more porosity by compaction (COPL) than by cementation (CEPL). In general compaction increased from the upper to the lower member of the Vinchina Formation (i.e with depth) and there is a clear trend of the compaction index ICOMPACT increase from the northern, shorter Los Pozuelos to the central, thicker La Troya sections. Surprisingly, the thickest southern El Yeso section shows anomalous results possibly due to the extensive development of cements as indicated by the highest CEPL values. Cements are related to the depositional environment (gypsum, calcite) and clast composition (zeolites). Early cements may have prevented further compaction while late cements may have produced open fabrics by dissolution of matrix and framework clasts. Both cases show that diagenetic processes are not homogeneous basin-wide and that the diagenetic pathways are not only controlled by the increase in burial depth but also by characteristics associated with depositional environments and framework clast composition.We also attempted to relate compaction to burial depth based on the nature (point, long, concave-convex and sutured contacts) and the number (contact and packing indexes) of grain-to-grain contacts in the studied sandstones. The sandstones present predominance of concave-convex grain-to-grain contacts and IC indexes mostly between 3 and 4. Sutured contacts are uncommon, and stylolites were not observed at all. Persistence of open fabrics, floating textures and point grain-to-grain contacts at different depths are the result of dissolution-cementation processes. A clear trend of increasing mechanical compaction with depth is only seen along the northern coarser-grained but thinner Los Pozuelos section. Although compaction seems to have increased with depth and also from north to south, the thickest but overall finer-grained El Yeso (southern) section do not show a clear pattern of change with depth. This behavior seems to be the result of at least four main factors: 1) mean sandstone grain-size, 2) thickness of muddy intervals, 3) development of early diagenetic cements, and 4) depth of burial.Comparison with comparable examples suggests that similar compactional characteristics are achieved between 3.5 and 6 km of maximum burial depths. However, cumulative thickness of the Vinchina basin-fill indicate that the base of the Vinchina Formation may have been buried at 8.5 km deep but preliminary backstripping models suggest that by 5 Ma this surface may have reached up to 10 km depth. Alternatively, recorded repetitive episodes of deformation, uplift, and erosion (progressive unconformities) in the Vinchina basin may have prevented the sedimentary pile to be deeply buried and therefore allow to reconcile the observed compactional textures with depth of burial.The results obtained in this work show that the analysis of mechanical compaction in sandstones constitutes a complex task and in occasions there is not a linear relationship between the contact indexes and depth of burial. Sandstone compaction is controlled not only by the depth of burial but also by other factors such as, time, geothermal flow, matrix content, the development of early cements, the ratio between ductile and rigid lithic fragments and, in tectonically active basins the development of progressive unconformities. Therefore, in most cases, it is very speculative to extend diagenetic conditions to basin-wide scales. In particular, the porosity-depth relationship must be based on primary porosity only.

Facies analysis, depositional sequences and platform evolution of the Sarvak Formation (late Albian-Turonian) in the Zagros Basin, West of Iran

The Sarvak Formation is a carbonate interval (as part of the Bangestan Group) deposited during the Cretaceous (late Albian-late Turonian), which hosts considerable hydrocarbon reserves. We logged five surface sections in the west of the Zagros Basin to depict the depositional environment and sequence stratigraphy of the Sarvak Formation. The formation comprises five 3rd order depositional sequences, including six facies belts deposited on a carbonate platform. The lower part of the formation comprises two depositional sequences (late Albian-early Cenomanian) deposited on a ramp. In contrast, the rest of the formation (Cenomanian to early Turonian), composed of three depositional sequences, were deposited on a rimmed shelf. The shift from ramp to shelf was presumably due to changes in the tectonic regime of the Arabian Plate from passive to active margin, and in part due to the obduction of the Neotethys’ ocean crust during the late Middle Cretaceous, accompanied by the development of shoal complex and rudistic reefs. This resulted in the development of some isolated carbonate platforms bordered by intrashelf basins, particularly in the Lurestan Zone. Active basement faults caused the exhumation of the platform and the development of unconformity on the Cenomanian-Turonian boundary. The five depositional sequences we studied were mainly controlled by eustatic change(s) during Albian, mid-Cenomanian to Turonian.

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

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

Stratigraphic record of continental breakup, offshore NW Australia

AbstractContinental breakup involves a transition from rapid, fault‐controlled syn‐rift subsidence to relatively slow, post‐breakup subsidence induced by lithospheric cooling. Yet the stratigraphic record of many rifted margins contain syn‐breakup unconformities, indicating that episodes of uplift and erosion interrupt this transition. This uplift has been linked to mantle upwelling, depth‐dependent extension and/or isostatic rebound. Deciphering the breakup processes recorded by these unconformities and their related rock record is challenging because uplift‐associated erosion commonly removes the strata that help constrain the onset and duration of uplift. We examine three major breakup‐related unconformities and the intervening rock record in the Lower Cretaceous succession of the Gascoyne and Cuvier margins, offshore NW Australia, using seismic reflection and borehole data. These data show the breakup unconformities are disconformable (non‐erosive) in places and angular (erosive) in others. Our recalibration of palynomorph ages from rocks underlying and overlying the unconformities shows: (i) the lowermost unconformity developed between 134.98–133.74 Ma (Intra‐Valanginian), probably during the localisation of magma intrusion within continental crust and consequent formation of continent–ocean transition zones (COTZ); (ii) the middle unconformity formed between ca. 134 and 133 Ma (Top Valanginian), possibly coincident with breakup of continental crust and generation of new magmatic (but not oceanic) crust within the COTZs; and (iii) the uppermost unconformity likely developed between ca. 132.5 and 131 Ma (i.e. Intra‐Hauterivian), coincident with full continental lithospheric breakup and the onset of seafloor spreading. During unconformity development, uplift was focussed along the continental rift flanks, likely reflecting flexural bending of the crust and landward flow of lower crust and/or lithospheric mantle from beneath areas of localised extension towards the continent (i.e. depth‐dependent extension). Our work supports the growing consensus that the ‘breakup unconformity’ is not always a single stratigraphic surface marking the onset of seafloor spreading; multiple unconformities may form and reflect a complex history of uplift and subsidence during continent–ocean transition.

Unconformity development in retroarc foreland basins: implications for the geodynamics of Andean-type margins

Unconformities in foreland basins may be generated by tectonic processes that operate in the basin, the adjacent fold–thrust belt or the broader convergent margin. Foreland basin unconformities represent shifts from high accommodation to non-depositional or erosional conditions in which the interruption of subsidence precludes the net accumulation of sediment. This study explores the genesis of long-duration unconformities (>1–20 myr) and condensed stratigraphic sections by considering modern and ancient examples from the Andes of western South America. These case studies highlight the potential geodynamic mechanisms of accommodation reduction and hiatus development in Andean-type retroarc foreland settings, including: (1) shortening-induced uplift in the frontal thrust belt and proximal foreland; (2) the growth and advance of a broad, low-relief flexural forebulge; (3) the uplift of intraforeland basement blocks; (4) tectonic quiescence with regional isostatic rebound; (5) the end of thrust loading and flexural subsidence during oblique convergence; (6) diminished accommodation or sediment supply due to changes in sea-level, climate, erosion or transport; (7) basinwide uplift during flat-slab subduction; and (8) dynamic uplift associated with slab window formation, slab break-off, elevated intraplate (in-plane) stress, or related mantle process. These contrasting mechanisms can be distinguished on the basis of the spatial distribution, structural context, stratigraphic position, palaeoenvironmental conditions, and duration of unconformities and condensed sections. Thematic collection: This article is part of the Fold-and-thrust belts collection available at: https://www.lyellcollection.org/cc/fold-and-thrust-belts

Exhumation history of late Mesozoic intrusions in the Tongling–Xuancheng area of the Lower Yangtze region, eastern China: Evidence from zircon (U–Th)/He and apatite fission track thermochronology

The Lower Yangtze region is one of the most important metallogenic belts in eastern China, yet there are few precise low-temperature geochronological data to constrain the evolution of the orefield. We collected samples from 12 late Mesozoic intrusions in the Tongling–Xuancheng area for zircon (U–Th)/He (ZHe) and apatite fission track (AFT) dating. These intrusions are divided into early and late periods on the basis of their emplacement ages. The ZHe and AFT ages of the early intrusions are 144.6–111.0 Ma and 123.1–89.4 Ma, respectively, and those of the late intrusions are 119.9–71.3 Ma and 96.7–66.9 Ma, respectively, with rare outliers. The ZHe and AFT age distributions and the modeled low-temperature thermal histories of these intrusions reveal that the Tongling–Xuancheng area underwent three stages of rapid cooling: during the earliest, middle, and end of the Early Cretaceous. During the earliest Early Cretaceous, rapid cooling resulted from regional compression and in situ cooling of magma. The middle Early Cretaceous phase of cooling occurred in a regional extensional regime characterized by the development of large rift basins and the emplacement of intrusions. The rapid cooling at ~ 100 Ma was concurrent with a global plate reorganization event that resulted in regional basin inversion and the development of unconformities between the Lower and Upper Cretaceous strata in East Asia. Although Cenozoic uplift is not seen in the cooling paths of our samples, regional compression since the Paleogene has uplifted the orebodies to near-surface level, favoring ore exploration and mining.

Triassic–Jurassic Accretionary History and Tectonic Origin of Stikinia From U‐Pb Geochronology and Lu‐Hf Isotope Analysis, British Columbia

AbstractThe timing of assembly and tectonic origins of terranes in the northern Cordillera of Alaska, British Columbia, and the Pacific Northwest are debated. Stikinia, a long‐lived arc terrane, has an enigmatic regional Mesozoic accretionary history and its tectonic origins remain unconstrained. Zircon U‐Pb geochronology and Lu‐Hf isotopic data on Triassic–Jurassic sedimentary and igneous rocks from central Stikinia shed light on the terrane‐scale effects of a latest Triassic–Early Jurassic collision between Stikinia and pericratonic Yukon‐Tanana terrane. Main age peaks from central Stikinia are 250–160 Ma, reflecting ongoing Mesozoic arc‐related igneous activity within Stikinia. Comparison of isotopic evolution and unconformity development between central Stikinia and northern Stikinia (Whitehorse trough) provide new constraints on regional latest Triassic–earliest Jurassic deformation. We attribute the shortening‐related deformation to variable along‐strike interactions during end‐on collision with the Yukon‐Tanana terrane, with significant crustal thickening at the northern apex of Stikinia that did not persist farther south. A small pre‐Devonian zircon population is significant, as the oldest exposed rocks in Stikinia are Early Devonian. Pre‐Devonian age peaks differ from those of the northern Yukon‐Tanana terrane, but resemble zircons from southern Wrangellia. These zircons are likely multi‐cyclic, derived from crust that originated in the Arctic region near the northern end of the Caledonide orogeny. We suggest that Stikinia was an independent crustal block prior to latest Triassic onset of collision with Yukon‐Tanana terrane. The ongoing, end‐on collision in turn promoted oroclinal assembly of the peri‐Laurentian terranes.

Diachronous development of Great Unconformities before Neoproterozoic Snowball Earth

The Great Unconformity marks a major gap in the continental geological record, separating Precambrian basement from Phanerozoic sedimentary rocks. However, the timing, magnitude, spatial heterogeneity, and causes of the erosional event(s) and/or depositional hiatus that lead to its development are unknown. We present field relationships from the 1.07-Ga Pikes Peak batholith in Colorado that constrain the position of Cryogenian and Cambrian paleosurfaces below the Great Unconformity. Tavakaiv sandstone injectites with an age of ≥676 ± 26 Ma cut Pikes Peak granite. Injection of quartzose sediment in bulbous bodies indicates near-surface conditions during emplacement. Fractured, weathered wall rock around Tavakaiv bodies and intensely altered basement fragments within unweathered injectites imply still earlier regolith development. These observations provide evidence that the granite was exhumed and resided at the surface prior to sand injection, likely before the 717-Ma Sturtian glaciation for the climate appropriate for regolith formation over an extensive region of the paleolandscape. The 510-Ma Sawatch sandstone directly overlies Tavakaiv-injected Pikes granite and drapes over core stones in Pikes regolith, consistent with limited erosion between 717 and 510 Ma. Zircon (U-Th)/He dates for basement below the Great Unconformity are 975 to 46 Ma and are consistent with exhumation by 717 Ma. Our results provide evidence that most erosion below the Great Unconformity in Colorado occurred before the first Neoproterozoic Snowball Earth and therefore cannot be a product of glacial erosion. We propose that multiple Great Unconformities developed diachronously and represent regional tectonic features rather than a synchronous global phenomenon.

Driving processes and recognition criteria for complex subaerial unconformities in a non‐marine succession: Implications for basin margin shaping during forced regressions

AbstractA regional sedimentological and sequence stratigraphic analysis is presented in this paper to provide new insights into the morphological complexity and sedimentary mechanisms of unconformities giving shape to basin margins. Although the development of unconformities entails significant sediment delivery to be considered when analysing ‘source to sink’ systems, as well as large palaeogeographical changes related to basin reconfiguration episodes, the complexity that unconformities can reveal towards basin margins has been scarcely explored. A sedimentological and architectural analysis combining outcrop and subsurface datasets shows five palaeoenvironmental evolutionary stages and four key stratigraphic surfaces in a constrainedca2 Ma Valanginian interval on the southern margin of the Neuquén back‐arc basin (Argentina). A complex‐type subaerial unconformity was identified, composed in turn of two subaerial unconformities exhibiting both single and composite motifs, different morphologies and facies shifts representative of large palaeogeographical changes. In the studied stratigraphic interval, two erosional stages occur linked to combinations of exclusively non‐marine‐driven processes involved in unconformities developing. The two subaerial unconformities differ in nature and distribution, representing a novel case of complex unconformities and stratigraphic architectures in non‐marine lowstand wedges. The Valanginian complex subaerial unconformity entails a high diachroneity along strike and depositional dip, implying that the hiatus created in landward settings occurred during relative sea‐level fall and rise stages during a period longer than in basinward areas. Disagreeing with classical sequential models, two third‐order sequence components of a complex lowstand wedge are preserved in proximal settings. Subsidence‐controlled accommodation and interplay between second‐order and third‐order cycles were combined, increasing the prospects of sediment storage and preservation potential of composite sequences towards landward areas. This work improves current comprehension about complex subaerial unconformities formation and related lowstand architectures in proximal settings, providing criteria to understand and revaluate lowstand wedge geometries, particularly for more complex examples, such as the case reported in this contribution.

LACUNA STRATIGRAFICA TRA LE FORMAZIONI DI RANZANO E DI ANTOGNOLA NELLE ZONE DI ROTEGLIA E MONTEBARANZONE (APPENNINO REGGIANO E MODENESE)

The Monte Piano — Blsmantova sequence (Middle Eocene — Late Miocene) in the Northern Apenmnes represents a new sedimentary cycle after the tectonic phase of the Early—Middle Eocene. This phase principally affected the structural Units of Ligurian domain. This sedimentary sequence (the postorogenic Ligurian Sequence), shows evidence of the progression of the Apennine orogenesis mainly through the development of unconformities of regional extension. In fact, this new sedimentary cycle began after the definitive closure of the Ligurian ocean (Oceanic stage) and sedimentation continued durIng the subsequent development of a collisional margin (Ensialic stage) between the European and African Adriatic plates.
 The stratigraphic and structural features of the Apenmne margin between Tresinaro Valley and Lavino Valley are described brteflv in this paper. Moreover, the contact between the Ranzano Formation and the Antognola Formation, parts of the postorogenic Ligurian Sequence, is discussed in terms of the micropaleontologica] data obtained from three stratigraphical sections situated In the lower valley of the Secchia River. The contact coincides with an unconformity that is marked by a biostratigraphical hiatus.
 In fact, foraminifera] associations indicate that the top of the Ranzano Formation is attributable to Zone PI 8 (Lower Oligocene) and that the base of the Antognola Formation is already attributable to Zone P 21 (Upper Oligocene). There was no evidence of the presence of Zone PI 9/20. The chronological range of the hiatus is estimated as 2.4 to 1 0 Ma, depending upon the chronological scale adopted and the exact position of the "top" of the Ranzano Formation and the base of the Antognola Formation.
 On the basis of the stratigraphic relations existing among the various units of the postorogenic Ligurian Sequence outcropping in the studied area and on the basis of the data collected on the contact between the Ranzano Formation and Antognola Formation, it is clear that the hiatus is attributable to submarine erosion of a part of the turbiditic deposits of the Ranzano Formation, which are affected by pronounced tectono—sedimentary phenomena. The tectonic movements developed in the Ligurian domain in a per10d preceding the deposition of the Antognola Formation. They are thus contemporary with the beginning of the deposition of the Macigno in the Tuscan paleogeographical domain.

Constraining the onset of flexural subsidence and peripheral bulge extension in the Miocene foreland of the southern Apennines (Italy) by Sr-isotope stratigraphy

In fold and thrust belts developing at convergent margins, the migration of the advancing wedge is accompanied by bulging of the downgoing plate, followed by the development of a foredeep basin filled by a thick succession of syn-orogenic sediments. The transition from forebulge to foredeep marks a key moment in the evolution of the orogenic system. In deep water environments, the record of this transition is typically complete and progressive. Conversely, in the shallow-water/continental environment of many collisional systems, the uplift of the forebulge area can imply emersion and erosion, obliterating the stratigraphic record of key steps of the evolution of the orogenic system. The southern Apennines constitute one of these collisional fold and thrust belts where the development of the forebulge has implied emersion and erosion, with the development of a Miocene forebulge erosional unconformity, accompanied by extensional deformation associated with the bending of the lithosphere during the forebulge stage. In this paper, we use strontium isotope stratigraphy to constrain with unprecedented time-resolution the age of the forebulge unconformity in areas presently incorporated in the northern sector of the southern Apennines fold and thrust belt. Integration of our results and those of previous studies indicates, at the regional scale, a younging toward the foreland of the forebulge unconformity across the belt. Our high-resolution ages also reveal a diachronous onset of the flexural subsidence over short distances, associated with the occurrence of horst and graben structures, possibly resulting from inherited paleotopography along with forebulge extension. This work highlights how high-resolution dating is critical to unravel the evolution of foreland basin systems at different scales.

GEOMETRIA E CARATTERI STRATIGRAFICI DELLA SEQUENZA CENOMANIANA NEL BACINO LOMBARDO (ALPI MERIDIONALI)

The Cenomanian Sequence in the area between the lakes of Lecco and Iseo (Lombardy Basin, Southern Alps) represents a depositional unit bounded by two main unconformities and relative conformities. The unconformities progressively develop from SW to NE, with hiatuses spanning in age the Early Cenomanian and the Late Cenomanian-Turonian respectively.
 A complete section of the Cenomanian Sequence consists of Early Cenomanian red marls with pelagic turbidites and pebbly mudstones (Marne rosse) followed by two calcareous chaotic layers enclosing a succession of thin bedded, fine grained siliciclastic turbidites (Torbiditi Sottili), of Middle to Late Cenomanian age.
 The association of chaotic layers with the enclosed wedge shaped body of turbidites and the development of unconformities towards the north, point out that the Cenomanian basin developed in a mobile area where deposition was controlled by sinsedimentary tectonics. Moreover, the Cenomanian basin is, so far, the youngest Cretaceous basin clearly documented in the Southern Alps, oriented east-west, therefore crosswise to the structures related to the Jurassic rifting. Stratigraphic data strongly suggest that the tectonic evolution of the northern margin of the Cenomanian Lombardy Basin was generated by compression with the development of antiform structures.The sandstone unit pinches out towards the north and east, and thins more gradually towards the south. It represents a wedge shaped body, which parallels E-W paleocurrent trends and onlaps onto the northern margin of the Cenomanian basin. The two chaotic layers are basinally extended and show facies changes and thinning from north to south. They derived from sliding and slumping of the northern slope of the basin.

Chronostratigraphy of the Baringo-Tugen Hills-Barsemoi (HSPDP-BTB13-1A) core – 40Ar/39Ar dating, magnetostratigraphy, tephrostratigraphy, sequence stratigraphy and Bayesian age modeling

The Baringo-Tugen Hills-Barsemoi 2013 drillcore (BTB13), acquired as part of the Hominin Sites and Paleolakes Drilling Project, recovered 228 m of fluvio-lacustrine sedimentary rocks and tuffs spanning a ~3.29–2.56 Ma interval of the highly fossiliferous and hominin-bearing Chemeron Formation, Tugen Hills, Kenya. Here we present a Bayesian stratigraphic age model for the core employing chronostratigraphic control points derived from 40Ar/39Ar dating of tuffs from core and outcrop, 40Ar/39Ar age calibration of related outcrop diatomaceous units, and core magnetostratigraphy. The age model reveals three main intervals with distinct sediment accumulation rates: an early rapid phase from 3.2–2.9 Ma; a relatively slow phase from 2.9–2.7 Ma; and the highest rate of accumulation from 2.7–2.6 Ma. The intervals of rapid accumulation correspond to periods of high Earth orbital eccentricity, whereas the slow accumulation interval corresponds to low eccentricity at 2.9–2.7 Ma, suggesting that astronomically mediated climate processes may be responsible for the observed changes in sediment accumulation rate. Lacustrine transgression-regression events, as delineated using sequence stratigraphy, dominantly operate on precession scale, particularly within the high eccentricity periods. A set of erosively based fluvial conglomerates correspond to the 2.9–2.7 Ma interval, which could be related to either the depositional response to low eccentricity or to the development of unconformities due to local tectonic activity. Age calibration of core magnetic susceptibility and gamma density logs indicates a close temporal correspondence between a shift from high- to low-frequency signal variability at ~3 Ma, approximately coincident the end of the mid-Piacenzian Warm Period, and the beginning of the cooling of world climate leading to the initiation of Northern Hemispheric glaciation c. 2.7 Ma. BTB13 and the Baringo Basin records may thus provide evidence of a connection between high-latitude glaciation and equatorial terrestrial climate toward the end of the Pliocene.