Maximum Depositional Age Research Articles

U-Pb and Hf Isotopic Analyses for Detrital Zircon of the Danzhou Group in the Western Jiangnan Orogenic Belt and Tectonic Implications

In order to better constrain the specific depositional age and provenance of the Danzhou Group and understand the geological evolution of the Jiangnan Orogenic Belt, we conducted a combined U-Pb and Hf-isotope analysis of detrital zircons from the Gongdong and Hetong formations of the Danzhou Group in the Longsheng area of the Western Jiangnan Orogenic Belt. Detrital zircons from the Gongdong Formation yield three age populations of 2658–2517 Ma, 2427–1678 Ma and 891–781 Ma, and the youngest ages suggest that the sedimentation began after ca. 783 Ma. U-Pb ages of detrital zircons from the Hetong Formation yield major populations at 2769–2502 Ma, 2492–2100 Ma, and 991–731 Ma, and the youngest ages redefine the maximum depositional age of this unit is 760 Ma, much younger than previously considered. Thus, the upper part of the Hetong Formation in the Longsheng area is newly subdivided into the Sanmenjie Formation, which is characterized by a large amount of 765–761 Ma volcanic rocks. The dominant 991–731 Ma detrital zircons for all samples were likely sourced from the Neoproterozoic igneous rocks of the southeast margin of the Yangtze Block. The subordinate 2494–1678 Ma detrital zircons were probably sourced from the Cathaysia Block. Minor amounts of 2769–2502 Ma detrital zircons may have been sourced from the Yangtze Block. Detrital zircons from the Gongdong Formation have mainly negative εHf (t) values (−1.1 to 21.8, 90%), suggesting that the detritus of the Gongdong Formation is dominated by the recycling of old crustal materials. The εHf (t) values of detrital zircons from the Hetong Formation have a large spread of −22.2 to +9.7, indicating that the source material of the Hetong Formation includes both the juvenile crustal materials and the recycled ancient crustal materials. The above age populations and Hf isotopic characteristics are consistent with the magmatic rocks in the Jiangnan Orogenic Belt and the Southeast Yangtze Block. Taking into account the lithostratigraphic features, provenances, and depositional ages, the Danzhou Group in the Western Jiangnan Orogenic Belt was deposited in a back-arc basin.

The Wattie Group of the Birrindudu Basin, northern Australia: a witness to the denudation of the Nuna-forming Isan/Chewings Orogeny

The Proterozoic Birrindudu Basin lacks a robust chronostratigraphic framework and remains one of the least explored basins in the Northern Territory, Australia. It shares lithological and chronological similarities with the McArthur Basin and the Tomkinson Province, which together form the ‘greater McArthur Basin’. The sedimentary successions of these basins have been divided into five basin-scale, non-genetic depositional packages—the Redbank, Goyder, Glyde, Favenc and Wilton packages. Of these, the Favenc Package is among the least studied. The Wattie Group in the Birrindudu Basin is interpreted as part of the Favenc Package based on geophysical and sedimentological correlations. New LA-ICP-MS detrital zircon U–Pb data provide new constraints on the age and provenance of the Wattie Group and establish relationships with the other Favenc Package sequences of the greater McArthur Basin. Our new data, combined with published data, yield maximum depositional ages for the formations of the Wattie Group, from oldest to youngest: the Wickham Formation (1623 ± 24 Ma), the Burtawurta Formation (1591 ± 70 Ma), the Hughie Sandstone (1585 ± 28 Ma), the Neave Formation (1611 ± 18 Ma), the Gibbie Formation (1705 ± 55 Ma) and the Seale Sandstone (1550 ± 24 Ma). The detrital zircon age distribution suggests that the Wattie Group primarily received detritus from south and southeastern regions, which we link to denudation and erosion of a southern orogen during the early Mesoproterozoic. A subtle northward shift in sediment source is apparent in stratigraphically younger units. This age-provenance shift is matched with a similar stratigraphic change in petrography, which we interpret as near-complete denudation of southern orogenic topography and a return to sediment sourced from proximal northern sources. Tectonically, we interpret that the Wattie Group represents erosion of topography caused by the Isan/Chewings Orogeny during the last major Nuna amalgamation.

Integrated geochronological and chemostratigraphic study of middle Miocene strata (Ogallala Group) at the eastern margin of the North American Great Plains

Global and regional shifts in climate and environmental conditions during the Miocene gradually gave rise to the grassland biomes that now dominate the modern North American Great Plains. Strata comprising the Ogallala Group provide critical information for understanding these transitions. Geologic mapping and scientific drilling at the eastern edge of the Ogallala Group in northeastern Nebraska, reveal a basal, pedogenically modified siltstone-dominated interval that was hitherto barely known and never firmly placed in a regional stratigraphic context. Herein, we distinguish this basal siltstone unit of the Ogallala Group in the eastern Niobrara River Valley from the overlying sand-dominated strata of the Valentine Formation on the basis of lithologic characteristics, trends in organic-carbon δ13C chemostratigraphic profiles, and U-Pb dating of detrital zircons. This siltstone unit bears some similarities to the middle Miocene Fort Randall Formation in its type area, which lies ∼75 km to the northwest in the Bijou Hills of South Dakota. Organic δ13C chemostratigraphic profiles generated from outcrops and cores of the basal siltstone in Nebraska indicate that the study area consistently had C3-dominated paleofloras while it was deposited, presumably in the middle Miocene. The youngest detrital zircons from the siltstone-dominated unit were probably deposited directly onto ancient landscapes by supervolcanic airfall events originating some 1,500 km to the west near the intersection of the boundaries of present Oregon, Nevada, and Idaho. These youngest zircons yield a maximum depositional age of ∼15 Ma. This age is in general agreement with the Barstovian (Ba2) mammal biostratigraphic age of the Fort Randall Formation in South Dakota. It is also significantly older than the ∼13 Ma youngest single grain zircon we recovered from sands of the overlying Valentine Formation. Thus, we propose that our integrated geochronologic and chemostratigraphic approach can date Late Cenozoic strata with constraints on the order of 1 m.y., portending broad applicability of our methods in reducing the uncertainties in dating terrigenous sediments in continental basins.

Diachronous Cretaceous Closure of the Bangong‐Nujiang‐Shyok Ocean (Westernmost Central Tibet)

AbstractPrecisely reconstructing the diachroneity of continental collision along various Paleo‐, Meso‐, and Neo‐Tethyan branches has proved to be challenging, which limited our understanding of the dynamics of terrane suturing and associated topographic inheritance in the Tibetan Plateau. In this study, we present stratigraphic and sedimentological evidence to support, for the first time, the presence of a late Early Cretaceous remnant deep‐water basin active at least until 110 Ma in the westernmost Bangong‐Nujiang suture in central Tibet. This basin was confined by shallow‐marine deposition in the north, east, and southeast, as indicated by obitolinid biostratigraphy and maximum depositional ages for six lithostratigraphic units, while the corresponding ocean had already closed in the east by Aptian‐Albian time. Detrital zircon U‐Pb age data (n = 965), forward numerical mixture modeling, and in situ εHf(t) values point at syn‐collisional magmatic rocks along strike in the east as the main sediment source, with additional variable input from uplifted Jurassic‐lowermost Cretaceous turbidites. Our paleogeographic reconstruction indicates that the closure of the Bangong‐Nujiang Ocean progressed markedly diachronously from east to west, culminating in the final closure of the Shyok oceanic branch during the Late Cretaceous (probably by 95 and no later than 85 Ma). This westward‐younging ocean closure corresponds to a shorter duration of continental convergence in the west, resulting in less crustal shortening and tectonic uplift than in the east. This may have played a pivotal role in shaping Cenozoic topography, characterized by higher elevations in the east compared to the west along the Bangong‐Nujiang‐suture valley.

Controls on late Miocene marine vertebrate bonebed genesis in northern Chile

The Bahía Inglesa Formation in the Atacama Region of Chile (27°S) hosts one the world's best-preserved Cenozoic marine vertebrate death assemblages attributed to harmful algal bloom (HAB)-mediated mass mortalities. However, the lack of a well-dated depositional model prevents understanding of the timing of fossil accumulation and associated paleoenvironmental conditions. We present a revised chronostratigraphic framework for the Bahía Inglesa Formation at the Cerro Ballena and Mina Fosforita paleontological sites based on detailed field sedimentology, paleontology, and UPb zircon and phosphate geochronology. We integrate maximum depositional ages (MDAs) of intercalated volcanic ash and sandstone beds with published basinal chronostratigraphy to provide the first correlation between both localities. Detrital zircon UPb ages suggest that the highly articulated cetacean carcasses preserved at Cerro Ballena were rapidly deposited in a barrier-protected shoreface environment at ca. 6.1 Ma during periods of voluminous volcanic ash and diatom accumulation on the shelf. Evidence of disarticulated and taxonomically diverse vertebrate fossils within the Mina Fosforita bonebed suggest deposition in an unsheltered offshore transition to inner shelf environment with strong wave action and rapid flooding conditions between ca. 7.4 and 6.8 Ma. We propose that the fossil accumulations at Cerro Ballena and Mina Fosforita were deposited during distinct sedimentation events controlled by local bay paleogeography and rapidly fluctuating relative sea-level conditions. We compare our chronostratigraphic results with 2969 published magmatic ages from the Central Andes and propose a correlation between the mass deaths of marine mammals in northern Chile and intensifications in volcanism, ocean fertilization, phosphogenesis, and HAB blooms during the late Miocene.

Detrital zircon provenance links between outcrop and subsurface, Rocas Verdes and Magallanes-Austral Basins, Patagonia

Analyzing how provenance signatures of tectonically complex ancient sedimentary basins vary between the outcrop and the subsurface provides a more complete sediment provenance history. The 500+-km-long outcrop belt of the Rocas Verdes and the Magallanes-Austral Basins in southern Patagonia and its subsurface to the east allow us to constrain relationships between longitudinal and transverse sediment sources feeding the basin. New detrital zircon U-Pb ages (n = 2558) from nine subsurface core samples from the Springhill, Piedra Clavada, and La Anita Formations along a 140 km E-W seismic reflection transect (50°S, 71.5°W–69.5°W) reveal the provenance of the basin’s eastern subsurface extent and the influence that tectonic activity can have on sediment capture. Detrital zircon ages (n = 1068) were also collected from four outcrop Springhill Formation samples 140 km NW from the transect near Lago San Martín. Maximum depositional ages (MDAs) from the Springhill Formation cores range from 159 to 157 Ma, which are older than correlative outcrop ages of 151–148 Ma. MDAs from the Piedra Clavada and La Anita Formations range from 93.7 to 91.5 Ma and from 79.3 to 78.3 Ma, respectively, and overlap in age or are slightly younger than outcrop ages. The unimodal Late Jurassic age distribution from the Springhill Formation suggests that it almost exclusively was sourced by recycling of the underlying El Quemado Complex. Early–Middle Jurassic ages in the Early Cretaceous Piedra Clavada Formation samples suggest the basin was either sourcing from the North Patagonian Massif to the N-NE and from the Deseado Massif to the NE or recycling exhumed volcaniclastic sequences in the northern margin of the basin. By the Late Cretaceous, the basin fill was more locally sourced, as suggested by the muted abundance of Early–Middle Jurassic ages in La Anita Formation samples and a second mode of Late Jurassic–Early Cretaceous ages locally derived from recycling of the exhumed El Quemado Complex from the Patagonian fold-and-thrust belt. We suggest that the latitudinal and local transverse drainages that sourced the Magallanes-Austral Basin outcrop belt also fed the eastern, subsurface extent of the basin. Results of this work highlight the importance of using several types of provenance methods and a three-dimensional approach to study the erosional history of sedimentary basins.

Detrital zircon age constraints on the deposition of the Sredogriv low-grade metasedimentry rocks, West Balkan Zone, Bulgaria

The Sredogriv low-grade metamorphic rocks belongs to the Montana Unit of the Western Balkan Zone in Northwestern Bulgaria. The low-grade rocks predominantly consist of metamorphosed in greenschist facies clastic and clastic-marly precursors and minor basic to acidic extrusive and intrusive presumably olistostromic rock bodies. Here, we present detrital zircon age constraint for clastic rock of the Sredogriv metamorphics of unknown till now depositional timing, together with an age of the clastic rock unconformably overlying these metamorphics. Detrital zircons in the Sredogriv metaconglomerate yielded a maximum depositional age of 523 Ma, additionally maximum age of deposition is defined at 263 Ma for a conglomerate of the Smolyanovtsi Formation from the first unconformable sedimentary cover. Mostly continental detrital material from magmatic source built the protolith of the Sredogriv metamorphics, which are recycled in the clastic cover rocks. Hence, the greenschist facies metamorphism of the Sredogriv metamorphics took place between 523 Ma and 263 Ma.

U-Pb detrital zircon geochronology of the late Paleozoic–early Mesozoic sedimentary successions from the Belogradchik Unit in the West Fore-Balkan, NW Bulgaria

U-Pb detrital zircon geochronology provides middle Permian–earliest Triassic (272–246 Ma) maximum depositional ages for the Zelenigrad and Vranska Formations of the Belogradchik Unit. These ages match the depositional span (305–249 Ma) of the sedimentary successions in the St. Iliya Heights of the Sakar-Strandzha Zone, in addition to their structural, textural and compositional similarities. The older zircon populations reveal erosion of Neoproterozoic basement, a feature characteristic for Gondwana-derived blocks. The gap between Cambrian and Carboniferous ages of the detrital zircon record of the Belogradchik Unit is ascribed to the Avalonia-type nature of the feeding provenance.

Influence of Triassic and Jurassic arc magmatism in Lower Jurassic Sierra de Santa Rosa Formation, northwestern México: Constraints from geochemistry and U-Pb geochronology

The Permo-Triassic and Lower Jurassic sedimentary succession of the El Antimonio Group in northern Sonora, Mexico is inferred to be deposited in a forearc basin and its age has been constrained by fossil assemblages and detrital zircon geochronology. In this work, petrography, geochemistry and detrital zircon U-Pb geochronology were undertaken on the Sierra de Santa Rosa Formation, the younger unit of the El Antimonio Group in the locality of the Sierra del Álamo, to constrain its tectonic setting, age, source area paleoweathering, and provenance. Sandstone of Sierra de Santa Rosa Formation is petrographically classified as arkose. The elemental ratios, REE patterns and Eu anomaly, as well as bivariate and ternary plots suggest igneous felsic sources subjected to weak to moderated chemical weathering under arid climatic conditions for detritus of the Sierra de Santa Rosa Formation. Detrital zircon U-Pb geochronology of seven sandstone samples of this unit record main populations of Proterozoic age (58% of the total grains) with age peaks at 1.68. 1.39 and 1.14 Ga and Triassic age (31% of total grains) with age peak at 219 Ma, and subordinate groups of Paleozoic, Jurassic and Archean ages. Main sources for the zircon grains are the regional nearby igneous-metamorphic Proterozoic basement rocks, recycled detritus from the Proterozoic and Paleozoic sedimentary cover, and the Permo-Triassic and Jurassic continental magmatic arcs of southwestern North America. The detrital zircon U-Pb geochronology of the dated samples allows to establish Sinemurian to early Toarcian maximum depositional ages for the Sierra de Santa Rosa Formation in the locality of the Sierra del Álamo.

Provenance of Paleogene Strata at Slim Buttes, South Dakota: Implications for post-Laramide Evolution of Western Laurentia

Slim Buttes is a 30 km long by 10 km wide set of buttes containing Paleogene strata in northwest South Dakota. At Reva Gap in northern Slim Buttes, Eocene-Oligocene terrestrial strata of Chadron and Brule Formations of the White River Group unconformably overlie the Paleocene Fort Union Formation. An angular unconformity separates the White River Group from overlying Oligocene and Miocene strata of the Arikaree Group. Using detrital zircon U-Pb ages, we determine the provenance of these rocks as part of a broader synthesis of post-Laramide sedimentation in the Rocky Mountains and western Great Plains. The Chadron Formation age spectrum is dominated by Cretaceous and Proterozoic grains that are interpreted to be locally recycled from the underlying Cretaceous and Paleocene strata. The Brule Formation has a maximum depositional age of ~34 Ma; Paleogene zircons dominate the age spectrum, and a wide variety of older zircons are also present. The Oligocene zircons are interpreted to have been sourced from volcanic systems in the Great Basin to the southwest, while the subsequent proportions of the zircons were derived from a variety of source areas in the Nevadaplano and Rocky Mountain areas to the southwest. Sparse amounts of Archean zircons are thought to represent the burial of Laramide uplifts throughout Wyoming at the time of Brule deposition, making for a regional paleotopography with little relief across the western interior of the United States. The Miocene-age Arikaree Group sand has a maximum depositional age of ~26 Ma and a multimodal detrital zircon age spectrum. The Arikaree Group provenance likely represents continued sourcing in the Great Basin volcanic systems and Nevadaplano, the beginnings of the re-exhumation of Laramide basement uplifts, and subsequent sediment evacuation out of the western interior and into the Gulf of Mexico to the southeast. Our findings indicate that the transport process and detrital zircon provenance signatures of these strata are decoupled, and each have their own independent evolution. The volcanic signature is primarily transported via aeolian processes (i.e. volcanic ash), and the recycled detrital zircon signature is primarily transported via fluvial processes.

Late Cambrian-early Ordovician extensional magmatism, uplift and sedimentation of the N Gondwana margin: new field, geochemical and geochronological data from the peri-Gondwanan Central Sakarya Terrane, Sakarya Zone, NW Turkey

ABSTRACT The timing and mechanism of the crustal extension that caused blocks of continental crust to rift and drift northwards from the northern Gondwana margin during the Early Palaeozoic are currently debated. Here, we present new field, geochemical and U-Pb zircon data from peri-Gondwanan Central Sakarya terrane in NW Turkey to shed light on the timing and petrogenesis of extensional magmatism and subsequent passive margin development of the northern Gondwana margin. Our 1/25.000 scale mapping of a selected area (250 km2) has revealed a south-vergent thrust stack comprising three slices. The Middle Slice of the thrust stack consists of a gneiss-amphibolite complex that has a basement-cover type stratigraphy (before regional metamorphism). The basement unit consists of meta-granitoids, intruded by meta-syenite and banded amphibolites, with geochemical characteristics reflecting I&S-type, A-type and within plate alkaline/transitional-type magmatism, respectively. The cover meta-clastic sediments have amphibolite lenses with MORB and back-arc basin-type geochemistry. Meta-granitoid bodies yielded zircon U-Pb ages of 507 ± 16 Ma, 503 ± 12 Ma, and 488 ± 2.6 Ma (Late Cambrian-Early Ordovician), whereas the meta-syenites gave ages of 473.9 ± 5.7 Ma and 471.4 ± 2.5 Ma (late Early Ordovician); also, age of the banded amphibolite was dated as 473.3 ± 4 Ma. The youngest detrital zircon population in the pebbly quartzite sample within cover meta-sediments (458 ± 3 Ma) constrains the maximum depositional age to around early Late Ordovician. The late Early Ordovician bimodal magmatism, a key focus of our study, is interpreted to have formed in response to the Rheic Ocean rifting along the northern margin of Gondwana. The Late Cambrian granitic magmatism, a precursor to bimodal magmatism, can be explained by lithospheric thinning and upwelling of the asthenosphere to form anatectic silicic melts. The unconformity between the basement and cover units may represent the initiation of a new phase of extension at the N Gondwana margin that culminated in the northward drifting of the Central Sakarya and the opening of Palaeotethys, associated with back-arc basic magmatism in Late Ordovician-Silurian.

Age and provenance relationships between the basal Great Valley Group and its underlying basement: implications for initiation of the Great Valley forearc basin, California, U.S.A.

ABSTRACT The Great Valley forearc (GVf) basin, California, records deposition along the western margin of North America during active oceanic subduction from Jurassic through Paleogene time. Along the western GVf, its underlying basement, the Coast Range Ophiolite (CRO), is exposed as a narrow outcrop belt. CRO segments are overlain by the Great Valley Group (GVG), and locally, an ophiolitic breccia separates the CRO from basal GVG strata. New stratigraphic, petrographic, and geochronologic data (3865 detrital and 68 igneous zircon U-Pb ages) from the upper CRO, ophiolitic breccia, and basal GVG strata clarify temporal relationships among the three units, constrain maximum depositional ages (MDAs), and identify provenance signatures of the ophiolitic breccia and basal GVG strata. Gabbroic rocks from the upper CRO yield zircon U-Pb ages of 168.0 ± 1.3 Ma and 165.1 ± 1.2 Ma. Prominent detrital-zircon age populations of the ophiolitic breccia and GVG strata comprise Jurassic and Jurassic–Early Cretaceous ages, respectively, with pre-Mesozoic ages in both that are consistent with sources of North America affinity. Combined with petrographic modal analyses that show abundant volcanic grains (> 50%), we interpret the breccia to be mainly derived from the underlying CRO, with limited input from the hinterland of North America, and the basal GVG to be derived from Mesozoic igneous and volcanic rocks of the Sierra Nevada–Klamath magmatic arc and hinterland. Analysis of detrital-zircon grains from the lower and upper ophiolitic breccia yields MDAs of ∼ 166 Ma and ∼ 151 Ma, respectively. Along-strike variation in Jurassic and Cretaceous MDAs from basal GVG strata range from ∼ 148 to 141 Ma, which are interpreted to reflect diachronous deposition in segmented depocenters during early development of the forearc. The ophiolitic breccia was deposited in a forearc position proximal to North America < 4 Myr before the onset of GVG deposition. A new tectonic model for early development of the GVf highlights the role of forearc extension coeval with magmatic arc compression during the earliest stages of basin development.

Tectonostratigraphic framework and provenance of a Mesoproterozoic rift succession: An example from the Espinhaço Supergroup, SE Brazil

During the Mesoproterozoic era, the amalgamation of the Rodinia supercontinent witnessed numerous intraplate rifting events far-field induced by “Grenvillian-type age” orogenic systems. The intimate connection between tectonics and sedimentation enables the reconstruction of these intraplate rifting cycles, linking them to accretionary events along craton margins. This is essential for understanding the paleotectonic history of continents during the Mesoproterozoic era. In southeastern Brazil, the Espinhaço Supergroup documents long-lasting superimposed rifting events that extended to the eastern border of the São Francisco paleocontinent from the Paleoproterozoic to the Mesoproterozoic era. Three unconformity-bound rift sequences (∼1.77–1.7 Ga; ∼1.57–1.5 Ga; and ∼ 1.19 Ga) have been identified within the Espinhaço Supergroup, terminated by sag-marine deposits of the Conselheiro Mata Group. In the southern extension of the southern Espinhaço range (Cipó range), a detailed sedimentological and stratigraphic study was conducted on a rift-related sequence with unknown stratigraphic positioning. This sequence is bounded at the base by eolian sediments of the Galho do Miguel Formation of the Southern Espinhaço Supergroup and at the top by Ediacaran-Cambrian carbonates and pelites of the Bambuí Group. Examination of the tectonically-induced depositional controls in the provenance signatures involved stratigraphic surveys coupled with U-Pb geochronological data of detrital zircons. The lower section of the rift-related sequence transitions eastward into marine sediments of the Conselheiro Mata Group within the Santa Rita Formation, now identified as the Lower Member. With a maximum depositional age of 1660 ± 32 Ma, this section directly overlies the Galho do Miguel Formation by an erosive unconformity, comprising syn-rift coarse-grained deposits from alluvial and fluvial systems within an inverted half-graben structure. The Lower Member of the Santa Rita Formation provides evidence of the Mesoproterozoic rifting event associated with the Conselheiro Mata Group. The marine sediments of the Undefined sequence have a maximum depositional age of 1462 ± 19 Ma, overlying the Lower Member of the Santa Rita Formation with an erosive unconformity. The stratigraphic position of this sequence remains speculative, possibly linked to a later extensional phase of the Mesoproterozoic Conselheiro Mata rifting event or one of the two continental rifting stages leading to the Neoproterozoic Macaúbas Group.

Ediacaran diamictite deposition in South China: Detrital zircon u-pb age evidence and macro sedimentary texture of the Aiqiling formation in southeastern Hunan Province

A better understanding of Ediacaran (Gaskiers) glaciations, including their ages, global distribution and types, is critical to our understanding of the Earth’s evolution following the Neoproterozoic “Snowball Earth”. The present study focuses on a diamictite that forms part of a continuous succession of Neoproterozoic clastic sedimentary rocks in the northwestern margin of the Cathaysia block in South China. Detailed sedimentary sequence established here shows that the diamictite, previously interpreted to be Cryogenian in age, is a part of the Ediacaran Aiqiling Formation. Detrital zircon U-Pb ages constrain its maximum deposition age to ca. 571 Ma, supporting its Ediacaran age. The macro sedimentary textures of the diamictite unit, including matrix-supported sediments with angular and randomly distributed clasts, combined with regional geochemical results, suggest a glaciogenic origin. The U-Pb detrital zircon age data also indicate an uplifting source area to the southeast that migrated northwestward since the Ediacaran.

40Ar/39Ar age of phengite from sandstone of the ophiolite-derived clastic sequence of the basin of the Rassokha River, collision belt of the Chersky Range

The Early Permian (275.3 ± 3.1 Ma) 40Ar/39Ar plateau age of detrital mica (Cr-phengite) from clasts in listvenite sandstones of the ophiolite-derived clastic sequence of the Rassokha Terrane of the Chersky Range probably corresponds (or is close) to the time of the formation of listvenites of the provenance (ophiolitic massifs of the range) and restricts the maximum deposition age of clastic rocks. A partial loss of Ar by mica as a result of deformations and postsedimentation transformation of rocks of the ophiolite-derived clastic sequence in the Early Permian is possible but it is less probable.

Position of the Precambrian-Cambrian boundary in the sections of Western Mongolia according to the malacological data

The Early Permian (275.3 ± 3.1 Ma) 40Ar/39Ar plateau age of detrital mica (Cr-phengite) from clasts in listvenite sandstones of the ophiolite-derived clastic sequence of the Rassokha Terrane of the Chersky Range probably corresponds (or is close) to the time of the formation of listvenites of the provenance (ophiolitic massifs of the range) and restricts the maximum deposition age of clastic rocks. A partial loss of Ar by mica as a result of deformations and postsedimentation transformation of rocks of the ophiolite-derived clastic sequence in the Early Permian is possible but it is less probable.

Pre-Choiyoi volcanism at Cordillera del Viento, southwestern margin of Gondwana (∼37° S): Geological characterisation, geochronology and regional implications

This contribution provides a comprehensive description and classification of fifteen lithofacies within the Carboniferous rocks attributed to the Arroyo del Torreón Formation and Sofía Dacite in order to understand the eruptive dynamics and the evolution of volcanism associated with the onset of the Gondwanan Orogenic Cycle at the Cordillera del Viento (Argentina). The outcrops of Carboniferous volcanic and volcaniclastic rocks linked to this cycle are restricted to the Cordilllera del Viento range. In consequence, they offer a valuable opportunity to research the volcanic arc products of the pre-Choiyoi magmatism.The Arroyo del Torreón Formation lithofacies interpretation and its distribution indicate that the volcanism commenced with local andesitic lava flows followed by a volcano-sedimentary sequence formed by multiple pumice-rich quasi-steady concentrated pyroclastic density currents alternating with episodes of epiclastic sedimentation. The latter are covered ignimbrites developed by boiling over eruptions. Crystal-rich tuffs overlie the ignimbrites and represent a transition to a plinian eruption. This eruption style continued, developing concentrated pyroclastic density currents with highly unsteadiness conditions and scarce fall-out deposits. Conversely, the Sofía Dacite, represent a notable shift in the volcanism style, characterized by dacitic to rhyolitic lava flows with minor pyroclastic beds. Zircon dating of a pumice-rich lapilli-tuff from the Arroyo del Torreón Formation yielded a minimum age of 327.77 ± 1.79 Ma that was interpreted as the maximum depositional age for this unit. Similarly, zircons from a porphyritic dacite representing the Sofía Dacite provided a Concordia age of 329.11 ± 1.31 Ma that was interpreted as the crystallisation age of this rock.Consequently, we propose a new stratigraphic division of the Arroyo del Torreón Formation into two members. The Cerro San Pedro Member will comprise the pyroclastic rocks at the base of the sequence, while the Sofía Member will include the upper lava-dominated portion of the unit. In this sense, the whole lithostratigraphic unit represents a distinctive Carboniferous basin associated with arc-related volcanism representative from the pre-Choiyoi magmatism.

The Early Carboniferous age of the Arrayán Formation in the Choapa Accretionary Complex: Implications for its fossil floral content, tectonic setting and evolution of the southwestern Gondwana margin (north-central Chile)

The Arrayán Formation comprises a thick, well stratified, strongly folded turbiditic succession, exposed along the Chilean coast, between 31°30′ and 32°S. It is interpreted as the frontally accreted portion of the Choapa Accretionary Complex formed in the south western Gondwana margin in Carboniferus and Permian times. The Arrayán deposits are unconformably overlain by the marine neritic and richly fossiliferous late Early to early Late Permian Huentelauquén Formation. The contact with the basally accreted metamorphic units of the Choapa Accretionary Complex is tectonic. A previous and a recently dated sample from the Arrayán Formation (PS-8, this paper) yielded mid-Early Carboniferous (Visean) maximum depositional ages of 340 ± 5 Ma and 342 ± 4 Ma respectively. This age (i) contradicts the so far accepted Late Devonian to Early Carboniferous age assigned to the Arrayán Formation, based on its fossiliferous content, (ii) indicates that the age of the psylophytal remains found in this formation are post-Visean, (iii) confirms that the Arrayán Formation is younger than the mid-Early Carboniferous and older than the late Early to early Late Permian Huentelauquén Formation, and (iv) locates its stratigraphic position between the late Early Carboniferous and Early Permian. Previous dates from the compositionally similar metaturbiditic Agua Dulce Formation, which is exposed next to the Arrayán Formation and grades into a mélange, yielded a similar age, which indicates that deposition of both units was coeval. The maximum depositional ages of the Arrayán and the Agua Dulce formations are older than the maximum depositional ages obtained from the metamorphic units in the Choapa Accretionary Complex, which confirms that frontal accretion began earlier than basal accretion, and that the latter reached the Permian and was coeval with deposition of the Huentelauquén Formation. The stratigraphic position and the zircon age distribution pattern suggest that the El Toco, Sierra El Tigre and the Arrayán formations are correlative. The Las Tórtolas Formation is somewhat younger than these formations. The age distribution pattern of the zircon grains in sample PS-8 shows two well-defined age peaks at ∼340 (Visean) and ∼480 Ma (Ordovician) and a barren interval between them. The barren interval coincides partly with the Middle Ordovician to Late Carboniferous passive stage proposed for this region of Gondwana. The Ordovician peak is present in all analyzed samples, between 22° and 39°20′S, whereas the Visean peak and the barren interval are less developed in samples north of 27°S. This similarity indicates the existence of roughly similar sources of sediments along this large section of the western Gondwana margin and, possibly, a rather similar paleogeographic context and depositional environment in the trench. A peak younger than 300 Ma in the Llano de Chocolate Beds indicates a younger depositional age and a different paleogeographic setting in the active continental margin, possibly in the forearc basin, like the Huentelauquén Formation. Broken-formation or mélange facies occur in all complexes, except in the El Toco Formation, and affect the frontally accreted portion of the subduction wedge. The much higher metamorphic grade shown in the Chañaral mélange compared to the much lower grade present in the Agua Dulce mélange suggests formation at different levels along a mega-splay or thrust rooted in the subduction channel. The overall mid-Mississippian age of the frontally accreted deposits analyzed in this study indicates that subduction was active already in the Early Carboniferous. The age, the tectonic setting and the geographic location of these deposits is totally different from the mostly older, marine deposits exposed to the east, in the Frontal Cordillera, which accumulated during a passive stage, in the Middle/Late Ordovician to late Early Carboniferous.

Revised stratigraphic position of a volcanic-ash-derived maximum depositional age in the Lower Cretaceous McMurray Formation

Abstract The stratigraphic framework developed for the McMurray Formation and Wabiskaw Member of the Clearwater Formation (Aptian, Lower Cretaceous) provides a consistent nomenclature and allows for correlation across the Athabasca Oil Sands Region. Event horizons (e.g. volcanic ash layers) within the stratigraphic framework provide crucial geochronologic ages that constrain the timing of deposition and improve upon biostratigraphic estimations. Here, we provide recognition criteria for genetic depositional units in the Firebag sub-basin with the intent of revising the stratigraphic position of a previously published volanic ash-derived maximum depositional age of 115.07 +/– 0.16 Ma. Results demonstrate that the previous placement of this event horizon at the top of the B1 parasequence set was not consistent with the accepted definition of the McMurray Formation–Wabiskaw Member boundary or application of the stratigraphic framework. Here, we establish that the ash-bearing coal horizon is at the top of the A2 parasequence set/channel belt of the McMurray Formation. The implications of this change include constraining the age of the A2 channel belt system to approximately 115 Ma which, under the previous study, would have been younger and unconstrained. We also discuss the implications for our understanding of the Aptian-Albian boundary in the context of allogenic drivers and global environmental change at that time.

Reconstructing the Zama (Mexico) discovery source to sink story, Part I; detrital zircon U–Pb and (U‐Th)/He provenance analysis and implications for sediment source terranes

AbstractThe Zama discovery was identified off the coast of Tabasco, Mexico, in the Sureste Basin of the Gulf of Mexico and is hosted in a three‐way closure in the Upper Miocene. This study conducted a detailed detrital zircon (DZ) U–Pb and (U‐Th)/He provenance analyses on samples from sandstone reservoirs in the Zama‐3 and Zama‐2ST1 wells. A total of 22 sandstone samples (11 from each well) were collected for DZ U–Pb and (U‐Th)/He dating from different reservoir zones, testing the hypothesis that different zones were whether originally derived from varied sedimentary source terranes and associated transport pathways to the Zama mini‐basin depositional site. Additional objectives include determination of maximum depositional ages, reconstruction of paleofluvial systems, and exploring the temporal evolution of the drainage region and hinterland tectonics. The DZ U–Pb age spectra from both Zama wells have remarkably homogenous DZ signatures with very similar DZ age modes and modal percentages, displaying dominant Permian/Chiapas Batholith (ca. 35%–45%), Mesoproterozoic/Oaxaquian (ca. 20%–35%), Early Palaeozoic/Acatlán (ca. 8%–20%), and Cenozoic magmatic arc (ca. 15%–25%) age modes, as well as some subsidiary (<5%) early Proterozoic/Archean and Early Cretaceous DZ age components, linked to recycled lower Palaeozoic strata and the Guerrero Terrane and Alisitos arc, respectively. Despite differences in paleocurrent directions, deduced from image logs, there are no systematic differences in DZ spectra, indicating a consistent sediment provenance and no changes in source area. All Zama samples analysed in the study are characterized by abundant syn‐depositional Late Miocene DZ grains, clustering between 8.6 and 10.2 Ma, corroborating a Tortonian (Late Miocene) depositional age, and yield rapid sediment accumulation rates of ca. 200 m in <1.4 Ma (13 m/Myr). Doubled zircon U–Pb and (U‐Th)/He age pairs are indicative of recycling of early Mesozoic rift strata and Paleogene and younger Chiapas basement. These new DZ U–Pb and (U‐Th)/He data have a nearly invariant Tortonian sediment provenance that is similar to the modern Grijalva River flowing generally northward out of the Chiapas highlands. The paleo‐Grijalva drainage, providing sediment to the Late Miocene Zama mini‐basin, was likely drastically larger than the present catchment as it involved 10 Ma plutonic sources that were subsequently downfaulted along the Pacific coast in the latest Miocene. Importantly, DZ U–Pb age components are consistent with Oaxaquia, early Acatlán, and Guerrero/Alisitos signatures and point to sourcing from the Chortis block during its tectonic eastward translation. Such a scenario would allow for a substantially larger Miocene paleo‐drainage that would have encompassed both Chiapas and portions of Chortis. The Miocene tectonic translation of Chortis and erosion of a large and tectonically active hinterland would also reconcile the dramatically larger Middle to Late Miocene sediment supply, funnelling a large influx of sediment into the southern Gulf of Mexico. Rapid Late Miocene sediment accumulation in the salt‐defined Zama mini‐basin must have involved sustained sediment flux via the paleo‐Grijalva drainage and was likely facilitated and focused by continued salt deflation due to sediment loading, as described in Part II. Thus, our work provides new scientific insights into one of the largest hydrocarbon discoveries in Mexico in recent years.