Maximum Depositional Age Research Articles

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.

Detrital zircon U-Pb-Hf isotopes of late Meso-Neoproterozoic metasedimentary rocks in the South Altyn Tagh: Implications for tectonic affinity of microcontinents in NW China

The Neoproterozoic tectonic correlation between the Central-South Altyn, Qilian, Qaidam, and East Kunlun blocks in northwestern China remains controversial, with competing models favoring separate blocks or a unified single block, and debatable paleo-positions in the Rodinia supercontinent. In this study, we present a systematic provenance study on the late Mesoproterozoic to Neoproterozoic metasedimentary rocks from the South Altyn Tagh. A mica quartz schist sample from the Bashikuergan group yielded a maximum depositional age of 1050 ± 31 Ma. Four paragneiss samples from the Altyn Complex yielded maximum depositional ages of 1106 Ma, 1065 Ma, 1054 Ma, and 870 Ma. Given that the Altyn Complex was intruded by numerous early Neoproterozoic granitoids (ca. 976–900 Ma), we propose that the sedimentary protoliths of the Altyn Complex and the Bashikuergan Group were deposited at two stages, i.e., ca. 1105–975 Ma and after 870 Ma. Provenance tracing indicates that these 1105–975 Ma sediments probably received detritus from the late Mesoproterozoic rocks of Western Australia, East Antarctica, and Central Indian Tectonic Zone in India. In contrast, the detritus of the paragneiss (deposited after 870 Ma) was likely sourced from local regions in the Altyn Tagh orogen. Combined with the comparable magmatic, sedimentary, and tectonic records, we propose that a few microcontinental fragments in northwestern China, including Central-South Altyn Tagh, Qilian, Qaidam, and East Kunlun blocks, constituted a unified block in the early Neoproterozoic and occupied a periphery position of the Rodinia supercontinent with a close paleogeographic affinity to South China and Northwest India.

Accretion of microcontinents and arcs in the southern Central Asian Orogenic Belt: insights from provenance analyses of early Paleozoic sedimentary records in Beishan

The Central Asian Orogenic Belt involves amalgamation of numerous magmatic arcs, accretionary complexes and continental fragments during long-lived subduction and accretion. However, the nature of key microcontinental terranes, as well as their continuity, remains unclear. This study conducted outcrop observation, heavy mineral assemblage, whole-rock geochemistry, detrital zircon U–Pb dating and Lu–Hf isotope analysis on sandstones in the Beishan orogenic belt. The findings from field observations, petrographic analysis and weathering indices show that the tectonic setting of the sandstone is a forearc sedimentary basin. The maximum depositional ages (MDAs) of sandstones are 503.8 ± 6.9 Ma and 431.9 ± 6.0 Ma. The provenance of the sandstone is complex, which probably originates from the Shuangyingshan and Hanshan blocks. By comparing the detrital zircon U–Pb ages with adjacent terranes such as Central Tianshan, Dunhuang and Tarim, it was observed that the Shuangyingshan block shares similar provenance with the Dunhuang terrane, while the Mazongshan–Hanshan shows affinity to the Central Tianshan terrane. This study highlights the complex nature of sediment provenance in the Beishan orogenic belt, influenced by tectonic processes involving thrusting, accretionary complexes and the presence of multiple terranes.

Long-term stability of sediment routing on an active continental margin: Insights from detrital zircon U-Pb ages and measured stratigraphy of Carboniferous to Miocene strata, Sierra de Narváez, NW Argentina

The complex Phanerozoic tectonic evolution of the western margin of South America, including extensional, neutral, and contractional deformation regimes, provides an opportunity to test how sedimentary systems were affected by variable tectonics on a long-lived active margin. Here, we report new geologic mapping, measured stratigraphic sections, paleocurrent measurements, and detrital zircon U-Pb ages from the Fiambalá area, NW Argentina, that together constrain the evolution of sediment routing and basin systems of the northern Sierras Pampeanas during Carboniferous to Miocene time. Carboniferous to Triassic strata of the Paganzo Group include fluvial, lacustrine, and eolian intervals deposited in an extensional setting with SSW-directed paleocurrent orientations. The Mesozoic El Pedernal Formation consists of a lower, basalt interval and an upper, conglomerate and sandstone interval, which has a new detrital zircon maximum depositional age of ca. 88 Ma. Observed strata of the Miocene Tambería Formation include fluvial to eolian sandstones of the lower member of the formation and alluvial conglomerates of the middle member. Whereas lower member Tambería fluvial strata have SSW-oriented paleocurrents, middle member conglomerates indicate broadly E-directed paleoflow.Detrital zircon age distributions of the Paganzo Group, El Pedernal Formation, and Tambería Formation are remarkably consistent, with dominant age peaks ca. 1.3–0.9 Ga and ca. 660-450 Ma. The ages and relative magnitudes of these peaks are consistent with derivation from igneous sources associated with Choiyoi magmatism (330-215 Ma), the Famatinian (490-450 Ma) and Pampean (540-510 Ma) orogens, basement of the MARA block (ca. 1.3–0.9 Ga), and recycling of Neoproterozoic to Cambrian sedimentary successions derived from central Gondwana (ca. 1.3–0.9 Ga, 650-590 Ma, 540-510 Ma).The consistent paleocurrent orientations and detrital zircon U-Pb ages of the Carboniferous Paganzo Group to the Miocene lower member of the Tambería Formation suggest that a S-directed axial drainage network with a consistent sediment source area persisted in the region for ∼300 Myr. This axial drainage network was succeeded by an E-directed transverse drainage network initiated between 15 Ma and 8 Ma due to growth of the Andean thrust wedge, coeval with tectonic compartmentalization and drainage modification of nearby foreland depocenters. These findings highlight the potential for long-term persistence of drainage networks on continental margins under changing tectonic regimes.

Glacially influenced provenance and Sturtian affinity revealed by detrital zircon U–Pb ages from sandstones in the Port Askaig Formation, Dalradian Supergroup

The Cryogenian ‘Sturtian’ snowball Earth glaciation ( c. 717–658 Ma) likely had a major role in shaping continental landscapes and biotic radiations of the late Neoproterozoic Era. However, an incomplete sedimentary record and inadequate syn-glacial age constraints make Cryogenian studies challenging. We present detrital zircon U–Pb ages for >2000 zircons from 11 sandstone samples taken at <200 m stratigraphic resolution throughout the Port Askaig Formation, a c. 1.1 km thick glaciogenic succession within the Dalradian Supergroup of Scotland and Ireland. Eight new maximum depositional age constraints, including a key constraint on deglaciation (<662.7 ± 7.8 Ma), support lithostratigraphic and stable isotope evidence that suggests the Port Askaig Formation preserves a relatively complete record of the global ‘Sturtian’ glaciation. An increasing contribution from Archean and Paleoproterozoic detritus to the sandstones through the lower c. 500 m of the Port Askaig Formation likely reflects the progressive glacial unroofing of the previously buried Lewisian Gneiss terrane. Archean and Paleoproterozoic grains then become scarce in the uppermost c. 300 m of the formation, which we attribute to glacial modification of the Laurentian continental margin landscape during the waning stage of ‘Sturtian’ glaciation. The disruption to sediment transport pathways caused by this modification, and evidenced by the detrital zircon data, points to partially warm-based ‘Sturtian’ ice sheets that were, to some degree, dynamic.

Source and age of the Ngaye banded iron formations, Adamawa Yadé Domain, Central Cameroon: Constraints from whole-rock geochemistry and U-Pb zircon geochronology

The Ngaye inlier of the Adamawa Yadé Domain is located in the Central African Fold Belt, adjacent to the Nyong Complex at the northern margin of the Congo craton, in central Cameroon. This area consists of metamorphosed granite-greenstone associations comprising amphibole- and pyroxene-rich banded iron formations (BIFs), amphibolites, and migmatitic gneisses. This study reports detailed petrographic, whole-rock geochemical, and LA-ICP-MS U-Pb zircon data for the Ngaye BIFs and associated amphibolites to better constrain their source, tectonic setting, and age. Amphibole-rich BIFs exhibit high REE-Y content and uncommon LREE-enriched patterns. In contrast, pyroxene-rich BIFs display combined low REE-Y content, seawater-like patterns, and positive Eu anomalies, suggesting a mixture of seawater and hydrothermal fluids during their deposition. The geochemical data of associated amphibolites suggest a back-arc setting for the Ngaye metavolcanosedimentary sequence, similar to that of the Nyong Complex. LA-ICP-MS U-Pb zircon dating of amphibole-rich BIFs indicate a maximum depositional age of ca. 2186 Ma and subsequent metamorphism at ca. 2038 Ma, overlapping with that of metavolcanosedimentary sequences from the Nyong Complex. Neoproterozoic age of ca. 598 Ma obtained for these BIFs is interpreted as the Pan-African metamorphic/hydrothermal imprint. This finding suggests that the post-depositional fluid overprint was probably related to the regional Pan-African tectono-thermal event.

U-Pb zircon and Sm-Nd geochronology of the dom Silvério group, SE Brazil: Implications for the evolution of the superposed Araçuaí and Minas-Bahia orogens

The Araçuaí orogen, the South American portion of the Brasiliano/Pan-African Araçuaí-West Congo orogenic system (AWCO), formed during the Ediacaran-Cambrian assembly of West-Gondwana. The basement of a substantial portion of the Araçuaí orogen comprises Archean and Paleoproterozoic rocks involved in the 2.1–2.0 Ga Minas-Bahia orogen. The Dom Silvério Group, which consists of mica schists containing layers and lenses of mafic, ultramafic, and Mn-rich rocks, as well as quartzites, graphite schists, marbles, and banded iron formations, defines a ca. 10 km-wide and 150 km-long belt in the southwestern portion of the Araçuaí orogen. This succession, currently interpreted as deep marine deposits or an ocean-floor assemblage of uncertain age - Paleoproterozoic or Neoproterozoic-, potentially records processes involved in critical evolutionary stages of the superposed Minas-Bahia and Araçuaí orogens. Aiming to characterize the age and tectonic significance of the Dom Silvério Group, we performed a geochronological and geochemical investigation on samples collected from selected lithotypes. U-Pb (LA-ICP-MS) determinations on detrital zircons revealed two distinct age spectra. Zircons extracted from the lower Dom Silvério schists define a spectrum characterized predominantly by Neoproterozoic age clusters with a maximum depositional age of 601 ± 10 Ma. Quartzites from the lower and upper sections of the group do not contain zircons younger than 1.9 Ga and show unimodal age distributions around 2067 Ma. The maximum depositional age estimated for these rocks is 2020 ± 6 Ma. The spectrum of the lower section schist indicates the turbidites of the Ediacaran Salinas Formation as their most probable correlative. In contrast, the quartzites and schists of lower and upper sections correlate with the Orosirian Itacolomi Group, the post-collisional assembly of the Minas-Bahia orogen. Sm-Nd isotope analyses and whole-rock geochemistry allow the discrimination of two generations of mafic rocks. The older generation, showing TDM model ages = 1.92 to 3.6 Ga and ƐNd(t) = -12.93 to +4.63, represents components of the basement. The younger generation, with TDM model ages = 1.0 Ga to 1.25 Ga and ƐNd(t) = +0.05 to +2.29, is similar to the ophiolitic rocks that occur imbricated with Ediacaran metasedimentary successions in the central portion of the Araçuaí orogen defining the Ribeirão da Folha accretionary belt. They show Th/Yb and Nb/Yb ratios of the EMORB-OIB mantle array and Nb/Y and Zr/Y ratios compatible with OIB and oceanic plateau basalts. The Dom Silvério Group is thus a complex that involves members of the Araçuaí and Minas-Bahia orogens. Their Ediacaran components materialize the southern extension of the Ribeirão da Folha accretionary wedge and, as such, provide further evidence for oceanic consumption during the development of the southern Araçuaí orogen.

Cretaceous magnetostratigraphy of the southern Simao Basin, SE Tibetan Plateau, and its paleogeographic implications

Precise stratigraphic chronology is the basis of many studies (e.g., tectonic, paleoclimate, and mineralization) in geoscience. Here, we carried out a detailed youngest detrital zircon U-Pb age-constrained magnetostratigraphic study on the middle-lower parts of a 2309-m-thick Upper Jurassic−Lower Cretaceous sequence from west Jiangcheng in the southern Simao Basin, southeastern Tibetan Plateau. A total of 2262 paleomagnetic samples were collected for magnetostratigraphy, and five siltstone/sandstone samples were collected for detrital zircon U-Pb analyses. Progressive thermal and alternating field demagnetizations isolated 1575 well-defined interpreted primary characteristic remanent magnetization directions, which yielded 25 normal polarity (N7−N31) and 25 reverse polarity (R8−R32) magnetozones. The interpreted maximum depositional ages calculated using the youngest detrital zircon U-Pb dates are 159.0 Ma, 154.0 Ma, 161.0 Ma, 149.3 Ma, and 139.5 Ma for the stratigraphic horizons at 2303 m, 1986 m, 1600 m, 1350 m, and 1110 m, respectively, in the section. Together with the previous magnetostratigraphic results of the upper part of the section, the observed polarity zones are best correlated with chrons C34n−M17r of the geomagnetic polarity timescale (GPTS2012), yielding magnetostratigraphic age estimates of ca. 143.5 Ma to ca. 65.0 Ma for the entire stratigraphic sequence. Paleogeographic analysis shows that the Simao Basin was in a low-latitude offshore environment during the Late Jurassic−earliest Cretaceous (before 142.3 Ma), a dominant fluvial environment during the early−middle Early Cretaceous (142.3−111.5 Ma), with widespread desertification at the end of this period, a saline lake sedimentary environment during the late Early Cretaceous−early Late Cretaceous (111.5−88.7 Ma), and a fluvial environment during the late Late Cretaceous (88.7−65.0 Ma). The changes in depositional environment are consistent with regional/global tectonic and paleoclimate changes, such as the dismission of Pangea, the termination of the megamonsoon, and the closure of the Meso-Tethys Ocean.

Late Mesoproterozoic passive continental margin in the northwestern Yangtze Block, South China: Insights from the Huodiya Group in the Hannan‐Micangshan Massif

Whether the Yangtze Block was involved in the global late Mesoproterozoic orogeny remains contentious. The Mesoproterozoic Huodiya Group exposed in the Hannan‐Micangshan Massif on the northwestern margin of the Yangtze Block could provide sufficient evidence to further clarify the regional tectonic affinity. However, quantitative insights into the depositional age, provenance and basin tectonic setting of the Huodiya Group still lack comprehensively constraining. This study presents a combined analysis of zircon U–Pb dating for meta‐sedimentary rocks of the Shangliang Formation of the upper Huodiya Group and intruded gabbroic dykes. Intergraded dating results constrain the late Mesoproterozoic maximum depositional age at ca. 1050 Ma. Ca. 900 Ma formation age for the intruded gabbroic dyke, plus the minimum formation age of the overlying Xixiang Group at ca. 950 Ma, further suggests its deposition should be prior to ca. 950–900 Ma. Concordant detrital zircon ages define pronounced age clusters of ca. 2229–1741 and 2950–2388 Ma and few age populations of ca. 3232–3082 and 1069–1033 Ma, consistent with the tectonothermal events in proximal domains at the northwestern and northern Yangtze Block. Detrital zircon age distribution patterns and cumulative curves, coupled with the stable carbonate platform and shallow‐marine facies sedimentation, suggest a passive continental margin setting for the Huodiya Group on the Yangtze Block margin at the late Mesoproterozoic. Comparative insights into the contemporaneous passive continental margin from north to southwest argue against the existence of a late Mesoproterozoic orogeny along the exterior periphery of the Yangtze Block.

The leaky chronometer: Evidence for systematic cryptic Pb loss in laser ablation U‐Pb dating of zircon relative to CA‐TIMS

AbstractU–Pb dating of zircon via laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) has become a primary method for determining the age of crystalline rocks and the provenance and maximum depositional age of (meta)sedimentary rocks. Although chemical abrasion has become a standard approach for mitigating Pb loss in zircon dated via high‐resolution thermal ionization mass spectrometry (CA‐TIMS), laser ablation dating is rarely accompanied by chemical abrasion. We evaluate the accuracy of U–Pb dates acquired via LA‐ICP‐MS relative to CA‐TIMS through analysis of a large database of paired analyses from the same zircon crystals. We show that laser ablation 206Pb/238U dates are systematically younger than their CA‐TIMS counterparts, with a greater shift in detrital zircon (−2.0%) versus igneous or metamorphic zircon (−0.9%). Although systematic biases related to “matrix effects” may be a contributing factor, our analysis suggests that unmitigated cryptic Pb loss is likely widespread in laser ablation U–Pb datasets.