Geochronology and geochemistry of the Huntington Formation, Olds Ferry terrane, Blue Mountains province, northern U.S. Cordillera: Implications for accreted terrane correlation and assembly

The Olds Ferry terrane is the more inboard of two accreted volcanic arc terranes in the late Paleozoic−early Mesozoic Blue Mountains province of the northern U.S. Cordillera. We present geologic, geochronologic, and geochemical data from the volcano-sedimentary Huntington Formation of the Olds Ferry arc that place the terrane within a firm temporal and tectonomagmatic context, and establish its identity as a fringing arc terrane along the Triassic to Early Jurassic Cordilleran margin. The Huntington Formation is divided into two unconformity-bounded informal members: a Norian (ca. 220 Ma) lower member comprising a sequence of mafic-intermediate volcanics, massive volcaniclastic breccias, and minor carbonates deposited unconformably onto the 237.7 Ma Brownlee pluton and intruded by the 210.0 Ma Iron Mountain pluton; and a Rhaetian through Pleinsbachian (<210−187.0 Ma) upper member composed of massive conglomerates, abundant rhyodacite to rhyolite effusive and pyroclastic flows, and interlayered sandstone turbidites, deposited with angular unconformity onto the lower member. An erosional hiatus and regional tilting produced an angular unconformity separating the Huntington Formation from the overlying basal conglomerates of the late Early to Middle Jurassic Weatherby Formation of the Izee forearc basin transgressive onlap sequence. Huntington Formation volcanic rocks are isotopically enriched relative to depleted mantle and coeval igneous rocks in the outboard Wallowa terrane. A temporal evolution to more radiogenic 87Sr/86Sr ratios (0.7036−0.7057) and εNd values (+5.4 to +3.1) in the upper member volcanics suggests increasing involvement of continental-derived material in their petrogenesis. Precambrian xenocrystic zircons in both lower and upper member volcaniclastic rocks strongly support a proximal location of the Olds Ferry terrane to cratonal North America during much of its history. The chronology and tectonostratigraphic architecture of the Olds Ferry terrane allows its robust correlation to other fringing-arc terranes along the U.S. and Canadian Cordillera.

LEONARDO DA VINCI’S AND NICOLAUS STENO’S GEOLOGY

Anniversaries for the two founding fathers of geology occurring in the same year prompted a comparative evaluation of how the two contributed to establishing the basic principles of the discipline. To do so, passages from their publications, codices and manuscripts have been quoted directly. The Stenonian principles (‘original horizontality’, ‘original continuity’, and ‘superposition of individual strata’) are present in Leonardo’s notebooks amazingly formulated, using similar wording when studying the same area more than 150 years earlier. Also, Stenonian priority in naming and explaining geological concepts and processes (e.g., faulting, folding, angular unconformity, relative chronology) are mirrored in Leonardo’s writings and pictorial works. While Steno enjoys priority in stepwise restoration of the geological history of a given region, Leonardo was the first to construct a 3D geological profile representation and geomorphologic maps. Lastly, the paper focuses on diverging stances of the two savants about the Noachian Deluge and the age of the Earth. Already 500 years ago, Leonardo had solved the question of marine fossil remains of organic origin found in the mountains implying the possibility of deep geologic time in a statement of ‘eternalism’. 350 years ago, Steno solved the same question in a different way in which he retained a basic role for the Deluge and assumed a short age for the Earth by focusing mainly on short-lived sedimentary and geomorphologic processes.

LWD DATA INVERSION FOR STRUCTURE WITH ANGULAR UNCONFORMITY

Non-structural traps and reservoir flanks are characterized by angular unconformities. Angular unconformity between dipping formation and sub-horizontal oil-water contact is common in the North Sea fields. This paper presents an approach to real-time inversion of LWD resistivity data for the scenario with angular unconformity. The approach utilizes artificial neural networks (ANNs) for calculating the tool responses in parametric surface-based 2D resistivity models. We propose a parametric model with two non-parallel boundaries suitable for scenarios with angular unconformity and pinch-out. Training of ANNs for this parametric model is performed using a database containing samples with the model parameters and corresponding tool responses. ANNs are the kernel of 2D inversion based on the Levenberg-Marquardt optimization method. To demonstrate applicability of our approach and compare with the results of 1D inversion, we analyze Extra Deep Azimuthal Resistivity tool responses in a 2D synthetic model. It is shown that 1D inversion determines either the position of the oil-water contact or dipping layers structure. At the same time, 2D inversion makes it possible to correctly reconstruct the positions of non-parallel boundaries. Performance of 2D inversion based on ANNs is suitable for real-time applications.

Detrital zircon provenance record of the Zagros mountain building from the Neotethys obduction to the Arabia-Eurasia collision, NW Zagros fold-thrust belt, Kurdistan region of Iraq

Recognition of new angular unconformity and synthesizing of new detrital zircon U-Pb and (U-Th)/He provenance records, including zircon (U-Th)/(He-Pb) double dating, from the NW Zagros elucidate the basin dynamics of the foreland wedge-top and intermontane units, as well as the tectonic processes in the source terranes in response to different geodynamic phases. In this contribution, we present field observations and detrital zircon provenance data from hinterland basins to reconstruct the basin dynamics and the underlying tectonic controls in the NW Zagros in the Kurdistan region of Iraq. Results reveal that the deposition of the suture zone units of the Red Beds Series (RBS; Suwais Group, Govanda Formation, Merga Group) occurred in an intermontane basin on top of folded Upper Cretaceous units with an angular unconformity. The RBS provenance data point at the Paleogene Walash-Naopurdan-Kamyaran (WNK) arc-related complex as a source area and show substantial decrease of magmatism by ~ 36 Ma, as reflected by the youngest ages peaks. New detrital zircon provenance data from the hinterland wedge-top units of the proto-Zagros foreland basin (the Tanjero, Kolosh, and Gercus Formations) exhibit exclusive derivation from the Upper Cretaceous Neotethys ophiolitic terranes, different from the provenance of the older Lower Cretaceous and Paleozoic units that are dominated by the Paleozoic and Neoproterozoic age spectra. These shifts in provenance between different tectonostratigraphic units argue for sediment route reversal from E to W in response to ophiolite obduction, arrival of the WNK complex and commencement of the continental collision during the late Eocene, followed by deposition of the RBS in the hinterland of the proto-Zagros fold-thrust belt, and paleodrainage connection with the post-collisional Neogene foreland basin.

Early Devonian tectonic conversion from contraction to extension in the Chinese Western Tianshan: A response to slab rollback

Extensive angular unconformities that occur across the Tianshan Orogen provide insight into the Paleozoic tectonic evolution of the southwestern Central Asian Orogenic Belt. This study presents new geochronological and geochemical data to unravel the age and origin of a pronounced angular unconformity in the Baluntai domain of the Chinese Western Tianshan. The angular unconformity separates a ductilely deformed metamorphic complex from overlying unmetamorphosed sequences. Detrital zircon age spectra suggest that the sedimentary samples received detritus from both the Precambrian basement and contemporaneous igneous rocks in the Central Tianshan and Yili blocks. The youngest detrital and metamorphic zircons in metasediments from the metamorphic complex reveal that their protoliths were deposited at ca. 419 Ma, and subsequently overprinted by metamorphism and ductile deformation at ca. 409 Ma. Along with ca. 532−407 Ma metavolcanics and foliated intrusions, the metamorphic complex constitutes a diachronous stratigraphic unit deposited during early Cambrian to late Silurian times, rather than a Precambrian basement as previously thought. The youngest detrital zircon age data from siliciclastic samples immediately above the angular unconformity constrain their deposition to ca. 407−397 Ma. In conjunction with a ca. 406 Ma rhyolitic tuff in the lower terrestrial siliciclastic unit and an early Carboniferous paleontological age of carbonates in the upper shallow-marine unit, it is suggested that the overlying unmetamorphosed sequences were accumulated during the early Devonian to the early Carboniferous. Thus, the angular unconformity in the Baluntai domain was formed between ca. 409 and 407 Ma. New and already published geochemical data reveal that the early Paleozoic (ca. 530−410 Ma) and late Paleozoic (ca. 410−320 Ma) igneous rocks are mainly medium- to high-K calc-alkaline I-type granitoids, which exhibit typical subduction-related geochemical signatures despite contrasting structural features. These geochronological and geochemical results are in accordance with those of similar early Devonian angular unconformities and Paleozoic igneous rocks in the southern Yili Block. The early Paleozoic igneous rocks display highly scattered zircon ɛHf(t) values, while the late Paleozoic rocks show a progressively increasing trend toward more positive values, attributed to slab rollback of the northward subducting South Tianshan Ocean under the Yili and Central Tianshan blocks. Thus, we propose that the formation of the angular unconformity was caused by tectonic conversion from contraction to extension due to slab rollback during the early Devonian. This study highlights the significance of geological and geochronological investigations of angular unconformities in the context of associated episodic magmatism and slab behavior in order to unravel distinct tectonic processes in the long-lived accretionary evolution of the Chinese Western Tianshan.

STRATIGRAPHIC AND STRUCTURAL RELATIONSHIPS THROUGH GEOLOGICAL AND GAMMAESPECTROMETRY ANALYSIS IN THE INTERFACE BETWEEN THE SÃO FRANCISCO CRATON AND THE ARAÇUAÍ OROGEN (SANTA RITA ANTICLINE, MINAS GERAIS, BRAZIL)

Geological mapping (1:25.000 scale), supported by gammaespectrometric analysis, was conducted in the Santa Rita Anticline region, a regional structure located in the contact between the Araçuaí Orogen external belt and the São Francisco Craton. The work aimed to detail the region's structural and stratigraphic relationships between the rocks of Espinhaço and São Francisco supergroups, which contact is not characterized by a thrust front, as it is in a large part of the Espinhaço Meridional Range (EMR). Five units from Espinhaço were mapped: Galho do Miguel Formation and Conselheiro Mata Group, represented by the Santa Rita, Córrego dos Borges, Córrego Bandeira and Córrego Pereira formations. By its turn, three units from the São Francisco Supergroup were mapped: undivided Macaúbas Group and Bambuí Group, represented by the Serra de Santa Helena and Lagoa do Jacaré formations. Restricted to the Espinhaço units, the Santa Rita Anticline consists in a structure with a south dipping axis and inflections that form saddle structures. The analysis of the gammaespectrometric responses provided clues that were confirmed in the field, highlighting an angular unconformity between the Córrego Pereira Formation and the Macaúbas Group and the local presence of isoclinal folds in Serra de Santa Helena rocks, promoting relevant questions regarding possible intermediary deformational events between the Statherian and Tonian tafrogenesis. The study contributes by raising questions about the evolution of the Araçuaí Belt in its contact with the São Francisco Craton and possible implications imposed by the Paramirim Keywords: Aerogeophysics, Litho-structural framework, Fold belt and craton transition, Espinhaço Meridional Range