REVUE DE MICROPALEONTOLOGIE最新文献

This research examines palynomorph assemblages preserved in sediments of the Iberian Pyrite Belt (IPB) that host the Neves-Corvo massive sulfide deposit and its importance for mineral exploration. The sediments belong to the Neves Formation, characterized by black shales and minor occurrences of siltstones and cherts, indicative of reduced environmental conditions, favorable to sulfide deposition in a hydrothermal context dominated by submarine felsic volcanism and graben structures. Analysis was performed in barren drill holes (e.g., Monte Novo), as opposed to productive sectors with massive sulfide mineralization and/or stockwork vein networks (e.g., Lombador, Corvo and Semblana deposits) and favorable sectors like Algaré. The recovered palynological assemblages are assigned to the LN Miospore Biozone, indicating a Late Famennian (latest Strunian) age. Several characteristic species of this age are identified, such as Retispora lepidophyta and Verrucosisporites nitidus in close association with Densosporites spitbergensis, Dictyotriletes fimbriatus, Retusotriletes cf. incohatus, Retusotriletes crassus, Vallatisporites spp. (including V. pusillites, and V. verrucosus). Despite some palynomorphs showing signs of breakage, folding, and indistinctness, no significant preservation or diversity differences were noted between barren and mineralized areas. The high abundance of marine phytoplankton in all studied black shales indicates ecologically stressed setting, dominated by dysoxic to anoxic conditions in a distal marine setting with active felsic volcanism and hydrothermal mineralizing events during Late Famennian time. This geologic context correlates with other IPB deposits and the global latest Devonian anoxic Hangenberg event.

The Kamitaki Complex, situated in the Sasayama area in Southwest Japan, has long been presumed to be a Permian subduction-related accretionary complex based on correlations from previous studies. However, because of the lack of fossil evidence, the exact age of the complex remained uncertain for a long time period. To address this gap in knowledge, a geological survey and microfossil mapping were conducted in the Kamitaki Complex to determine its age and geological context.

A geological survey revealed that the Kamitaki Complex mainly consists of clastic rocks, and a mixture of sandstone, basalt, and chert blocks within the mudstones. The Kamitaki Complex is tectonically intercalated into the Lopingian (Late Permian) accretionary complex of the Ultra-Tamba Terrane and Late Triassic accretionary complex of the Tamba Terrane. The lithological and structural characteristics of the Kamitaki Complex confirm that it is an accretionary complex. Microfossil mapping yielded depositional ages, with radiolarian fauna such as Eptingium nakasekoi, Pseudostylosphaera japonica, Cryptostephanidium japonicum, and Oertlispongus cf. diacanthus identified in mudstones suggesting an Anisian (early Middle Triassic) age. In contrast, radiolarian fauna found in cherts, including Pseudoalbaillella? aff. longicornis, and Follicucullus cf. porrectus, indicate an early Capitanian (late Guadalupian, middle Permian) depositional age. These findings suggest that the Kamitaki Complex records a trenchward migration of the oceanic plate in a pelagic environment from the early Capitanian and an accretion at the trench during the Anisian period.

Conventionally, the plate boundary between the Panthalassa and Paleo-Asia during the latest Permian to Middle Triassic was of the transform type, primarily because no subduction-related accretionary complexes from this period have been identified in the Japanese Islands. However, the discovery of Kamitaki Complex, an Anisian accretionary complex, provides evidence of Middle Triassic subduction activity along the eastern margin of Paleo-Asia. According to the internal structure and age polarity of the Ultra-Tamba and Tamba terranes, an accretionary complex developed over a prolonged period (approximately 120 million years) in a tectonic setting that persisted along the eastern margin of Paleo-Asia from the late Guadalupian to the earliest Cretaceous period.

New perspectives in the archaeometric investigation of the chert used in lithic tool assemblages allow us to learn more about the socio−economic behavior of prehistoric human groups. The case study presented here is from the Artofago Cave in South Tuscany, Italy. Among other findings, an Upper Paleolithic lithic complex was excavated from a large fireplace near the entrance.

The purpose of this study is to gather data aimed at determining the raw material used in the production of these lithic tools, thereby enhancing research into its geographic source area. Here we highlight the significance of the geological age of the rock as one of key factors enabling us to formulate hypotheses about its geological provenance. It showcases a successful application of radiolarian biostratigraphy in dating cherts from the archaeological record. Specifically, 67 chert samples from the Upper Paleolithic lithic complex of the Artofago cave were treated with HF to isolate the radiolarian assemblages. The samples had been carefully chosen in advance between broken chert pieces (chert debris) in the waste material given that the analysis is partially destructive.

Chert samples can be assigned to a Middle − Late Jurassic age, more specifically middle Callovian−early Oxfordian to late Kimmeridgian−early Tithonian (UAZ 8-11).

These ages are comparable with those of the Diaspri di Monte Alpe Formation of the Ligurian Vara Unit, the uppermost tectonic unit of the Northern Apennines.

This study presents the first data on radiolarian fauna from Miocene deposits of the submarine Vityaz Ridge (SVR) and paraxial zone of the Kuril-Kamchatka Trench. Twenty-two dredge samples were studied, and 214 radiolarian taxa were identified. Taxonomic composition allowed their assignment to Miocene assemblage zones, including Lipmanella japonica conica-Gondwanaria dogieli, Pentactinosphaera hokurikuensis, Dendrospyris sakaii, Eucyrtidium inflatum Subzone a, Lychnocanoma magnacornuta, and Lychnocanoma parallelipes zones. These radiolarian assemblages correlate with studied sequences of many deep-sea cores in the northern Pacific and some sections of onshore Japan. As a result, we designed a biostratigraphic scheme of Miocene radiolarians for the SVR and reconstructed the environmental conditions in this area. In particular, two Miocene climatic optima that were previously established in the northern Pacific were identified in the Middle and Upper Miocene sediments of the southern plateau and Middle Miocene sediments of the northern plateau of the SVR.

Palynological analysis of 200 ditch cuttings of wells M6 and M7 from the western Niger Delta yield a diverse assemblage of pollen, spores, freshwater algae, foraminiferal wall linings and dinoflagellate cysts. Based on pollen and spore marker species, the wells date middle – late Miocene (P720-P860). The regular occurrence of dinoflagellate cysts enhances the subdivision of the sequences into eight and seven tentative dinoflagellate cysts assemblage zones for the M6 and M7 wells respectively. The proposed Niger Delta Dinoflagellate Cysts (C-I) zones are viz: the Lingulodinium machaerophorum Assemblage zone; Homotryblium spp./Sumatradinium spp. Assemblage zone; Operculodinium centrocarpum Assemblage zone; Nematosphaeropsis labyrinthus Assemblage zone; and the Protoperidinium spp./Selenopemphix spp. Assemblage zone. The boundaries are marked by remarkable dinocysts events such as first downhole occurrences or highest stratigraphic occurrences, supplemented with last downhole occurrences or lowest stratigraphic occurrences as the samples were ditch cuttings. The common presence of dinoflagellate cysts and the dominance of Nematosphaeropsis labyrinthus and Impagidinium spp. indicate open marine conditions during the deposition of the studied section.

Reconstructing ocean plate stratigraphy (OPS) and revealing the places of origin and ages of oceanic rocks of the Jurassic–Early Cretaceous accretionary complex in Eastern Asia are important to clarify the accretion process and the paleogeography and tectonic setting of the Panthalassa Sea during the Paleozoic to Mesozoic times. In this context, many geological investigations of the Middle Jurassic–Early Cretaceous units of the Northern Chichibu accretionary complex (except for an Early Jurassic one) and Mikabu Unit have been performed. In this study, geological mapping, faunal analysis of radiolarians, and geochemical analysis of basaltic rocks were conducted for the Hebiki Unit of the Northern Chichibu accretionary complex to obtain prime geological information such as its stratigraphy, ages, and origin of basaltic rock. The Early Jurassic Hebiki Unit consists of chaotic mélange, which contains sandstone, siliceous shale, chert, and basaltic rocks as blocks in a highly sheared shale matrix. Late Permian and Late Triassic radiolarians were newly identified from chert blocks in this study. Based on the radiolarian evidence from this study and previous ones that identified Jurassic radiolarians from siliceous shale, the OPS of the Hebiki Unit are reconstructed as follows: Upper Permian and Upper Triassic (upper Carnian to Rhaetian) chert, and Lower Jurassic (Pliensbachian) siliceous shale in ascending order. Meanwhile, the basaltic rocks of the Hebiki Unit are divided into two types in terms of the major and trace element composition and REE profiles: low and high Nb/Zr types. The low and high types are interpreted as oceanic island basalt (OIB) and mid-ocean ridge basalt (MORB), respectively. Considering the reconstructed OPS of the four tectonostratigraphic units (Kashiwagi, Kamiyoshida, Sumaizuku, and Hebiki) of the Northern Chichibu accretionary complex, the four units are considered to have accreted successively.