Stratigraphy and ecostratigraphic distribution of foraminiferal morphogroups from the Upper Jurassic of the Makar’yev section (Unzha River,Volga River basin)

Investigation of the Upper Callovian to Lower Kimmeridgian microfossils from the Makar’yev reference section (Unzha River, East European Platform) has been carried out. The section is characterized by ammonite debris and abundant associations of benthic and planktic foraminifers. It is a perfect object for stratigraphic and paleoecological researches. The biostratigraphic distribution of foraminifers from the Makar’yev section allows one to identify standard foraminifera zones of the East European Platform, as well as to upgrade some of them. The analysis of vertical and lateral ammonites and foraminiferal distribution, completed with litho stratigraphy, has precised the stratigraphic volume and position of boundaries of several lithological units.An improved stratigraphic scheme for the Kostroma area of the Moscow Depression is proposed. Analysis of the composition, structure, and dynamic changes of the foraminiferal assemblages has been performed. The morphofunctional analysis of foraminiferal genera has for the first time identified how foraminiferal morphogroups differing in their life style and feeding strategy varied with short-term paleoenvironmental changes. These morphogroup changes allow establishing four ecostratigraphic levels. These paleoecological data have been calibrated along with geochemical factors. They have shown a crisis of foraminiferal association during the Late Oxfordian and Early Kimmeridgian. A similar crisis has also been discovered in the north of Siberia, which may be an argument for its global distribution. The analysis of the taxonomic composition and the density of foraminiferal associations, in parallel with the structure of the association, has revealed a succession of transgressive and regressive events during the Late Callovian-Early Kimmeridgian. It allows the typification of each assemblage in relation with each event and underlines the occurrence of second-order sea-level fluctuations (middle part of the Middle Oxfordian and the earliest Kimmeridgian).     

Sedimentological and petrochemical studies of Jurassic clastic rocks,Habo Dome Basin,Kachchh Mainland,Northwest India: Implications for depositional environment,provenance, and tectonic setting

Mesozoic rocks are extensively and excellently preserved in the western Indian shield in several basins. The Kachchh Mainland Basin (KMB), comprising six small sub‐basins, is the main repository of these sediments. Habo Dome Basin, situated in the easternmost part of KMB and largest among the six basins, hosts clastics of the Chari Formation of Jurassic age. The fluctuating transgressive–regressive facies cycle, developed during the Callovian and Late Early Oxfordian in the Habo Dome Basin, was mainly controlled by local tectonics and not by global eustatic fluctuations. Near magmatic relationships are displayed by various elements of the clastic rocks of Habo Dome Basin. Two litho‐chemical groups have been identified in Habo Dome Basin, which are cyclically repeated over entire lithostratigraphic sequence, indicating alternate pulses of sediment inputs from two different sources under palpitating tectonic conditions. Provenance indicator elements and their ratios coupled with source modeling indicate predominantly felsic source with basic and alkalic components. Integrated analysis of petrograhic and geochemical characteristics suggests two source terranes for these rocks: a granitoid source with significant basic volcanics (Banded Gneissic Complex) and a granite–gneissic source with minor alkaline volcanics (Nagarparkar Massif) lying to northeast and NNW respectively. The petrochemistry of Habo Dome clastics suggests their deposition in a fault controlled sink which was influenced by sea level changes. Drifting of the Indian plate resulted in the opening of series of rifted basins in the Kachchh Mainland during Late Triassic/Early Jurassic, which were closed later during collision of Indian plate with Eurasia at early Eocene. The Habo Dome Basin which opened up as a half graben in response to the initial stress regime, remained tectonically unstable until the cessation of pre and post collisional stress regimes.     

Depositional environments and iron ooid formation in condensed sections (Callovian–Oxfordian, south-eastern Paris basin, France)

Carbonate platforms across Western Europe were superseded at the Middle–Upper Jurassic (Callovian–Oxfordian) boundary either by alternating marl–limestone and widespread marl deposits or by condensed sections containing iron ooids. The characteristics of marine condensed sections in the south-eastern part of the Paris Basin (France) and their distribution pattern are examined here, and a model of iron ooid formation is developed. Iron ooids are found from the shoreface to the offshore zone. They are most abundant in the median-to-distal offshore transition zone, where they originally formed. They also occur commonly, albeit often as reworked grains, in the proximal offshore zone, to which they were transported. The contemporaneous, thick, predominantly marl sections that occur laterally are devoid of iron ooids and were deposited in deeper settings (distal offshore zone). The iron ooids are composed of goethite. Typically, they have a nucleus made up of a clump of goethite crystals and a laminated cortex. Three distinctive nanostructures are identified in the cortex laminae: (i) a nanograined crystalline structure typical of primary goethite; (ii) a secondary nanoflaked structure thought to have formed mechanically by reorientation of the goethite crystals; and (iii) a coalesced structure acquired by subsequent diagenetic recrystallization. The iron ooids formed successively (i) by lamina growth when goethite precipitated in the surface layer of the sediment (nanograined structure) and (ii) by interruption of growth when the ooids were remobilized by hydrodynamic agents, as reflected by the flaked nanostructure; (iii) these two nanostructures were sometimes transformed into a coalesced structure by recrystallization when ooids were buried.     

Characterization and correlation of Upper Jurassic (Oxfordian) bentonite deposits in the Paris Basin and the Subalpine Basin, France

Abstract Explosive volcanic activity is recorded in the Upper Jurassic of the Paris Basin and the Subalpine Basin of France by the identification of five bentonite horizons. These layers occur in Lower Oxfordian (cordatum ammonite zone) to Middle Oxfordian (plicatilis zone) clays and silty clays deposited in outer platform environments. In the Paris Basin, a thick bentonite (10–15 cm), identified in boreholes and in outcrop, is dominated by dioctahedral smectite (95%) with trace amounts of kaolinite, illite and chlorite. In contrast, five bentonites identified in the Subalpine Basin, where burial diagenesis and fluid circulation were more important, are composed of a mixture of kaolinite and regular or random illite/smectite mixed-layer clays in variable proportions, indicating a K-bentonite. In the Subalpine Basin, a 2–15 cm thick bentonite underlain by a layer affected by sulphate–carbonate mineralization can be correlated over 2000 km². Euhedral zircon, apatite and biotite crystals have been identified in all the bentonites. The geochemical composition of the bentonites in both basins is characterized by high concentrations of Hf, Nb, Pb, Ta, Th, Ti, U, Y, Zr and low concentrations of Cr, Cs and Rb. Biostratigraphical and geochemical data suggest that the thick bentonite in the Paris Basin correlates with the thickest bentonite in the Subalpine Basin, located 400 km to the south. These horizons indicate that significant explosive volcanic events occurred during the Middle Oxfordian and provide potential long-distance isochronous marker beds. Immobile element discrimination diagrams and rare-earth element characteristics indicate that the original ash compositions of the thickest bentonites correspond to a trachyandesitic source from a within-plate alkaline series that was probably related to North Atlantic rifting.     

阿姆河右岸B区中部卡洛夫-牛津阶高精度层序地层划分及层序发育模式

土库曼斯坦阿姆河盆地是世界上著名的大型含油气沉积盆地,然而目前盆地内卡洛夫—牛津阶的层序划分和地层对比依然存在争议。基于Vail经典层序地层学及沉积学相关理论,以阿姆河右岸B区29口井的岩心资料及50口井的测井资料为依据,结合地震、薄片、地球化学等手段,开展了阿姆河右岸B区卡洛夫-牛津阶的层序地层研究。将阿姆河右岸B区卡洛夫-牛津阶划分为5个三级层序和15个四级层序,其中卡洛夫阶包括2个三级层序(SQ1和SQ2),牛津阶分为3个三级层序(SQ3-SQ5)。在高精度层序划分的基础上,建立了研究区的层序地层格架,在垂向上,各级层序格架内部高位域生屑砂屑灰岩及礁滩体沉积更为发育;在平面上,根据不同层序沉积几何体和沉积相的分布,明确提出了卡洛夫期和牛津期发育两种不同的碳酸盐岩台地类型的观点：SQ1-SQ2时期(卡洛夫期)研究区应为缓坡型台地,初始具有西高东低的地貌,坡度较缓,沉积速率差别不大,主要发育缓坡台地层序地层模式;SQ3-SQ5时期(牛津期)则演化为镶边台地,研究区沉积速率远小于A区,随着海平面变化形成了差异巨大的西高东低沉积地貌。