Palynomorph and palynofacies indications of age, depositional environments and source potential for hydrocarbons: Lower Cretaceous Zubair Formation, southern Iraq

Assemblages of miospores and dinoflagellate cysts encounterd in core samples three boreholes drilled in southern Iraq an Aptian-mid Albian age-range for the Zubair Formation, the largest oil reservoir in southern Iraq and neighbouring Kuwait. Four types of palynofacies have been identified, and are interpreted to indicate delta top swamp and marsh, delta front, prodelta and marine platform environments respectively. Amorphous matter comprises more than 50% of the organic component of some of the palynofacies in the upper part of the Zubair Formation. The rocks with which they are associated are highly rated as a potential source of liquid hydrocarbons. Those deposits yielding less than this (down to 12%) are considered to have moderate potential.     

Cretaceous sedimentary basins in Sichuan,SW China: Restoration of tectonic and depositional environments

This study documents sediment infill features and their responses to the tectonic evolution of the Sichuan Basin and adjacent areas. The data include a comparison of field outcrops, well drillings, inter-well correlations, seismic data, isopach maps, and the spatial evolution of sedimentary facies. We divided the evolutionary history of the Sichuan Cretaceous Basin into three stages based on the following tectonic subsidence curves: the early Early Cretaceous (145–125 Ma), late Early Cretaceous to early Late Cretaceous (125–89.8 Ma), and late Late Cretaceous (89.8–66 Ma). The basin underwent NW–SE compression with northwestward shortening in the early Early Cretaceous and was dominated by alluvial fans and fluviolacustrine sedimentary systems. The central and northern areas of the Sichuan Basin were rapidly uplifted during the late Early Cretaceous to early Late Cretaceous with southwestward tilting, which resulted in the formation of a depression, exhibited southwestward compression, and was characterized by aeolian desert and fluviolacustrine deposits. The tectonic framework is controlled by the inherited basement structure and the formation of NE mountains, which not only affected the clastic supply of the sedimentary basin but also blocked warm-wet currents from the southeast, which changed the climatic conditions in the late Late Cretaceous. The formation and evolution of Cretaceous sedimentary basins are closely related to synchronous subtle far-field tectonism and changes in climate and drainage systems. According to the analysis of the migration of the Cretaceous sedimentation centers, different basin structures formed during different periods, including periods of peripheral mountain asynchronous thrusting and regional differential uplift. Thus, the Sichuan Cretaceous sedimentary basin is recognized as a superimposed foreland basin.     

Upper Cretaceous depositional environments and bivalve assemblages of far-east Asia: the Himenoura Group, Kyushu, Japan

The depositional environments and bivalve assemblages are determined for the Upper Cretaceous Hinoshima Formation of the Himenoura Group, Kamishima, Amakusa Islands, Kyushu, Japan. The Hinoshima Formation is characterized by a thick transgressive succession that varies from incised-valley-fill deposits to submarine slope deposits with high aggradation rates of depositional systems. The incised valley is filled with fluvial, bayhead delta, brackish-water estuary, and marine embayment deposits, and is overlain by thick slope deposits.Shallow marine bivalves are grouped into five fossil assemblages according to species composition: Glycymeris amakusensis (foreset beds of a bayhead delta), Nippononectes tamurai (foreset beds of a bayhead delta), Ezonuculana mactraeformis–Nucula formosa (central bay), Glycymeris amakusensis–Apiotrigonia minor (slope), and Inoceramus higoensis–Parvamussium yubarensis (slope). These bivalve assemblages all represent autochthonous and parautochthonous conditions except for a Glycymeris amakusensis–Apiotrigonia minor assemblage found in debris flow and slump deposits. The life habitats of these bivalves and the compositions of the assemblages are discussed in terms of the ecological history of fossil bivalves of the mid- to Late Cretaceous.     

The Baganuur coal deposit, Mongolia: depositional environments and paleoecology of a Lower Cretaceous coal-bearing intermontane basin in Eastern Asia

The investigations involved geophysical, sedimentological, palynological, chemical and mineralogical studies, supported by field-based infrared spectrometry. The Baganuur Basin, Central Mongolia, is among the rift or pull-apart-basins, which subsided at the boundary between the Jurassic and the Lower Cretaceous in East Asia. During the Berriasian, peat accumulation began in the area under study in Central Mongolia. The palynoflora is akin to the Siberian palynological province. Based on the phytoclast assemblages and the ratios of total sulfur content to total organic content, marine transgressions into this intermontane basin may be ruled out. The coal interseam sediments were laid down prevalently under neutral to slightly alkaline conditions; only in some carbonaceous sediments, the pH of intrastratal solutions was lowered. Suboxic to anoxic conditions persisted during almost the entire Lower Cretaceous period in the Baganuur Basin. Based on the distribution of fining- and coarsening-upward sequences and the organic matter, the basin fill has been subdivided into seven depositional units (A: fluvial–swamp, B: fluvial–lacustrine, C: deltaic–fluvial, D: fluvial, E: fluvial–deltaic–lacustrine/floodplain (?), F: lacustrine–deltaic–swamp, G: swamp–fluvial). A conspicuous change in the fluvial–lacustrine regime and an increase in the sediment supply may be observed at the boundary between depositional units B and C. A strong uplift triggered the onset of an intensive delta sedimentation. Lithoclasts, heavy minerals (e.g., apatite, zircon, garnet, anatase, brookite, epidote, sphene, tourmaline) and phyllosilicates (e.g., kaolinite, smectite, mica, chlorite) attest to a mixing of detrital material. One provenance area was abundant in acidic plutonic rocks as shown by the granitic lithoclasts, the other in volcanic rocks, which produced the vitroclastic debris deposited as tephra fallout. Post-depositional alteration of the siliciclastic interseam sediments was favored by a distinctive facies association of transmissive and sealing horizons. It led to a re-deposition of Ca, U and Sr in the siliciclastics. Post-depositional alteration of the organic material converted it into lignite to subbituminous C coal.     

Evolution of depositional settings in the Torrey area during the Smithian (Early Triassic,Utah, USA) and their significance for the biotic recovery

This work focuses on well‐exposed Lower Triassic sedimentary rocks in the area of Torrey (south‐central Utah, USA). The studied Smithian 8 deposits record a large‐scale third‐order sea‐level cycle, which permits a detailed reconstruction of the evolution of depositional settings. During the middle Smithian, peritidal microbial limestones associated with a rather low‐diversity benthic fauna were deposited seaward of the tidal flat siliciclastic red beds. Associated with siliceous sponges, microbial limestones formed small m‐scale patch reefs. During the late middle to late Smithian interval, the sedimentary system is characterized by tidal flat dolostones of an interior platform, ooid‐bioclastic deposits of a tide‐dominated shoal complex, and mid‐shelf bioclastic limestones. Microbial deposits, corresponding to sparse stromatolites formed in the interior platform, are contemporaneous with a well‐diversified marine fauna living in a seaward shoal complex and mid‐shelf area. The nature and distribution of these Smithian microbial deposits are not related to any particular deleterious environmental condition, highlighting that observed patterns of biotic recovery after the end‐Permian mass extinction were directly influenced by depositional settings. Facies evolution and stratal stacking patterns allow us to identify large, medium and small‐scale, as well as elementary depositional sequences. Large‐ and medium‐scale sequences are consistent with sea‐level changes, whereas small‐scale and elementary sequences are better explained by autocyclic processes. Copyright © 2015 John Wiley & Sons, Ltd.     

Petrography and geochemistry of the Late Cretaceous redbeds in the Gan-Hang Belt,southeast China: implications for provenance,source weathering,and tectonic setting

ABSTRACT

The distinct basin and range tectonics in southeast China were generated in a crustal extension setting during the late Mesozoic. Compared with the adjacent granitoids of the ranges, the redbeds of the basins have not been well characterized. In this article, provenance, source weathering, and tectonic setting of the redbeds are investigated by petrographic and geochemical studies of sandstone samples from the Late Cretaceous Guifeng Group of the Yongchong Basin in the Gan-Hang Belt, southeast China. Detrital grains are commonly subangular to subrounded, poorly sorted, and are rich in lithic fragments. The variable pre-metasomatic Chemical Index of Alternation (CIA* = 62–85), Chemical Index of Weathering (CIW = 70.90–98.76, avg. 85.62), Plagioclase Index of Alteration (PIA = 60.23–98.35, avg. 79.91), and high Index of Compositional Variability (ICV = 0.67–3.08, avg. 1.40) values collectively suggest an overall intermediate degree of chemical weathering and intense physical erosion of the source rocks, but a relatively decreased degree of chemical weathering during the late stage (Lianhe Formation) of the Guifeng Group is observed. Several chemical ratios (e.g. Al₂O₃/TiO₂, La/Th, Cr/Th, Th/Sc, Zr/Sc) also suggest a dominant felsic source nature, significant first-cycle sediment supply, and low sedimentary recycling. Such features are consistent with active extension tectonic setting. Sandstone framework models and geochemical characteristics suggest the provenance is related to passive margin (PM), active continental margin (ACM), and continental island arc (CIA) tectonic settings. Sediment derivation from the Neoproterozoic metamorphic rocks and Silurian–Devonian granites indicates a PM provenance, whereas sediments derived from the Early Cretaceous volcanic-intrusive complexes suggest an ACM and CIA nature. Therefore, the Late Cretaceous redbeds were deposited in a dustpan-like half-graben under the back-arc extension regime when southeast China was possibly influenced by northwestward subduction of the Palaeo-Pacific plate beneath East Asia.     

Late Cretaceous redbeds from the Gan-Hang Belt in Southeast China: petrography and geochemistry implications for provenance,source weathering,and tectonic setting

ABSTRACT

The distinct basin and range tectonics in Southeast China were generated by crustal extension associated with subduction of the Palaeo-Pacific plate during the late Mesozoic. Compared with adjacent granitoids of the ranges, the redbeds of the basins have not been well characterized. In this article, provenance, source weathering, and tectonic setting of the redbeds are investigated by petrographic and geochemical studies of sandstones from the Late Cretaceous Guifeng Group in the Yongchong Basin, Southeast China. Detrital grains are subangular to subrounded, poorly sorted, and rich in lithic fragments. Variable Chemical Index of Alternation values (59.55–79.82, avg. 66.79) and high Index of Compositional Variability (ICV) values (0.67–3.08, avg. 1.40) indicate an overall low degree of chemical weathering and rapid physical erosion of source rocks. Such features are consistent with an active extension tectonic setting. Other chemical indices (e.g. Al₂O₃/TiO₂, Th/U, Cr/Th, Th/Sc, Zr/Sc) also suggest significant first-cycle sediment input to the basin and a dominant felsic source nature. Thus, the Guifeng Group possibly underwent moderate to low degrees of weathering upwards. Sandstone framework models and geochemical characteristics suggest the provenance was likely a combination of passive margin (PM) and active continental margin (ACM) with minor continental island arc (CIA) tectonic settings. Sediment derivation from Neoproterozoic metamorphic rocks and Cambrian to Triassic granitoids indicates PM provenance, whereas sediments derived from Jurassic to Cretaceous granitoids suggest ACM and CIA nature. Therefore, the Late Cretaceous redbeds were deposited in a dustpan-like half-graben basin under the back-arc extension regime when Southeast China was possibly influenced by northwestward subduction of the Palaeo-Pacific plate beneath East Asia.     

Clay Mineralogical and Geochemical Studies of Detrital Rocks of the Upper Cretaceous Salt-bearing Strata,Simao Basin,SW China:Implication for Provenance and Source Weathering

正1 Introduction Geological studies established on several sections in Lanping-Simao basin have shown that the salt-bearing strata of Mengyejing formation(Yunlong Fm.in Lanping basin)are constituted by an alternation of salt layers and interbedded facies.The latter consists mainly of mudstones,and mudstone-rich conglomerate.The mineralogy and geochemistry of salt-bearing beds and