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2020 Vol.  9 No.  2
Published: 2020-04-20

Palaeoclimatology
Biopalaeogeography
128 Asia-Gondwana connections indicated by Devonian fishes from Australia: palaeogeographic considerations
Gavin Charles Young, Jing Lu

Middle Palaeozoic vertebrate fossil occurrences are summarised for Australia, with reference to faunal connections between Asia and East Gondwana, as first indicated by fish distributions of Lower Devonian fossil sites. Major endemic groups discussed are pituriaspid (Australian) and galeaspid (Asian) agnathans, wuttagoonaspids (Australian) and antarctaspid (Antarctic, Australian, Asian) arthrodires, yunnanolepid and sinolepid antiarchs (South China, Indochina terrane, Australia), and early tetrapodomorphs (South China, Australia). More widespread groups that lived in shallow marine environments (lungfishes, buchanosteid arthrodires, antiarch Bothriolepis) also show species groups shared between South China and East Gondwana. Exchange of continental facies fishes (e.g. tristichopterid tetrapodomorphs) may have been interrupted by marine transgression in the Frasnian, but were restored in the late Famennian with the appearance of Grenfellaspis in eastern Australia, the only sinolepid antiarch known from outside Asia. The hypothesis of Gondwana dispersion and Asian accretion, to explain the collage of geological terranes forming modern east and southeast Asia, implies increasing dissimilarity with increasing age, but the Siluro-Devonian early vertebrate evidence is inconsistent with this. Previous cladistic analysis of Asian terranes predicted galeaspid agnathans on the Indochina terrane, and their subsequent discovery at Ly Hoa, Vietnam, confirms that Indochina and South China had come together across the Song Ma suture by Middle Devonian time.

2020 Vol. 9 (2): 128-149 [Abstract] ( 165 ) [HTML 1KB] PDF (10923 KB)   ( 37 )
Biopalaeogeography
150 Morphology and features of Cambrian oncoids and responses to palaeogeography of the North China Platform
En-Zhao Xiao, Ming-Xiang Mei, Shu Jiang, Tehseen Zafar
The Cambrian strata in the North China Platform are fully exposed. A wide variety of carbonate oncoids with different shapes occur in the Xuzhuang and Zhangxia formations (Miaolingian Series) from six Cambrian sections in the study area. A comprehensive study involving outcrop description, microscopic observation, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and carbon and oxygen isotope analysis is conducted to determine the facies, morphology, internal structure, and geochemical properties of the oncoids. The oncoids are divided into six types based on their morphology and internal structure. Microscopic and ultrastructural observations reveal typical microbial fossils (Girvanella) and microbially-related sediments (framboidal pyrite), indicating the biogenicity of the oncoids. Additionally, the XRD and carbon and oxygen isotope analysis results suggest that the formational environments of these oncoids are different due to terrestrial influences. Statistical data on the oncoids from the six sections show that there are obvious differences in the types of oncoids and the proportions of different varieties in each section. The spatial differences in the oncoid morphologies are associated with different palaeogeographic settings. The rough oncoid growth patterns developed in nearshore environments were influenced by terrigenous debris and steep terrain, whereas the delicate oncoid growth patterns developed in offshore environments were less affected by terrestrial factors and were featured by more stable depositional processes related to microbial mats.
2020 Vol. 9 (2): 150-167 [Abstract] ( 94 ) [HTML 1KB] PDF (15864 KB)   ( 33 )
168 Palynomorph assemblage biozonation of Paleogene strata in Bende-Umuahia Area, Niger Delta Basin, southeastern Nigeria
Okechukwu Nicodemus Ikegwuonu, Obianuju Patricia Umeji, Osita Igwebuike Chiaghanam, Kingsley K. Nwozor, Otobong Sunday Ndukwe, Kingsley Chukwuebuka Chiadikobi
Cenozoic sediments form extensive outcrops in the Niger Delta Basin. Detailed palynostratigraphic study was undertaken across Paleogene sequences exposed in Bende-Umuahia Area in up-dip sectors of the Niger Delta Basin, southeastern Nigeria, to establish different palynomorphs assemblage zones, with their corresponding ages. Palynological analysis was carried out on 27 selected outcrop samples, using the conventional maceration technique for recovering acid-insoluble organic-walled microfossils from sediments. Three Cenozoic lithostratigraphic units, including Imo Formation, Ameki Formation, and Ogwashi Formation, are exposed in the study area. Lithologies are sandstone, carbonaceous shale, mudstone, limestone, and the lignite. A total of 65 species of sporomorphs and 51 dinoflagellate cysts were identified. The recovered spores and pollen grains were used to establish six informal palynomorph assemblage zones, labeled as zone A - zone F, based on the first and the last occurrences of two or more species. These palynomorph assemblage zones include: (1) zone A - middle Paleocene Scabratriporites simpliformis-Bombacidites annae zone; (2) zone B - late Paleocene Foveotricolporites crassiexinus-Mauritidiites crassiexinus zone; (3) zone C - early Eocene Striatopollis catatumbus-Momipites africanus zone; (4) zone D - middle Eocene Margocolporites umuahiaensis-Gemmastephanocolporites brevicolpites zone; (5) zone E - late Eocene Cicatricosisporites dorogensis-Perfotricolpites nigerianus zone; and, (6) zone F - Oligocene-early Miocene Verrucatosporites usmensis-Magnastriatites howardii zone. The erected palynozones were correlated and compared with existing biozones in subsurface, down-dip sectors of the Niger Delta Basin, with pantropical palynological zones in tropical areas of Africa, and with palynofloral provinces of northern South America. A comparison of palynozones studied in southeastern Nigeria with other international palynozones, in this study, will assist in establishing the correlation of sediments for these areas.
2020 Vol. 9 (2): 168-180 [Abstract] ( 97 ) [HTML 1KB] PDF (9381 KB)   ( 27 )
Lithofacies palaeogeography and sedimentology
181 Ancient rip current records and their implications: An example from the Cretaceous Ukra Member, Kutch, India
Subir Sarkar, Amlan Koner
Poorly-sorted conglomerate patches rich in granules or sturdy fossils or both, and reddish mud matrix within the interstices stand out amidst fine-grained siliciclastic shelf sediments of the trangressive systems tract (TST) of the Lower Cretaceous Ukra Member, Kutch Basin, India. The siliciclastic shelf sediments contrast the conglomerates with their remarkable lateral extension. The fossils belong to a low-diversity group of sedentary bivalves that can be traced into the shoreface facies assemblage. The shelf sandstones are almost always sculpted by wave structures, especially hummocky cross-stratification while textures in the conglomerates suggest that the sediment settling was generally from suspensions. Textural variations in conglomerates reflect an immediate variation in flow viscosity prior to the downloading. The current structures obtained from the conglomerates record offshoreward palaeocurrent, in contrast to the shore-parallel palaeocurrent in the TST. The hummocky cross-stratified (HCS) beds are interpreted as seasonal storm deposits, while the conglomerate patches are taken as rip current deposits induced by waves of much longer periods. The glauconite-rich shale that alternates with conglomerates is probable fair-weather products. The conglomerates could not be recognized either in the coarse-grained shoreface deposits occupying the lower part of the overall fining-upward TST or in the coarsening-upward and glauconite-depleted highstand systems tract (HST). In contrast to the TST, the HST is dominantly tide-imprinted, having shore-normal palaeocurrent direction. It appears that intensification of waves and weakening of tides during transgression favored strong rip currents generation, which had presumably caused severe damage to the sea coast and to the shell banks growing preferably at the necks of the rip current channels. Rapid lateral facies transitions in the shoreface deposits at the basal part of the TST suggest enhanced irregularity in the coastline, possibly because of the mega cusps indented upon it. Frequency and intensity of storms enhanced during periods of global warming caused the transgression of the Early Cretaceous Ukra Sea.
2020 Vol. 9 (2): 181-197 [Abstract] ( 77 ) [HTML 1KB] PDF (11222 KB)   ( 26 )
198 Large soft-sediment deformation structures (SSDS) in the Permian Barren Measures Formation, Pranhita-Godavari Valley, India: Potential link to syn-rift palaeoearthquake events
Biplab Bhattacharya, Abhirup Saha
Records of palaeoearthquakes in sedimentary rocks are often debated due to the potential confusion in distinguishing seismic versus aseismic trigger mechanisms causing liquefaction. The present paper documents some unique soft-sediment deformation structures (SSDS), characterized by their extremely large size, complex morphology and preservation in coarse-grained pebbly sandstone. The SSDS are present in the Permian Barren Measures Formation, a syn-rift depositional unit within the Lower Gondwana succession in the Pranhita-Godavari Valley, eastern Peninsular India. The ~210 m thick succession is represented by stacked fining-up retrogradational cycles, characterized by coarse-grained trough cross-stratified pebbly sandstones near the base and fine-grained heterolithic sandstones-mudstones at the top. Each cycle signifies a change from fluvial- to tidal-influenced depositional systems. Coarse-grained pebbly sandstone beds near the base of each cycle record most SSDS, including complexly deformed layers, pseudonodules, load and flame structures, various water-escape structures like vertical/inclined sediment columns (sedimentary dykes) and contorted beds, and syn-sedimentary faults. The deformed beds are underlain and overlain by the undeformed beds. Complexly deformed SSDS are often sharply truncated at the top by undeformed beds manifesting syn-sedimentary character, which signifies that deformation took place just after deposition of the affected beds, but before deposition of the overlying beds while sedimentation was continuous. Facies analysis reveals the absence of processes like storms/pounding waves, slumps, rapid dumping (massive beds), impact shaking, volcanisms, tsunami waves or sediment gravity flows in the study area, thus negating their possibility as triggering agents for the liquefaction. The complex nature and large size of the deformation structures imply extensive liquefaction near the sediment-water interface. In addition, the deformed beds comply with most of the criteria of typical seismites. In half-graben type Gondwana basins, such seismites can be linked to palaeoearthquakes, which signify the phases of syn-rift fault reactivation, basinal sagging and associated accommodation changes.
2020 Vol. 9 (2): 198-215 [Abstract] ( 80 ) [HTML 1KB] PDF (18836 KB)   ( 27 )
216 Thickening-upward cycles in deep-marine and deep-lacustrine turbidite lobes: Examples from the Clare Basin and the Ordos Basin
Lei-Fu Zhang, Da-Zhong Dong
Deep-marine and deep-lacustrine reservoirs have been targets for conventional and unconventional oil and gas exploration and development for decades. Thickening-upward cycles in the deep-marine Carboniferous Ross Sandstone Formation outcrops in western Ireland and the deep-lacustrine Triassic Yanchang Formation outcrops in southeast Ordos Basin have been investigated and correlated in this study. Typical thickening-upward cycles consisting of, from bottom to top: (1) laminated shales/shales with interbedded siltstone beds; (2) interbedded sandstones/siltstones and mudstones; (3) structureless massive sandstones, are well recognized in these outcrops and are interpreted as turbidite lobes. A continuously prograding lobe-element model is proposed to explain the repeated stacking of thickening-upward cycles. Thickening-upward cycles developed within deep-marine and deep-lacustrine environments are highly comparable in many aspects, such as sedimentary structures, sheet-like geometries and amalgamation features. A frequent and strong degree of amalgamation is developed within the massive sandstone at the top of each thickening-upward cycle, suggesting a layer-by-layer depositional manner. Field observations and comparison with deep-marine counterparts support the occurrence of turbidity flows in the Yanchang Formation, Ordos Basin.
2020 Vol. 9 (2): 216-231 [Abstract] ( 91 ) [HTML 1KB] PDF (10383 KB)   ( 26 )
232 Sedimentary characteristics of microbialites influenced by volcanic eruption: A case study from the Lower Cretaceous Shipu Group in Zhejiang Province, East China
Xiao-Fang Wang, Xiu-Cheng Tan, Shao-Nan Zhang, An-Jiang Shen, Chang Li, Guang Hu, Xin Wang, Zi-Liao Chen, Li-Yin Pan, Jie Zhang, Wei Chen
This study describes a sequence of microbialites and volcanics of the Lower Cretaceous Shipu Group, an example of microbialites influenced by volcanic activity. It is located at Shipu town in eastern Zhejiang Province on the coast of southeastern China. Based on macroscopic outcrop observations, microscopic examination of thin sections, electron probe microanalysis (EPMA), field emission scanning electron microscopy (FESEM) imaging analysis, and energy dispersive X-ray spectrometry (EDS) analysis, nine microbialite-tuffite assemblages have been recognized in the section. Their thickness increased gradually upwards as volcanism decreased. There are ooids, bioclastic grains, intraclasts and tuffaceous grains in the grain shoal with local dolomitization. Above the grain shoal, microbial reefs develop either individually or conjoining with adjacent ones, and consist of stromatolites and serpulid tubes with common recrystallization. Tubes of serpulids are calcified and the tube wall is micrite. The tube and intertube parts are filled by sparry calcite. Colonial serpulids are surrounded by microbes to form stromatolites. Black layers of stromatolites contain many calcite crystals with fan-shaped growth pattern and preserved organic matter. Microbes are so well preserved in crystal lattices that the original microstructure of the microbes can be clearly observed by FESEM imaging analysis. Microbial reefs develop at a local high point near or above fair-weather wave-base where waves removed fine volcanic ashes. Interreef deposits are coarse tuffite due to physical differentiation. Volcanic activity could provide rich nutrition for microbes, but too much fine volcanic ash inhibits microbial growth. As a result, a moderate supply of volcanic ash favors the development of microbialites.
2020 Vol. 9 (2): 232-245 [Abstract] ( 80 ) [HTML 1KB] PDF (14722 KB)   ( 33 )
Palaeoclimatology
246 Continental chemical weathering during the Early Cretaceous Oceanic Anoxic Event (OAE1b): a case study from the Fuxin fluvio-lacustrine basin, Liaoning Province, NE China
Xiao-Tao Xu, Long-Yi Shao, Bo Lan, Shuai Wang, Jason Hilton, Jian-Yi Qin, Hai-Hai Hou, Jie Zhao
This study focuses on Early Cretaceous mudstones from the Shahai and Fuxin formations in the Fuxin continental basin. We analyse chemical weathering, land surface temperatures and palaeoclimates based on chemical weathering indices, and emphasize the implications of continental chemical weathering on nutrient fluxes into lakes and oceans. According to Cr and Ni abundance, Al2O3-TiO2, La/Sc-Th/Co and V-Ni-Th*10 plots, as well as rare earth element (REE) analysis, mudstone samples from the Shahai and Fuxin formations were derived from the same type of provenance comprising mainly felsic igneous rocks. Chemical weathering trends reflected by the Chemical Index of Alteration (CIA), Weathering Index of Parker (WIP) and the Mafic Index of Alteration for Oxidative weathering environments (MIA(O)) are consistent with each other and allow the geological succession to be divided into four stages. Land surface temperatures of the Shahai and Fuxin formations are estimated based on the linear relationship of CIA to temperature, and also can be divided into four stages consistent with those determined from chemical weathering trends. During Stage A (early part of the late Aptian) chemical weathering and land surface temperatures were relatively low and showed characteristic high fluctuations, while Stage B (latest Aptian) represented a transitional period where weathering rates and temperatures increased, and high amplitude fluctuations continued. Conditions changed markedly in Stage C (early Albian) with very high and stable weathering, and warm, humid climates, while in Stage D (middle and late Albian) conditions returned to low chemical weathering and land surface temperatures. These stages of chemical weathering and land surface temperature fluctuations represent responses to global climate fluctuations during the Early Cretaceous, with the early Albian high weathering intensities and warm, humid climates combining to create high nutrient levels that would have flushed through rivers into lakes and ultimately oceans. This correlates stratigraphically with the development of Early Cretaceous black shales during Ocean Anoxic Event 1b, showing the importance of continental weathering regimes as a causal mechanism for lake and ocean anoxia.
2020 Vol. 9 (2): 246-266 [Abstract] ( 105 ) [HTML 1KB] PDF (5563 KB)   ( 32 )
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