Regional Stratigraphic Correlation Research Articles

新潟堆積盆地，加茂地域における下部〜中部中新統の年代層序と堆積盆発達史

The Kamo area is situated in the eastern margin of the Niigata sedimentary basin, Northeast Japan Arc. The Lower to Middle Miocene successions in the Kamo area are divided into the Tonoiri, Otani, and Nanatani Formations in ascending order. In this study, we show newly determined fission-track ages, chronostratigraphy, and regional stratigraphic correlations of the Lower to Middle Miocene successions, and discuss basin development and tectonics in the Kamo area. The age of each formation is as follows—approximately 18-17 Ma, 17-15.2 Ma and 15.2-12.3 Ma for the Tonoiri, Otani, and Nanatani Formations, respectively. On the basis of the age framework of the successions studied, we can correlate the successions with stratigraphic successions in other areas of the northern Niigata sedimentary basin. In particular, a glauconite sandstone bed in the uppermost part of the Nanatani Formation can be correlated to that intercalated in the boundary horizon between the Nanatani and Teradomari Stages in the Niigata sedimentary basin and that in the boundary horizon between the Nishikurosawa and Onnagawa Stages in the Akita region. Lithofacies and paleobathymetrical data show rapid subsidence and transgressions during the deposition of the Tonoiri and Otani Formations. Furthermore, the Tonoiri and Otani Formations show abrupt lateral changes in lithologies and thickness, which suggests that differential subsidence occurred in the basin, probably in response to rifting of the Eastern Japan Sea Rift System during the Early Miocene between 18 and 15.2 Ma. In contrast, the overlying Nanatani Formation has almost a uniform lithology with minor lateral changes in thickness. This suggests that the rifting may have ceased at around 15.2 Ma and subsequent uniform subsidence seems to have occurred between 15.2 and 12.3 Ma. Consequently, the syn-rift stage sediments can be assigned to the Tonoiri and Otani Formations and the post-rift stage sediments can be assigned to the Nanatani Formation.

The Pleistocene terrace staircase of the River Thame, central-southern England, and its significance for regional stratigraphic correlation, drainage development, and vertical crustal motions

The Thame is one of the principal left-bank affluents of the Thames, the largest river in southern England; it joins the Upper Thames at Dorchester, ∼20 km downstream of Oxford. Its terraces include a younger group of four, which date from the late Middle Pleistocene and Late Pleistocene, are disposed subparallel to the modern river, and represent drainage within the modern catchment. At higher levels there are three older terraces, the Three Pigeons, Tiddington and Chilworth terraces, which are assigned to MIS 16, 14 and 12. With much gentler downstream gradients, these are fragmentary remnants of much more substantial fluvial deposits, indicating a much larger river that was disrupted by the Anglian (MIS 12) glaciation. This interpretation supersedes an earlier view that the glacigenic deposits in the Thame headwaters correlate with the Blackditch terrace, the highest of the younger group, which has hitherto provided an argument that the glaciation in this region occurred in MIS 10. It is suggested that the headwaters of the pre-Anglian ‘Greater Thame’ river were located near Northampton and that the Milton Sands of that area represent an upstream counterpart of the Chilworth terrace deposits. It is envisaged that this early Middle Pleistocene drainage geometry, located between the Jurassic limestone and Chalk escarpments, developed as a result of the increase in uplift rates that followed the Mid-Pleistocene Revolution (MPR). It is suggested that before this time, including during the Early Pleistocene, the modern Thame catchment and adjacent regions drained southeastward through the Chalk escarpment, but these small rivers lacked the erosional power to cut through the Chalk in pace with the faster uplift occurring in the early Middle Pleistocene, and so became diverted to the southwest, subparallel to the Chalk escarpment, to form the pre-Anglian ‘Greater Thame’ tributary of the Upper Thames. The post-MPR uplift is estimated to decrease northwestward from 90 m in the Middle Thames to 75 m near the Thame–Thames confluence and to 65 m upstream of Oxford. The post-Anglian (post-450 ka) component of uplift decreases northward from 33 m near the Thame–Thames confluence to an estimated ∼20 m in the Northampton area; the relative stability of the latter area makes feasible the proposed correlation between the Milton Sands and the pre-Anglian River Thame. Limited post-Anglian uplift in the Northampton area is also inferred from the upstream convergence of the terraces of the modern rivers Nene and Great Ouse. These observed lateral variations in vertical crustal motions reflect lateral variations in crustal properties (including heat flow, crustal thickness, and thickness of underplating at the base of the crust) that are known independently. This study thus provides, for the first time, an integrated explanation of the Pleistocene drainage development across a large region of central-southern England.

Chondrichthyans from the Lower Ferron Sandstone Member of the Mancos Shale (Upper Cretaceous: Middle Turonian) of Emery and Carbon Counties, Utah, USA

The Lower Ferron Sandstone Member of the Mancos Shale in southeastern Utah preserves a chondrichthyan assemblage of at least 13 taxa that include: Hybodus sp., Ptychodus cf. P. mammillaris Agassiz, 1843, Ptychodus whipplei Marcou, 1858, cf. Chiloscyllium sp., Scapanorhynchus raphiodon (Agassiz, 1843), Cretodus crassidens (Dixon, 1850), cf. Leptostyrax sp., cf. Cretalamna appendiculata (Agassiz, 1835), Squalicorax sp., Pseudohypolophus mcnultyi (Thurmond, 1971), Protoplatyrhina hopii Williamson, Kirkland and Lucas, 1993, Ischyrhiza schneideri (Slaughter and Steiner, 1968), and Ptychotrygon triangularis (Reuss, 1844). Although this assemblage is typical of other Turonian chondrichthyan faunas in North America, fossil teeth are preserved in two unique facies associations that consist of arenitic sandstones with mud interclasts and rounded chert, feldspar, and quartz pebbles. the coarser beds within these facies associations are previously interpreted to represent storm events and turbidity flows associated with a sea level lowstand. Chondrichthyan teeth occurring within these coarser beds are indicative of extensive transport and reworking and attest to the durable nature of chondrichthyan teeth for biostratigraphic and paleoecological interpretations. Similar studies of chondrichthyan teeth in shelf marine settings may also provide new insights for facies interpretations related to sequence stratigraphy and regional stratigraphic correlations.

Caldera-forming eruptions of the Quaternary Kone Volcanic Complex, Ethiopia

Quaternary volcanism in the northern Main Ethiopian Rift is characterized by rift-axis eruptive centres with complicated collapse structures. Despite their association with major explosive eruptions, few modern studies have been carried out on them. We present the results of fieldwork and laboratory analyses of one of the largest of these centres, the Kone Volcanic Complex (KVC). It consists of nested calderas, trachyte lava flows, trachyte and alkali rhyolite pyroclastic deposits, alkali rhyolite lava domes and numerous small transitional to alkali basalt lava vents and flows. The largest subsidence structure, Birenti Caldera, has a diameter of ∼11 km and an area of ∼95 km 2. Post-collapse volcanism in the interior of Birenti Caldera produced a predominantly trachytic cone that was partially destroyed by a further eruption, forming the well-preserved but irregularly-shaped Kone Caldera (diameter of ∼8 km and an area of ∼24 km 2). Rhyolite lava dome eruptions and tectonically-controlled basalt lava effusion represent the most recent volcanism in the KVC. We report here the first detailed geologic map, along with petrologic and stratigraphic analyses, in order to reconstruct the eruptive history of this large but previously little-known volcanic complex. The findings have implications for understanding the relationships between rifting, tectonics and volcanism in the northern Main Ethiopian Rift, and for future efforts to trace widespread tephra horizons for regional stratigraphic correlation.

Cenozoic sediments in the southern Tarim Basin: implications for the uplift of northern Tibet and evolution of the Taklimakan Desert

Abstract Cenozoic sedimentary successions along the southern margin of the Tarim Basin, western China, reach up to 10 km in thickness. The two studied sections, the Yecheng and Aertashi, comprise c . 4.5 km and c . 7.0 km of clastic sedimentary rocks respectively. The base of the Yecheng section has been dated palaeomagnetically to be about 8 Ma. Age control of the Aertashi section is based on 87 Sr/ 86 Sr measurements (for the basal marine bed), together with magnetostratigraphy and regional stratigraphic correlation. The lower part of each section is mainly composed of fine-grained mudstone and fine sandstone, which makes up the Wuqian Group (Miocene). The palaeoenvironment is low-energy, meandering and braided streams. The middle part is composed of red mudstone, sandstone with thin conglomerate beds, which make up the Artux Formation (Pliocene). The palaeoenvironment is a distal- to mid-fan environment. The uppermost part of the section, known as the Xiyu Formation (Plio-Pleistocene), consists of cobble and boulder conglomerate intercalated with massive siltstone lenses, which formed as proximal alluvial fan and aeolian deposits. Neogene red beds passing upward into upward-coarsening conglomerate and debris-flow deposits record the change in palaeoslope related to uplift of the northern margin of Tibetan Plateau. The formation of aeolian dunes at c . 8 Ma, and underlying playa lake deposits (as at Aertashi), may indicate an arid, enclosed basin in the southern Tarim after this time. Sedimentological characteristics, together with grain size distribution and geochemistry of siltstone bands in the Xiyu and Artux Formations, point to an aeolian origin. This indicates that the Taklimakan Desert and the regional climate regime may have been fully developed by the Early Pliocene. The onset of aeolian sedimentation in the southern Tarim Basin coincided with uplift of the northern Tibetan Plateau, as inferred from the lithofacies change. Tibetan Plateau uplift resulted in the shift of sedimentary environments northwards into the southern Tarim Basin, and could well have triggered the onset of full aridity in the Taklimakan region as a whole.

Sedimentary environment of Ediacaran glacigenic diamictite in Guozigou of Xinjiang, China

The Talisayi Formation found in the Guozigou region on the southern side of Sayram Lake of Xinjiang, China is a Neoproterozoic glacigenic diamictite succession that has not been adequately researched. According to its isotopic geochronology and regional stratigraphic correlation, the Talisayi Formation is equivalent to the Ediacaran glacial sediments. The present paper provides the geochemical data of the Talisayi Formation that reveals the tectonic setting and sedimentary environment in this area during the Ediacaran period. The oxides of Si, K, Na and Al and their ratios of matrix materials from glacigenic diamictites and bulk rock of siltstone and argillaceous siltstones indicate that the sedimentary environment of this formation evolved from a tectonically active region to a passive continental margin. Values of CIA (carbonate-corrected Chemical Index of Alteration) ranging from 54 to 74 show a low-middle weathering action under a glacial environment. Total organic carbon (TOC), V/Cr, Ni/Co and U/Th ratios suggest that the sediments deposited in shallow water with an oxic and weak hydrodynamic condition. There is an abrupt rise in TOC, V/Cr, Ni/Co and U/Th ratios respectively in the overlying Lower Cambrian dolomitic limestones and in the bottom of Talisayi Formation, indicating a rapidl increase in biologic productivity and a variation in redox conditions, which was probably caused by seawater mixing of deep anoxic water and surface oxic water.

Quantitative evaluation of the biostratigraphic distribution of acanthomorphic acritarchs in the Ediacaran Doushantuo Formation in the Yangtze Gorges area, South China

Doushantuo acanthomorphic acritarchs are large morphologically complex organic-walled microfossils broadly constrained between the ∼635 Ma Nantuo glaciation and the ∼551 Ma Miaohe Biota. They are potential biostratigraphic tools for subdivision and correlation of the Ediacaran System in South China. However, major variations in sedimentary facies and stratigraphic thickness present challenges in understanding the spatial and temporal distribution of these acritarchs. Further, the distribution of acritarchs in the Doushantuo Formation is associated with the presence of early diagenetic chert and phosphate, implying a certain degree of preservational bias and/or environmental control. The purpose of this paper is to document the stratigraphic distribution of Doushantuo acritarchs and to quantitatively evaluate their biostratigraphic significance and possible taphonomic–environmental biases, based on high-resolution paleontological data from six sections over ∼100 km in the Yangtze Gorges area, South China. A total of 1082 acritarch fossils were recorded from 84 chert horizons in the six sections of the study area. These chert horizons are not uniformly distributed throughout the Doushantuo Formation, thus the presence/absence of early diagenetic chert does play a role in controlling the distribution of acritarchs. The sampled chert horizons can be grouped into two stratigraphic intervals in the lower and upper Doushantuo Formation, respectively, based on regional stratigraphic correlation. Quantitative analysis shows that the two intervals are distinct taxonomically and largely independent of taphonomic or facies controls. Thus, the two intervals can be regarded as assemblage biozones. The lower biozone (biozone 1) is numerically dominated by Tianzhushania spinosa ( n = 587; 68.3%) and Meghystrichosphaeridium magnificum ( n = 74; 8.6%), whereas the upper biozone (biozone 2) is dominated by Ericiasphaera rigida ( n = 104; 47.1%) and Tianzhushania spinosa ( n = 34; 14.1%). The three most common genera, Meghystrichosphaeridium, Tianzhushania, and Ericiasphaera, have been identified in all sections. Correspondence analysis (CA), detrended correspondence analysis (DCA), discriminant analysis (DA), and pairwise comparisons of samples (Spearman rank coefficient and Jaccard–Chao index), all consistently support biostratigraphic zonations. Thus, the distribution of the Doushantuo acanthomorphs is primarily controlled by biostratigraphic position of samples, with facies and taphonomic differences playing a secondary role. Our case study suggests that acanthomorphic acritarchs can offer a viable tool for regional correlation of the Ediacaran System.

New SHRIMP U-Pb age from the Wuqiangxi Formation of Banxi Group: Implications for rifting and stratigraphic erosion associated with the early Cryogenian (Sturtian) glaciation in South China

In South China, the Wuqiangxi Formation of the Banxi Group and its equivalents underlie the early Cryogenian (Sturtian) glacial deposits but their thickness varies from 2000 m. In the Guzhang section of western Hunan, the Wuqiangxi Formation is only 152 m thick, and an ash bed 58 m below the glacial diamictite yielded a SHRIMP U-Pb age of 809.3±8.4 Ma. In contrast, 90 km south of the Guzhang section towards the basin in Zhijiang area where the Wuqiangxi Formation is ∼2200 m thick, an age of 725±10 Ma has been reported from the top of this unit, 300 m below the glacial diamictite. These ages provide new evidence for the regional stratigraphic correlation across the Nanhua basin, and suggest unusually large (>2 km) stratigraphic erosion potentially associated with the Sturtian glaciation in South China. The magnitude of erosion may imply significant uplifting and tectonotopography at the onset of the Sturtian glaciation.

The Finnmarkian Orogeny revisited: An isotopic investigation in eastern Finnmark, Arctic Norway

The Scandinavian Caledonides have been viewed as resulting from either a single Silurian (i.e. Scandian) event or from polycyclic orogenies involving several collisions on the margin of Baltica. Early studies of the Kalak Nappe Complex (KNC) in Finnmark, Arctic Norway, led to the hypothesis of an Early Cambrian-Early Ordovician (520-480 Ma) Finnmarkian Orogeny, though the nature of this tectonic event remains enigmatic. In this contribution we have employed in situ UV laser ablation Ar-Ar dating of fine-grained phyllite and schist from the eastern Caledonides of Arctic Norway to investigate the presence of pre-Scandian tectonometamorphic events. U-Th-Pb detrital zircon and whole rock Sm-Nd analyses have been used to test the regional stratigraphic correlations of these metasedimentary rocks. These results indicate that the Berlevåg Formation within the Tanafjord Nappe, previously assumed to be part of the KNC, was deposited after 1872 Ma and prior to a low temperature hydrothermal event at 555 ± 15 Ma. It has a likely provenance on the Baltica continent, lacks any Grenville-Sveconorwegian detrital zircons, and thus cannot be part of the KNC which contains abundant detritus in this age range. Instead the Berlevåg Formation is interpreted as part of the Laksefjord Nappe Complex, which structurally underlies the KNC. Laser-ablation argon-argon dating also shows that late Caledonian (i.e. Scandian) tectonometamorphism affected both the KNC and its immediate footwall at c. 425 ± 15 Ma. This is corroborated by a step-heating argon-argon muscovite age of 424 ± 3 Ma which is interpreted as dating cooling. However, within two samples from the KNC, an earlier (Middle-Late Cambrian) metamorphic event is also recorded. A biotite-grade schist yielded an Ar-Ar inverse isochron age of 506 ± 17 Ma from whole rock surfaces, in which the mineral domains are too fine-grained to date individually. An early generation of muscovite from a coarser-grained amphibolite-facies sample yielded an inverse isochron of 498 ± 13 Ma. Both isochron ages have atmospheric argon intercept values. Previous studies have documented similar Cambrian ages in the Caledonian nappes below the KNC. These results suggest correlative tectonometamorphic events in the eastern KNC and its footwall at c. 500 Ma. This Cambrian event may reflect the arrival of the Kalak Nappe Complex as a previously constructed exotic mobile belt onto the margin of Baltica. Combined with recent studies from the western Kalak Nappe Complex, the results do not support the traditional constraint on the Finnmarkian Orogeny sensu stricto. However they vindicate classic tectonic models involving a Cambrian accretion event.

Late Cretaceous Foraminiferal Faunas from the Saiqu “mélange” in Southern Tibet

Abstract: As one of the mélanges in the southern side of the Yarlung‐Zangbo suture zone, the Saiqu mélange in southern Tibet is important for understanding the evolution of the Neo‐Tethys ocean. The age of the Saiqu mélange, however, has been debated due to the lack of reliable fossil evidence in matrix strata. Based on lithological similarities with platform strata in southern Tibet and limited fossils from exotic blocks, previous studies variously ascribed the Saiqu mélange to be Triassic in general, Late Triassic, or Late Cretaceous. Here we reported planktonic foraminiferal faunas from the matrix strata of the Saiqu mélange. The new fossils yield a Late Cretaceous age, which is so far the best age constraint for the mélange. Regional stratigraphic correlation indicates that the Cretaceous Oceanic Red Beds (CORBs) in Saiqu may be time equivalent to the CORBs of the Zongzhuo Formation in neighboring regions. Thus the Saiqu mélange should be correlated to the Upper Cretaceous Zongzhuo Formation rather than the Triassic Xiukang Group, as previously suggested.

Plio-Pleistocene microtephra in DSDP site 231, Gulf of Aden

We reconstruct a Plio-Pleistocene microscopic tephrostratigraphy for DSDP Site 231 in the Gulf of Aden. Systematic microtephrostratigraphy increases the potential for identifying tephra horizons for regional stratigraphic correlation and age control, as well as providing information about eruptive histories. Microtephra reveal three main pulses of volcanism c. 4.0–3.2 Ma, 2.4 Ma and 1.7–1.3 Ma, corresponding to peaks in volcanic activity recorded in the East African Rift System. Previous studies of DSDP Site 231 have reported six visible tephra horizons (up to 25 cm thick) with geochemical compositions matching East African tuffs. We find 68 additional microtephra horizons through microscopic examination of 1050 samples (each integrating c. 3 ka) in over 200 m of marine sediments. We report the major and minor element geochemical compositions of individual glass shards in six of these microtephra horizons and establish a robust correlation at 168.73 m to the Lokochot Tuff (3.58 Ma), which together with previously identified tephra, provides a tightly constrained chronostratigraphy for the mid Pliocene.

Provenance and chemostratigraphy of the Neoproterozoic West Congolian Group in the Democratic Republic of Congo

Lithogeochemical, chemostratigraphic, Rb–Sr and Sm–Nd isotope as well as U–Pb single zircon age data from the principal stratigraphic units of the West Congolian Group (West Congo Supergroup) in the West Congo Belt, Democratic Republic of Congo, presented in this reconnaissance study provide new insights into regional stratigraphic correlation and sediment provenance. The oldest unit (Sansikwa Subgroup) is a continental rift basin fill that culminated in a marine glacial diamictite (Lower Mixtite Formation), which is correlated with the global 750–720 Ma Sturtian glaciation. This is followed by a post-glacial, marine carbonate-dominated succession (Haut Shiloango Subgroup), a second diamictite unit (Upper Mixtite Formation) for which a syn-Marinoan (636 Ma) age is favoured, a post-glacial carbonate succession (Schisto-Calcaire Subgroup) and finally molasse sediments that experienced orogenic deformation at c. 566 Ma, as indicated by new 40Ar– 39Ar data. The youngest unit (Inkisi Group) is post-orogenic, derived from the West Congo Belt, and thus not part of the same supersequence. The proposed regional to global correlation of the various units is in line with progressively younger detrital zircon age spectra up-section. Overall, these age spectra indicate a Neoarchaean basement source (Congo Craton) and a Palaeoproterozoic source from the known Kimezian Supergroup to the west of the belt, which corresponds in age with the Eburnean of southern Africa. In addition, we identified a hitherto unknown late Mesoproterozoic source to the west of the belt, the age of which is comparable to the Espinhaço Supergroup of the Araçuai Belt in Brazil.

Sequence stratigraphy of the Middle Cambrian Daegi Formation (Korea), and its bearing on the regional stratigraphic correlation

The sediments that comprise the Middle Cambrian Daegi Formation in eastern central Korea are interpreted to have been deposited in a carbonate ramp setting. The Daegi Formation consists of nine lithofacies; argillaceous rock, ribbon rock, oolitic limestone, skeletal limestone, microbial limestone, intraclastic limestone, micritic limestone, bioturbated limestone and crystalline limestone, which are grouped into three facies associations presenting, from base up, outer ramp, grain shoal and inner ramp facies associations. Two depositional sequences can be distinguished in the Daegi Formation. Sequence 1 in the lower part of the Daegi Formation comprises a lower transgressive systems tract (TST) and an upper highstand systems tract (HST). The TST consists of terrigenous deposits with subordinate limestones, while the HST contains various types of limestone. Sequence 2 comprises the upper half of the Daegi Formation and was deposited during a TST. The sequence stratigraphic analysis of the Daegi Formation suggests that the sediments were deposited during one 3rd-order sea-level change, during the middle to late Middle Cambrian. The sequence stratigraphic framework of the Daegi Formation can be excellently correlated with coeval deposits in North China, the Zhangxia and the lower part of the overlying Gushan formations, which belong to the same tectonic block as the Korean Peninsula, the Sino-Korean Block (SKB). The almost identical sequence stratigraphic frameworks in two separate basins suggest that the sequence stratigraphic development on the SKB was mainly influenced by eustatic changes during the Middle Cambrian.

Mid-Cretaceous stratigraphy of the James Ross Basin, Antarctica

Abstract The extensive Cretaceous sedimentary sequence exposed within the James Ross Basin, Antarctica, is critical for regional stratigraphic correlations in the Southern Hemisphere, and also for our understanding of the radiation and extinction of a range of taxonomic groups. However, the nature and definition of Cenomanian-Turonian strata on the NW margins of James Ross Island has previously been difficult, due both to marked lateral facies changes and to stratigraphical discontinuities within the extensive Whisky Bay Formation. Facies variation and local unconformities were the result of fault-controlled deep-marine sedimentation along the basin margin. In this study the Albian-Cenomanian boundary is defined for the first time in the upper levels of the Lewis Hill Member of the Whisky Bay Formation. However, there is a Cenomanian-late Turonian unconformity between the Lewis HIll and Brandy Bay members of the Whisky Bay Formation. Equivalent lithostratigraphical units exposed further to the SW on James Ross Island appear to be more complete with the early Cenomanian-late Turonian interval represented by the upper parts of the Tumbledown Cliffs and the lower part of the Rum Cove members of the Whisky Bay Formation. The Turonian-Coniacian boundary is provisionally placed at the junction between the Whisky Bay and Hidden Lake formations. The revised stratigraphic ages for this section show that the Late Cretaceous radiations of a number of major plant and animal groups can be traced back to at least the Turonian stage. This raises the possibility that their dissemination might be linked to the global Cretaceous thermal maximum.

Sequence stratigraphic interpretation of the Chilga basin sediments, Northwest Ethiopia

Sequence stratigraphic interpretation of passive margins and marine environments reveal stratigraphic records that result from the influence of long-term change in eustatic sea level, tectonic subsidence, and climate. On the other hand, sequence stratigraphic interpretation of continental rift basin sediments reveal stratigraphic records that are dominantly controlled by short-term local tectonic conditions and climatic fluctuations. The Chilga co ntinental rift basin, located in northwest Ethiopia, consisting of sediments composed of claystones, siltstones and silty sandstones, volcanic ashes, lignite beds, and vertebrate and plant fossil-rich sandstones provides ample information for such a sequence stratigraphic interpretation. This paper presents a sequence stratigraphic interpretation of the Chilga basin sediments using integrated approaches such as regional stratigraphic correlation and interpretation of depositional environment derived from petrography, x-ray diffraction analysis, and lithologic associations. Results indicate that the Chilga basin preserves three 4 th order sedimentary cycles within 1.0 Ma. Sequence stratigraphic analysis of the Chilga continental rift basin is a working model for the many unstudied Ethiopian rift and non-rift basin sediments, which are dominantly composed of fine-grained sediments. Key words/phrases : Basin, Chilga, continental rift, Ethiopia, sequence stratigraphy SINET: Ethiopian Journal of Science Vol. 28 (1) 2005: 75-92

Pedogenesis of late Quaternary deposits, northern Kyonggi Bay, Korea: Implications for relative sea-level change and regional stratigraphic correlation

Late Pleistocene and early Holocene paleosols are described from construction pits and boreholes in the northern Kyonggi Bay, west coast of Korea. Thin-section petrography, chemistry, clay mineralogy and geotechnical properties of the paleosols were examined to infer relative sea-level fluctuations during the late Quaternary. Relict laminae and gradational contacts with underlying tidal rhythmites confirm that the late Pleistocene paleosol is a pedogenically altered tidal deposit. Illuvial clay coatings, rootlets and enrichment of kaolinite and chemically stable oxides indicate that pedogenesis occurred on a well-drained, stable surface during the last glacial period. Redoximorphic features, such as drab-colored root traces and sphaerosiderite, however, suggest that waterlogged conditions developed temporarily due to a rise of the water table at the end of the last glacial period. As sea level rose, an early Holocene paleosol formed in freshwater bog deposits, burying the late Pleistocene paleosol. The early Holocene paleosol has abundant sphaerosiderites and organic material. The presence of weakly developed illuvial clay coatings, oxidized siderites and pellets, however, indicates oxidizing condition associated with water-table fall occurred during the early Holocene. Despite the similarity in the degree of chemical and clay mineralogical weathering, the late Pleistocene paleosol is distinguished from its early Holocene counterpart by thicker and more abundant illuvial clay coatings, more Fe 2O 3 and a greater degree of consolidation. Duration of pedogenesis might be a primary cause for the difference of the pedofeatures with climate playing a secondary role. The widespread presence of similar late Pleistocene paleosols highlights its significance as a key stratigraphic marker for regional correlations along the west coast of Korea.

Relative paleointensity of the geomagnetic field during the past 0.8 Ma from Nihewan Basin, Hebei Province, China

We conducted a detailed rock magnetic study on upper 80 m of Jing’erwa core from the Nihewan basin. The results indicate that the sediments from Jing’erwa core are suitable for relative paleointensity study, and anhysteretic remanent magnetization (ARM) can be used as the normalization parameter of the natural remanent magnetization (NRM). Our relative paleointensity data of upper 80 m of Jing’erwa core provide a continuous record of the intensity variation during the last 0.8 Ma, which correlates well with the results from marine sediments cores in the Pacific Ocean. This means that the sediments records are reliable for relative paleointensity of Earth’s magnetic field, and suggests that these sediments have recorded the real changes of geomagnetic field, which would provide a new method for regional stratigraphic correlation.

Tectonic-stratigraphic division and blind fold structures in Nansha Waters, South China Sea

Extensive multiple-channel reflection seismic data have been collected in the Nansha (Spratley Islands) Waters, the southern margin of the South China Sea. Stratigraphic correlation is shown with focus on a comprehensive geophysiXcal survey line run from offshore NW Sabah to offshore SE Vietnam. According to the varying tectono-stratigraphy from southeast to northwest, five tectonic belts can be determined, i.e. the Palawan-Borneo Nappe, Nansha Trough, Nanwei-Liyue Compressive Belt, Zheng'he Extensional Belt and the Circum-Southwest Subbasin Belt. In the Palawan-Borneo Nappe, Neogene-Quaternary deposits were highly upthrust northwestwards, resulting in a series of moderately to tightly folded anticlines separated by open synclines. The Nansha Trough was a narrow, deep-water belt filled with thick, undisturbed Neogene-Quaternary deposits. The Nanwei-Liyue Compressive Belt was dominated by strongly folded paleo-anticlines overlain by an undeformed sedimentary cap with a pronounced hiatus of Paleogene sediments. The Zheng'he Extensional Belt consisted of a rugged topography and Paleogene half-grabens bounded by listic faults. The major extensional faults were reactivated to cut through the overlying Neogene-Quaternary deposits. Over the Circum-Southwest Subbasin Belt, Neogene-Quaternary deposits draped on the largely subsided fault blocks related to the Late Oligocene–Mid-Miocene seafloor spreading. Based on the regional stratigraphic correlation, the prominent paleo-anticlines found within the Nanwei-Liyue Compressive Belt are deduced to consist of mainly Mesozoic marine sediments that were compressed in the Late Mesozoic Era. Therefore, the Nansha Microcontinent Block is shown to be a collision complex assembled during Late Mesozoic Era.

Stratigraphy, geochronology, and geochemistry of the Georgie River area, northwest British Columbia, and implications for mineral exploration

Georgie River area is underlain by greenschist facies volcanic, clastic, and plutonic rock. High precision U–Pb isotopic ages on zircon enable regional stratigraphic correlation by constraining the Lower to lower Middle Jurassic depositional history. A sample from pyroxene-phyric tholeiitic basalt is Upper Triassic and (or) Lower Jurassic in age; its upper age limit is constrained by a crosscutting intrusion dated at 189.8 ± 0.3 Ma. The Bulldog Creek pluton, at 193.0 ± 0.3 Ma, is significantly older than the previously reported K–Ar age. Hazelton Group strata include a coarse-grained volcaniclastic and volcanic unit overlain by a fine-grained clastic and bimodal volcanic flow unit. The coarse volaniclastic unit is coeval and probably cogenetic with the Outram Lake porphyry intrusion (189.7 ± 0.3 Ma) and near the top includes andesitic tuff (189.8 ± 0.6 Ma). These data constrain the age of deposition of the unit and support correlation with the Lower Jurassic Hazelton Group and members of the Texas Creek plutonic suite. Age of the fine clastic and bimodal volcanic unit is no older than the youngest detrital zircons (ca. 185 Ma) near its base. That age and the age of a rhyolite unit (176.6 ± 1.1 Ma) at the top support correlation with the Middle Jurassic Salmon River Formation of the Hazelton Group. Resolution of the litho- and chronostratigraphy of the area reveal close similarities in age, rock types, succession, and composition with strata that host the base- and precious metal-rich stratiform Eskay Creek deposit, revealing the potential for Eskay-type volcanogenic massive sulphide (VMS) deposits in the Georgie River area.

Early Proterozoic geomagnetic field in western Laurentia: implications for paleolatitudes, local rotations and stratigraphy

Abstract The east-west trending, post-orogenic subvertical Cleaver Dykes from the Great Bear Magmatic Arc are dated by the U–Pb (baddeleyite) method to be 1740 +5/−4 Ma. They have a mean direction of magnetization of D, I=136.5 and 57.4°, k=64, α95=4.5°, based on samples from 17 diabase dykes, a mean location 65.7°N, 118.03°W, and a corresponding paleopole at 19.4°N, 83.3°W, A95=6.1°. Studies of baked contacts, country rocks distant from dykes, and later intrusions that cut them, show that the dykes became magnetized soon after intrusion. The paleopole is not significantly different from that obtained from the 1759±1 Ma post-orogenic Jan Lake Granite from the Hudsonian Orogen in southeast Saskatchewan. These paleopoles fall among a cluster previously determined from post-orogenic overprints in a wide variety of rocks and geological settings across Laurentia. Their mean is 20.8°N, 94.5°W, A95=5.2°. Collectively they record a widespread remagnetization episode following the Hudsonian Orogeny, during which the polarity of the field remained constant. This single polarity interval (the Cleaver Superchron) has a mean age of approximately 1747 Ma, and its duration is at least 13 million years but is probably much longer. With further refinement, it could become a global statigraphical marker. There are 18 paleopoles from red beds and igneous rocks which record the paleofield in western Laurentia mainly over an extended interval we estimate to be at least 100 million years between about 1960 and 1830 Ma during the Hudsonian Orogeny. Over this interval, western Laurentia remained at a low and essentially constant latitude. Reversals of the geomagnetic field occurred opposed magnetizations being of approximately equal frequency; there are numerous magnetochrons which could, in future, assist in regional stratigraphic correlations. On the basis of six studies, we estimate that the reference paleopole for this interval is at 1.1°S, 71.7°W, A95=12.1° (N=6). Paleopoles from the other 12 studies are spread over a 100° arc indicating large variable rotations about local vertical axes which probably resulted from transcurrent faulting caused by indentation of the bordering Archean Slave and Superior cratons during the Hudsonian Orogeny. These reference paleopoles show that during the Hudsonian Orogeny, western Laurentia was in paleolatitudes of 15–30°. During post-Hudsonian time (ca. 1750 Ma) paleolatitudes were about 20° higher. Paleowind directions inferred by others, indicate that Laurentia during these intervals was in the northern hemisphere.