Continental Margin Deposits Research Articles

Lateral redox variability in ca. 1.9 Ga marine environments indicated by organic carbon and nitrogen isotope compositions.

The stepwise oxygenation of Earth's surficial environment is thought to have shaped the evolutionary history of life. Microfossil records and molecular clocks suggest eukaryotes appeared during the Paleoproterozoic, perhaps shortly after the Great Oxidation Episode at ca. 2.43 Ga. The mildly oxygenated atmosphere and surface oceans likely contributed to the early evolution of eukaryotes. However, the principal trigger for the eukaryote appearance and a potential factor for their delayed expansion (i.e., intermediate ocean redox conditions until the Neoproterozoic) remain poorly understood, largely owing to a lack of constraints on marine and terrestrial nutrient cycling. Here, we analyzed redox-sensitive element contents and organic carbon and nitrogen isotope compositions of relatively low metamorphic-grade (greenschist facies) black shales preserved in the Flin Flon Belt of central Canada to examine open-marine redox conditions and biological activity around the ca. 1.9 Ga Flin Flon oceanic island arc. The black shale samples were collected from the Reed Lake area in the eastern part of the Flin Flon Belt, and the depositional site was likely distal from the Archean cratons. The black shales have low Al/Ti ratios and are slightly depleted in light rare-earth elements relative to the post-Archean average shale, which is consistent with a limited contribution from felsic igneous rocks in Archean upper continental crust. Redox conditions have likely varied between suboxic and euxinic at the depositional site of the studied section, as suggested by variable U/Al and Mo/Al ratios. Organic carbon and nitrogen isotope compositions of the black shales are approximately -23‰ and +13.7‰, respectively, and these values are systematically higher than those of broadly coeval continental margin deposits (approximately -30‰ for δ13Corg and +5‰ for δ15Nbulk). These elevated values are indicative of high productivity that led to enhanced denitrification (i.e., a high denitrification rate relative to nitrogen influx at the depositional site). Similar geochemical patterns have also been observed in the modern Peruvian oxygen minimum zone where dissolved nitrogen compounds are actively lost from the reservoir via denitrification and anammox, but the large nitrate reservoir of the deep ocean prevents exhaustion of the surface nitrate pool. Nitrogen must have been widely bioavailable in the ca. 1.9 Ga oceans, and its supply to upwelling zones must have supported habitable environments for eukaryotes, even in the middle of oceans around island arcs.

Global sea level changes or local tectonics? Pliocene, Miocene and Oligocene biomarkers in cored sedimentary rocks from IODP Expedition 317, Canterbury Basin, New Zealand

The various influences of global sea level (eustasy) and local tectonic changes on sedimentation processes in continental margin deposits are important topics in sedimentary research. International Ocean Discovery Program Expedition 317 to the Canterbury Basin, on the eastern margin of the South Island of New Zealand, provided an opportunity to study sediment geochemistry in contrasting depositional settings, from mid-shelf to upper continental slope sedimentary rocks from the Eocene to the Holocene. This paper provides the first examination of the organic geochemical record from Pliocene, Miocene, and Oligocene sediment samples recovered during the expedition from the U1351, U1352, and U1353 sites, using bulk geochemistry and hydrocarbon distributions, including biomarkers. The main aim of this research was to correlate changes in organic accumulation with local tectonic activity and global climate changes that occurred in the Miocene. There is good preservation of C11 to C35n-alkanes, with varying predominance of odd-over-even chain length n-alkanes, and much variation in the terrigenous/aquatic ratio over the cores. Based on biomarker and Rock Eval parameters, thermal maturity varies from low in the Pliocene, to early oil window in the deeper Miocene and Oligocene sedimentary rocks. The pristane/phytane ratios for the three cores indicate anoxic to oxic depositional environments. Marine organic matter input is indicated by the high C30 sterane index, whilst high relative amounts of oleanane, C24 tetracyclic terpane, and C29 steranes indicates major terrigenous organic matter input. Local tectonic activity is suggested to have had a significant influence on the accumulation of the organic matter in the Canterbury Basin. The influence of eustasy and global sea temperature variations are suggested for the periods < 14 Ma, 12–7 Ma, and ∼ 6 Ma. An increase in terrigenous organic matter input during the late Miocene correlates with the uplift of the Southern Alps and an increase in the continental slope angle for the South Island of New Zealand.

The geology of the Capiru Group, Curitiba Terrane, Southern Ribeira Belt (Brazil)

ABSTRACT The Capiru Group is a Neoproterozoic metasedimentary unit that crops out in the Curitiba terrane, Southern Ribeira Belt, Brazil. A detailed geological mapping of this unit was performed during 2016 to 2020. Original interpretations and the new geological map were compiled and integrated with the past field data of the state (MINEROPAR) and national (CPRM) geological surveys, and to unpublished and recent published studies. This compilation represents the most updated synthesis of the Capiru Group, and provides the basis for recognition and individualization of lithostratigraphic units. The lithostratigraphic units reflect depositional/tectonic events, which involved: a) passive continental margin deposition on the Curitiba microplate, b) compressional episodes during the Brasiliano - Pan African Orogeny (Late Neoproterozoic), c) extensional processes that followed the collision stage (Cambrian), and finally, the d) extensional tectonics related to the Gondwana break-up during the Lower Cretaceous, that culminated to the Proto-Atlantic Ocean opening.

Detrital zircon provenance and depositional links of Mesozoic Sierra Nevada intra-arc strata

Abstract A compilation of new and published detrital zircon U-Pb age data from Permo-Triassic to Cretaceous intra-arc strata of the Sierra Nevada (eastern California, USA) reveals consistent sedimentary provenance and depositional trends across the entire Sierra Nevada arc. Detrital zircon age distributions of Sierra Nevada intra-arc strata are dominated by Mesozoic age peaks corresponding to coeval or just preceding arc activity. Many samples display a spread of pre-300 Ma ages that is indistinguishable from the detrital age distributions of pre-Mesozoic prebatholithic framework strata and southwestern Laurentian continental margin deposits. Synthesis of detrital zircon age data with tectonostratigraphic constraints indicates that a marine to subaerial arc was established in Triassic time, giving way to widespread shallow- to deep-marine deposition in latest Triassic to Early Jurassic time that continued until the emergence of the arc surface in the Early Cretaceous. No data presented herein require the existence of Mesozoic exotic terranes and/or outboard arcs that were previously hypothesized to have been accreted to the Sierra Nevada. We conclude that Sierra Nevada intra-arc strata formed within a coherent depositional network that was intimately linked to the southwestern United States Cordilleran margin throughout the span of Mesozoic arc activity.

Pace, magnitude, and nature of terrestrial climate change through the end-Permian extinction in southeastern Gondwana

Abstract Rapid climate change was a major contributor to the end-Permian extinction (EPE). Although well constrained for the marine realm, relatively few records document the pace, nature, and magnitude of climate change across the EPE in terrestrial environments. We generated proxy records for chemical weathering and land surface temperature from continental margin deposits of the high-latitude southeastern margin of Gondwana. Regional climate simulations provide additional context. Results show that Glossopteris forest-mire ecosystems collapsed during a pulse of intense chemical weathering and peak warmth, which capped ∼1 m.y. of gradual warming and intensification of seasonality. Erosion resulting from loss of vegetation was short lived in the low-relief landscape. Earliest Triassic climate was ∼10–14 °C warmer than the late Lopingian and landscapes were no longer persistently wet. Aridification, commonly linked to the EPE, developed gradually, facilitating the persistence of refugia for moisture-loving terrestrial groups.

Kurucaşile – Cide Dolayının Jeolojisi

Cide-Kurucaşile area on the İstanbul Zone is an important region for understanding the geological evolution of NW Anatolia and the Black Sea. The lowest visible unit of the region is the Carboniferous coal-bearing terrestrial sediments. Permian aged terrestrial Çakraz Formation and the overlying Triassic lacustrine Çakrazboz Formation unconformably overlie the Palaeozoic sequence. Dogger aged Himmetpaşa Formation represents a short-period marine invasion. The transgression that started in the Kimmerician on the Dogger and older units has led to the development of a carbonate platform, which has risen and demised after the Berriasian. A second carbonate platform, much more limited than the previous one, developed in the Late Barremian, and then the whole region deepened. The Cenomanian is a period of regional uplift and erosion. The first effective volcanic activity in the region started in the Middle Turonian. During this period, extensional faults coeval with the deposition of volcanic and volcanogenic clastics, and colluvial wedges developed in front of them, and the region gained an irregular topography. In the Middle-Late Santonian, the volcanism became silent for a while, the region rapidly subsided and became a completely deep marine environment. These events indicate that the continental crust of the region was totally broken up and the oceanic crust began to develop in the Western Black Sea back arc basin. In the Campanian, the volcanism reactivated much more intensely, but ceased after a short time, leaving its place to pelagic sediments forming the southern passive continental margin deposits of the Black Sea. In the Maastrichtian, the region was tectonically compressed and uplifted and provided turbiditic material to the Black Sea in the north and the Devrek Basin in the south. Siliciclastic turbidites have been deposited in the region until the Middle Eocene. At the end of the Middle Eocene, following uplifting of the entire region and the destruction of marine environments, the north-vergent thrusts were developed due to intense compression and the region became a fold-thrust belt. This compressional tectonic regime is probably still active.

Zinc- and cadmium-isotope evidence for redox-driven perturbations to global micronutrient cycles during Oceanic Anoxic Event 2 (Late Cretaceous)

This study uses organic-rich sediments from the Tarfaya Basin, Morocco, to assess the Cd- and Zn-isotope response to dramatic global palaeoenvironmental change during the Cenomanian–Turonian interval (Late Cretaceous). These organic-rich continental-margin deposits include an expression of Oceanic Anoxic Event 2 (OAE 2, ∼94 Ma), an interval associated with the spread of low-oxygen marine environments and widespread burial of organic-rich sediments. Due to placement of the Tarfaya Basin in a region of upwelling and relatively constant local environmental conditions, the stratigraphic variations in δ114Cd and δ66Zn values largely reflect changes in the seawater isotopic composition of the sub-surface proto-North Atlantic Ocean. Positive shifts of ∼0.2–0.3‰ away from background values in δ114Cd and δ66Zn are observed during the main phase of the positive carbon-isotope excursion associated with OAE 2. These isotopic shifts are coeval with decreases in Cd/TOC and Zn/TOC ratios and thus imply that drawdown of isotopically light Cd and Zn from seawater inventories was a result of extensive burial of these metals in organic-rich marine sediments globally. Low δ66Zn values during the Plenus Cold Event, a cooler episode during OAE 2, are similar in timing and magnitude to variations found in the English Chalk (Eastbourne, UK) and support the inference of a global control on these isotopic excursions. The δ66Zn values during the Plenus Cold Event are taken to record global oxygenation, possibly including the remobilization of isotopically light Zn from continental-margin sediments. A considerably smaller change in δ114Cd values for this interval implies that the Cd- and Zn-isotope systems can provide information about slightly different environmental processes, with global seawater composition with respect to Zn also being influenced by the magnitude of oxic removal sinks and isotopically light Zn input fluxes from sediments and hydrothermal fluids.

Sea-level and climate signatures recorded in orbitally-forced continental margin deposits over the last 1 Myr: New perspectives from the Bohai Sea

Sediment accumulation in continental margins during the Quaternary Period was substantially influenced by both sea-level fluctuations and climate changes. However, the response of sediment accumulation to these changes on an orbital timescale, remains poorly understood, for two main reasons: the scarcity of sedimentary records with a high-resolution chronology, and the difficulty in distinguishing the role of sea-level from regional climate signals. Here we present the results of sediment color reflectance (c*), grain size and total organic carbon (TOC) content of core BH08 (212.4 m in length, spanning the last ~1 Myr) recovered from the Bohai Sea, China. The chronology of core BH08 on the orbital timescale is constrained by both magnetostratigraphy and astronomical tuning. Sedimentary facies analysis suggests that the sedimentary sequence is dominated by the alternation of a deltaic system and floodplain deposition. Principal components analysis of the grain-size data reveals two principal components (PCs), including PC1, reflecting the 31–500 μm fraction, and PC2, reflecting the 18–66 μm fraction. Comparison of sample scores on PC1, PC2 and c* with the alternation of sedimentary environments reveals that PC1 and c* reflect cycles of deltaic and floodplain deposition at ~100-kyr and ~40-kyr periodicities, while PC2 exhibits a ~20-kyr periodicity, in addition to the ~100-kyr and ~40-kyr cycles. We infer that PC1 and c* are mainly sea-level dependent, whereas PC2 is controlled by the combination of changes in monsoon climate and sea level. We suggest that Milankovitch-scale monsoon climate fluctuations controlled the sediments supply to the Bohai Sea during the last ~1 Myr, while the redistribution of sediments by marine processes (e.g., tidal currents) has partially obscured the monsoonal signal in the grain-size proxy (e.g., PC1) which instead is sensitive to sea-level change. Our results provide an example of the influence of climate and sea-level on sediment accumulation in a shallow continental margin setting influenced by monsoonal climate in an icehouse world.

Distinguishing between Deep-Water Sediment Facies: Turbidites, Contourites and Hemipelagites

The distinction between turbidites, contourites and hemipelagites in modern and ancient deep-water systems has long been a matter of controversy. This is partly because the processes themselves show a degree of overlap as part of a continuum, so that the deposit characteristics also overlap. In addition, the three facies types commonly occur within interbedded sequences of continental margin deposits. The nature of these end-member processes and their physical parameters are becoming much better known and are summarised here briefly. Good progress has also been made over the past decade in recognising differences between end-member facies in terms of their sedimentary structures, facies sequences, ichnofacies, sediment textures, composition and microfabric. These characteristics are summarised here in terms of standard facies models and the variations from these models that are typically encountered in natural systems. Nevertheless, it must be acknowledged that clear distinction is not always possible on the basis of sedimentary characteristics alone, and that uncertainties should be highlighted in any interpretation. A three-scale approach to distinction for all deep-water facies types should be attempted wherever possible, including large-scale (oceanographic and tectonic setting), regional-scale (architecture and association) and small-scale (sediment facies) observations.

Comparative geochemical study of the cherty rocks of the Taukha terrane (Sikhote-Alin) and its paleogeodynamic significance

The article presents the results of a comparative geochemical study of cherts and siliceous–clayey rocks composing the tectono-sedimentary complexes of various structural levels of the Taukha terrain of the Sikhote-Alin Late Jurassic–Early Cretaceous accretionary prism. The data obtained show that the same-aged parts of the cross-sections of the cherty–terrigenous sequences of the terrane lower and middle structural levels are characterized by significantly different contents and distribution patterns of major petrogenic oxides, trace, and rare-earth elements due to their accumulation in various facies of oceanic zones. The set of geochemical data indicates that the formation of both tectono-sedimentary complexes was carried out differently, gradually replacing each others’ facies oceanic zones, and started in the near-spreading ridge area, then in the pelagial, and completed in the continental-margin deposition environment. Based on the time interval of the facies conditions changing for each complex (i.e., time of transition from one facies zone to another), the speeds of a paleo-oceanic plate motion (and, correspondingly, the speeds of spreading) for individual intervals of the Jurassic were calculated. Differences in the spreading speeds at various sites of a paleorift zone caused the turning of a paleocontinent margin contour and spreading ridge axis at the perpendicular position, which in turn caused change of the geodynamic mode on the eastern margin of the Paleo-Asian continent at the Jurassic–Cretaceous transition.

New 40Ar/39Ar age constraints on cooling and unroofing history of the metamorphic host rocks (and igneous intrusion associates) from the Bulfat Complex (Bulfat area), NE-Iraq

The Northern Zagros Suture Zone (NZSZ), formed as a result of the collision between Arabian and Sanandaj-Sirjan microplate, is considered as part of the Zagros orogenic belt. NZSZ is marked by two allochthonous thrust sheets in upward stacking order: lower and upper allochthon. The Bulfat complex is a part of the upper allochthon or “Ophiolite-bearing terrane” of Albian-Cenomenion age (97–105 Ma). Voluminous highly sheared serpentinites associated with ophiolites occur within this upper allochthon. In addition, the Gemo-Qandil Group is characterized by gabbroic to dioritic Bulfat intrusion with a crystallization age spanning from ~45 to ~ 40 Ma, as well as extensive metapelites with contact to the Walash-Naupurdam metavolcanic rocks. Due to the deformation in the Sanandaj-Sirjan Zone along the eastern side of the Iraqi segment of NZSZ, the Gemo-Qandil Group was regionally metamorphosed during late Cretaceous (~ 80 Ma). This tectono-compressional dynamics ultimately caused an oscillatory deformation against Arabian continental margin deposits as well. During these events, gabbro-diorite intrusion with high-grade contact metamorphic aureoles occurred near Bulfat. Thus, there is an overlap between regional and contact metamorphic conditions in the area. The earlier metamorphic characteristic can be seen only in places where the latter contact influence was insignificant. Generally, this can only observed at a distance of more than 2.5 km from the contact. According to petrographic details and field observations, the thermally metamorphosed metapelitic units of the metasediment have been completely assimilated, with only some streaks of biotite and relicts of initial foliation. They strongly resemble amphibolite-grade slices from the regional metamorphic rocks in the region. Metapelitic samples far from the intrusion give similar biotite cooling ages as the intrusive rocks. Thus, they may be affected by the same thermal event. 40Ar/39Ar dating of biotite in metapelite rocks of Bulfat by step-wise heating with laser gave average weighted isotopic ages of 34.78 ± 0.06 Ma. This is interpreted as crystallization/recrystallization age of biotite possibly representing the time of cooling and uplift history of the Bulfat intrusion. Cooling and exhumation rates for the Bulfat gabbro-diorite rocks were estimated as ~ 400 °C/Ma and ~ 3.3 mm/year respectively. According to petrographic details, field observations and Ar/Ar dating concerning the contact metamorphism near Bulfat due to the gabbro-diorite intrusion, no significant deformation is visible during exhumation processes after the Paleogene tectono-thermal event, indicating that isotopic ages of 34.78 ± 0.06 Ma could mark the timing of termination of the island arc activity in the Ophiolite-bearing terrane (upper allochthon).

Sediment storage in the Southern Alps of New Zealand: New observations from tracer thermochronology

Careful study of the processes transporting sediment across Earth's surface is critical for robust interpretation of the sedimentary record. Here we consider the specific influence of cyclic glaciation on the export of sediment from mountain landscapes to ocean basins. Using detrital apatite fission-track tracer thermochronology, we present new observations of sediment provenance from six large river systems draining the eastern flank of the Southern Alps, New Zealand. Detrital cooling ages in all six rivers reflect erosion of partially-reset and fully-unreset bedrock exposed in lower catchment areas and indicate that sediment is not currently contributed in proportion to long-term (>106 yr) erosion patterns. Instead, detrital cooling ages are better explained by either localized erosion along the eastern mountain front or intermontane sediment storage. Of these two alternatives, only intermontane sediment storage is further consistent with suspended sediment flux measurements in eastern rivers. Our observations are consistent with prior interpretations of Holocene sediment retention, and contrast with tracer thermochronology from continental margin deposits indicating sediment was rapidly exported to the continental shelf during the late Pleistocene. Collectively, this evidence argues for a reactive sediment routing system east of the main drainage divide that responds to cyclic glaciation by retaining sediment onshore following deglaciation and evacuating sediment reservoirs offshore during the subsequent glacial advance. Our research demonstrates the importance of intermontane sediment storage on the transmission of high-frequency (∼104–5 yr) climate signals to offshore sedimentary archives while highlighting a novel approach to detailing sediment provenance in tectonically active mountain ranges.

Geochemical and zircon U-Pb and Lu-Hf isotopic constraints on the origin of supracrustal rocks from the mid-Qilian terrane: A comparison between supracrustal rocks on the two sides of the eastern segment of the Altyn Tagh Fault

Comparisons between the tectonic units on two sides of the sinistral ENE-striking Altyn Tagh Fault (ATF) are important tracers to find out the corresponding relationships among different terranes. This study focuses on the tectonic units along the eastern segment of the ATF. The Paleoproterozoic Beidahe Group in the mid-Qilian terrane, south side of the eastern segment of the ATF, is mainly composed of metasedimentary rocks and amphibolites. These rocks are petrologically and mineralogically similar to those from the Dunhuang Group, north side of the eastern segment of the ATF. The detrital zircon U-Pb ages indicate that the protolith sediments of the Beidahe metasedimentary rocks were mainly sourced from a provenance dominated by magmatic rocks in ages of 2.85–2.55Ga, 1.70–1.55Ga and 1.42–1.03Ga. This sedimentary sequence was deposited no earlier than 1.03Ga as passive continental margin deposits. Geochemical and zircon U-Pb and Lu-Hf isotopic data suggest that the Neoarchean-Mesoproterozoic basement rocks from the Tarim Craton most likely provide the majority of source materials for their protolith. Amphibolites from the Beidahe Group have high total Fe2O3 and Al2O3 contents but low Mg# values, indicating a high degree of compositional evolution and contamination of the older crust. They have flat REE patterns but positive LILE and negative Nb-Ta and Ti anomalies, suggesting a back-arc basin basalts (BABBs) affinity for their protolith. The amphibolites have a crystallization age of 1.61Ga and zircon εHf(t) values ranging from −7.7 to +10.6. The zircon εHf(t) values and whole-rock εNd(t) values indicate an involvement of juvenile and pre-existing old materials in the formation of their parental magmas. Comparisons between the Beidahe and Dunhuang Groups indicate that the metasedimentary rocks from these two groups are both sourced from the Tarim Craton but have different depositional ages, and that the amphibolites have similar crystallization ages but different tectonic settings for their source rocks. The field relationships and geochemical and geochronological characteristics of the metasedimentary rocks and amphibolites from the Beidahe Group support the interpretation that the observed recent proximity of these two rock complexes might be the result of tectonic juxtaposition rather than a primary intrusive contact.

U[sbnd]Pb detrital zircon ages from some Neoproterozoic successions of Uruguay: Provenance, stratigraphy and tectonic evolution

The Neoproterozoic volcano-sedimentary successions of Uruguay have been the subject of several sedimentologic, chrono-stratigraphic and tectonic interpretation studies. Recent studies have shown, however, that the stratigraphy, age and tectonic evolution of these units remain uncertain. Here we use new UPb detrital zircon ages, combined with previously published geochronologic and stratigraphic data in order to provide more precise temporal constraints on their depositional age and to establish a more solid framework for the stratigraphic and tectonic evolution of these units. The sequence of events begins with a period of tectonic quiescence and deposition of extensive mixed siliciclastic-carbonate sedimentary successions. This is followed by the development of small fault-bounded siliciclastic and volcaniclastic basins and the emplacement of voluminous granites associated with episodic terrane accretion. According to our model, the Arroyo del Soldado Group and the Piedras de Afilar Formation were deposited sometime between ∼1000 and 650 Ma, and represent passive continental margin deposits of the Nico Pérez and Piedra Alta terranes, respectively. In contrast, the Ediacaran San Carlos (<552 ± 3 Ma) and Barriga Negra (<581 ± 6 Ma) formations, and the Maldonado Group (<580–566 Ma) were deposited in tectonically active basins developed on the Nico Pérez and Cuchilla Dionisio terranes, and the herein defined Edén Terrane. The Edén and the Nico Pérez terranes likely accreted at ∼650–620 Ma (Edén Accretionary Event), followed by their accretion to the Piedra Alta Terrane at ∼620–600 Ma (Piedra Alta Accretionary Event), and culminating with the accretion of the Cuchilla Dionisio Terrane at ∼600–560 Ma (Cuchilla Dionisio Accretionary Event). Although existing models consider all the Ediacaran granites as a result of a single orogenic event, recently published age constraints point to the existence of at least two distinct stages of granite generation, which are spatially and temporally associated with the Edén and Cuchilla Dionisio accretionary events.

Oceanic sediments of the Amur Terrane: Their age and tectonic significance

The oceanic pelagic and hemipelagic siliceous and siliceous-clayey sediments play a subordinate role in the Amur Terrane, where they constitute thin tectonic slices separated by thicker terrigenous continental-margin deposits. The analysis of the radiolarian assemblages revealed the Middle-Late Jurassic age of hemipelagic siliceous mudstone and the Early Cretaceous age of similar continental-margin sediments. These new data contribute to the knowledge of the terrane's stratigraphy and demonstrate the progressively younger age of the stratigraphic boundaries between the different sedimentary facies in the southeastern direction. The multiple stacking of the oceanic and continental-margin sediments is characteristic of the accretionary complexes, and precisely such an interpretation of the tectonic nature of the Amur Terrane is consistent with its composition and position in the regional structure.

Precambrian key tectonic events and evolution of the North China craton

Abstract The North China craton (NCC) is one of oldest cratons in the world, with crust up to c . 3.8 Ga old, and has a complicated evolution. The main Early Precambrian geological events and key tectonic issues are as follows. (1) Old continental nuclei have been recognized in the NCC, and the oldest remnants of granitic gneiss and supracrustal rocks are 3.8 Ga old. The main crustal growth in the NCC took place at 2.9–2.7 Ga. The NCC can be divided into several microblocks, which are separated by Archaean greenstone belts that represent continental accretion surrounding the old continental nuclei. (2) By 2.5 Ga, the microblocks amalgamated to form a coherent craton by continent–continent, arc–continent or arc–arc collisions. The tectonic processes in Neoarchaean and modern times appear to differ more in degree than in principle. Extensive intrusion of K-granite sills and mafic dykes and regional upper amphibolite- to granulite-facies metamorphism occurred, and marked the beginning of cratonization in the NCC. Coeval ultramafic–mafic and syenitic dykes of c . 2500 Ma in Eastern Hebei indicate that the NCC became a stable, thick and huge continent at the end of the Archaean, and probably was a part of the Neoarchaean supercontinent that has been suggested by previous studies. (3) In the period between 2500 and 2350 Ma, the NCC was tectonically inactive, but the development of a Palaeoproterozoic volcanic and granitic rocks occurred between 2300 and 1950 Ma. The volcanic–sedimentary rocks are termed Palaeoproterozoic mobile belts; these have a linear distribution, and were affected by strong folding and metamorphism at 1900–1850 Ma, and intruded by granites and pegmatites at 1850–1800 Ma. The Palaeoproterozoic mobile belts formed and evolved within the craton or continental margin (epicontinental geosyncline). Some 2.30–1.95 Ga rift-margin, passive continental margin deposits, analogous arc or back-arc assemblages, as well as HP and HT–UHT metamorphic complexes seem to be comparable with many in the late Phanerozoic orogenic belts. Regarding Palaeoproterozoic orogeny in other cratons, it is possible that a global Palaeoproterozoic orogenic event occurred, existed and resulted in the formation of a pre-Rodinian supercontinent at c . 2.0–1.85 Ga. (4) In contrast, the c . 1800 Ma event is an extension–migmatization event, which includes uplift of the lower crust of the NCC as a whole, the emplacement of mafic dyke swarms, continental rifting, and intrusion of an orogenic magmatic association. This event has been considered to be related to the break-up of the pre-Rodinian supercontinent at 1.8 Ga, attributed to a Palaeoproterozoic plume. (5) As HP and HT–UHT metamorphic rocks occur widely in the NCC, their high pressure of 10–14 kbar has attracted attention from researchers, and several continental collisional models have been proposed. However, it is argued that these rocks have much higher geothermal gradient and much slower uplift rate than those in Phanerozoic orogenic belts. Moreover, HP and HT–UHT rocks commonly occur together and are not distributed in linear zones, suggesting that the geological and tectonic implications of these data should be reassessed.

Significance of the trace fossil Zoophycos in Pliocene deposits, Antarctic continental margin (ANDRILL 1B drill core)

AbstractZoophycos is a complex three dimensional trace fossil that is abundant in deep ocean sediments worldwide, but has not been described previously from Cenozoic continental margin deposits of Antarctica. In the ANDRILL 1B core drilled through the north-west McMurdo ice shelf, Zoophycos occurs in a 17 m thick unit of interglacial sediments bounded above and below by glacial surfaces of erosion. This unit was deposited during the transition from the relatively warm Early Pliocene characterized by productive open waters to the cooler Late Pliocene with fluctuating subpolar ice sheets. Globally, Late Cenozoic Zoophycos are most abundant at great depths (&gt; 1000 m), and where sedimentation rates and TOC levels are low; the Zoophycos producer, probably a worm-like animal, was (is) a slow colonizer. Application of these preferences to the ANDRILL 1B core indicates that the Zoophycos-bearing unit was deposited episodically, with sufficient time between events to allow for the slow processes of colonization and construction. The foray of Zoophycos producer into the relatively shallow ANDRILL 1B depths (200–1000 m) during the Pliocene documents “emergence” of benthic animals, supporting suggestions that the unique modern Antarctic and Southern Ocean faunas result from both “emergence” and “submergence” during the Cenozoic.

Tectonic implications of Late Paleozoic stratigraphic distribution in Northeast China and adjacent region

An analysis of the distribution of the Late Paleozoic strata on Northeast China and adjacent region reveals a zonal pattern of the distribution around the core of the Jiamusi-Mongolia Block. The main part of Late Paleozoic marine strata in this area is considered the continental margin deposits of the Jiamusi-Mongolia Block by analyzing the stratigraphic contact relationship, lithofacies, etc. The results are exhibited in a series of tectonic paleogeographic maps. This presents an important proof for the foundation of the Jiamusi-Mongolia Block, and confines the forming time of Jiamusi-Mongolia Block to the Late Silurian.

Carboniferous continental margin deposits in southern Tuscany, Italy: results from geological mapping of the geotopes Farma Valley and San Antonio Mine Area

AbstractRelics of the Carboniferous shelf, slope and basin margin section occur as intensely tectonized slices within the middle Tertiary Monticiano‐Roccastrada metamorphic core complex in southern Tuscany. These sedimentary successions are interpreted as predominantly siliciclastic second‐order sequence stratigraphic cycles, deposited in a successor basin (Farma Basin) established near the Southern Convergence Front of the westmediterranean Hercynides. The units developed as highstand systems tracts above a downlap surface and buried pelagites of a starved, moderately deep‐water basin (Risanguigno Basin). The Risanguigno Basin was characterized by dys‐ and anoxic conditions and underwent partial tectonic inversion during the early Carboniferous. Detailed mapping has revealed that Carboniferous turbidites deposited on the slope, base of slope and Farma Basin margin are present in the eastern part of the Farma Valley and in the San Antonio Mine Area. Coeval lithologies—storm influenced deposits of the inner shelf and lower shoreface—crop out in the western part of the Farma Valley. Both units, considered to be related to storm‐triggered offshore bottom currents bypassing the outer shelf, are covered by offlapping shallow marine and littoral clastics probably of Permian age. Subsequent tectonic uplift and partial inversion of the Farma Basin resulted in subaerial exposure, erosion of the upper part of the basin fill and deposition of the terrestrial Verrucano Group (middle Triassic), which sealed these early Carboniferous to late Permian sequences considered to represent a late Palaeozoic preligurian failed rift. Copyright © 2008 John Wiley &amp; Sons, Ltd.

A sedimentary prelude to Marinoan glaciation, Cryogenian (Middle Neoproterozoic) Keele Formation, Mackenzie Mountains, northwestern Canada

Continental margin deposition between the Sturtian- and Marinoan-aged global glacial periods is represented in NW Canada by a siliciclastic-to-carbonate grand cycle. The Keele Formation, a 300–600 m thick carbonate and siliciclastic deposit, comprises the upper part of this interglacial cycle. Sedimentary facies in the lower, carbonate-dominated member of the Keele span a broad spectrum of depositional environments, from below storm-wave-base to peritidal to continental, and are disposed across a relatively narrow paleogeographic belt. Lateral arrangement of facies and the gradual stratigraphic transitions between them, suggest deposition in a ramp setting; local slump structures and occasional debrites in deep-water facies imply that the ramp was distally steepened. Fluvial facies are concentrated at a single locality, indicating that terrigenous-clastic sediment was derived from a point source or sources. Most strata of the carbonates are “Bahamian” in character and similar to Phanerozoic warm-water carbonates in that they contain abundant calcite ooids of interpreted original aragonitic mineralogy and have well-developed muddy peritidal facies. Nevertheless, close juxtaposition of deep- and shallow-water facies in Keele rocks indicates high-amplitude sea-level change during deposition, on the order of ∼100 m, suggesting accumulation in a climatic icehouse period. This interpretation implies that although the globe possessed polar and high-altitude glaciers at this time, sea-surface temperature, at least at tropical latitudes, warmed dramatically between Neoproterozoic glacial episodes. The onset of Marinoan glaciation here is heralded by an abrupt shift to nearshore marine siliciclastic sedimentary rocks. These deposits form the upper member of the Keele Formation (informally called the Keele clastic wedge) and directly underlie Ice Brook Formation diamictites. Carbonates are rare and are restricted to mudstones and flat-pebble conglomerates that lack any indication of tropical conditions.