Marine Sequences Research Articles

Volcanic rift margin model for the rift-to-drift setting of the late Neoproterozoic-early Cambrian eastern margin of Laurentia: Chilhowee Group of the Appalachian Blue Ridge

New data support a model of a volcanic rifted margin for eastern Laurentia and the breakup of the supercontinent Rodinia. Upper Neoproterozoic–lower Cambrian rocks of the Chilhowee Group in the Blue Ridge Province of eastern North America are subdivided into two facies assemblages separated by an unconformity. Historically, the rocks have been correlated as a tripartite division: 1) basal sandstone and conglomerate (Cochran, Unicoi, and Weverton Formations), 2) middle siltstone and shale (Nichols Shale, Hampton Shale, and the Hampton and Harpers Formations), and 3) an upper sandstone and shale (Nebo Quartzite, Murray Shale, Hesse Quartzite, and Helenmode, Erwin, and Antietam Formations). Sedimentary analyses show that boundaries of the newly defined facies assemblages transect the named stratigraphic units. Assemblage A consists of fluvial-lacustrine deposits with interbedded subaerial basalt flows overlain by marine deposits. Fluvial strata formed in anastomosing braided-river channels and are interbedded with mudstones deposited in shallow lacustrine plains. Overlying shallow marine deposits consist of upward-coarsening successions capped by over-thickened sandstones. Shale-rich turbidite deposits characterized by subdued bioturbation and common slump features occur at the top. Assemblage B consists of sandstone and shale deposited on a stable shelf. These upward-coarsening parasequences are stacked to form transgressive-regressive system tracts reflecting long-term sea-level fluctuations. A working hypothesis is that these rocks are comparable to those present in seismic reflection profiles and drill cores of the volcanic rift margins of the Atlantic and Indian Oceans. Assemblage A deposits formed during rapid subsidence associated with the formation of seaward-dipping reflectors. Flood basalts within fluvial sandstones are the landward facies of more voluminous basalt flows in the opening basin. Overlying marine deposits of Assemblage A were coeval with basaltic volcanism and deeper-water deposits to the east. Assemblage B unconformably overlies Assemblage A rocks and laps onto much older continental rocks. It was deposited on the craton into the late Cambrian. Much of the lateral lithologic variability and differences in character and thickness of the Chilhowee Group is attributed to the depositional setting that was influenced by the underlying rift architecture. It was further complicated by juxtaposition of rocks telescoped along numerous Paleozoic thrust faults. This model predicts that coeval rocks in outboard thrust sheets are finer-grained, mafic volcanic-rich marine sequences (Assemblage A), overlain by shelf-margin and basin deposits that lack volcanic rocks (Assemblage B). This model provides more refined sedimentary criteria for examination of other volcanic rift margins.

Stratigraphy, palaeontology and geochemistry of the late Neoproterozoic Aar Member, southwest Namibia: Reflecting environmental controls on Ediacara fossil preservation during the terminal Proterozoic in African Gondwana

Common, Ediacaran fossils are well preserved in a Late Neoproterozoic (ca. 545Ma) shallow marine sequence, described here as the Aar Member of the Dabis Formation (Kuibis Subgroup, Nama Group), near Aus in southwest Namibia. This 31–38m thick, shale-dominant unit records the transition from fluvial-shallow marine Kliphoek Sandstone to open marine limestone of the Mooifontein Member of the Zaris Formation, deposited on a subsiding continental margin during a major, regional transgression. Thin sandstone beds contain fossils at a number of levels throughout the Aar Member. Concentrations of Pteridinium were mostly transported in flood-derived sheets, while some Ernietta assemblages are preserved close to in situ. Rangea has also been transported, and is mostly confined to thin sandstone lenses incised into mudstone. Limestone beds, common throughout, include at least two marker horizons that can be followed regionally and show local evidence of storm reworking. Systematic sampling and analyses of limestone reveals enrichment in both 13C and 18O higher in the section, with negative δ13C near the base rising to moderate positive values near the top. The negative-to-positive transition in δ13C values is more pronounced in the east, with all of the lower Aar Member samples consistently depleted in 13C. While this may reflect greater degrees of alteration by meteoric or dewatering fluids, the same carbonates are notably enriched in 18O relative to those at the same stratigraphic position to the west. The overall rise in 13C is attributed to greater proportional burial of organic matter and release of oxygen to surface environments, while the spatial variability is likely the result of a strong surface-to-deep carbon isotopic gradient in seawater. A number of the fossils, especially Rangea, are encrusted with jarosite, an iron-bearing sulphate mineral and common weathering product of pyrite. This observation suggests that preservation of the fossils may have resulted from the rapid encrustation of pyrite on the surface of the organisms as they decomposed and were consumed by sulphate-reducing bacteria within the sandy, near shore sediments. Insofar as pyrite formation requires iron, which is soluble and reactive in anoxic solutions, it is likely that the deeper subtidal environments lacked oxygen. In situ pyritized forms like Ernietta may have developed the capacity to survive under episodically anoxic or sub-oxic environmental conditions, while Pteridinium and Rangea lived within an oxygenated estuarine or fluvial setting and were transported during storms to anoxic, ferruginous environments where they were exquisitely preserved.

Thick deltaic sedimentation and detachment faulting delay the onset of continental rupture in the Northern Gulf of California: Analysis of seismic reflection profiles

AbstractThe transition from distributed continental extension to the rupture of continental lithosphere is imaged in the northern Gulf of California across the obliquely conjugate Tiburón‐Upper Delfin basin segment. Structural mapping on a 5–20 km grid of seismic reflection lines of Petroleos Mexicanos demonstrates that ~1000% extension is accommodated on a series of NNE striking listric‐normal faults that merge at depth into a detachment fault. The detachment juxtaposes a late‐Neogene marine sequence over thinned continental crust and contains an intrabasinal divide due to footwall uplift. Two northwest striking, dextral‐oblique faults bound both ends of the detachment and shear the continental crust parallel to the tectonic transport. A regional unconformity in the upper 0.5 s (two‐way travel time) and crest erosion of rollover anticlines above the detachment indicates inversion and footwall uplift during the lithospheric rupture in the Upper Delfin and Lower Delfin basins. The maximum length of new crust in both Delfin basins is less than 40 km based on the lack of an acoustic basement and the absence of a lower sedimentary sequence beneath a wedge‐shaped upper sequence that reaches >5 km in thickness. A fundamental difference exists between the Tiburón‐Delfin segment and the Guaymas segment to the south in terms of presence of low‐angle normal faults and amount of new oceanic lithosphere, which we attribute to thermal insulation, diffuse upper‐plate extension, and slip on low‐angle normal faults engendered by a thick sedimentary lid.

Deglacial to postglacial palaeoenvironments of theCelticSea: lacustrine conditions versus a continuous marine sequence

Recent work on the last glaciation of theBritishIsles has led to an improved understanding of the nature and timing of the retreat of theBritish−IrishIce Sheet (BIIS) from its southern maximum (Isles ofScilly), northwards into theCeltic andIrish seas. However, the nature of the deglacial environments across theCelticSea shelf, the extent of subaerial exposure and the existence (or otherwise) of a contiguous terrestrial linkage betweenBritain andIreland following ice retreat remains ambiguous. Multiproxy research, based on analysis of 12BGSvibrocores from theCelticDeepBasin (CDB), seeks to address these issues.CDBcores exhibit a shell‐rich upward fining sequence ofHolocene marine sand above an erosional contact cut in laminated muds with infrequent lonestones. Molluscs,in situForaminifera and marine diatoms are absent from the basal muds, but rare damaged freshwater diatoms and foraminiferal linings occur. Dinoflagellate cysts and other non‐pollen palynomorphs evidence diverse, environmentally incompatible floras with temperate, boreal andArctic glaciomarine taxa co‐occurring. Such multiproxy records can be interpreted as representing a retreating ice margin, with reworking of marine sediments into a lacustrine basin. Equally, the same record may be interpreted as recording similar conditions within a semi‐enclosed marine embayment dominated by meltwater export and deposition of reworked microfossils. As assemblages from these cores contrast markedly with proven glaciomarine sequences from outside theCDB, a glaciolacustrine interpretation is favoured for the laminated sequence, truncated by aLateWeichselian transgressive sequence fining upwards into fully marine conditions. Reworked rare intertidal molluscs from immediately above the regional unconformity provide a minimum datec. 13.9 cal. kaBPfor commencement of widespread marine erosion. Although suggestive of glaciolacustrine conditions, the exact nature and timing of laminated sediment deposition within theCDB, and the implications this has on (pen)insularity ofIreland following deglaciation, remain elusive.

Hydrogeochemical and mineralogical investigations of arsenic- and humic substance-enriched aquifers

• Arsenic contaminated and humic-substance-enriched aquifers in the Chianan Plain were investigated. • Aqueous As was closely correlated with TOC, Alk, fluorescence intensity and Eh. • Arsenic species were under non-equilibrium chemical conditions. • Iron oxyhydroxides and sulfides contained the major portion of As. • As-rich black shale is the major source of As in the Chianan Plain. This study investigated the hydrogeochemical and mineralogical characteristics of arsenic-contaminated and humic-substance-enriched aquifers in the Chianan Plain, Taiwan, which is an endemic area for blackfoot disease (BFD). Factorial analysis (FA) was used to evaluate the hydrochemical characteristics of 83 groundwater samples in the Chianan Plain, and 462 geological core samples obtained from 9 drilling wells were collected to analyze their arsenic and iron contents. The major mineral phases and chemical components were determined using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy and energy dispersive spectrometry (SEM–EDS). Partition of arsenic among various hosting solids in sediments was determined by sequential extraction. The results of FA showed that the hydrochemical characteristics of the groundwater samples could be grouped by 4 factors: salinization, arsenic, sulfide, and iron. Arsenic was positively correlated with alkalinity, dissolved organic/inorganic carbon, and fluorescence intensity [humic acids, (HAs)]. As(V) has a higher chelating affinity with HAs than does As(III), resulting in higher As(V) concentrations distributed throughout the reducing environment. High levels and correlations of As and HAs may cause BFD in the Chianan Plain. No correlation was found between the measured and calculated redox potentials of the various redox couples. The As(III)/As(V) was under a chemical non-equilibrium condition. The vertical distribution of the sedimentary As (solid phase) typically increased with depth, but the aqueous As concentrations were higher in the second aquifer (depth of 80–120 m). Arsenic content (solid phase) was higher in the clay/silt sediments and marine formations. The major minerals identified by XPS and SEM–EDS were goethite, hematite, magnetite, pyrite, and siderite, agreeing with the SI values calculated by PHREEQC. Arsenic content was strongly correlated with sulfur (weight%; R 2 = 0.76, p < 0.05), but was weakly correlated with iron (weight%). However, a moderate correlation ( R 2 = 0.44–0.75; p < 0.001) between As (s) and Fe (s) in the sediments was found in the transitions in the marine and non-marine formations, especially in the fine grains. The chelation of humic complex, competition for sorption sites of organic carbon, reduction dissolution of Fe oxides are mainly responsible for the groundwater As mobility in the Chianan Plain, especially for the marine sequence.

Late-metamorphic veins record deep ingression of meteoric water: A LA-ICPMS fluid inclusion study from the fold-and-thrust belt of the Rhenish Massif, Germany

The fluid evolution of late-metamorphic quartz vein systems in the fold-and-thrust belt of the Rhenish Massif has been studied by microthermometry and LA-ICPMS microanalysis. The quartz veins are hosted by very low-grade organic-matter-rich metasiltstones and metapelites of the Hunsrück nappe complex. The veins record two major stages of textural evolution, a massive vein filling assemblage with elongate-blocky quartz, chlorite, albite and apatite, and a later open space filling assemblage with euhedral crystals of quartz, ankerite/dolomite and minor calcite and sulfides. The euhedral quartz crystals host well-preserved fluid inclusion assemblages, which made it possible to reconstruct the chemical evolution of the fluid system with time. The fluid inclusions are all aqueous two-phase with low salinity, and three successive fluid generations record a systematic decrease in salinity from 4.7–5.3 to 3.1–3.9 and then to 1.4–1.7wt.% eqv. NaCl. The salinity decrease is paralleled by a decrease in homogenization temperatures from 210–225°C to 148–164°C and 124–139°C. Using LA-ICPMS microanalysis of individual fluid inclusions, reproducible elemental concentrations of Li, Na, K, Rb, Cs, Mg, Ca, Ba, Sr, B, Al, As, Sb, S, Cl and Br could be determined. The concentrations of the alkali metals and of boron, arsenic and antimony are correlated with the fluid salinity. Element concentrations are highest in the early fluid generation and decrease systematically in the latter two generations. The systematic changes in fluid composition (salinity, homogenization temperature and elemental concentrations) are best explained by fluid mixing between a hot metamorphic fluid of moderate salinity (that carried elevated concentrations of B, As and Sb) and a cooler dilute fluid that was of meteoric origin. The fluid evolution therefore signals deep ingression of meteoric water during late-metamorphic exhumation and uplift. The molar Cl/Br ratios of the fluid inclusions lie on a linear array that extends from close to seawater to values that are substantially below seawater. Because evaporitic sources are conspicuously absent in the shallow-water marine sequence of the Rhenish Massif, the elevated Br concentrations (and Cl/Br ratios below seawater) are best explained by fluid–rock interaction and liberation of Br from the organic matter in the metasediments.

NOTES ON FOSSIL CHITONS. 5. POLYPLACOPHORA FROM THE PLIOCENE OF WESTERN LIGURIA, NORTHWEST ITALY

This study describes the chiton fauna (Mollusca, Polyplacophora) from deposits of the Pliocene marine sequence of Western Liguria in northwestern Italy between Genova (Genoa) and Ventimiglia. The studied fossils consist of 9,657 valves from nine sites (Bussana, Rio Sant'Antonino, Garlenda, Salea, Caranchi, Rio Torsero, Zinola, Sestri Ponente, and Borzoli; see Fig. 1) taken from the lower clayey formation named Argille di Ortovero (Ortovero Clay). From these we identified 31 species, 22 of which were already known, 5 are identified only at generic level, and four are described as new: Leptochiton josei sp. n., Ischnochiton ligusticus sp. n., Connexochiton roccai sp. n., and Lepidochitona pliocinerea sp. n. Only three species ( Lepidopleurus cajetanus, Chiton corallinus and Acanthochitona fascicularis ) occur in all the studied sites but, even so, it is difficult to evaluate their relative prevalence. Ch. corallinus and Lepidop. cajetanus are most common, representing 46% and 31% respectively of the total valves found. Some species found are particularly noteworthy; Lept. alveolus previously not known as a fossil, except a dubiously identified record from the Eocene/Oligocene of Washington; Lept. bedullii and I. martinelli are known only from few Pliocene localities. Four species are described as new, and this increases the number of Mediterranean Pliocene determined species known to 37. Connexochiton roccai sp. n. represent the first report of Connexochiton as a fossil. Seventeen (55%) of the species found are still living in the Mediterranean Sea and 13 of these also occur in the eastern Atlantic. Only one species, Lept. alveolus , occurs as a living species in the Atlantic but is absent from the Mediterranean Sea. Five determined species are known only from Mediterranean Pliocene, and 12 are recorded for the first time from the Ligurian Pliocene. The analysis of the Ligurian samples suggests a shallow water depositional environment, possibly from lower infralittoral to circalittoral depth, and suggests a mixture of faunal elements from different depths and environments.

Transgressive–regressive sequences on the slope of an isolated carbonate platform (Middle–Late Permian, Laibin, South China)

From the Middle to Late Permian, the Laibin area in Guangxi, South China, was situated on the slope of an isolated carbonate platform, on which continuous marine successions were deposited. Two global stratotype sections for the boundary between the Guadalupian (Middle Permian) and Lopingian (Late Permian) are located at Penglaitan and Tieqiao in the Laibin area, respectively, and thus are chosen for study. At the two locations, 14 facies are recognized in the Maokou and Heshan Formations, and they are further grouped into four facies associations (basin, lower slope, upper slope, and platform margin). Six main transgressive–regressive (TR) sequences are identified in strata from the Roadian (Middle Permian) to the Wuchiapingian (Late Permian). They are conformable marine sequences that were little influenced by regional uplift (Dongwu Movement) and so provide a good record of the sea-level changes in South China at this time. Based on the significant taxonomic selection and controversial marine faunal loss in the end-Guadalupian mass extinction, and the Middle-Late Permian sea-level changes recorded by the TR sequences in the Laibin area, it is suggested that this extinction event might have been triggered by the reduction and loss of shallow-marine habitat area caused by the end-Guadalupian regression. The global cooling and Emeishan volcanism also occurring at this time could have further enhanced this extinction event.

High-precision temporal calibration of Late Permian vertebrate biostratigraphy: U-Pb zircon constraints from the Karoo Supergroup, South Africa

Therapsid and other tetrapod fossils from the South African Karoo Supergroup provide the most detailed and best studied terrestrial vertebrate record of the Middle and Late Permian. The resulting biostratigraphic scheme has global applicability. Establishing a temporal framework for these faunas has proven difficult: magnetostratigraphy has been hampered by a Jurassic overprint, and intercorrelation with Permian marine sequences has been equivocal. Here we report U-Pb zircon isotope dilution–thermal ionization mass spectrometry (ID-TIMS) dates for five volcanic ashes interbedded with fossils from the Pristerognathus , Tropidostoma , and Cistecephalus vertebrate biozones of the Beaufort Group. This temporal framework allows correlation to marine zonations and improves understanding of rates of faunal evolution and patterns of basin evolution. Our results identify no correlative vertebrate extinctions in the Karoo Supergroup to the marine end-Guadalupian mass extinction and raise the question of whether there is any record of a terrestrial extinction related to the Emeishan large igneous province.

Unconventional hydrocarbons: Australia's old rocks prove their worth

Australia’s search for petroleum began in the onshore basins where extensive areas of Paleozoic marine sequences, with some high-quality source rock intervals and spectacular outcrop, encouraged sporadic exploration for many decades. For these efforts, there were some rewards, including the discovery of the Mereenie oil field in Ordovician rocks, the Amadeus Basin in 1960s, and the Blina discovery in Devonian carbonates in the Canning Basin during the early 1980s. Since the late 1980s, however, the focus of exploration has shifted offshore where more and larger discoveries were made in the Mesozoic marginal basins, which today contain about 90% or more of Australia’s conventional oil and gas. Now, however, the focus has shifted back to the onshore, recognising the potential for shale and tight gas and oil in these older rocks. The onshore basin area under exploration license has nearly doubled from 2010–12; several major international companies have joined local explorers in testing the worth of Australia’s lower Paleozoic and Proterozoic petroleum systems, and new discoveries have been made in several basins. Geoscience Australia and its partners in the state and NT surveys are undertaking new assessments and studies across a number of these basins.

Diversity of Middle Eocene Coralline Red Algae from the Prang Limestone (Shella Formation) of Jaintia Hills, Meghalaya, NE Himalaya, India with special emphasis on palaeoenvironment

Palaeocene-Eocene carbonates of shallow marine sequence belonging to Shella Formation of Jaintia Group are well developed in the south of the Jaintia Hills, Meghalaya and represent an excellent outcrop of shelf sediments in North Eastern Himalaya, India. These outcrops are exposed along the southern fringe of Shillong Plateau. The Prang Limestone is the uppermost lithostratigraphic unit of the Shella Formation within the Jaintia Group. It is overlain by sand-shale alternation of the Kopili Formation and underlain by the Narpuh Sandstone. A rich assemblage of larger foraminifera including Nummulites, Discocyclina etc. has been recorded earlier from the Prang Limestone and based on the foraminiferal assemblage, a Middle Eocene to Early Late Eocene age has been assigned to the Prang Limestone. Thin section analysis of the Prang Limestone revealed the abundance of well preserved coralline red algae along with other well preserved and significant biogenic elements. Morpho-taxonomical study reveals that the algal flora is represented by seven species belonging to the families Hapalidiaceae (Subfamily: Melobesioideae), Corallinaceae (Subfamily: Mastophoroideae) and Sporolithaceae. The dominance of melobesioid and mastophoroid forms is the characteristic feature of this algal assemblage. Coralline algae occur mostly in encrusting growth form with orbitoid foraminifera and occasionally the coralline algae form rhodoliths. On this basis of algal forms it can be inferred that they thrived in a shallow, warm, shelf environment of normal salinity.

Vertebrate Tracksites in the Middle Jurassic-Upper Cretaceous of South Tunisia

Four vertebrate tracksites from the Middle Jurassic and Upper Cretaceous in the Tataouine basin of southern Tunisia are described. Approximately 130 tridactyl footprints distributed over an area of 200 square meters, preserved on Callovian beds exposed at the Beni Ghedir site, represent the oldest evidence of a dinosaur fauna in Tunisia. In addition, three tracksites—Chenini, Ksar Ayaat, and Jebel Boulouha—have been discovered in the Cretaceous beds of the upper Continental Intercalaire, previously considered as a strictly marine depositional sequence. In addition to dinosaur tracks, the Chenini tracksite (late Albian) includes poorly preserved crocodilian tracks, and footprints assigned to a pleurodiran turtle have been recovered at the Ksar Ayaat locality (early Cenomanian). The Jebel Boulouha tracksite is dominated by well-preserved tridactyl tracks referred to small-sized theropods. Depositional settings of each tracksite have been defined on stratigraphic and sedimentologic data, and tracks were ascribed to different ichnocoenoses in relation to their paleoenvironments. This new and differentiated track record gives important information on how the fossil vertebrate fauna changed in southern Tunisia during mid-Jurassic to mid-Cretaceous times. These data provide a unique and useful census of tetrapod associations along the southern margin of the peri-Mediterranean area.

Mid-Late Quaternary vegetation and climate change reconstructed from palynology of marine cores off southwestern New Zealand

Little is known about how mid-latitude Southern Hemisphere terrestrial vegetation responded during glacial terminations and the warmer phases of the Late Quaternary, especially beyond the last glacial cycle where records are commonly fragmentary and poorly-dated. The timing, magnitude and sequence of environmental changes are investigated here for terminations (T) I, II and V and their subsequent warm interglacials of MIS 1, 5e and 11 by direct correlation of terrestrial palynomorphs (pollen and spores) and marine climate indicators in marine piston cores MD06-2990/2991 recovered from the East Tasman Sea, west of South Island, New Zealand. The climate there is strongly influenced by the prevailing mid-latitude westerly wind belt that generates significant amounts of orographic rainfall and the proximity of the ocean which moderates temperature variability. Chronological constraint for the cores is provided by δ¹⁸O stratigraphy, radiocarbon chronology and the identification of two widespread silicic tephra horizons (25.6 ka Kawakawa/Oruanui Tephra (KOT); ~345 ka Rangitawa Tephra (RtT)) sourced from the central North Island. Similar vegetation changes over the last two glacial cycles at MD06-2991 and in the adjacent nearby on land record of vegetation-climate change from Okarito Bog permit transfer of the well resolved Marine Isotope Stage (MIS) chronology to Okarito for the pre radiocarbon dated interval (~139-28 ka). Placing both sequences on a common age scale nonetheless assumes there is minimal lag between pollen production and final deposition on the seafloor. However, the timing of Late Pleistocene palynomorph events and KOT between independently dated marine and terrestrial sedimentary sequences are found in this study to be indistinguishable, which supports the direct transfer of terrestrially derived ages to the marine realm and vice versa. Vegetation change in southwestern New Zealand is of similar structure during T-I and T-II, despite different amplitudes of forcing (i.e., insolation rise, CO₂ concentrations). In a climate amelioration scenario, shrubland-grassland gave rise to dominantly podocarp-broadleaf forest taxa, with accompanying rises in mean annual air temperature (MAAT) estimated from Okarito pollen typically synchronous with nearby ocean temperatures. The T-II amelioration commenced after ~139 ka in response to increasing boreal summer insolation intensity, with prominent ocean-atmosphere warming over the period from ~133-130 ka. In contrast, northern mid-high latitude paleoclimate records display cooling over Heinrich Stadial 11 (~135-130 ka), and are prominently warm from ~130-128 ka, while southwestern New Zealand and the adjacent ocean displays cooling. Such millennial-scale climate asynchrony between the hemispheres is most likely a result of a systematic, but non-linear re-organisation of the ocean-atmosphere circulation system in response to orbital forcing. The subsequent MIS 5e climatic optimum in Westland was between ~128-123 ka, with maximum temperatures reconstructed in the ocean…

Controls of Late Jurassic-Early Cretaceous tectonic event on source rocks and seals in marine sequences, South China

Thermal evolution of source rocks and dynamic sealing evolution of cap rocks are both subjected to tectonic evolution. The marine sequences in South China have experienced superposed structural deformation from multiple tectonic events. To investigate the effectiveness of preservation conditions, it is of great importance to understand the controls of key tectonic events on the dynamic evolution of cap rocks. This paper discusses the controls of Late Jurassic-Early Cretaceous (J3-K1) tectonic event on source and cap rocks in marine sequences in South China based on the relationships between J3-K1 tectonic event and the burial history types of the marine sequences, the hydrocarbon generation processes of marine source rocks, the sealing evolution of cap rocks, the preservation of hydrocarbon accumulations, and the destruction of paleo-oil pools. The study has the following findings. In the continuously subsiding and deeply buried areas during the J3-K1 period, marine source rocks had been generating hydrocarbons for over a long period of time and hydrocarbon generation ended relatively late. At the same time, the sealing capacity of the overburden cap rocks had been constantly strengthened so that hydrocarbons could be preserved. In the areas which suffered compressional deformation, folding and thrusting, uplifting and denudation in J3-K1, the burial history was characterized by an early uplifting and the hydrocarbon generation by marine source rocks ended (or suspended) during the J3-K1 period. The sealing capacity of the cap rocks was weakened or even vanished. Thus the conditions for preserving the hydrocarbon accumulations were destroyed. The continuously subsiding and deeply buried areas during the J3-K1 period are the strategic precincts of the petroleum exploration in marine sequences in South China.

MACROFAUNAL RESPONSE TO THE END-TRIASSIC MASS EXTINCTION IN THE WEST-TETHYAN KOSSEN BASIN, AUSTRIA

Bivalves are the most common macrofauna present in marine sequences spanning the end-Triassic mass extinction and document the initial ecological response to the crisis. In the west-Tethyan Kossen Basin, marine bivalves occur within distinctive low diversity episodic shell beds at the time of initial crisis and d 13C minimum, and continue for 1 m (,,20 kyr) upward into the peak extinction phase devoid of macrofauna. The paleoecology, shell mineralogy, and paleobiogeographic context of these well-preserved bivalves suggest they are part of eurytopic opportunistic paleocommunities flourishing in a time of crisis and are consistent with some, but not all, of the paleoenvironmental scenarios hypothesized in the context of synchronous volcanic activity. The best model-to-data fit is found for an ocean acidification scenario, which predicts an increased extinction risk for taxa with thick calcareous skeletons and aragonite mineralogy due to reduced CaCO 3 saturation of seawater for a period of ,20 kyr. Other kill mechanisms, such as climatic change and reduced salinity in nearshore marine settings, are less well supported but not fully incompatible with our data and might have acted in concert with ocean acidification.

Potential methane reservoirs beneath Antarctica

Once thought to be devoid of life, the ice-covered parts of Antarctica are now known to be a reservoir of metabolically active microbial cells and organic carbon. The potential for methanogenic archaea to support the degradation of organic carbon to methane beneath the ice, however, has not yet been evaluated. Large sedimentary basins containing marine sequences up to 14 kilometres thick and an estimated 21,000 petagrams (1 Pg equals 10(15) g) of organic carbon are buried beneath the Antarctic Ice Sheet. No data exist for rates of methanogenesis in sub-Antarctic marine sediments. Here we present experimental data from other subglacial environments that demonstrate the potential for overridden organic matter beneath glacial systems to produce methane. We also numerically simulate the accumulation of methane in Antarctic sedimentary basins using an established one-dimensional hydrate model and show that pressure/temperature conditions favour methane hydrate formation down to sediment depths of about 300 metres in West Antarctica and 700 metres in East Antarctica. Our results demonstrate the potential for methane hydrate accumulation in Antarctic sedimentary basins, where the total inventory depends on rates of organic carbon degradation and conditions at the ice-sheet bed. We calculate that the sub-Antarctic hydrate inventory could be of the same order of magnitude as that of recent estimates made for Arctic permafrost. Our findings suggest that the Antarctic Ice Sheet may be a neglected but important component of the global methane budget, with the potential to act as a positive feedback on climate warming during ice-sheet wastage.

Cenozoic stratigraphy of Taiwan: Window into rifting, stratigraphy and paleoceanography of South China Sea

Shallow marine sequences of the northern South China Sea (SCS) are uplifted and exposed by plate convergence in the Taiwan mountain belt. These deposits provide detailed geological information about the rifting event, stratigraphy, sedimentology, paleoclimate and paleoceanography of the shallow SCS to compare with what are recorded in the ODP 1148 deep-sea core. Seismic surveys and marine micropalentological studies show that Eocene sequences in the offshore Taiwan Strait and onland Taiwan mountain belt are all deposited in rifting basins and are covered unconformably by the Late Oligocene-Neogene post-rifting strata. Between syn-rifting and post-rifting sequences, there is a regional break-up unconformity throughout the island. Early Oligocene and Late Eocene strata are missing along the break-up unconformity equivalent to the T7 unconformity in the Pearl River Mouth Basin off south China. This may suggest that the SCS oceanic crust could have initiated between 33 and 39 Ma. Neither obvious stratigraphic gap nor slumping features are found in the Oligocene-Miocene transition interval of Taiwan. This observation highly contrasts with what has been documented from the ODP 1148 deep-sea core. This suggests that the stratigraphic gap and slumping features could only be recorded in the SCS deep sea region, but not in the shallow shelf near Taiwan. Compared to the Middle Miocene paleoceanographic re-organization events in the SCS deep sea, the geological history of the Taiwan shallow sequence shows changes of in sedimentation and faunal composition. Due to the Antarctic glacial expansion at ∼14 Ma, Middle to late Miocene strata of the Western Foothills show progressive regression sedimentations associated with a decrease of benthic foraminiferal abundance and a sharp faunal turnover event. Many Early-Middle Miocene endemic benthic foraminifers were extinct in 14-13 Ma and new benthic foraminifers of the Kuroshio Current fauna appeared from 10.2 Ma, comparable with new occurrence of Modern benthic foraminifers at 9 Ma in the Java Sea area. This reveals that the Western Boundary Kuroshio Current in the North Pacific could initiate from 10-9 Ma due to closures of the Indo-Pacific seaways by convergent tectonics between the Australian Continent and the Indonesian Arc in 12-8 Ma. Subduction of the SCS oceanic lithosphere since the Middle Miocene resulted in formation of the Hengchun Ridge accretionary prism and the North Luzon Arc. Occurrence of these two bathymetric highs (−2400 m) since the Middle Miocene and closures of the inter-arc passages in the North Luzon arc in the last 3.5 Ma would control the water exchanges between the West Pacific and the deep SCS. Accordingly, the tectonic evolution in the Central Range-Hengchun Peninsula accretionary prism and the arc-forearc Coastal Range not only control directly the route for water exchanges between the West Pacific and the SCS, but also indirectly shows a great influence on the geochemistry of deep SCS waters. The latter is best shown by much negative carbon isotope values of benthic foraminifers in the ODP 1148 deep-sea core than the West Pacific records in the last 14 Ma.

Substrate-controlled ichnofacies along a marine sequence boundary: The Intra-Valanginian Discontinuity in central Neuquén Basin (Argentina)

The Intra-Valanginian Discontinuity in the Neuquén Basin (Argentina) marks a dramatic sea-level fall within Lower Cretaceous strata, and can be mapped for tens of kilometers both along depositional strike and dip. In the proximal region of the lowstand configuration, the subaerial segment of the sequence boundary is demarcated by alluvial conglomerates (basal Mulichinco Formation) onto deep-marine black shales (Vaca Muerta Formation). In the distal marine realm, the time-equivalent marine sequence boundary separates black shales and marls beneath from shallow-marine carbonate deposits above. This paper documents and discusses substrate-controlled trace fossils demarcating the marine sequence boundary across more than 30km in a proximal–distal transect in order to shed light onto processes affecting the sea-floor during the generation, biogenic modification, and preservation of this regionally extensive omission surface.The downdip expression of the marine sequence boundary is well-exposed across a 20-km long, strike-oriented outcrop. Everywhere in this region the discontinuity is demarcated by robust Thalassinoides with well-defined walls and circular cross-sections, suggesting that the marly substrate was relatively firm during excavation. The burrow system is filled with bioclasts of echinoids and bivalves not present in underlying strata, up to pebble-size intraclasts that were eroded from the underlying succession, and authigenic minerals (glauconite, phosphates). This trace-fossil suite illustrates the Glossifungites Ichnofacies. The updip expression of the marine sequence boundary in the study area was recorded in a cored well located about 20km from the outcrops. As in the distal region, a first generation of passively filled Thalassinoides excavated into a firm marly substrate is attributed to the Glossifungites Ichnofacies. However, this suite is cross-cut at the top by Gastrochaenolites and other unidentified borings. Bored intraclasts also occur above the omission surface. This trace-fossil suite represents the Trypanites Ichnofacies.The Intra-Valanginian Discontinuity records a complex history after the onset of the dramatic sea-level fall. Extensive erosion and exhumation of previous sediments, as well as early cementation, combined to produce different types of substrates during the generation of the omission surface (firmgrounds and hardgrounds). Colonization of firm substrates probably started firstly in proximal regions, when relative sea-level was still falling. During early transgression, firmgrounds were occupied in distal regions and borers colonized the hardgrounds developed in proximal regions. Thus, the surface demarcates a composite surface, involving a sequence boundary plus a transgressive surface, draped in turn by a mixing of sediments suggesting siliciclastic starvation. The generation and preservation of the features associated with the Intra-Valanginian Discontinuity are thought to result from the combination of a ramp-type pre-existing morphology, coupled with a high-amplitude sea-level fall and a relatively deep-marine setting prior to the formation of the omission surface.

Allochthonous Mesozoic marine sequences of northeastern Asia and western North America: Correlation of the stratigraphic levels and geodynamic depositional settings

The problem of correlating the isolated outcrops of allochthonous Middle Mesozoic deposits developed in northeastern Asia and western North America includes several aspects: (1) the stratigraphic subdivision of the sequences using the radiolarian assemblages and taking into account the complex nappe-thrust structure of the region; (2) the analysis of their facies composition with reconstructing the geodynamic depositional settings; (3) the search for features in common (with respect to the previous two aspects) in all these localities. The first widely applied radiolarian analysis revealed that the Middle Mesozoic marine rocks from separate nappes and slices of tectono-stratigraphic sections in East Asia enclose 20 different-age (Norian to Hauterivian) radiolarian assemblages. The correlation of these assemblages revealed the wide lateral distribution of their host Middle Mesozoic marine sequences in the regions under consideration. The significance of the presented materials is determined by the following facts: (1) using the radiolarian analysis, the Middle Mesozoic marine sequences of northeastern Asia are first subdivided into stage and substage units with defining their analogs in western North America; (2) despite the discrete distribution of Mesozoic allochthons, it is shown that the defined stratigraphic units are widespread in northeastern Asia; (3) it is established that these allochthons consist of rocks formed in different geodynamic depositional settings: ocean floor, island arc, fore-arc, and marginal-sea domains. This spectrum of heterogeneous rocks is traceable practically through the entire northeastern Asia region.

An extraterrestrial trigger for the Early Cretaceous massive volcanism? Evidence from the paleo-Tethys Ocean

The Early Cretaceous Greater Ontong Java Event in the Pacific Ocean may have covered ca. 1% of the Earth's surface with volcanism. It has puzzled scientists trying to explain its origin by several mechanisms possible on Earth, leading others to propose an extraterrestrial trigger to explain this event. A large oceanic extraterrestrial impact causing such voluminous volcanism may have traces of its distal ejecta in sedimentary rocks around the basin, including the paleo-Tethys Ocean which was then contiguous with the Pacific Ocean. The contemporaneous marine sequence at central Italy, containing the sedimentary expression of a global oceanic anoxic event (OAE1a), may have recorded such ocurrence as indicated by two stratigraphic intervals with 187Os/188Os indicative of meteoritic influence. Here we show, for the first time, that platinum group element abundances and inter-element ratios in this paleo-Tethyan marine sequence provide no evidence for an extraterrestrial trigger for the Early Cretaceous massive volcanism.