Marine Sedimentary Sections Research Articles

Local melt contamination and global climate impact: Dating the emplacement of Karoo LIP sills into organic-rich shale

The emplacement of Large Igneous Provinces (LIPs) is commonly correlated with global climate change, and environmental and biological crises. To establish this complex causative link, chemical proxies from marine sedimentary sections must be temporally tied to LIP activity through high-precision geochronology. The temporal link is often ambiguous, especially as periods of environmental change are commonly shorter than the full duration of magmatic and volcanic activity in a LIP. Most importantly, temporal relations need to be established through precise and accurate U-Pb geochronology; however, mafic rocks are often undersaturated in zircon or baddeleyite. New U-Pb age determinations of pegmatitic pods from Karoo LIP dolerite sills into the Ecca Group of the Karoo Basin yield single crystal dates with uncertainties below 50 ka, which allow for a new, detailed assessment of this intrusive event. These samples yield complicated age spectra and Hf geochemistry, which indicate that localized assimilation of sedimentary rocks enabled zircon saturation within these mafic melts. Zircon crystallization age interpretations from these data indicate that sill emplacement within the Ecca Group was coeval across the basin within uncertainty of U-Pb dating, and therefore represents a period of particularly high magma flux. Since the Ecca Group is locally exceptionally rich in organic matter, the widespread and rapid sill-related contact metamorphism generated a significant volume of thermogenic volatiles. Given the temporal overlap of the emplacement of sills between 183.147 ± 0.059 Ma and 183.187 ± 0.133 Ma and the timing of the global Toarcian Oceanic Anoxic Event (TOAE), our data support a causal link between this discrete period of high flux emplacement of Karoo sills into the Ecca Group and the global climate change that took place during the Early Jurassic.

Mercury anomalies and carbon isotope excursions in the western Tethyan Csővár section support the link between CAMP volcanism and the end-Triassic extinction

The end-Triassic extinction is one of the major Phanerozoic mass extinctions and it appears to have been linked to coeval rapid and severe environmental change, thought to be triggered by volcanism in the Central Atlantic Magmatic Province (CAMP). However, direct stratigraphic evidence to substantiate this linkage and to help develop scenarios for the cascade of events is still scarce. Mercury is an increasingly widely used proxy to trace the volcanic activity associated with large igneous provinces (LIPs) in distal sedimentary sections, but so far Hg records are available from only a handful of Triassic–Jurassic boundary (TJB) sections. One of the few well-studied marine sedimentary sections with a continuous sedimentary record across the TJB is located at Csővár (Hungary) and it exposes an extended succession of carbonates deposited in an intraplatform basin on the western Tethyan shelf. Previously, this section yielded one of the first convincing records of carbon isotope excursions (CIEs) across the TJB, albeit from low-resolution sampling. Here we report a new, high-resolution δ13Ccarb curve, supplemented with Hg measurements. A series of successive negative carbon isotope excursions (termed NCIE-1 to 6) attests to carbon cycle perturbations in the TJB interval. Four excursions appear significant after statistical smoothing. Of these, NCIE-3 exhibits the highest amplitude and is biostratigraphically constrained to the topmost Triassic, hence reliably correlated with the initial CIE, a globally recognised excursion closely preceding the TJB, and coincident with the end-Triassic extinction (ETE) horizon. The Hg concentration data provide the longest record available to date from a single section across the TJB. It shows very low values below NCIE-3 that are interpreted as the pre-volcanic background, followed by a prominent Hg peak that is nearly coincident with the most significant carbon isotope spike (NCIE-3). The slight lag suggests that onset of a major extrusive phase of CAMP (marked by a significant rise in Hg) closely followed the very onset of carbon cycle perturbation at that time (expressed by an abrupt change in the δ13Ccarb signal), possibly from biogenic methane release. Subsequent and recurring smaller Hg psuggest a pulsatory nature of prolonged volcanic activity. Organic content in the section is consistently low and sedimentary Hg concentrations are therefore normalized against Fe content, a reliable proxy in the lack of significant lithological changes. The maximum sedimentary Hg concentration at Csővár is greater than that in any other TJB section, although not unprecedented if other events are considered. Three hypotheses are explored to explain the high values; i) the hit-or-miss model could suggest that deposition of the sampled beds was fortuitously coincident with major eruptions, ii) the presence and preservation of cryptotephra could account for the unusually high sedimentary Hg enrichment, and iii) changes in the proportion of Hg-carrier phases throughout the studied succession, e.g. from magnetite to pyrite dominance, could have enhanced the potential of Hg capture and deposition. Collectively, the new data provide direct stratigraphic and geochemical evidence for the link between CAMP volcanism and carbon cycle perturbations and strengthen the case of their causal relationship with the end-Triassic extinction.

Tanystropheus and other archosauromorph reptile remains from the Middle and Late Triassic of Villány (Villány Hills, Hungary)

Tanystropheus and other possible archosauromorph fossils have been discovered from Middle to Upper Triassic shallow marine sedimentary sections in Villany (Villany Hills, southern Hungary). Four fragmentary cervical vertebrae can be assigned to Tanystropheus sp. based on characteristic features including the strongly elongate and hollow vertebral body with extremely reduced neural spine. Besides the cervicals, various teeth, classified into four different morphotypes including longitudinally striated, carinated and ziphodont ones, are thought to belong to archosauromorphs, since they markedly differ from the frequently found teeth of fish and sauropterygians. In addition, three enigmatic cranial bones that might represent some circumorbital elements, have been found as well, and are referred to here as Sauropsida indet. These fossils, originated from the same tectonic unit as those from the Anisian of Bihor (Romania), are of great importance for a better understanding of the poorly known semi-aquatic to terrestrial vertebrate fauna of the Middle to Late Triassic of central Europe.

A 30 Ma history of the Amazon River inferred from terrigenous sediments and organic matter on the Ceará Rise

The history of the Amazon River is a much-discussed subject, and the timing of the development of a transcontinental system in particular is a matter of some controversy, with estimations varying between the Early Miocene and the Pliocene or even the Pleistocene. To shed further light on this, we studied the sediment provenance of an Oligocene to Late Pleistocene marine sedimentary section from the Ceará Rise (ODP Site 925), a topographic high in the central Atlantic Ocean, using major element concentrations and Nd isotopic composition in 85 samples. In addition, the carbon isotopic composition of bulk organic matter and changes in the distribution of glycerol dialkyl glycerol tetraethers (GDGTs) were used to identify periods of increased river outflow. On the basis of these results, we suggest that the history of the development of the Amazon River is characterized by specific steps. During the late Oligocene/Early Miocene (30–18.3 Ma), the terrigenous mass accumulation rates (TARs) were high, and sediment and GDGT compositions suggest that a large river system existed, which at times received weathering products from a younger and probably Andean sediment source. A shift to a younger Andean sediment provenance after 8.7 Ma indicates that the Amazon River became permanently connected with the Andes. Between 18.3 and 4.5 Ma, TARs were generally low, and GDGTs were derived for the most part from in situ production in marine waters. Around 4.5 Ma, the river expanded, probably due to ongoing tectonic activity, and uplift in the Andes increased Andean rock erosion. This led to a strong increase in terrigenous sediment deposition and enhanced organic matter preservation on the Ceará Rise, and the delivery of terrestrial (both soil and riverine) branched GDGTs to the Ceará Rise.

Sea level and climatic-induced facies variations in the Middle Cambrian House Range Embayment, western Laurentia

The Middle Cambrian House Range Embayment (HRE) represents a deep-water incursion into an extensive carbonate platform along the passive margin of western Laurentia. The distribution and source of HRE mixed siliciclastic-carbonate sediments are poorly understood because many key elements were missed during past field observations or coarse correlation. This study describes the HRE complex lithological variations, and improves current understanding concerning the global climatic and sea level fluctuations that drove these variations. Magnetic susceptibility (χ), gamma radiation (GR) and carbonate content (CaCO3%) are used to construct high-resolution correlation among three overlapping Middle Cambrian marine sedimentary sections: 1) the Drumian Global boundary Stratotype Section and Point (GSSP) in the Drum Mountains, western Utah, 2) the Marjum Pass section in the House Range, western Utah, and 3) the Packrat section Great Basin National Park in the Snake Range, eastern Nevada. Middle Cambrian HRE lithological variations followed broad facies patterns. Carbonate sediments dominated the HRE edges and decreased inward, while shale, dominated the HRE trough and decreased outward. The carbonate/detrital ratio varied along the area between the HRE Edge and trough due to; 1) sea level fluctuations, (2) the depositional setting at each section, and 3) the allocyclic, climatic variations that acted on the carbonate platform of western Laurentia. Sea level and climatic fluctuations drove temporal facies variations that are well correlated among the three sections, while the depositional setting for each section drove spatial facies variations that account for carbonate and detrital sediment sources and their trajectories into the HRE. The GSSP is a shallow trough section; Marjum Pass is a lower slope section of the northern HRE ramp; and the Packrat is a lower slope section situated along the southern HRE ramp. Sea level and climatic cycles during the Middle Cambrian Drumian Stage, induced long-term and short-term facies variations, respectively, that were superimposed on each other.

Hydro-climate changes over southwestern Arabia and the Horn of Africa during the last glacial–interglacial transition: A pollen record from the Gulf of Aden

We studied a marine sedimentary section deposited at a high sedimentation rate in the Gulf of Aden (core MD92-1002) to document environmental and hydrological changes since the end of the last glacial period and better understand the onset and decay of the Holocene Humid Period (HHP). Pollen analysis reveals that the period from the LGM to 13.5ka was hyper arid with sparse vegetation cover characterized by xerophitic taxa from the Saharo-Sindian phyto-geographical region. Humidity increased since 14.9ka to reach a maximum between ~9 and 7.5ka, as revealed by peak abundances of the tropical mangrove Rhizophora and high values of a pollen-based humidity index. The HHP ended up gradually, starting to decline as soon as 7.5ka and collapsing definitively at 4ka. This decline is characterized by the appearance of Mediterranean taxa and the return of arid conditions, comparable to that of today. Comparison of our pollen record with three speleothem records from the Arabian Peninsula makes it possible to reconstruct a northward and westward shift of the Inter-Tropical Convergence Zone at the onset of the HHP.

Astronomical tuning of the end-Permian extinction and the Early Triassic Epoch of South China and Germany

Abstract The timing of the end-Permian mass extinction and subsequent prolonged recovery during the Early Triassic Epoch can be established from astronomically controlled climate cycles recorded in continuous marine sedimentary sections. Astronomical-cycle tuning of spectral gamma-ray logs from biostratigraphically-constrained cyclic stratigraphy through marine sections at Meishan, Chaohu, Daxiakou and Guandao in South China yields an integrated time scale for the Early Triassic, which is consistent with scaling of magnetostratigraphy from climatic cycles in continental deposits of the Germanic Basin. The main marine mass extinction interval at Meishan is constrained to less than 40% of a 100-kyr (kilo-year) cycle (i.e., δ C 13 is estimated to have lasted δ C 13 from − 2 ‰ to 4‰ across the Smithian–Spathian boundary at Chaohu was completed in 50 kyr. The earliest marine reptiles in the Mesozoic at Chaohu that are considered to represent a significant recovery of marine ecosystems did not appear until 4.7 myr (million years) after the end-Permian extinction. The durations of the Griesbachian, Dienerian, Smithian and Spathian substages, including the uncertainty in placement of widely used conodont biostratigraphic datums for their boundaries, are 1.4 ± 0.1 , 0.6 ± 0.1 , 1.7 ± 0.1 and 1.4 ± 0.1 myr , implying a total span for the Early Triassic of 5.1 ± 0.1 myr . Therefore, relative to an assigned 251.902 ± 0.024 Ma for the Permian–Triassic boundary from the Meishan GSSP, the ages for these substage boundaries are 250.5 ± 0.1 Ma for base Dienerian, 249.9 ± 0.1 Ma for base Smithian (base of Olenekian stage), 248.2 ± 0.1 Ma for base Spathian, and 246.8 ± 0.1 Ma for the base of the Anisian Stage. This astronomical-calibrated timescale provides rates for the recurrent carbon isotope excursions and for trends in sedimentation accumulation through the Early Triassic of studied sections in South China.

Integrated stratigraphy and palaeoenvironment of the P/E boundary interval, Rakhi Nala section, Indus Basin (Pakistan)

Marine sedimentary section across the Paleocene/Eocene (P/E) boundary interval is preserved in the Dungan Formation (Lower Indus Basin), Pakistan. Four dinoflagellate zones in the P/E interval of the Rakhi Nala section (Lower Indus Basin) are identified and correlated. The quantitative analysis of the dinoflagellate cyst assemblages together with geochemical data are used to reconstruct the palaeoenvironment across the P/E interval. The dinocyst assemblages allow the local correlation of the Dungan Formation (part) of the Sulaiman Range with the Patala Formation (part) of the Upper Indus Basin and global correlation of the Zone Pak-DV with the Apectodinium acme Zone of the Northern and Southern hemispheres. The onset of the carbon isotopic excursion (CIE) associated with Paleocene Eocene Thermal Maximum (PETM) is used globally to identify the P/E boundary. The CIE for the total organic carbon (fine fraction) δ13CFF is of a magnitude of −1.7‰ is recorded for the first time in the Indus Basin. The Apectodinium acme precedes and straddles the onset of the CIE in the Indus Basin. This Apectodinium acme is also accompanied by a planktonic and benthonic foraminifera “barren zone.” The CIE in the Indus Basin, coupled with the changes in the dinocyst distribution and the benthonic and planktonic foraminifera assemblages, provides evidence of the changes associated with the PETM in this little-known part of the world. The benthonic foraminiferal assemblage indicates bathyal environment of deposition at the time of P/E boundary interval; the presence of dominantly open marine dinoflagellates and high planktonic foraminiferal ratio suggest that the water column at this site was well connected with the rest of the Tethys.

Ca isotopes in carbonate sediment and pore fluid from ODP Site 807A: The Ca 2+(aq)–calcite equilibrium fractionation factor and calcite recrystallization rates in Pleistocene sediments

The calcium isotopic compositions (δ 44Ca) of 30 high-purity nannofossil ooze and chalk and 7 pore fluid samples from ODP Site 807A (Ontong Java Plateau) are used in conjunction with numerical models to determine the equilibrium calcium isotope fractionation factor ( α s−f) between calcite and dissolved Ca 2+ and the rates of post-depositional recrystallization in deep sea carbonate ooze. The value of α s−f at equilibrium in the marine sedimentary section is 1.0000 ± 0.0001, which is significantly different from the value (0.9987 ± 0.0002) found in laboratory experiments of calcite precipitation and in the formation of biogenic calcite in the surface ocean. We hypothesize that this fractionation factor is relevant to calcite precipitation in any system at equilibrium and that this equilibrium fractionation factor has implications for the mechanisms responsible for Ca isotope fractionation during calcite precipitation. We describe a steady state model that offers a unified framework for explaining Ca isotope fractionation across the observed precipitation rate range of ∼14 orders of magnitude. The model attributes Ca isotope fractionation to the relative balance between the attachment and detachment fluxes at the calcite crystal surface. This model represents our hypothesis for the mechanism responsible for isotope fractionation during calcite precipitation. The Ca isotope data provide evidence that the bulk rate of calcite recrystallization in freshly-deposited carbonate ooze is 30–40%/Myr, and decreases with age to about 2%/Myr in 2–3 million year old sediment. The recrystallization rates determined from Ca isotopes for Pleistocene sediments are higher than those previously inferred from pore fluid Sr concentration and are consistent with rates derived for Late Pleistocene siliciclastic sediments using uranium isotopes. Combining our results for the equilibrium fractionation factor and recrystallization rates, we evaluate the effect of diagenesis on the Ca isotopic composition of marine carbonates at Site 807A. Since calcite precipitation rates in the sedimentary column are many orders of magnitude slower than laboratory experiments and the pore fluids are only slightly oversaturated with respect to calcite, the isotopic composition of diagenetic calcite is likely to reflect equilibrium precipitation. Accordingly, diagenesis produces a maximum shift in δ 44Ca of +0.15‰ for Site 807A sediments but will have a larger impact where sedimentation rates are low, seawater circulates through the sediment pile, or there are prolonged depositional hiatuses.

Palaeomagnetic study of Lower Jurassic marine strata from the Neuquén Basin, Argentina: A new Jurassic apparent polar wander path for South America

A thorough palaeomagnetic study in four marine sedimentary sections from the Neuquén Basin in west-central Argentina was carried out. Sections are several hundreds meters-thick and consist of ammonite-bearing beds with intercalated volcanics. Two magnetic components carried by titanomagnetites were identified in the studied sections, one soft corresponding most likely to a present-day remagnetisation and a harder one which is interpreted as the primary magnetisation of the sections, based on optical studies and the results of field tests of palaeomagnetic stability, bearing an Early Jurassic age. As a result, two new palaeomagnetic poles for stable South America are presented: one for the Hettangian–Sinemurian located at 223° E, 51° S, A 95 = 6°, N = 25, and the other for the Pliensbachian–Toarcian located at 67° E, 74° S, A 95 = 5°, N = 52. These and other poles from the literature were used in this study to construct a refined Late Triassic to Jurassic APW path for stable South America, which differs significantly from previous models in showing a cusp between the Sinemurian and the Pliensbachian, indicative of large apparent polar wander. The same feature is observed in other continents of Pangea, like Eurasia. Palaeolatitudes of the Neuquén Basin indicate that South America was subject to considerable N–S movements during the Late Triassic and lowermost Early Jurassic. These latitudinal movements of Pangea are consistent with displacements recorded for marine faunas from South America and Eurasia.

EPR stratigraphy applied to the study of two marine sedimentary sections in southwestern Venezuela

This work explores the application of the electron paramagnetic resonance (EPR) technique, a high sensitivity spectral method, as a means of characterizing stratigraphic facies and identifying depositional environments. We have studied two marine sedimentary sections, of mid-Cretaceous and mid-Eocene ages, that are located in southwestern Venezuela. The different paramagnetic species (e.g. manganese, free radicals and different forms of Fe) have been identified and their relative concentrations measured from the EPR spectra. These results were then integrated to those from petrographic and magnetic susceptibility analyses (room and high temperature) as well as to some other rock magnetic data such as Q n and S-ratios. Because Mn 2+ content seems to be finely tuned to variations of redox conditions, it could serve as a sensitive index of paleoenvironmental changes in stratigraphic columns with large lithological contrasts. Conversely, the presence and concentration of distinct Fe species proves to be quite useful characterizing different depositional environments at those stratigraphic sections that exhibit similar lithologies throughout. The integration of EPR results with rock magnetic data allows the identification of diagenetic events that could have taken place after sediment deposition.

Correction to “Trace element abundances in the Rock Canyon Anticline, Pueblo, Colorado, marine sedimentary section and their relationship to Caribbean plateau construction and oxygen anoxic event 2”

[1] In the paper “Trace element abundances in the Rock Canyon Anticline, Pueblo, Colorado, marine sedimentary section and their relationship to Caribbean plateau construction and oxygen anoxic event 2” by L. J. Snow et al. (Paleoceanography, 20, PA3005, doi:10.1029/2004PA001093), the word oxygen in the title should be ocean: “Trace element abundances in the Rock Canyon Anticline, Pueblo, Colorado, marine sedimentary section and their relationship to Caribbean plateau construction and ocean anoxic event 2.”

Trace element abundances in the Rock Canyon Anticline, Pueblo, Colorado, marine sedimentary section and their relationship to Caribbean plateau construction and oxygen anoxic event 2

Rapid eruption of submarine lava flows during formation of the Caribbean plateau correlates closely with ocean anoxic event 2 (OAE2), which bracketed the Cenomanian/Turonian boundary (∼93.5 Ma). These events also correspond with a positive excursion in carbon isotopic composition of seawater. Hydrothermal activity associated with large‐scale submarine volcanism may have been responsible for this abrupt change in ocean chemistry. We determined the distribution of major, minor, and trace element abundances in the Rock Canyon marine sedimentary section (Pueblo, Colorado). After normalizing element concentrations to Zr to remove the variable contribution of terrigenous material to these sediments, we detected an interval of concentrated metal abundance anomalies that coincides with the abrupt beginning of the positive δ13C isotope excursion. The metal abundance anomalies indicate that intermittent hydrothermal activity, in the form of both water/rock exchange and magmatic degassing, introduced large concentrations of trace metals into the Cretaceous ocean at the same time that extinctions of benthic species, turnover in plankton communities, and increases in isotopically light organic carbon burial occurred. The stratigraphic position of this interval of trace metal anomalies matches events associated with OAE2 and indicates that intermittent hydrothermal activity on a massive scale triggered abrupt changes in carbon burial and deep ocean oxygen contents.

Oligocene to Recent tectonic history of the Central Solomon intra-arc basin as determined from marine seismic reflection data and compilation of onland geology

Systematic analysis of a grid of 3450 km of multichannel seismic reflection lines from the Solomon Islands constrains the late Tertiary sedimentary and tectonic history of the Solomon Island arc and its convergent interaction with the Cretaceous Ontong Java oceanic plateau (OJP). The OJP, the largest oceanic plateau on Earth, subducted beneath the northern edge of the Solomon arc in the late Neogene, but the timing and consequences of this obliquely convergent event and its role in the subduction polarity reversal process remain poorly constrained. The Central Solomon intra-arc basin (CSB), which developed in Oligocene to Recent time above the Solomon arc, provides a valuable record of the tectonic environment prior to and accompanying the OJP convergent event and the subsequent arc polarity reversal. Recognition of regionally extensive stratigraphic sequences—whose ages can be inferred from marine sedimentary sections exposed onland in the Solomon Islands—indicate four distinct tectonic phases affecting the Solomon Island arc. Phase 1: Late Oligocene–Late Miocene rifting of the northeast-facing Solomon Island arc produced basal, normal-fault-controlled, asymmetrical sequences of the CSB; the proto-North Solomon trench was probably much closer to the CSB and is inferred to coincide with the trace of the present-day Kia-Kaipito-Korigole (KKK) fault zone; this protracted period of intra-arc extension shows no evidence for interruption by an early Miocene period of convergent “soft docking” of the Ontong Java Plateau as proposed by previous workers. Phase 2: Late Miocene–Pliocene oblique convergence of the Ontong Java Plateau at the proto-North Solomon trench (KKK fault zone) and folding of the CSB and formation of the Malaita accretionary prism (MAP); the highly oblique and diachronous convergence between the Ontong Java plateau and the Solomon arc terminates intra-arc extension first in the southeast (Russell subbasin of the CSB) during the Late Miocene and later during the Pliocene in the northwest (Shortland subbasin of the CSB); folds in the CSB form by inversion of normal faults formed during Phase 1; Phinney et al. [Sequence stratigraphy, structural style, and age of deformation of the Malaita accretionary prism (Solomon arc-Ontong Java Plateau convergent zone)] show a coeval pattern of southeast to northwest younging in folding and faulting of the MAP. Phase 3: Late Pliocene–early Pleistocene arc polarity reversal and subduction initiation at the San Cristobal trench. Effects of this event in the CSB include the formation of a chain of volcanoes above the subducting Australia plate at the San Cristobal trench, the formation of the broad synclinal structure of the CSB with evidence for truncation at the uplifted flanks, and widespread occurrence of slides and “seismites” (deposits formed by seismic shaking). Phase 4: Pleistocene to Recent continued shortening and synclinal subsidence of the CSB. Continued Australia-Pacific oblique plate convergence has led to deepening of the submarine, elongate basin axis of the synclinal CSB and uplift of the dual chain of the islands on its flanks.

The Neuquén composite section: magnetostratigraphy and biostratigraphy of the marine lower Jurassic from the Neuquén basin (Argentina)

Paleomagnetic and biostratigraphic data from five marine sedimentary sections in the Argentine Neuquén basin have allowed to construct the Lower Jurassic Neuquén Composite Section. This composite section comprises 14 reversed and 11 normal polarity Zones, in relation to 17 Andean ammonite Assemblage Zones, spanning the Hettangian–Toarcian (Early Jurassic). It represents the first paleomagnetic data of Lower Jurassic marine successions in the Southern Hemisphere. The Neuquén composite Section was correlated to the International Mesozoic Polarity Time Scale which, for the Lower Jurassic, comprises 54 polarity Zones and 16 Standard ammonite Zones. The correlation between the regional and the international biomagnetostratigraphic scales supports but also refines the correlation between the Andean and Standard ammonite zonations. Correlation between the Neuquén composite Section and the polarity sequences recorded in each section helped to assign several unfossiliferous stratigraphical levels to the corresponding biozones.

Constraints on the thermal and tectonic evolution of Greymouth coalfield

The southern end of the Paparoa Range in Westland, South Island, New Zealand, comprises an asymmetrical, southward plunging, faulted (Brunner‐Mt Davy) anticline, the eastern limb of which is common with the western limb of an asymmetrical (Grey Valley) syncline forming a Neogene foreland basin (Grey Valley Trough). The faulted anticline is a classic inversion structure: compression during the Neogene, associated with the development of the modern Australia‐Pacific plate boundary, caused a pre‐existing normal fault zone, about which a late Cretaceous‐Oligocene extensional half graben had formed (Paparoa Trough), to change its sense of displacement. The resulting basement loading formed the foreland basin, containing up to 3 km of mainly marine sedimentary section. Fission track results for apatite concentrates from 41 shallow drillhole and outcrop samples from the Greymouth Coalfield part of the Brunner‐Mt Davy Anticline are reported and interpreted, to better establish the timing and amount of inversion, and hence the mechanism of inversion. The fission track results integrated with modelling of vitrinite reflectance data, show that the maximum paleotemperatures experienced during burial of the Late Cretaceous and mid‐Eocene coal‐bearing succession everywhere exceeded 85°C, and reached a peak of 180°C along the axis of the former basin. Cooling from maximum temperatures occurred during three discrete phases: 20–15 Ma, 12–7 Ma, and c. 2 Ma to the present. The amount of denudation has been variable across the inverted basin, decreasing westward from a maximum of c. 2.5 km during the first deformation phase, c. 1.2 km during the second phase, and 1.4 km during the third phase. It appears that exhumation over the coalfield continued for about 2 m.y. beyond the biostratigraphically determined time ranges of each of two synorogenic unconformities along the western limb of the Grey Valley Syncline. Stick‐slip behaviour on the range front fault that localised the inversion is inferred. The tectonic evolution of the anticline‐syncline pair at the southern end of the Paparoa Range, is therefore identical in style, and similar in timing, to the development of the Papahaua Range‐Westport Trough across the Kongahu Fault Zone, in the vicinity of Buller Coalfield.

Miocene calibration for calcareous nannofossils from low-latitude Ocean Drilling Program sites and the Jamaican conundrum

Prior work with marine sedimentary sections exposed on Jamaica had been interpreted as evidence for a curious pattern of diachrony between the tropics and subtropics in the ages of certain biostratigraphic zonal boundaries of middle to late Miocene age. We offer an alternative to the Jamaican low-latitude nannofossil calibration, using a combination of paleomagnetic, biostratigraphic, and lithostratigraphic data derived from tropical sections recovered by the Ocean Drilling Program (ODP) in the eastern Pacific and western Atlantic Oceans. Our nannofossil calibration is based on magnetic polarity stratigraphy obtained from the Pacific Ocean and is consistent with the notion that Milankovitch periodicities governed cyclic changes in the lithology of sediments from the Ceara Rise, a sea-floor high in the Atlantic Ocean located offshore of the Amazon delta. Our results thus appear to be representative of the tropics and so call into question the ages that had been assigned to nannofossil zonal boundaries based on results from Jamaican sections.

Forearc response to subduction of the Cocos Ridge, Panama-Costa Rica

Stratigraphic, paleontologic, and structural data from two Panama-Costa Rica forearc peninsulas located landward of the aseismic Cocos Ridge along the southernmost Middle America Trench document rapid Pliocene subsidence and basin infilling followed by Quaternary deformation and uplift. On the Burica Peninsula, an approximately 3,000-m-thick, Pliocene-Pleistocene clastic sedimentary sequence, characterized by fine-grained turbidite deposits and volumetrically minor coarser-grained channel-fill, slump, and debris-flow deposits, bears depth-diagnostic foraminifera that document rapid Pliocene subsidence and deposition on a south-dipping paleoslope. New paleobathymetric and age estimates from foraminifera in these rocks indicate shallowing of depositional paleodepths from 2,000+ to 1,200 m during the late Pliocene. Present-day exposure of these rocks indicates an average Quaternary uplift rate of ∼1 mm/yr. The turbidite section on the Burica Peninsula is interpreted to represent a trench-slope deposit and is correlated to a similar, marine sedimentary section on the adjacent Osa Peninsula of Costa Rica. Structures developed within Pliocene-Pleistocene strata of the outer-forearc Burica and Osa Peninsulas record minor (<5%) subhorizontal, margin-perpendicular shortening and margin-perpendicular extension. Bedding orientations on the Osa and Burica Peninsulas generally record arcward tilting by 10°-30° of these outer-forearc blocks. On the Burica Peninsula, the Medial fault a north-striking, high-angle, right-lateral strike-slip (?) fault, has truncated and reoriented the Pliocene-Pleistocene sedimentary sequence by fault-bend (forced) folding. A linear, margin-parallel fault, herein named the Ballena-Celmira fault zone, bounds the arcward side of the Osa and Burica Peninsulas and has accommodated vertical displacements between the outer forearc (Burica-Osa area) and inner forearc (Terraba area) throughout the Tertiary. Structural and seismicity observations from the forearc region suggest that Neogene deformation of the leading edge of the arc in this area reflects the effects of isostatic uplift, and possibly increased interplate shear stresses, as a result of subduction of the Cocos Ridge beneath this part of the arc since 1 m.y. ago.

Calcareous Nannofossil Biostratigraphy of Chititu Formation, Wrangellia Terrane, Alaska: ABSTRACT

The tectonostratigraphic terrane (suspect terrane) concept has been a major catalyst in attempts to unravel the complex tectonic history of Alaska. Paleomagnetic work has suggested that the Wrangelia terrane in south-central Alaska had a paleolatitude of 32/degree/N /plus minus/ 9/degree/ in the Late Cretaceous, but was accreted to North America prior to the Eocene. A plate velocity of at least 24 cm/yr is implied by this latitude and time framework. If confirmed, this velocity will be one of the fastest plate velocities yet established. Calcareous nannofossils from the Chititu Formation, Wrangelia terrane, have been investigated in order to provide a refined biostratigraphic framework for establishing the age of paleolatitudes determined for the Chititu. Thirty-one species of nannofossils were identified in a section of the Chititu measured on MacColl Ridge. These species allow assignment of the section to the internationally recognized Reinhardties anthophorus (CC 15: late early to late Santonian) and Lucianorhabdus cayeuxii (CC 16: late santonian to earliest Campanian) biozones. Calcareous nannofossils clearly have potential for use in age determinations in Wrangellia and other Alaskan tectonostratigraphic terranes with suitable marine sedimentary sections.

地磁気層序と放射年代―地磁気編年にともなう問題点

This paper briefly reviews the history and present status of magnetostratigraphy, and re-examines the usefulness of magnetostratigraphy in absolute chronology. At present, two different kinds of time scale are used as standards for magnetostratigraphic age determination. The first is that time scale developed from a combination of paleomagnetic polarity data and radiometric age dates derived from volcanic rock sequences on land. The second is based on the interrelationships between the sequence of geomagnetic reversals and marine planktonic microfossils. Usually, these time scales are assumed to be equivalent.Based on the collection of radiometric dates made on magneto- and biostratigraphically-studied sedimentary sections, the relationships between the geomagnetic-radiometric and geomagnetic-microbiostratigraphic time scales were examined. A distinct discrepancy was noticed between the radiometric age and the estimated absolute age from the magnetic-microbiostratigraphic scale, as indicated in Fig. 5. This discrepancy is worthy of serious consideration by uses of the geomagnetic time scale in absolute chronology. Further examination of various methods of measurement is needed to increase the reliability of magnetostratigraphy, microbiostratigraphy and radiometric chronology.In Japanese marine sedimentary sections, there exist many pyroclastic intercalations that may provide suitable materials for such examinations.