Pelagic Sequences Research Articles

Oligocene planktonic foraminiferal biostratigraphy and paleoclimatic interpretation from Hole 538A, DSDP Leg 77, Gulf of Mexico

A relatively extended Oligocene pelagic sequence with good to medium recovery, drilled during DSDP Leg 77 in the Gulf of Mexico, yielded rich and well diversified planktonic foraminiferal faunas. Planktonic foraminifera recorded in Hole 538A span the interval from Zone P19 through P22. Evolutionary lineages were observed among the globoquadrinids, the globigernitids, and the “ Globigerina” ciperoensis and Globigerinoides primordius groups. Quantitative analysis of planktonic foraminiferal assemblages shows that faunas fluctuate in abundance and species diversity throughout the sequence. A few of these fluctuations that could be related to selective dissolution are mainly confined to the early-mid Oligocene. A climatic curve was constructed using a warmer indicators, Turborotalia pseudoampliapertura, Globoquadrina tripartita, Dentoglobigerina globularis, Dentoglobigerina baroemoenensis, “Globigerina” ciperoensis and Globigerinoides groups, and Cassigerinella chipolensis; and as cooler indicators, Catapsydrax spp., Globorotaloides spp., Subbotina angiporoides group, Globigerina s. str., and the tenuitellids. Three major intervals are identifiable in the climatic curve: Interval 1 (lower) up to Zone P20 predominantly cooler; Interval 2 (intermediate) up to the upper part of Zone P21a with warm and cool fluctuations; and Interval 3 (upper), warmer, with large positive peak, due to abundant “ G” angulisuturalis, at the beginning of Zone P21b with recooling midway in Zone P22. In intervals 1 and 2 planktonic foraminiferal faunas are dominated by temperate forms. Interpretation of planktonic foraminiferal data suggests that cooler water conditions characterize the early-mid Oligocene; during the mid Oligocene (most of Zone P21a) water masses exhibit peculiar characteristics transitional to the warmer waters prevailing during the late Oligocene. Warmer conditions were not definitely settled in Zone P22, however, as indicated by the cooler episode following the warmest peak. These climatic trends are inconsistent with those inferred from oxygen isotopes except at small scale. In fact, oxygen isotope values for Oligocene Atlantic Ocean are too heavy (thus too cool) in comparison with the high abundance and diversity of warm taxa, especially in Zone P22. When values are lighter (warmer), as in Zone P19, abundance and diversity of warm indices are too low. To explain such a cool isotope values in presence of highly diversified and abundant warm planktonic foraminifera, we suggest (1) that the oxygen isotope ratio used for estimating Oligocene paleotemperatures might be 1/%. heavier than Eocene values and further increased for the late Oligocene. This hypothesis implies the presence of a relatively extended ice cap in Antarctica in the early and mid Oligocene, and probably an increase in ice volume during the late Oligocene; (2) heavier isotope values might be related to an increase in salinity, or (3) by a combination of both ice cap and increase in salinity.

Depositional history of the Sulu Sea from ODP Sites 768, 769 AND 771

Holes were drilled at three Sites in the Sulu Sea on Ocean Drilling Program Leg 124. Site 768 lies in the deeper part of the SE sub‐basin and Sites 769 and 771 lie on the flanks of the Cagayan ridge. The results indicate that the Sulu Basin originated in the late early Miocene (c.18.8 Ma) in a backarc setting. The Cagayan Ridge was a site of early to early middle Miocene arc volcanism with the deposition of a thick sequence of andesitic to basaltic volcaniclastic deposits. In the basin center an early Miocene pelagic sequence is interrupted by a thick unit of rhyolitic to dacitic pyroclastic flows. Middle to late Miocene sedimentation is more continental in character with thick quartz‐rich turbidites in the basin center. Only the hemipelagic claystone related to these terrigenous turbidites were deposited on the Cagayan Ridge. A decrease in the supply of clastic detritus from arc and continental sources and a change in the level of the carbonate compensation depth in the upper Pliocene resulted in pelagic carbonate deposition throughout the late Pliocene and Pleistocene.

Seismic stratigraphy of the Raggatt Basin, southern Kerguelen Plateau: Tectonic and paleoceanographic implications

During Cenozoic and late Mesozoic time, sediment accumulated in the Raggatt Basin on the southern Kerguelen Plateau. We describe the seismic stratigraphy of the Raggatt Basin, utilizing multichannel seismic (MCS) data obtained by the Bureau of Mineral Resources, Geology and Geophysics (Australia) and the Institut de Physique du Globe de Strasbourg (France). Seven major seismic stratigraphic sequences in the Raggatt Basin overlie a basement complex of Early Cretaceous age. The underlying basement complex is characterized by two types of seismic images: acoustic basement and a layered basement which includes dipping reflectors. The seismic stratigraphic sequences include terrestrial and shallow-water units K1 and K2 of Early to Late Cretaceous age, which fill depressions in the basement; a thick unit, K3, of Late Cretaceous to Paleocene age, which is of mixed shallow-water and open-marine facies and in places has a mounded (carbonate) upper surface; a depression-filling pelagic deposit, P1, of Late Cretaceous to Eocene age; a thick pelagic unit, P2, which is mainly Eocene in age; and two post-Eocene pelagic sequences, PN1 and NQ1, which are relatively thin and more limited in areal extent than the underlying sequences. Highlights in the geologic history of the Raggatt Basin include formation and erosion of the basement complex in a subaerial or shallow-water environment in Early Cretaceous time; differential subsidence, probably thermal in origin, and development of carbonate mounds and a major western boundary transform fault in Late Cretaceous time; renewed subsidence near the Cretaceous-Tertiary boundary possibly related to formation of the Labuan Basin/Diamantina Zone by sea-floor spreading; and vigorous Antarctic Circumpolar Current activity beginning by Oligocene time, probably related to the breakup of the northern Kerguelen Plateau and Broken Ridge.

Quantitative estimate of resedimentation in the pelagic sequence of the equatorial Pacific

A study of deep-sea sedimentation processes was undertaken in the east-central equatorial Pacific Ocean near DSDP Site 574, using lithologic information from the site and a grid of 250 km of high-resolution seismic reflection profiles. These data are used to quantitatively evaluate pelagic drape and basin-filling processes in a pelagic carbonate environment. Sediment accumulation influenced by the relief of the depositional surface, seafloor currents, and sediment supply was examined in eight seismic-stratigraphic intervals. As expected, the thinnest sediment sections occur over topographic highs and areas of steep dips, while the thickest portions occur in lows and over gradual slopes. Syn-depositional sediment redistribution varies between about 4 and 17% of the volume of the defined intervals.

Geochemical and paleoenvironmental variations across the Cretaceous/Tertiary boundary at Braggs, Alabama

The Cretaceous/Tertiary (K/T) boundary in southern Alabama occurs in a sequence of interbedded shallow-marine limestones and marls deposited during a Late Maastrichtian regression and subsequent Danian transgression. The presence of a diverse assemblage of Cretaceous and Paleocene benthic micro- and macrofossils has allowed detailed examination of paleoenvironmental changes in this shallow-marine setting at the time of the K/T boundary extinctions. Although extensive diagenesis has resulted in the recrystallization and cementation of whole rock carbonate, a few molluscan macrofossils have retained their original calcitic structure. The oxygen isotopic record of these macrofossils shows a gradual cooling of 2–3°C over a 3 m.y. period which began in the late Maastrichtian and continued into the Danian. The characteristic depletion in δ 13C across the K/T boundary displayed by planktonic microfossils from pelagic sequences is not recorded at Braggs. Major changes in whole rock trace and minor element chemistry reflect the decrease in terrigenous mineral fluxes to this location brought about by the rapid Early Paleocene transgression.

The detection and illustration of regular sedimentary cycles using Walsh power spectra and filtering, with examples from the Lias of Switzerland

Power-spectral analysis provides an objective method for the detection of regular cyclicity in data recording some oscillating parameter (or time series). Digitization of measured stratigraphic sections is a convenient method for generating long time series, during the examination of pelagic sequences for cyclicity related to orbital variations or Milankovitch cycles. The application of Walsh power-spectral analysis to stratigraphic sections is outlined and band-pass filtering using the Walsh transform introduced. It is shown that variations in sedimentation rate, hiatuses, bioturbation and variations in compaction have the effect of damping or destroying spectral peaks. The wavelength ratios of spectral peaks are often used, during studies of Milankovitch cycles, to decide which orbital cycles are involved. However, in some circumstances hiatuses are able to alter wavelength ratios. Averaged Walsh power spectra were generated for three Liassic limestone/shale sequences from Breggia Gorge, Switzerland. The 21 000 year (21 ka) precession cycle was detected in all three cases, the 100 ka eccentricity cycle in two cases and the 41 ka obliquity cycle in one. Filtering revealed that the supposed 21 ka sedimentary cycles are grouped into packets of four or five small and large amplitude cycles. The packets reflect the 100 ka modulation of the 21 ka precession cycle. Local disruption of the packaging can be used to locate hiatuses in pelagic strata.

Eocene and early oligocene micropaleontology and paleoenvironments in SE Umbria, Italy

A detailed study, based on calcareous nannofossils and planktonic and benthic Foraminifera, has been carried out on five sections in the “Scaglia” Group of the Umbrian region (Visso and Cascia areas, Central Italy). These sections are characterized by continuous pelagic sedimentation and are constituted mainly of marls, marly limestones and limestones of Early Eocene to Early Oligocene age. Calcareous turbidites, with shallow-water skeletal debris, discharged from the Latium-Abruzzi carbonate platform, are interbedded within the pelagic sequences. The biostratigraphic study, based on calcareous nannofossils and planktonic Foraminifera, confirmed the continuous succession of standard events and revealed some new ones. These results allow us to refine the biostratigraphic resolution, with a very good correlation between calcareous nannofossil and planktonic foraminiferal zonations. The major features and events recognized are: • The Early-Middle Eocene boundary (Zones P9–P10) is characterized by the first occurrence (FO) of hantkeninids, by abundant thick-walled Globigerinatheka (G. senni) and by spinose acarininids. A decrease has been noted in planoconvex morozovellids and an increase of low-spired subbotinids. • The benthic assemblage shows a great variability, with an increase in the Zone P11 of the large Heterolepa and Cibicidoides. This is an event of worldwide significance. • In the calcareous nannofossils, the FO of Nannotetrina sp. was used to define the lower boundary of Zone CP13 instead of N. fulgens, because poor preservation prevented identification at the specific level. • The FO of Reticulofenestra umbilica, marking the lower boundary of Zone CP14, occurs in the upper part of Zone P11. • The Zone P13 was recognized indirectly because of the absence of the index species Orbulinoides beckmanni. The base of the zone is close to the FO of Turborotalia cerroazulensis, and is defined by an increase of Catapsydrax sp. and by the last occurrence (LO) of Acarinina bullbrooki. The top of the zone is defined by an increase of the high-spired subbotinids of the S. corpulenta group and the disappearance of Morozovella spinulosa • The Middle/Late Eocene boundary (Zones P14/P15) is characterized by the extinction of the spinose acarininids and of the smooth-walled Planorotalites. From Zone P13 to Zone P15 there is a replacement in the benthic assemblages. In fact, Pleurostomella, Heterolepa and Anomalinoides capitata characterize these zones, bolivinids become abundant and uvigerinids appear around the P14/P15 boundary. • The Late Eocene is characterized by an increase of subbotinids with abortive chambers and of the Turborotalia cerroazulensis lineage. Hantkeninids and cribohantkeninids are rare, particularly C. inflata. The FO of Istmolithus recurvus has been noted at the base of Zone P16. Within this zone, Globigerinatheka index and Turborotalia cunialensis coexist. The LO of G. index is associated with a strong decrease of representatives of the genus Globigerinatheka. At the same level of the LO of G. index, the coeval extinctions of Discoaster barbadiensis and Discoaster saipanensis are noted. Just above this level, within Zone P17, the LO of T. cerroazulensis lineage ( T. cocoaensis and T. cunialensis) has been recognized. The Eocene/Oligocene boundary is defined by planktonic Foraminifera at the extinction level of hantkeninids and cribrohantkeninids, shortly following that of the T. cerroazulensis group.

Permian age from radiolarites of the Hawasina nappes, Oman Mountains

The Hawasina napper of the Oman Mountains yielded Permian radiolarians from cherts stratigraphically overlying a thick volcanic basement (Al Jil Formation) at the base of the Hamrat Duru Group. This fauna represents the first Permian radiolarians and radiolarites in the central and western Tethyan realm. A Permain age for pelagic sequences within the Hawasina Complex of Oman has major significance for regional paleogeographic reconstruction. A clear differentiation between platform (reefal sediments) and basin (radiolarites) from the base of the Late Permian (255 Ma) is implied. It suggests a flexure of the platform during Permian time; the present data implies that a zone of rifting was already developed adjacent to the northeast Gondwana platform margin during the Late Permian. The Hamrat Duru Basin corresponds to an opening intracontinental rift area (sphenochasm) between Arabia and northeast Gondwana, a reentrant of the paleo-Tethys.

Metabentonite geochemistry: magmatic cycles and graptolite extinctions at Dob's Linn, southern Scotland

ABSTRACTThe Moffat Shale Group is a condensed, variable and partly pelagic sequence of mudrocks of Llandeilo—Llandovery age. The sequence has a five-fold lithological subdivision based mainly on the occurrence of grey mudstones within a succession otherwise dominated by fully euxinic black graptolitic mudrocks. Associated with the black mudrocks, especially in the Llandovery, are metabentonite beds which achieve a climax, both in thickness and in number, within the top quarter of the mudrock sequence. A geochemical and mineralogical study has confirmed a volcanic origin for the metabentonites. Major element data highlight a carbonate-dominated environment above the gregarius—convolutus Zones boundary. Phosphorus levels reach a peak at the same boundary, as well as at the Caradoc—Ashgill boundary where phosphorite horizons are known from Wales and Norway. Immobile trace elements have highlighted regular changes in source magma composition. Prolonged periods of crystal fractionation in magmas of intermediate composition gave rise, on eruption, to large volumes of silicic ash which had a deleterious effect on graptolite species and led to local extinctions. Regular fluctuations in ash composition from silicic to intermediate are ascribed to alternating fractionation and magma mixing cycles.

Beryllium-10 data from redeposited late miocene pelagic sediments (East Java, Indonesia)

10Be measurements of samples from a (hemi)pelagic sequence of Middle Miocene to Lower Pliocene age (Java, Indonesia) allow estimating the rate of sedimentation and the timespan during which deposition occurred. Large variations of 10Be values between adjacent samples can be satisfactorily ascribed to short term variations of the rate of sedimentation, with in addition downslope mass transport of sedimentary units that had previously been deposited higher on the submarine slope.

Thickness distribution of reddish brown clay in the western North Pacific

Thickness and stratigraphy of reddish brown clay were compiled from lithologic descriptions of various types of cored surface sediments in the western North Pacific. Thin brown clays usually overlie gray clay in hemipelagic depositional environments and the thickness of the brown clay increases offshore, as results of the decreasing rate of sedimentation of terrigenous organic matter. The hemipelagic gray clay disappears in pelagic environments and several tens of meters of pelagic red clay overlies the basement sedimentary sequences of bedded chert and chalk in the pelagic seafloor. In the boundary region of the hemipelagic and pelagic sequences at the outer open floor of the trench basin, the pelagic facies of the red clay-bedded chert-chalk assemblage underlie the hemipelagic gray clay. This stratigraphy might indicate subduction of pelagic facies below the hemipelagic sequences.

Rare-earth elements abundance and distribution in pelagic sediments by instrumental neutron activation analysis

The geochemical behavior of REE has been tested in the Umbro-Marchean (Italy) pelagic sequences of Cretaceous-Paleocene age. REE were determined by INAA in a number of limestone, marl and clay samples. Both chondrite and shale normalized patterns are discussed: the observed REE amounts and distributions are mainly attributed to highly complex diagenetic processes.

Sedimentation, Metallogenesis, and Tectonic Origin of the Basal Galice Formation Overlying the Josephine Ophiolite, Northwestern California

Basal sediments overlying the Late Jurassic Josephine ophiolite consist of interbedded siliceous argillite, radiolarian chert, and rare graywacke overlain by a thick flysch sequence; together the pelagic rocks and flysch comprise the Galice Formation in northwestern California. The transition between pelagic rocks and overlying flysch is comprised of radiolarian argillites containing abundant silt-size detritus. Deposition of many of the argillites and cherts by turbidity currents is indicated by sedimentary structures such as graded bedding. Some of the cherts are tuffaceous. Metalliferous rocks (up to 22% and 6% MnO) occur in some sections and have geochemical characteristics indicative of a substantial hydrothermal component formed by precipitation from heated seawater circulated through the underlying ophiolite. These rocks are found approximately 8-21 m above the base of the sequence, suggesting an origin from off-axis hydrothermal vents. A massive sequence of metalliferous sediments in one of the sections studied is probably a fossil mound. The presence of graywackes within the pelagic sequence, the local occurrence of an olistostrome rather than pelagic rocks directly overlying the ophiolite, and the common high detrital contents indicate deposition of the basal Galice on a topographic high (spreading ridge) within a sediment-choked basin. Previous work strongly suggests a back-arc basin setting.

Early organic diagenesis: The significance of progressive subsurface oxidation fronts in pelagic sediments

Porewater and solid phase geochemical data at two contrasting NE Atlantic stations are reported. Station 10552, on the Cape Verde abyssal plain, is a site of slow pelagic accumulation ( ca. 0.4 cm kyr −1). Molecular oxygen is present in the sediment column to at least 2 m, and probably much deeper, labile organic-carbon is almost totally consumed in the upper few centimetres of the sediment. By contrast, at station 10554 on the Madeira abyssal plain, the pelagic sequence has been interrupted by the occasional deposition of organic-rich turbidites. Porewater oxygen and nitrate profiles show that subsurface organic metabolism of the organic-carbon associated with the uppermost turbidite layer is a significant fraction of the overall metabolism in the sediment column. This metabolism occurs at a relatively thin reaction front which progresses deeper into the turbidite with time. This phenomenon exerts a controlling influence on the present nutrient profile and redox succession. In a less extreme form, substrate distributions of this latter type are not uncommon in Atlantic sediments. A model has been developed which is controlled by both oxygen and nitrate data. This model permits a vertical profile of metabolic activity to be derived, and also gives estimates of the reaction rate constants and solid phase mixing rates at these two contrasting stations. About 30% of the total activity at station 10554 is located within the turbidite at the deepening reaction front; this is a non-steady-state condition. In fact, it is found that the integrated metabolic activity at the two stations is not dissimilar ( ca. 1–2 × 10 −13 moles cm −2 sec −1 ). The striking differences in redox profile are therefore primarily attributable to differences in the distribution of metabolic activity within the column.

Cretaceous rhythmic bedding sequences: a plausible link between orbital variations and climate

Simple oscillation cycles have been recognized in Cretaceous pelagic sequences as a marked periodicity in carbonate and/or organic carbon content. The estimated periodicities of these cycles resemble Milankovitch-type variations in orbital elements. However, during warm geologic periods the nature of the link between orbital variations and the sedimentary record has been problematic. Climate model studies show that the intensity of continental margin precipitation in specific low latitude regions is a function of land-sea thermal contrast which is sensitive to orbital variations. Cretaceous climate model simulations are characterized by regions of intense precipitation, the locations of which are controlled by geographic configuration. In particular, in the model simulation, the northern margin of the Tethys Ocean has all the climatic characteristics necessary to exhibit a sensitivity to orbital variations. Detailed studies of the biota, sedimentology and geochemistry of the bedding rhythms in Greenhorn Cyclothem in the Western Interior Seaway of North America are best interpreted in terms of variations in precipitation, runoff and stable stratification of the seaway. Combined the geologic and model studies support a link between orbital periodicities, variations of intense precipitation associated with specific paleogeographic situations and Cretaceous bedding patterns. This hypothesis has important implications for the sensitivity of the atmosphere-hydrosphere-lithosphere system to external forcing.

Radiometric time scale for the upper Eocene and Oligocene based on K/Ar and Rb/Sr dating of volcanic biotites from the pelagic sequence of Gubbio, Italy

Several samples of unaltered volcanic biotite were recovered from five biostratigraphically and magnetostratigaphically controlled bentonite horizons in the continuous upper Eocene–Oligocene pelagic carbonate sequence of Gubbio, northern Italy. K/Ar and Rb/Sr dates on these samples offer a unique opportunity to calibrate with closely spaced points the late Paleogene biostratigraphic and magnetostratigraphic time scales. Until now, time scales have been constructed by interpolating between less certain and more widely spaced data points. Our combined K/Ar and Rb/Sr dates are here used to calibrate the following five stratigraphic levels, from top to bottom: Level 5—upper part of Globorotalia opima opima planktonic foraminiferal zone, upper part of indistinct NP23-NP24 nannoplankton zone and CP18 nannoplankton zone, bottom of magnetic anomaly 9N: 28.0 ± 0.3 Ma. Level 4— Globigerina ampliapertura and Cassigerinella chipolensis zone, lower part of NP23 zone and bottom of CP18 zone, upper part of anomaly 12R: 32.0 ± 0.8 Ma. Level 3— Pseudohastigerina micra zone, upper part of NP21 and CP16b zones, top of anomaly 13N: 35.4 ± 0.4 Ma. Level 2—upper part of Globigerinatheka semiinvoluta zone, upper part of NP18 and CP15a zones, top of magnetic anomaly 16N: 36.0 ± 0.4 Ma. Level 1—lower part of G. semiinvoluta zone, between zones NP18 and NP19, middle part of zone CP15a, upper part of anomaly 17N: 36.4 ± 0.3 Ma. From these data, the interpolated age for the much-debated Eocene-Oligocene boundary is 35.7 Ma with a 2-standard-deviation uncertainty of 0.4 Ma.

Late Paleozoic microfaunas from southernmost Chile and their relation to Gondwanaland forearc development

Newly discovered radiolaria and fusulinids of late Carboniferous–earliest Permian age document ages for off-scraped pelagic sequences within the Chilean parts of the Gondwanaland forearc. Well-preserved radiolarians are illustrated and compared with the only other known correlatives from Japan. These findings provide new insight for paleoenvironments of oceanic assemblages within the forearc as well as new constraints for its temporal development.

The Josephine ophiolite, northwestern California

Research Article| September 01, 1984 The Josephine ophiolite, northwestern California GREGORY D. HARPER GREGORY D. HARPER 1Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1984) 95 (9): 1009–1026. https://doi.org/10.1130/0016-7606(1984)95<1009:TJONC>2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation GREGORY D. HARPER; The Josephine ophiolite, northwestern California. GSA Bulletin 1984;; 95 (9): 1009–1026. doi: https://doi.org/10.1130/0016-7606(1984)95<1009:TJONC>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The 157-m.y.-old Josephine ophiolite is a complete ophiolite consisting of the following units: harzburgite tectonite, cumulate ultramafics, cumulate gabbro interlayered with cumulate ultramafics, cumulate gabbro, high-level gabbro and diorite, a 1.5-km-thick sheeted dike complex, and 0.4 km of pillow lava and breccia. The ophiolite is conformably overlain by the “Galice Formation,” consisting of a thin, siliceous pelagic sequence overlain by a thick sequence of turbidites having both volcanic arc and continental-margin sources. The ophiolite and overlying “Galice Formation” were intruded by numerous 150- to 151-m.y.-old calc-alkaline dikes and sills followed shortly by deformation and regional metamorphism under conditions of prehnite-pumpellyite to lower-greenschist facies.The Josephine Peridotite makes up the base of the ophiolite and covers >800 km2. It is an alpine-type peridotite that represents upper-mantle material and the residue of partial melting. Ultramafic rocks overlying the Josephine Peridotite are olivine + spinel ± clinopyroxene cumulates that grade upward into clinopyroxene and orthopyroxene ± olivine ± plagioclase cumulates, and finally to clinopyroxene + plagioclase cumulates. High-level gabbros and diorites are characterized by highly variable textures, complex intrusive relationships, strongly zoned plagioclase, and abundant mafic dikes; plagiogranites occur at this level as dikes, small intrusions, and screens between mafic dikes, but they are rare.The sheeted dike complex consists of mafic dikes averaging 0.6 m in thickness. Chilled margins are locally well preserved, and many of the dikes have been split and intruded by younger dikes, resulting in “half” and “marginless” dikes. The dike complex is composed of 100% sheeted dikes near the center but contains gabbro screens near the base and pillow lava and breccia screens in its upper parts. The dikes are inclined at ∼ 40° to bedding after correcting for folding and are inferred to have been rotated shortly after formation. The extrusive sequence consists predominantly of pillow lavas but contains abundant pillow breccias and some massive lavas. Some of the breccias formed as talus deposits, whereas others are graded and matrix-supported, indicating transport and redeposition, possibly in an area of rugged submarine topography. The dikes and lavas have geochemical affinities to island-arc tholeiites but are in some aspects transitional to mid-ocean–ridge basalts.The dikes and lavas are cogenetic with the underlying plutonic sequence, as indicated by similarity in crystallization sequence, contradictory intrusive relationships between sheeted dikes and high-level plutonic rocks, and lack of diabase dikes in the lowermost cumulates and harzburgite. The upper portions of the ophiolite were hydrothermally metamorphosed at the spreading center, resulting in downward changes in mineralogy, texture, and δ18O; in addition, metalliferous sediments and massive sulfides that occur overlying and within the pillow lavas probably were formed by precipitation from thermal springs. A geothermal gradient of 250–350 °C/km is inferred from mineral assemblages.On the basis primarily of its regional setting, the ophiolite is inferred to have formed in a back-arc basin; in particular, it is within a thrust sheet sandwiched between a coeval magmatic arc complex and a slightly older arc complex (remnant arc). This interpretation is consistent with abundant volcanic-arc detritus in graywackes overlying the ophiolite. The island-arc magmatic affinities of the lavas and dikes may be the result of formation of the ophiolite during the earliest stages of back-arc spreading. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Lithology, field relationships, and structure of the Pounamu Ultramafics from the Whitcombe and Hokitika Rivers, Westland, New Zealand

Abstract The Pounamu Ultramafics comprise a series of parallel or en echelon lenses and layers of ultramafic and mafic rocks defining a northeaststriking, 2 km wide zone (the Pounamu Ultramafic Belt — PUB) in garnet zone Haast Schists of North Westland. In their most coherent form, the Pounamu Ultramafics comprise serpentinite, gabbro, polymict igneous breccia, and metabasite. Gabbros are intruded by feldsparphyric dikes and unconformably overlain by breccia. Breccia or metabasite is overlain sequentially by a pelagic sequence including: interlaminated limestone and chert, phosphate lenses, and a massive porphyroblastic biotite schist; and a terrigenous series of thin-bedded, upward younging units of graded quartzofeldspathic sandstone and siltstone, similar to distal turbidite lithologies from the western Torlesse terrane. Similar upward grading quartzofeldspathic metasediments occur beneath the ultramafic unit. The Pounamu Ultramafics are interpreted as an ophiolite segment representing the basement to ...

The Snow Mountain Volcanic Complex: An On-Land Seamount in the Franciscan Terrain, California

The Snow Mountain Volcanic Complex in the Franciscan terrain of northern California is a 1 1/2 to 2 km-thick sequence of altered submarine pillow lavas, pillow breccias, diabase, and interbedded sediments, forming a klippe that structurally overlies nearby rocks of the Franciscan assemblage, the Coast Range ophiolite, and Great Valley sequence. All of the Snow Mountain rocks bear a weak overprint of blue-schist-facies metamorphism. The volcanic rocks were deposited on top of 200-300 m of interbedded turbiditic greywacke, shale, and radiolarian chert, which are interpreted as an interfingering abyssal fan and pelagic sequence. The volcanic pile is therefore supracrustal and is not an ophiolitic fragment formed at a mid-ocean spreading center; rather, it is probably the remnant of a former seamount. The lavas are mostly titanium-rich transitional basalts; minor associated rocks include rhyolite and peralkaline rhyolite. Relict clinopyroxenes in the basaltic rocks are all titanium-aluminum-rich augites. Thes...