Provenance signature of a forearc basin modified by spreading ridge subduction: Detrital zircon geochronology and detrital modes from the Paleogene Arkose Ridge Formation, southern Alaska

Upper Paleocene–Eocene boulder conglomerate, cross-stratified sandstone, and laminated carbonaceous mudstone of the Arkose Ridge Formation exposed in the southern Talkeetna Mountains record fluvial-lacustrine deposition proximal to the volcanic arc in a forearc basin modified by Paleogene spreading ridge subduction beneath southern Alaska. U-Pb ages of detrital zircon grains and modal analyses were obtained from stratigraphic sections spanning the 2,000 m thick Arkose Ridge Formation in order to constrain the lithology, age, and location of sediment sources that provided detritus. Detrital modes from 24 conglomerate beds and 54 sandstone thin sections aredominated by plutonic and volcanic clasts and plagioclase feldspar with minor quartz, schist, hornblende, argillite, and metabasalt. Westernmost sandstone and conglomerate strata contain <5% volcanic clasts whereas easternmost sandstone and conglomerate strata contain 40 to >80% volcanic clasts. Temporally, eastern sandstones andconglomerates exhibit an upsection increase in volcanic detritus from <40 to >80% volcanic clasts. U-Pb ages from >1400 detrital zircons in 15 sandstone samples reveal three main populations: late Paleocene–Eocene (60-48 Ma; 16% of all grains), Late Cretaceous–early Paleocene (85–60 Ma; 62%) and Jurassic–Early Cretaceous (200–100 Ma; 12%). A plot of U/Th vs U-Pb ages shows that >97% of zircons are <200 Ma and>99% of zircons have <10 U/Th ratios, consistent with mainly igneous source terranes. Strata show increased enrichment in late Paleocene–Eocene detrital zircons from <2% in the west to >25% in the east. In eastern sections, this younger age population increases temporally from 0% in the lower 50 m of the section to >40% in samples collected >740 m above the base. Integration of the compositional and detrital geochronologic data suggests: (1) Detritus was eroded mainly from igneous sources exposed directly north of the Arkose Ridge Formation strata, mainly Jurassic–Paleocene plutons and Paleocene–Eocenevolcanic centers. Subordinate metamorphic detritus was eroded from western Mesozoic low-grade metamorphic sources. Subordinate sedimentary detritus was eroded from eastern Mesozoic sedimentary sources. (2) Eastern deposystems received higher proportions of juvenile volcanic detritus through time, consistent with construction of adjacent slab-window volcanic centers during Arkose Ridge Formation deposition. (3)Western deposystems transported detritus from Jurassic–Paleocene arc plutons that flank the northwestern basin margin. (4) Metasedimentary strata of the Chugach accretionaryprism, exposed 20-50 km south of the Arkose Ridge Formation, did not contribute abundant detritus. Conventional provenance models predict reduced input of volcanic detritus to forearc basins during exhumation of the volcanic edifice and increasing exposure ofsubvolcanic plutons (Dickinson, 1995; Ingersoll and Eastmond, 2007). In the forearc strata of these conventional models, sandstone modal analyses record progressive increases upsection in quartz and feldspar concomitant with decreases in lithic grains, mainly volcanic lithics. Additionally, as the arc massif denudes through time, theyoungest detrital U-Pb zircon age populations become significantly older than the age of forearc deposition as the arc migrates inboard or ceases magmatism. Westernmost strata of the Arkose Ridge Formation are consistent with this conventional model. However, easternmost strata of the Arkose Ridge Formation contain sandstone modes that record an upsection increase in lithic grains accompanied by a decrease in quartz and feldspar, and detrital zircon age populations that closely match the age of deposition. This deviation from the conventional model is due to the proximity of the easternmost strata to adjacent juvenile volcanic rocks emplaced by slab-window volcanic processes. Provenance data from the Arkose Ridge Formation show that forearc basins modified by spreading ridge subduction may record upsection increases in non-arc, syndepositional volcanic detritusdue to contemporaneous accumulation of thick volcanic sequences at slab-window volcanic centers. This change may occur locally at the same time that other regions of the forearc continue to receive increasing amounts of plutonic detritus as the remnant arc denudes, resulting in complex lateral variations in forearc basin petrofacies and chronofacies.

Quaternary and Paleogene calcareous nannofossils of ODP Leg 115

The occurrences of ten datum events for the Quaternary and top Pliocene nannofossils are identified at nine Leg 115 sites. A quantitative investigation of Paleogene nannofossils in 470 samples selected from 11 holes at 9 sites yielded 197 taxa, including one new species and 10 unidentified taxa that are likely to be new species. Regional differences in the timing of some biostratigraphically important events are recognized, and a set of datum events useful for biostratigra- phy in the tropical Indian Ocean is presented. Biogeographical differences are minor for Paleogene cores from the tropical sites (Sites 707-716); however, the Quaternary and late early Oligocene floras observed at the two subtropical sites (Sites 705 and 706) differ significantly from the corresponding floras of the tropical sites. Bathymetrically controlled dissolution is recognized by the reduction of species diversity in the Paleogene flora. Selective dissolution of nannofossils is also evidenced by the percentage reduction of three holococcolith taxa, Lanternithus minutus, Zygrhablithus bijugatus, and Holococcolith type A as well as by the increase of Coccolithus pelagicusand Cribrocentrum reticulatumin the deeper sites.

Cretaceous-Paleogene biomagnetostratigraphy of ODP Leg 121 sites

Broken Ridge, in the eastern Indian Ocean, is a shallow-water volcanic platform which formed during the Early to middle Cretaceous at which time it comprised the northern portion of the Kerguelen-Heard Plateau. Rifting during the middle Eocene and subsequent seafloor spreading has moved Broken Ridge about 20?N to its present location. The sedimentary section of Broken Ridge includes Turonian-lower Eocene limestone and chalk with volcanic ash, an interval of detrital sands and gravels associated with middle Eocene rifting and uplift, and a middle-late Oligocene unconformity overlain by a thin section of Neogene-Holocene pelagic calcareous ooze. This paper summarizes the available post-cruise biostratigraphic and magnetostratigraphic data for the Cretaceous-Paleogene section on Broken Ridge. The synthesis of this information permits a more precise interpretation of the timing of events in the history of Broken Ridge, in particular the timing and duration of the middle Eocene rifting event. Paleontologic data support rapid flexural uplift of Broken Ridge in response to mechanical rather than thermal forces. Other highlights of the section include a complete Cretaceous/Tertiary boundary and an opportunity for first-order correlation of Paleogene diatom stratigraphy with that of the calcareous groups.

Early Oligocene continental climate ofthe Paleogene Basin in Hungary and Slovenia

This paper concentrates on the Early Oligocene palaeoclimate of the southern part of Eastern and Central Europe and gives a detailed climatological analysis, combined with leaf-morphological studies and modelling of the palaeoatmospheric CO2 level using stomatal and d13 C data. Climate data are calculated using the Coexistence Approach for Kiscellian floras of the Palaeogene Basin (Hungary and Slovenia) and coeval assemblages from Central and Southeastern Europe. Potential microclimatic or habitat variations are considered using morphometric analysis of fossil leaves from Hungarian, Slovenian and Italian floras. Reconstruction of CO2 is performed by applying a recently introduced mechanistic model. Results of climate analysis indicate distinct latitudinal and longitudinal climate patterns for various variables which agree well with reconstructed palaeogeography and vegetation. Calculated climate variables in general suggest a warm and frost-free climate with low seasonal variation of temperature. A difference in temperature parameters is recorded between localities from Central and Southeastern Europe, manifested mainly in the mean temperature of the coldest month. Results of morphometric analysis suggest microclimatic or habitat difference among studied floras. Extending the scarce information available on atmospheric CO2 levels during the Oligocene, we provide data for a well-defined time-interval. Reconstructed atmospheric CO2 levels agree well with threshold values for Antarctic ice sheet growth suggested by recent modelling studies. The successful application of the mechanistic model for the reconstruction of atmospheric CO2 levels raises new possibitities for future climate inference from macro-flora studies.

Cretaceous and Paleogene calcareous nannofossils from ODP Leg 123

Two of five holes drilled at two separate sites during Leg 123 of the Ocean Drilling Program intersected thick and relatively complete sections of Upper Cretaceous-Paleogene nannofossiliferous sediments. Although dominated by turbidite deposition in the upper part, Hole 765C contains a thick and relatively complete Albian-Oligocene section, including a particularly thick Aptian interval, with abundant and fairly well-preserved nannofossils. Several unconformities are confidently interpreted in this section that span much of the Santonian, late Campanian, Maestrichtian, late Eocene, and early Oligocene. Hole 766A contains a thick and relatively complete Albian-lower Eocene section having generally abundant and well-preserved nannofossils. Several unconformities also have been identified in this section that span much of the Coniacian, early Campanian, Maestrichtian, and late Eocene through early Pliocene. The chronostratigraphic position and length of all these unconformities may have considerable significance for reconstructing the sedimentary history and for interpreting the paleoceanography of this region. A particularly thick section of upper Paleocene-lower Eocene sediments, including a complete record across the Paleocene/Eocene boundary, also was cored in Hole 766A that contains abundant and diverse nannofossil assemblages. Although assemblages from this section were correlated successfully using a standard low-latitude zonation, difficulties were encountered that reduced biostratigraphic resolution. Several lines of evidence suggest a mid-latitude position for Site 766 during this time, including (1) high assemblage diversity characteristic of mid-latitude zones of upwelling and (2) absence of certain ecologically controlled markers found only in low latitudes.

Stable carbon and oxygen isotope ratios of Paleogene planktonic foraminifera from the Maud Rise

The oxygen and carbon isotopic composition has been measured for numerous Paleogene planktonic foraminifer species from Maud Rise, Weddell Sea (ODP Sites 689 and 690), the first such results from the Antarctic. The results provide information about large-scale changes in the evolution of temperatures, seasonally, and structure of the upper water column prior to the development of a significant Antarctic cryosphere. The early Paleocene was marked by cooler surface-water conditions compared to the Cretaceous and possibly a less well developed thermocline. The late Paleocene and early Eocene saw the expansion of the thermocline as Antarctic surface waters became warm-temperate to subtropical. The late Paleocene to early Eocene thermal maximum was punctuated by two brief excursions during which time the entire Antarctic water column warmed and the meridional temperature gradient was reduced. The first of these excursions occurred at the Paleocene/Eocene boundary, in association with a major extinction in deep sea benthic foraminifers. The second excursion occurred within the early Eocene at ~54.0 Ma. These excursions are of global importance and represent the warmest intervals of the entire Cenozoic. The excursions were associated with fundamental changes in deep-water circulation and global heat transport. The thermal maximum of the early Eocene ended with the initiation of a long-term cooling trend at 52.0 Ma. This cooling trend was associated with reduced seasonality, and diminished structure and/or duration of the seasonal thermocline. The cooling trend was punctuated by three major cooling steps at 43.0, 40.0, and -36.0 Ma.

Paleogene calcareous nannofossil record of the Tasmanian gateway

The opening of the Tasmanian Gateway between Australia and Antarctica has long been considered a critical element in the initiation of the Antarctic Circumpolar Current, thermal isolation of Antarctica, and Cenozoic global cooling. The timing for the opening of the gateway to shallow-water circulation and subsequently to deep-water circulation was poorly known, however, and the dating of these events was a major objective of ODP Leg 189. Nannofossil data from Leg 189 sites and DSDP Site 281 in the Tasmanian Gateway suggest a 41-42 Ma age for the initiation of widespread glauconite deposition in the region, which coincided with a sharp drop in sedimentation rate. This is interpreted to be the opening of the gateway to shallow-water circulation, which occurred within the middle of the 51-33 Ma long-term cooling. The change from siliciclastic sediments to pelagic carbonates, the most conspicuous sedimentological and paleontological change in the region for the last 70 Ma and presumably the indicator for the opening of the gateway to deep-water circulation, is dated at about 31 Ma. This event is more than 2 my younger than the major high-latitude cooling in the earliest Oligocene, and thus cannot be the cause for the latter.

Paleogene and Neogene larger foraminifera of ODP Leg 133 holes

At several sites drilled during Ocean Drilling Program (ODP) Leg 133 on the Queensland Plateau, larger shallow-water benthic foraminifers have been recovered from neritic carbonates and from turbidites that consist of shallow-water-derived material. Within neritic sediments, the occurrence of different faunal associations provides a tool for biostratigraphic subdivision. Three main phases of neritic deposition occurred on the Queensland Plateau. An Eocene episode is characterized by subtropical to temperate associations (Operculina-Nummulites Facies). It is unconformably followed by a late Oligocene to middle Miocene episode that contains tropical to subtropical associations (Spiroclypeus Facies, Larger Foraminifer-Coral Facies, Austrotrillina Facies, Flosculinella-Amphistegina Facies, Marginopora Facies, and Miogypsina Facies). After the middle Miocene, most of the Queensland Plateau carbonate platform was drowned. The post-middle Miocene to Holocene reefs, which are characterized by a geographically more restricted distribution, shed neritic material including larger benthic foraminifers into adjacent basinal areas. This process is associated with a partial reworking of middle Miocene deposits containing Lepidocyclina (Nephrolepidina).

Heavy minerals from Paleogene sediments at DSDP Leg 81 Holes

Five heavy mineral associations occur in the Paleocene and Eocene sediments recovered during Leg 81 of the Deep Sea Drilling Project (DSDP) in the SW Rockall area. Association 1, consisting of augite, iddingsite, and olivine, was derived from the basaltic rocks of the northern part of the Rockall Plateau. Association 2 consists of epidote group minerals, including piedmontite, and amphiboles of actinolite, actinolitic hornblende, and magnesio-hornblende compositions, and was derived from the metamorphic basement of south Greenland. Association 3 comprises garnet, augite, apatite, and edenitic and pargasitic amphiboles and has a provenance in the southern Rockall Plateau. Associations 4 (garnet, apatite, edenitic/pargasitic amphiboles) and 5 (garnet, apatite) are intrastratal solution derivatives of Association 3, with successive removal of first pyroxene and then amphibole with increasing depth of burial. Throughout the SW Rockall Plateau area there is a significant change in the spectrum of the above assemblages in the lower part of the Eocene. This change has been noted at Sites 403, 404, 553, and 555 and is defined by the last appearance of Association 2. This level therefore marks the cessation of sediment supply from southern Greenland and is the result of the final separation of Rockall and Greenland immediately prior to magnetic Anomaly 24.

Foraminiferal assemblages and stable isotopes across the early Paleogene carbonate facies of Perth Abyssal Plain off Western Australia

Bulk carbonate content, planktic and benthic foraminiferal assemblages, stable isotope compositions of bulk carbonate and Nuttallides truempyi (benthic foraminifera), and non-carbonate mineralogy were examined across ~30 m of carbonate-rich Paleogene sediment at Deep Sea Drilling Project (DSDP) Site 259, on Perth Abyssal Plain off Western Australia. Carbonate content, mostly reflecting nannofossil abundance, ranges from 3 to 80% and generally exceeds 50% between 35 and 57 mbsf. A clay-rich horizon with a carbonate content of about 37% occurs between 55.17 and 55.37 mbsf. The carbonate-rich interval spans planktic foraminiferal zones P4c to P6b (~57-52 Ma), with the clay-rich horizon near the base of our Zone P5 (upper)-P6b. Throughout the studied interval, benthic species dominate foraminiferal assemblages, with scarce planktic foraminifera usually of poor preservation and limited species diversity. A prominent Benthic Foraminiferal Extinction Event (BFEE) occurs across the clay-rich horizon, with an influx of large Acarinina immediately above. The delta13C records of bulk carbonate and N. truempyi exhibit trends similar to those observed in upper Paleocene-lower Eocene (~57-52 Ma) sediment from other locations. Two successive decreases in bulk carbonate and N. truempyi delta13C of 0.5 and 1.0? characterize the interval at and immediately above the BFEE. Despite major changes in carbonate content, foraminiferal assemblages and carbon isotopes, the mineralogy of the non-carbonate fraction consistently comprises expanding clay, heulandite (zeolite), quartz, feldspar (sodic or calcic), minor mica, and pyrolusite (MnO2). The uniformity of this mineral assemblage suggests that Site 259 received similar non-carbonate sediment before, during and after pelagic carbonate deposition. The carbonate plug at Site 259 probably represents a drop in the CCD from ~57 to 52-51 Ma, as also recognized at other locations.