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Iron oxide-copper-gold (IOCG) and iron oxide-apatite (IOA) deposits are important sources of Cu and Fe, respectively. They contain abundant Fe-oxides and may contain Au, Ag, Co, rare earth elements (REEs), U, Ni, and V as economically important by-products. In Peru, the Mina Justa IOCG deposit is located next to the giant Marcona IOA deposit. Constraining the timing of Fe and Cu mineralization at Mina Justa is fundamental to understanding the duration and type of processes that generated this mineral deposit, and ultimately to testing the genetic link with other deposits in the area. Previous authors used alteration minerals to indirectly date Cu mineralization at Mina Justa at around 100 Ma. We report Ar/Ar dates of actinolite, U-Pb dates of magnetite, apatite, and titanite collected by in situ laser-ablation-multicollector-inductively coupled plasma-mass spectrometry, and Re-Os thermal ionization mass spectrometry dates for sulfides. These results indicate that Cu mineralization at Mina Justa occurred at ca. 160 Ma and that Fe mineralization is older and coeval with the neighboring Marcona IOA deposit, consistent with Cu mineralization overprinting IOA-style mineralization at Mina Justa. [ABSTRACT FROM AUTHOR]

Controls on high-frequency sequences formed during super-greenhouse conditions in the Late Cretaceous Western Interior Seaway remain equivocal because of the active foreland basin tectonic setting and the lack of direct evidence of polar glaciations to support a glacio-eustatic origin. This paper quantifies eustatic sea-level changes based on high-resolution sequence stratigraphic analysis and improved chronometry of shallow marine deposits of the Late Cretaceous Gallup Sandstone in New Mexico, USA. Backstripping techniques remove tectonic and compactional subsidence and enable quantification of the magnitude of eustatic sea-level change, that allow evaluation of the dominant controls on the high-frequency sequences to resolve the role of orbitally controlled, climate-driven eustasy versus tectonics. Sixty-five parasequences, constituting 29 parasequence sets and 12 sequences are identified in the ~1.2 m.y. duration of the Late Cretaceous Gallup system. New 40Ar/39Ar dating of bentonites constrains the durations of the individual parasequences, parasequence sets, and sequences, and that these match Milankovitch periodicity, indicating an orbital climate control. The magnitudes of sea-level changes between parasequences range between -28 m and +22 m, which are compatible with hypotheses of both aquifer and glacio-eustasy. Aquifer-eustasy predicts a reciprocal relationship between floodplain cycles and shallow marine sequences, such that aquifer drawdown and falling water tables should correlate to rising sea levels (highstands), whereas increased aquifer storage and rising water tables should correlate to falling sea levels (lowstands). Our preliminary observations show synchronous, versus reciprocal, relationships that may be more compatible with a glacio-eustatic origin. The results of this study support the hypothesis that the Cretaceous greenhouse was marked by high-frequency, low-amplitude glaciations driven by orbital climate cycles, but further work is required to evaluate the contribution of aquifer-eustasy. [ABSTRACT FROM AUTHOR]

A 2100 Ma, 420 cm-thick metasaprolite, which is devoid of plagioclase but contains abundant microcline, occurs in the Archean McGrath Gneiss beneath the overlying Paleoproterozoic Denham Formation. Despite being recrystallized under amphibolite-facies conditions and chemically modified by potassium metasomatism, mass fluxes related to weathering and metasomatism of the McGrath metasaprolite can be estimated by judicious application of several geochemical parameters and the A–C*N–K plot (Fedo et al., 1995). The metasaprolite yields values of 67–94 for the Plagioclase Index of Alteration, 62–72 for the Chemical Index of Alteration, and 47 for the Feldspar Index of Weathering, and the total mass flux removed by weathering is a minimum of 2.4 moles/cm 2 . Such values correspond to an intermediate degree, or intensity, of weathering compared to other, more intensely weathered paleosols in the region, such as those beneath the Baraboo and Sioux quartzites. Potassium metasomatism occurred at 1742 Ma during the geon 17 Yavapai tectonothermal event, when a minimum of 0.33 moles/cm 2 K 2 O was added to the metasaprolite. Although the absence of pedogenic textures precludes interpretation of climatic conditions during weathering of the McGrath Gneiss, the level of atmospheric p CO 2 can be estimated by application of Sheldon's (2006) method, which yields a minimum of 3.7 × PAL for a weathering duration of 100,000 years. [ABSTRACT FROM AUTHOR]

Abstract: Eighteen lavas erupted from cinder cones behind the volcanic front in southeastern Guatemala and western El Salvador have been dated using the 40Ar/39Ar method on groundmass separates. Plateau ages range between 1069.5±22.1and 37±12.0ka. The majority of dated lavas are younger than 600ka. Importantly, cinder cone lavas become progressively younger toward the volcanic front/trench. In addition, the lavas become systematically depleted in Cs, Ba, rare earth elements, Cs/Zr, La/Zr, and δ 18O with time. The baseline values of these same geochemical parameters increase with distance from the volcanic front. The cinder cone lavas are closely associated with rifting south of the North American–Caribbean plate boundary. The trenchward migration of behind-the-front volcanism may be linked to a westward migration of this rifting. Existing evidence suggests there was an important change in the face of volcanism behind the volcanic front in southeastern Guatemala: from a small number of large polygenetic volcanoes to abundant smaller monogenetic cones. The temporal/spatial changes in the geochemistry of cinder cone lavas is attributed to decreasing contamination by Paleozoic basement rocks, both with time and with diminishing distances to the volcanic front. [Copyright &y& Elsevier]

The 40Ar/39Ar radioisotope system is widely used to date eruption and cooling of volcanic tephra-marker horizons that commonly provide the only means of correlating and assigning numerical ages to stratigraphy in which they are contained. This chronometer bridges the gap between 14C and longer-lived isotopic systems that are too imprecise for dating young samples. However, 40Ar/39Ar ages obtained from coevally erupted biotite and sanidine do not always match. Here, we use an independent chronometer, 238U-230Th disequilibrium, to demonstrate that 40Ar/39Ar age disparity is not caused by differences in pre-eruption crystallization times. Our findings indicate that the presence of extraneous 40Ar in biotite, and its absence in sanidine, may result from violations of two assumptions implicit in 40Ar/39Ar geochronology on volcanic samples: (1) Prior to eruption, minerals are devoid of 40Ar due to rapid loss to an "infinite reservoir" such as the atmosphere, and (2) closure to volume diffusion is geologically instantaneous and coincident with eruption. We propose a mechanism whereby the presence of extraneous Ar in certain minerals is explained by the relative sequence of four events in a magmatic system: (1) crystallization, (2) mineral closure with respect to Ar diffusion, (3) isotopic equilibration of magmatic and atmospheric Ar, and (4) quenching of the system by eruption. These data have potentially far-reaching implications for studies that depend on geochronological data, necessitating re-evaluation of interpretations based solely on biotite with no independent age control, particularly in young samples where the effects are most pronounced. [ABSTRACT FROM AUTHOR]

Abstract: A brief period of enhanced 10Be flux that straddles the interstadial warm period known as Dansgaard–Oeschger event 10 in Greenland and its counterpart in Antarctica, the Antarctic Isotope Maximum 10 is but one consequence of the weakening of Earth''s magnetic field associated with the Laschamp excursion. This 10Be peak measured in the GRIP ice core is dated at 41,250 y b2k (= before year 2000 AD) in the most recent GICC05 age model obtained from the NorthGRIP core via multi-parameter counting of annual layers. Uncertainty in the age of the 10Be peak is, however, no better than ±1630 y at the 95% confidence level, reflecting accumulated error in identifying annual layers. The age of the Laschamp excursion [Guillou, H., Singer, B.S., Laj, C., Kissel, C., Scaillet, S., Jicha, B., 2004. On the age of the Laschamp geomagnetic excursion. Earth Planet. Sci. Lett. 227, 331-343.] is revised on the basis of new 40Ar/39Ar, unspiked K–Ar and 238U–230Th data from three lava flows in the Massif Central, France, together with the 40Ar/39Ar age of a transitionally magnetized lava flow at Auckland, New Zealand. Combined, these data yield an age of 40,700±950 y b2k, where the uncertainty includes both analytical and systematic (40K and 230Th decay constant) errors. Taking the radioisotopic age as a calibration tie point suggests that the layer-counting chronologies for the NorthGRIP and GISP2 ice cores are more accurate and precise than previously thought at depths corresponding to the Laschamp excursion. [Copyright &y& Elsevier]

Calbuco (41.3°S) is a highly active composite frontal arc volcano in the Andean Central Southern Volcanic Zone (CSVZ). It is currently ranked by Chile's Geological Survey (SERNAGEOMIN) as among the most dangerous volcanic centers in the Southern Andes, with sub-Plinian explosive activity occurring as recently as 2015. Compositional and geochronologic data from 65 samples constrain the Pleistocene-Holocene eruptive evolution of Calbuco. New 40Ar/39Ar ages from lava flows, revised mapping, and existing 14C dates for Holocene tephras reveal that much of the current edifice is <20 ka in age. Thus, Calbuco has produced a greater Holocene eruptive flux, 3.4 km3/ka averaged over the last 10 ka and 4.8 km3/ka over the last 4 ka, than other arc volcanoes both in the CSVZ and globally. Morphologic and stratigraphic observations along with new whole-rock geochemical data suggest that this remarkable magma flux is expressed as repeated eruptions of lavas and tephra with a narrow SiO 2 range (54–58 wt%). This compositional monotony, conspicuous resorbed amphibole, and elevated fluid-mobile trace element abundances differ from the post-glacial compositions and mineralogy observed at other CSVZ arc front volcanoes. Mid-crustal processing at or just below the depth where amphibole is stable plays a key role in the genesis of andesite at Calbuco. The precision of 40Ar/39Ar dates from Latest Pleistocene to Holocene low K 2 O mafic lavas remains a challenge to constructing eruptive chronologies for comparison to records of glacier expansion and retreat. Notwithstanding, our findings from Calbuco imply that petrologic factors, including mantle melt flux, depth of magma storage, and melt H 2 O content, may promote distinctive eruptive responses at adjacent CSVZ volcanoes following rapid retreat of the Patagonian Ice Sheet at 18 ka. • New 40Ar/39Ar ages from lava flows reveal that Calbuco Volcano grew at a fast rate (3.4 km3/ka) during the Holocene. • The precision of 40Ar/39Ar dates for young, low K 2 O mafic lavas remains a challenge to constructing eruptive chronologies. • Factors like magma storage depth may promote unique eruptive responses at adjacent volcanoes following fast deglaciation. [ABSTRACT FROM AUTHOR]

The article presents a study examining transient rhyolite melt extraction to produce a shallow granitic pluton. It mentions that rhyolitic melt that fuels explosive eruptions often originates in the upper crust via extraction from crystal-rich sources, implying an evolutionary link between volcanism and residual plutonism. It reports that thermo-petrological simulations linked to a zircon saturation model map spatiotemporal melt flux distributions.

Fluctuations in depositional conditions during the onset of severe climate events in Earth history predispose stratigraphic archives to hiatuses, often hindering complete reconstructions of paleoclimate events and their triggers. Several studies have proposed that a hiatus of unknown duration exists at the base of Oceanic Anoxic Event 2 (OAE2) in the North American Western Interior Basin at the base Turonian global boundary stratotype section and point (GSSP) in Pueblo, Colorado, which potentially influences integrated radioisotopic, biostratigraphic, and astrochronologic age models of the Cenomanian-Turonian boundary interval. To quantify the duration of this hiatus, refine the chronology of OAE2, and assess marine geochemical perturbations associated with the onset of the event, we present new 40Ar/39Ar dates from regional bentonites along with a new proximal- distal chemostratigraphic transect of the epeiric Western Interior Basin (WIB), including initial osmium isotope (Osi) and stable carbon isotope (δ13C) data. The new 40Ar/39Ar age determinations confirm and further constrain previous estimates of Cenomanian-Turonian boundary timing. Further, the regional chemostratigraphic synthesis demonstrates the conformity of the OAE2 successions correlated to Pueblo, shows that the duration of the lag between the onset of the Osi and δ13C excursions is ~60 k.y., and thus constrains the magnitude of the pre-OAE2 hiatus in Pueblo to less than this value. The new astronomically tuned, conformable Osi record across the onset of OAE2 captures a geologically rapid onset of large igneous province volcanism, consistent with other records, such that the addition of CO2 to the ocean-atmosphere system may have driven changes in marine carbonate chemistry. Additionally, the refined chronostratigraphy of OAE2 and the Cenomanian-Turonian boundary in the central WIB improves correlation with other records, such as those in the Eagle Ford Group, Texas. The correlations highlight that discrepancies among OAE2 age models from globally distributed sections commonly stem from differing definitions of the event and uncertainties associated with astronomical tuning, in addition to stratigraphic preservation. [ABSTRACT FROM AUTHOR]

The complex urban geology of the historic center of Naples is characterized by the emplacement of products from two adjoining volcanic areas that were reworked by alluvial and coastal processes. To expand the geologic knowledge of this densely populated area, multidisciplinary research was carried out on the recently drilled San Marcellino borehole, as part of a larger borehole collection project. Critical revision of 290 previously drilled sequences, and a complete set of tephrostratigraphic, sedimentologic, and paleoecologic data obtained on the San Marcellino stratigraphy, informed a hydrogeological conceptual model of water circulation and paleoenvironmental reconstruction of the city historic center. Both before and after the Neapolitan Yellow Tuff eruption, ca. 15 ka, the landscape was characterized by a marine to transitional paleoenvironment. This signifies a good balance between sea-level rise and volcaniclastic inputs, consisting of both primary and remobilized deposits in an area that is also subject to bradyseismic oscillations. Amongst the primary pyroclastic deposits, the products of an explosive event from Campi Flegrei about 16 ka, with an eastward-directed sustained-column phase, were recognized. Collectively, the multidisciplinary data obtained in this study favor the presence of a double-aquifer conceptual model, characterized by a volcanic and a marine-sedimentary aquifer. [ABSTRACT FROM AUTHOR]

The Eocene Huitrera Formation of northwestern Patagonia, Argentina, is renowned for its diverse, informative, and outstandingly preserved fossil biotas. In northwest Chubut Province, at the Laguna del Hunco locality, this unit includes one of the most diverse fossil floras known from the Eocene, as well as significant fossil insects and vertebrates. It also includes rich fossil vertebrate faunas at the Laguna Fría and La Barda localities. Previous studies of these important occurrences have provided relatively little sedimentological detail, and radioisotopic age constraints are relatively sparse and in some cases obsolete. Here, we describe five fossiliferous lithofacies deposited in four terrestrial depositional environments: lacustrine basin floor, subaerial pyroclastic plain, vegetated, waterlogged pyroclastic lake margin, and extracaldera incised valley. We also report several new 40Ar/39Ar age determinations. Among these, the uppermost unit of the caldera-forming Ignimbrita Barda Colorada yielded a 40Ar/39Ar age of 52.54 ± 0.17 Ma, ~6 m.y. younger than previous estimates, which demonstrates that deposition of overlying fossiliferous lacustrine strata (previously constrained to older than 52.22 ± 0.22 Ma) must have begun almost immediately on the subsiding ignimbrite surface. A minimum age for Laguna del Hunco fossils is established by an overlying ignimbrite with an age of 49.19 ± 0.24 Ma, confirming that deposition took place during the early Eocene climatic optimum. The Laguna Fría mammalian fauna is younger, constrained between a valley-filling ignimbrite and a capping basalt with 40Ar/39Ar ages of 49.26 ± 0.30 Ma and 43.50 ± 1.14 Ma, respectively. The latter age is ~4 m.y. younger than previously reported. These new ages more precisely define the age range of the Laguna Fría and La Barda faunas, allowing greatly improved understanding of their positions with respect to South American mammal evolution, climate change, and geographic isolation. [ABSTRACT FROM AUTHOR]

We present a review of the geomorphology of the Tyrrhenian Sea coast of central Italy integrated by a novel structural-geomorphological study coupled with statistical analysis of topographic culminations and comparison with a Digital Elevation Model, aimed at reconstructing a suite of paleo-surfaces corresponding to remnant portions of marine terraces. We performed geochronological, sedimentological, micromorphological and mineralogical investigations on the deposits forming the different paleo-surfaces between Civitavecchia and Anzio towns, in order to provide chronostratigraphical, paleogeographical and paleoenvironmental constraints. Using the newly achieved dataset we correlate these paleo-surfaces with the coastal terraces formed during past sea-level highstands, as recognized by previous studies, and we refine their correlation with the Marine Isotope Stage (MIS) timescale. In particular, we have extended our geomorphological analyses landward in the area between the Tiber River mouth and Anzio, in order to include the oldest paleo-surface developed above the deposits of the last large explosive eruption at 365 ± 4 ka in the Colli Albani Volcanic District. Results of this study allow us to recognize a set of higher paleo-surfaces at elevation ranging 108 thought 71 m a.s.l., which we interpret as one tectonically displaced, widespread coastal terrace originated during the MIS 9.1 highstand. We correlate the previously identified paleo-surfaces of 66–62 m a.s.l. and 56–52 m a.s.l. with the equivalent coastal terraces developed during the sea-level highstands of sub-stages 7.5 and 7.3/7.1. Moreover, based on data from literature on relative elevation of maximum sea level during the highstands of MIS 11 through MIS 5.1, we assess the regional uplift and the concurrent tectonic displacements that have occurred since 900 ka in this area. • We reconstruct a suite of six paleo-surfaces along the central Tyrrhenian Sea coast. • Paleoenvironmental data identify these paleo-surfaces as remnants of coastal terraces. • We provide 40Ar/39Ar age constraints to the terraced sedimentary deposits. • We correlate these terraces with the Marine Isotope Stage record. • We identify previously unrecognized MIS 9 sea-level markers in this region. • We assess regional uplift and tectonic displacements in the last 400 ka. [ABSTRACT FROM AUTHOR]

Basins in orogenic hinterlands are directly coupled to crustal thickening and extension through landscape processes and preserve rec ords of deformation that are unavailable in footwall rocks. Following prolonged late Meso zoic--early Cenozoic crustal thickening and plateau construction, the hinterland of the Sevier orogen of western North America underwent late Cenozoic extension and formation of metamorphic core complexes. While the North American Cordillera is one of Earth's best-studied orogens, estimates for the spatial and temporal patterns of initial extensional faulting differ greatly and thus limit understanding of potential drivers for deformation. We employed (U-Th)/(He-Pb) double dating of detrital zircon and (U-Th)/He thermo chronology of detrital apatite from precisely dated Paleogene terrestrial strata to quantify the timing and magnitude of exhumation and explore the linkages between tectonic unroofing and basin evolution in northeastern Nevada. We determined sediment provenance and lag time evolution (i.e., the time between cooling and deposition, which is a measure of upper-crustal exhumation) during an 8 m.y. time span of deposition within the Eocene Elko Basin. Fluvial strata deposited between 49 and 45 Ma yielded Precambrian (U-Th)/He zircon cooling ages (ZHe) with 105-740 m.y. lag times dominated by unreset detrital ages, suggesting limited exhumation and Proterozoic through early Eocene sediment burial (<4--6 km) across the region. Minimum nonvolcanic detrital ZHe lag times decreased to <100 m.y. in 45-43 Ma strata and to <10 m.y. in 43-41 Ma strata, illustrating progressive and rapid hinterland unroofing in Eocene time. Detrital apatite (U-Th)/He ages present in ca. 44 and 39 Ma strata record Eocene cooling ages with 1--20 m.y. lag times. These data reflect acceleration of basement exhumation rates by >1 km/m.y., indicative of rapid, large-magnitude extensional faulting and metamorphic core complex formation. Contemporaneous with this acceleration of hinterland exhumation, syntectonic freshwater lakes developed in the hanging wall of the Ruby Mountains--East Humboldt Range metamorphic core complex at ca. 43 Ma. Volcanism driven by Farallon slab removal migrated southward across northeastern Nevada, resulting in voluminous rhyolitic eruptions at 41.5 and 40.1 Ma, and marking the abrupt end of fluvial and lacustrine deposition across much of the Elko Basin. Thermal and rheologic weakening of the lithosphere and/or partial slab removal likely initiated extensional deformation, rapidly unroofing deeper crustal levels. We attribute the observed acceleration in exhumation, expansion of sedimentary basins, and migrating volcanism across the middle Eocene to record the thermal and isostatic effects of Farallon slab rollback and subsequent removal of the lowermost mantle lithosphere. [ABSTRACT FROM AUTHOR]

Within extended orogens, records that reflect the driving processes and dynamics of early extension are often overprinted by subsequent orogenic collapse. The Copper Mountains of northeastern Nevada preserve an exceptional record of hinterland extensional deformation and high-elevation basin formation, but current geochronology and thermochronology are insufficient to relate this to broader structural trends in the region. This extension occurred concurrent with volcanism commonly attributed to Farallon slab removal. We combine thermochronology of both synextensional hanging-wall strata and footwall rocks to comprehensively evaluate the precise timing and style of this deformation. Specifically, we apply (U-Th)/(He-Pb) double dating of minerals extracted from Eocene-Oligocene Copper Basin strata with multi-mineral (U-Th)/He and 40Ar/39Ar thermochronology of rocks sampled across an ~20 km transect of the Copper Mountains. We integrate basement and detrital thermochronology records to comprehensively evaluate the timing and rates of hinterland extension and basin sedimentation. Cooling and U-Pb crystallization ages show the Coffeepot Stock, which spans the width of the Copper Mountains, was emplaced at ca. 109-108 Ma, and then cooled through the 40Ar/39Ar muscovite and biotite closure temperatures by ca. 90 Ma, the zircon (U-Th)/He closure temperature between ca. 90 and 70 Ma, and the apatite (U-Th)/He closure temperature between 43 and 40 Ma. Detrital apatite and zircon (U-Th)/(He-Pb) double dating of late Eocene fluvial and lacustrine strata of the Dead Horse Formation and early Oligocene fluvial strata of the Meadow Fork Formation, both deposited in Copper Basin, shows that Early Cretaceous age detrital grains have a cooling history that is analogous to proximal intrusive rocks of the Coffeepot Stock. At ca. 38 Ma, cooling and depositional ages for Copper Basin strata reveal rapid exhumation of proximal source terranes (cooling rate of ~37 °C/m.y.); in these terranes, 8-12 km of slip along the low-angle Copper Creek normal fault exhumed the Coffeepot Stock in the footwall. Late Eocene-early Oligocene slip along this fault and an upper fault splay, the Meadow Fork fault, created a half graben that accommodated ~1.4 km of volcaniclastic strata, including ~20 m of lacustrine strata that preserve the renowned Copper Basin flora. Single-crystal sanidine 40Ar/39Ar geochronology of interbedded tuffs in Copper Basin constrains the onset of rapid exhumation to 38.0 ± 0.9 Ma, indicating that surface-breaching extensional deformation was coincident with intense proximal volcanism. Coarse-grained syndeformational sediments of the Oligocene Meadow Fork Formation were deposited just prior to formation of an extensive regional Oligocene-Miocene unconformity and represent one of the most complete hinterland stratigraphic records of this time. We interpret this history of rapid late Eocene exhumation across the Copper Mountains, coeval volcanism, and subsequent unconformity formation to reflect dynamic and thermal effects associated with Farallon slab removal. The final phase of extension is recorded by late, high-angle normal faults that cut and rotate the early middle Miocene Jarbidge Rhyolite sequence, deposited unconformably in the hanging wall. These results provide an independent record of episodic Paleogene to Miocene exhumation documented in Cordilleran metamorphic core complexes and establish that substantial extension occurred locally in the hinterland prior to province-wide Miocene extensional break-up. [ABSTRACT FROM AUTHOR]

Following a recent chronostratigraphic revision of 17 fossiliferous sites hosting assemblages constituting local faunas of the Aurelian Mammal Age for peninsular Italy, we provide a re-structured biochronological framework and discuss the current validity and significance of the middle Pleistocene Faunal Units (FU) for this region. Contrasting with the previous model of a wide faunal renewal during Marine Isotope Stage (MIS) 9 (∼ 330 ka), the First Occurrences (FO) of several species of the Torre in Pietra FU are significantly backdated and referred to the Fontana Ranuccio FU (530–400 ka). We show that the faunal renewal was more gradual and occurred earlier than previously assumed. Many taxa that are typical of the late Pleistocene register their FO in the Fontana Ranuccio FU, latest Galerian, which is characterized by the almost total disappearance of Villafranchian taxa and by the persistence of typical Galerian taxa such as Dama clactoniana , Bison schoetensacki and Ursus deningeri , and by the FO of Stephanorhinus kirchbergensis , S. hemitoechus , Hippopotamus amphibius , Cervus elaphus eostepahnoceros , Ursus spelaeus , Canis lupus, and Vulpes vulpes. The next Torre in Pietra FU is characterized only by the FO of Megaloceros giganteus and Mustela putorius. However, we observe that MIS 9 marks the actual moment when the faunal assemblages of this region are represented only by those taxa characterizing the late middle Pleistocene and late Pleistocene. For this reason, we propose to still consider the Torre in Pietra (lower levels) local fauna as a conventional boundary for the Galerian-Aurelian transition. Finally, we remark that the strong faunal renewal in MIS 13, with five FOs, coincides with the temperate climatic conditions due to the absence of marked glacial periods that could have favored the FO and the subsequent spread of these taxa. [ABSTRACT FROM AUTHOR]

The Andean Southern Volcanic Zone comprises > 30 active arc front volcanoes that grew over periods of hundreds of thousands of years. Quantifying the rates at which these volcanoes grow is key to appreciating geological hazards, clarifying petrologic evolution, and exploring possible relationships between volcanism, ice loading, and climate. The integration of precise geochronology and geologic mapping, together with new lava compositions and volume estimates, reveal the evolution of the Antuco-Sierra Velluda volcanic complex at 37.2°S. Thirty-one new 40 Ar/ 39 Ar age determinations illuminate a punctuated eruptive history that spans at least 430 kyr. Sierra Velluda comprises ~ 130 km 3 and began to grow prior to 426.8 ka. A lacuna in the volcanic record between 343.5 and 150.4 ka coincides with glaciations associated with marine isotope stages (MIS) 10 and 8, although shallow intrusions were emplaced at 207.0 and 190.0 ka. Antuco began to grow rapidly on the northeast flank of Sierra Velluda, erupting > 60 km 3 of lava during three phases: (1) an early phase that began at 150.4 ka, (2) a post-MIS 2 phase between 16.3 and 6.2 ka, and (3) a post-sector collapse phase after 6.2 ka. Volcanism has been continuous during the last 100 kyr, with an average rate of cone growth during this period of ~ 0.46 km 3 /kyr that has accelerated by about 50% during the past 6 kyr. Whereas Sierra Velluda erupted basaltic andesitic to andesitic (53.5 to 58.7 wt% SiO 2 ) lavas, during the last expansion of glaciers between ~ 130 and 17 ka, Early Antuco erupted a wider spectrum of lavas, ranging from basaltic andesite to dacite (52.0 to 64.5 wt% SiO 2 ). Notably, eruptions following the last glacial termination at 17 ka produced basalts and basaltic andesites (50.9–53.7% SiO 2 ), and following the 6.2 ka cone collapse they have been exclusively olivine basalt (50.9–53.0% SiO 2 ) with > 5 wt% MgO. Thermodynamic and trace element modeling suggests that lavas from Sierra Velluda and Early Antuco reflect extensive fractional crystallization of parental basaltic magmas with low water content (~ 1 wt%) at pressures between 0.9 and 1.5 kbar. In contrast, eruptions following rapid deglaciation tapped asthenospheric mantle-derived basalt that has been extensively modified by assimilation of partial melts of lower crustal rocks. [ABSTRACT FROM AUTHOR]

The Fucino Basin, central Italy, with its long and continuous history of Quaternary sediment accumulation, is one of the richest Mediterranean Middle Pleistocene tephra records. Here, we present a new detailed investigation of tephra layers (tephras) of the 250–170 thousand years before present (ka) interval, corresponding to the entire Marine Isotope Stage (MIS) 7 and parts of the MIS 8 and MIS 6. The investigated tephras have been characterised in terms of major, minor and trace elements, Sr–Nd isotopic compositions and 40Ar/39Ar ages. For correlation purposes, glass compositions and several new 40Ar/39Ar ages of selected proximal pyroclastic units spanning the same temporal interval from Vulsini (Latera Volcanic Complex), Sabatini, and Vico volcanic systems, central Italy, were measured. The late MIS 8-early MIS 6 Fucino tephras were backtracked to their corresponding volcanic sources, which include the Vulsini, Vico, Sabatini, Roccamonfina, Ischia and Campi Flegrei volcanic systems. While some of these tephras have been correlated to specific eruption units, other layers are currently not documented or described in near-vent sections, thus highlighting previously unrecognised events generated by these volcanic systems. Furthermore, the new high precision 40Ar/39Ar ages provide improved temporal constraints for Fucino making it one of the most detailed and chronologically best constrained tephra records for central Mediterranean MIS 7 tephrochronology. The Fucino record thus provides new integrative information for reconstructing the explosive history of Italian volcanoes during the investigated time interval. Furthermore, the geochronological constraints provide the basis for future paleoclimatic investigations at local and regional scale. • Fucino proves to be the richest Mediterranean Middle Pleistocene tephra record. • The Fucino provides a detailed history of the explosive volcanism during MIS 7. • High precision 40Ar/39Ar ages constrain the Mediterranean MIS 7 tephrochronology. • A robust, independent age-model for the MIS 7 paleoclimatic record is provided. [ABSTRACT FROM AUTHOR]

Discovery of seafloor volcanism west of Buldir Volcano, the westernmost emergent volcano in the Aleutian arc, demonstrates that surface expression of active Aleutian volcanism falls below sea level just west of 175·9°E longitude, but is otherwise continuous from mainland Alaska to Kamchatka. Lavas dredged from newly discovered seafloor volcanoes up to 300 km west of Buldir have end-member geochemical characteristics that provide new insights into the role of subducted basalt as a source component in Aleutian magmas. Western Aleutian seafloor lavas define a highly calc-alkaline series with 50-70% SiO2. Most samples have Mg-numbers [Mg# = Mg/(Mg + Fe)] greater than 0·60, with higher MgO and lower FeO* compared with average Aleutian volcanic rocks at all silica contents. Common basalts and basaltic andesites in the series are primitive, with average Mg# values of 0·67 (±0·02, n = 99, 1SD), and have Sr concentrations (423 ± 29 ppm, n = 99) and La/Yb ratios (4·5 ± 0·4, n = 29) that are typical of island arc basaltic lavas. A smaller group of basaltic samples is more evolved and geochemically more enriched, with higher and more variable Sr and La/Yb (average Mg# = 0·61 ± 0·1, n = 31; Sr = 882 ± 333 ppm, n = 31; La/Yb = 9·1 ± 0·9, n = 16). None of the geochemically enriched basalts or basaltic andesites has low Y (<15 ppm) or Yb (<1·5 ppm), so none show the influence of residual or cumulate garnet. In contrast, most western seafloor andesites, dacites and rhyodacites have higher Sr (>1000 ppm) and are adakitic, with strongly fractionated trace element patterns (Sr/Y = 50-350, La/Yb = 8-35, Dy/Yb = 2·0-3·5) with low relative abundances of Nb and Ta (La/Ta > 100), consistent with an enhanced role for residual or cumulate garnet + rutile. All western seafloor lavas have uniformly radiogenic Hf and Nd isotopes, with εNd = 9·1 ± 0·3 (n = 31) and εHf = 14·5 ± 0·6 (n = 27). Lead isotopes are variable and decrease with increasing SiO2 from basalts with 206Pb/204Pb = 18·51 ± 0·05 (n = 11) to dacites and rhyodacites with 206Pb/204Pb = 18·43 ± 0·04 (n = 18). Western seafloor lavas form a steep trend in 207Pb/204Pb-206Pb/204Pb space, and are collinear with lavas from emergent Aleutian volcanoes, which mostly have 206Pb/204Pb > 18·6 and 207Pb/204Pb > 15·52. High MgO and Mg# relative to silica, flat to decreasing abundances of incompatible elements, and decreasing Pb isotope ratios with increasing SiO2 rule out an origin for the dacites and rhyodacites by fractional crystallization. The physical setting of some samples (erupted through Bering Sea oceanic lithosphere) rules out an origin for their garnet + rutile trace element signature by melting in the deep crust. Adakitic trace element patterns in the dacites and rhyodacites are therefore interpreted as the product of melting of mid-ocean ridge basalt (MORB) eclogite in the subducting oceanic crust. Western seafloor andesites, dacites and rhyodacites define a geochemical end-member that is isotopically like MORB, with strongly fractionated Ta/Hf, Ta/Nd, Ce/Pb, Yb/Nd and Sr/Y. This eclogite component appears to be present in lavas throughout the arc. Mass-balance modeling indicates that it may contribute 36-50% of the light rare earth elements and 18% of the Hf that is present in Aleutian volcanic rocks. Close juxtaposition of high-Mg# basalt, andesite and dacite implies widely variable temperatures in the western Aleutian mantle wedge. A conceptual model explaining this shows interaction of hydrous eclogite melts with mantle peridotite to produce buoyant diapirs of pyroxenite and pyroxenite melt. These diapirs reach the base of the crust and feed surface volcanism in the western Aleutians, but are diluted by extensive melting in a hotter mantle wedge in the eastern part of the arc. [ABSTRACT FROM AUTHOR]

The development of integrated astronomical and radioisotopic time scales from rhythmic strata of the Western Interior Basin (WIB) has played a fundamental role in the refinement of Late Cretaceous chronostratigraphy. In this study, X-ray fluorescence (XRF) core scanning is utilized to develop a new elemental data set for cyclo-stratigraphic investigation of Cenomanian-Turonian strata in the WIB, using material from the Aristocrat-Angus-12-8 core (north-central Colorado). The XRF data set yields the first continuous 5-mm-resolution analysis of lithogenic, biogenic, and syngenetic-authigenic proxies through the uppermost Lincoln Limestone Member, the Hartland Shale Member, and the Bridge Creek Limestone Member, including oceanic anoxic event 2 (OAE 2). The 40Ar/39Ar ages from ashes in three biozones, including a new age from the Dunveganoceras pondi biozone (uppermost Lincoln Limestone Member), provide geochronologic constraints for the cyclostratigraphic analysis. Astrochronologic testing of the 5-mm-resolution XRF weight percent CaCO3 data via average spectral misfit analysis yields strong evidence for astronomical influence on climate and sedimentation. Results from the Bridge Creek Limestone Member are consistent with the previously published astrochronology from the U.S. Geological Survey #1 Portland core (central Colorado), and identification of an astronomical signal in the underlying Hartland Shale Member now permits extension of the WIB astrochronology into the earlier Cenomanian, prior to OAE 2. High rates of sedimentation in the Angus core during the interval of OAE 2 initiation, as compared to the Portland core, allow recognition of a strong precessional control on bedding development. As a consequence, the new results [ABSTRACT FROM AUTHOR]

This study revises and improves the chrono-stratigraphic framework for late Turonian through early Campanian time based on work in the Western Interior U.S. and introduces new methods to better quantify uncertainties associated with the development of such time scales. Building on the unique attributes of the Western Interior Basin, which contains abundant volcanic ash beds and rhythmic strata interpreted to record orbital cycles, we integrate new radioisotopic data of improved accuracy with a recently published astrochronologic framework for the Niobrara Formation. New 40Ar/39Ar laser fusion ages corresponding to eight different ammonite biozones are determined by analysis of legacy samples, as well as newly collected material. These results are complemented by new U-Pb (zircon) chemical abrasion-isotope dilution-thermal ionization mass spectrometry ages from four biozones in the study interval. When combined with published radioisotopic data from the Cenomanian-Turonian boundary, paired 206Pb/238U and 40Ar/39Ar ages spanning Cenomanian to Campanian time support an astronomically calibrated Fish Canyon sanidine standard age of 28.201 Ma. Stage boundary ages are estimated via integration of new radioisotopic data with the floating astrochronology for the Niobrara Formation. The ages are determined by anchoring the long eccentricity bandpass from spectral analysis of the Niobrara Formation to radioisotopic ages with the lowest uncertainty proximal to the boundary, and adding or subtracting time by parsing the 405 k.y. cycles. The new stage boundary age determinations are: 89.75 ± 0.38 Ma for the Turonian-Coniacian, 86.49 ± 0.44 Ma for the Coniacian-Santonian, and 84.19 ± 0.38 Ma for the Santonian-Campanian boundary. The 2σ uncertainties for these estimates include systematic contributions from the radioisotopic measurements, astrochronologic methods, and geologic uncertainties (related to stratigraphic correlation and the presence of hiatuses). The latter geologic uncertainties have not been directly addressed in prior time scale studies and their determination was made possible by critical biostratigraphic observations. Each methodological approach employed in this study--new radioisotopic analysis, stratigraphic correlation, astrochronology, and ammonite and inoceramid biostratigraphy--was critical for achieving the final result. [ABSTRACT FROM AUTHOR]

The Salamanca Formation of the San Jorge Basin (Patagonia, Argentina) preserves critical records of Southern Hemisphere Paleo cene biotas, but its age remains poorly resolved, with estimates ranging from Late Cretaceous to middle Paleocene. We report a multi-disciplinary geochronologic study of the Salamanca Formation and overlying Río Chico Group in the western part of the basin . New constraints include (1) an 40Ar/39Ar age determination of 67.31 ±0.55 Ma from a basalt fl ow underlying the Salamanca Formation, (2) micropaleontological results indicating an early Danian age for the base of the Salamanca Formation, (3) laser ablation HR-MC-ICP-MS (high resolutionmulti collector-inductively coupled plasmamass spectrometry) U-Pb ages and a highresolution TIMS (thermal ionization mass spectrometry) age of 61.984 ±0.041(0.074) [0.100] Ma for zircons from volcanic ash beds in the Peñas Coloradas Formation (Río Chico Group), and (4) paleomagnetic results indicating that the Salamanca Formation in this area is entirely of normal polarity, with reversals occurring in the Río Chico Group. Placing these new age constraints in the context of a sequence stratigraphic model for the basin, we correlate the Salamanca Formation in the study area to Chrons C29n and C28n, with the Banco Negro Inferior (BNI), a mature widespread fossiliferous paleosol unit at the top of the Salamanca Formation, corresponding to the top of Chron C28n. The diverse paleo botanical assemblages from this area are here assigned to C28n (64.67- 63.49 Ma), 2-3 million years older than previously thought, adding to growing evidence for rapid Southern Hemisphere fl oral recovery after the Cretaceous-Paleogene extinction. Important Peligran and "Carodnia" zone vertebrate fossil assemblages from coastal BNI and Peñas Coloradas exposures are likely older than previously thought and correlate to the early Torrejonian and early Tiffanian North American Land Mammal Ages, respectively. [ABSTRACT FROM AUTHOR]

Abstract: New paleomagnetic measurements, coupled with Argon–Argon (40Ar/39Ar) radioisotopic dating, are revolutionizing our understanding of the geodynamo by providing detailed terrestrial lava records of the short-term behavior of the paleomagnetic field. As part of an investigation of the Koolau Volcano, Oahu, and the short-term behavior of the geomagnetic field, we have sampled a long volcanic section located on the volcano''s buttressed flank within Halawa Valley. Prior paleomagnetic and Potassium–Argon (K–Ar) investigations of the Koolau Volcanic Series revealed excursional directions The alkaline composition of lava flows, easy access, and close geographical proximity to K–Ar dated lava flows made this newly studied 120-m thick sequence of flows an excellent candidate for detailed paleomagnetic analysis. At least eight samples, collected from each of 28 successive flow sites, were stepwise demagnetized by both alternating field (5 mT to 100 mT) and thermal (from 28 °C to 575–650 °C) methods. Mean directions were obtained by principal component analysis. All samples yielded a strong and stable ChRM trending towards the origin of vector demagnetization diagrams based on seven or more demagnetization steps, with thermal and AF results differing insignificantly. Low-field susceptibility vs. temperature (k–T) analysis conducted on individual lava flows indicated approximately half with reversible curves. Curie point determinations from these analyses revealed a temperature close to or equal to 580 °C, indicative of almost pure magnetite ranging from single domain (SD) to pseudosingle domain (PSD) grain sizes for most of the flows. The mean directions of magnetization of the entire section sampled indicate a reversed polarity, with ∼10 m of the section characterized by excursional directions (5 lava flows). The corresponding VGPs are located off the southeastern part of Africa, close to Madagascar. 40Ar/39Ar incremental heating experiments on groundmass from nine flow sites at different stratigraphic levels yields isochrons between 2.64±0.23 to 2.37±0.17 Ma with a weighted mean age of 2.514±0.039 Ma, which, combined with the overall reversed polarity and the absence of polarity reversals, strongly suggests that the excursion corresponds to Cryptochron C2r.2r-1 [S. Candle and D.V. Kent, A New Geomagnetic Polarity Time Scale for the Late Cretaceous and Cenozoic, Jour. Geophys Res., 100(1995), 6093–6095.]. Current dating is insufficiently precise to unambiguously define this excursion as a Microchron (<10 kyr). This is potentially the first terrestrial record of Cryptochron C2r.2r-1 and the age is 2–3% older than in the reported timescale [S. Candle and D.V. Kent, A New Geomagnetic Polarity Time Scale for the Late Cretaceous and Cenozoic, Jour. Geophys Res., 100(1995), 6093–6095.]. Our finding places important constraints on the evolution of the entire Koolau shield edifice, since most of the lavas recorded a reversed polarity. [Copyright &y& Elsevier]

• 1890 Ma minimum age of Paleoproterozoic Pembine ophiolite. • Post-Penokean 1817 Ma mafic dikes indicate extension prior to geon 17 Yavapai accretion. • Mafic dikes strongly deformed and metamorphosed during Yavapai accretion. • Subsequent geon 16 Mazatzal and geon 14 Wolf River overprinting was lower grade. • Stabilization of Proterozoic provinces at geon 11–10 related to widespread magmatic underplating. New geochronologic data in the southern Lake Superior region provide key information on the timing and nature of tectonic activity that pre-and post-date initial Paleoproterozoic growth of Laurentia during the geon 18 Penokean orogeny. The obducted Pembine ophiolite formed along the edge of a Paleoproterozoic ocean basin at least 30 m.y. prior to Penokean island arc/microcontinent accretion beginning at 1860 Ma. Following Penokean orogenesis, intrusion of mafic dikes at 1817 ± 2 Ma indicate a period of extension that coincided with a 30 m.y. gap in orogenic felsic magmatism at 1835–1805 Ma (between the Penokean and Yavapai orogenies) and likely represents relaxation of Penokean compression and a tectonic switch to intra-arc extension related to initiation of Yavapai subduction. Subsequent Yavapai arc accretion (1750–1720 Ma) resulted in pervasive ductile deformation of the dikes and host rocks at temperatures of ~700 °C, previously attributed to Penokean deformation. Geon 16 Mazatzal overprinting of the accreted Penokean and Yavapai provinces was widespread but of overall lower metamorphic grade (greenschist facies), and the thermal effects of the 1476–1470 Ma shallow level Wolf River batholith was limited to a 10–15 km wide contact zone surrounding the intrusion. In contrast to the Archean Superior Province to the north, Paleoproterozoic terranes in the southern Lake Superior area experienced widespread low-temperature reheating and cooling of shallow crustal levels at ca. 1.1–1.0 Ga attributed primarily to magmatic underplating with little subsequent Neoproterozoic exhumation. In the southern Lake Superior region widespread magmatic underplating likely thickened, strengthened, and stabilized Proterozoic Penokean-Mazatzal lithosphere but destabilized Archean cratonized Superior Province lithosphere to the north. [ABSTRACT FROM AUTHOR]

Coastal fluvial and deltaic deposits are locally incomplete recorders of elapsed time, but where detailed three-dimensional stratigraphic geometries can be constructed from well sections or high-resolution seismic data, and dated using high-precision chronostratigraphic methods, such as 14C dating, the construction of three-dimensional time-space (Wheeler) diagrams reveals important insights regarding the relationships between basin architecture, accommodation rates, sedimentation rates, stratigraphic preservation, and the nature of autogenic and allogenic processes. Analysis of the time-stratigraphic history of point-sourced delta systems, using strike and dip cross sections, is used to evaluate the completeness of the stratigraphic record to resolve how basins fill, how vertical stratigraphic successions record time, and which surfaces record the longest hiatuses. The paper includes analyses and comparison of Quaternary and Cretaceous river and delta systems. Supply-dominated systems fed by rivers that are free to avulse, like the Holocene highstand Mississippi Deltas in the Gulf of Mexico, show a strong autogenic record. In the Mississippi deltas, a quasi-continuous Holocene stratigraphic record can be reconstructed but this record is distributed among a series of laterally offset delta complexes and component delta lobes. Although 1D vertical sections may sample less than 25% of the total time, and in many cases less than a few percent, a hiatus in one section typically correlates to deposits elsewhere. Deltas fed by entrenched rivers, and especially the lowstand shelf edge systems in the Gulf of Mexico, as well as the Mahakam in Kalimantan Indonesia, form a series of laterally-offset lobes at the shelf edge that look superficially similar in their pattern and areal scale to the autogenic lobes of the Mississippi delta, but the time stratigraphic relationships are very different. Rather than a quasi-complete time-stratigraphic record, each shelf edge lobe is interpreted as an allogenic lowstand delta, deposited over tens of thousands, versus thousands of years, and are bounded by a prolonged non-depositional hiatus, time equivalent to highstand deposition confined to the inner shelf. Analysis of strike cross sections within an allogenic lobe of the Lagniappe lowstand delta reveals an autogenic signal, similar, but at a smaller scale than the Mississippi. In 1D, the progradational part of an upward coarsening deltaic parasequence records the time it takes the delta to prograde past a given point, which is estimated to be a few hundred years, representing only a few percent of the time associated with progradation and transgression of a parasequence. Nevertheless, this part of the succession records the highest rates of deposition (typically 10–50 m/ka). Much of the time associated with a parasequence is tied up at the top surfaces that record periods of bypass, as the lobe progrades farther seaward, as well as transgressive surfaces of erosion and deepening. The trangressive units deposit at much slower rates (1 m/ka) recorded by highly bioturbated condensed sections. Analysis of fluvial and deltaic floodplains and channel belts also show a highly incomplete record (as little as 5% of elapsed time), reflecting the observation that rivers only occupy a small part of any given floodplain at any given time, especially when rivers are entrenched and floodplains may be exposed to form paleosols. Entrenched river deposits, such as valley fills, may record only a few percent of the total time, indicating that most of the valley records bypass and erosion. Aggrading floodplains, which may be more common during highstands of sea level, may contain more complete records, including peats and coals, with up to 50% of time recorded by sediment. A tentative reconstruction of the development of the fluvial Canterbury Plain of South Island, New Zealand, is offered as an example of the highly discontinuous nature of alluvial sedimentation. [ABSTRACT FROM AUTHOR]

Accurate and precise ages of large silicic eruptions are critical to calibrating the geologic timescale and gauging the tempo of changes in climate, biologic evolution, and magmatic processes throughout Earth history. The conventional approach to dating these eruptive products using the 40 Ar/ 39 Ar method is to fuse dozens of individual feldspar crystals. However, dispersion of fusion dates is common and interpretation is complicated by increasingly precise data obtained via multicollector mass spectrometry. Incremental heating of 49 individual Bishop Tuff (BT) sanidine crystals produces 40 Ar/ 39 Ar dates with reduced dispersion, yet we find a 16-ky range of plateau dates that is not attributable to excess Ar. We interpret this dispersion to reflect cooling of the magma reservoir margins below ∼475 °C, accumulation of radiogenic Ar, and rapid preeruption remobilization. Accordingly, these data elucidate the recycling of subsolidus material into voluminous rhyolite magma reservoirs and the effect of preeruptive magmatic processes on the 40 Ar/ 39 Ar system. The youngest sanidine dates, likely the most representative of the BT eruption age, yield a weighted mean of 764.8 ± 0.3/0.6 ka (2σ analytical/full uncertainty) indicating eruption only ∼7 ky following the Matuyama-Brunhes magnetic polarity reversal. Single-crystal incremental heating provides leverage with which to interpret complex populations of 40 Ar/ 39 Ar sanidine and U-Pb zircon dates and a substantially improved capability to resolve the timing and causal relationship of events in the geologic record., Competing Interests: The authors declare no conflict of interest.

We present four new 40 Ar/ 39 Ar ages of tephra layers from an aggradational succession (Valle Giulia Formation) near the mouth of the Tiber Valley in Rome that was deposited in response to sea-level rise during Marine Isotopic Stage (MIS) 13. These new ages, integrated with seven previously determined ages, provide the only extant independent, radioisotopic age constraint on glacial termination VI and on the duration of MIS 13 sea-level rise. The new geochronologic constraints suggest a long duration for the period of sea-level rise (533 ± 2 through 498 ± 2 ka) encompassing two consecutive positive peaks of the δ 18 O curve (substages 13.3 and 13.1). Consistently, the litho-stratigraphic features of the sedimentary record account for two aggradational phases separated by an intervening erosional phase. Moreover, the ages obtained for this study give us the opportunity to compare the timing of the sea-level fluctuations inferred from the stratigraphic record and that provided by the astrochronologic calibration of the Oxygen isotopic curves, and to assess the calibrations of 40 Ar/ 39 Ar standards. Results of this comparison indicate that the best match is for an age of 1.186 Ma for the Alder Creek Rhyolite sanidine and 28.201 Ma for the Fish Canyon Tuff sanidine.