Age and palaeoenvironmental constraints on the genesis of the Yandi channel iron deposits, Marillana Formation, Pilbara, northwestern Australia

The fluvial goethite‐hematite channel iron deposits (CID) of fhe Pilbara region of Western Australia, formerly known as the ‘Robe Pisolite’, represent a major source of iron ore mined in the Hamersley Province. The CID occupy Early Tertiary palaeochannels that are typically less than 1 km but range to several kilometres in width and from 1 to ∼⃒100 m thick. The Robe palaeochannel is the longest, with CID partly preserved over a distance of 150 km, whereas the Marillana palaeochannel contains CID along 80 km of its length. The CID range from goethitic mudstone to fine hematite‐goethite gravel and intraformational conglomerate varying in distribution along the channels in a range of massive, bedded and altered types. The granular type that forms the bulk of the ore is typically ooidal, comprising pelletoids with goethite cortices around hematite nuclei (which often consist of fossilised wood), with typically abundant coarse goethitised wood fragments, a varied proportion of peloids, minor pisoids, and a diverse porous goethitic matrix. The Robe deposits tend to have more spherical pelletoids, with more regularly developed layering, and smaller cores than CID from the Yandi deposits of the Marillana valley. Generally, the presence of ferruginised fossil wood is the principal diagnostic field criterion used to distinguish CID from other comparable goethite‐hematite detrital deposits. Electron microprobe X‐ray maps reveal that lattice Al and Si increase from the hematitic nuclei to the matrix, and then the cortex. In the Robe CID, alumina in the nuclei varies between 0.5 and 2.8%, whereas at Yandi, alumina in the nuclei ranges from 0.2 to 0.7%. The goethite cortex of a granule is usually richer in both alumina and silica than its nucleus, in a cortex : nucleus ratio of ∼⃒2:1 at Robe and 3:1 at Yandi. The nucleus and cortex of individual pelletoids generally exhibit a similar trace‐element distribution, which can often be related to the surrounding country rock and, with the exception of Al, is different to that shown by the matrix. A visual classification (labelled co‐manual) based on the relative amount of cortex (co), matrix (ma) and nucleus (nu) enables an estimate of the deleterious minor elements, mainly aluminium (al), for separating samples into ore, near‐ore and non‐ore.

Miocene fluvial goethite/hematite channel iron deposits (CID) are part of the Cenozoic Detritals 2 (CzD2), of the Western Australian Pilbara region. They range from gravelly mudstones through granular rocks to intraformational pebble, cobble and rare boulder conglomerates, as infill in numerous meandering palaeochannels in a mature surface that includes Precambrian granitoids, volcanics, metasediments, BIF and ferruginous Palaeogene valley fill. In the Hamersley Province of the Pilbara, the consolidated fine gravels and subordinate interbedded conglomerates, with their leached equivalents, are a major source of export iron ore. This granular ore typically comprises pedogenically derived pelletoids comprising hematite nuclei and goethite cortices (ooids and lesser pisoids), with abundant coarser goethitised wood/charcoal fragments and goethitic peloids, minor clay, and generally minimal porous goethitic matrix, with late-stage episodic solution and partial infill by secondary goethite, silica and siderite (now oxidised) in places. Clay horizons and non-ore polymictic basal and marginal conglomerates are also present. The accretionary pedogenic pelletoids were mostly derived from stripping of a mature ferruginous but apparently well-vegetated surface, developed in the Early to Middle Miocene on a wide variety of susceptible rock types including BIF, basic intrusives and sediments. This deep ferruginisation effectively destroyed most remnants of the original rock textures producing a unique surface, very different to those that produced the underlying CzD1 (Palaeogene) and the overlying CzD3 (Pliocene – Quaternary). The peloids were derived both intraformationally from fragmentation and reworking of desiccated goethite-rich muds, and from the regolith. Tiny wood/charcoal fragments replaced in soil by goethite, and dehydrated to hematite, formed nuclei for many pelletoids. Additionally, abundant small (≤10 mm) fragments of wood/charcoal, now goethite, were probably replaced in situ within the consolidating CID. This profusion of fossil wood, both as pelletoid nuclei and as discrete fragments, suggests major episodic wild fires in heavily vegetated catchments, a point supported by the abundance of kenomagnetite – maghemite developed from goethite in the pelletoids, but less commonly in the peloids. The matrix to the heterogeneous colluvial and intraformational components is essentially goethite, primarily derived from modified chemically precipitated iron hydroxyoxides, resulting from leaching of iron-rich soils in an organic environment, together with goethitic soil-derived alluvial material. Major variations in the granular ore CID after deposition have resulted from intermittent groundwater flow in the channels causing dissolution and reprecipitation of goethite and silica, particularly in the basal CID zones, with surface weathering of eroded exposures playing a role in masking some of these effects. However, significant variations in rock types in both the general CID and the granular ore CID have also resulted from the effects of varied provenance.

Neogene planktonic foraminiferal biostratigraphy and evolution: Equatorial to subantarctic, South Pacific

The results of palynological study of the reference section of upper Paleogene and Neogene deposits of the Kulunda Plain, exposed by borehole 2 (settlement Ozeryanka, Novosibirsk Oblast) are presented. In the Tavda Formation, a dinoflagellate cysts assemblage of late Priabonian age and a palynoassemblage with Quercus gracilis–Q. graciliformis of the late Eocene were identified. Nine palynoassemblages have been identified from continental Oligocene and Neogene deposits: Carya spackmania–Carpinus perfectus–Tilia of the beginning of the second half of early Oligocene; Betula–Corylus–Pinus s/g Haploxylon of the second half of early Oligocene; Juglans sieboldianiformis–Pterocarya stenopteroides–Fagus of the end of the early Oligocene, possibly the beginning of the late Oligocene; Castanea–Quercus–Myrica of late Oligocene; Pinus s/g Haploxylon–Abietinieaepollenites sellowiiformis–Cupressaceae of early Miocene, presumably the end of late Oligocene; Alnus–Ulmus–Polypodiales of early–middle Miocene; Betula–Quercus–Ulmus of middle Miocene and Alnus–Polypodiales–Sigmopollis of middle–late Miocene; Betula–Artemisia–Amaranthaceae of late Miocene. Layers with freshwater dinocysts Pseudokomewuia sp. 1 were found at the top of the Zhuravka Formation. The deposition environment in the late Eocene, Oligocene and Miocene in the south of the West Siberian Plain has been reconstructed. The marine transgression in the Priabonian extended to the north of the modern Kulunda Plain. The first half of the Early Oligocene in the region experienced a hiatus. After a significant cooling at the Eocene–Oligocene boundary, climatic conditions again became warm and humid as evidenced by the distribution the growth of mesophytic coniferous-broadleaf forests with hickory. In the second half of the early Oligocene, the climate became colder and more humid, and the proportion of elements of the Arcto- Tertiary flora increased in plant communities. During the end of the early Oligocene and the late Oligocene, the climate became warmer, and broadleaf trees dominated the forests. At the end of the late Oligocene, the climate again became more humid, but remained warm, and pine forests predominated in phytocenoses, with the participation of ancestral forms of modern Cathaya. The cooling at the turn of the late Oligocene–early Miocene led to the predominance of conifers in forests; in the early Miocene, the proportion of small-leaved tree species increased sharply, and the participation of pine trees decreased. In the middle Miocene, the climate remained quite warm, but drier, and cypress trees disappeared from the plant communities. In the late Miocene, open plant communities are formed.

Lithostratigraphy and planktonic foraminiferal biostratigraphy of the late Eocene-Middle Miocene sequence in the area between Wadi Al Zeitun and Wadi Al Rahib, Al Bardia area, northeast Libya

Variations in elemental and mineralogical compositions of Late Oligocene, Early and Middle Miocene coal seams in the Kale-Tavas Molasse sub-basin, SW Turkey

Influence of periatlantic climates and paleoclimates on the distribution and mineralogical composition of bauxites and ferricretes

Recovery and interpretation of the 18O/16O of Miocene oolitic goethites in multi-generational mixtures of Fe (III) oxides from a channel iron deposit of Western Australia

Late cretaceous to recent palaeoenvironments of the Saudi Arabian Red Sea

D/H of late Miocene meteoric waters in Western Australia: Paleoenvironmental conditions inferred from the δD of (U-Th)/He-dated CID goethite

<p>During the Cenozoic Era (the last 65 Ma), Antarctica’s climate has evolved from ice free conditions of the ‘Greenhouse world’, which at its peak (~ 55 Ma) supported near-tropical forests, to the ‘Icehouse’ climate of today with permanent ice sheets, and a very sparse macroflora. This long-term cooling trend is punctuated by a number of major, abrupt, and in some cases, irreversible climate transitions. Reconstructing past changes in vegetation, sea surface temperature, hydroclimate and the carbon cycle require robust geological proxies that in turn can provide insights into climatic thresholds and feedbacks that drove major transitions in the evolution of Antarctica’s ice sheets. Biomarkers allow climate and environmental proxy reconstructions for this region, where other more traditional paleoclimate methods are less suitable. This study has two aims. Firstly to assess the suitability and applicability of biomarkers in Antarctic sediments across a range of depositional settings and ages, and secondly to apply biomarker-based climate proxies to reconstruct environmental and climate conditions during key periods in the development of the Antarctic Ice Sheets. The distribution and abundances of n-alkanes are assessed in Oligocene and Miocene sediments from a terrestrial outcrop locality in the Transantarctic Mountains, and two glaciomarine sediment cores and an ice-distal deep marine core from the western Ross Sea. Comparisons are made with n-alkane distributions in Eocene glacial erratics and sedimentary rocks of the Mesozoic Beacon Supergroup, both likely sources of reworked material. A shift in dominant chain length from n-C₂₉ to n-C₂₇ occurs between the Late Eocene and Early Oligocene, considered a response to a significant climate cooling. Samples from glaciofluvial environments onshore, and subglacial and ice-proximal environments offshore display a reworked n-alkane distribution, characterised by low carbon preference index (CPI), high average chain length (ACL) and high n-C₂₉/n-C₂₇ values. Whereas, samples from lower-energy, more benign lacustrine and ice-distal marine environments predominantly contained contemporary material. Palynomorphs and biomarker proxies based on n-alkanes and glycerol dialkyl glycerol tetraethers (GDGTs) are applied to a Late Oligocene and Early Miocene glaciomarine succession spanning the large transient excursion of the Mi-1 glaciation (~23 Ma) in DSDP Site 270 drill core from the central Ross Sea. While the Late Oligocene is marked by relatively warm conditions, regional cooling initiated a transition into Mi-1. This was likely driven by a combination of decreasing atmospheric CO₂ and an orbital geometry favouring low seasonality and cool summers, leading to an intensification of proto-Antarctic bottom water production as the Ross Sea deepened and cooled. Mi-1 manifests as a regionally cool period, with minimum subsurface temperatures of ~4°C and onshore mean summer temperatures of ~8°C. A negative n-alkane δ¹³C excursion of up to 4.8‰ is interpreted as a vegetation response to cold, restricted growing seasons, with plants driven to lower altitudes and more stunted growth forms. However, ocean temperatures remained too warm for marine-based ice sheets to advance onto the outer continental shelf and over-ride the drill site. The large increase in ice volume associated with this event, implied by global δ¹⁸O records, was probably held on a higher, terrestrial West Antarctica of greater extent than present day. The relative lack of ice rafted debris during Mi-1, suggests the presence of a marginal marine-terminating ice sheet with fringing ice shelves to the south of DSDP site 270, calving icebergs lacking a basal debris layer, similar to those calving from the Ross Ice Shelf today. This extensive ice cover may explain a large decrease in marine n-alkanes at this time restricting marine productivity on the continental shelf. The biomarker data for the Early Miocene in DSDP 270 indicates a relative warming in both terrestrial and marine temperatures following the transient Mi-1 glacial expansion, but an overall baseline cooling of climate between Late Oligocene and the Early Miocene in the Ross Sea embayment. Isoprenoid GDGTs are used to reconstruct a Cenozoic subsurface ocean temperature compilation for the Ross Sea, a key source region of ocean deep water. The ocean temperature TEXL86 calibration and BAYSPAR in standard subsurface mode were considered, through comparison with independent microfossil and sedimentological data, the most appropriate for use in this region. Ocean temperatures cool prior to the Eocene/Oligocene transition and remain cool for the rest of the Cenozoic, with the exception of short periods of relative warmth in the Late Oligocene and Mid-Miocene Climate Optimum, and long-term trends broadly mirror that of the foraminiferal δ¹⁸O record from the deep Pacific. The Δ Ring Index is used to assess non-thermal influences on GDGT distributions, and displays a long term shift from more positive to more negative deviations. This correlates with %GDGT-0, and also relates to a declining trend in the Methane Index, which reflect the contribution of methanogenic and methanotrophic archaea. These changes suggest that these archaea contributed more to the archaeal community in the early to mid Cenozoic, potentially indicating a more anoxic depositional environment in the Ross Sea. The Branched to Isoprenoid Tetraether index (BIT) steadily declines over the Cenozoic, reflecting increasingly hyper-arid conditions onshore, with less active glaciofluvial systems, limited soil development and less ice-free land.</p>

The Gulf of California is a young example of crustal stretching and transtensional shearing leading to the birth of a new oceanic basin at a formerly convergent margin. Previous studies focused along the southwestern rifted margin in Baja California indicated rifting was initiated after subduction and related magmatism ceased at ca. 14–12.5 Ma. However, the geologic record on the Mexico mainland (Sinaloa and Nayarit States) indicates crustal stretching in the region began as early as late Oligocene. The timing of cooling and exhumation of pre- and synrift plutonic rocks can provide constraints on the timing and rate of rifting. Here, we present results of a regional study on intrusive rocks in the southern Gulf of California sampled along the conjugate Baja California and Nayarit-Sinaloa rift margins, as well as plutonic rocks now exposed on submerged rifted blocks inside the gulf. Forty-one samples were dated via U/Pb zircon and 40Ar/39Ar mineral ages, providing emplacement age and thermochronological constraints on timing and rate of cooling. We found an extensive suite of early and middle Miocene plutons emplaced at shallow depths within the basement Cretaceous–Paleocene Peninsular Range and Sinaloa-Jalisco Batholiths. Early Miocene granitoids occur in an elongated WNW-ESE belt crossing the entire southern gulf from southern Baja California to Nayarit and Sinaloa. Most have an intermediate composition ( 75 SiO2 wt%) was emplaced 20.1–18.3 Ma, near the end of the early Miocene. Age span and chemical composition of the early Miocene silicic plutons essentially overlap ignimbrites and domes exposed in the southern Sierra Madre Occidental and in southern Baja California, suggesting that eruptive sources for the early Miocene ignimbrite flare-up may also have been located within the southern Gulf of California. Early Miocene plutons cooled below the 40Ar-39Ar biotite closure temperature (350–400 °C) in less than 2.5 m.y., which we interpret as evidence of a regional extensional event leading to the opening of the Gulf of California. A less widely distributed suite of intermediate-composition, middle Miocene granitoids (15–13 Ma) was sampled from the central-western part of the gulf, west of the Pescadero Basin, and these correspond to an episode of scarce volcanism recorded by the middle and upper members of the onshore Comondu Group in Baja California. Our widely spaced sampling of the generally sediment-covered igneous crust suggests that middle Miocene primary volcanic rocks are much less abundant than implied by previous models in which the gulf was the site of a robust Comondu arc. Thermobarometry data also indicate a very shallow depth (<5 km) of emplacement for the middle Miocene plutonic rocks. Some of these rocks also show a distinctive inequigranular texture indicative of at least two crystallization stages at different pressure. Early and middle Miocene granitoids away from the gulf axis yielded 40Ar-39Ar cooling ages very close to U-Pb zircon ages, demonstrating rapid cooling to <350 °C, which we attribute to their shallow emplacement and, possibly, to exhumation soon after intrusion. Since Comondu-age and middle Miocene magmatism in the gulf region coincided with rapid cooling of young plutons that predate the end of subduction, we suggest that intense crustal stretching controlled the pattern and timing of Comondu-age magmatism, rather than the middle Miocene magmatism controlling the locus of <12 Ma extension.

The distribution of foraminifera in the Uscari, Rio Banano, Moin Formations of the Limon Basin, Costa Rica, is the subject of this study. Heterostegina antillea and Lepidocyclina (L.) waylandvaughani place the basal Uscari Formation of Quebrada Terciopelo in Zone N3 of the latest Oligocene. The overlap of Orbulina universa and Globorotalia fohsi peripheroronda places the type section Uscari Formation in the early Middle Miocene (zones N9 to N10). The overlapping ranges of planktonic foraminifera and the ostracods Radimella ovata in the sandstones places that unit in Zone N18 of Early Pliocene. Globorotalia truncatulinoides excelsa indicates a Pleistocene age for the Moin Formation. The algal-foraminiferal limestones of the basal Uscari Formation were deposited on a shallow shelf receiving volcaniclastic sediment from the emerging islands of the Central American arc. During the Late Oligocene and Early Miocene the Linon Basin quickly subsided. In the Quebrada Terciopelo reference section of the Uscari Formation, Early Miocene foraminiferal indicators of a middle to upper bathyal environment include Siphogenerina transversa and Melonis pompilioides; late early and early Middle Miocene faunas (Uvigerina peregrina, Bolivina pseudoplicata, and Buliminella bassendorfensis indicate progressive shoaling. For the Early Miocene Rio Reventazon section of the Uscari Formation, species such as Cibicides wuellerstorfi and Melonis pompilioides indicate depths near 2000 meters. The type section of the Uscari Formation in Quebrada Uscari is dominated by an outer shelf assemblage containing Hanzawaia concentrica and Fursenkoina pontoni. The Early Pliocene sandstones of the Rio Banano Formation were deposited on a shallow, current-swept continental shelf. These rocks contain a mixture of open marine, nearshore, and a few reefal species, including Cibicidoides floridanus, Amphistegina gibbosa, Articulina mayori, and Elphidium discoidale. The claystones of the Pleistocene Moin Formation contain outer shelf to upper slope species, such as Gyroidina soldani, Cibicidoides floridanus, and Cassidulina curvata. The foraminiferal record of the Rio Banano sandstones suggests land was emergent in the southern Limon Basin, separating the Caribbean and Pacific basins, by 5 mybp, before such an event occurred in Panama.

Causes of lineage decline in the Aplodontidae: Testing for the influence of physical and biological change

Miocene terrestrial mammals are poorly known from the Atlantic Coastal Plain. Fossils of the Order Carnivora from this time and region are especially rare. We describe a carnivoran mandible with a p4 from the late Oligocene or early early Miocene Belgrade Formation in Jones County, North Carolina. Comparisons are made with carnivoran jaws with similar premolar and molar lengths from the late Oligocene and Miocene of North America and Eurasia. These indicate that the North Carolina jaw is assignable to the Ailuridae, a family whose only living member is the red panda. The jaw is tentatively referred to Amphictis, a genus known elsewhere from the late Oligocene and early Miocene of Europe and the early Miocene (Hemingfordian) of North America.The North Carolina mandible compares best with the late Oligocene (MP 28) Amphictis ambiguus from Pech du Fraysse, France, the oldest known member of the Family Ailuridae, and with the early Miocene (MN 1–MN 2a) A. schlosseri from southwestern Germany. This identification is compatible with a late late Arikareean (Ar4, early Miocene, MN 2-3 equivalent) age assignment for the other terrestrial mammals of the Belgrade Formation.