Pliocene Age Research Articles

The kinematic significance of rotation-related deformation features in a fault-defined wedge associated with the North Anatolian Fault, central Turkey

Along most of its 1150km length the North Anatolian Fault Zone (NAFZ) of northern Turkey separates the Anatolian platelet from the Eurasian plate. In its eastern sector this fault zone displays several major splays, including the Kırıkkale-Erbaa (Sungurlu) Fault Zone. The region investigated in this study forms a wedge delimited by the NAFZ, the Kırıkkale-Erbaa splay and the Elmadag Imbricate Thrust Zone and this tectonically defined wedge also hosts the greater part of an important Cenozoic sedimentary trough, the Çankiri-Çorum Basin. Detailed study of an area in the southern part of this basin, between Delice and Salmanli, has provided evidence for the complex neotectonic evolution of this region, including details of the nature and behaviour of the Kırıkkale-Erbaa Fault (KEF). Evidence of significant neotectonic deformation has been observed in basin-fill strata of Eocene to Late Pliocene age, mostly within a zone of high strain adjacent to the KEF. These features include young thrusting of Eocene limestones on to Miocene units, tight upright and overturned folds within the fault zone that involve Eocene–Pliocene strata, and kinematic indicators from the KEF master fault plane. These and related morpho-tectonic features demonstrate that this young phase of deformation is predominantly compressive/transpressive in character in this area.It is concluded that the local compressive/transpressive stress field has been created by anticlockwise rotation of a wedge-shaped block (fault wedge) lying between the North Anatolian Fault Zone and its splay, the KEF, with compression maximised in a narrow zone close to the latter. Moreover, the north–south trending Elmadağ Imbricate Thrust Zone together with a buried structural high within the local basement has blocked the westwards translation of this tectonic wedge, thus creating further localised compression and inducing southwestwards ‘escape’ of the fault wedge block. Previously recorded differences in strain rates associated with the North Anatolian Fault Zone and the KEF, together with the north-convex curvature of the North Anatolian Fault Zone in this region are considered to be the factors primarily responsible for this anticlockwise rotation. This study demonstrates that previous geotectonic history plays a determinant role in the neotectonic behaviour of the individual structural elements present within the Anatolian collage, a conclusion that may be relevant to the understanding of strike-slip dominated domains in comparable geotectonic settings elsewhere.

Structural evolution of the northwestern Zagros, Kurdistan Region, Iraq: Implications on oil migration

ABSTRACT The studied area in Kurdistan Region of Iraq lies across an important topographic/structural boundary between the southern lowlands and the northern, folded and imbricated Zagros Mountains. It also encompasses a prominent change in structural orientation of the northern Zagros, from a general NW-SE “Zagros” to an E-W “Taurus” trend. Geological mapping and structural observations, both in the mountains (Mesozoic–Palaeogene) and in the lowlands (Neogene), led to the following conclusions. (1) The oldest recorded deformation is a layer-parallel shortening, coupled with southwest-vergent shear that was followed by major folding of ca. 10 km wavelength and ca. 1,000 m amplitude. Even the Upper Miocene–Pliocene Bakhtiari Formation has steep to overturned beds in some parts, and synclines preserve syn-tectonic strata of Neogene–Pliocene age. Box folding is associated with crestal collapse, internal thrusting in the core and with formation of systematic joint sets. (2) On the southern limb of the major folds, thrusting of variable offset can be observed. The thrusts on the southern and northern limbs are considered responsible for the major uplift during main folding. (3) En-échelon fold-relay patterns suggest left-lateral shear along the EW-oriented segment and right-lateral shear along the NW-oriented segment. (4) A quick-look qualitative analysis of striated fault planes suggests a variable shortening trend from NE-SW to N-S, and some rare NW-SE shortening all associated with thrust faults. (5) The general structural setting of the area is linked to the north-eastwards to northwards propagation of the Arabian Margin beneath Eurasia. The ca. 30° bend in the mountain chain may be explained by the original shape of the Arabian Margin, or by pre-existing tectonic zones of E-W orientation in the northern part. Several observations suggest that there was no oroclinal bending (i.e. major rotation) of different parts of the chain, but the structures simply molded on their local buttress (almost) according to present orientations. However, a limited amount of rigid-body rotation in the different segments cannot be ruled out. The changing shortening directions generated several structural combinations on both the NW-SE Zagros and the E-W Taurus segments of the arc, many of which are still preserved. (6) Spectacular bitumen seepage in Upper Cretaceous and Palaeocene limestone originates from fractures or geodes of these formations. Many of these bitumen-filled voids are linked to the above-described Late Neogene–Recent shortening-folding process; therefore hydrocarbon migration into these voids is interpreted to be very young. This contradicts earlier ideas about massive Late Cretaceous breaching and bleeding off of hydrocarbons in this region.

Alternating diatomaceous and volcaniclastic deposits in Milos Island, Greece. A contribution to the upper Pliocene–lower Pleistocene stratigraphy of the Aegean Sea

In the northern part of Milos Island, South Aegean Volcanic Arc, a stratigraphic succession spanning the late Pliocene and early Pleistocene exceeds 300m in thickness and records an alternation of products from submarine volcanic explosive events and fossiliferous, both siliceous and carbonate marine sedimentary rocks. The accumulation of volcaniclastic deposits took place under subaqueous marine conditions as evidenced by sedimentary features, scarce fossil remains and the local presence of burrow traces.Each major volcanic event was represented by the accumulation of thick-bedded pumice and glassy tuffs, succeeded by the deposition of marine sediments composed mainly of laminated diatomaceous marlstone and sandy limestone. The silica content of the diatomaceous sediments varies from 39% to 70%, mostly in the form of biogenic opal-A. SiO2/Al2O3 ratios ranging from 5 to 21 indicate a varied contribution of reworked volcanic grains. The sequential arrangement of basinal diatomaceous marlstone and shoreface sandy limestone deposits suggests that the depth of water was not great, which is also consistent with features observed in the volcaniclastic deposits.Calcareous nannofossils from the upper part of the succession of northern Milos allow biostratigraphic correlation with nannofossil biozones MNN19a–MNN19b across the Gelasian/Calabrian boundary within the early Pleistocene. A late Pliocene (Piacenzian) age has been estimated for the lower part of the section, where biozone MNN16a has been documented. Diatoms, planktonic foraminifera and calcareous nannofossil assemblages in the diatomaceous beds point to deposition in a partially closed, relatively shallow marine environment. In this setting, warm and stratified water conditions can be concluded for the Piacenzian diatomaceous beds whereas highly productive cool waters are inferred from the Gelasian/Calabrian diatomite.

Technical note: Late Pliocene age control and composite depths at ODP Site 982, revisited

Abstract. Ocean Drilling Program (ODP) Site 982 provided a key sediment section at Rockall Plateau for reconstructing northeast Atlantic paleoceanography and monitoring benthic δ18O stratigraphy over the late Pliocene to Quaternary onset of major Northern Hemisphere glaciation. A renewed hole-specific inspection of magnetostratigraphic reversals and the addition of epibenthic δ18O records for short Pliocene sections in holes 982A, B, and C, crossing core breaks in the δ18O record published for Hole 982B, now imply a major revision of composite core depths. After tuning to the orbitally tuned reference record LR04, the new composite δ18O record results in a hiatus, where the Kaena magnetic subchron might have been lost, and in a significant age reduction for all proxy records by 130 to 20 ky over the time span 3.2–2.7 million years ago (Ma). Our study demonstrates the general significance of reliable composite-depth scales and δ18O stratigraphies in ODP sediment records for generating ocean-wide correlations in paleoceanography. The new concept of age control makes the late Pliocene trends in SST (sea surface temperature) and atmospheric pCO2 at Site 982 more consistent with various paleoclimate trends published from elsewhere in the North Atlantic.

Pliocene Stratigraphy at Paredones Blancos: Significance of a Massive Crushed-Rhodolith Deposit on Isla Cerralvo, Baja California Sur (Mexico)

EMHOFF, K.F.; JOHNSON, M.E.; BACKUS, D.H., and LEDESMA-VÁZQUEZ, J., 2012. Pliocene stratigraphy at Paredones Blancos: Significance of a massive crushed-rhodolith deposit on Isla Cerralvo, Baja California Sur (Mexico).A white blaze across coastal cliffs is the hallmark of Paredones Blancos on Isla Cerralvo, Mexico. Cliff-forming strata include three roughly 10-m-thick units with a lateral coherence of only 0.75 km. The middle unit is a massive deposit of crushed rhodoliths. The other two units consist of matrix-supported conglomerate with cobbles and boulders of granodiorite, basaltic andesite, and hornblende diorite. By volume, the matrix accounts for >80% of those units, mostly composed of grus from weathered granite. Thin sections were studied from samples collected at five levels through the middle rhodolith unit to determine carbonate purity as a ratio between organic CaCO3 and inorganic minerals. The bottom and top margins of the deposit show higher levels of mixed clastics, with 37% inorganic mineral content, as compared with 11% toward the middle. Original depositional environments are interpreted as a rhodolith bank adjacent to a large fan delta built seaward from a wide canyon mouth. The stratigraphic sequence records a rise in sea level that brought rhodolith debris to the flooded canyon mouth above the basal conglomerate, and a drop that emplaced another conglomerate above the rhodolith deposit. A Middle Pliocene age is based on co-occurrence of Clypeaster bowersi and Argopecten revellei within the carbonates.

Cosmogenic nuclide dating of cave deposits of Mount Granier (Hauts de Chartreuse Nature Reserve, France): morphogenic and palaeogeographical implications

Mount Granier lies in the northeast corner of the Chartreuse Mountains. It contains a vast cave system, whose uppermost levels were thought to be of pre-Quaternary age. 10Be/26Al burial dating of two cave deposits in the oldest passages, widely spaced but belonging to a same gallery level which formed after the last folding phase, gave ages of 4.3 Ma and 3.4 Ma, indicating that these levels are of at least Pliocene age. In addition, the rates of denudation estimated according to the analytical data are unusually low in this alpine context but show an acceleration during the pre-Quaternary period of interest, ranging from 7 m/Ma to 14 m/Ma. Data from karst deposits serve as reference and comparison site for Alpine chronology as well as for cave genesis and palaeogeographical reconstructions, similar to that of the Siebenhengste massif in Switzerland.

Temperature, Pressure, and Composition of the Mantle Source Region of Late Cenozoic Basalts in Hainan Island, SE Asia: a Consequence of a Young Thermal Mantle Plume close to Subduction Zones?

Basaltic lavas from Hainan Island near the northern edge of the South China Sea have an age range of between late Miocene (about 13 Ma) and Holocene, with a peak age of late Pliocene to middle Pleistocene. The basaltic province is dominated by tholeiites with subordinate alkali basalts. Most analysed samples display light rare earth element (LREE) enriched REE patterns and ocean island basalt (OIB)-like incompatible element distributions. The basalts contain abundant undeformed high-Mg olivine phenocrysts (up to Fo90·7) that are high in CaO and MnO, indicating high-magnesian parental magmas. Independent barometers indicate that clinopyroxenes in the basalts crystallized over a wide range of pressures of 2–25 kbar (dominantly at 10–15 kbar) and that the melt cooled from about 1350°C to 1100°C during their crystallization. The compositional characteristics of the basalts indicate that their generation most probably involved both low-silica and high-silica melts, as represented by the alkali basalts and tholeiites, respectively. Our results show that the source region for the Hainan basalts is highly heterogeneous. The source for the tholeiites is mainly composed of peridotite and recycled oceanic crust, whereas the source for the bulk of the low-Th alkali basalts consists predominantly of peridotite and low-silica eclogite (garnet pyroxenite). Some high-Th (≥ 4 ppm) alkali basalts may have been produced by partial melting of low-silica garnet pyroxenite (eclogite). We estimated the primary melt compositions for the Hainan basalts using the most forsteritic olivine (Fo90·7) composition and the most primitive bulk-rock samples (MgO > 9·0 wt % and CaO >8·0 wt %), assuming a constant Fe–Mg exchange partition coefficient of KD = 0·31 and Fe3+/FeT = 0·1. The effective melting pressure (Pf) and melting temperature (T) of the primary melts are Pf = 18–32 kbar (weighted average = 23·8 ± 1·8 kbar) and T = 1420–1520°C for the tholeiites, and Pf = 25–32 (weighted average = 28·3 ± 1·4 kbar) and T = 1480–1530°C for the alkali basalts. The Pf –T data form an array that plots systematically above the dry lherzolite solidus but below the base of the lithosphere (∼55 km) and intersects the dry peridotite solidus at a pressure of about 50 kbar. The mantle potential temperature beneath Hainan Island, based on the estimate primary melt compositions, varies from about 1500 to 1580°C with a weighted average of 1541 ± 10°C. The high-magnesian olivine phenocrysts, high mantle potential temperature, and the presence of recycled oceanic crust in the source region provide independent support for the Hainan plume model that has previously been proposed largely based on geophysical observations. The Hainan plume thus provides a rare example of a young mantle plume associated with deep slab subduction.

Porphyry and Epithermal Deposits and 40Ar/39Ar Geochronology of the Baguio District, Philippines

The Baguio district is located in the Central Cordillera of northern Luzon, Philippines. It contains numerous mineralized porphyry copper-gold, epithermal gold-silver and skarn gold-lead-zinc deposits. The district is floored by Cretaceous-Eocene metavolcanic and metasedimentary rocks, which are overlain by marine to terrestrial sedimentary and volcanic rocks of early Miocene to Pliocene ages. Tertiary arc magmatism related to east-directed subduction of the South China Sea plate along the Manila Trench beneath northern Luzon produced a major batholith, the Central Cordillera Intrusive Complex, which defines the eastern boundary of the Baguio district. The intrusive complex was emplaced in the early Miocene based on 40Ar/39Ar age determinations for the Lucbuban gabbro (22.6 ± 0.5 Ma) and Virac granodiorite (20.23 ± 0.38 and 20.2 ± 0.7 Ma). Recent exploration has led to the discovery of several new porphyry copper-gold and skarn prospects in the western Baguio district. Mineralization was preceded by the intrusion of a suite of hornblende megacrystic andesite dikes in the central part of the district from 4.55 ± 0.15 to 3.45 ± 0.19 Ma. Porphyry copper-gold and skarn deposits at Black Mountain and Mexico, on the western side of the district, formed between 3.09 ± 0.15 and 2.81 ± 0.24 Ma. The Santo Tomas II Cu-Au-(Pd) porphyry deposit was emplaced into the central southern part of the district at about 1.5 Ma, based on secondary biotite 40Ar/39Ar ages of 1.48 ± 0.05 Ma and 1.47 ± 0.05 Ma. On the eastern side of the district, porphyry-style mineralization at the Hartwell and Ampucao porphyry Cu-Au prospects returned 40Ar/39Ar ages of 1.09 ± 0.10 and 0.51 ± 0.26 Ma, respectively. Quartz-carbonate-base metal sulfide style epithermal gold-silver veins are well-developed on the eastern side of the Baguio district, and crosscut porphyry-style mineralization at the Acupan, Baguio gold and Nugget Hill deposits. Epithermal veining is inferred to have occurred in the past million years, based on a reported K-Ar age determination of 0.65 ± 0.07 Ma from the Acupan gold mine. A large advanced argillic alteration zone (the Baguio lithocap) crops out in the northwest part of the district, and formed between 1.4 and 0.9 Ma, based on previous K-Ar dating. Pliocene-Pleistocene mineralization in the Baguio district was triggered by the east-directed subduction of the Scarborough Ridge. Ridge subduction caused the subduction angle along the Manila Trench beneath northern Luzon to decrease, which facilitated crustal thickening and exhumation. Ridge subduction also promoted the development of northwest-trending faults in the upper plate that interacted with major arc-parallel north- and arc-normal northeast-trending fault sets associated with the Philippine fault system. Fertile magmas were emplaced into transtensional strike-slip relay basins, resulting in the accumulation of more than 35 Moz of gold and several million tonnes of copper over approximately 3 m.y

Pliocene and Quaternary depositional model of the Algarve margin contourite drifts (Gulf of Cadiz, SW Iberia): Seismic architecture, tectonic control and paleoceanographic insights

The contourite drifts off southwest Iberia that formed as a result of the interaction of the Mediterranean Outflow Water (MOW) with the continental middle slope were studied in the Algarve margin using multichannel reflection seismic lines, oil-wells, piston cores, and a bathymetric compilation of four datasets. The seismostratigraphic interpretation of a dense array of oil industry seismic and stratigraphic correlation allowed the identification of five seismic units of Early Pliocene through Holocene in the Faro and Albufeira drifts and four correlative seismic units in the Lagos and Sagres drifts and three in the Portimão drift. A three-phased evolutionary model for the contourite formation is proposed. Firstly, a precursory phase of Pliocene age made up of sheeted drifts represents an initial phase of deposition under bottom-current activity that is correlated with the first stages of an enhanced MOW at about 3.5Ma. Secondly, the building up phase of Early Pleistocene age is related to a strengthening of the MOW close to the onset of the Northern Hemisphere glaciations at about 2.6Ma during which were deposited low-mounded drifts. Thirdly, the growing phase from Middle-Pleistocene through Holocene suggests the presence of a stronger MOW since the beginning of the Middle-Pleistocene Transition at about 1.3–1.0Ma, accounting for the deposition of mounded drifts and formation of the Álvares Cabral moat. Seismostratigraphic interpretation and isochron maps allowed for the establishment of the main oceanographic, climatic, morphologic and tectonic factors that controlled the drifts deposition: i) the Pliocene and Quaternary MOW circulation forced by climate changes; ii) the sea bottom topography inherited from the Late Miocene, mainly shaped by the Portimão, Lagos and Bartolomeu Dias canyons system; iii) the interaction between along-slope and down-slope processes since the Pliocene; and iv) Pliocene–Quaternary fault-activity and diapirism.

New rodent faunas from Middle Miocene and Mio- Pliocene in the Cabriel Basin (Valencia, Spain)

Five new localities with micromammal remains have been located in the Cabriel Basin. These localities are situated in the eastern area of the Cabriel Basin, in the Juan Vich ravine, where the oldest deposits of this basin crop out. The localities JV2, JV3 and JV6 are of Middle Miocene age, JVTLI1 of Middle Miocene or Late Miocene age and JVTS1 of Late Miocene or Early Pliocene age. The beginning of the sedimentary history of the basin was attributed to Late Turolian (Late Miocene). The new sedimentary record from the Juan Vich ravine and the mammalian findings extend the stratigraphic and paleontological knowledge of the Basin.

Latest Miocene-Pliocene Larger Foraminifera and depositional environments of the carbonate bank of La Désirade Island, Guadeloupe (French Antilles)

In this paper we present first results of the study of planktonic Foraminifera, large benthic Foraminifera and carbonate facies of La Désirade, aiming at a definition of the age and depositional environments of the Neogene carbonates of this island. The study of planktonic Foraminifera from the Detrital Offshore Limestones (DOL) of the Ancienne Carrière allows to constrain the biochronology of this formation to the lower Zone N19 and indicates a latest Miocene to early Pliocene (5.48–4.52 Ma) age. Large benthic Foraminifera were studied both as isolated and often naturally split specimens from the DOL, and in thin sections of limestones from the DOL and the Limestone Table (LT). The assemblages of Foraminifera include Nummulitidae, Amphisteginidae, Asterigerinidae, Peneroplidae, Soritidae, Rotalidae (Globigerinidae: Globigerinoides, Sphaeroidenellopsis, Orbulina) and incrusting Foraminifera ( Homotrema and Sporadotrema). The genera Amphistegina, Archaias and Operculina are discussed. Concerning the Nummulitidae we include both “ Paraspiroclypeus” chawneri and “ Nummulites” cojimarensis, as well as a newly described species, Operculina desiradensis new species, in the genus Operculina, because the differences between these 3 species are rather on the specific than the generic level, while their morphology, studied by SEM, is compatible with the definition of the genus Operculina (D’Orbigny emend. Hottinger) . The three species can be easily distinguished on the basis of their differences in spiral growth: while O. desiradensis has an overall logarithmic spiral growth, O. cojimarensis and especially O. chawneri show a tighter and more geometric spiral growth. O. cojimarensis and O. chawneri were originally described from Cuba in outcrops originally dated as Oligocene and later redated as early Pliocene. Therefore, O. chawneri was considered until now as restricted to the early Pliocene. However, in the absence of a detailed morphometric and biostratigraphic study of the Caribbean Neogene nummulitids, it is difficult to evaluate the biochronologic range of these species. The history of the carbonates begins with the initial tectonic uplift and erosion of the Jurassic igneous basement of La Désirade, that must have occurred at latest in late Miocene times, when sea-level oscillated around a long-term stable mean. The rhythmic deposition of the Désirade Limestone Table (LT) can be explained by synsedimentary subsidence in a context of rapidly oscillating sea-level due to precession-driven (19–21 kyr) glacio-eustatic sea-level changes during the latest Miocene-Pliocene. Except for a thin reef cap present at the eastern edge of the LT, no other in-place reefal constructions have been observed in the LT. The DOL of western Désirade are interpreted as below wave base gravity deposits that accumulated beneath a steep fore-reef slope. They document the mobilisation of carbonate material (including Larger Foraminifera) from an adjacent carbonate platform by storms and their gravitational emplacement as debris and grain flows. The provenance of both the reefal carbonate debris and the tuffaceous components redeposited in the carbonates of La Désirade must be to the west, i.e. the carbonate platforms of Marie Galante and Grande Terre.

New age determination of the Cenozoic Lunpola basin, central Tibet

AbstractKnowing when the Tibetan Plateau reached its present elevation is important for understanding the uplift history of Tibet. Recently, Rowley & Currie (2006) suggested that central Tibet exceeded 4000 m from 35 Ma to the Pliocene using the oxygen-isotope composition of calcareous minerals in Lunpola basin sediments. However, they adopted a poor age assignment for the Dingqing Formation in the Lunpola basin based on previous microfossil studies. In this study, we present SIMS U–Pb zircon dates from a bentonite layer intercalated within the middle to lower Dingqing Formation. Twenty-six measurements yield a highly reliable U–Pb age of 23.5 ± 0.2 Ma (2σ, MSWD = 1.1), suggesting that the deposition age of the Dingqing Formation is late Oligocene to early Miocene, much older than the Miocene–Pliocene age used by Rowley & Currie (2006). This age robustly constrains the age of Cenozoic sedimentary strata in central Tibet, and hence provides an important basis for estimating the palaeoelevation in the high Tibet during the geological past.

Geological evolution of the Karakoram Ranges

The Karakoram terrane in Northern Pakistan is geologically equivalent to the Qiangtang terrane of central Tibet but unlike Tibet shows extreme topographic relief, high uplift-exhumation and erosion rates and exposes deep crustal metamorphic and igneous rocks of the lower crust. The Karakoram terrane has been above sea-level since Early to Mid-Cretaceous, the age of the youngest marine sedimentary rocks. During the Late Jurassic to Mid-Cretaceous northdipping subduction of Tethyan oceanic lithosphere beneath the Karakoram resulted in widespread Andean I-type granite-granodiorite magmatism (Hunza, Hushe plutonic units) with possible concomitantlow-pressure andalusite-sillimanite grade Buchan-type metamorphism.The Kohistan intra-oceanic island arc developed above a second north-dipping subduction zone to the south during the Aptian- Eocene (~120-45 Ma) and was accreted to the southern margin of Asia concomitantly with obduction onto India during the Late Cretaceous.Final closure of Neo-Tethys and collision of India with Asia occurred at ca. 50 Ma. Continuous crustal thickening occurred across the southern Karakoram resulting in widespread regional Barrovian facies kyanite- and sillimanite-grade metamorphism with U-Pb zircon and monazite ages ranging from Latest Cretaceous (~63 Ma) to Pliocene times (4-3 Ma). Crustal melting resulted in intrusion of several sets of leucogranite dykes (Hunza dykes) at ~50 Ma and 35 Ma. The major phase of crustal melting in the Baltoro plutonic unit spanned at least 13 million years from 26-13 Ma (Late Oligocene to Mid-Miocene), and resulted in intrusion of a large-scale biotite monzogranite to garnet two-mica leucogranite intrusion of batholithic proportion. Some extra heat contribution from the upper mantle may have been required to account for the large size of the granite batholith and the lamprophyric dykes intruded around the margins. The youngest metamorphism and partial melting is known from the deep crustal metamorphic-migmatite gneiss domes (Dassu dome) of Pliocene age in the Baltoro region and the Late Miocene Sumayar leucogranite in Hunza. Geological and U-Pb geochronological data show that the Karakoram Range was a dynamic active mountain range with semi-continuous crustal thickening, deformation, metamorphism and partial melting for at least the last 63 million years and continues to this day.

Pliocene coastal palaeomorphology and ostracod faunas of the Bass Strait hinterland, southeast Australia

Early to late Pliocene sedimentary strata present across the northern Bass Strait hinterland, southeastern Australia yield extensive fossil proxy data relevant to the interpretation of high sea level coastal palaeomorphology. Within the Pliocene Whalers Bluff Formation exposed in coastal cliffs near the township of Portland, Victoria, marine microfossil faunas delineate two broad cycles of deposition. Both these sedimentary cycles are bound below by unconformity surfaces. Within the lower sedimentary cycle, a basal stress-tolerant (low diversity) marginal marine microfossil fauna devoid of ostracods and suggestive of bottom-water hypoxia, is succeeded by a diverse shallow marine ostracod fauna dominated by stenohaline species indicative of a sheltered (but open) oceanic embayment. This lower sedimentary cycle has an early Pliocene (Zanclean) age. Equivalent shallow marine (e.g. coastal embayment) deposits occur broadly across the coastal hinterland of southeastern Australia—reflecting the generally higher global sea levels of this time. The upper cycle in the cliff exposures at Portland is late Pliocene (Piacenzian) in age. Equivalent deposits across the Bass Strait hinterland are restricted to former incised river valley settings. Euryhaline estuarine/coastal lagoon Ostracoda are present throughout the upper cycle in the Portland cliffs. These are associated with a low diversity microfauna at the base of the upper cycle and a high diversity microfauna towards the top of the cycle. Early Pliocene coastal marine deposits can be distinguished from late Pliocene coastal marine deposits across the northern Bass Strait hinterland on the basis of the presence or absence of certain open marine (‘stenohaline’) ostracod species.

Diversification in the Andes: Age and origins of South American Heliotropium lineages (Heliotropiaceae, Boraginales)

The uplift of the Andes was a major factor for plant diversification in South America and had significant effects on the climatic patterns at the continental scale. It was crucial for the formation of the arid environments in south-eastern and western South America. However, both the timing of the major stages of the Andean uplift and the onset of aridity in western South America remain controversial. In this paper we examine the hypothesis that the Andean South American groups of Heliotropium originated and diversified in response to Andean orogeny during the late Miocene and a the subsequent development of aridity. To this end, we estimate divergence times and likely biogeographical origins of the major clades in the phylogeny of Heliotropium, using both Bayesian and likelihood methods. Divergence times of all Andean clades in Heliotropium are estimated to be of late Miocene or Pliocene ages. At least three independent Andean diversification events can be recognized within Heliotropium. Timing of the diversification in the Andean lineages Heliotropium sects.Heliothamnus, Cochranea, Heliotrophytum, Hypsogenia, Plagiomeris, Platygyne clearly correspond to a rapid, late Miocene uplift of the Andes and a Pliocene development of arid environments in South America.

Palynology and palynofacies analyses of the Gray Fossil Site, eastern Tennessee: Their role in understanding the basin-fill history

The Gray Fossil Site (GFS) includes multiple karst sub-basins that are filled with lacustrine sediments. Early paleontologic work on one of the sub-basins (GFS-2) indicates a late Miocene/early Pliocene age based on an assemblage of well-preserved vertebrate fossils. However, detailed palynological analysis of the 38.7 m deep GFS-1 core recovered from another sub-basin indicates an older age. The presence of Caryapollenites imparalis, C. inelegans and C. prodromus association suggests a Paleocene to Eocene age for the GFS-1 core section. This age is also supported by the absence of pollen of the Poaceae, the grass family that is not commonly present until the Neogene. Age constraints from palynologic data suggest that the GFS has a more complex basin-fill history than previously suspected, and that multiple depo-centers within the basin may have been periodically active through the Cenozoic. Palynofacies analysis of the GFS-1 core indicates that phytoclasts and opaques are the most abundant organic constituents and have diluted both the palynomorph population and amorphous organic matter. Two possible scenarios can account for this observation: 1) an oxidizing depositional paleoenvironment; and 2) a localized high flux of charcoal following wildfires and subsequent increased runoff.

Contribution to petrological and stratigraphical understanding of the Cordillera de Guanacaste lava flows, Costa Rica

Geochemical data are presented for lavas from four pair Quaternary stratovolcanoes (Cordillera de Guanacaste), small basic outcrops trenchward of the volcanic front (VF), and paleovolcanic units (Pliocen- Pleistocene age) of northern Costa Rica. The more evolved lavas of the calc-alkaline series are from Rincon de la Vieja and Miravalles andesitic volcanoes, and from Canas Dulces (dacite/rgyolite) and Bijagua (andesite) domes-field. They usually have high Zr, Ba and Rb contents. The Quaternary volcanoes grew over a regional basaltic to dacitic basement (Miocene-Pleistocene), called Pre-Neovolcanic lavas (PNL). The Intra-ignimbrite lava flows (IIL, Pliocene age) have a clearly different trend with respect to the volcanoes, especially in their high TiO2 and P2O5 content. Both old lava units show a tholeiitic tendency, transitional to calc-alkaline series. The less evolved laves correspond to Orosi, Cacao, and Tenorio (Parcelas cone) volcanoes and also to small isolated basic lavas and cinder cones (IBL) located 20-26 krn in front of the VF. These rocks are high-alumina tholeiitic basalts/basaltic andesites or transitional to alkalic basalts. Crystal fractionation is the dominant mechanism involved in generating the range of magmatic compositions. The geochemistry suggests that Guanacaste lavas are cogenetic but some source heterogeneites are required to explain the observed petrological features.

Petrología de las rocas intrusivas neógenas de Costa Rica

The purpose of this paper is to present a synthesis of the petrology of the Neogene plutonic rocks of Costa Rica. Two series are distinguished: an alkaline one of Miocene/Pliocene age, restricted to a small area and a calc-alkaline series of Miocene and Pliocene age and with a wide areal extension. The plutonic bodies are generally small and have been emplaced during various intrusive stages near to the surface. The mineralogical composition of the calc-alkaline rocks is very uniform. There are, however great variations in their modal composition and texture, not only from one intrusive unit to another but also between the different intrusive bodies of the same unit. Chemically there exists a difference between the calc-alkaline plutonic rocks of the Cordillera de Talamanca and those of the Cordillera de Tilarán, specially with regard to the alkali content and the iron enrichment, which might be due to crustal thickening during the Upper Miocene. The origin of these cale-alkaline plutonic rocks is explained in terms of a two stage model. The first stage comprises the formation of tholeiitic and Calc-alkaline melts which are underplated. The second stage yields intermediate to acid magmas by partial melting of the increased crust.

The Osa Group: an auriferous Pliocene sedimentary unit from the Osa Peninsula, southern Costa Rica

Recent geological mapping in the Osa Peninsula and micropaleontological studies have led to the recognition of a new lithostratigraphic unit- the Osa Group. This group replaces the Charco Azul and the Armuelles Formation in the Osa Peninsula as shown on the DGMP 1:200,000 geological map, and includes tje informally defined Punta La Chancha formation. It is defined as a sequenceof semi-consolidated to lithified, greenish-grey, graded, graywacke-type conglomerates, sandstones, siltstones and claystones of Mid to Upper Pliocene age lying unconformably on a Nicoya Complex basement, and unconformably overlain by unconsolidated Quaternary sediments of the Puerto Jimenez Group. The Osa Group was deposited in a wide variety of sedimentary enviroments ranging from fresh-water fluviatile; brackish lagoonal-estuarine-deltaic; shallow shelf down to deep marine basin. Economically important placer gold is widespread and tendsto be concentrated locally in the conglomeratic members, especially at the basal unconformity. It is suggested that the Osa Group sediments were deposited in intermittently subsiding pull-apart basins governed by wrench faulting in a geological setting similar to that on and around the Osa Peninsula at present. The present-day Panama- Coiba Fracture Zone in the Gulf of Panama can be extrapolated northwarsd via three submarine canyons on the continental slope, and thence into a system of braided northwest-trending transcurrent faults parallel to Costa Rica's Pacific coastline. The landforms and bathymetry of the region surrounding the Osa Peninsula are considered to be largely controlled by (intermittent) dextral strike-slip movement on the main fractures. Within this tectonic setting the Golfo Dulce basin and the Coto Colorado-Chiriqui lowlands form pull-apart or tipped-wedge basins, whereas the Osa and the Burica Peninsula, and the offshore Coiba Ridge, are tilted horst blocks.

Age, composition, and areal distribution of the Pliocene Lawlor Tuff, and three younger Pliocene tuffs, California and Nevada

The Lawlor Tuff is a widespread dacitic tephra layer produced by Plinian eruptions and ash flows derived from the Sonoma Volcanics, a volcanic area north of San Francisco Bay in the central Coast Ranges of California, USA. The younger, chemically similar Huichica tuff, the tuff of Napa, and the tuff of Monticello Road sequentially overlie the Lawlor Tuff, and were erupted from the same volcanic field. We obtain new laser-fusion and incremental-heating 40 Ar/ 39 Ar isochron and plateau ages of 4.834 ± 0.011, 4.76 ± 0.03, ≤4.70 ± 0.03, and 4.50 ± 0.02 Ma (1 sigma), respectively, for these layers. The ages are concordant with their stratigraphic positions and are significantly older than those determined previously by the K-Ar method on the same tuffs in previous studies. Based on offsets of the ash-flow phase of the Lawlor Tuff by strands of the eastern San Andreas fault system within the northeastern San Francisco Bay area, total offset east of the Rodgers Creek–Healdsburg fault is estimated to be in the range of 36 to 56 km, with corresponding displacement rates between 8.4 and 11.6 mm/yr over the past ∼4.83 Ma. We identify these tuffs by their chemical, petrographic, and magnetic characteristics over a large area in California and western Nevada, and at a number of new localities. They are thus unique chronostratigraphic markers that allow correlation of marine and terrestrial sedimentary and volcanic strata of early Pliocene age for their region of fallout. The tuff of Monticello Road is identified only near its eruptive source.