Early Pliocene Age Research Articles

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.

Fossilisation of a silica diagenesis reaction front on the mid-Norwegian margin

The diagenetic transformation of biogenic silica from opal-A to opal-CT was recognised on seismic reflection data over an area of 78 × 10 3 km 2 on the mid-Norwegian margin. The opal-A/CT diagenetic boundary appears as a positive, high amplitude reflection that generally cross-cuts the hosting stratigraphy. We demonstrate that it is not a sea bottom simulating reflection (BSR) and also that is not in thermal equilibrium with the present day isotherms. We present arguments that three styles of deformation associated with the opal-A/CT reflection – polygonal faulting, regional anticlines and synclines and differential compaction folding – indicate that the silica diagenesis reaction front is fossilised at a regional scale. Isochore maps demonstrate the degree of conformity between the opal-A/CT reflection and three seismic horizons of Late Miocene to Early Pliocene age that potentially represent the paleo-seabed when ‘fossilisation’ of the reaction front took place. The seismic interpretational criteria for recognition of a fossilised diagenetic front are evaluated and the results of our study are integrated with previous studies from other basins of the NE Atlantic in order to determine if the arrest of silica diagenesis was diachronous along this continental margin.

Diatom-based correlation of early to mid-Pliocene drillcores from the southwestern Ross Sea, Antarctica

The ANDRILL AND-1B drillcore in southern Victoria Land Basin, Antarctica recovered an upper Miocene to early Pleistocene continental shelf stratigraphic section, including a spectacular Pliocene and Pleistocene sequence of alternating diamictite and diatom-rich sediments. This punctuated sequence reflects variation between glacial sediments deposited by ice advance over the site and open-marine diatom productivity and sedimentation. An early Pliocene age for the base of diatom-rich sediment is constrained through integrated diatom biostratigraphy and magnetostratigraphy. The presence of marine diatom Shionodiscus tetraoestrupii from the top of the core to 583.64mbsf indicates the lowest diatom-bearing sediments are younger than 5Ma. This drillcore provides important new chronostratigraphic control for paleoenvironmental changes also recorded in three drillcores from the western coast of the McMurdo Sound area, DVDP-10 and -11 in Taylor Valley and CIROS-2 from the seaward edge of Ferrar Fjord. The refined continental shelf biostratigraphy developed from AND-1B provides a framework for regional correlation to understand better the timing and character of large paleoenvironmental changes in the western Ross Sea that involve the history of the West Antarctic Ice Sheet (WAIS). Multivariate analysis, along with traditional biostratigraphic approaches, enables the correlation and comparison of coeval intervals in these drillcores. A composite stratigraphic sequence from these four cores suggests the early and mid-Pliocene Ross Sea experienced extended intervals of open-marine conditions with minimal sea-ice cover and high diatom production and sedimentation. This new information provides important constraints on Antarctic paleoclimate and ice sheet history during an important interval when global climate was warmer than today. The history preserved in these four drillcores will be an important tool to guide and test future ice sheet and climate models.

Comment on “The oldest South American Cricetidae (Rodentia) and Mustelidae (Carnivora): Late Miocene faunal turnover in central Argentina and the Great American Biotic Interchange” by D.H. Verzi and C.I. Montalvo [Palaeogeography, Palaeoclimatology, Palaeoecology 267 (2008) 284–291

In a recent paper (Verzi, D.H., Montalvo, C.I., 2008. The oldest South American Cricetidae (Rodentia) and Mustelidae (Carnivora): late Miocene faunal turnover in central Argentina and the Great American Biotic Interchange. Palaeogeography, Palaeoclimatology, Palaeoecology 267, 284–291) the potentially oldest South American Cricetidae (Rodentia) and Mustelidae (Carnivora) have been reported from a Late Miocene (Huayquerian) assemblage located in central Argentina (Caleufú site, La Pampa province). These findings expand the biochron of these important families in South America 3–4 Ma earlier than previous records. However, several observations prevent us from accepting the validity of these assumptions. In this paper we discuss the age of the Caleufú assemblage, and the assignment of the GHUNLPam 21722 to Mustelidae. The Caleufú assemblage contains a mixture of Pliocene and Late Miocene faunal elements, and has been assigned to the Huayquerian only due to the “stage of evolution” of some rodents. The lack of isotopic or paleomagnetic data coupled with the isolation of this locality and the absence of a local stratigraphic succession inhibit its correlation with other palaeontological comparable sites and a robust inference about its chronology. The “stage of evolution” of a taxon is not a biostratigraphic tool, thus we cannot rule out the possibility that the Caleufú assemblage has an Early Pliocene (Montehermosan) age. The GHUNLPam 21722, a poorly preserved specimen, shows several characters (e.g., 4 lower incisors, mental foramina below the incisors absent, third lower incisor smaller than the first and second ones) that do not match with Mustelidae (or Carnivora) but instead strongly resemble those observed in didelphimorphian marsupials. The discussed caveats regarding Caleufu assemblage chronology and taxonomy indicate that new and detailed studies are needed before the hypotheses advanced by Verzi and Montalvo can be accepted.

Tectonics, sea-level changes and palaeoenvironments in the early Pleistocene of Rome (Italy)

The historical site of the Monte Mario lower Pleistocene succession (Rome, Italy) is an important marker of the Pliocene/Pleistocene boundary. Recently, the Monte Mario site was excavated and restudied. A spectacular angular unconformity characterizes the contact between the Monte Vaticano and the Monte Mario formations, which marks the Pliocene/Pleistocene boundary. Biostratigraphical analyses carried out on ostracod, foraminifer, and calcareous nannofossil assemblages indicate an Early Pliocene age (topmost Zanclean, 3.81–3.70 Ma) for the underlying Monte Vaticano Formation, whereas the Monte Mario Formation has been dated as early Pleistocene (Santernian, 1.66–1.59 Ma). Palaeomagnetic analyses point to C2Ar and C1r2r polarity chrons for the Monte Vaticano and the Monte Mario formations, respectively. The Monte Mario Formation consists of two obliquity-forced depositional sequences (MM1 and MM2) characterized by transgressive systems tracts of littoral marine environments at depths, respectively, of 40–80 m and 15–20 m. The data obtained from foraminifer and ostracod assemblages allow us to reconstruct early Pleistocene relative sea-level changes near Rome. At the Plio/Pleistocene transition, a relative sea-level drop of at least 260 m occurred, as a result of both tectonic uplift of the central Tyrrhenian margin and glacio-eustatic changes linked to early Pleistocene glaciation (Marine Isotope Stage 58).

Tectonism and eustasy across a Late Miocene strandplain: The Loxton–Parilla Sands, Murray Basin, southeastern Australia

Sediments of the Loxton–Parilla Sands form a progradational strandplain that stretches for over 350 km across the Murray Basin, in southeastern Australia. The strandplain comprises over 600 individual ridges, with stratigraphic and strontium isotope data indicating these strandlines are Late Miocene to Early Pliocene in age, with deposition beginning before 7.2 Ma and terminating at approximately 5.0 Ma. Post-Pliocene uplift along the Padthaway High due to regional, compressive, intraplate tectonism, was responsible for altering the orientation of strandlines and drainage systems, as well as contributing to increased erosion across the strandplain. This uplift both restricted and starved the coastline of sand-sized sediment, changing the depositional environment from one of progradation to active erosion. This produced a series of scarps along the southwestern Murray Basin and ultimately contributed to the formation of the Kanawinka Escarpment, a large erosional cliff feature whose formation was not exclusively the result of fault-related tectonism. This evidence for Pliocene tectonism along southeastern Australia, together with the depositional ages inferred from strontium isotope analysis, indicates that, contrary to previous work, the strandlines of the Loxton–Parilla Sands do not represent an unbroken record of Late Miocene–Pliocene glacioeustatic change. We estimate that approximately 3 Ma of depositional history may be missing from the sedimentary record between the southwestern-most Loxton–Parilla Sands strandlines and those of the Pleistocene Bridgewater Formation.

Phosphate‐rich deposits associated with the Mio‐Pliocene unconformity in south‐east Australia

AbstractPhosphates are present on the surface of the Mio‐Pliocene unconformity in the Otway, Port Phillip and Gippsland basins of south‐east Australia. The phosphates occur as lenticular lag deposits and include reworked phosphatic intraclasts, vertebrate bone and teeth. In situ phosphatized burrows are also found in sediments of Late Miocene and Early Pliocene age. The phosphatic intraclasts on the unconformity are interpreted as reworked phosphatized burrows derived from latest Miocene sediments (6 to 5 Ma). The phosphatization of these intraclasts is temporally related to the unconformity. The timing of phosphogenesis coincides with a period of transgression across the south‐east Australian margin following Late Miocene uplift. This transgression is responsible for initial marine erosion of the underlying Miocene sequence, creation of a period of very slow sedimentation that was favourable to phosphate formation and subsequent deposition of the latest Miocene through to Pliocene sediments. The continental weathering of the uplifted highlands adjacent to the sedimentary basins, global phosphorus enrichment in the Late Miocene oceans and localized upwelling may all have contributed to phosphatization in south‐eastern Australia.

New insights in the evolution of Antarctic glaciation from depth conversion of well-log calibrated seismic section of Prydz Bay

The understanding of the evolution of the Antarctic Ice Sheet is crucial for the comprehension of the history of past global climate. The debate regarding the age of the transition to modern “dry- and cold-based” ice sheet after the Neogene polythermal conditions has taken place over more than 20 years. An evident change in the geometry of the depositional systems of the Prydz Bay continental margin demarked the initiation of the Prydz Channel Fan and has been inferred to correspond to this transition. The improvement in the age placement of this change contributes to unravel the last stages of the Antarctic glacial history. We predicted the spatial distribution of P-wave velocity data along both dip- and strike-oriented seismic profiles that intersect 3 Ocean Drilling Program (ODP) sites on the Prydz Bay continental shelf. We used this information to assist the correlation of the existing litho- and bio-stratigraphic information among the drilling sites and to produce an accurate geometric reconstruction of the Neogene shelf units through depth-migration of the seismic data. The revised stratigraphy that we obtained suggests an early late to late early Pliocene age for the seismic reflector at the base of the Prydz Channel Fan. This age, younger than previously proposed, is consistent with the age inferred for similar geometric changes identified in different Antarctic margins.

Distribution of Glacial Deposits, Soils, and Permafrost in Taylor Valley, Antarctica

ABSTRACTWe provide a map of lower and central Taylor Valley, Antarctica, that shows deposits from Taylor Glacier, local alpine glaciers, and grounded ice in the Ross Embayment. From our electronic database, which includes 153 sites from the coast 50 km upvalley to Pearse Valley, we show the distribution of permafrost type and soil subgroups according to Soil Taxonomy. Soils in eastern Taylor Valley are of late Pleistocene age, cryoturbated due to the presence of ground ice or ice-cemented permafrost within 70 cm of the surface, and classified as Glacic and Typic Haploturbels. In central Taylor Valley, soils are dominantly Typic Anhyorthels of mid-Pleistocene age that have dry-frozen permafrost within the upper 70 cm. Salt-enriched soils (Salic Anhyorthels and Petrosalic Anhyorthels) are of limited extent in Taylor Valley and occur primarily on drifts of early Pleistocene and Pliocene age. Soils are less developed in Taylor Valley than in nearby Wright Valley, because of lesser salt input from atmospheric deposition and salt weathering. Ice-cemented permafrost is ubiquitous on Ross Sea, pre–Ross Sea, and Bonney drifts that occur within 28 km of the McMurdo coast. In contrast, dry-frozen permafrost is prevalent on older (≥115 ky) surfaces to the west.

Contamination of seismicity catalogs by quarry blasts: An example from İstanbul and its vicinity, northwestern Turkey

Scientists have proposed two fault systems of different ages in the Sea of Marmara: the Thrace-Eskişehir Fault Zone of Early Miocene–Early Pliocene age and the North Anatolian Fault Zone of Late Pliocene–Recent age. Different seismicity rates and extensions of these faults onto land near İstanbul have been suggested. One of the reasons for these differences is the contamination of seismicity catalogs by seismic events from quarries operated in İstanbul and its vicinity, including Gaziosmanpaşa (Cebeci and Kemerburgaz), Çatalca, Ömerli, Gebze, and Hereke. In this study, we investigated waveforms of 179 seismic events (1.8 < Md < 3.0) from the KOERI, NEMC digital database. We determined differences between earthquakes and quarry blasts based on time- and frequency-domain analyses of their seismograms (amplitude peak ratio, power ratio, and spectral amplitude ratio) and used these differences as discriminants. The results of this study indicate that 15% and 85% of the investigated seismic events are earthquakes and quarry blasts, respectively.

Micromys caesaris, a new murid (Rodentia, Mammalia) from the Late Pliocene of the Guadix Basin, southeastern Spain

The Genus Micromys includes a single extant species, Micromys minutus (Pallas, 1771), which lives in Europe and North Asia. This genus is known in the fossil record since the late Miocene; eight fossil species have been described in Europe and Asia, most of them of late Miocene and early Pliocene age. The evolution of this genus during the late Pliocene is barely known. Although it is present in numerous localities of this age, remains of Micromys are usually scarce and generally assigned to the species M. minutus or M. praeminutus Kretzoi, 1959.

The Pliocene Pleshet Formation near Khassif, southern Israel, and its shallow marine ostracode fauna

Neogene sediments, several hundred meters thick, are exposed in the Gaza - Be’er Sheva Canyon, southern Israel. Isolated Khassif sections at the edge of the canyon south of Ofaqim are only up to 12m thick. A micropaleontological study undertaken on more than 40 samples reveals, in addition to benthonic foraminifers, a diversified fauna of 31 species of ostracodes belonging to 22 genera. Representatives of the genus Aurila are the most common forms. Nearly all taxa are stenohaline, shallow marine species. The occurrence of the euryhaline Cyprideis cf. torosa marks a shallowing event in the middle parts of both sections. Other sea level changes are evidenced in the exposures by erosional unconformities, as well by changing lithologies of calcareous sandstones and marls versus conglomerates. A Pliocene age is attributed to the sections based on the distribution of common ostracodes inMediterranean region. The stratigraphic ranges of Cistacythereis robusta and Oblitacythereis mediterranea suggest an Early Pliocene age for these exposures.Marine Pliocene ostracodes are described in the present study for the first time from Israel. The age and lithology of the Khassif sections allow their correlationwith the sediments of the Pleshet Formation from the Gaza - Be’er Sheva Canyon.

Disruption of the Hellenic arc: Late Miocene extensional detachment faults and steep Pliocene‐Quaternary normal faults—Or what happened at Corinth?

Extensional faults exposed in the Peloponnesus and mainland Greece, most of which are described here for the first time, record a transition from regional extension of the Aegean domain to the modern tectonic system. The East Peloponnesus Detachment System trends north‐northwest from the southern Peloponnesus to ∼30 km north of the Gulf of Corinth, dips gently northeast, and is late Miocene–early Pliocene in age. It has a minimum displacement of 25–30 km and appears to be the youngest of the regional‐scale extensional systems with significant displacement that formed parallel to the Hellenic arc. The partially coeval East Sterea Extensional System, which extends from the Gulf of Corinth to the Aegean Sea, contains low‐angle normal faults that both crosscut and trend parallel to older structures of the Hellenic arc. Late Miocene to early Pliocene displacement within this zone disrupted the arc‐parallel structures of the Hellenides. Upper Pliocene‐Quaternary normal faults, which trend approximately east‐west and generally dip steeply at the surface, continue the disruption of the Hellenic arc. Much of the subsidence within the Gulf of Corinth appears to be unrelated to the younger faults and is instead related to the motion on the East Peloponnesus Detachment, which crosscuts the modern graben.

Vertebrate paleontology, biostratigraphy, geochronology, and paleoenvironment of Qaidam Basin in northern Tibetan Plateau

Qaidam Basin is the largest terrestrial basin of the Tibetan Plateau and has the most continuous sedimentary record in the Cenozoic. Although constituting the first known fossil vertebrates of the Tibetan Plateau and discovered in early scientific expeditions in the 1930s, mammalian fossils from Qaidam Basin have not played a significant role in studying the basin stratigraphy. Our explorations in the Qaidam Basin during the past 8 years have resulted in the first well documented collection of fossil vertebrates from there. In addition to being the best organisms for terrestrial biostratigraphy in the Cenozoic, the new fossil vertebrates offer a unique window into the ancient biotas that chronicle the late Cenozoic uplift of the plateau and associated environmental changes. Here we establish for the first time a faunal sequence of early Oligocene to early Pliocene age. Four mammalian faunas are recognized: Olongbuluk Mammal Fauna (middle Miocene), Tuosu Mammal Fauna (early late Miocene), Shengou Mammal Fauna (early late Miocene), and Huaitoutala Mammal Fauna (early Pliocene). All four mammal faunas can be directly or indirectly tied to a paleomagnetic section in the Olongbuluk–Huaitoutala area, permitting robust calibration of the chronology of these faunas. At least four fish faunas are also found: Lulehe Fish Fauna (late early to early late Oligocene), Shengou Fish Fauna (early late Miocene), Eboliang Fish Fauna (late late Miocene to early Pliocene), and Yahu Fish Fauna (early Pliocene). The fish faunas represent the only records of their respective times in the Tibetan Plateau and thus by themselves cannot be used for age determination. However, these fish faunas are either integrated into paleomagnetic sections or directly tied to mammal faunas of known ages, permitting independent assessment of their chronology. Zoogeographically, Qaidam mammal faunas show increasing endemism through time, presumably related to topographic barriers formed by the growth and uplift of the Tibetan Plateau and to climatic changes triggered by the same tectonic process. Fishes that were trapped in internally drained basins developed pachyostotic morphologies caused by hypersaline environments, which are associated with the increasing aridity of the Tibetan Plateau since early Pliocene.

Implications of soils on mid-Miocene-aged drifts in the McMurdo Dry Valleys for ice sheet history and paleoclimate reconstruction

Strongly developed soils on unconsolidated deposits of mid-Miocene age occur in the uplands above 1500 m elevation in the Asgard Range and Quartermain Mountains in the McMurdo Dry Valleys. Inferred from the preservation and distribution of their surfaces, these deposits have been largely unmodified since a widespread glacial erosional event prior to 10 Ma. Composition of the till plays a key role in soil development on these ancient surfaces. Soils derived from dolerite-rich till have significantly greater depths of staining and ghosts, color-development equivalents of the Bw horizon, and extractable Fe than sandstone-rich tills. However, no significant differences were found in depth of visible salts, salt stage, electrical conductivity of the horizon of salt enrichment, and profile quantities of salts, which implies a similar age for the soils. The soils on the Miocene-aged deposits are less developed than soils of early Quaternary and Pliocene age at lower elevation in the Dry Valleys, inconsistent with the conclusion that these are relict Miocene surfaces. We suggest that the lower stage of soil development on the surfaces of Miocene age deposits primarily reflects higher erosion (deflation) rates than soils on Pliocene age deposits at lower elevation. If so, several meters of material may have been removed on 10-million-year timescales and that many of the erosion features ascribed to ancient glacial erosion could simply be the result of subaerial erosion under cold desert conditions. In this case, the soils would only reflect the climate of the last few million years. Although the soils are classified primarily as Typic Anhyorthels and Typic Anhyturbels, some of the soils on doleritic till have saltpans enriched in NaNO 3 or Na 2SO 4 and are classified in petronitric or petrogypsic subgroups.

Tectonic subsidence v. erosional lowering in a controversial intramontane depression: the Jiloca basin (Iberian Chain, Spain)

The Jiloca basin is a large intramontane, NNW–SSE-trending topographical depression in which the relative role of tectonic subsidence and erosional lowering is currently a matter of discussion. Geometry and facies of the sedimentary infill at its central sector have now been characterized from compiled borehole data, which allows discussions of how the evolutionary model is constrained. The central Jiloca depression contains a Late Pliocene to Pleistocene sedimentary sequence made up of alluvial fan, pediment mantle and episodic palustrine deposits, overlying a carbonate unit that could represent an early lacustrine stage of Late Miocene–Early Pliocene age. The geometry of these units is partially controlled by NW–SE-striking normal faults. Both the morphological depression and the sedimentary basin truncate previous folds, whose traces beneath the Neogene–Quaternary infill have been interpreted from the geology of the basin margins, borehole data and hydrogeological criteria. The northern and southern sectors of the Jiloca depression are bounded by faults showing measurable hectometric-scale throws (Calamocha and Concud faults). Moreover, in the central sector, the ∼ 350–400 m tectonic uplift of Sierra Palomera has been interpreted from a morphostructural reconstruction of the tilted block which separates the Teruel and Jiloca graben, being similar to the height of the Sierra Palomera mountain front. All these features are consistent with a tectonic basin developed within the framework of the Neogene–Quaternary extensional evolution of eastern Spain. In contrast, they are hardly compatible with genetic models based on erosional deepening, either topographic lowering by numerous nested Tertiary erosion pediplains, or sub-alluvial Pliocene–Quaternary karstic corrosion.

Carbonate mound development at the SW Rockall Trough margin based on high resolution TOBI and seismic recording

In 2002, high-resolution sidescan sonar images of a mound area at the SW Rockall Trough margin were recorded with the TOBI deep towed sidescan sonar. Processed TOBI images with a pixel resolution of 6 m provide a unique overview of the carbonate mound distribution and related sedimentary features around the mounds. Three morphologically distinct areas can be recognised on the TOBI mosaic. In area I (between 500 and 600 m water depth), upslope of the mounds, giant sediment waves are found with wavelengths up to 500 m, with wave crests of more than 10 km long and with an amplitude of up to 30 m, reflected with a high backscatter on the TOBI image. Area II (between 600 and 1000 m water depth), along the upper margin flank, is characterised by clustered and isolated carbonate mounds, forming elongated ridges with an orientation perpendicular to the slope. Sedimentary structures such as flow ridges, sediment waves, local scouring at the foot of some mounds and draping of sediment around mounds indicate the influence of strong near-bed currents, oriented in two main current directions parallel to the margin, as well as in a similar direction as the mound clusters. The mound clusters are several kilometres long, can be over 380 m high and are dissected by downslope directed channels. On the TOBI image, the mounds appear as regions of high backscatter caused by the presence of cold water coral colonies on the sediment and by distinct shadows. All mounds have their tops at a specific depth level (500–600 m water depth). Area III (between 900 and 1200 m water depth) below the mounds is characterised by disturbed sediments with distinct slump scars and flow ridges. 2D high resolution seismic profiling perpendicular and parallel to the slope across the mounds does in general not reveal strong internal reflectors within the mounds. In contrast, three strong reflectors can be observed in the sedimentary sequence underneath the mounds. The upper reflector C10 (of early Pliocene age) is an erosional unconformity, above which most of the mounds and sediment waves seem to have developed. The second erosional unconformity C20 (of middle Miocene age) is formed by an apparent irregular surface that locally is dissected by some small faults. Reflectors C10 and C20 have been found in two different mound clusters in the mound area, indicating that mound development possibly started after formation of reflector C20. The third reflector forms the acoustic basement, which is locally dissected by some small faults that are located below the mounds. At least two stages of mound formation are recognised on the seismic profiles.

Signatures of storms, oceanic floods and forearc tectonism in marine shelf strata of the Quinault Formation (Pliocene), Washington, USA

AbstractMarine shelf strata of the Quinault Formation reflect the influences of storm–flood processes and convergent margin tectonism on sedimentation and palaeocommunity distributions in an active forearc basin of Early Pliocene age, western Washington, USA. The sedimentologic, ichnologic and invertebrate megafaunal character of coastal sea cliff exposures in the Pratt Cliff–Duck Creek area, Quinault Indian Nation, reveal five different sedimentary facies – scoured, Rosselia, bioturbated, mixed and Acharax. These facies document the shifting interplay and intensities among storms, waves and river‐flood plumes during transgression in inner to mid‐shelf settings. Storm sedimentation on the inner shelf is recorded north of Pratt Cliff by amalgamated, proximal tempestites of the scoured facies, which grade up‐section to thick deposits of hummocky cross‐stratified sandstone, indicative of strong wave influences. These hummocky beds alternate, in metre‐scale packages, with banded mudstone and siltstone that have distinctive sedimentologic and ichnofaunal characteristics (Rosselia facies). In particular the mudstone and siltstone occur as 1–15 cm‐thick, rhythmic, parallel beds that are laterally continuous, internally homogeneous to faintly laminated, and thus similar in nature to fine‐grained, oceanic flood deposits reported from shelf settings offshore the modern Eel River, northern California. The Quinault flood deposits are dominated by the ubiquitous trace fossil Rosselia socialis, comprising vertical, mud‐packed, flaring burrows with a sand‐filled central shaft which has been inferred as the feeding‐dwelling structure of a vermiform invertebrate adapted to high sedimentation rates in inner‐shelf settings. Fairweather conditions in between the higher energy periods of storms, waves and floods are recorded north of Pratt Cliff by the mixed facies, which is interpreted as representing the sand and mud zone of the inner‐ to mid‐shelf transition. Quieter, deeper, mid‐shelf, fairweather settings are typified by the bioturbated facies south of Pratt Cliff, where lower sedimentation rates and lower physical energies produced extensively bioturbated deposits of sandy siltstone punctuated, in places, by isolated sandy beds of distal tempestites.Quinault strata also chronicle stratigraphic signatures of subduction of the Juan de Fuca plate beneath western Washington during the Pliocene. For example, the imprint of geochemically unusual authigenic carbonates and a chemosynthetic palaeocommunity (Acharax facies) have been interpreted as a methane seep on the Quinault seafloor. Furthermore, a mobile rockground epifauna of pholadid bivalves became established on abundant, dark mudstone cobbles and pebbles sourced from the Hoh Assemblage, a Miocene accretionary prism that was actively deforming as well as interacting with Quinault forearc sediments during the Pliocene. Hoh mudstone clasts were supplied to the Quinault shelf via seafloor‐piercing diapirs and eroding mélange shear zones, exposures of which today occur in fault contact with Quinault strata along the coast from Taholah to the Raft River.

Neogene diatoms from the Sandy Ridge section, Alaska Peninsula: Significance for stratigraphic and paleogeographic reconstructions

Fossil diatoms from the Milky River Formation of the Sandy Ridge section, Alaska Peninsula are analyzed. Data on stratigraphic distribution of diatoms and silicoflagellates are presented. Based on presence of stratigraphically important forms (Neodenticula kamtschatica, Thalassiosira oestrupii, Cosmiodiscus insignis), the assemblages studied are correlated with the North Pacific Cenozoic diatom zones. An assemblage similar to that of subzone “b” of the Neodenticula kamtschatica Zone (Barron and Gladenkov, 1995) indicates the latest Miocene-initial early Pliocene (5.5-4.8 Ma) age of its host deposits. Additional species of biostratigraphic importance (Thalassiosira temperei, Th. latimarginata, and others) refine ages of different parts of the section. In particular, age of stratigraphically lowest beds bearing Astarte (mollusks valuable for establishing the Bering Strait opening) were dated at 5.5-5.4 Ma. Some paleogeographic inferences are made from analysis of composition of the diatom assemblages.

Pliocene marine deposits at Punta Maldonado, Guerrero state, Mexico

Diverse and abundant Foraminifera and Ostracoda assemblages were recovered from a measured stratigraphic section at Punta Maldonado, Guerrero state, Mexico. The planktonic species indicate an early Pliocene age, between 5.3 and 3.6 Ma; an early late Pliocene (around 2.4 Ma) planktonic assemblage also was recorded from isolated deposits. These ages contradict the Cretaceous–Paleogene age previously assigned to the sedimentary succession at Punta Maldonado. All indicators—benthic assemblages, ichnofacies, lithology, grain size, primary structures, mineralogy, body rock geometry, and facies—suggest deposition in the foreshore and offshore transition zones of a storm-dominated shallow siliciclastic shelf. The Ostracoda and Foraminifera indicate deposition around the outer neritic/upper bathyal boundary, which suggests an uplift of 320–400 m in the area during the Pliocene. This study represents the first report of Pliocene marine rocks in the southwestern coast of Mexico; the data presented contribute to regional geotectonic models.