Miocene basin development and volcanism along a strike-slip to flat-slab subduction transition: Stratigraphy, geochemistry, and geochronology of the central Wrangell volcanic belt, Yakutat-North America collision zone

New geochronologic, geochemical, sedimentologic, and compositional data from the central Wrangell volcanic belt (WVB) document basin development and volcanism linked to subduction of overthickened oceanic crust to the northern Pacific plate margin. The Frederika Formation and overlying Wrangell Lavas comprise >3 km of sedimentary and volcanic strata exposed in the Wrangell Mountains of south-central Alaska (United States). Measured stratigraphic sections and lithofacies analyses document lithofacies associations that reflect deposition in alluvial-fluvial-lacustrine environments routinely influenced by volcanic eruptions. Expansion of intrabasinal volcanic centers prompted progradation of vent-proximal volcanic aprons across basinal environments. Coal deposits, lacustrine strata, and vertical juxtaposition of basinal to proximal lithofacies indicate active basin subsidence that is attributable to heat flow associated with intrabasinal volcanic centers and extension along intrabasinal normal faults. The orientation of intrabasinal normal faults is consistent with transtensional deformation along the Totschunda-Fairweather fault system. Paleocurrents, compositional provenance, and detrital geochronologic ages link sediment accumulation to erosion of active intrabasinal volcanoes and to a lesser extent Mesozoic igneous sources. Geochemical compositions of interbedded lavas are dominantly calc-alkaline, range from basaltic andesite to rhyolite in composition, and share geochemical characteristics with Pliocene-Quaternary phases of the western WVB linked to subduction-related magmatism. The U/Pb ages of tuffs and Ar-40/Ar-39 ages of lavas indicate that basin development and volcanism commenced by 12.5-11.0 Ma and persisted until at least ca. 5.3 Ma. Eastern sections yield older ages (12.5-9.3 Ma) than western sections (9.6-8.3 Ma). Samples from two western sections yield even younger ages of 5.3 Ma. Integration of new and published stratigraphic, geochronologic, and geochemical data from the entire WVB permits a comprehensive interpretation of basin development and volcanism within a regional tectonic context. We propose a model in which diachronous volcanism and transtensional basin development reflect progressive insertion of a thickened oceanic crustal slab of the Yakutat microplate into the arcuate continental margin of southern Alaska coeval with reported changes in plate motions. Oblique northwestward subduction of a thickened oceanic crustal slab during Oligocene to Middle Miocene time produced transtensional basins and volcanism along the eastern edge of the slab along the Duke River fault in Canada and subduction-related volcanism along the northern edge of the slab near the Yukon-Alaska border. Volcanism and basin development migrated progressively northwestward into eastern Alaska during Middle Miocene through Holocene time, concomitant with a northwestward shift in plate convergence direction and subduction collision of progressively thicker crust against the syntaxial plate margin.

山东山旺中新世孢粉植物及古气候和古生态研究

本文对山旺中新世孢粉植物群进行了详细的研究。在22，95米厚的山旺硅藻土剖面上系统地采集了130块孢粉样品，样品之间的采样距离为10-30厘米。鉴定出孢粉类型111种。以此为基础，对山旺碰藻土沉积期间的古气候和古生态进行了高分辨率定量的恢复和重建。 根据每种孢粉类型的生物地层信息，山旺中新世孢粉植物群形成的时代为中中新世。这个年龄值也与同位素测年的结果16.78-14.11 Ma，以及根据哺乳动物划分的生物地层带MN4-MN5相一致。 运用共存分析法定量重建了山旺中新世孢粉植物群形成时的各项气候参数的定量值。所包括的10个古气候参数值分别为： 年平均温度(MAT) 15.6-17.2ºC，最冷月平均温度(TCM)5.0-6.6 0 C，最热月平均温度(TWM) 24.6-27.8 ºC，温度的年较差( TD) 20. 0-25.5 0 C，年平均降雨量(MAP) 1162-1308mm，最热月份的平均降雨量( PwarmM)108-lllmm，最湿月份的平均降雨量(PwetM) 148-180mm，最干月份的平均降雨量(PDM) 16-59mm，最干及最 湿月份降雨量的差值(DP) 81-153mm，平均相对湿度(RH) 72-74%，共存分析的结果表明，在整个22.95米厚的研究剖面上，从底部到顶部的古植物群形成期间的古气候有一个轻微的波动，其中年平均温度的波动范围大约在3度左右，最热月平均温度的波动范围在l度左右， 最冷月平均温度的波动范围在4度左右， 年平均降雨量的波动范围在100毫米左右。 山旺中新世时的气候应属于Koppen's的“Cfa”气候类型，与现今武汉与芜湖地区的气候相似。 与现代山旺地区气候上的区别主要在于年平均降雨量、降雨量的分配，以及最冷月份的平均温度。 通过应用多元分析的方法，对山旺中新世时期的植被进行了重建。山旺中新世时期的古植被为“混交中生林”，在研究剖面从底部到顶部的时间轴上，“混交中生林”经历了“湿润环境下的混交中生林”，“干旱环境下的混交中生林”，“湿润湖岸环境下的混交中生林”，“山地、湿润及碱性环境下的混交中生林”，和“碱性环境下的混交中生林”等5个阶段的变化。山旺剖面所代表的山旺湖周围的地形在断裂构造运动的控制下，也经历了一系列的变化。

Palaeoenvironmental analysis based on alluvial ostracod assemblages of the Cenicero section (lower Miocene, NW Ebro Basin)

In this work we perform for the first time a palaeoenvironmental and biostratigraphic analysis of the lower Miocene alluvial deposits of the Cenicero section (NW sector of the Ebro Basin; N Iberian Peninsula), based on the ostracod and micromammal assemblages. One of the main characteristics of this section is the unusual abundance on non-reworked ostracods present in the studied samples compared to other European sequences of similar age and sedimentary environment. This fact has allowed us to develop precise palaeoenvironmental reconstructions. The variations of the identified ostracod assemblages, defined by species such as Cyclocypris laevis, Ilyocypris bradyi, Ilyocypris gibba, Limnocythere sp. or Pseudocandona parallela, record the development of small, ephemeral and shallow ponds in a distal alluvial and/or floodplain environment. Towards the upper part of the section the ponds appear to be less ephemeral, being the aquatic systems more stable for ostracods development. Variations in the water temperature and salinity have been observed along the section, which are related to changes in the local pluviometric regime. On the other hand, the presence of micromammals in one of the studied samples has allowed the precise dating of this section. Thus, the presence of Armantomys daamsi dates the Cenicero section as Agenian (lower Miocene), local zone Y2 (MN2).

Climatic reconstruction at the miocene shanwang basin；China；using leaf margin analysis；clamp；coexistence approach；and overlapping distribution analysis

IEECAS SKLLQG

High resolution stratigraphy and miocene facies correlation in Lisbon and Setúbal Peninsula (Lower Tagus basin, Portugal)

Correlation between facies associations (marine, estuarine and distal fluviatile environments) and disconformities, observed between Foz da Fonte (SW of Setúbal Peninsula) and Santa Iria da Azóia (NE of Lisbon) are presented. The precise definition of the marine-continental facies relationships improved very much the chronology of the depositional sequence boundaries. Tectonic and eustatic controls are discussed on the basis of subsidence rates variation.

Palaeo-geomorphological significance of miocene and pliocene euryhaline ostracoda in the Nepean 1 borehole, Port Phillip Basin, SE Australia

Ostracoda able to tolerate significant fluctuations in salinity occur as fossils in latest Late Miocene and earliest Late Pliocene strata of the Nepean 1 borehole within the Port Phillip Basin. The main euryhaline species present are Osticythere baragwanathi (Chapman & Crespin), Leptocythere hartmanni (McKenzie), Xestoleberis cedunaensis Hartmann and Tanella gracilis Kingma. Euryhaline ostracods occur in both high diversity, high abundance fossil ostracod assemblages and in low diversity, high abundance  assemblages. These latest Late Miocene and earliest Late Pliocene fossil ostracod assemblages accumulated close to the maximum point of flooding during respective marine transgressions onto the northern Bass Strait hinterland. The presence of euryhaline Ostracoda in these assemblages  is reminiscent of ostracod occurrences associated with modern day SE Australian barrier coastlines. Euryhaline ostracods are not common in earlier Cenozoic sediments of SE Australia and Osticythere baragwanathi makes its first appearance in the latest Late Miocene beds of the Nepean 1 borehole. This latter occurrence is interpreted to reflect the initial phase of development for the modern SE Australian barrier coastline and in particular, the initial formation of Port Phillip Bay as a coastal geomorphological feature.

Record of the deep marine Clinocythereis australis Ayress and Swanson, 1991 (Ostracoda) from the upper Miocene Tambo River Formation, Gippsland Basin, Australia : Palaeo-oceanographic and biostratigraphic implications

Fossils of the deep marine ostracod, Clinocythereis australis Ayress & Swanson, 1991 occur within the Tambo River Formation, Gippsland Basin, southeastern Australia and record an approximately 6 Ma phase of late Miocene coastal ocean upwelling within this region. The presence of deep marine faunal elements within late Miocene Mitchellian strata is in contrast to the absence of such faunal elements in latest Miocene Cheltenhamian and younger marine strata of the Bass Strait hinterland. The absence of deep marine faunal elements in post-Mitchellian onshore strata is due to the Kosciusko Uplift, which transformed Bass Strait into a wholly shallow seaway placing adjacent coastal regions beyond the reach of ocean upwelling influences.