End Of Middle Miocene Research Articles

Petrography and palynology of Late Oligocene and Middle Miocene coals in the Gelibolu peninsula, NW Turkey

In this study, coal samples from two different coal zones located close to each other were examined. They occur in the Late Oligocene Osmancık and Miocene Gazhanedere formations in the Gelibolu peninsula, in the SW part of the Thrace Basin (NW Turkey). This paper focuses on the organic-petrographic composition and depositional conditions of the two coal zones by using coal petrography, including maceral ratios, and palynology. The maceral composition of the Oligocene coal samples is similar to that of the Miocene coal samples. The coal samples from both locations are characterized by a high content of huminite group macerals, ranging from 46% to 78%. The prevailing maceral from this group is gelinite (31%-65%), which is indicative of a high degree of gelification of organic matter. Relatively low contents of liptinite (not exceeding 9%) and inertinite (not exceeding 8%) were also determined. The tissue preservation of organic matter was poor. The content of mineral matter is inferred from microscopic studies and is variable but generally high, varying from 6% to 46%, similar to that of other Turkish coals; it consists mostly of quartz, calcite, clay minerals, and pyrite. This input of mineral matter is probably related to periodic inundations of swamp. The mean reflectance values of samples ranged from 0.42% to 0.50% Ro, suggesting that the rank of coal is subbituminous according to ASTM classification. Low tissue preservation index (TPI) and high gelification index values suggest that both coal zones were deposited in a limnic environment. The presence of framboidal pyrite and low TPI reflect high pH values (pH 6-7) and reducing conditions during peat deposition. Two different palynological assemblages may be distinguished based on the percentages of palynomorphs. The first assemblage including the stratigraphical marker species Dicolpopollis kockelii (Calamus) suggests the Late Oligocene. The age of the second assemblage is probably the end of Middle Miocene.

Formation stages of the unique sangaredi bauxite-bearing group, Futa Jallon-Mandingo province, West Africa

The middle Miocene continental fluvial sediments of the Sangaredi Group in the western districts of the Futa Jallon-Mandingo bauxite-bearing province were initially more widespread. Their thickness in the axial zones of the main drainage systems (Paleocogon, Paleotomine, etc.) attained 150 m. The lower, early Miocene steps of topography formed during the first phase of regional neotectonic evolution were buried beneath these sediments by the end of middle Miocene. The sediments of the Sangaredi Group underwent substantial epigenetic alteration in the glei geochemical setting with removal of iron and bleaching. At the second stage of neotectonic uplifting and dissection, this low-Fe and high-Al protolith was again affected by lateritic weathering with formation of unique sedimentary-lateritic bauxites. The underlying bedrock was also affected by bleaching and lateritization with formation of high-quality infiltration-metasomatic bauxites with geliform cryptocrystalline structure and massive porcelainlike or oolitic texture. Both classes of bauxites are high-quality and of great economic importance.

Late Cretaceous to Recent kinematics of SE Anatolia (Turkey)

Abstract Five different deformation phases have been recognized in the SE Anatolian orogen and the Arabian Platform based on palaeostress inversion studies using fault-slip data sets. The timing and duration of these phases are determined using various criteria including the age of the affected strata, syndepositional structures, cross-cutting structures and overprinting slickensides. The oldest deformation phase is characterized generally by NE–SW-directed extension. The extension is thought to have resulted from slab-roll back processes during the Maastrichtian to Middle Eocene interval ( c . 60 Ma to 40–35 Ma). The second deformation phase is characterized by east–west to NW–SE-directed compression and thought to result from cessation of roll-back processes possibly due to subduction of younger oceanic crust or increase in the convergence rate between Africa and Eurasia during the post-Middle Eocene to Late Oligocene interval ( c . 40–35 Ma to 25 Ma). The third deformation phase is characterized by east–west to NW–SE-directed extension possibly due to slab detachment that initiated in Iran and migrated westwards during the latest Oligocene to Middle Miocene period (25–11 Ma). The fourth deformation phase is characterized by approximately north–south-directed compression due to collision and further northwards indentation of Arabian Plate by the end of Middle Miocene (11–3.5 Ma). The fifth and present deformation phase is characterized by NE–SW compression which might result from tectonic re-organization in the region since the Middle Pliocene ( c . 3.5 Ma to recent).

On the systematics and phylogeography of eight-barbel loaches of the genus Lefua (Cobitoidea: Nemacheilidae): mtDNA typing of L. pleskei

Comparative analysis of personal sequence data for the mtDNA control region (926 to 928 bp) from eight-barbel loaches inhabiting eight localities in the Amur River basin (4) and the Sea of Japan (4) and the GeneBank/NCBI data for the Lefua individuals from the other regions of the world showed that eight-barbel loaches from Primorskii krai water basins were marked by a specific group of mtDNA haplotypes. This finding is considered as supporting the species status of L. pleskei. Genetic distances within L. pleskei are small (on average 0.355) and close to those within L. nikkonis (on average 0.48%). The distances between this species pair are the least (on average 2.15%) among all other pairs compared. In MP, ML, and Bayesian trees, L. pleskei and L. nikkonis haplotypes formed a common clade with high statistically significant support. In all tree variants, L. costata mtDNA haplotypes were located out of the group of interest. A clade consisting of highly diverged lineages of Lefua sp. and L. echigonia haplotypes occupied even more independent position. The mtDNA haplotypes of L. pleskei and L. costata from the Amur River basin were evolutionary young and derived from the haplotypes found in these species from the Sea of Japan (L. pleskei) or the Yellow Sea (L. costata) basins. It is thereby suggested that both species rather recently migrated into the Amur River system. According to the molecular clock data, basal diversification of the eight-barbel loach lineages took place at the end of middle Miocene (about 11 to 12 Myr ago), while divergence of L. pleskei and L. costata ancestral forms probably occurred approximately, 5 Myr ago. Since all main lineages of eight-barbel loaches were found in the Sea of Japan basin (continental coastline and the islands), the divergence order and dispersal patterns of the Lafua species might have been largely determined by the geological development pattern of this water body and the adjacent territories.

Ignimbrite flare‐up and deformation in the southern Sierra Madre Occidental, western Mexico: Implications for the late subduction history of the Farallon plate

The Sierra Madre Occidental (SMO) of western Mexico is one of the largest silicic volcanic provinces on Earth, but the mechanism for the generation of such a large volume of ignimbrites has never been clearly defined. We present new 40Ar/39Ar ages, geologic mapping, and structural data for the southern part of the SMO demonstrating that most of this volcanic province was built in two episodes of ignimbrite flare‐up in Oligocene (31.5–28 Ma) and early Miocene (23.5–20 Ma) time, and that extensional deformation occurred mostly before the transfer of Baja California to the Pacific plate. Extensive ignimbrite successions, with 40Ar/39Ar ages clustering at ∼23 and ∼21 Ma, cover most of the southern SMO, thus correlating in age with ignimbrites exposed in southern Baja California and central Mexico. Grabens with a 020° to N‐S orientation developed in the east almost concurrently with this volcanic episode. Half grabens and NNW striking listric normal fault systems formed at the end of middle Miocene as far as 150 km from the present coast. A belt of left‐lateral transpressional structures formed along the southern boundary of the SMO during the same period. We link these magmatic and tectonic events to the evolution and dynamics of the Farallon and North America plates during the Miocene. Particularly, we propose that a first detachment of the lower part of the Farallon plate in early Miocene time produced a transient thermal event and partial melting of the crust via mafic underplating. Middle Miocene extension would be related to a second detachment event, resulting from the slowing subduction that preceded the final capture of the Magdalena microplate by the Pacific plate at 12.5 Ma. Transpression at the southernmost end of the SMO occurred along the inland projection of the Magdalena‐Cocos plate boundary and may be explained by a difference in subduction rate and by a temporal convergence between the two plates in the eve of the end of subduction of the Magdalena plate.

Evidence on Mixing of Mantle and Crustal Derived Magmas in Bodrum (Mugla) Area Volcanic Rocks, Southwest Turkey

The study area is located at Turgutreis-OrtakentYalikavak-Akyarlar areas, the west of Bodrum (Mugla) in the Southwest Turkey where calcalkaline, alkaline and locally shoshonotic volcanism were effective after the end of Middle Miocene. These volcanics has been attributed to continental rifting due to tensional regime (e.g. Ercan et al., 1984). Tensional tectonics in the region affected thickened and partially melted continental crust, and caused the mixing of crustal calcalkalineand mantle derived alkaline magmas. Two volcanic cycles were effective in the Bodrum Peninsula during Middle-Upper Miocene times. First cycle volcanites are tuffagglomerate, high-K calcalkaline andesite-daciterhyodacite-rhyolite suite developed at the end of Middle Miocene. This cycle ceased with shoshonitic latite and trachyandesite, and second cycle developed with shoshonitic basalt, trachybasalt and trachyte as small outcrops and dykes in Upper Miocene (Ercan et al., 1984). The volcanites studied, namely andesite, latite, dacite, rhyodacite and rhyolite (Fig. 1), show generally porphyric and sometimes felsitic, hyalopilitic, microlitic, pilotaxitic textures. Phenoand microphenocrystals are plagioclase, biotite, hornblende, quartz, augite and sanidine. Groundmass is made up generally by feldspar microlites and/or glass, and sometimes alteration products. Plagioclases are subhedral laths and rarely resorbed and fragmented crystals. Some of them commonly display sieve texture with mottled extinction or patchy zoning, and inclusions of pale grown glass, sometimes with an overgrowth rim which may be separated from the cores by a dark grey zone of 'clouded' plagioclase charged with micron-sized melt inclusions. Plagioclases also show discontinuous and sometimes oscillatory zoning. Hornblendes are subhedral to anhedral, but often 'ghost like' crystals, pseudomorphed by opaques, pyroxene, feldspars and quartz or simply rimmed by opaques. Biotites are subhedral to anhedral with common reaction rims or breakdown products, or pseudomorphed by opaques. Breakdown of biotite forms opaques or a granular intergrowth of opx + ilmenite + magnetite + K-feldspar. These reaction products of hornblend and biotite were caused by dehydration above the hornblend and biotite stability limit as result of magma mixing (e.g. Nixon, 1988). The rocks are generally high-K calcalkaline in compos i t ion , having 4 7 % K20 (Fig. 1), 15.8-17.8% AlaO3, 26.3-47.1 Mg-number, low Ni, Cr, Co and high La, Ce contents. Furthermore low Na20/K20, Nb, TiO2 (<1%) and high LILE contents resembles volcanites developed at active continental margins. Geochemical variations show that the rocks are cogenetic and related to each other by fractional crystallisation, assimilation and magma mixing processes during the evolution of crustal magma chambers. Major and trace element variations can be explanied by fractionating phenocrystal phases. Low Y and high La/Y may imply that