Late Miocene Environmental Crises and Their Impacts:Desiccation of the Mediterranean and Its bearing on World Desertification and Hominid Appearance

正The ancient Mediterranean was once dried up around6~5.3million years ago.It is a landmark event,which had great impact on global environment and climate.However,few references are available in the literatures on this topic,     

Geochemical constraints on the genesis of the Pb–Zn deposit of Jalta (northern Tunisia): Implications for timing of mineralization,sources of metals and relationship to the Neogene volcanism

The occurrence of Pb–Zn deposits of Jalta district (northern Tunisia) as open space fillings and cements and breccia in the contact zones between Triassic dolostones and Miocene conglomerates along or near major faults provides evidence of the relationship between the mineralization and tectonic processes. Pb isotopes in galena from the deposits yielded average ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios of 18.821, 15.676 and 38.837, respectively, implying a well-mixed multi-source upper crustal reservoir of metals. Magmatism and compressional tectonism during the Alpine orogeny favored Pb–Zn mineralization in the Jalta district. The enrichment in Pb, Zn, Cd and Co of the Triassic carbonates and enrichments in Pb, Zn and Cd in Triassic clayey shales is associated with hydrothermal alteration around faults. Alunite in the deposit has δ³⁴S values (−2.5 to −1.5‰ VCDT), which could have been formed at and above the water table in a kind of steam-heated environment, where fluids containing H₂S mixed with fluids containing K and Al. The H₂S could have been produced by TSR of sulfates at high temperature at depth and then leaked upward through deep-seated faults, whereas the K and Al could have been acid-leached from Miocene volcanic rocks.     

Before the flood: Miocene otoliths from eastern Amazon Pirabas Formation reveal a Caribbean-type fish fauna

The Pirabas Formation of Early Miocene age represents the final stage of the central western Atlantic carbonate platform in northeastern South America, predating the emplacement of the Amazon delta system. The otolith-based fossil fish fauna is represented by 38 species typical of a shallow marine environment. A total of 18 species are described new to science from the families Congridae, Batrachoididae, Bythitidae, Sciaenidae and Paralichthyidae. The fish fauna was associated with high benthic and planktic primary productivity including seagrass meadows, calcareous algae and suspension-feeders. The break of todays shallow marine bioprovince at the Amazonas delta mouth is not evident from the fish fauna of the Pirabas Fm., which shows good correlation with the Gatunian/proto-Caribbean bioprovince known from an only slightly younger time window in Trinidad and Venezuela. Differences observed to those Early Miocene faunal associations are interpreted to be mainly due to stratigraphic and geographic and not environmental differences. We postulate that the emergence of the Amazonas river mouth close to its present day location has terminated the carbonate cycle of the Pirabas Fm. and pushed back northwards a certain proportion of the fish fauna here described.     

Petrogenetic implications and geochronology of middle Miocene Tannayama igneous rocks,Goto Islands,Japan Sea southern margin,northwestern Kyushu,Japan

The subduction of “hot” Shikoku Basin and the mantle upwelling related to the Japan Sea opening have induced extensive magmatism during the middle Miocene on both the back-arc and island-arc sides of southwest Japan. The Goto Islands are located on the back-arc side of northwestern Kyushu, and middle Miocene granitic rocks and associated volcanic, hypabyssal, and gabbroic rocks are exposed. The igneous rocks at Tannayama on Nakadori-jima in the Goto Islands consist of gabbronorite, granite, granite porphyry, diorite porphyry, andesite, and rhyolite. We performed detailed geological mapping at a 1:10 000 scale, as well as petrographical and geochemical analyses. We also determined the zircon U–Pb age dating of the igneous rocks from Tannayama together with a granitic rock in Yagatamesaki. The zircon U–Pb ages of the Tannayama igneous rocks show the crystallization ages of 14.7 Ma ± 0.3 Ma (gabbronorite), 15.9 Ma ± 0.5 Ma (granite), 15.4 Ma ± 0.9 Ma (granite porphyry), and 15.1 Ma ± 2.1 Ma (rhyolite). Zircons from the Yagatamesaki granitic rock yield 14.5 Ma ± 0.7 Ma. Considering field relationships, new zircon data indicate that the Tannayama granite formed at ~16–15 Ma, and the gabbronorite, granite porphyry, diorite porphyry, andesite, and subsequently rhyolite formed at 15–14 Ma, which overlaps a plutonic activity of the Yagatamesaki. The geochemical characteristics of the Tannayama igneous rocks are similar to those of the tholeiitic basalts and dacites of Hirado, and the granitic rocks of Tsushima in northwestern Kyushu. This suggests that the Tannayama igneous rocks can be correlated petrogenetically with the igneous rocks in those areas, with all of them generated by the upwelling of hot mantle diapirs during crustal thinning in an extensional environment during the middle Miocene.     

Geologic age of the lower Josoji Formation,Shimane Peninsula,Southwest Honshu,Japan: Implications for an abrupt change to deep-water during the earlier opening stage of the Japan Sea

The lower part of the Josoji Formation, Shimane Peninsula, contains clues for figuring out changes in deep-water characteristics during the opening of the Japan Sea. The foraminiferal assemblage includes early to middle Miocene biostratigraphic index taxa such as planktonic foraminiferal Globorotalia zealandica and Globorotaloides suteri. The occurrence of these two species, together with the absence of praeorbulinids, suggests that the lower part of the Josoji Formation is assigned to the top of planktonic foraminiferal Zone N7/M4 (16.39 Ma). The benthic foraminiferal assemblage, which is characterized by Cyclammina cancellata and Martinottiella communis, clearly suggests that the lower Josoji Formation was deposited at bathyal depths, and that it developed in association with the abrupt appearance of deep-sea calcareous forms. Such bathyal taxa are the main constituents of the Spirosigmoilinella compressa–Globobulimina auriculata Zone of the Josoji Formation and also of the Gyrodina–Gyroidinoides Zone at Ocean Drilling Program Site 797 in the Japan Sea. The base of these benthic foraminiferal zones can be correlated with the base of the nannofossil Sphenolithus heteromorphus Base Zone (= CNM6/CN3); thus, its estimated age is 17.65 Ma. This biostratigraphic information suggests that the lower Josoji Formation was deposited from shortly before 17.65–16.39 Ma in upper limit age. Evidence that fresh to brackish and shallow-water basins formed in the rifting interval of 20–18 Ma in the Japan Sea borderland suggests that the abrupt appearance of deep-sea calcareous foraminifera occurred about 1 my earlier in this area than in other sedimentary basins and suggests that a significant paleoceanographic change occurred in the proto-Japan Sea at 17.65 Ma.     

Foraminiferal sequence biostratigraphy of the Oligo‐Miocene Janjukian strata from Torquay,southeastern Australia

The foraminiferal fauna from two holes near the coastal section of marls and limestones at Torquay is dominated by inner to mid‐shelf benthic forms, especially the cibicidids, discorbids and miliolids. Planktonic species are rare and rarely age‐diagnostic. A cluster analysis of the species occurrences and relative abundances identified four assemblages, A to D up‐section. These assemblages also closely correspond to lithological changes characterising lithostratigraphic units: Angahook Formation (assemblage A), lower and upper Jan Juc Formation (B, C) and Puebla Clay (D). Biofacies trends based on the relative abundances of inner and outer neritic taxa led to the recognition of third‐order sequences and boundaries equivalent to TB1.1 to TB1.4, confirming a previous identification on sedimentological grounds. They demonstrate that foraminiferal assemblages were directly influenced by third‐order sea‐level fluctuations and can be used to predict third‐order sequences. The long‐uncertain regional Oligocene‐Miocene boundary is placed at the Jan Juc Formation—Puebla Clay contact, across which there was a major faunal change. Several benthic forms disappeared, at least temporarily: Cibicidoides perforatus, Amphistegina lessonii and Pararotalia mackayi. The typical Miocene planktonic taxa Globoquadrina dehiscens and Globoturborotalita brazieri made their first appearance. These events were associated with an increase in inner neritic benthos signalling a low sea‐level, consistent with the contemporaneous global pattern.     

Astronomical Dating of the Middle Miocene Hanjiang Formation in the Pearl River Mouth Basin,South China Sea

The Hanjiang Formation of Langhian age(middle Miocene) in the Pearl River Mouth Basin (PRMB),South China Sea consists of deltaic siliciclastic and neritic shelf carbonate rhythmic alternations,which form one of the potential reservoirs of the basin.To improve stratigraphic resolutions for hydrocarbon prospecting and exploration in the basin,the present study undertakes spectral analysis of high-resolution natural gamma-ray(NCR) well-logging record to determine the dominant frequency components and test whether Milankovitch orbital signals are recorded in rhythmic successions.Analytical results indicate the orbital cycles of precession(～19 ka and～23 ka), obliquity(～41 ka),and eccentricity(～100 ka and～405 ka),which provide the strong evidence for astronomically driven climate changes in the rhythmic alternation successions.Within biochronological constraint,a high-resolution astronomical timescale was constructed through the astronomical tuning of the NGR record to recent astronomically calculated variation of Earth’s orbit. The astronomically tuned timescale can be applied to calculate astronomical ages for the geological events and bioevents recognized throughout the period.The first downhole occurrences of foraminifers Globorotalia peripheroronda and Globigerinoides sicanus are dated at 14.546 Ma and 14.919 Ma,respectively,which are slightly different from earlier estimates in the South China Sea. When compared with the global sea-level change chart,the astronomical estimate for the sequences recognized based on microfossil distributions have the same end time but the different initiation time. This is probably due to the local or regional tectonic activities superimposed on eustatic rise which postponed the effect of global sea-level rising.Astronomical timescale also resolves the depositional evolution history for the Langhian Stage(middle Miocene) with a variation that strongly resembles that of Earth’s orbital eccentricity predicted from 13.65 Ma to 15.97 Ma.We infer that the main factor controlling the variability of the sedimentation rate in the Hanjiang Formation is related to the～405-ka-period eccentricity.     

Fault systems and paleostresses in the Vestfirdir Peninsula. Relationships with the Tertiary paleo-rifts of Skagi and Snaefells (Northwest Iceland)

Abstract

The Vestfirdir Peninsula of northwestern Iceland mainly consists of tholeiitic lava flows, 8–14 Ma old, gently dipping to the southeast. A detailed study of strike-slip and normal faulting allowed identification of two main paleostress regimes. Two sets of normal faults were recognized. The largest set trends ENE-WSW to NNE-SSW; the minor set trends NW-SE to WNW-ESE. Concerning the major extension, the reconstruction of paleostress trajectories shows a gradual change in trend from ESE-WNW, in the northern half of the peninsula, to NNW-SSE to the south. The minor extension also shows a gradual change from NNE-SSW to ENE-WSW trends, from north to south. The nearly constant perpendicularity between the major and minor trends of extension is accounted for by permutation of stress axes within the general pattern of extension related to oceanic rifting. The progressive azimuthal change of the major extension trend, from northeast to southwest across the peninsula, is interpreted as the expression of a change in trend of the extinct Skagi-Snaefells rift, a structure that cannot be reconstructed directly through geological mapping. The average trends of extension in the southern Vestfirdir Peninsula, N150°E, suggest a N60°E trend for the ancient Snaefells rift segment. Likewise, the N100–110°E trends of extension in northern Vestfirdir suggest a N10–20°E trend for the Skagi paleo-rift., © Elsevier, Paris     

Magnetostratigraphy of the Ouarzazate Basin: Implications for the timing of deformation and mountain building in the High Atlas Mountains of Morocco.

Two magnetostratigraphic profiles (450 samples) have been carried out to constrain the age of synorogenic formations in the southern foreland of the High Atlas of Morocco. The Amekchoud profile covers the Aït Ouglif and Aït Kandoula alluvial formations that form the bulk of the Ouarzazate basin fill, indicating an age between the upper Langhian and the Messinian (Miocene). Data obtained in the previously unexplored Hadida formation profile covers the oldest terms of the foreland basin succession, but the low quality of the magnetic record only allows proposing a tentative age between the middle Lutetian and an undetermined middle to late Eocene. The correlation of the Amekchoud profile is based on the recognition of the long C5n chron (Tortonian) in the middle part of the section studied and a new vertebrate locality of upper Tortonian age found in the upper part. These results indicate a discontinuous record of foreland basin development in the southern Atlas domain from mid Eocene to late Miocene times, punctuated by an intermediate large hiatus of 20-25 ma (late Eocene to mid Miocene). Thrusting in the Sub-Atlas frontal thrust belt began before the Aït Ouglif and Kandoula formations, probably during the Oligocene, and extends up to recent times. The alternation of periods of deposition with others of no sedimentary record, which does not coincide with specific tectonic events, results probably from the interference of orogenic deformation and the mantle-related thermal uplift events that have been described for the Moroccan Atlas.     

Sedimentary architecture and growth pattern of turbidite systems in distal part of a median fan; example of the Upper Miocene sedimentary sequence of the Lower Congo basin

The understanding of deep-water turbidite systems implies a preliminary detailed analysis of the architectural elements which compose them. Using 3D seismic data, three architectural elements are recognized including a new one: the “meandering erosional nested channels”. The spatial organisation and the relative stratigraphic position of these “elementary bricks” allow to define four stages which form the sedimentary history of the distal part of a upper Miocene turbidite system of the Lower Congo basin: 1, depositional stage with frontal splay development; 2, erosional channel and prograding system; 3, depositional stage with vertical aggradation of the channel and 4, abandonment phase with channel avulsion.