New data on the Albian-Cenomanian radiolarians from the Karai Formation (South India)

Radiolarians from the lower part of the Karai Formation (upper Albian-middle Cenomanian) are studied in detail for the first time. Among over 50 radiolarian species identified in the formation, there are Acaeniotyle amplissima (Foreman), Savaryella novalensis (Squinabol), S. quadra (Foreman), Vitorfus campbelli Pessagno, Archaeodictyomitra montisserei (Squinabol), Holocryptocanium barbui Dumitrica, Pseudoeucyrtis sp. cf. Ps. spinosa (Squinabol), Stichomitra communis Squinabol, Tubilustrionella transmontanum (O’Dogherty), and others. The discovered radiolarians are divided into the Halesium triacanthum-Orbiculiforma nevadaenis (late Albian-early Cenomanian), Crucella latum-Cryptamphorella micropora (late Albian?-early Cenomanian), and Becus sp. B-Godia concava (terminal Albian-middle Cenomanian) assemblages. In general, the Albian-Cenomanian radiolarians of South India are comparatively less diverse than the concurrent assemblages of the Mediterranean region and California. In taxonomic composition and morphological peculiarities, they are comparable with the Aptian-Albian radiolarians of Western Australia (Ellis, 1993). Consequently it can be postulated that sea basins of South India were situated during the Albian-Cenomanian in the temperate latitudes of the Southern Hemisphere.     

Mineralogical and Geochemical Constraints on the Sediment Sources of Late Stone Age Pottery from the Birimi Site,Northern Ghana

The mineralogy and bulk chemical compositions of 15 Kintampo (Late Stone Age) potsherds from the Birimi site on the Gambaga Escarpment and eight samples of local sediment were determined with the intent of characterizing these wares and identifying the material used in their manufacture. Sediment from clay pits still used by potters north of the escarpment contains iron‐rich laterite clasts (100 × XFeO_t = 100 × FeO_t/[FeO_t + Al₂O₃ + SiO₂] ≥10). Sedimentary clasts in stream sediments are relatively siliceous and iron‐poor (100 × XFeO_t < 10). Bulk geochemical data together with the compositions of lithic clasts (laterite, siltstone/sandstone) link the pottery to sediment sources, including escarpment sediments not presently used by Ghanaian potters. Fresh granite clasts found in some of the sherds were not found in the analyzed sediment samples, although some of their distinctive mineralogical constituents (e.g., variably barian alkali feldspar) are present. The analytical data suggest that pots found at Birimi were made locally by mixing escarpment sediment with clay and stream sediment brought in from below the escarpment. This contrasts with present‐day practice, whereby the pots themselves are imported. The place where Birimi pottery was made and the outcrop source of aluminous sediment (mudstone with an “escarpment” trace element signature) used in these wares, however, remain unidentified. © 2013 Wiley Periodicals, Inc.     

Lake George revisited: New evidence for the origin and evolution of a large closed lake,Southern Tablelands,NSW, Australia

Lake George contains the longest continuous sedimentary record of any Australian lake basin, but previous age models are equivocal, particularly for the oldest (pre-Quaternary) part of the record. We have applied a combination of cosmogenic nuclide burial dating, magnetostratigraphy and biostratigraphy to determine the age of the basal (fluvial) unit in the basin, the Gearys Gap Formation. Within the differing resolutions achievable by the three dating techniques, our results show that (i) the Gearys Gap Formation, began accumulating at ca 4 Ma, in the early Pliocene (Zanclean), and (ii) deposition had ceased by ca 3 Ma, in the mid late Pliocene (Piacenzian). Whether the same age control provides an early Pliocene (Zanclean) age for the formation of the lake basin is uncertain. During the Piacenzian, the vegetation at the core site was a wetland community dominated by members of the coral fern family Gleicheniaceae, while the surrounding dryland vegetation was a mix of sclerophyll and temperate rainforest communities, with the latter including trees and shrubs now endemic to New Guinea–New Caledonia and Tasmania. Mean annual rainfall and temperatures are inferred to have been ～2000–3000 mm, although probably not uniformly distributed throughout the year, and within the mesotherm range (>14°C <20°C), respectively. Unresolved issues are: (1) Does the basal gravel unit predate uplift of the Lake George Range and therefore provide evidence that one of the proposed paleo-spillways of Lake George, that above Geary's Gap, has been elevated up to 100–200 m by neotectonic activity over the past 4 million years? (2) Did a shallow to deepwater lake exist elsewhere in the lake basin during the Pliocene?     

Tectonic and seismic conditions for changes in spring discharge along the Garni right lateral strike slip fault (Armenian Upland)

Abstract

This study presents the relationship between local seismicity and springs discharge, in which the microseismicity modifies the state of stress of the aquifer matrix and these modifications of the local stress control the aquifer behaviour from an example of a basaltic reservoir crossed by a seismic fault.

Along the right-lateral Garni seismic wrench fault, the distribution in space and time of more than 500 micro-earthquakes has been studied providing a pattern of stress distribution. Together with this tectonic survey, the hydrographs of springs issuing from an aquifer located in basaltic lava flows and crossed by the fault have been gauged for 4 years. According to its tectonic, geologic and hydrodynamic properties, the reservoir has been divided in to several parts. The hydrological behaviour of the reservoir and of each of its parts has been modelled, based on the rainfall and hydrodynamic properties of the basaltic reservoir and of nearby reservoirs.

The model allows us to define the ‘normal behaviour’ of the springs, when their discharge is not affected by an earthquake. Anomalies to the normal hydrometric curves are defined, and correlated to small (M < 3.5) earthquakes along the Garni fault. We propose that the circulation of underground water in the area around the fault depends largely on the fracture pattern of the basalts and the aquifer basement, as pointed out from field observations. Changes in spring discharges are explained by variations in the state of stress around the Garni fault, induced by the alternation of elastic strain and stick-slip movement on the fault, and by creep far from the fault. A model of stress distribution is proposed which explains changes in fracture charateristics before and after earthquakes. © Elsevier, Paris     

Middle-Late Alpine thermotectonic evolution of the southern Rhodope Massif,Greece

Abstract

The transition from the Alpine tectonic assembly to the exhumation of the units in the Rhodope metamorphic province in northernmost Greece has been refined by ⁴⁰Ar/³⁹Ar laserprobe mica analyses. Preservation of pre-Alpine (~ 280 Ma and 145 Ma) muscovite cooling ages at the western margin of the Rhodope indicate that subsequent events failed to reset the argon system thermally in white mica in the outcropping basement of this region. The central and eastern Rhodope are characterized by white mica cooling ages of 40–35 Ma with ages gradually decreasing to ca. 15 Ma near the eastern margin of the Strymon Valley. The Eo-Oligocene ages reflect the regional exhumation of the metamorphosed units to shallow crustal levels, with corresponding temperatures below ca. 350 °C, by 40–35 Ma. The younger cooling ages are attributed to the initiation and subsequent operation of the Strymon-Thasos detachment system since ca. 30 Ma. This study provides a crucial contribution to future regional tectonic models for the Rhodope region as it recognizes an early stage of development of the Strymon-Thasos detachment system, and has constrained the regional exhumation of the Rhodope metamorphic province since 40 Ma indicating that the regionally observed amphibolite facies metamorphism had terminated by this time. © 2000 Editions scientifiques et médicales Elsevier SAS     

Characteristics of tropical cyclones making landfall on the Pacific coast of Mexico: 1970-2010

This study evaluates impacts from tropical cyclone (TC) landfalls on populated areas located along the Pacific Ocean coast of Mexico. The period of interest is from 1970 through 2010 and an international disaster database is used to identify the impact from the landfalling TCs. More than 30 landfall events occurred during the period; we examined the top 25 TCs based on rainfall accumulation, as well as the top 10 TC-related disasters based on the affected population. Each event resulted in affected population from 20 000 to more than 800 000. Strong winds and heavy rainfall, during periods of one to three days, are associated with property damage and loss of lives. Our results indicate that excessive rainfall accumulations and daily rates, over highly populated areas, are important elements associated with the occurrence of disasters. Six of the top 10 TC-related disasters occurred during El Niño and three during neutral conditions; however, looking at the top 25 events, 10 occurred during El Niño and 10 during neutral conditions. Three case studies that occurred during El Niño events (Liza in 1976, Pauline in 1997, and Lane in 2006) are documented in more detail as they affected areas with different population densities in the southern and northwestern coasts of Mexico.     

Temporal relationship between Holocene human occupation and vegetation change along the northwestern margin of the Central African rainforest

This paper discusses the relationship between patterns of human settlements and environmental change during the Holocene along the northwestern margins of the equatorial rain forest of central Africa. Palaeoenvironmental data from high-resolution sediment cores from lacustrine deposits, plant macro-remains from forest soils, and archaeological data are harnessed to discuss the differential impact of climate and/or humans on the central African rain forest. It is shown that climate change impacted the rain forest well before the widespread expansion of human settlements all over the study area.     

Geochemistry of Brines from Salt Ore Deposits in Western Tarim Basin

In the geological evolution of the Tarim Basin, many transgressions and relictions happened. So there have been plentiful sources of salt. Moreover, because of uttermost drought, a lot of salt has been deposited. It is possible to find potash salt in this area. In our fieldwork, we have found salt and brine in western Tarim Basin. Based on a geological survey and the characteristics of sedimentary facies and paleogeography, this paper deals with the geochemical parameters and discusses the possibility of formation of potash salt in terms of the chemical analyses of samples collected from western Tarim Basin. Results of brine analysis lead to some conclusions: most of these salt brines have eluviated from very thick halite beds, mainly chloridetype salt and this kind of halite does not reach the stage of potash deposition in all aspects; WKSL (Wukeshalu) occupies a noticeable place, and we should attach importance to this district because there have been some indicators of the occurrence of potash deposits as viewed from the contents of Br and K. Finally, low Br contents are recognized in the Tarim Basin as a result of salt aggradation, and this point of view has been proved by the results of this experiment and the data available. It cannot depend upon the index of Br to judge the evolution stage of halite. We must look for other facies of potash except marine facies.     

Tectonic controls on metamorphism, partial melting, and intrusion: timing and duration of regional metamorphism and magmatism in the Ni de Massif, Turkey

Mafic high-pressure granulite, eclogite and pyroxenite xenoliths have been collected from a Mesozoic volcaniclastic diatreme in Xinyang, near south margin of the Sino-Korean Craton (SKC). The high-pressure granulite xenoliths are mainly composed of fine-grained granoblasts of Grt+Cpx+Pl+Hbl±Kfs±Q±Ilm with relict porphyritic mineral assemblage of Grt+Cpx±Pl±Rt. P–T estimation indicates that the granoblastic assemblage crystallized at 765–890 °C and 1.25–1.59 GPa, corresponding to crustal depths of ca. 41–52 km with a geotherm of 75–80 mW/m². Calculated seismic velocities (V_p) of high-pressure granulites range from 7.04 to 7.56 km/s and densities (D) from 3.05 to 3.30 g/cm³. These high-pressure granulite xenoliths have different petrographic and geochemical features from the Archean mafic granulites. Elevated geotherm and petrographic evidence imply that the lithosphere of this craton was thermally disturbed in the Mesozoic prior to eruption of the host diatreme. These samples have sub-alkaline basaltic compositions, equivalent to olivine– and quartz–tholeiite. REE patterns are flat to variably LREE-enriched (La_N/Yb_N=0.98–9.47) without Eu anomaly (Eu/Eu*=0.95–1.11). They possess 48–127 ppm Ni and 2–20 ppm Nb with Nb/U and La/Nb ratios of 13–54 and 0.93–4.75, respectively, suggesting that these high-pressure granulites may be products of mantle-derived magma underplated and contaminated at the base of the lower crust. This study also implies that up to 10 km Mesozoic lowermost crust was delaminated prior to eruption of the Cenozoic basalts on the craton.