Structure and Quaternary tectonic history of the Woodlark triple junction region,Solomon Islands

The Woodlark triple junction region, a topographically and structurally complex triangular area of Quaternary age, lies east of Simbo Ridge and southwest of the New Georgia island group, Solomon Islands, at the junction of the Pacific, Australian and Solomon Sea plates. SeaMARC II side-scan imagery and bathymetry in conjunction with seismic reflection profiles, 3.5 kHz records, and petrologic, magnetic and gravity data show that the active Woodlark spreading centre does not extend into this region.South of the triple junction region, the Woodlark spreading centre reoriented at about 2 Ma into a series of short ESE-trending segments. These segments continued to spread until about 0.5 Ma, when the lithosphere on their northern sides was transferred from the Solomon Sea plate to the Australian plate. Simultaneously the Simbo transform propagated northwards along the western side of the transferred lithosphere, forming a trench-trench-transform triple junction located NNW of Simbo island and a new leaky plate boundary segment that built Simbo Ridge.As the Pacific plate approached, the area east of northern Simbo Ridge was tilted northwards, sheared by dominantly right-lateral faults, elevated, and intruded by arc-related magmas to form Ghizo Ridge. Calc-alkalic magmas sourced beneath the Pacific plate built three large strato-volcanic edifices on the subducting Australian plate: Simbo at the northern end of Simbo Ridge, and Kana Keoki and Coleman seamounts on an extensional fracture adjoining the SE end of Ghizo Ridge.A sediment drape, supplied in part from Simbo and Kana Keoki volcanoes, mantles the east-facing slopes of northern Simbo and Ghizo Ridges and passes distally into sediment ponded in the trench adjoining the Pacific plate. As a consequence of plate convergence, parts of the sediment drape and pond are presently being deformed, and faults are dismembering Kana Keoki and Coleman seamounts.The Woodlark system differs from other modern or Tertiary ridge subduction systems, which show wide variation in character and behaviour. Existing models describing the consequences of ridge subduction are likely to be predictive in only a general way, and deduced rules for the behaviour of oceanic lithosphere in ridge subduction systems may not be generally applicable.     

Computer Monte Carlo simulation in quantitative resource estimation

The method of making quantitative assessments of mineral resources sufficiently detailed for economic analysis is outlined in three steps. The steps are (1) determination of types of deposits that may be present in an area, (2) estimation of the numbers of deposits of the permissible deposit types, and (3) combination by Monte Carlo simulation of the estimated numbers of deposits with the historical grades and tonnages of these deposits to produce a probability distribution of the quantities of contained metal.Two examples of the estimation of the number of deposits (step 2) are given. The first example is for mercury deposits in southwestern Alaska and the second is for lode tin deposits in the Seward Peninsula.The flow of the Monte Carlo simulation program is presented with particular attention to the dependencies between grades and tonnages of deposits and between grades of different metals in the same deposit.     

Prospects of large-scale noble metal mineralization in the Upper Selemdzha minerogenic zone (by the example of the Malomyr and Sagur-Semertak ore clusters)

The Upper Selemdzha minerogenic zone, which has the potential for noble metal mineralization in the Upper Amur, has been studied. It is confined to the thick strata of Paleozoic terrigeneous-sedimentary rocks (sandstones, siltstones, clay schists, packages of their thin rhythmical interlayering, sedimentary breccias, and conglomerates) in the Tokura subzone of the Selemdzha-Kerbinsk zone in the Amur-Okhotsk area of the Mongol-Okhotsk system, overlain by Early Cretaceous volcanogenic sedimentary rocks and intruded by Early Cretaceous quartz diorites, granodiorites, and dacites. In the Malomyr and Sagur-Semertak ore clusters, there are a few known potential deposits of noble metals and ore occurrences; when studied and prospected, they can exhibit vein-veinlet noble metal mineralization with rich contents of gold and platinoids in ore columns and metasomatic deposits.     

An analysis of storm-induced landslides in colluvial soils overlying mudrock sequences, southeastern Ohio, USA

In late-June, 1998, a series of thunderstorms dropped 16.5 cm (6.5 in.) of rain in a 72-h period over southeastern Ohio, causing extensive flooding, six deaths, and 179 million dollars in property damage. The storms also triggered more than 60 shallow landslides along a 64 km (40-mi) stretch of Interstate 77 between Buffalo and Marietta, Ohio. Almost all of the landslides were translational, occurring along the contact between colluvial soils and the underlying bedrock (shales, claystones, mudstones). Six sites were selected in the affected area for a detailed study of the shallow landslides. At each site, a record was made of the stratigraphy, slope geometry, number of landslides, landslide dimensions, hydrologic conditions, and influence of vegetation on landslide distribution. Both colluvial soils and underlying bedrock were sampled for laboratory investigations, which included determination of natural water content, grain size distribution, Atterberg limits, permeability, slake durability, and shear-strength parameters. Data from laboratory tests were used to perform sensitivity and stability analyses with respect to varying slope angles, strength parameters, and thicknesses of saturated colluvial soil. The results of the study indicate that the shallow landslides along Interstate 77 occurred when the colluvial soils reached 90% to 100% saturation, depending upon the slope angles.     

Long-term abundance patterns of macroalgae in relation to environmental variables in the Tagus Estuary (Portugal)

Seasonal and inter-annual patterns of macroalgal abundance in a Tagus Estuary oyster reef are described. Macroalgal abundance was estimated as canopy percent cover by three permanent point intercept transects over a 7-year period. Four categories were defined, corresponding to bare substrate and three different macroalgal functional-form groups: (1) ULVA, foliose group, included Ulva spp.; (2) GRACIL, terete corticated macrophyte group, included only Gracilaria gracilis; and (3) FILAM, small (<10 cm) filamentous group, including eight species. A canonical correspondence analysis (CCA) showed that: (1) ULVA were associated with long and hot days, being usually dominant during spring and especially summer; (2) FILAM were associated with mild temperatures and relatively long days, abundant in spring but showed frequent peaks in summer; and (3) GRACIL were also favoured by spring season, although associated to lower temperature and less daylight hours than FILAM. GRACIL and FILAM were present throughout the year. On the contrary, ULVA were absent or with low cover during colder periods. A negative correlation between GRACIL and FILAM seems to indicate competition between the two categories. The applied models explained 23.3% of the temporal variance in category abundance. Rainfall negatively affected macroalgal cover, as indicated by the positive correlation between rainfall and bare substrate. Our conclusions are in agreement with previous studies that consider algae as excellent environmental integrators, even on a small scale, due to a strong link between the macroalgal communities and relevant environmental variables. It is also relevant that this study used open-access databases of environmental variables, which open up new possibilities for mining existing data resources in new ways. Due to large inter-annual variability, long-term studies are essential to understand population dynamics in estuarine phytobenthic communities.     

The impact of extreme turbidity events on the nursery function of a temperate European estuary with regulated freshwater inflow

Estuaries are used as nursery grounds by numerous marine species despite being usually subject to strong anthropogenic disturbances. Abundances of marine recruits (fish and crustacean decapods) and their main prey (mysids) were monitored by monthly sampling, from June 1997 to February 2009, in the lower reaches of the Guadalquivir estuary (SW Spain). During that period, unusually high and persistent turbidity events (HPTEs) were observed twice. Both HPTEs started with strong and sudden freshwater discharges after relatively long periods of very low freshwater inflow. Data from this time-series were used to test the hypothesis that HPTEs may negatively impact the nursery function of estuaries either by decreasing prey availability or by decreasing survival/arrival of marine recruits. During HPTEs, the commonest mysid (Mesopodopsis slabberi), a key species in the estuarine food web, showed a significant decrease in abundance. Likewise, some marine recruits that prey on M. slabberi and whose peaks of abundance within the estuary occur in summer–autumn (Engraulis encrasicolus and Pomadasys incisus) were less abundant during HPTEs. It is also suggested that HPTEs might have triggered a shift in the distribution of the most euryhaline prey (Neomysis integer) and predator (Dicentrarchus punctatus and Crangon crangon) species, towards more saline waters. This could have contributed to an increase in the inter-specific competition (for food/habitat) within the estuarine nursery area. The results discussed in this study call attention to the need to reduce as much as possible the anthropogenic pressures that may stimulate the occurrence of high and persistent turbidity events (HPTEs) in order to preserve the nursery function of temperate estuaries.     

Upper Jurassic overlapping-fans slope-apron system: Brae oilfield,North Sea

The Brae oilfield reservoir in the North Sea comprises Upper Jurassic resedimented conglomerates and sandstones interbedded with organic-rich silstone and mudstone thin-bedded turbidites. The system represents a series of small overlapping fans that form a thick (300 m) slope-apron accumulation of sediments deposited in a narrow (<10 km wide) belt along an active fault zone. The complex lateral and vertical distribution of facies was due mainly to variable tectonic activity, and partly also to sediment supply and sea-level changes.     

The Ni–Antimonide Mineral Assemblage: A Product of Recrystallization of Sn–Pb–Zn Ores of the Yuzhnoe Deposit,Sikhote-Alin,Russia

Doklady Earth Sciences - A mineral assemblage with nisbite NiSb2 and breithauptite NiSb unique for a Mesozoic (Sn)–Pb–Zn vein deposit is found during mineralogical–geochemical...     

Abundance and distribution of ichthyoplankton in Narragansett Bay, Rhode Island, 1989–1990

An ichthyoplankton survey (18 stations in seven sampling sectors) was conducted in Narragansett Bay in 1990 to provide information on abundance, distribution, and seasonal occurrence of eggs and larvae of estuarine fishes, including seasonal migrants. An additional goal was to examine changes in species composition, abundance, and distribution occurring since the last baywide survey in 1972–73. The taxonomic composition of eggs and larvae in 1990 (41 species in 25 families from 684 plankton samples) and in 1972–73 (43 species in 28 families from 6900 samples) was similar. Maximum abundance of fish eggs occurred in June and larvae in July, minimum abundance in September to February. Species diversity was greatest in May–July and lowest during January in both surveys. However, egg and larval densities in 1990 were considerably lower than in 1972–73. Bay anchovy, tautog, and cunner accounted for 86% of the eggs and 87% of the larvae in the bay in 1990. These three species accounted for only 55% of the eggs and 51% of the larvae in 1972–73, with menhaden accounting for another 18% of the eggs and 34% of the larvae. Searobins, scup, and butterfish eggs were common in 1973 (19%) but rare in 1990 (2%). Ichthyoplankton abundance for several of the most abundant species was significantly lower (p<0.05) in the Providence River, upper bay, and Greenwich Bay in 1990 than in 1972–73. Density of fish eggs and larvae in the lower portions of the bay was lower in 1990 for some species but not others. Distribution data suggested a general down-bay shift in density in 1990. *** DIRECT SUPPORT *** A01BY085 00015