Eastern Sub-basin Research Articles

Morpho-tectonic study on late-stage spreading of the Eastern Subbasin of South China Sea

Morpho-tectonic study on late-stage spreading of the Eastern Subbasin of South China Sea

Water Constituents in the North-western Black Sea from Optical Remote Sensing and In situ Data

Satellite-based optical observations of surface waters have been used to assess the main environmental interactions in the north-western Black Sea, as a contribution to the EROS 21 project. Such observations allow evaluations of the presence and abundance of water constituents (primarily phytoplankton pigments), providing essential information on the processes which are taking place in the area and on their spatial and temporal scales. An analysis of ocean colour imagery was performed, using historical data collected by the CZCS (1978–1986), and by the MOS before, during and after the EROS 21 oceanographic cruise which took place in April/May 1997. The time series of CZCS-derived parameters (i.e. chlorophyll-like pigment concentration) originates from the archive generated by the OCEAN project. The MOS data were processed to apply sensor calibration, to correct for atmospheric contamination so as to assess water-leaving radiances for each visible channel, and to estimate geophysical parameters such as pigment concentration and in-water optical depth, which is correlated to suspended matter concentration. The atmospheric correction was performed with a novel algorithm developed especially for MOS application. The bio-optical algorithms used to derive in-water parameters were obtained by comparison with the concurrent in situ measurements of optically active parameters collected in the north-western Black Sea in the framework of the EROS 21 project. The multi-satellite data set highlights the differences between western and eastern sub-basins, inshore and offshore domains, northern and southern near-coastal areas. In the Danube delta area, the water constituents trace complex interactions of near-coastal and basin-wide features of the Black Sea.

Phytoplankton assemblages in two sub-basins of Lake Como

This paper reports the results of a two-year study on the chemical characteristics and phytoplankton assemblages in the western (Como) and eastern (Abbadia Lariana) sub-basins of Lake Como. The data were collected from September 1997 to August 1999 in the frame of an investigation financed by the Provincial Administration of Como and Lecco. Over the period of study TP concentrations ranged between 6-46 μg P l-1 at Abbadia, and 5-50 μg P l-1 at Como. Reactive phosphorus ranged between 6-31 μg P l-1 at Como, and 6-25 μg P l-1 at Abbadia. By the end of the study, 85 taxa belonging to six algal groups were identified. Chlorophyta (Chlorophyceae and Conjugatophyceae) (30) were the best represented, followed by Bacillariophyceae (23), Cyanoprokaryota (8), Cryptophyceae (6), Chrysophyceae (6) and Dinophyceae (6). The time variation of chlorophyll a (0.5-9.72 μg l-1), phytoplankton density (177-64925 cell. ml-1) and biovolume (21-6665 mm3 m-3) showed the same trend, with the lowest and maximum values recorded during winter and from spring to summer, respectively. In order to compare the seasonal succession in the two sub-basins, cluster analysis and non-metric multidimensional scaling ordination were used jointly. Multivariate analysis showed i) a substantial homogeneity in the time seasonality of the dominant phytoplankton assemblages in the two sub-basins and ii) a clear differentiation of summer phytoplankton from the other seasonal phases. From a purely quantitative point of view, there were dissimilarities between the two stations. The western sub-basin showed a greater algal production than Abbadia, especially in spring and summer. Despite the strong reduction of TP since the 1980s, the results of investigations carried out during the 1990s indicated a substantial stability of phytoplankton . It is emphasised that a reduction in biomass and shift in species composition will be possible only by assuring a severe control of the nutrient loads.

Morpho-tectonic study on late-stage spreading of the Eastern Subbasin of South China Sea

Three NE-trending linear structural zones with different strikes are present in the Eastern Subbasin of the South China Sea. They are distributed in the 350-km-wide central region of both sides of the Scarborough seamount chain, representing a morphological indication of the basement faulting. These three zones correspond respectively to three spreading episodes: the magnetic anomalies 6c —6a (24—21 Ma), 6a — 5e (21 — 19 Ma) and 5e — 5d (5c) (19 — 16 Ma). Instability, subsection and asymmetry characterize the seafloor spreading of the subbasin. The spreading directions change in a continuous way in each of the zones, but abrupt changes by 3°—5° occur when crossing the boundary between the zones, reflecting that the spreading direction has evolutionary characteristics of both gradual and sudden changes. NW-trending transform faults of the spreading become progressively densely distributed from the east to the west, cutting the NE-trending zones into several segments, between which the strikes of the NE-trending zones have marked changes. Such features indicate that the spreading axis is associated with subsection along the strike. Around 21 Ma (magnetic anomaly 6a), there was an important event of spreading acceleration, with the full rate rapidly increasing from 30.54 km/Ma to 42.88 km/Ma. This rate increment event corresponds to the sudden changes in the spreading characteristics of basement faulting, sedimentation, volcano activities, etc. The asymmetry of spreading over the eastern part of the Eastern Subbasin is generally larger than that over the western part, and the spreading rate is markedly larger on the southern side than on the northern side. As a result, the oceanic basin is wide in the east and narrow in the west, forming a significantly asymmetric pattern.

Bransfield Basin fine-grained sediments: late-Holocene sedimentary processes and Antarctic oceanographic conditions

The Antarctic Peninsula is sensitive to climatic change due to its northerly position and to the relatively reduced volume and character of its ice cover. High-resolution palaeoclimatic records from the Ant arctic Peninsula ice cores extend back only 500 years. A climatic record of 2850 years in the Bransfield Basin is investigated through the analysis of sediment gravity cores from the floor of the central subbasin (core GEBRA-1) and the slope of the eastern subbasin (core GEBRA-2). Sedimentological, mineralogical and geo chemical properties have been systematically measured, together with Accelerator Mass Spectrometry (AMS) radiocarbon dating. The fine-grained sediments result from two main processes: hemipelagic settling from resuspensions and primary productivity, and turbidity currents. Hemipelagic sediments were selected to investi gate the oceanographic and climatic conditions of the northern Antarctic Peninsula region during the last three millennia. Cold climatic periods are characterized by millimetric laminations and/or black layers with higher organic carbon, nitrogen and opal contents. Warm periods are recorded as massive to diffuse laminated facies with lower biogenic contents. The results include the ‘Little Ice Age’ (LIA) cold pulse as well as several 200– 300 year long fluctuations within the LIA and before this major climatic event of the Holocene.

Optical remote sensing of coastal plumes and run-off in the Mediterranean region

Sea surface colour data, derived from the Coastal Zone Colour Scanner (CZCS) archive, have been used to assess the space/time variability of coastal plumes and run-off in the Mediterranean Sea. A time series of 2645 scenes, collected by the CZCS from 1979 to 1985, was processed to apply sensor calibration algorithms, correct for atmospheric contamination, and derive chlorophyll-like pigment concentration. Individual images, remapped on a 1-km2 pixel grid, were generated for each available day, and then mean values calculated pixel by pixel to form monthly, seasonal and annual composites. The results obtained must be taken with caution, due to the CZCS limitations in the quantitative assessment of bio-optical pigments when high concentrations of dissolved organics or suspended sediments are present, e.g. along littorals or within plumes. Marked differences appear in the distribution of water constituents between coastal zones and open sea, northern and southern near-coastal areas, western and eastern sub-basins. The oligotrophic character of the basin contrasts with areas of high concentration related to river plumes—Ebro (Ebre), Po, Rhone, Nile—, coastal run-off patterns, and persistent mesoscale features (e.g. coastal filaments and eddies). Seasonal variability appears to be high, with higher concentrations occurring over most of the basin in the cold season, when climatic conditions are favourable to coastal run-off and vertical mixing. Atmospheric forcing (wind and rainfall over continental margins) could play an important role in establishing the observed space/time distribution of water constituents. The impact of continental interactions (fluvial and coastal run-off), or that of exchanges between coastal zone and open sea, could have paramount influence on the biogeochemical fluxes in the entire basin.

Gravity interpretation of Chiang Mai Basin, northern Thailand: concentrating on Ban Thung Sieo area

The interpretation of gravity data from the Chiang Mai Basin area indicates that this Tertiary basin can be divided into five sub-basins. The largest is located just east of Chiang Mai City. This sub-basin is bordered in the west and northwest by steep major dip-slip faults and in the east by a more gentle dipping contact between the Tertiary sequence and the basement rocks of the eastern mountain range. South of this large sub-basin, in an area around Ban Thung Sieo, there are two other nearly parallel sub-basins separated by a basement horst which runs in a northeast-southwest direction; 2.5 D modelling suggests that the depth to the Cretaceous basement along the axis of the eastern sub-basin varies from 1.3 km at the northern end to 2.3 km at the southern end. This eastern sub-basin is bordered in the east by a fault contact with Paleozoic rocks of the eastern mountain range. As for the western sub-basin, the modelling suggests that its western edge consists of a fault contact between the rocks of western Doi Suthep mountain range and the Tertiary sequence. The maximum depth to the Cretaceous basement for this sub-basin is around 1.9 km.

Basin Modeling of Tadla Basin, Morocco, for Hydrocarbon Potential

The Tadla basin, Morocco, is the easternmost major structural subdivision of the Meseta south of the Central Massif. After the basin remained relatively stable through the Paleozoic, movements during the Variscan orogeny divided the basin into two subbasins, a western subbasin where compressional forces formed large anticlines and an eastern subbasin where intense folding and faulting are associated with uplift and erosion. The sedimentary basin fill includes numerous organic shales, which provide the source beds. Also deposited were numerous shallow-water to continental clastics and to a lesser extent carbonates, providing several possible reservoir rocks for hydrocarbons. The results from computer modeling show that the chief formations of hydrocarbon generation are Carboniferous and Silurian. Most of these formations may have generated hydrocarbons before Variscan movements, which probably destroyed preexisting oil and gas traps. The only source rocks in the generation stage from Mesozoic to the present are the upper Visean shales. The western part of the Tadla basin was the most favorable area for the generation and maturation of hydrocarbons of the upper Visean shales.

Longitudinal and temporal trends in the water chemistry of the North Branch of the Moose River

Surface water acidification is potentially a problem in regions with low ionic strength drainage waters. Atmospheric deposition of sulfuric acid has generally been implicated as the causative agent of this problem, although other sources of acidity may contribute. The Adirondack region of New York State is an area with “acid-sensitive” surface waters and an abundance of acidic lakes. The intent of this study was to evaluate the processes regulating the acid/base chemistry of a series of lakes draining a large heterogeneous watershed in the Adirondack region of New York. The study site, the North Branch of the Moose River, is heterogeneous in its soil and geological characteristics. This variability was reflected through differences in water chemistry that occurred within the basin. The northern headwaters generally drain subcatchments with shallow, acidic soils. The resulting water chemistry was acidic (equivalence of acidic anions exceeded equivalence of basic cations) with high concentrations of Al and dissolved organic carbon (DOC). As this water migrated through a large lake (Big Moose Lake) with a moderate hydrologic retention time (∼0.5 yr), considerable loss of DOC was evident. As acidic water was transported through the drainage area, it mixed with waters that were enriched in concentrations of basic cations from the eastern subbasins. As a result, there was a successive increase in the acid neutralizing capacity (ANC) and a decrease in Al concentrations as water migrated from the northern reaches to the outlet of the watershed. In addition to these general trends, short-term changes in water chemistry were evident. During low flow summer periods concentrations of basic cations were elevated, while concentrations of SO 4 2− and NO 3 − were relatively low. These conditions resulted in less acidic waters (higher ANC) with relatively low concentrations of Al. During high flow winter/spring conditions, elevated concentrations of SO 4 2− and NO 3 − were evident, while concentrations of basic cations were reduced resulting in low pH (low ANC) waters with high concentrations of Al. Variability in the processes regulating the pH buffering of waters was apparent through these short-term changes in water chemistry. In the northern subbasin short-term fluctuations in ANC were minimal because of the buffering of Al under low pH conditions. Seasonal changes in the ANC were more pronounced in the eastern subbasin because of the predominance of inorganic carbon buffering in the circumneutral pH waters. Lakes in the west-central Adirondacks have characteristically short hydraulic residence times and elevated nitric acid inputs. As a result these waters may be more susceptible to surface water acidification than other “acid-sensitive” lake districts in eastern North America. Given the apparent interregional differences, extrapolation of chemical trends in the Adirondacks to other areas may be tenuous.

Mineralized veins and breccias of the Cripple Creek District, Colorado

The Cripple Creek district has yielded nearly 21 million troy ounces of gold since its discovery in 1891. The orebodies occur as narrow veins within Precambrian and Tertiary rocks and as bulk tonnage deposits within tectonic and hydrothermal breccias.The district is localized within and adjacent to a 27.9- to 29.3 + or - 0.7-m.y.-old nested diatreme-intrusive complex. Two magmas, phonolitic and alkali basaltic in composition, generated volcanic flows, subvolcanic intrusions, and phreatomagmatic breccias. Magma mixing is suggested by intermediate composition latite-phonolite and syenite. Subsidence of the diatreme complex rocks is indicated by (1) a thick fluvial-lacustrine sedimentary sequence in the eastern subbasin, (2) the presence of carbonaceous debris, ripple-laminated rocks, and dessication cracks in sedimentary rocks at depths more than 300 m below the present surface, (3) by the fracture systems near the diatreme subbasin margins that reflect basement rock influence, and (4) by flat-dipping veins near intrusive bodies or small breccia bodies (e.g., the Cresson diatreme).The vein deposits as exemplified by those of the Ajax mine cut Precambrian crystalline rocks and Tertiary rocks of the diatreme complex. Within the Precambrian rocks the veins are radial to the margins of the diatreme system and are sheeted zones with rock dissolution and open-space fillings. Where the veins cut the Cripple Creek breccia, they are an irregular anastomosing fracture zone. The major veins exhibit remarkable vertical continuity, extending to more than 1,000 m below the present surface. Vein-related hydrothermal alteration occurs in a narrow selvage that extends outward no more than five times the vein width. Secondary K-feldspar, dolomite, roscoelite, and pyrite occur within an inner zone adjacent to the veins, whereas an outer zone contains sericite, montmorillonite, magnetite, minor secondary K-feldspar, and pyrite. There is no expansion of the alteration zones in the upper level mine exposures.Five stages of minerals are recognized in the Ajax mine veins: (1) quartz-fiuorite-adularia-pyrite-(dolomite-marcasite), (2) base metals-quartz-pyrite, (3) quartz-fluorite-pyrite-hematite-rutile, (4) quartz-pyrite-rutile-calaverite-acanthite, and (5) quartz-fluorite-dolomite. The proportions of each stage vary within and between veins, but the ore mineralogy is consistent throughout the vertical extent of the developed vein systems. Horizontally, gold values ranged between 0.5 and 1.0 oz Au per short ton.Fluid inclusion analyses have documented the presence of early stage 1 saline fluids (33- >40 equiv. wt % NaCl) with the higher salinities found in the upper 300 m of the Ajax mine levels; the fluids were boiling and contained CO 2 . Stage 2 and 3 fluid inclusions exhibit progressively lower homogenization temperatures, and salinities are markedly lower (0-8.3 equiv. wt % NaCl). The telluride ore was deposited from weakly boiling, dilute fluids (1.4-3.5 equiv. wt % NaCl) with temperatures below 150 degrees C.The bulk tonnage deposits, as exemplified by the Globe Hill area, consist of mineralized tectonic and hydrothermal breccias cutting pyroxene-bearing alkali trachyte. Four structural events occurred at Globe Hill: (I) emplacement of hydrothermal breccia bodies along a north-west-trending 1,800- by 700-m zone; (II) intersecting tectonic adjustments along steep variable-strike zones on the western margin of the stage 1 breccias; (III) intrusive breccia emplacement at the major stage II fault intersection; and (IV) hydrothermal brecciation centered to the immediate east of the Globe Hill pit and characterized by a matrix consisting of anhydrite, carbonate, fluorite, pyrite, and base metal sulfides.Two hydrothermal events generated gold-silver mineralization and associated wall-rock alteration in the bulk tonnage deposits. The precious and base metals occur with alteration products in breccia clasts or in matrix minerals within the hydrothermal and tectonic breccias. The fluids responsible for alteration-mineralization were boiling as indicated by wide ranges of filling temperatures in fluid inclusions of the same mineral grain, extensive development of explosion texture in quartz and celestite, and large variations of liquid/vapor ratios in fluid inclusions within individual crystal growth zones. Temperatures were below 200 degrees C as indicated by minimum filling values. Capping of boiling shallow hydrothermal fluids appears to have been enhanced by the alkali trachyte porphyry intrusion at Globe Hill, which acted as a permeability barrier to upward-migrating fluids. Vapor-dominated fluids developed over-pressuring, leading to hydrothermal brecciation and low-grade gold deposits. On the other hand, the vein systems in the Cripple Creek district formed along structures open to the surface; hence, hydrothermal brecciation did not occur.

Margins of the Southwest sub-basin of the South China Sea—A frontier exploration target?

The deep South China Sea Basin is about 500 km narrower (when measured along fracture zone directions) in the southwest sub-basin than in the eastern sub-basin. Reconstruction of the South China Sea Basin to predrift configurations thus creates an unacceptable 500-km overlap of continental or transitional crust in the western half of the proto-South China Basin. In order to maintain continuity through the plate-tectonic evolution of this basin, about 500 km of differential crustal extension must have occurred in the west in relation to that in the east. Virtually no deep-penetration seismic data exist across the northwest and southwest foundered and extended continent-transition crust. However, plate tectonics allows us to place constraints on the effective β even in the absence of direct information on crustal thickness, heat flow, or subsidence history. The problem here is essentially the inverse of that normally presented. Assuming various endmember models and knowing the effective β (i.e., extension), we can predict the cumulative subsidence (and hence maximum sediment thicknesses), the regional thermal history, and crustal thicknesses. The total continental plus transitional margin crust in the western sub-basin represents a differential stretching factor about twice that inferred for rifted margins of the eastern sub-basin. The actual mean stretching factor for the western sub-basin margins may be as high as β = 5 or 6. The anomalous crustal extension inferred for the western half of the basin leads us to speculate on the likelihood that numerous rift basins and their associated sediments may have been subjected to unusual thermal conditions that would be favorable for the early maturation of hydrocarbons. Although this region remains virtually unexplored, the foregoing analysis suggests it should perhaps be one focus for additional frontier exploration. This portion of the South China Sea margin (young, thin sediments) offers the ideal natural laboratory for the further testing of continental margin thermal subsidence models.

THE PETROLEUM POTENTIAL OF THE EASTERN ARROWIE BASIN AND FROME EMBAYMENT

The eastern Arrowie Basin is a descriptive term for the present-day structurally defined Cambro- Ordovician basin approximately coinciding with the Curnamona Shelf depositional feature which formed on the eastern stable flank of the Adelaide Trough. Remnants of the Arrowie depositional basin are preserved in outcrops of the Flinders Ranges and in the subsurface to the east and west of the ranges. The eastern subsurface sub-basin occupies the area around Lake Frome in South Australia and across the border in New South Wales. Much of the eastern sub-basin is overlain by the Jurassic and Cretaceous rocks of the Frome Embayment of the Eromanga Basin.The area has been explored sporadically over at least the past 20 years but no deep wells have been drilled to test exploration targets. This assessment of the prospectivity of the basin is assisted by shallow stratigraphic and mineral drilling, by extrapolation from outcrop, and by regional geophysics.In the deepest part of the eastern sub-basin the total thickness of the Cambro-Ordovician sediments is expected to exceed 2500 m. Potential reservoirs can be expected within both carbonate and sandstone rocks. Good secondary porosity exists in sandstones of the Middle to Late Cambrian Lake Frome Group and good porosity development is predicted within possible ooid shoal deposits.Source material has been demonstrated in outcrop samples with Total Organic Carbon measured at up to 0.35 per cent. It is suggested that petroleum generation probably occurred in Cambrian sediments in the Adelaide Trough (now the area of the Flinders Ranges) and the petroleum migrated east and west to the stable shelf areas. Sufficient depth of burial may also have been attained on the Curnamona Shelf to allow local generation.In the past, the very poor quality of available seismic data has frustrated the mapping of the subsurface in sufficient detail to justify exploration drilling. Recent experimental seismic surveys, however, have produced data of a much better quality. Potential exists for anticlinal, fault and unconformity traps. There is also some potential for stratigraphic traps such as ooid shoal deposits. At this stage an exploration programme to evaluate the potential of this area is being carried out by the licence holders.

Wave-Dominated Deltaic Systems of Upper Cretaceous San Miguel Formation, Maverick Basin, South Texas: ABSTRACT

Sandstone units of the Upper Cretaceous San Miguel Formation in south Texas are wave-dominated delta sequences deposited during a major marine transgression. San Miguel sediments were deposited in the Maverick basin within the Rio Grande embayment. Cross sections and sandstone maps reveal that during deposition of the San Miguel, the Maverick basin consisted of two subbasins. A western subbasin received sediments from the northwest; the eastern subbasin received sediments from the north. Net-sandstone patterns show that the thickest parts of the sandstone bodies are generally strike oriented. Where not eroded, sand-feeder (fluvial) systems are indicated by dip-aligned components on the updip sides of the sandstone units. According to these net-sandstone patterns, the San Miguel deltas vary considerably and make up a wide spectrum of wave-dominated delta types. The most common vertical sequences in the San Miguel coarsen upward from silt and clay to fine sand. Burrows dominate, and the few primary structures are of small scale. Large-scale cross-beds are observed only in outcrop. Strand-plain or barrier-island facies sequences, which prevail in most wave-dominated delta deposits, are incomplete in the San Miguel. In most places, only the lower shoreface is preserved. The upper parts of the sequences, which normally bear large-scale primary structures, were lost by marine reworking during subsequent transgression. Intense burrowing destroyed any primary structures at the tops of the truncated sequences. End_of_Article - Last_Page 1611------------