Western Sub-basin Research Articles

Decadal scale variability of sea surface temperature in the Mediterranean Sea in relation to atmospheric variability

Twenty-four years of AVHRR-derived sea surface temperature (SST) data (1985–2008) and 35 years of NOCS (V.2) in situ-based SST data (1973–2008) were used to investigate the decadal scale variability of this parameter in the Mediterranean Sea in relation to local air–sea interaction and large-scale atmospheric variability. Satellite and in situ-derived data indicate a strong eastward increasing sea surface warming trend from the early 1990s onwards. The satellite-derived mean annual warming rate is about 0.037°C year–1 for the whole basin, about 0.026°C year–1 for the western sub-basin and about 0.042°C year–1 for the eastern sub-basin over 1985–2008. NOCS-derived data indicate similar variability but with lower warming trends for both sub-basins over the same period. The long-term Mediterranean SST spatiotemporal variability is mainly associated with horizontal heat advection variations and an increasing warming of the Atlantic inflow. Analysis of SST and net heat flux inter-annual variations indicates a negative correlation, with the long-term SST increase, driving a net air–sea heat flux decrease in the Mediterranean Sea through a large increase in the latent heat loss. Empirical orthogonal function (EOF) analysis of the monthly average anomaly satellite-derived time series showed that the first EOF mode is associated with a long-term warming trend throughout the whole Mediterranean surface and it is highly correlated with both the Eastern Atlantic (EA) pattern and the Atlantic Multidecadal Oscillation (AMO) index. On the other hand, SST basin-average yearly anomaly and NAO variations show low and not statistically significant correlations of opposite sign for the eastern (negative correlation) and western (positive correlation) sub-basins. However, there seems to be a link between NAO and SST decadal-scale variations that is particularly evidenced in the second EOF mode of SST anomalies. NOCS SST time series show a significant SST rise in the western basin from 1973 to the late 1980s following a large warming of the inflowing surface Atlantic waters and a long-term increase of the NAO index, whereas SST slowly increased in the eastern basin. In the early 1990s, there is an abrupt change from a very high positive to a low NAO phase which coincides with a large change in the SST spatiotemporal variability pattern. This pronounced variability shift is followed by an acceleration of the warming rate in the Mediterranean Sea and a change in the direction (from westward to eastward) of its spatial increasing tendency.

Control of regional structural styles and faulting on Northeast Mexico spring distribution

Groundwater-dependent, spring-fed ecosystems of the Cuatrocienegas Basin, Coahuila, Mexico, host >70 endemic species. These desert springs occur primarily aligned along the base of an anticline that bisects the Cuatrocienegas Basin, but the hydrogeologic controls of the springs are poorly understood. The hypothesis that spring locations are controlled by subsurface geology, such as buried anticlines or faulting, versus stratigraphic controls is tested by evaluating: (1) regional structural styles; (2) fracture models of analogous structures; (3) hydrogeologic data; and (4) geophysical surveys. Jurassic and Cretaceous siliciclastic and carbonate rocks deposited on the Coahuila Block west of the Cuatrocienegas Basin have dips <10° and lack faults because of a structurally rigid granodiorite basement. To the east of the Coahuila Block and around the Cuatrocienegas Basin, the Coahuila Folded Belt has anticlines associated with basement-involved faults, 10–25° backlimb dips, and forelimb dips up to vertical or slightly overturned. Springs in the western sub-basin that represent 85% of total basin discharge are located on zones of highest anticipated fracture density predicted by fracture models of analogous anticlines. Spring waters reveal elevated temperature (32–35°C) and low tritium (<1 tritium unit). Gravimetry and time-domain electromagnetic surveys correspond with a best-fit Cuatrocienegas Basin hydrogeologic model of fractures associated with reverse faulting controlling spring locations in the western Cuatrocienegas Basin. Springs in the eastern sub-basin are located where ephemeral streams have eroded through confining beds along the base of alluvial fans and lack faulting. Regional variations in structural style are an important control on the location of springs in the Cuatrocienegas Basin.

Sequence-stratigraphic interpretation of structurally controlled deposition: Middle Miocene Kareem Formation, southwestern Gulf of Suez, Egypt

ABSTRACT The structural evolution and sequence-stratigraphic architecture of the syn-rift Middle Miocene (Langhian) Kareem Formation were studied in the Western Sub-Basin (WSB) of the Gulf of Suez, Egypt. Biostratigraphic data, facies interpretations, and log data from thirteen wells were used to construct six tectono-sedimentary scenarios and lithofacies-distribution maps, which illustrate the paleogeography during six systems tracts. Abrupt thickness and facies changes reflect deep basins and adjacent high-relief areas created by differential fault-block movements. Within the WSB seven individual fault blocks record complex, tectonically controlled deposition of the Kareem Formation, which we subdivide into two third-order sequences. Earlier tectonic interpretations, facies analysis, and stratigraphic correlations from individual fault blocks were used to interpret the varying depositional settings during six systems tracts and, as a consequence, to discern the structural evolution of the WSB during the Middle Miocene.

Chemical evolution of shallow playa groundwater in response to post-pluvial isostatic rebound, Honey Lake Basin, California–Nevada, USA

The 1,750-km2 endorheic Honey Lake basin (California–Nevada, USA) was part of the 22,000-km2 Pleistocene Lake Lahontan pluvial lake system which existed between 5,000 and 40,000 years BP. The basin consists of two subbasins separated by a low elevation divide. Groundwater in the western subbasin has a maximum total dissolved solids (TDS) content of only ∼1,300 mg/L; however eastern subbasin groundwater has a maximum TDS of ∼46,000 mg/L. This TDS distribution is unexpected because 94% of surface water TDS loading is to the western subbasin. In situ reactions and upwelling thermal groundwater contributing to groundwater chemistry were modeled using NETPATH. The TDS difference between the subbasins is attributed to post-Lake Lahontan isostatic rebound about 13,000 years ago. Prior to rebound the subbasins did not exist and the low point of the basin was in the eastern area where hydraulic isolation from the larger Lake Lahontan and frequent desiccation of the basin surface water resulted in evaporite mineral deposition in accumulating sediments. After rebound, the terminal sink for most surface water shifted to the western subbasin. Although most closed basins have not been impacted by isostatic rebound, results of this investigation demonstrate how tectonic evolution can impact the distribution of soluble minerals accumulating in shallow basins.

Structural and basin evolution in Miocene time, southwestern Gulf of Suez, Egypt

Seismic-facies analysis and the subsurface tectonic evolution of the Western Sub-Basin (WSB)oftheGulfofSuez,Egypt,werestudiedbyseismicreflectiondataof45seismiclines.Eight of these lines were selected to illustrate the structural framework and depositional history of the WSB supported by the composition, velocity and vertical seismic profile (VSP) logs of ten wells. The analysis of two way time (TWT) structure-contour maps and rose diagrams revealed that the MiocenestrataaredissectedbyNE-SWtrendingfaultsoftheMorganAccommodationZone(MAZ). Northofthiszone,ageneraldiptowardsthenortheastisrecognized,southofthiszone,theregional dip is towards the southwest. The complex structural framework was the main factor controlling facieschangesofthesyn-depositionalMioceneunitsalongthestudyarea.

Cenozoic tectonic and depth/age evolution of the Indonesian gateway and associated back-arc basins

We reconstruct the tectonic and paleo-age and -depth history of the Indonesian seaway and associated southeast Asian back-arc basins to create a comprehensive paleogeographic framework for the region during the Cenozoic. We combine published tectonic models with revised models of the evolution of back-arc basins based on available marine magnetic anomaly data and a recently published high-resolution (1 min) marine gravity anomaly grid which reveals many fine-scale tectonic features previously not resolved. Reconstructions of the now-subducted Ceno-Tethys ocean floor are integrated with a revised absolute plate motion reference frame for the Indian Ocean based on moving hotspots. All marginal seas north of Australia formed in a back-arc setting, with the Caroline and Celebes seas opening north of a northward dipping subduction zone, and the Solomon Sea south of a southward dipping subduction zone north of Australia. We suggest that the evolution of the two sub-basins of the Caroline Sea from 36 to 25 Ma followed different seafloor spreading patterns, with the western sub-basin being influenced by the Manus hotspot. Our model suggests that collision between the Melanesian and the Caroline arcs led to cessation of oceanic crustal production in the Caroline Basin shortly before 25 Ma. Combined geophysical and geological observations suggest that several major tectonic events occurred north of Australia at around 45 Ma, roughly at the time when Australia started moving northwards at fast rates similar to the present. We suggest that this event is related to a relocation of the subduction zone NW of Australia under the Philippine Sea plate due to a collision and subsequent accretion of old Pacific plate material to the northward subducting Australian plate. Our gridded oceanic paleo-depth maps capture the history of marginal seas north of Australia, and the stepwise destruction of the Ceno-Tethys ocean floor in the northern Wharton Basin and the west Pacific, providing boundary conditions for both paleoceanograpic and geodynamic models. A regional analysis of basement depth anomalies reveals that median negative anomalous depth of several back-arc basins is of the order of 650–800 m, with a range of 300–1100 m, accompanied by negative regional heatflow anomalies, suggesting that mantle-driven dynamic topography plays a larger role in modulating regional basement depth than previously suggested. Regional shear wave mantle tomography cross sections highlight that all marginal basins that exhibit large negative dynamic topography are underlain by massive buried slab material in the lower portion of the upper mantle and/or below the transition zone, supporting the notion that the observed paired negative dynamic topography and heatflow anomalies are due to basin formation above slab burial grounds.

Groundwater basin of the Tulum Valley, San Juan, Argentina: A morphohydrogeologic analysis of its central sector

The geometry of a sector in the groundwater basin of the Tulum Valley has been studied to determine the shape, thickness, and vertical and horizontal distribution of the grain size, as well as the depositional environmental conditions of the Quaternary deposits that fill the valley. The geomorphologic features of the area have been investigated on the basis of aerial photographs checked with fieldwork. Three subsurface sections were prepared for a hydrogeological analysis of the area. These cross-sections were prepared by combining information from descriptions of well samples and interpretations of geophysical logs of wells and electric resistivity surveys. Within the studied area, the floor of the groundwater basin is asymmetrically shaped; the Quaternary deposits, which lie on an impervious or poorly pervious electrically conductive hydrogeologic basement of Late Tertiary age, reach a thickness of 670 m in the west and only 215 m in the eastern extreme. The Tulum Valley Basin is divided into two subbasins by a fault system trending NNE–SSW, which plays an important role in the configuration of the basin and the distribution of the Quaternary sediments units, as well as the distribution of aquifers in the subsurface. The western subbasin has a thicker cover and coarser grain sizes than the eastern one, where the sediments have more fine-grained intercalations and hardpans. The latter are probably pedogenic in origin.

Carbon dioxide sequestration in the Campine Basin and the adjacent Roer Valley Graben (North Belgium): an inventory

Abstract The Campine Basin offers a variety of potential CO 2 storage sites. Within the Roer Valley Graben area Triassic Buntsandstein rocks have an estimated CO 2 storage capacity of several hundred million tons. Mesozoic shales and younger sediments provide adequate sealing. Westphalian D sandstones may possess storage opportunities in the southern graben area, and the karstified/dolomitized parts of the Dinantian carbonates in the western sub-basin have an estimated CO 2 storage capacity of 130 × 10 6 tons. In places, the Dinantian reservoirs may be combined with carbonate reservoirs in the underlying Devonian strata. Sealing is provided by Namurian shales and Westphalian coals and shales. Along the western and southern edge of the basin the Dinantian reservoir is sealed by Cretaceous chalks and marls. Besides aquifers, the available coal qualifies for CO 2 storage. Six Six methane target areas contain a producible enhanced coalbed methane (ECBM), volume of 53–79 × 10 9 m 3 . This provides a minimum CO 2 sequestration potential of 400 × 10 6 tons. ECBM development factors include the sedimentological setting of the coal sequence as well as changes in porosity, permeability and stress conditions induced by former mining activities. The CO 2 storage capacity in abandoned coalmines is limited. Nevertheless, CO 2 storage in the abandoned mines may be an option when it is combined with coalmine methane extraction or ECBM production in neighbouring areas.

Origins and Paleoceanographic Significance of Layered Diatom Ooze from Bransfield Strait in the Northern Antarctic Peninsula around 2.5 kyrs BP

We used diatom and porewater data of two piston cores from the central subbasin and one from the western subbasin in the Bransfield Strait in the northern Antarctic Peninsula to elucidate the depositional mechanism of the layered diatom ooze. The layered diatom ooze is characterized by an abundance of organic carbon, biogenic silica, sulfde sulfur, and lower porewater sulfate concentration. This lack of pore-water sulfate concentration in the diatom ooze interval may reflect development of reducing micro-environment in which bacterially mediated sulfate reduction occurred. The negative relationship between the total organic carbon and sulfate contents, however, indicates that sulfate reduction was partly taking place but does not control organic carbon preservation in this unit. Rather, well-preserved Chaetoceros resting spores in the layered diatom ooze indicate a rapid sedimentation of the diatom as a result of repetitive iceedge blooms on the Bransfield shelf during the cold period (around 2500 yrs BP) when the permanent seaice existed on the shelf, During this period, it is expected that the downslope-flowing cold and dense water was also formed on the Bransfield shelf as a result of sea ice formation, playing an important role for the formation of layered diatom ooze in the Bransfield subbasins.

A Laboratory Model of Exchange and Mixing between Western Boundary Layers and Subbasin Recirculation Gyres*

Chaotic advection is suggested as a possible mechanism for fluid exchange and mixing among a western boundary current and subbasin recirculation gyres. Applications include the North Atlantic Deep Western Boundary Current and its adjacent mesoscale recirculation gyres. Visualization and quantification of certain aspects of chaotic advection in a laboratory analog are described. Depending on the strength of the forcing, recirculating fluid offshore of the western boundary layer may be contained in a single gyre (not favorable for chaotic advection) or twin gyre with a ‘‘figure-eight’’ geometry (favorable for chaotic advection). When time dependence is imposed on these steady flows by varying the forcing periodically, the resulting fluid exchange, stirring, and mixing is most dramatic in the case of the twin gyre. A template for these processes can be formed by highlighting certain material contours (invariant manifolds) using dye and other techniques. These objects can be used to identify blobs of fluid (turnstile lobes) that are carried into and out of the gyres. The associated transports and flushing times can be estimated. The preferential stirring and mixing in the twin-gyre case is quantified by calculating the effective diffusivity of the flow field based on snapshots of the dye fields at longer times. The experiment suggests how tracers in a western boundary current might be transported into and out of neighboring recirculations and where regions of strong zonal or meridional transport might occur.

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.

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.

The transvaal sequence: an overview

The 15 000 m of relatively unmetamorphosed clastic and chemical sedimentary and volcanic rocks of the 2550-2050 Ma Transvaal Sequence as preserved within the Transvaal and correlated Griqualand West basins of South Africa, and in the Kanye basin of Botswana are described. Immature clastic sedimentary and largely andesitic volcanic rocks of the Wolkberg, Godwan and Buffelsfontein Groups and the Bloempoort and Wachteenbeetje Formations probably represent rift-related sequences of Ventersdorp age. The thin sandstones of the Black Reef Formation, developed at the base of both the Kanye and Transvaal basin successions and correlated with the basal Vryburg siltstones of the Griqualand West Sequence, are considered here to be the basal unit of the Transvaal Sequence. The Black Reef fluvial deposits grade up into the epeiric marine carbonates of the Malmani Subgroup. These stromatolitic dolomites and interdbedded cherts were laid down within a steepened carbonate ramp setting; transgressions from an initial Griqualand West compartment towards the northeast covered both the Kanye and Transvaal basins. Iron formations of the succeeding Penge Formation and Griqualand West correlates are envisaged as relatively shallow water shelf deposits within the carbonate platform model; siliceous breccias of the Kanye basin are interpreted as reflecting subaerial brecciation of exposed silica gels. The Duitschland Formation overlying the Penge iron formations is seen as a final, regressive clastic and chemical sedimentary deposits as the Malmani-Penge sea retreated from the Transvaal basin. The interbedded sandstones and mudstones of the uncomformity-bounded Pretoria Group probably represent a combination of alluvial fan and fluviodeltaic complexes debouching into the largely lacustrine Transvaal and Kanye basins. A strong glacial influence in the lower Pretoria Group is reflected in the correlated Makganyene diamicities of the Griqualand West Sequence. Sedimentation across all three basins was interrupted by the extrusion of the Hekpoort-Ongeluk andesites. Upper Pretoria Group sediments of the Silverton and Magaliesberg Formations probably reflect a marine transgression. These rocks are not present in the Griqualand West basin, and were affected by Bushveld Complex-related thermal doming in the Transvaal basin; post-Magaliesberg sedimentation continued thereafter in separate eastern and western fluviodeltaic-lacustrine sub-basins. The largely volcanic Rooiberg Group (sensu lato) began with catastrophic basin floor collapse and Leeuwpoort Formation fluvial sedimentation in the western sub-basin. The succeeding Smelterskop and Makeckaan Formations reflect a transition from fluvial deposition to volcanism, and are succeeded by the widespread and voluminous, predominantly felsitic lavas of the Dullstroom, Damwal and Selonsrivier Formations. The correlated Loskop, Glentig and Rust de Winter Formations which overlie the felsites conformably, represent the final sedimentary phase of the Transyaal basin.

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.

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.

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------------