Central Sub-basin Research Articles

Vegetation and climate dynamics during the Holocene and Eemian interglacials derived from Lake Baikal pollen records

The last interglacial (LI) and Holocene changes in annual precipitation ( P ann), the mean temperature of the warmest ( T w) and coldest ( T c) month and the moisture index ( α) were reconstructed from continuous pollen records from Lake Baikal. The Holocene core (52°31′N, 106°09′E) presented in this study was recovered from a depth of 355 m in the 25-km wide underwater Buguldeika saddle separating the southern sub-basin of Lake Baikal from its central sub-basin. The biome reconstruction shows that tundra and steppe biomes have highest scores during ca. 15,000–13,300 cal. years B.P. and that taiga becomes a dominant vegetation type after ca. 13,300 cal. years B.P. Our quantitative reconstruction indicates an onset of relatively warm and wet conditions soon after ca. 10,000 cal. years B.P. The warmest and wettest climate with T w ∼ 16 °C, P ann ∼ 480 mm and α ∼ 0.9–1 has been reconstructed for ca. 9000–7000 cal. years B.P. In the Lake Baikal region this interval is characterized by the appearance and spread of hunter communities (Kitoi culture). Consistently a hiatus in the regional archaeological record (4900–4200 years B.C. or 6850–6150 cal. years B.P.) coincides with the interval of a major climate deterioration which followed the ‘climatic optimum’. An attempt to find a relationship between the archaeological record and a spread of steppe and meadow communities in the Lake Baikal region demonstrates that despite a long habitation of the area the human impact on vegetation was local rather than regional and likely did not affect the pollen record from Lake Baikal. The reconstructed peaks in the steppe biome scores during the last 9000 years are consistent with short (one to five hundred year) episodes of weak Pacific (summer) monsoon supporting our interpretation that the Holocene vegetation changes around Lake Baikal are associated with large-scale circulation processes controlling regional water balance rather than with human activities. Thus, our study proves the suitability of Lake Baikal pollen data for the reconstruction of natural vegetation and climate dynamics through the whole period from the onset of the LI to the present. Comparison of the recent and the last interglacial suggests that the Holocene ‘climatic optimum’ was less pronounced (e.g. lower summer and winter temperatures and annual precipitation sums) than that of the LI. On the other hand, pollen records demonstrate that the Holocene ‘forest phase’ already lasts some thousand years longer than that of the LI. The interglacial vegetation dynamics derived from the Lake Baikal pollen records can be satisfactorily explained by reconstructed changes in summer and winter temperatures and in available moisture. The interglacial vegetation around Lake Baikal is dominated by the boreal forests, which are associated with a generally warm and wet climate. The high sea level associated with decreased ice volume appears to have had a greater impact on the Siberian environments during the last and the recent interglacial than the direct effect of lower-than-present winter insolation. Reconstructed changes in the winter temperature correlate well with changes in the sea level and global ice volume, while the summer temperatures derived from the Lake Baikal pollen records track changes in the summer insolation.

Geomorphological and sedimentological evolution of a lake basin under strong volcano-tectonic influence: The seismic record of Lago Calafquén (south-central Chile)

The Chilean Lake District is a geodynamically active area located at the foot of the Andean cordillera and permanently influenced by the Chilean active continental margin. The study area is prone to steady mass movements that are triggered by active volcanism, strong earthquakes and climatic forces. On the glacial-born lake Lago Calafquén high-resolution seismic investigation was performed to select suitable coring sites for sedimentary analyses, to revise the existing bathymetry and to integrate the lake bottom topography into the geomorphological context of the surroundings. The sub-bottom profiling reveals a complex structure of the lake floor, which results in a division into an eastern and a central sub-basin. The eastern basin encompasses the deep basin plain which is a depositional area for dense suspension-loaded underflows. The morphology of the basement of the central sub-basin is affected by large landslides. Large rock-slope failures of debuttressed sidewalls fill the primary lake floor of the southern side covered by more than 40 m of sediment. A high-resolution seismic analysis of the sedimentary infill reveals deposits and structures of large mass movements and turbidites, obviously triggered by strong earthquakes. Incidents related to rebound effects by isostatic uplift are also considered to provoke remarkable magnitudes that trigger mass movement processes in the central sub-basin. Furthermore, volcanic impact frequently interrupts normal lacustrine sedimentation of the sediment succession by deposition of tephra layers. The serialized geodynamic impacts on Lago Calafquén cause severe damage to the sedimentary archive. Thus, suitable coring sites for palaeoenvironmental reconstruction should be selected carefully.

Additional evidence of LIW entrainment across the Algerian subbasin by mesoscale eddies and not by a permanent westward flow

The circulation of the Levantine Intermediate Water (LIW) in the Algerian subbasin (western basin of the Mediterranean sea) has been much debated for more than fifteen years now. Together with the old circulation diagrams, several numerical models claim that a branch of LIW is permanently flowing westwards across the Algerian subbasin, i.e. directly from the Channel of Sardinia towards the Strait of Gibraltar. Only a few models support the fact that the unique continuous flow of LIW is structured as an alongslope counterclockwise vein, which is thus directed northwards off Sardinia in the Algerian subbasin, and hence support the diagram published by Millot in 1987 [Millot, C. (1987a) Circulation in the Western Mediterranean. Oceanologica Acta 10(2), 143–149]. According to this diagram, any little mixed LIW found in the central subbasin corresponds to fragments which have been pulled away from the vein and entrained there by mesoscale eddies originated from the Algerian Current. The ELISA experiment (1997–1998), as a follow-up of other ones conducted since about 15 years, was designed partly to validate the diagram. In addition to about 40 current meters set in place for one year, four main campaigns were conducted with a sampling strategy guided in real time by infrared satellite information. The data set we present clearly provides additional evidence that the little mixed LIW found in the central Algerian subbasin has been entrained there by the mesoscale eddies and not by a permanent westward flow.

Tectonic development of the Bransfield Basin and its prolongation to the South Scotia Ridge, northern Antarctic Peninsula

The Bransfield Basin is located off the northern tip of the Antarctic Peninsula. The analysis of multichannel seismic reflection profiles allows the shallow structure of the Bransfield Basin and its eastwards prolongation through the Scotia Sea to be imaged. The Bransfield Basin is asymmetrical, with most of the sediment input coming from the passive Antarctic Peninsula margin, which is deformed by normal faults generally dipping less than 45° northwestwards. Much less sediment comes from the conjugate South Shetland Islands margin, which is dominated by high-angle normal faults, generally dipping between 45° and 60°, constituting the inner boundary of the South Shetland Block tectonic horst. The detachment and moving apart of the South Shetland Block from the Antarctic Peninsula led to the opening of the Bransfield Basin, where incipient oceanic spreading has been reported previously. The South Shetland Block outer boundary to the northwest is made of the South Shetland Trench, which to the east passes into the sinistral transpressive Scotia–Antarctic plate boundary marked by the prominent South Scotia Ridge. Following cessation of spreading in the Phoenix–Antarctic Ridge northwest of the South Shetland Block during middle Pliocene, roll-back played a major role in the development of the Bransfield Basin, with maximum extension in the Central subbasin. In addition, the Eastern Bransfield Basin accommodates most of the sinistral transtensional deformation along the South Scotia Ridge. Normal and reverse faults were simultaneously active along the inner and outer boundaries, respectively, of the continental crust fragment constituting the South Shetland Block. The development of the Bransfield Basin offers a good example of the tectonic processes involved in continental fragmentation associated with backarc basin development.

Surface-water quality for the Salí River watershed in NW Argentina

Spatial and temporal variations of Na, Mg, Si, K, Ca, SO42–, Cl–, HCO3–, Ag, Al, As, Au, B, Ba, Be, Bi, Br, Ce, Cd, Co, Cr, Cs, Cu, Dy, Er, Eu, F, Fe, Ga, Ge, Gd, Hf, Hg, Ho, I, La, Li, Mn, Mo, Nb, Nd, Ni, P, Pb, Pr, Rb, Sb, Sm, Sn, Sr, Ta, Tb, Te, Th, Ti, Tl, U, V, W, Y, Yb, Zn, and Zr were monitored through 37 sampling stations to determine the main aspects influencing the surface-water quality in the Sali River watershed (Tucuman Province, NW Argentina). The influence of the regional geological setting on water chemistry allows to distinguish three sub-basins. The interaction with sedimentary rocks was found to be dominant in the northern and central-eastern sub-basin as well as in the southern sub-basin, whereas the metamorphic-granitic basement of the Pampean Ranges was noted in the central sub-basin. In addition, anthropogenic activities affect the spatial variation of K, P, Mn, Rb, and Pb as well as dissolved oxygen concentrations and Eh. Temporal water-quality variation is related to the spatial distribution of precipitation and to the seasonal character of the main local industries (sugar cane, alcohol, citrus), increasing P and K concentrations and decreasing dissolved oxygen concentration and Eh in winter. Cl–, Na, SO42–, Al, As, B, Fe, Mn, Se and U concentrations exceed the regulated drinking-water thresholds at several sampling stations.

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.

Provenance of lithogenic surface sediments and pathways of riverine suspended matter in the Eastern Mediterranean Sea: evidence from 143Nd/ 144Nd and 87Sr/ 86Sr ratios

In order to characterize the provenance of lithogenic surface sediments from the Eastern Mediterranean Sea (EMS), residual (leached) fraction of 34 surface samples have been analysed for their 143Nd/ 144Nd and 87Sr/ 86Sr isotope ratios. The sample locations bracket all important entrances of riverine suspended matter into the EMS as well as all sub-basins of the EMS. The combined analyses of these two isotope ratios provide a precise characterization of the lithogenic fraction of surface sediments and record their dilution towards the central sub-basins. We reconstruct provenance and possible pathways of riverine dispersal and current redistribution, assuming more or less homogenous isotopic signatures and flux rates of the eolian fraction over the EMS. Lithogenic sediments entering the Ionian Sea from the Calabrian Arc and the Adriatic Sea are characterized by high 87Sr/ 86Sr isotope ratios and low ε Nd(0) values (average 87Sr/ 86Sr=0.718005 and ε Nd(0)=−11.06, n=5). Aegean Sea terrigenous sediments show an average ratio of 87Sr/ 86Sr=0.713089 ( n=5) and values of ε Nd(0)=−7.89 ( n=5). The Aegean isotopic signature is traceable up to the southwest, south, and southeast of Crete. The sediment loads entering the EMS via the Aegean Sea are low and spread out mainly through the Strait of Casos (east of Crete). Surface sediments from the eastern Levantine Basin are marked by the highest ε Nd(0) values (−3.3, n=6) and lowest 87Sr/ 86Sr isotope ratios (average 0.709541, n=6), reflecting the predominant input of the Nile sediment. The influence of the Nile sediment is traceable up to the NE-trending, eastern flank of the Mediterranean Ridge. The characterization of the modern riverine dispersal and eolian flux, based on isotope data, may serve as a tool to reconstruct climate-coupled variations of lithogenic sediment input into the EMS.

Adriatic Sea surface circulation as derived from drifter data between 1990 and 1999

The Adriatic Sea surface circulation for the period 1990–1999 is studied using the data of more than 200 satellite-tracked drifters. The spatial structure and the temporal variability of the surface currents, at meso- to seasonal scales, are described in terms of Eulerian and Lagrangian statistics estimated from the low-pass filtered drifter velocities. Maps of mean currents, subtidal velocity variance and mean kinetic energies were produced using a 40-km averaging scale. The mean flow map confirms that the global cyclonic circulation in most of the Adriatic basin is broken into three re-circulation cells in the northern, central and southern sub-basins (the latter two being controlled by the bathymetry of the Jabuka and South Adriatic Pits, respectively). An isolated cyclonic gyre prevails near the head of the basin. Mean velocities in the cyclonic gyres can exceed 25 cm s −1 in the coastal areas where the velocity variance is also maximum (reaching 500 cm 2 s −2). Values near 2×10 7 cm 2 s −1, 2 days and 18 km were obtained for the diffusivity and the Lagrangian integral time and spatial scales in the along-basin direction, respectively. In the across-basin direction, the statistics are typically 50% of the above values. Geographical and seasonal variations of the Lagrangian statistics can be substantial. It was found that the fluctuating velocities (or the mesoscale eddies) have a preferential cyclonic sense of rotation. The gyres and the coastal currents are mostly prevailing in summer and fall. In winter and spring, they are less intense but the southern one tends to re-circulate more around the South Adriatic Pit. The mean eddy kinetic energy is maximum in winter and fall throughout the central and southern sub-basins. The drifters showed that the southeastward flow along the Italian Peninsula has a width varying between 45 and 70 km and a mean core speed of 25–35 cm s −1. In the northern and central sub-basins, maximum velocities are found within 5–10 km off the coast in winter and spring, while a weaker maximum is seen more offshore (15–25 km) during the other seasons. In the southern Adriatic, the current is wider in summer, fall and winter, whereas in spring, it becomes thinner (maximum core speed near 10 km from shore and width of about 50 km). The maximum core speed is generally larger in summer than in winter.

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.

An inter-basinal comparison of the sedimentology of Late Holocene to recent sediments in the Rift Valley, Lake Turkana, Kenya

Grain size variations, sediment chemistry and general mineralogical assemblages of sediments in Lake Turkana reflect provenance. Allogenic sediments in Lake Turkana are mainly supplied by the Omo and Kerio-Turkwel Rivers. Minor inputs are from seasonal streams and strong southeasterly winds. The depth profiles of the grain size distributions in lake sediment cores exhibit enantiomorphism, which is interpreted as being diagnostic both of shifts in the equilibrium energy regime of the transporting media and of the dominant provenance of particular size grades within the specific basins of the lake. The North Sub-basin is dominated by fine-grained sediments, which reflect the texture of the volcanic rocks of the Omo River drainage basin. The Central Sub-basin sediments reflect, as sources, the coarser metamorphic terrane of the Kerio-Turkwel Rivers drainage basin. Kaolinite and fine-grained iron oxides are brought into the lake mainly by the two large fluvial input systems: the Omo River in the North Sub-basin and the Kerio-Turkwel Rivers in the Central Sub-basin. Some fine-grained overflow of this material makes its way into the South Sub-basin. Illite in the North and Central Sub-basins is strongly related to transport of material from near-shore sediments and, in the Central Sub-basin and northern reaches of the South Sub-basin, from the Kerio and Turkwel Rivers input. Smectite and calcite are mainly authigenic. In the South Sub-basin, however, the relatively coarser detrital particles are derived from silt and sand-sized in situ biogenic (calcitic and siliceous) debris and ˦olian-transported particles from regions southeast of the lake. The ˦olian fraction accounts largely for the ubiquitous and distinct very fine sand size grade, and consists of quartz, feldspar and blue-green amphiboles.

Tectonic and eustatic controls on sequence stratigraphy of the Pliocene Loreto basin, Baja California Sur, Mexico

Research Article| February 01, 2000 Tectonic and eustatic controls on sequence stratigraphy of the Pliocene Loreto basin, Baja California Sur, Mexico Rebecca J. Dorsey; Rebecca J. Dorsey 1Department of Geological Sciences, 1272 University of Oregon, Eugene, Oregon 97403-1272 Search for other works by this author on: GSW Google Scholar Paul J. Umhoefer Paul J. Umhoefer 2Department of Geology, Northern Arizona University, Flagstaff, Arizona 86011 Search for other works by this author on: GSW Google Scholar Author and Article Information Rebecca J. Dorsey 1Department of Geological Sciences, 1272 University of Oregon, Eugene, Oregon 97403-1272 Paul J. Umhoefer 2Department of Geology, Northern Arizona University, Flagstaff, Arizona 86011 Publisher: Geological Society of America Received: 15 Jun 1998 Revision Received: 30 Mar 1999 Accepted: 23 Apr 1999 First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (2000) 112 (2): 177–199. https://doi.org/10.1130/0016-7606(2000)112<177:TAECOS>2.0.CO;2 Article history Received: 15 Jun 1998 Revision Received: 30 Mar 1999 Accepted: 23 Apr 1999 First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Rebecca J. Dorsey, Paul J. Umhoefer; Tectonic and eustatic controls on sequence stratigraphy of the Pliocene Loreto basin, Baja California Sur, Mexico. GSA Bulletin 2000;; 112 (2): 177–199. doi: https://doi.org/10.1130/0016-7606(2000)112<177:TAECOS>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The Loreto basin formed by rapid westward tilting and asymmetric subsidence within a broad releasing bend of the Loreto fault during transtensional deformation along the western margin of the active Gulf of California plate boundary. Sedimentary rocks range in age from ∼5(?) to 2.0 Ma and consist of siliciclastic and carbonate deposits that accumulated in nonmarine, deltaic, and marine settings. The basin is divided into the central and southeast subbasins, which have distinctly different subsidence histories and stratigraphic evolution. Sedimentary rocks of the Loreto basin are divided into four stratigraphic sequences that record discrete phases of fault-controlled subsidence and basin filling. Sequence boundaries record major changes in tilting geometries and sediment dispersal that were caused by reorganization of basin-bounding faults. Sequence 1 consists of nonmarine conglomerate and sandstone that accumulated in alluvial fans and braided streams. The sequence 1–2 boundary is a marine flooding surface in both subbasins, and parasequences within sequence 2 consist of progradational Gilbert deltas that are capped by transgressive marine shell concentrations and flooding surfaces. The sequence 2–3 boundary is a low-angle erosional unconformity in the southeast subbasin and a thin interval of downlap in the central subbasin. Sequence 3 is characterized by bioclastic limestones that were derived from the uplifted portion of the hanging-wall tilt block. The sequence 3–4 boundary is an angular unconformity in the southeast subbasin and an abrupt marine flooding surface in the central subbasin. Sequence 4 consists dominantly of in situ shallow-marine carbonate deposits.By comparing parasequences of sequence 2 with marine oxygen-isotope curves, we can discriminate between eustatic and tectonic controls on stratigraphic evolution. In the central subbasin, sequence 2 accumulated during a short phase of extremely rapid subsidence (8 mm/yr); it contains 14 paracycles that do not match the O-isotope curve, and there are no unconformities. In the southeast subbasin, sequence 2 accumulated at a rate of ∼1.5 mm/yr; it contains 4 paracycles that appear to match the O-isotope curve, and sequence boundaries are unconformities. Thus, we conclude that during sequence 2 deposition: (1) extremely rapid subsidence in the central subbasin outpaced eustatic sea-level changes, and Gilbert delta paracycles were produced by episodic fault-controlled subsidence; and (2) subsidence in the southeast subbasin was slower than the rate of eustatic sea-level changes, and the internal stratigraphic cyclicity preserves a record of eustatic rather than tectonic events. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Tectonic and eustatic controls on sequence stratigraphy of the Pliocene Loreto basin, Baja California Sur, Mexico

The Loreto basin formed by rapid westward tilting and asymmetric subsidence within a broad releasing bend of the Loreto fault during transtensional deformation along the western margin of the active Gulf of California plate boundary. Sedimentary rocks range in age from ∼5(?) to 2.0 Ma and consist of siliciclastic and carbonate deposits that accumulated in nonmarine, deltaic, and marine settings. The basin is divided into the central and southeast subbasins, which have distinctly different subsidence histories and stratigraphic evolution. Sedimentary rocks of the Loreto basin are divided into four stratigraphic sequences that record discrete phases of fault-controlled subsidence and basin filling. Sequence boundaries record major changes in tilting geometries and sediment dispersal that were caused by reorganization of basin-bounding faults. Sequence 1 consists of nonmarine conglomerate and sandstone that accumulated in alluvial fans and braided streams. The sequence 1–2 boundary is a marine flooding surface in both subbasins, and parasequences within sequence 2 consist of progradational Gilbert deltas that are capped by transgressive marine shell concentrations and flooding surfaces. The sequence 2–3 boundary is a low-angle erosional unconformity in the southeast subbasin and a thin interval of downlap in the central subbasin. Sequence 3 is characterized by bioclastic limestones that were derived from the uplifted portion of the hanging-wall tilt block. The sequence 3–4 boundary is an angular unconformity in the southeast subbasin and an abrupt marine flooding surface in the central subbasin. Sequence 4 consists dominantly of in situ shallow-marine carbonate deposits. By comparing parasequences of sequence 2 with marine oxygen-isotope curves, we can discriminate between eustatic and tectonic controls on stratigraphic evolution. In the central subbasin, sequence 2 accumulated during a short phase of extremely rapid subsidence (8 mm/yr); it contains 14 paracycles that do not match the O-isotope curve, and there are no unconformities. In the southeast subbasin, sequence 2 accumulated at a rate of ∼1.5 mm/yr; it contains 4 paracycles that appear to match the O-isotope curve, and sequence boundaries are unconformities. Thus, we conclude that during sequence 2 deposition: (1) extremely rapid subsidence in the central subbasin outpaced eustatic sea-level changes, and Gilbert delta paracycles were produced by episodic fault-controlled subsidence; and (2) subsidence in the southeast subbasin was slower than the rate of eustatic sea-level changes, and the internal stratigraphic cyclicity preserves a record of eustatic rather than tectonic events.

PALAEOZOIC PRE-SALT SEDIMENTS IN THE PRECASPIAN PETROLIFEROUS PROVINCE

The PreCaspian (North Caspian) Basin is located on the SE margin of the Russian Platform and extends as far south as the north coast of the intra-continental Caspian Sea. Regional surface elevations here are as much as 24 m below sea-level. The basin is one of the World's deepest, with basin-fill sediments exceeding 20 km in thickness. The Proterozoic section the pre-salt Palaeozoic section and the Upper Permian (salt-bearing) interval are all about 5-km thick although as a result of salt movements the salt-bearing sequence may be thicker. The basin has a roughly oval outline and is 900-km wide 600-km in length and covers around 500,000 sq. km. It is divided into the Central and SE Sub-basins by the approximately east-west trending Astrakhan-Aktyubinsk system of basement highs. A series of massive carbonate build-ups rim the basin margins.

Water Quality in the Shingle Creek Basin, Florida, before and after Wastewater Diversion

AbstractShingle Creek is a major inflow to Lake Tohopekaliga, Florida. Water quality and the trophic status of Lake Tohopekaliga are affected strongly by the water quality of Shingle Creek. This report documents 10 yr of water quality data in Shingle Creek at the lake outfall; for a pre‐ (October 1981–December 1986) and a post‐wastewater discharge (January 1987–September 1991) removal period. Nutrient budgets for the subbasins were calculated from an intensive research program (January 1983–December 1985) to document instream impacts attributable to wastewater, determine the role of the cypress swamp in the middle subbasin, and document relationships between water quality and land uses. Rapid urbanization converted forested uplands and agricultural lands to housing and commercial land use during the study. Stormwater runoff in Florida has been identified as a major pollution source. Treatment of stormwater pollution, through Best Management Practices (BMPs), has been regulated by the State of Florida in this area since 1982. By 1988, 84% of the urban landuse in the upper basin was subject to stormwater treatment prior to being discharged to the creek. Potential increases in urban derived nutrient inputs were offset by stormwater management, and alum treatment and diversion of municipal wastewater. Nitrogen loads, and P loads and variance decreased significantly during the 10‐yr period, despite rapid urbanization in the northern and central subbasins. Nutrient export from the subbasins was influenced by the dominant land use. The middle subbasin contains a swamp that contributed the greatest P and Cl− loads because of the increase in discharge to the swamp from sources other than the canal. The northern urban subbasin received the wastewater discharges and served as a net sink for N and P exported from the subbasin.

Correlation, frequency of emplacement and source directions of megaturbidites on the Madeira Abyssal Plain

This paper describes the sediments of the whole Madeira Abyssal Plain including the MADCAP area to the south. Previous studies have centred on the central sub-basin which occupies most of the plain and which has become one of the most studied areas of seafloor. The plain is made up of a series of large turbidites separated by thin pelagic layers which can be used to provide a high resolution stratigraphy back to about 750,000 years. Many of the turbidites were deposited close to oxygen isotope stage boundaries suggesting a strong relationship to sea-level/climate change, although the exact initiation mechanism for the flows is not known. Data from the northern and southern extensions of the plain shows the effects of different entry points for the four main turbidite groups; volcanic rich turbidites enter from the NE and E, calcareous turbidites from the W, one group of organic rich turbidites enters from the SE and one group from the NE and E. Different turbidite sources operate for limited periods before switching to one of the other source areas. It is very rare for two sources to provide turbidites at the same isotope stage boundary.

Comparative sequence stratigraphy and structural styles of the Slyne Trough and Hebrides Basin

The Slyne Trough to the west of Ireland is divided into three sections: North, Central and South. The South and Central sub-basins are separated by a seismic ‘dead zone’ along the line of the Clew Bay disturbance. The Central and North sub-basins are separated by a zone transferring the fault bounding the main basin from the western margin in the Central sub-basin to the eastern margin in the North sub-basin. Five stratigraphic sequences from the Slyne Trough, which span the Rhaetian to Middle Jurassic, are recognized in the 27/13–1 well, and they are compared to the equivalent sequences in the Hebrides Basin. The Late Jurassic Hebridean sequences are expected in the deeper parts of the Central Slyne Trough. The Slyne Trough and Hebrides Basin sediments are mainly tectonically controlled, although major eustatic events in the Early Toarcian and Late Bathonian are recognized.

Interpretation of Fault-Controlled Ramp Structures in Sedimentary Basins--Example from Caspian Sea Using Landsat TM Data: ABSTRACT

Lineaments on a series of edge-enhanced images (TM data) from a region around the Caspian Sea form a geomorphically significant linear trend along the major Caucasus-Kopeh Dagh fault line. This fault represents the line of collision between the Cimmerian continents and the Turan plate on the south and north, respectively. The lineament zone manifests a ramp structure that forms a relatively narrow topographic high in the Caspian Sea. Paleogeographic studies of the Caspian Sea suggest that the basin is part of the eastern Paratethys, which began to develop in the early Paleogene during the Alpine-Himalayan uplift. On the basis of the lineaments and associated geomorphic features, the Caspian Sea can be divided into southern, central, and the northern Caspian subbasins. The Caucasus-Kopeh Dagh fault line trends N80/degrees/W and separates the southern Caspian from the central subbasin, approximately along 40/degrees/N latitude. The boundary between the central and the northern subbasins is also a linear topographic high which trends N70/degrees/E and lies approximately at 44/degrees/N latitude. The southern and central subbasins have subequal areal extension covering 35.64% and the 36.63% of the whole sea, whereas the northern subbasin occupies only 27.73% of the basin.

Mineral precipitation and diagenesis in the sediments of the Lake Bogoria basin, Kenya Rift Valley

Summary Lake Bogoria is a perennial, saline, alkaline closed lake fed by hot springs and ephemeral runoff. Detailed studies of the sedimentology, mineralogy and chemistry of the late Quaternary sediments, both from exposures around the shoreline and from lake bottom cores, have provided abundant evidence for early diagenesis and mineral precipitation. Marginal fan-deltaic clastics exhibit carbonate (calcite, dolomite), iron oxide, opaline silica and zeolite (analcime, natrolite) cements, especially where groundwater is shallow and subject to evaporative concentration. Travertines associated with microbial activity around hydrothermal springs are predominantly calcite; high temperature, inorganically precipitated travertines are aragonitic. Most are relic deposits. In places, spring carbonates have been silicified (opal-A) or altered to fluorite in response to changing groundwater composition. Fan-delta conglomerates have been hydrothermally altered by alkaline spring waters yielding zeolites (analcime, thomsonite, clinoptiloite, mordenite), clay minerals, opaline silica and magadiite. The lacustrine sediments include prodeltaic clastics, organic oozes and sodium carbonate evaporites. Trona and nahcolite predominate in the central sub-basin. The local occurrence of thermonatrite may be associated with sub-lacustrine hydrothermal activity. In the north subbasin, evaporites are exclusively gaylussite. Sodium silicates (magadiite, kanemite) occur in black organic oozes and formed in an anoxic environment rich in sodium carbonate and dissolved SiO 2 . Other authigenic minerals in the lacustrine sediments include pyrite, siderite, fluorite, zeolites (analcime, mordenite, clinoptilolite), opaline silica (formed from a gel precursor around sub-lacustrine springs?) and vivianite.

Tectonic and geologic evolution of the Espanola Basin, Rio Grande Rift: Structure, rate of extension, and relation to the state of stress in the western United States

The Española basin of the Rio Grande rift began as a broad crustal downwarp in latest Oligocene time. Most of the basin is 2–3 km deep, but localized faulting allowed accumulation of up to 5 km of sedimentary fill in a central sub-basin. The localized early faulting ended before filling of the central Española basin was completed about 10 m.y. ago. Movement on faults that define the present western margin of the Española basin began ~ 10 m.y. ago. Jemez Mountain volcanism, in the western Española basin, also began at about this same time. West tilting of up to 30° occurred due to movement along pervasive N-trending intrabasin faults about 7.5 m.y. ago in conjunction with continued movement along the western border faults. Volcanism continued after this tilting, forming many of the large volcanic constructs of the Jemez Mountains. Regional uplift of the entire northern Rio Grande rift began ~ 7 m.y. ago. Movement on the Pajarito fault zone began about 5 m.y. ago and continues to the present. This fault zone defines the western margin of the velarde graben, a narrow central sub-basin where recent movement has been concentrated. Some volcanism also has occurred within the southern Velarde graben. Total extension across the Española basin since ~ 26 m.y. ago is estimated to have been ~ 5.5 km (roughly 10%) or between 3.5 and 8 km assuming high-angle planar faulting. The ~ 0.2 mm/yr averaged long term rate of extension has been separated into three periods of activity: 1. (1) ~ 0.14 mm/yr from 26 to 10 m.y. ago. 2. (2) ~ 0.5 mm/yr from 10 to 5 m.y. ago. 3. (3) ~ 0.14 mm/yr from 5 m.y. ago to present. A change in least principal stress direction from WSW-ENE to WNW-ESE that occurred throughout the western United States about 10 m.y. ago coincides with a roughly 3.5 times increase in the rate of extension, preferential development and movement of N- to NE-trending normal faults, and a few degrees of clockwise rotation of rocks in the western Española basin. Similar to the Española basin, initial basins of the southern Rio Grande rift were broad downwarps and rifting was greatly accelerated after ~ 10 m.y. ago. Accelerated uplift of the northern Rio Grande rift also occurred at about this time indicating that activity in the entire Rio Grande rift was modulated by this change in extension direction ~ 10 m.y. ago that appears related to Pacific-North American plate interactions. This modulation coupled with major faulting (~ 10 m.y. ago) preceding uplift (~ 7 m.y. ago) in the Española basin suggest a passive rifting process for the Rio Grande rift whereby stresses due to plate interactions elsewhere cause faulting in the lithosphere which leads to the development of a “passive” asthenospheric uplift. Furthermore, the roughly 20 m.y. pre-uplift history of sediment accumulation in basins of the central and southern rift, and the inherited character, trend, and geometry of the Rio Grande rift as a whole are also more consistent with a passive rifting process.

Geology, structure, and tectonics of the Pajarito fault zone in the Española basin of the Rio Grande rift, New Mexico

The Pajarito fault zone forms the western border of the Velarde graben, the presently active, central subbasin of the Espanola basin section of the Rio Grande rift in north-central New Mexico. The fault zone is a north-northeast-trending zone of predominantly down-to-the-east faults that cut Miocene to Pliocene volcanic rocks along the eastern flank of the Jemez Mountains. Where the fault zone cuts the 1.1-m.y.-old Tshirege Member of the Bandelier Tuff, it has produced a steep, 50- to 100-m-high fault scarp. The total displacement across the fault zone during its 5-m.y. history is between 200 and 600 m. Rates of displacement for the time periods 0–5, 0–1.1, and 1.1–5 m.y. ago range from 0.02 to 0.136 mm/yr. Abrupt facies changes between older volcanics and volcaniclastic sediments of the Jemez Mountains appear to have controlled the local position, trend, and character of the Pajarito fault zone. The fault zone bows and/or steps eastward where two large volcanic complexes are present but is found farther west in between and at either end of the volcanic complexes. One complex was sufficiently massive to interfere with the development of the Velarde graben. Slickensides on mesoscopic faults in the Tshirege Member of the Bandelier Tuff indicate that the Pajarito fault zone has undergone extension in two directions during the past 1.1 m.y., approximately parallel and perpendicular to the local trend of the fault zone. These directions indicate that the Pajarito fault zone has reoriented the regional minimum and intermediate stress directions to perpendicular and parallel, respectively, to the local trend of the fault zone, and that both minimum and intermediate stress directions are tensional. A tectonic history for the Pajarito fault zone area of the Espanola basin begins with relatively stable accumulation of prerift and synrift sediments from Eocene to Oligocene time. Sedimentation concomitant with faulting, unrelated to the Pajarito fault zone, filled deep central depressions within the Espanola basin. This faulting ceased prior to the end of the filling of the basin, around 10 m.y. ago in the local area. Jemez Mountain volcanism began about this time, before movement along the western-margin border faults of the Espanola basin caused west-tilting of old volcanics and sediments, about 7.5 m.y. ago. Volcanism continued under relatively stable conditions until ∼5 m.y. ago. At this time, the Pajarito fault zone and Velarde graben formed. Faulting has continued to the present, localized along this central subbasin.