Adjacent Offshore Region Research Articles

A SYNTHESIS OF THE GEOLOGY AND PETROLEUM GEOLOGY OF THE IRANIAN PORTION OF THE SOUTH CASPIAN BASIN AND SURROUNDING AREAS

The South Caspian Basin, the northern Alborz Mountains, the Gorgan plain and the Moghan plain constitute the northernmost and youngest petroleum system in Iran. This region was part of the Paratethys realm from Oligocene to Pliocene time. The Oligocene – Miocene Maikop/Diatom Total Petroleum System of the South Caspian Basin produces major volumes of hydrocarbons in Azerbaijan and Turkmenistan, and the Iranian sector of the basin has consequently undergone exploration due to its generally similar geology. The 20 km thick, dominantly Cenozoic sedimentary cover in the basin is reduced to less than 3 km in the northern foothills of the Alborz Mountains, and scattered surface oil seepages in the latter region are believed to be generated by Cretaceous and Miocene source rocks. In the Moghan plain to the southwest of the South Caspian Basin, anticlinal folds of Oligo‐Miocene Zivar Formation sandstones may be prospective for hydrocarbon exploration. Mud volcanoes in the Gorgan plain and in adjacent offshore regions at the SE margin of the South Caspian Basin are associated with hydrocarbon seepages, and appear to be sourced by Cretaceous and Cenozoic shales and mudstones. Major structural features in the southern part of the South Caspian Basin include Cenozoic mud diapirs, folds and gravity structures.

A Statistical Analysis of Total Lightning Flashes and Peak Current from High-Resolution ENTLN Measurements in South China during 2017

Abstract In this study, the spatiotemporal characteristics of total lightning, including cloud-to-ground (CG) and intracloud (IC), flashes and the peak current of CG flashes are investigated using the observations from the Earth Networks Total Lightning Network (ENTLN) during 2017 over Guangdong Province, China, and its adjacent offshore regions, which are the capital of lightning activity and convective rainfall in China. The ENTLN detects 2 825 558 IC and 569 104 CG flashes, accounting for 83.2% and 16.8% of annual-total lightning flashes, respectively, and they are distributed during 224 lightning days or 3316 lightning hours over the regions. The lightning activity peaks in June (IC) and July (CG) with a diurnal peak at 1500 LST. On average, about 70 lightning days with 20 flashes per kilometer squared per year occur inland, and double that many occur offshore. High lightning days and flash density are found in the central and southwestern mountainous region and the Guangdong–Hong Kong–Macao Greater Bay area, where convective activity is high. An analysis of the CG observations reveals that negative CG (NCG) produce an average of 1.96 strokes, with 5% of them having more than six return strokes. The peak currents of NCG strokes increase with increasing multiplicity. Results also show that the peak current and accumulated value of NCG flashes are the appropriate indicators to characterize the intensity of lightning activity in terms of spatiotemporal variations, especially in the mountainous region. It is concluded that IC observations provide more distinct spatial distribution of lightning activity than CG flashes; and those appropriate indicators should be used to represent the peak current of multistroke NCG flashes.

The effect of Oceanic South Atlantic Convergence Zone episodes on regional SST anomalies: the roles of heat fluxes and upper-ocean dynamics

The South Atlantic Convergence Zone (SACZ) is an atmospheric system occurring in austral summer on the South America continent and sometimes extending over the adjacent South Atlantic. It is characterized by a persistent and very large, northwest-southeast-oriented, cloud band. Its presence over the ocean causes sea surface cooling that some past studies indicated as being produced by a decrease of incoming solar heat flux induced by the extensive cloud cover. Here we investigate ocean–atmosphere interaction processes in the Southwestern Atlantic Ocean (SWA) during SACZ oceanic episodes, as well as the resulting modulations occurring in the oceanic mixed layer and their possible feedbacks on the marine atmospheric boundary layer. Our main interests and novel results are on verifying how the oceanic SACZ acts on dynamic and thermodynamic mechanisms and contributes to the sea surface thermal balance in that region. In our oceanic SACZ episodes simulations we confirm an ocean surface cooling. Model results indicate that surface atmospheric circulation and the presence of an extensive cloud cover band over the SWA promote sea surface cooling via a combined effect of dynamic and thermodynamic mechanisms, which are of the same order of magnitude. The sea surface temperature (SST) decreases in regions underneath oceanic SACZ positions, near Southeast Brazilian coast, in the South Brazil Bight (SBB) and offshore. This cooling is the result of a complex combination of factors caused by the decrease of solar shortwave radiation reaching the sea surface and the reduction of horizontal heat advection in the Brazil Current (BC) region. The weakened southward BC and adjacent offshore region heat advection seems to be associated with the surface atmospheric circulation caused by oceanic SACZ episodes, which rotate the surface wind and strengthen cyclonic oceanic mesoscale eddy. Another singular feature found in this study is the presence of an atmospheric cyclonic vortex Southwest of the SACZ (CVSS), both at the surface and aloft at 850 hPa near 24°S and 45°W. The CVSS induces an SST decrease southwestward from the SACZ position by inducing divergent Ekman transport and consequent offshore upwelling. This shows that the dynamical effects of atmospheric surface circulation associated with the oceanic SACZ are not restricted only to the region underneath the cloud band, but that they extend southwestward where the CVSS presence supports the oceanic SACZ convective activity and concomitantly modifies the ocean dynamics. Therefore, the changes produced in the oceanic dynamics by these SACZ events may be important to many areas of scientific and applied climate research. For example, episodes of oceanic SACZ may influence the pathways of pollutants as well as fish larvae dispersion in the region.

Ages and character of igneous rocks of the Da Lat Zone in SE Vietnam and adjacent offshore regions (Cuu Long and Nam Con Son basins)

The Da Lat Zone in SE Vietnam consists of a Mesozoic orogenic belt that is characterised by abundant Cretaceous igneous rocks that were intruded into or deposited on Permo-Triassic rocks (represented by the Chau Thoi Formation) and Jurassic metasedimentary rocks. The three Cretaceous magmatic phases of the Dinh Quan, Deo Ca, and Ankroet suites of the Da Lat Zone were previously subdivided on the basis of geochemical and isotopic differences and related to within-plate and continental arc magmatism during Paleo-Pacific subduction. Limited dating gave varied estimates for crystallisation ages of the different suites. Here we present new whole-rock chemical data and LA-ICP-MS U-Pb zircon analyses for Cretaceous plutonic and volcanic rocks. The results indicate volcanic arc magmatism started in the Early Cretaceous, represented by the Dinh Quan suite at c. 113–135 Ma, and continued into the mid to Late Cretaceous (Deo Ca, Nha Trang, Dak Rium and Don Duong formations) with magmatism from c. 90 to 109 Ma. Granitoid basement rocks of the Cuu Long Basin yielded similar ages of c. 90 to 92 Ma, representing offshore equivalents. The Ankroet suite represents the youngest magmatic phase and is interpreted as within-plate magmatism from c. 75 to 87 Ma. The Cretaceous rocks generally have very few older zircons and their rarity is interpreted to indicate magmas were generated predominantly by melting of juvenile arc crust.

Lithospheric architecture and geodynamics of the Archean Dharwar craton and surrounding terranes: New insights from satellite gravity investigation

The construction and destruction of continents, cratons and supercontinents on the globe involved multiple events of plume, rifting, subduction-accretion, and collision through the long history of evolution of our planet. Archean cratons preserve the records of continent building in the Early Earth and understanding their lithospheric architecture is fundamental to building geodynamic models. The Dharwar craton in southern India is among the major cratons on the globe. Recent geological and geochronological studies indicate that the craton formed through the assembly of a number of micro-blocks, although the terrane boundaries and lithospheric structure have not been well-defined. Here, we analyze satellite gravity data, covering the entire Dharwar craton and its surrounding regions, with a view to understand the regional lithospheric architecture and the geodynamic evolution. Our results on the residual gravity field, based on finite element approach, can clearly demarcate the boundaries of the major geotectonic units in the Dharwar craton and surrounding regions. We identify the presence of a conspicuous residual gravity low (−55 mGal) that runs north–south for about 700 km, paralleling the western margin of the Dharwar craton. This gravity low passes through the Chitradurga schist belt and covers the Koyna seismogenic region in the north to Coimbatore in the south. The gravity patterns observed over this region, might suggest a deep-seated paleo- rift zone, which we term as ‘Koyna-Coimbatore rift’. The prominent gravity low associated with this rift structure, can be explained by the presence of a 70 km thick low velocity/low density zone (LVZ) in subcrustal lithospheric mantle, suggesting asthenospheric flow into the upper mantle. The Western Dharwar Craton (WDC) might extend further north encompassing the Koyna-Warna seismic zone. Another major finding from this study is delineation of the Central Dharwar Cratonic (CDC) block between the Chitradurga shear zone and the Kolar greenstone belt, which is characterized by a prominent residual gravity high zone (+20 mGal), coinciding with the exposures of the Closepet granitoids and their equivalents, possibly marking a collisional suture between two microblocks. The 2½D residual gravity modeling across an E-W profile, which cuts all the major geotectonic features of WDC, CDC and Eastern Dharwar Craton (EDC), reveals popping-up of the CDC terrane, below which Moho has upwarped to a depth of 36 km and the lithosphere is considerably thinner at about 120 km, compared to 48 km and 175 km respectively in the WDC. This feature corresponds to lithospheric destruction of almost 100–120 km below the Dharwar craton, with magma underplating. On the eastern side of the craton, the positive residual gravity contours that correspond to the Eastern Ghats Belt rocks, take a sharp turn into the adjacent offshore region northeast of Ongole, and continue further south, until the granulite terrane north of Chennai, indicating the absence of the Eastern Ghats Belt rocks east of Cuddapah Basin. Our gravity model is able to demarcate three collisional boundaries between (i) WDC and CDC, (ii) CDC and Cuddapah Basin, and (iii) Nellore Schist Belt and adjoining east coast terrane. The extensive lithospheric destruction with removal of nearly 100 km of the cratonic root is correlated to prolonged subduction during Paleo-Neoproterozoic, together with mantle plumes at different times playing a subordinate role, thus indicating combined mechanical, thermal and chemical erosion.

Thallium dynamics in the southern North Sea

In open ocean waters thallium (Tl) belongs to the group of conservative elements, even though deviations from this trend have been observed in NW German coastal waters. Here, we report on tidal, seasonal and spatial dynamics of Tl along with Mo and Mn in the water column of a backbarrier tidal flat close to the island of Spiekeroog, the Jade system (Inner Jade and Jade Bay) and the adjacent offshore region. Dissolved thallium (Tldiss) displays strong tidal and seasonal variations (∼25–60 pM) unrelated to salinity. In all study areas, Tldiss clearly deviates from conservative behavior. In general, Tldiss is low during low tide (with a loss of up to 50%) and inversely related to Mndiss, except in summer. The tidal Tl variations as well as the loss of Tl in the water column may be due to Tl removal from pore waters in reducing sediments and drainage of Tl-free but Mn-rich pore waters into the water column during low tide. The negative Tl anomaly can be traced offshore for more than 40 km to the island of Helgoland.The redox chemistry of Tl is not well studied, and Tl removal from pore waters was previously suggested to only occur under anoxic/sulfidic conditions. By contrast, our preliminary pore water results suggest that Tl could be removed already under slightly reducing (suboxic) conditions, likely along with microbially induced Mn reduction in the sediments. Therefore, this study supports the biological involvement in the aqueous cycling of Tl. We propose the use of Tldiss next to Mndiss as valuable indicator of suboxic or anoxic pore water discharge to the coastal realm.

A Revised Local Magnitude (ML) Scale for New Zealand Earthquakes

In this study, a new local magnitude ( M L) scale is developed for New Zealand and adjacent offshore regions. SeisComP3 (SC3) has been in use for earthquake analysis in New Zealand since September 2012 with the original Richter (1935) log A attenuation relationship for calculating M L. The attenuation characteristics of New Zealand differ significantly from southern California, where M L was originally defined, and therefore result in M L that is consistently high when compared with moment magnitude ( M w). Using M w from 528 regional moment tensor solutions along with peak observed amplitudes, a new log A curve is derived, along with station correction factors that define a revised M L scale for New Zealand earthquakes that is more consistent with M w. The new log A curve is similar to the original Richter (1935) definition at hypocentral distances of ∼100–200 km but differs significantly at closer and farther distances. The new M L is more consistent with M w across New Zealand, including crustal earthquakes and earthquakes below the crust. The California Institute of Technology (Caltech)–U.S. Geological Survey seismic processer (CUSP) system was used for earthquake analysis prior to SC3, and previous studies have derived regression relationships relating CUSP M L with M w. After applying the regression relationships to CUSP M L, we found very good agreement between CUSP M L and the new SC3 M L, which is important for developing a consistent M L between different catalogs. Online Material: Table of station corrections for the New Zealand seismograph network.

Update of Regional Moment Tensor Analysis for Earthquakes in New Zealand and Adjacent Offshore Regions

Abstract New Zealand is one of the more seismically active countries in the world with over 15,000 earthquakes each year in New Zealand and the adjacent offshore region. Routine regional moment tensor (RMT) analysis was implemented by GeoNet in 2007, and nearly 1300 RMT solutions have been calculated for the New Zealand region from August 2003 through early September 2012. The New Zealand RMT catalog contains events with moment magnitude ( M w ) 3.2–7.3 and centroid depth 2–375 km, and includes RMT solutions from aftershock sequences for two major earthquakes: 15 July 2009 M w 7.8 Dusky Sound and 3 September 2010 M w 7.1 Darfield. The RMT solutions provide important constraints on the active tectonics of New Zealand including stress and strain, and the RMT catalog, along with 155 Global Centroid Moment Tensor Project solutions, is used to examine the relationship between M w and local magnitude ( M L ). For shallow focus (≤33 km depth) the M L / M w relationship is found to be similar across New Zealand except for the Fiordland region. For all of New Zealand except Fiordland, M L =(0.93±0.06) M w +(0.54±0.24), and for Fiordland, M L =(0.83±0.04) M w +(1.09±0.13). For deep‐focus earthquakes (>33 km depth) M L is consistently larger than M w by more than a full magnitude unit for some events. The discrepancy between M L and M w is found to be depth dependent with ( M L − M w )=−0.94exp(− h /75.37)+0.80, where h is focal depth in km.

Coupling between populations of copepod taxa within an estuarine ecosystem and the adjacent offshore regions

Population dynamics in open systems are complicated by the interactions of local demography and local environmental forcing with processes occurring at larger scales. A local system such as an estuary or bay may contain a zooplankton population that effectively becomes independent of regional dynamics or the local dynamics may be closely coupled to a broader scale pattern. As an alternative, the details of migration and advection may mean that dynamics in a local system are coupled to other specific areas rather than tracking the overall dynamics at a larger scale. We used a reconstructed time series (1973–1987) for copepod taxa to examine the extent to which zooplankton dynamics in Galway Bay reflect processes in broader areas of the NE Atlantic. Continuous Plankton Recorder (CPR) counts were used to establish time series for nine offshore ecoregions, with the regions themselves defined using underlying patterns of chlorophyll variability. The open nature of Galway Bay was reflected in strong associations between bay zooplankton counts and offshore CPR data in a majority of cases (7/10). For each zooplankton taxon, there were large differences among regions in the degree of association with Galway Bay time series. Akaike weights indicated that one ecoregion tended to be the dominant link for each taxon. This indicates that the zooplankton of the Bay reflect more than the local modification of a regional signal and that different zooplankton in the bay may have separate source regions. The data from Galway Bay also fall within a ‘sampling shadow’ of the CPR. Later years of the time series showed evidence for changes in phenology, with spring zooplankton peaks generally occurring earlier in the year for smaller species.

Composition of trawl catch fauna off the mouth of the Rio Baluarte, southeastern Gulf of California

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 403:145-153 (2010) - DOI: https://doi.org/10.3354/meps08434 Composition of trawl catch fauna off the mouth of the Rio Baluarte, southeastern Gulf of California Juan Madrid-Vera1,*, Eva Visauta-Girbau2, Hugo Aguirre-Villaseñor1 1Instituto Nacional de Pesca, Centro Regional de Investigación Pesquera, Mazatlán, Sinaloa, Mexico 2Institut de Ciències del Mar (ICM-CSIC), Psg. Marítim de la Barceloneta 37-49, 08003 Barcelona, Spain *Email: juanchomvera@yahoo.com.mx ABSTRACT: From April to December 2007, experimental trawls (n = 44, 4 to 22 m depth) were made off the mouth of the Rio Baluarte ecosystem (MRB) and compared to experimental trawls (n = 54, 7 to 40 m) in the adjacent offshore region (AOR) from 2002 to 2007. For both ecosystems, a total of 143 species belonging to 5 phyla were collected. Fishes comprised ca. 80% of the bycatch biomass, while molluscs and echinoderms contributed <20% bycatch. Eight species accounted for 50% of the numerical abundance: the crab Portunus asper (11%), the Panama grunt Pomadasys panamensis (8%), the curvina Stellifer ericymba (7%), the comb sand star Astropecten armatus (6%), the fishes Orthopristis chalceus (6%) and Larimus effulgens (4%), and the crabs Callinectes arcuatus (4%) and Hepatus kossmani (3%). Species with a high or medium survival rate with respect to trawling activity were well represented in the samples, e.g. the crabs C. arcuatus, H. kossmani, and Euphilax robustus, the hermit crab Petrochirus californiensis, and the sea star Luidia brevispina. For the MRB and AOR, the estimated mean biomass was 2.08 and 0.72 t km–2, respectively. The population of the blue shrimp Litopenaeus stylirostris and white shrimp L. vannamei was composed of recruits, juveniles, spawners, and old individuals, whereas for grunts it was composed of recruits and juveniles, but rarely old individuals. The high diversity, population structure, and productivity of the studied fauna has positive effects, such as enabling the recruitment for coastal marine fisheries. This is an important reason to protect the river mouth and the adjacent shallow waters. KEY WORDS: Gulf of California · Trawling · Bycatch · Closed season · Population structure Full text in pdf format PreviousNextCite this article as: Madrid-Vera J, Visauta-Girbau E, Aguirre-Villaseñor H (2010) Composition of trawl catch fauna off the mouth of the Rio Baluarte, southeastern Gulf of California. Mar Ecol Prog Ser 403:145-153. https://doi.org/10.3354/meps08434Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 403. Online publication date: March 22, 2010 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2010 Inter-Research.

Composition, abundance and distribution of holoplanktonic polychaetes from the expedition “El Golfo 6311-12” of Scripps Institution of Oceanography

Holoplanktonic polychaetes from the zooplankton samples taken during El Golfo 63/11-12 Cruise conducted by Scripps Institution of Oceanography, University of California, San Diego, were studied. Eighty-seven samples were reviewed, which were collected over an extensive area including both coasts, western and eastern, of the Baja California peninsula, the continental coast of the Gulf of California, and some localities in the northeastern Pacific. Samples were obtained in the epipelagic region by oblique plankton hauls (using conical nets 5 m in length, 0.5 and 1.0 m in mouth diameter, and 0.50 mm mesh), from 47 localities. Two hundred and twenty-three polychaetes belonging to 17 species of six holoplanktonic families were identified. Ecological characterization of the species showed that Phalacrophorus uniformis, Typhloscolex muelleri, Sagitella kowalewski and Travisiopsis dubia were dominant, Tomopteris nationalis was constantly seen, and the remaining twelve species were rare. The richness and distribution of these holoplanktonic worms were poor compared with other collections obtained by different cruises in the adjacent offshore regions of the survey area. These results are discussed from a biogeographical point of view.

Thermal and tectonic evolution of the Falkland Islands: implications for hydrocarbon exploration in the adjacent offshore region

Apatite fission track and vitrinite reflectance data combine to suggest that the Falkland Islands experienced three discrete episodes of heating and cooling during the Phanerozoic, each of which can be related to major changes in the plate tectonic setting of the islands. Combined with published data from the adjacent offshore areas, the results provide unique insights into the behaviour of the region during the phases of convergent and divergent tectonic movements that have affected the region since the Permian. Initial cooling of the Falkland Islands in the Late Permian was initially restricted to West Falkland, and was the result of differential uplift and erosion of West Falkland relative to East Falkland, coinciding with D2 Gondwanian deformation. Subsequent Early Jurassic cooling of both East and West Falkland was associated with plume-related thermal uplift preceding the separation of East and West Gondwana and the development of the Falkland Plateau Basin. The Falkland Islands experienced renewed heating during the Mesozoic, probably due to increased burial, suggesting that the offshore basins originally extended onto the present onshore region. Late Cretaceous/Early Tertiary cooling of the Falkland Islands was contemporaneous with the development of a transpressional plate boundary along the southern margin of the Falkland Plateau resulting in erosion of the islands that were situated on a flexural forebulge.

Interactions between mantle upwelling, drainage evolution and active normal faulting: an example from the central Apennines (Italy)

In this paper we show that the processes that have shaped the Quaternary surface development of the Apennines in central Italy are all consequences of a single subcrustal process, the upwelling of the mantle. The relationship between gravity and topography shows that mantle convection is responsible for a long-wavelength (150–200km) topographic bulge over the central Apennines, and stratigraphic evidence suggests this bulge developed in the Quaternary. Active normal faulting is localized at the crest of this bulge and produces internally-draining fault-bounded basins. These basins have been progressively captured by the aggressive headward erosion of major streams that cut down to the sea on the flanks of the regional bulge. The only surviving closed basins are those on the Apennine watershed most distant from the marine base level, where continued normal faulting is still able to provide local subsidence that defeats their capture by the regional drainage network. Understanding the competition between regional capture and local, fault-related subsidence of intermontane basins is crucial for recognizing potentially hazardous active faults in the landscape and also for interpreting the sediment supply to adjacent offshore regions. Central Italy provides a good modern analogue for processes that are probably common in the geological record, particularly on rifted margins and intracontinental rifts, but may not have been fully appreciated.

Thermal regime, hydrocarbon maturation and geodynamic events along the western margin of India since late Cretaceous

The passive continental margin of western India and the adjacent offshore region are associated with a transitional type thinned crust. It contains several sedimentary basins where substantial recoverable oil/gas reserves exist. The northern Cambay graben, northern and eastern parts of the Bombay offshore and the Konkan coast region that are situated close to western margin exhibit reasonably high heat flow and geothermal gradients beneath which the asthenosphere is upwarped to a depth of 30–70 km. Temperatures at the depth of 3 km are estimated to be in the range of 105–260°C. Curie depth analysis from MAGSAT studies in an area between latitudes 11°N and 19°N and longitudes 65°E and 73°E also indicates a high geothermal gradient of about 30°C/km within the upper crustal column. We suggest that the occurrence of oil and gas in these areas may be due to catastrophic and geodynamic events which took place in the last 130 Ma. India’s super-mobility, continental breakups, possible bolide impact and Deccan volcanic episode at the western margin resulted in substantial lithospheric heating, accompanied by subcrustal melting and rise of isotherms, to eventually enhance the hydrocarbon maturation process. The study indicates that all other sedimentary basins situated on the western margin are also thermally mature and may have high potential for the occurrence of hydrocarbons.

Crustal stress in and around Norway: a compliation of in situ stress observations

Abstract In-situ rock stresses yield information about geodynamic processes in the crust, and are important input data for almost all kinds of geomechanics work. Compilations of rock stress measurements, such as the World Stress Map, have been used to characterize the regional stress field. On the basis of new data from the Norwegian region, a map of the maximum horizontal stress ( σ H ) direction, for both on- and offshore Norway, has been established. Four main stress provinces with different regional stress trends are identified. The Barents Sea and northern Norway province exhibits a very consistent N-S σ H direction, with high horizontal stresses. In the Norwegian Sea and mid-Norway province the σ H direction has rotated towards NW-SE. Also here, high horizontal stresses are measured and a compressional regime appears. In the northern North Sea and western Norway province the σ H direction is more scattered. A WNW-ESE direction dominates, but a NNE-SSW trend, parallel to the major structures in the area, also appears. The stress regime is primarily compressional, but in the Stord area normal faulting earthquakes are observed. The southern North Sea province yields a very scattered σ H direction. However, a NW-SE trend, which aligns with the stress system in western Europe, is identified. The high variation, both laterally and with depth, indicates near-isotropic horizontal stresses and influence by local features. The different stress determination techniques applied (overcoring, borehole breakouts and earthquake focal mechanisms) yield approximately the same stress orientation, and regional and internal variation, although they cover different depth intervals. They further manifest high horizontal stresses in the Norwegian area, generally exceeding the vertical stress. This indicates the presence of compressional tectonic stresses penetrating deep into the crust. The first-order stress pattern in Norway and adjacent offshore regions show a rotation in σ H direction from N-S in the Barents Sea to WNW-ESE in the North Sea. This rotation can be explained by the ridge push effect alone, but other effects may also be contributing factors. The increased scatter in σ H direction in the south is believed to be due to changes in the tectonic stress magnitudes, or related to relative changes in angle between the continental margin and the ridge push force.

The permafrost regime in the Mackenzie Delta, Beaufort Sea region, N.W.T. and its significance to the reconstruction of the palaeoclimatic history

AbstractObserved depths to the base of the ice‐bearing permafrost (!BPF) as determined from geophysical well‐log and survey information in the Mackenzie Delta and offshore Beaufort Sea continental shelf show significant variation. Depths range from over 700m beneath the northeastern portion of Richards Island and the adjacent offshore region to the north, to less than 100m in the modern Mackenzie Delta and offshore Mackenzie Bay regions. Along the Tuktoyaktuk Peninsula, permafrost thicknesses exceed 600m but tend to thin to less than 100m in a southeasterly direction.The wide variation in the thickness and distribution of permafrost is due in part to differences in the subsurface deltaic lithologies and to the complexity of the past surface temperature history, exhibited, for example, by fluctuations of the mean annual ground temperature. Geological cross‐sections showing variation in the Tertiary and Upper Cretaceous deltaic sequences indicate a good correlation with the permafrost distribution. In particular, where the sand and gravel units of the Iperk and Kugmallit Sequences predominate, permafrost is thick owing to the high thermal conductivities of the ice‐rich sediments. Conversely, where the low thermal conductivity shales and mudstones of the Richards Sequence are present, permafrost is accordingly thinner.Numerical models of permafrost aggradation in the Mackenzie Delta region suggest that Early and Late Wisconsinan surface temperatures were of the order −18°C. A mid‐Wisconsinan rise in temperature to approximately −8°C, similar to mean surface temperature today, is also indicated. This proposed palaeoclimate is sufficient to account for the observed maximum permafrost thicknesses in the region.

Ecology of juvenile and adolescent banana prawns, Penaeus merguiensis, in a mangrove estuary and adjacent off-shore area of the Gulf of Carpentaria. I. Immigration and settlement of postlarvae

The hydrology of the Norman River and adjacent off-shore region in the Gulf of Carpentaria is dominated by the alternation of a summer north-west monsoon and a winter south-east trade wind. The seasonality in both air temperature and rainfall is reflected in marked annual cycles of water temperature, salinity, and dissolved oxygen and nutrient (nitrate and silicate) levels. Near the mouth of the river; water temperature ranged from 15.0�C in June to 32.7�C in February, while salinity ranged from c. 0‰, in December and February to 38.2‰, in September. Postlarval banana prawns, P. merguiensis, were found in the river from November to May. but immigration was essentially restricted to the period immediately preceding the wet season (mid November). Maximum numbers ofjuvenile prawns occurred approximately 2 weeks later. A marked tidal periodicity in the movement of postlarvae was observed; postlarvae were present in the water column at the mouth of the river only during the flood tide with maximum numbers moving into the river at the time of maximum flood tide velocity. Incoming postlarvae settled out in the shallow water along the banks of the river (within 10 m of the shoreline), inland as far as 85 km. A lager catch of smaller prawns 48-72 km up-stream (salinity 10-20‰,) suggested that this area was the main settlement area for postlarvae. The population, however, contracted towards the river mouth during the wet season. concentrating the prawns in an area extending only a few kilometres up-river. Most prawns were caught on the muddy substrate adjacent to the mangrove (Avicennia sp.) fringe which lines the numerous bends in the river. Mechanisms for transporting larvae into the estuary and the relationship between the seasonality of physical and chemical parameters and immigration of postlarvae are discussed.

Phosphorite pellets and rock from the western continental margin and adjacent coastal terrace of South Africa

Well-rounded, spherical and ovoidal phosphorite pellets and irregular phosphatic rocks are located on the western continental margin and on the adjacent coastal terrace of South Africa. These minor (<1% of total unconsolidated sediment) and isolated offshore occurrences of pelletal phosphorite are the southernmost extension of rich phosphate deposits off South West Africa. The onland phosphorites formed in a sheltered, shallow-water lagoon or estuary during the Miocene. Wind-generated upwelling and high biological productivity in the estuary and in the immediately adjacent offshore region provided a plentiful supply of P to the bottom sediments. Fine layers (2–15 mm) of pure phosphorite were precipitated directly from the overlying water column, whereas phosphatic sandstones were formed by apatite precipitating within the interstices of quartzitic sediment mantling the estuary floor. Because the solubility of apatite decreases with increasing temperature, a process of cyclic precipitation may have been operative in shallow water during low tides when solar heating was at a maximum. The phosphorite pellets originated by accretion, usually around nuclei of quartz, feldspar or biogenic material at the sediment—water interface while rolling back and forth in shallow water. Pellets were also derived by fragmentation of partially lithified apatitic bedrock. Phosphorite pellets and rocks on the middle and outer shelf probably formed under conditions similar to the onland deposits during a later regression.