Growth patterns of the acheloos and evinos deltas,Western Greece

The coastal plain complex on the north side of the Gulf of Patras has been built by the overlapping Acheloos and small Evinos deltas. Upstream river channels are braided; on the lower part of the delta they are meandering. Classical writings indicate that the present Acheloos channel has been occupied for at least 2300 years; air photographs show three former major distributary channels. The present annual sediment discharge (suspended: 3–4 million tons; bedload: 2 million tons) is sufficient to build all the modern delta plain by progradation of these four distributaries in the last 5000–7000 years of relatively stable sea level.Old maps and air photographs show that river mouths prograde by periodic plugging of sub-distributaries within 1 km of the mouth. Where the river progrades into deep water (> 20 m) a steep sandy pro-delta slope forms and a shoal mouth bar is developed by wave reworking. The freshwater muddy plume of the Acheloos extends up to 15 km offshore. Analysis of about 30 bottom samples shows that very silty sediment settles within a few kilometers of the river mouth, becoming progressively more clayey offshore. Marine reworking of abandoned distributaries takes place rapidly (0.5 km in the first ten years, and a further 0.5 km in the next 30 years). Sand is redistributed to form parallel accretionary spits and barrier beaches. A shallow pro-delta platform up to 2 km wide has formed by such reworking along the southern margin of both deltas. Gradual subsidence (less than 0.5 mm/year) leads to flooding of shallow lagoons behind barrier beach systems.Subaerial delta facies have been mapped in the field, and extensively sampled in artificial channel cuts. Normal tidal range is only 16 cm, but storm tides up to 1 m high flood low supratidal flats that locally extend several kilometres inland.Broad immediately subtidal silt flats are found on the inner part of the western Evinos pro-delta platform. Silt is drifted in suspension by strong winter easterly winds from the Evinos mouth, and the pro-delta platform is sufficiently wide and shallow to damp down most large waves that would subsequently remove the silt.Where sand supply is limited, the coastline is marked by a sandy low-tide terrace, and a berm or storm ridge built of variable proportions of sand and reeds. Only near delta mouths is sand supply sufficient for broad barrier beach-dune systems to form.     

Fluxes of metals to a manganese nodule radiochemical,chemical, structural,and mineralogical studies

Fluxes of metals to the top and bottom surfaces of a manganese nodule were determined by combining radiochemical (²³⁰Th,²³¹Pa,²³²Th,²³⁸U,²³⁴U) and detailed chemical data. The top of the nodule had been growing in its collected orientation at 4.7 mm Myr^?1 for at least 0.5 Myr and accreting Mn at 200 μg cm^?2 kyr^?1. The bottom of the nodule had been growing in its collected orientation at about 12 mm Myr^?1 for at least 0.3 Myr and accreting Mn at about 700 μg cm^?2 yr^?1. Although the top of the nodule was enriched in iron relative to the bottom, the nodule had been accreting Fe 50% faster on the bottom.²³²Th was also accumulating more rapidly in the bottom despite a 20-fold enrichment of²³⁰Th on the top.The distribution of alpha-emitting nuclides calculated from detailed radiochemical measurements matched closely the pattern revealed by 109-day exposures of alpha-sensitive film to the nodule. However, the shape and slope of the total alpha profile with depth into the nodule was affected strongly by²²⁶Ra and²²²Rn migrations making the alpha-track technique alone an inadequate method of measuring nodule growth rates.Diffusion of radium in the nodule may have been affected by diagenetic reactions which produce barite, phillipsite and todorokite within 1 mm of the nodule surface; however, our sampling interval was too broad to document the effect. We have not been able to resolve the importance of nodule diagenesis on the gross chemistry of the nodule.     

A method of determining optimum scales for averaging the earth's topography in quantitative studies of atmospheric cyclo- and anti-cyclogenesis

A method for optimum modelling of the earth's topography, corresponding to differing meteorological phenomena, is presented.The optimum averaging scale for mountains in terms of orographic cyclo- and anti-cyclogenesis is shown to be of the order of 150 km. The use of larger or smaller averaging scales decreases the correlation between the degree of cyclo- and anti-cyclogenesis and the parameters which describe the orography.A quantitative relation between orographic cyclo- and anti-cyclogenesis and the form of orography determined by the Laplacian ² Z ₀ of the terrain function Z ₀ = Z ₀(x, y) is presented.     

Dyke magnetization, magnetostratigraphy and upper-crustal structure in the Reydarfjordur area of eastern Iceland

A dyke swarm approximately 12 m.y. old and comprising 103 basaltic dykes was sampled during a palaeomagnetic investigation of the Reydarfjördur area, eastern Iceland. Of 100 dykes yielding statistically significant palaeomagnetic directions 36 are normal, 43 reversed and 21 are intermediate; 5% of the dykes are olivine-phyric. The magnetic stratigraphy of the lava pile in this area is partially known from earlier palaeomagnetic studies, and six additional sections measured by field magnetometer and a palaeomagnetic section of 40 flows (20 reversed, 16 normal, and 4 intermediate) are reported here. These sections can be correlated by means of petrologic horizons (silicic tuffs, olivine basalts and porphyritic units) and exhibit stratigraphic thickening towards the centre of the Reydarfjördur dyke swarm. The portion of the basalt pile cut by the dyke swarm can be related both petrologically and palaeomagnetically to the dykes, and the number of exposed dykes is found to be of the same order, but smaller than, the number of lava flows in that comparable portion of the lava pile. This implies that in general the exposed dyke swarm is related to the exposed lava pile which is unlikely to extend much further downdip than the known extension updip. The lava pile in Iceland evolves as large lenticular units formed at eruptive axes, and distributed en échelon along the length of the axial zone with spacings comparable to the thickness of the lithosphere.     

Palaeomagnetic study of the Swedish rapakivi suite: Proterozoic tectonics of the Baltic Shield

The major Proterozoic igneous intrusions in the Swedish sector of the Baltic Shield are the Ragunda complex (1293 m.y., palaeomagnetic pole 165°E, 54°N) and the Nordingrågabbro-granite-anorthosite complex (1385 ± 30 m.y.). The latter body has been partially remagnetised by later post-Jotnian dolerites (1254 m.y.), and sites influenced by the dolerites have a stable magnetisation with a mean direction D = 45°, I = ?39°, (α₉₅ = 4.3°). Elsewhere, the gabbro-anorthosite facies have a magnetisation of dual polarity predating the dolerite and recoverable at various stages of thermal and/or a.f. cleaning with a mean of D = 48°, I = 37° (α₉₅ = 5.3°); medium and high coercivity remanence resides in large magnetite grains and fine, predominantly hematite, rods in feldspar megacrysts. The Nordingrårapakivi granite yields a mean, also including dual polarities, of D = 221°, I = ?25° (α₉₅ = 13°), and the Gävle granite yields a mean of D = 26°, I = 17° (α₉₅ = 13°).New data define the a.p.w. path for the Baltic Shield after final uplift and cooling of the ca. 1800 m.y. Svecofennian mobile belt and prior to intrusion of the post-Jotnian dolerites at 1250 m.y.; this (ca. 1500–1200 m.y.) path defines a double loop similar in size and shape to the contemporaneous path for the Laurentian Shield and the paths can be superimposed to define relative positions of the shields. They were in juxtaposition prior to 1200 m.y. with the optimum reconstruction obtained by rotation of approximately 64° about a Euler pole at 1°E, 36°N. Pre-1500 m.y. palaeomagnetic data are also shown to fit this same unique reconstruction. The main geological correlations are an alignment of the Lower/Middle Proterozoic major strike-slip zones, the structural trends within the pre-1700 m.y. mobile belts, and the Grenville and Sveconorwegian (ca. 1100 m.y.) mobile belts. The anorogenic magmatism characteristic of Proterozoic times became gradually more restricted to one active margin of the continental reconstruction as temperature gradients decreased and the crust consolidated. All of these Proterozoic tectonic/magmatic trends are parallel to the long axis of the continental reconstruction.     

Aspects of Caledonian palaeomagnetism and their tectonic implications

Palaeomagnetic results from the Lower Palaeozoic inliers of northern England cover the upper part of the (Middle Ordovician) Borrowdale Volcanic Series (palaeomagnetic pole 208°E, 18°S, A₉₅ = 9.4°), minor extrusive units relating to the Caradoc and Ashgill stages of Ordovician times, intrusive episodes of Middle Ordovician and Middle Silurian to Late Devonian age, and the Shap Granite of Devonian (393 m.y.) age (palaeomagnetic pole 313°E, 33°S, A₉₅ = 5.6°).A complete assessment of Ordovician to Devonian palaeomagnetic data for the British region shows that the pole was nearly static relative to this region for long intervals which were separated by shifts occupying no more than a few millions of years. The mean palaeomagnetic poles are: Ordovician (6°E, 16°S), Lower Silurian (58°E, 16°N), Middle Silurian/Lower Devonian (318°E, 6°N) and Middle/Upper Devonian (338°E, 26°S); the first two shifts separating these mean poles can be explained predominantly in terms of rotational movements of the crustal plate but the last involved appreciable movement in palaeolatitude.Comparison of Lower Palaeozoic palaeomagnetic data from the British region with contemporaneous data from continental Europe/North America on the Pangaean reconstruction reveals a systematic discrepancy in palaeolatitude between the two regions prior to Middle Devonian times. This discrepancy was eliminated during a few millions of years of Lower/Middle Devonian times (ca. 395 m.y.) and can be explained in terms of ca. 3500 km of sinistral strike-slip movement close to the line of the orthotectonic Caledonides. This motion is linked both in time and place to the impingement of the Gondwanaland and Laurentian supercontinents during the Acadian orogeny; this appears to have displaced the British sub-plate until it became effectively locked between the Baltic and Laurentian regions. Although movement of the dipole field relative to the British region in Lower Palaeozoic times is now well defined, nearly one fifth of the total data show that the geomagnetic field was more complex than dipolar during this interval. Until the significance of these anomalies is fully resolved, the tectonic model derived from the palaeomagnetic data cannot be regarded as unambiguous.     

An iron-rich deposit from the Northeast Pacific

An iron-rich deposit dredged from the upper flank of Dellwood Seamount in the Northeast Pacific has been analyzed for major and trace elements, rare-earth contents and uranium isotopic composition. In terms of mineralogy and overall chemical composition, the deposit resembles other iron-rich deposits variously attributed to volcanic hydrothermal activity. Both the relative concentrations of the rare-earth elements and the isotopic composition of uranium rule out seawater as the sole source of elements in this deposit. The rare-earth element pattern indicates that these elements were derived from the underlying basalt. The²³⁴U/²³⁸U ratio is significantly higher than in seawater and can best be explained by preferential leaching of²³⁴U generated by decay from its parent²³⁸U in the underlying rock and subsequent redeposition of the excess²³⁴U together with the Fe and minor metals. These data are consistent with a model for the origin of submarine metal-rich solutions involving mobilization of elements from the interior of slowly cooling basalts by circulating seawater.     

Palaeomagnetism of silurian lavas of Somerset and Gloucestershire,England

Detailed alternating field demagnetisation of Upper Llandovery volcanics of the Mendip Hills and Gloucestershire has isolated remanence directions interpreted as primary from each of five sites. Well-defined high-coercivity secondary magnetisation is present in six samples of one site and low-coercivity secondary remanence is present in all samples from another site; the former component was apparently acquired in Permo-Triassic times. Primary directions of magnetisation show marked improvement in precision after correction for penecontemporaneous folding, and show a late Llandovery reversal in the sense R → N.The group mean directions of magnetisation isD = 243.5°,I = 47.5° (precision parameterk = 29). Petrographic examination confirms observations from magnetic properties that relict titanomagnetite (oxidation classes 3 to 5) is the remanence carrier in most samples. Hematite, probably mostly late magmatic in origin, is widely developed in all samples, but only the principal remanence carrier where it has thoroughly replaced the titanomagnetite. Low-coercivity remanence is apparently caused by weathering effects but there is no clear visible cause for secondary high-coercivity remanence carried by some samples.The mean virtual geomagnetic pole position is close to Upper Silurian/Lower Devonian pole positions from other parts of Britain and defines a minimum apparent polar shift of 60° between late Ordovician and Upper Llandovery times. Reference to absolute age dates suggests that this shift took place between ca. 447 and 434 m.y. followed by slight polar movement between ca. 434 and 394 m.y.     

A palaeomagnetic study of the coast-parallel Jurassic dyke swarm in southern Greenland

Results are reported from palaeomagnetic samples collected in two traverses across the coast-parallel dyke swarm of southern Greenland. This swarm probably resulted as the consequence of initial rifting between Greenland and Labrador, and a reversal of magnetisation has been found which is correlated on the basis of KAr age determinations (～168 m.y.) with the Mateke event of the Middle Jurassic (Bajocian). All of fifteen sites show significant grouping of directions after a.f. cleaning; three have anomalous directions of magnetisation while the remainder (nine normal, three reversed) give a combined mean direction of D = 336°, I = 66° (α₉₅ = 4.6°) with a palaeomagnetic pole at 191°E, 72°N. The dykes exhibit the same corelation between polarity and deuteric oxidation state as that found in Tertiary volcanics. There is a systematic change in magnetisation across the dyke swarm in south Greenland from normal to anomalous to reversed directions; this is interpreted as due to lateral migration of the response to the regional stress field with time. The pole position lies in the vicinity of Jurassic poles from North America after closing the Labrador Sea according to the reconstruction of Bullard, Everett and Smith, but the scatter of these latter poles precludes a confirmation of this reconstruction for Middle Jurassic and earlier times.     

Rare earth elements in the sedimentary cycle: A summary

The relative and absolute concentrations of rare earth elements (REE) in authigenic and biogenic phases of deep-sea sediments are quite different. Competition between these phases for REE has resulted in fractionation from the parent material, the latter consisting predominantly of terrigenous material, but with a contribution from marine volcanism. The strongest feature of this fractionation is a depletion of Ce, relative to La, in CaCO₃, opalline silica, phillipsite, phosphorite, barite, and montmorillonitic clays; and a Ce enrichment in Fe/Mn nodules. The distribution of REE in different masses of seawater strongly reflects their fractionation in sediments. Whereas the relative concentration of REE in rivers resembles that of shale, their removal from seawater by authigenic and biogenic phases results in: (1) a decrease of their total concentration; (2) a depletion of Ce; and (3) an enrichment of heavy REE relative to light REE. The order of fractionation for water masses in the Atlantic Ocean is:Antarctic intermediate water > North Atlantic deep water > Antarctic bottom water> shelf water > river water ～ shale.The shale-normalized pattern for the sum of REE in the authigenic and biogenic phases of pelagic sediment and in seawater resembles that of an admixture of shale and basalt corresponding presumably to the realtive inputs from continents and marine volcanism respectively. The estimated rate of accumulation of each REE in the sediment, however, is approximately 12 times the estimated rate of input of REE from these two sources.