Subglacial bed conditions during Late Pleistocene glaciations and their impact on ice dynamics in the southern North Sea

Passchier, S., Laban, C., Mesdag, C.S. & Rijsdijk, K.F. 2010: Subglacial bed conditions during Late Pleistocene glaciations and their impact on ice dynamics in the southern North Sea. Boreas, Vol. 39, pp. 633–647. 10.1111/j.1502‐3885.2009.00138.x. ISSN 0300‐9483. Changes in subglacial bed conditions through multiple glaciations and their effect on ice dynamics are addressed through an analysis of glacigenic sequences in the Upper Pleistocene stratigraphy of the southern North Sea basin. During Elsterian (MIS 12) ice growth, till deposition was subdued when ice became stagnant over a permeable substrate of fluvial sediments, and meltwater infiltrated into the bed. Headward erosion during glacial retreat produced a dense network of glacial valleys up to several hundreds of metres deep. A Saalian (MIS 6) glacial advance phase resulted in the deposition of a sheet of stiff sandy tills and terminal moraines. Meltwater was at least partially evacuated through the till layer, resulting in the development of a rigid bed. During the later part of the Saalian glaciation, ice‐stream inception can be related to the development of a glacial lake to the north and west of the study area. The presence of meltwater channels incised into the floors of glacial troughs is indicative of high subglacial water pressures, which may have played a role in the onset of ice streaming. We speculate that streaming ice flow in the later part of the Saalian glaciation caused the relatively early deglaciation, as recorded in the Amsterdam Terminal borehole. These results suggest that changing subglacial bed conditions through glacial cycles could have a strong impact on ice dynamics and require consideration in ice‐sheet reconstructions.     

A SEARCH FOR DIRECT HYDROCARBON INDICATORS IN THE FORMBY AREA1

It has been proposed that the high-frequency, low-amplitude magnetic anomalies found over some hydrocarbon deposits are due to long-term microseepage of hydrocarbons into iron-rich sedimentary roof rocks, with subsequent precipitation of diagenetic magnetite or pyrrhotite at or near the water-table. Aerogeophysical data sets, comprising both magnetic and gamma-ray spectrometer measurements, over the Formby oil-field, Lancashire, U.K., have been analysed for hydrocarbon-related anomalies. Detailed ground magnetic traverses were also made to investigate some of the aeromagnetic anomalies. No hydrocarbon-induced magnetic anomalies were detected. The majority of the high-frequency events occurring in the aeromagnetic data correlated with cultural features, others were attributed to artefacts of the data processing. In particular there were no extensive areas of high-frequency, low-amplitude anomalies as might be expected from authigenic magnetic minerals. Borehole chippings from inside and outside the oil-field were examined. High magnetic susceptibilities were due to contamination during drilling. Magnetite, which was identified by mineralogical analysis, was found to be of detrital origin. If any diagenetic magnetic material was present it was in such small quantities as to be insignificant. The oil-field did not possess a characteristic gamma-ray spectrometric signature. Anomalies were found to be related to hydrological conditions and to the distribution of surficial deposits. Attempts to suppress the influence of the drift geology had only limited success. It is concluded that the effectiveness of high-resolution aeromagnetic onshore surveys for direct hydrocarbon detection has yet to be established. In particular, the anomaly found over the Formby oil-field was caused by the cumulative effect of borehole casing. Similar cultural contamination by oil-field equipment may explain some of the anomalies discovered over hydrocarbon deposits in North America. It is also unlikely that the spectrometric method can be applied diagnostically in any but the most simple and drift-free geological environments.     

Petrology and Isotopic Composition of a Grenvillian Basement Fragment in the Northern Appalachian Orogen: Blair River Inlier, Nova Scotia, Canada

Mesoproterozoic metaplutonic rocks in northern Cape Breton Island,Nova Scotia, occur in a tectonic inlier within the Appalachianorogen. Although they have been multiply metamorphosed and variablydeformed, the petrology and geochemistry of these rocks provideinsight into the tectonomagmatic evolution of easternmost Laurentia.Anorthosite, syenite, and granitoid plutons (1100–980Ma) intruded the Sailor Brook and Polletts Cove River gneisses.New Nd isotopic data are presented from a biotite-rich partof the Sailor Brook gneiss (     

Geochemical Constraints from Zoned Hydrothermal Tourmalines on Fluid Evolution and Sn Mineralization: an Example from Fault Breccias at Roche, SW England

Hydrothermal fluid evolution north of the St Austell granite,southwest England, has been studied through geochemical analysisof tourmaline from a fault breccia of <2 cm width withinmassive quartz–tourmaline rocks at Roche. Brecciated tourmalinegrains have overgrowths of <400 µm width [Fe/(Fe +Mg) = 0·31–0·99] with four chemically distinctzones (1–4, towards the margins). Variations in overgrowthcomposition were caused by episodic mixing between Mg-, Al-richmagmatic hydrothermal fluids (dominant in zone 1), with an increasingcomponent of more oxidizing, Fe-rich formation waters (zones2 and 4). More oxidizing conditions are supported by high Sncontents in zone 2 (<0·35 wt %), with Sn probablypresent as Sn⁴⁺ rather than Sn²⁺, the usual form in hydrothermalfluids. From X-ray maps, zones 1 and 3 occur exclusively asovergrowths on pre-existing grains, indicating that overgrowthformation was kinetically favoured over tourmaline nucleation.In zones 2 and 4, nucleation and growth occurred, possibly asa result of supersaturation with respect to tourmaline duringincreased mixing with formation waters. Tourmaline is associatedwith the main episode of mineralization in many important mineraldeposits, often unaffected by alteration. This method of studyinghydrothermal fluid evolution may therefore have uses in exploration,particularly for tourmaline-breccia-hosted ores in Cu-porphyrydeposits. KEY WORDS: breccia; Cornwall; hydrothermal; tin; tourmaline     

Carbonate mineral cycles generated by foraminiferal and pteropod response to Pleistocene climate: west Florida ramp slope

Carbonate ramps are gently sloping depositional surfaces where shallow-water, coarse-grained facies pass basinward into fine-grained, deep-water sediments, with no abrupt change in slope. The objectives of this study are: (i) to integrate the depositional processes recorded in the Pleistocene stratigraphy of the west Florida outer ramp into a palaeoclimatic and palaeoceanographic framework for the eastern Gulf of Mexico; and (ii) to examine the origin of mineralogical and sedimentary cycles in the light of pteropod and planktonic foraminiferal populations corresponding to climatic oscillations. Aragonitic, pteropod-rich sediments with large amounts of insoluble residue occur in sediments deposited during glacial intervals; sandy calcitic sediments with abundant planktonic foraminifera accumulate during interglacials. These cycles reflect variations in biological productivity of pelagic pteropods and planktonic foraminifera, rather than preferential dissolution of either aragonitic or calcitic fractions. Species assemblages suggest that the productivity cycles are linked to changes in upwelling intensity at the margins of the Loop Current and variations in water mass salinities, as well as terrigenous dilution from the Mississippi Delta. These cycles are the response to Pleistocene glacial-interglacial oscillations, controlled by Milankovitch orbital parameters. Although the organisms contributing to deep-water carbonate environments have changed through geological time, facies patterns, as well as sedimentary textures and structures, identified in the west Florida sediments provide criteria for recognition of ancient ramps. An understanding of the processes on a modern ramp slope, such as west Florida, may prove valuable in palaeoclimatic and palaeoenvironmental analysis of epicontinental carbonate sequences and ramps in the rock record.     

From Input Controls to Quota Management in the Tasmanian Rock Lobster Fishery

In an effort to control escalating fishing effort, a quota management system (QMS) of individual transferable quotas (ITQs) has recently been introduced in the Tasmanian rock lobster fishery. We outline and compare the input controls system in place before the introduction of a QMS, and we discuss the operation of the QMS to the end of its first full season in February 1999. We find cautiously for a successful introduction of a QMS as catch, effort and catch rate data tend towards what fishery managers believe to be bio economically sustainable levels.     

The Volume and Rare Earth Concentrations of Magmas Generated during Finite Stretching of the Lithosphere

We present a model of lithospheric stretching and associatedmelting to predict the volume and rare earth element compositionof basaltic magmas generated during rifting. The model differsfrom previous approaches in two ways: (1) we assume a twolayerlithospheric stretching model in which the amount of stretchingin the crust and lower lithosphere may differ; (2) we allowfor multiple instantaneous episodes of extension to occur overa specified time interval. Melt volumes are computed from thesyn- and post-rift subsidences that account for the thicknessof the sedimentary sequence. The concentrations of rare earthelements are calculated using an incremental melting model incorporatingvariations of the melt fraction with depth. The predicted meltthickness and rare earth concentrations are most sensitive tevariations in the basal mantle temperature, magnitude of subcrustalstretching, and time dependence of deformation. Given moderatelyhigh mantle temperatures (>1450C) and large amounts of stretchingin the upper mantle, the model is capable of generating alkalinemelts, and predicts the transition to tholeiitic magmatism observedin many continental rifts. We compare some of the model predictionswith observations on the igneous and tectonic history of theNorth Sea Rift and Labrador margin (eastern Canadian margin). KEY WORDS: rifting; partial; melting; basalt; rare earth ^*Corresponding author