Urban geology: Glasgow's hidden industrial heritage

The growth of Glasgow from a small town into a heavily industrialised conurbation depended greatly on its local geological riches. Extensive mining and quarrying of a range of minerals took place from the 18th century onwards. The early underground stoop and room (pillar and stall) workings, and the backfilled quarries, together with variably consolidated natural superficial deposits, have bequeathed to the city a heritage of unwanted engineering problems which cannot easily be quantified. Recent work by the British Geological Survey (BGS) illustrates the scale of both geological and man-made problems.     

Postglacial sediment yield to Chilliwack Lake,British Columbia,Canada

Tunnicliffe, J., Church, M. & Enkin, R. J. 2012 (January): Postglacial sediment yield to Chilliwack Lake, British Columbia, Canada. Boreas, Vol. 41, pp. 84–101. 10.1111/j.1502‐3885.2011.00219.x. ISSN 0300‐9483. Seismic records and evidence from sediment cores at Chilliwack Lake provide the basis for a long‐term (postglacial) sediment budget for a 324‐km² Cordilleran catchment. Chilliwack Lake (11.8 km² surface area), situated in the North Cascade Mountains, near Chilliwack, British Columbia, was formed behind a valley‐wide recessional moraine in the final phase of post‐Fraser alpine glaciation. Seismic surveys highlight the postglacial lacustrine record, which is underlain by a thick layer of sediments related to deglacial sedimentation. Sediment cores provide details of grain‐size fining from the delta to the distal lake basin. The cores also show a record of intermittent fire and debris flows. Magnetic measurements of lake sediments provide information on grain size, as well as a dating framework. The total postglacial lake‐floor deposit volume is estimated to be 397 ± 27 × 10⁶ m³. Including estimates of fan and delta deposition, the specific postglacial yield to the lake is calculated to be ～86 ± 13 Mg km² a^?1. The sediment volume in the uppermost (Holocene) lacustrine layer is 128 ± 9 × 10⁶ m³, representing ～41 ± 4 Mg km² a^?1 in the Holocene. Compared with other Cordilleran lakes of similar size, particularly those with glacial cover in the watershed, Chilliwack Lake has experienced relatively modest rates of sediment accumulation. This study provides an important contribution to a growing database of long‐term (postglacial) sediment yield data for major Cordilleran lakes, essential for advancing our understanding of the pace of landscape evolution in formerly glaciated mountainous regions.     

The origin and significance of groundwater-seepage gypsum from Bristol Dry Lake, California, USA

Gypsum and anhydrite fabrics observed in trenches and deep (500 m) cores from Bristol Dry Lake, California, USA, exhibit a vertical alignment of crystals similar to the fabric seen in bottom-nucleated brine pond gypsum. However, geochemical and sedimentological evidence indicate that the gypsum formed in Bristol Dry Lake precipitated displacively within the sediment where groundwater saturated with respect to gypsum recharges around the playa margin (groundwater-seepage gypsum). Evidence for displacive growth of gypsum is: (i) the geometry of the deposit, (ii) stable isotopic data and the water chemistry of the brine, and (iii) inclusions of matrix which follow twin planes and completely surround crystals as they grow. The bulk of the gypsum precipitated in the playa occurs around the edges of the playa in the playamargin facies and completely rings the lake. Sulphate concentrations in the groundwater increase toward the gypsum zone in the playa margin. Basinward of this zone, sulphate concentrations decrease sharply to trace element levels in the basin centre brine. Authigenic gypsum is rare in the centre of the playa. Stable (δ¹⁸O values measured for gypsum waters of crystallization (GWC) are similar to the values calculated for groundwater in the playa margin and alluvial fan sediments (?– 6%0), whereas measured brine δ¹⁸O values range from + 0·5 to + 3·7%0. Deuterium values measured for groundwater are ?– 70%0, GWC are ?– 60 to – 65%0 and brine values are ?– 57%0. The geometry of the deposit and the chemical data suggest that the water precipitating the gypsum is more closely associated with the groundwater than the brine. However, some mixing between groundwater and brine is likely. Within 100 m of the surface, the gypsum dehydrates to anhydrite, although the same vertically aligned fabric is retained through the diagenetic process. The similarity of displacive vertically aligned gypsum and anhydrite fabrics seen in Bristol Dry Lake to subaqueously deposited gypsum in modern brine ponds indicates that the criteria used to define subaqueous fabrics must be better constrained.     

Fluvial and lacustrine palaeoenvironments of the Miocene Siwalik Group, Khaur area, northern Pakistan

The Miocene Siwalik Group (upsection, the Chinji, Nagri, and Dhok Pathan Formations) in northern Pakistan records fluvial and lacustrine environments within the Himalayan foreland basin. Thick (5 m to tens of metres) sandstones are composed of channel bar and fill deposits of low-sinuousity (1·08–1·19), single-channel meandering and braided rivers which formed large, low-gradient sediment fans (or ‘megafans’). River flow was dominantly toward the south-east and likely perennial. Palaeohydraulic reconstructions indicate that Chinji and Dhok Pathan rivers were small relative to Nagri rivers. Bankfull channel depths of Chinji and Dhok Pathan rivers were generally ≤ 15 m, and up to 33 m for Nagri rivers. Widths of channel segments (including single channels of meandering rivers and individual channels around braid bars) were 320–710 m for Chinji rivers, 320–1050 m for Nagri rivers, and 270–340 m for Dhok Pathan rivers. Mean channel bed slopes were on the order of 0·000056–0·00011. Bankfull discharges of channel segments for Chinji and Dhok Pathan rivers were generally 700–800 m³s^?1, with full river discharges possibly up to 2400 m³s^?1. Bankfull discharges of channel segments for Nagri rivers were generally 1800–3500 m³s^?1, with discharges of some larger channel segments possibly on the order of 9000–32 000 m³s^?1. Full river discharges of some of the largest Nagri braided rivers may have been twice these values. Thin (decimetres to a few metres) sandstones represent deposits of levees, crevasse channels and splays, floodplain channels, and large sheet floods. Laminated mudstones represent floodplain and lacustrine deposits. Lakes were both perennial and short-lived, and likely less than 10 m deep with maximum fetches on the order of a few tens of kilometres. Trace fossils and body fossils within all facies indicate the former existence of terrestrial vertebrates, molluscs (bivalves and gastropods), arthropods (including insects), worms, aquatic fauna (e.g. fish, turtles, crocodiles), trees, bushes, grasses, and aquatic flora. Palaeoenvironmental reconstructions are consistent with previous palaeoclimatic interpretations of monsoonal conditions.     

Intra- and extrabasinal controls on fluvial deposition in the Miocene Indo-Gangetic foreland basin, northern Pakistan

The Miocene Siwalik Group (upsection, the Chinji, Nagri, and Dhok Pathan Formations) in northern Pakistan records evolving fluvial systems within the Himalayan foreland basin. Sedimentological variations are evaluated with respect to local, regional, and global controls on fluvial deposition and basin filling. Thick (5 m to tens of metres) sandstones are composed of channel bar and fill deposits of low-sinuosity, meandering and braided rivers which formed large, low-gradient sediment fans (or ‘megafans'). River flow was dominantly toward the south-east. The proportion of thick sandstones varies in all Siwalik sections on three scales, and reflects similar variations in palaeochannel size and grain size: (1) small-scale variations are generally tens of metres thick, and reflect the alternation of thick sandstones (channel-belt deposits) and mudstone-dominated strata (overbank deposits) through the section; (2) medium-scale variations are roughly one-hundred to a few hundreds of metres thick, and primarily correspond to changes in channel-deposit thickness, but also to the degree of superposition of channel deposits and/or to changes in the number of channel-belt deposits per unit of section; and (3) large-scale variations (formation-scale) are greater than one km thick, and primarily correspond to changes in channel-deposit thickness. Time-scales of small-, medium-, and large-scale variations appear to be on the order of 10⁴, 10⁵ and 10⁶ years, respectively. The Chinji-Nagri transition is characterized by increases in channel-deposit proportion, sandstone thickness, palaeochannel size and discharge, mean grain size of sandstones, and sediment accumulation rates; and a decrease in avulsion period. The Nagri-Dhok Pathan transition is characterized by decreases in channel-deposit proportion, sandstone thickness, palaeochannel size and discharge, mean grain size of sandstones, and avulsion period; and a further increase in sediment accumulation rates. Formation boundaries across the Potwar Plateau decrease in age toward the west. The Chinji-Nagri transition ranges in age from ～ 10·9–12·7 Ma, and the Nagri-Dhok Pathan transition ranges in age from ～9·3–10·1 Ma. Small-scale variations are attributable to repeated river avulsions triggered by autocyclic processes and/or mountain-front tectonism (e.g. faulting, earthquakes). Medium-scale variations are attributable to local changes in the position of large sediment fans, also triggered by autocyclic processes and/or mountain-front tectonism. The Chinji-Nagri transition records the diversion or establishment (possibly due to river piracy) of a larger river system in the area. River diversion or piracy probably took place within the mountain belt and is attributable to increasing and spatially variable mountain-belt uplift rates, and possibly the development of associated mountain-front deformational structures. The Nagri-Dhok Pathan transition records the diversion of the larger river system out of the area and the establishment of a smaller river system. This diversion is attributable to progressively increasing rates of mountain-belt uplift and basin subsidence. The regional palaeoclimate throughout the time interval studied was apparently constant, and eustatic sea level changes apparently had no effect on deposition in the area.     

Bedload stresses and sediment transport theory—a correction

A previous proposal (Leeder, 1977) to test the magnitude of the solid-transmitted stresses due to bedload transport failed to confirm Bagnold's theory because the applied fluid stress contribution was not corrected for sidewall drag. Once this correction is made there is a reasonable correspondence of theory and experiment.     

Rapid Variscan exhumation and the role of magma in core complex formation: southern Brittany metamorphic belt, France

ABSTRACT The high-grade migmatitic core to the southern Brittany metamorphic belt has mineralogical and textural features that suggest high-temperature decompression. The chronology of this decompression and subsequent cooling history have been constrained with ⁴⁰Ar/³⁹ Ar ages determined for multigrain concentrates of hornblende and muscovite prepared from amphibolite and late-orogenic granite sheets within the migmatitic core, and from amphibolite of the structurally overlying unit. Three hornblende concentrates yield plateau isotope correlation ages of c. 303–298 Ma. Two muscovite concentrates record well-defined plateau ages of c. 306–305 Ma. These ages are geologically significant and date the last cooling through temperatures required for intracrystalline retention of radiogenic argon. The concordancy of the hornblende and muscovite ages suggest rapid post-metamorphic cooling. Extant geochronology and the new ⁴⁰Ar/³⁹Ar data suggest a minimum time-integrated average cooling rate between c. 725 °C and c. 125 °C of c. 14 ± 4°C Ma^-1, although below 600 °C the data permit an infinitely fast rate of cooling. Mineral assemblages and reaction textures in diatexite migmatites suggest c. 4 kbar decompression at 800–750 °C. This must have pre-dated the rapid cooling. Emplacement of two-mica granites into the metamorphic belt occurred between 345 and 300 Ma. The youngest plutons were emplaced synkinematically along shallow-dipping normal faults interpreted to be reactivated Eo-Variscan thrusts. A penetrative, west-plunging stretching lineation developed in these granites suggests that extension was orogen-parallel. Extension was probably related to regional uplift and gravitational collapse of thermally weakened crust during constrictional (escape) tectonics in this narrow part of the Variscan orogen. This followed slab breakoff during the terminal stages of convergence between Gondwana and Laurasia; detachment may have been consequent upon a change in kinematics leading to dextral displacement within the orogen. Dextral ductile strike-slip displacement was concentrated in granites emplaced synkinematically along the South Armorican Shear Zone. Rapid cooling is interpreted to have resulted from tectonic unroofing with emplacement of granite along decollement surfaces. The high-grade migmatitic core of the southern Brittany metamorphic belt represents a type of metamorphic core complex formed during orogen-parallel extensional unroofing and regional-scale ductile flow.