Relationship of diagenetic chlorite rims to depositional facies in Lower Cretaceous reservoir sandstones of the Scotian Basin

The relationship between diagenetic chlorite rims and depositional facies in deltaic strata of the Lower Cretaceous Missisauga Formation was investigated using a combination of electron microprobe, bulk geochemistry and X‐ray diffraction data. The succession studied comprises several stacked parasequences. The delta progradational facies association includes: (i) fluvial or distributary channel sandstones (some with tidal influence); (ii) thick‐bedded delta‐front graded beds of sandstone interpreted as resulting from fluvial hyperpycnal flow during floods and storms; and (iii) more distal muddier delta‐front and prodeltaic facies. The transgressive facies association includes lag conglomerate, siderite‐cemented muddy sandstone and mudstone, and bioclastic sandy limestone. Chlorite rims are absent in the fluvial facies and best developed in thick sandstones lacking mudstone baffles. Good quality chlorite rims are well correlated with Ti in bulk geochemistry. Ti is a proxy for Fe availability, principally from the breakdown of abundant detrital ilmenite (FeTiO₃). Under conditions of sea floor diagenesis, the abrupt decrease in sedimentation rate at transgressive surfaces caused progressive shallowing of the sulphate‐depletion level and of the overlying Eh‐controlled diagenetic zones, resulting in conditions suitable for diagenetic formation of berthierine to migrate upwards through the packet of reservoir sandstones. This early diagenetic berthierine suppressed silica cementation and later recrystallized to chlorite. Thick euhedral outer chlorite rims were precipitated from formation water in sandstone lacking muddy baffles on this chlorite substrate and inhibited late carbonate cementation. This study thus shows that the preservation of porosity by chlorite rims is a two‐stage process. Rapidly deposited delta‐front turbidite facies create early diagenetic conditions that eventually lead to the formation of chlorite rims, but the best quality chlorite rims are restricted to sandstones with high permeability during burial diagenesis.     

Magnetic properties of oxidized and reduced smectites

 Four smectites with different total Fe contents (two nontronites, one ferruginous smectite, and one montmorillonite) were reduced to obtain a range of Fe(II)/Fe(III) ratios and their magnetic properties measured with a SQUID (superconducting quantum interference device) as a function of applied magnetic field strength at 5 K and as a function of temperature in a field of 0.1 T. The unaltered nontronite and ferruginous smectite specimens showed antiferromagnetic coupling, whereas the coupling in the reduced samples was ferromagnetic; the paramagnetic Curie temperature increased with increasing Fe(II) content. Data collected after cooling samples in both the presence and absence of an external magnetic field of 0.1 T showed that at low temperatures the reduced (ferromagnetic) nontronite and ferruginous smectite samples exhibit a memory effect of previous magnetic field exposure consistent with superparamagnetic or spin glass behavior. The superparamagnetic/ferromagnetic transition temperature, T _f , increased linearly with increasing Fe(II) content for each of the nontronites, but the relationship between T _f and Fe(II) content differed for different clays, thus demonstrating that T _f is sensitive to isomorphous substitutions in the clay structure. The montmorillonite was paramagnetic in both oxidized and reduced forms. Received: 23 March 1999 / Revised, accepted: 27 August 1999     

Petrology,phase equilibria modelling and zircon U–Pb geochronology of Paleoproterozoic mafic granulites from the Fuping Complex,North China Craton

The Fuping Complex is one of the important basement terranes within the central segment of the Trans‐North China Orogen (TNCO) where mafic granulites are exposed as boudins within tonalite–trondhjemite–granodiorite (TTG) gneisses. Garnet in these granulites shows compositional zoning with homogeneous cores formed in the peak metamorphic stage, surrounded by thin rims with an increase in almandine and decrease in grossular contents suggesting retrograde decompression and cooling. Petrological and phase equilibria studies including pseudosection calculation using thermocalc define a clockwise P–T path. The peak mineral assemblages comprise garnet+clinopyroxene+amphibole+quartz+plagioclase+K‐feldspar+ilmenite±orthopyroxene±magnetite, with metamorphic P–T conditions estimated at 8.2–9.2 kbar, 870–882 °C (15FP‐02), 9.6–11.3 kbar, 855–870 °C (15FP‐03) and 9.7–10.5 kbar, 880–900 °C (15FP‐06) respectively. The pseudosections for the subsequent retrograde stages based on relatively higher H₂O contents from P/T–M(H₂O) diagrams define the retrograde P–T conditions of <6.1 kbar, <795 °C (15FP‐02), 5.6–5.8 kbar, <795 °C (15FP‐03), and <9 kbar, <865 °C (15FP‐06) respectively. Data from LA‐ICP‐MS zircon U–Pb dating show that the mafic dyke protoliths of the granulite were emplaced at c. 2327 Ma. The metamorphic zircon shows two groups of ages at 1.96–1.90 Ga (peak at 1.93–1.92 Ga) and 1.89–1.80 Ga (peak at 1.86–1.83 Ga), consistent with the two metamorphic events widely reported from different segments of the TNCO. The 1.93–1.92 Ga ages are considered to date the peak granulite facies metamorphism, whereas the 1.86–1.83 Ga ages are correlated with the retrograde event. Thus, the collisional assembly of the major crustal blocks in the North China Craton (NCC) might have occurred during 1.93–1.90 Ga, marking the final cratonization of the NCC.     

Formation and preservation of fresh lawsonite: Geothermobarometry of the North Makran Blueschists,southeast Iran

A low‐grade metamorphic “Coloured Mélange” in North Makran (SE Iran) contains lenses and a large klippe of low temperature, lawsonite‐bearing blueschists formed during the Cretaceous closure of the Tethys Ocean. The largest blueschist outcrop is a >1,000 m thick coherent unit with metagabbros overlain by interlayered metabasalts and metavolcanoclastic rocks. Blueschist metamorphism is only incipient in coarse‐grained rocks, whereas finer grained, foliated samples show thorough metamorphic recrystallization. The low‐variance blueschist peak assemblage is glaucophane, lawsonite, titanite, jadeite±phengitic mica. Investigated phase diagram sections of three blueschists with different protoliths yield peak conditions of ~300–380°C at 9–14 kbar. Magnesio‐hornblende and rutile cores indicate early amphibolite facies metamorphism at >460°C and 2–4 kbar. Later conditions at slightly higher pressures of 6–9 kbar at 350–450°C are recorded by barroisite, omphacite and rutile assemblages before entering into the blueschist facies and finally following a retrograde path through the pumpellyite–actinolite facies across the lawsonite stability field. Assuming that metamorphic pressure is lithostatic pressure, the corresponding counterclockwise P–T path is explained by burial along a warm geothermal gradient (~15°C/km) in a young subduction system, followed by exhumation along a cold gradient (~8°C/km); a specific setting that allows preservation of fresh undecomposed lawsonite in glaucophane‐bearing rocks.     

Volcanic particles in agriculture and gardening

Volcanic pyroclasts of small size, such as lapilli and small pumice stones, are widely used in agriculture, gardening, and for pot plants as natural inorganic mulch. The technique of using pyroclasts to enhance topsoil stems from the eighteenth century, and specifically from the ad 1730–1736 eruption on Lanzarote. Critical observations on plant development during and after the eruption showed that the vegetation died when buried under a thick layer of lapilli, but grew vigorously when covered thinly. While the agriculture of Lanzarote was restricted to cereals before the eruption, it diversified to many kinds of fruit and vegetables afterwards, including the production of the famous Malvasía wines in the Canaries. The population of Lanzarote doubled in the years after the eruption, from about 5000 in 1730 to near 10 000 in 1768, predominantly as a result of the higher agricultural productivity. This outcome led to widespread use of lapilli and pumice fragments throughout the islands and eventually the rest of the globe. Lapilli and pumice provide vesicle space for moisture to be retained longer within the planting soil, which can create an environment for micro‐bacteria to thrive in. Through this route, nutrients from volcanic matter are transported into the surrounding soil where they become available to plant life. The detailed processes that operate within the pyroclasts are less well understood, such as the breakdown of nutrients from the rock matrix and transport into the soil by biological action. Further studies promise significant potential to optimize future agricultural efforts, particularly in otherwise arid areas of the globe.     

Displacement ring load Green's functions for saturated porous transversely isotropic tri‐material full‐space

Based on the Biot's poroelastic theory and using scalar potential functions both the ring load and point load displacement Green's functions for a transversely isotropic saturated porous full‐space composed of an upper half‐space, a finite thickness middle layer and a lower half‐space is analytically presented for the first time. It is assumed that each region consists of a different transversely isotropic material. The equations of poroelastodymanics in terms of the solid displacements and the pore fluid pressure are uncoupled with the help of two scalar potential functions, so that the governing equations for the potential functions are either a second order wave equation or a repeated wave‐heat transfer equation of sixth order. With the aid of Fourier expansion with respect to circumferential direction and Hankel integral transforms with respect to the radial direction in cylindrical coordinate system, the response is determined in the form of line integrals in the real space, followed by theorem of inverse Hankel integral transforms. The solutions degenerate to a single phase elastic material, and the results are compared with previous studies, where an excellent agreement may be observed with the results provided in the literature. Some examples of displacement Green's functions are finally given to illustrate the solution. Copyright © 2016 John Wiley & Sons, Ltd.     

Impact of lithology and soil properties on abandoned dryland terraces during the early stages of soil erosion by water in south‐east Spain

Soil erosion by water in abandoned dry terraces is one of the most important environmental problems in semiarid areas, enhancing biological degradation and reducing possible resources that can be obtained. However, little is known about the effects of the types of lithology and soil properties on the early stages of soil erosion. Therefore, the main aim of this research was to assess the effect of different lithologies (marls, limestones, and metamorphic—phyllites, schists, and greywackes—materials) and soil properties on the early stages of soil erosion by water in abandoned dry terraces, compared with similar terraces still in agricultural use. Soil analyses (texture, aggregate stability, and bulk density) and 22 rainfall simulations were carried out under dry conditions. During the experiments, local inclination, vegetation and stone cover, total organic matter, and antecedent soil moisture were also quantified. The results showed that the highest soil loss (41.41 g/m² in cultivated plots and 17.05 g/m² in the abandoned plots) and runoff (3.79 L/m² in the abandoned plot) occurred on marl substrata. Marls also showed the shallowest infiltration front (9 cm) and lowest infiltration rate (4.3 cm/min). Limestones and, especially, metamorphic areas, showed a lower degree of soil erosion, higher infiltration rates, and deeper infiltration fronts.     

Reloca Slide: an ~24 km3 submarine mass‐wasting event in response to over‐steepening and failure of the central Chilean continental slope

Reloca Slide is the relict of an ~24‐km³ submarine slope collapse at the base of the convergent continental margin of central Chile. Bathymetric and seismic data show that directly to the north and south of the slide the lower continental slope is steep (~10°), the deformation front is shifted landwards by 10–15 km, and the frontal accretionary prism is uplifted. In contrast, ~80 km to the north the lower continental margin presents a lower slope angle of about 4° and a wide frontal accretionary prism. We propose that high effective basal friction conditions at the base of the accretionary prism favoured basal accretion of sediment and over‐steepening of the continental slope, producing massive submarine mass wasting in the Reloca region. This area also spatially correlates with a zone of low coseismic slip of the 2010 Maule megathrust earthquake, which is consistent with high basal frictional coefficients.     

Flood hazards at ford stream crossings on ephemeral channels (south‐east coast of Spain)

Most road‐stream crossings over ephemeral channels are vulnerable to extreme hydrologic events. Ford stream crossings (FSCs) are usually dangerous for the road traffic during periods of high flow, in particular under flash flood conditions. The present paper analyzes the flood hazards on the Mediterranean coast in the Region of Murcia (south‐east Spain), affecting this type of road‐stream crossing over dry channels, according to hydraulic variables and bedload transport rates estimated for discharges at bankfull and flood‐prone stages. Under such conditions, the safety of people and vehicles was obtained using numerical models, developed by previous researchers; in particular, water levels and flow velocities across ford reaches were compared with different trend curves between water depths and corresponding critical velocities for children and adults, and for various prototype vehicles. Specifically, two approaches to assess this type of hazards were proposed: a specific Hydraulic Hazard Index and an algorithm for estimating the flood hazard from criteria of bed stability and bedload transport capacity (Flood Hazard at Fords, FHF). In addition, different exposure levels were established, using a Flood Vulnerability Index, based on the FHF, the road category, and the annual average daily traffic. The FHF model gave the best results with regard to the magnitude of the damage observed in recent flash floods for flow stages similar to those simulated. According to the danger thresholds established for this index, half‐bankfull flows represent here a high risk: 27.3% of FSCs for mini‐cars and 18.2% for large cars. At bankfull, the FHF exhibits very high values for mini‐cars (77.3% of FSCs) and for large passenger vehicles (50% of FSCs), while at the floodprone stage, extreme FHF values are reached for all kinds of vehicles at most of the ford crossings.