Normal faulting as a control on the stratigraphic development of shallow marine syn-rift sequences: the Nukhul and Lower Rudeis Formations, Hammam Faraun fault block, Suez Rift, Egypt

Tectono-stratigraphic analysis of the East Tanka fault zone (ETFZ), Suez Rift, indicates that the evolution of normal fault segments was an important control on syn-rift depositional patterns and sequence stratigraphy. Sedimentological and stratigraphic analysis of the Nukhul Formation indicates that it was deposited in a narrow (ca 1–2 km), elongate (ca 5 km), fault-bounded, tidally influenced embayment during the low subsidence rift-initiation phase. The Nukhul Formation is composed of transgressive (TST) and highstand (HST) systems tract couplets interpreted as reflecting fault-driven subsidence and the continuous creation of accommodation in the hangingwall to the ETFZ. The overlying Lower Rudeis Formation was deposited during the high subsidence rift-climax phase, and is composed of forced regressive systems tract (FRST) shallow marine sandbodies, and TST to HST offshore mudstones. Activity on the ETFZ led to marked spatial variability in stratal stacking patterns, systems tracts and key stratal surfaces, as footwall uplift, coupled with regressive marine erosion during deposition of FRST sandbodies, led to the removal of intervening TST–HST mudstone-dominated units, and the amalgamation of FRST sandbodies and the stratal surfaces bounding these units in the footwall. This study indicates that the evolution of normal fault segments over relatively short (i.e. <1 km) length-scales has the potential to enhance or suppress a eustatic sea-level signal, leading to marked spatial variations in stratal stacking patterns, systems tracts and key stratal surfaces. Crucially, these variations in sequence stratigraphic evolution may occur within time-equivalent stratal units, thus caution must be exercised when attempting to correlate syn-rift depositional units based solely on stratal stacking patterns. Furthermore, local, tectonically controlled variations in relative sea level can give rise to syn-rift stacking patterns which are counterintuitive in the context of the structural setting and perceived regional subsidence rates.     

VELOCITY–DISCHARGE RELATIONSHIPS DERIVED FROM DYE TRACER EXPERIMENTS IN GLACIAL MELTWATERS: IMPLICATIONS FOR SUBGLACIAL FLOW CONDITIONS

Repeated dye tracer tests were undertaken from individual moulins at Haut Glacier d'Arolla, Switzerland, over a number of diurnal discharge cycles during the summers of 1989–1991. It was hoped to use the concepts of at-a-station hydraulic geometry to infer flow conditions in subglacial channels from the form of the velocity–discharge relationships derived from these tests. The results, however, displayed both clockwise and anticlockwise velocity–discharge hysteresis, in addition to the simple power function relationship assumed in the hydraulic geometry approach. Clockwise hysteresis seems to indicate that a moulin drains into a small tributary channel rather than directly into an arterial channel, and that discharges in the two channels vary out of phase with each other. Anticlockwise hysteresis is accompanied by strong diurnal variations in the value of dispersivity derived from the dye breakthrough curve, and is best explained by hydraulic damming of moulins or sub/englacial passageways. Despite the complex velocity–discharge relationships observed, some indication of subglacial flow conditions may be obtained if tributary channels comprise only a small fraction of the drainage path and power function velocity–discharge relationships are derived from dye injections conducted during periods when the supraglacial discharge entering the moulin and the bulk discharge vary in phase. Analyses based on this premise suggest that both open and closed channel flow occur beneath Haut Glacier d'Arolla, and that flow conditions are highly variable at and between sites.     

Application of K-feldspar–jadeite–quartz barometry to eclogite facies metagranites and metapelites in the Sesia Lanzo Zone (Western Alps, Italy)*

The eclogite facies assemblage K-feldspar–jadeite–quartz in metagranites and metapelites from the Sesia-Lanzo Zone (Western Alps, Italy) records the equilibration pressure by dilution of the reaction jadeite+quartz=albite. The metapelites show partial transformation from a pre-Alpine assemblage of garnet (Alm₆₃Prp₂₆Grs₁₀)–K-feldspar–plagioclase–biotite±sillimanite to the Eo-Alpine high-pressure assemblage garnet (Alm₅₀Prp₁₄Grs₃₅)–jadeite (Jd_80–97Di_0–4Hd_0–8Acm_0–7)–zoisite–phengite. Plagioclase is replaced by jadeite–zoisite–kyanite–K-feldspar–quartz, and biotite is replaced by garnet–phengite or omphacite–kyanite–phengite. Equilibrium was attained only in local domains in the metapelites and therefore the K-feldspar–jadeite–quartz (KJQ) barometer was applied only to the plagioclase pseudomorphs and K-feldspar domains. The albite content of K-feldspar ranges from 4 to 11 mol% in less equilibrated assemblages from Val Savenca and from 4 to 7 mol% in the partially equilibrated samples from Monte Mucrone and the equilibrated samples from Montestrutto and Tavagnasco. Thermodynamic calculations on the stability of the assemblage K-feldspar–jadeite–quartz using available mixing data for K-feldspar and pyroxene indicate pressures of 15–21 kbar (±1.6–1.9 kbar) at 550±50 °C. This barometer yields direct pressure estimates in high-pressure rocks where pressures are seldom otherwise fixed, although it is sensitive to analytical precision and the choice of thermodynamic mixing model for K-feldspar. Moreover, the KJQ barometer is independent of the ratio PH2O/P_T. The inferred limiting a(H₂O) for the assemblage jadeite–kyanite in the metapelites from Val Savenca is low and varies from 0.2 to 0.6.     

VARIABILITY IN THE CHEMICAL COMPOSITION OF IN SITU SUBGLACIAL MELTWATERS

Meltwaters collected from boreholes drilled to the base of the Haut Glacier d'Arolla, Switzerland have chemical compositions that can be classified into three main groups. The first group is dilute, whereas the second group is similar to, though generally less concentrated in major ions, than contemporaneous bulk glacial runoff. The third group is more concentrated than any observed bulk runoff, including periods of flow recession. Waters of the first group are believed to represent supraglacial meltwater and ice melted during drilling. Limited solutes may be derived from interactions with debris in the borehole. The spatial pattern of borehole water levels and borehole water column stratification, combined with the chemical composition of the different groups, suggest that the second group represent samples of subglacial waters that exchange with channel water on a diurnal basis, and that the third group represent samples of water draining through a ‘distributed’ subglacial hydraulic system. High NO⁻₃ concentrations in the third group suggest that snowmelt may provide a significant proportion of the waters and that the residence time of the waters at the bed in this particular section of the distributed system is of the order of a few months. The high NO⁻₃ concentrations also suggest that some snowmelt is routed along different subglacial flowpaths to those used by icemelt. The average SO²⁻₄: (HCO⁻₃ + SO²⁻₄) ratio of the third group of meltwaters is 0.3, suggesting that sulphide oxidation and carbonate dissolution (which gives rise to a ratio of 0.5) cannot provide all the HCO⁻₃ to solution. Hence, carbonate hydrolysis may be occurring before sulphide oxidation, or there may be subglacial sources of CO₂, perhaps arising from microbial oxidation of organic C in bedrock, air bubbles in glacier ice or pockets of air trapped in subglacial cavities. The channel marginal zone is identified as an area that may influence the composition of bulk meltwater during periods of recession flow and low diurnal discharge regimes. © 1997 by John Wiley & Sons, Ltd.     

THE USE OF BOREHOLE VIDEO IN INVESTIGATING THE HYDROLOGY OF A TEMPERATE GLACIER

A GeoVision Micro™ colour video camera was used to investigate the internal structure of 11 boreholes at Haut Glacier d'Arolla, Switzerland. The boreholes were distributed across a half-section of the glacier, with closest spacing towards the glacier margin. The boreholes were used to investigate the hydrology of the glacier through automatic monitoring of borehole water level and electrical conductivity (EC) at the glacier bed. EC profiling was undertaken in several boreholes to determine the existence of water quality stratification. Temporal variations in EC stratification were used to infer borehole water sources and patterns of water circulation. Borehole video was used to confirm the conclusions made from these indirect sources of evidence, and to provide an independent source of information on the structure and hydrology of this temperate valley glacier. The video showed variations in water turbidity, englacial channels and voids, conditions at the glacier bed and down-borehole changes in ice structure. Based on the video observations, englacial channels accounted for approximately 0·1% of the vertical ice thickness, and englacial voids for approximately 0·4%. Overall, the video images provided useful qualitative and semi-quantitative data that reinforce interpretations of a range of physical and chemical parameters measured in boreholes. © 1997 by John Wiley & Sons Ltd.     

Fluids on differentiated asteroids: Evidence from phosphates in differentiated meteorites GRA 06128 and GRA 06129

Abstract– Paired meteorites Graves Nunatak 06128 and 06129 (GRA) represent an ancient cumulate lithology (4565.9 Ma ± 0.3) containing high abundances of sodic plagioclase. Textures and stable isotope compositions of GRA indicate that superimposed on the igneous lithology is a complex history of subsolidus reequilibration and low‐temperature alteration that may have extended over a period of 150 Myr. In GRA, apatite is halogen‐rich with Cl between 4.5 and 5.5 wt% and F between 0.3 and 0.9 wt%. The Cl/(Cl+F+OH) ratio of the apatite is between 0.65 and 0.82. The Cl and F are negatively correlated and are heterogeneously distributed in the apatite. Merrillite is low in halogens with substantial Na in the 6‐fold coordinated Na‐site (≈2.5%) and Mg in the smaller octahedral site. The merrillite has a negative Eu anomaly, whereas the apatite has a positive Eu anomaly. The chlorine isotope composition of the bulk GRA leachate is +1.2‰ relative to standard mean ocean chloride (SMOC). Ion microprobe chlorine isotope analyses of the apatite range between ?0.5 and +1.2‰. Textural relationships between the merrillite and apatite, and the high‐Cl content of the apatite, suggest that the merrillite is magmatic in origin, whereas the apatite is a product of the interaction between merrillite and a Cl‐rich fluid. If the replacement of merrillite by apatite occurred at approximately 800 °C, the fluid composition is f(HCl)/f(H₂O) = 0.0383 and a HCl molality of 2.13 and f(HCl)/f(HF) = 50–100. It is anticipated that the calculated f(HCl)/f(H₂O) and a HCl molality are minimum values due to assumptions made on the OH component in apatite and basing the calculations on the apatite with the lowest X_Cl. The bulk δ³⁷Cl of GRA is a >2σ outlier from chondritic meteorites and suggests that parent body processes resulted in fractionation of the Cl isotopes.     

An empirical estimate of the diffusion rate of oxygen in diopside

The intracrystalline diffusion rate of oxygen in diopside was constrained based on natural isotopic variations from a granulite facies marble from Cascade Slide, Adirondacks (New York, USA). The oxygen isotope compositions of the diopsides, measured as a function of grain size, are nearly constant (20.9 ± 0.3‰ vs. SMOW) over the entire measured size range (0.3–3.2 mm diameter). The δ¹⁸O values of the cores of calcite grains are 23.0‰. Temperature estimates based on the Δ¹⁸O(calcite-diopside) are 800d?C, in agreement with the highest previous thermometric estimates for these rocks. The lack of isotopic variation in the diopsides as a function of grain size requires that the oxygen intracrystalline diffusion rate in diopside from the Adirondack samples was very slow. The maximum diffusion rates (D_800d?C parallel to the c-axis) were calculated with an infinite reservoir model (IRM) and a finite reservoir model (FRM) that incorporates mineral modal abundances and initial isotopic variations. For an assumed activation energy (Q) = 100 kJ/mol, the IRM diffusion rate estimate of 1.6 times 10^-20cm²/s is two orders of magnitude faster than from the FRM; at Q=500kJ/mol, the D_800d?C estimate for both methods is c. 5.6 times 10^-20 cm²/s. The present results require that a hydrothermal fluid significantly enhances the diffusion rate of oxygen in diopside if previous data are correct. The δ¹⁸O(SMOW) and δ¹³C(PDB) values of the calcite, measured in situ with a CO₂ laser, are 22.9 ± 0.3, 0.1±0.3‰ in the grain cores, 22.1 ±0.3, 0.2 ±0.1‰ at the grain boundaries and 21.7 ±0.4, -0.6±0.1‰ abutting diopside grains. The δ¹⁸O and δ¹³δC values measured conventionally are: crystal cores, 22.96, -0.95‰; abutting diopside grains, 22.38, -0.93‰; bulk, 22.79, -0.95%. Use of the bulk δ¹⁸O(calcite) values for thermometry yields unreasonably high temperatures. The lower δ¹⁸O values at the calcite grain boundaries are not due to retrograde diffusional exchange with the diopside, they are thought to be a result of a late retrograde fluid infiltration.     

EXPERIMENTAL INVESTIGATIONS OF THE WEATHERING OF SUSPENDED SEDIMENT BY ALPINE GLACIAL MELTWATER

The magnitude and processes of solute acquisition by dilute meltwater in contact with suspended sediment in the channelized component of the hydroglacial system have been investigated through a suite of controlled laboratory experiments. Constrained by field data from Haut Glacier d'Arolla, Valais, Switzerland the effects of the water to rock ratio, particle size, crushing, repeated wetting and the availability of protons on the rate of solute acquisition are demonstrated. These ‘free-drift’ experiments suggest that the rock flour is extremely geochemically reactive and that dilute quickflow waters are certain to acquire solute from suspended sediment. These data have important implications for hydrological interpretations based on the solute content of glacial meltwater, mixing model calculations, geochemical denudation rates and solute provenance studies.