Calculated Phase Relations in the System Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O with Applications to UHP Eclogites and Whiteschists

Pressure–temperature grids in the system Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O and its subsystems have been calculatedin the range 15–45 kbar and 550–900°C, usingan internally consistent thermodynamic dataset and new thermodynamicmodels for amphibole, white mica, and clinopyroxene, with thesoftware THERMOCALC. Minerals considered for the grids includegarnet, omphacite, diopside, jadeite, hornblende, actinolite,glaucophane, zoisite, lawsonite, kyanite, coesite, quartz, talc,muscovite, paragonite, biotite, chlorite, and plagioclase. Compatibilitydiagrams are used to illustrate the phase relationships in thegrids. Coesite-bearing eclogites and a whiteschist from Chinaare used to demonstrate the ability of pseudosections to modelphase relationships in natural ultrahigh-pressure metamorphicrocks. Under water-saturated conditions, chlorite-bearing assemblagesin Mg- and Al-rich eclogites are stable at lower temperaturesthan in Fe-rich eclogites. The relative temperature stabilityof the three amphiboles is hornblende > actinolite > glaucophane(amphibole names used sensu lato). Talc-bearing assemblagesare stable only at low temperature and high pressure in Mg-and Al-rich eclogites. For most eclogite compositions, talccoexists with lawsonite, but not zoisite, in the stability fieldof coesite. Water content contouring of pressure–temperaturepseudosections, along with appropriate geotherms, provides newconstraints concerning dehydration of such rocks in subductingslabs. Chlorite and lawsonite are two important H₂O-carriersin subducting slabs. Depending on bulk composition and pressure–temperaturepath, amphibole may or may not be a major H₂O-carrier to depth.In most cases, dehydration to make ultrahigh-pressure eclogitestakes place gradually, with H₂O content controlled by divariantor higher variance assemblages. Therefore, fluid fluxes in subductionzones are likely to be continuous, with the rate of dehydrationchanging with changing pressure and temperature. Further, eclogitesof different bulk compositions dehydrate differently. Dehydrationof Fe-rich eclogite is nearly complete at relatively shallowdepth, whereas Mg- and Al-rich eclogites dehydrate continuouslydown to greater depth. KEY WORDS: dehydration; eclogites; phase relations; THERMOCALC; UHP metamorphism; whiteschists     

Calculated Phase Relations in the System NCKFMASH (Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O) for High-Pressure Metapelites

Petrogenetic grids in the system NCKFMASH (Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O)and the subsystems NCKMASH and NCKFASH calculated with the softwareTHERMOCALC 3.1 are presented for the P–T range 7–30kbar and 450–680°C, for assemblages involving garnet,chloritoid, biotite, carpholite, talc, chlorite, kyanite, staurolite,paragonite, glaucophane, jadeite, omphacite, diopsidic pyroxene,plagioclase, zoisite and lawsonite, with phengite, quartz/coesiteand H₂O in excess. These grids, together with calculated compatibilitydiagrams and P–T and T–X_Ca and P–X_Ca pseudosectionsfor different bulk-rock compositions, show that incorporationof Ca into the NKFMASH system leads to many of the NKFMASH invariantequilibria moving to lower pressure and/or lower temperature,which results, in most cases, in the stability of jadeite andgarnet being enlarged, but in the reduction of stability ofglaucophane, plagioclase and AFM phases. The effect of Ca onthe stability of paragonite is dependent on mineral assemblageat different P–T conditions. The calculated NCKFMASH diagramsare powerful in delineating the phase equilibria and P–Tconditions of natural pelitic assemblages. Moreover, contoursof the calculated phengite Si isopleths in P–T and P–X_Capseudosections confirm that phengite barometry in NCKFMASH isstrongly dependent on mineral assemblage. KEY WORDS: phase relations; metapelites; NCKFMASH; THERMOCALC; phengite geobarometry     

Internal architecture and sedimentary evolution of coarse-grained, turbidite channel-levee complexes, Early Eocene Regência Canyon, Espírito Santo Basin, Brazil

Successions of Early Eocene coarse-grained turbidites up to 400 m thick fill fault-controlled canyons along the eastern Brazilian continental margin. They form part of a Late Albian to Early Eocene transgressive succession characterized by onlapping, deepening-upward sedimentation. In the Lagoa Parda oil field (Regência Canyon, Espírito Santo Basin) the turbidite facies consist mostly of unstratified conglomerate and sandstone, with interbedded bioturbated mudstone and thin-bedded, stratified sandstone. Within the main Regência Canyon, the coarser grained facies occur within 38 deeply incised channels. The fills are 9 to >50 m thick, 210 to >1050 m wide and >1 km long. The finer grained facies build asymmetrical levees that are higher and thicker on the left side (looking downstream) of their channels, probably as an effect of the Coriolis force (to the left in the Southern Hemisphere). Nine levee successions up to 50 m thick are associated with the 20 youngest channels. The deposits filling the low-sinuosity Lagoa Parda channels record successive channel abandonment through relatively rapid avulsions. Avulsions of unleveed channels took place randomly, but channels with well-developed levees show preferential avulsion to the right (looking downstream), opposite to the direction of preferential levee growth. Lagoa Parda channels can be grouped into three complexes 20–100 m thick. These complexes have an estimated duration of about 140 000 years. It is suggested that control of the development of individual channel complexes was related to variation in sediment supply, in turn probably related to climatic changes. The deposition of each channel complex would have followed an increase in sediment supply into the Regência Canyon through delta/fan-delta and littoral drift systems, which in turn would have responded to phases of higher denudation rates in the high-relief, ancestral coastal ranges of south-eastern Brazil. Overall, the three Lagoa Parda channel complexes form a turbidite succession characterized by channel fills that become narrower, thinner and finer grained upward. These trends were induced mostly by a longer term (>400 000 years) decrease in sediment supply, which in turn resulted from the combined effects of a long-term (second-order) trend of sea-level rise, and the decreasing fault activity at the basin margin and source area.     

Geostrophic sand ridge, dune fields and associated bedforms from the Northern KwaZulu-Natal shelf, south-east Africa

Subaqueous dunes are formed on the KwaZulu-Natal outer-shelf due to sediment transport by the Agulhas Current (geostrophic current). These dunes occur within two dune fields at depths of ? 35 to ? 70 m. The net sediment transport direction is south, but short-period reversals form northward-migrating bedforms. The dune fields are physically bounded by late Pleistocene beachrock and aeolianite ledges. A bedform hierarchy has been recognized in the dune fields comprising a system of three generations of climbing bedforms. The outer dunefield has given rise to a sand ridge (H=12 m; L=4 km; W=1.1 km; and an 8° lee slope) whereas the inner dune fields have achieved large-scale dune status. Bedload parting zones within the dune fields occur where the sediment transport direction switches from north to south due to reversals in the geostrophic flow; these zones occur at depths of ? 60, ? 47 and ? 45 m. An interpretative stratigraphic model is presented on what such geostrophite deposits would look like in the ancient sedimentary record.     

The Bude Formation (Lower Westphalian), SW England: siliciclastic shelf sedimentation in a large equatorial lake

The 1300-m-thick turbiditic Bude Formation was deposited in a lake, Lake Bude, but disagreement persists over whether the environment was a deltaic or deep-water fan. The tectonic setting of the lake was the northern flank of a northerly advancing Variscan foreland basin, close to the Westphalian palaeo-equator. Palaeocurrents indicate sediment sourcing from all quadrants except the south. There is a dm-m scale cyclicity, whereby sandstone bodies comprising amalgamated event beds alternate with mudstone intervals containing non-amalgamated event beds. The ‘ideal’ cycle is a symmetrical coarsening-up/fining-up cycle, consisting of three facies (1, 2 and 3) arranged in 12321 order. Facies 3, in the middle of the cycle, is an amalgamated sandstone body up to 10 m thick which interfingers laterally with thin (cm) mudstone layers. The sandstone body comprises amalgamated beds of very fine sandstone which are largely massive and up to 0.4 m thick. Channels are absent except for scours up to 0.2 m deep which truncate the interfingering mudstone layers. Sandstone bodies are inferred to be tongue-shaped in three dimensions. Facies 1 and 2, completing the 12321 cycle, are respectively dark-grey fine and light-grey coarse, varved(?) mudstone containing thin (< 0.4 m) sandstone event beds. Fossils and burrows indicate that facies 1 and 2 were deposited, respectively, in brackish (rarely marine) and fresh water. Hence, the ideal cycle (12321) reflects an upward decrease then increase in salinity (brackish-fresh-brackish); this is attributed to the lake sill being periodically overtopped by the sea, due to glacio-eustatic sea-level oscillations. The resulting oscillations in lake depth produced the coarsening-up/fining-up (regressive-transgressive) cyclicity, the central sandstone body representing the regressive maximum. Event beds are interpreted as river-fed turbidites deposited during catastrophic storm-floods. Combined-flow ripples and other wave-influenced structures occur in event beds throughout the ideal cycle, suggesting deposition of the entire Bude Formation above storm wave base. The proposed environment is a shelf, of continental-shelf dimensions, but lacustrine instead of marine. Sandstone bodies are interpreted to be river-connected tongues or lobes. The absence of cycles containing nearshore or emergent facies is attributed to: (i) the lake sill preventing the water level from falling below sill level, thereby insulating the lake floor from eustatically forced emergence; and (ii) relatively distal deposition, beyond the reach of shoreline progradations. The lack of palaeoflow from the south is attributed to a (now eroded?) deep-water trough lying to the south, in front of the northerly advancing orogen. Some facies 2 laminated mudstone beds grade laterally into massive and/or contorted beds, interpreted as in-situ seismites (Facies 4), consistent with an active foreland basin setting. Development of seismites was possibly facilitated by gas bubbles and/or weak cohesion in the (fresh water) bottom mud. The late Quaternary Black Sea, with its broad northwestern shelf, is probably a good physiographical analogue of Lake Bude, and was likewise fresh at times.     

A CROSS-HOLE AND FACE-TO-BOREHOLE IN-SEAM SEISMIC EXPERIMENT AT INVINCIBLE COLLIERY,AUSTRALIA*

An experimental cross-hole and face-to-borehole in-seam seismic survey was carried out at Invincible Colliery in the Western Coalfield of New South Wales. Objectives were to determine propagation characteristics of the Lithgow seam and to establish the infrastructure for seismic mapping hole-to-hole in Australia. The seam supports leaky P-, S- and P-SV-modes. These modes propagate with group velocities (at 60 Hz) of 3.1, 1.5 and 1.2 km/s respectively. Particle motion polarization is well developed, as is dispersion of the SH-mode. Attenuation rates are high. The seam is lossy (Q of approximately 20). Two prominent structures were mapped by mode conversion. One is believed to be a fracture zone, the other a zone of intense roof thrusting. The old workings and a minor strike-slip fault, which intersected raypaths, were found to be relatively transparent to P- and S-waves at 60 Hz. Telemetry delay and shot-break timing errors of the exploder box are significant. The resulting traveltime scatter is reduced by means of a least-squares “statics” procedure. The group velocity estimation algorithm (based on Fourier transform) yields dispersion characteristics which can be matched with theoretical results for a simple model of a coal seam waveguide. The experiment demonstrates the capability to retrieve in-seam seismic data of diagnostic quality over an appreciable distance (2 km). The experience gained in both survey layout and data processing will be beneficial to future seam wave surveying of Australian coal mines.     

A storm and tidally-influenced prograding shoreline—Upper Cretaceous Milk River Formation of Southern Alberta, Canada

The Santonian-Campanian Milk River Formation of Southern Alberta represents the transition from an open shelf, through a storm-dominated shoreface into a non-marine sequence of shales and sandstones, with coal. The open shelf deposits consist of interbedded bioturbated mudstones with sharp-based hummocky cross-stratified sandstones. There are no indications of fairweather reworking of the sandstones, which are therefore interpreted as having been deposited below fairweather wavebase. The shoreface sequence consists of a 28 m thick sandstone. It has a very sharp, loaded base, and is dominated by swaley cross-stratification, a close relative of hummocky cross-stratification. Angle of repose cross-bedding is preserved in scattered patches only in the top 5 m of the sand body. Channels up to 180 m wide and 7 m deep are cut into this sand body, with channel margins characterized by lateral accretion surfaces. Regional dispersal trends, as well as local palaeocurrent readings suggest flow toward the NW. Within the channels there is some herringbone cross-bedding and at least two examples of neap-spring bundle cycles, suggesting that the channels are tidally-influenced. Above the channels there is a sequence of carbonaceous shales with in situ root casts and lignitic coal seams. No marine, brackish or lagoonal fauna was identified, and the sequence appears to represent a distal floodplain. The sequence from interbedded hummocky cross-stratified sandstones and bioturbated mudstones into a 10–20 m thick, sharp-based shoreface sandstone characterized by swaley cross-stratification is uncommon. The scarcity or absence of angle of repose cross-bedding in the shoreface, and the dominance of swaley cross-stratification suggests that the shoreface was so storm-dominated that almost no fairweather record was preserved. Other examples of swaley cross-stratified shorefaces are reviewed in the paper.     

Distribution, morphology, and origin of sedimentary furrows in cohesive sediments, Southampton Water

Patches of sedimentary furrows are developed at several locations in the cohesive estuarine sediments of Southampton Water (water depth 1–12 m). These furrows apparently result from short periods of erosion followed by long periods of deposition. Although all the furrows are similar, regularly spaced, parallel troughs, 0.5–15 m wide aligned with the dominant current, furrows in different patches have different characteristics. In some areas furrow width is 1/5–1/15 of furrow spacing (termed ‘narrow’), whereas in other areas furrow width is about 1/2 of the spacing (termed ‘wide’). Narrow furrows have developed where sediment accumulation rates are greater than 3–6 cm yr^?1; wide furrows where accumulation rates are lower. Cockle shells, and other coarse sediments, concentrated on the furrow floors and on floors of smaller (2–10 cm wide) minifurrows, play an important role in furrow formation and evolution as they act to widen the furrows when mobilized during current episodes. Uniform sedimentation across the profile during slack periods tends to narrow the furrow. Some of the larger furrows have remained in the same position for 12 years, while mini-furrows have duration scales of a few months or less. Well-developed furrows are also found in a recently dredged channel. Bedforms similar to those described here may be preserved in the sedimentary record. While no analogues to the larger furrows are presently known, minifurrows may be morphologically similar to the ‘gutter casts’ described from ancient rocks.