Mineralogy and Petrology of a Tactite near Helena, Montana

The aluminous pyroxene, fassaite, occurs in two small tabularbodies within mafic plutonites of the Boulder Batholith nearits north-east margin twelve miles east of Helena, Montana.First described by Knopf & Lee (1957), the bodies are contact-metasomatizedlimestone septa, now magnesian-tactites, consisting chieflyof fassaite, spinel, garnet, vesuvianite, and clintonite. Lesscommon minerals include pargasite, diopside, wollastonite, sphene,perovskite, anorthite, forsterite, calcite and chlorite. Sometwenty-five microprobe analyses of the fassaite show it is variablein composition and largely consists of the components CaMgSi₂O₆(53–83 per cent), CaAl₂SiO₆ (7–25 per cent), CaFeAlSiO₆(8–28 per cent), and CaTiAl₂O₆ (0–7 per cent). Thestoichiometry generally requires that most of the iron is ferric,consistent with Mössbauer data taken on a typical sample.If fassaite analyses from these and other contact metamorphicrocks are plotted on a triangular diagram with Ca(Mg,Fe)Si₂O₆,CaAl₂SiO₆ and CaFeAlSiO₆ as end-members, the distribution ofpoints offers no positive evidence for a solvus gap betweenfassaite and diopside as proposed by Ginzburg (1969). The mostaluminous fassaites occur with spinel-clintonite ± grossularand have 25 per cent of the Si replaced by Al, making them truepolymorphs of a garnet (i.e. Gr₄₂And₂₃Pp₃₅). No unusual cationordering is detected in these fassaites by single-crystal X-rayphotographs or Mössbauer measurements. Smede's (1966) estimate of 3–4 km of stratigraphic coverfor the Boulder Batholith indicates pressures of approximately1 kb, in agreement with the occurrence of andalusite + K-feldsparin a hornfels at the Kokaruda Ranch complex. The partial assemblagesof grossular, epidote, perovskite, anorthite-wollastonite, anorthite-calcite,and fassaite-calcite require X_CO2 = 0·12 ± 0·08and T = 570 ± 10 °C at these pressures. These pressuresand temperatures place this occurrence in the upper portionsof the hornblende-hornfels facies after Turner (1968), althoughthe low pressures and water-rich fluids permit assemblages (wollastonite,calcite-forsterite-diopside) that Turner lists as characteristicof the pyroxene-hornfels facies.     

Platform and off-platform carbonates of the Upper Cambrian of western Maryland, U.S.A.

Upper Cambrian carbonates in western Maryland are comprised of platform facies (Conococheague Limestone) west of South Mountain and basin facies (Frederick Limestone) east of South Mountain. Conocheague platform carbonates contain interbedded non-cyclic and cyclic facies. Non-cyclic facies consist of cross-stratified grainstones, thrombolitic bioherms, and graded, thin-bedded dolostones. These were deposited in shallow, subtidal shelf lagoons. Cyclic facies are composed of repeated sequences of cross-stratified grainstone; ribbon-rock; wavy, prism-cracked laminite; and planar laminated dolostone. The cyclic facies are shallowing-upward cycles produced by lateral progradation of tidal flats over shallow, nearshore subtidal environments. Cyclic and non-cyclic facies are interbedded in the Conococheague in a layer cake fashion, but no higher-order cyclicity can be found. The Frederick Limestone is dominated by monotonously thick sequences of graded, thin-bedded limestones, interbedded with massive peloidal grainstones and beds of breccia up to 10 m thick in the lower Frederick. The breccias contain transported megaclasts of Epiphyton-Girvanella boundstones. The basal Frederick was deposited in a slope-to-basinal setting east of a rimmed shelf. An Epiphyton-Girvanella marginal reef along the shelf edge was the source of the blocks in the breccias. The upper Frederick Limestone formed on a carbonate ramp.     

Lithified carbonate sediment and zeolitic tuff in basalts, Mid-Atlantic Ridge

Basaltic boulders dredged from the Mid-Atlantic Ridge contain lithified coccolith-foraminiferal ooze in fractures and small pockets. Textural and isotopic studies of this sediment provide no evidence for high temperature metamorphism. The lithified carbonate sediment occurs together with palagonitized basaltic glass and zeolitic tuff, and appears to have been squeezed into cracks and other voids within the congealed margins of partly-cooled basalt pillows which intruded soft carbonate ooze. Thermal metamorphism probably was precluded by rapid heat dissipation in convecting pore waters and by the thermal stability of calcareous microplankton. Low temperature alteration of basaltic glass appears to have provided the chemical milieu for precipitation of calcite cement as well as zeolites in the sediment.     

Stability Relations of the Pyroxenes and Olivine in Certain High Grade Metamorphic Rocks

The stability relations of mineral assemblages consisting ofvarious combinations of Capyroxene, orthopyroxene, olivine,quartz, dolomite, calcite, and graphite are treated by analyticalmeans with emphasis on reactions of the following type: MgSiO₃CaCO₃SiO₂CaMgSi₂O₆CO₂ (1) 3MgSiO₃CaCO₃ Mg₂SiO₄CaMgSi₂O₆CO₂ (2) and the associated reactions involving ferrous iron. In particular,reaction (1) is discussed in detail in terms of uncertaintiesinherent in the thermochemical data and the limits placed onits equilibrium constant by the geological conditions of theoccurrence of the observed assemblages. It is shown from theequation of equilibrium of (1) that the composition of orthopyroxeneis a linear function of the carbon dioxide fugacity and thatthe latter is lowered by the presence of the FeSiO₃ component.The latter conclusion is compatible with the rarity of the associationcalcite-orthopyroxcne and its apparent confinement to iron-richrocks. An analysis of the same assemblage involved in reaction (1)indicates that when graphite coexists with the iron-rich pyroxene,the fugacity of oxygen falls near the center of the magnetitefield for a wide range of CO₂ fugacities and temperatures. It is shown that reaction (2) is also displaced strongly tothe right under most metamorphic conditions, which is compatiblewith fairly common occurrence of the association of Capyroxeneand olivine. Finally, a critique is presented on the possibility of the useof the pyroxene-bearing assemblages as indicators of metamorphicgrade and it is concluded that this is not rigorously possiblesince the systems almost invariably possess too many degreesof freedom.     

A REVIEW OF GRAIN-SIZE PARAMETERS

This paper comprises a review of the many graphical and mathematical techniques that have been proposed for the statistical summary of grain-size data. Satellitic problems, such as laboratory techniques, choice of size scales, and interpretation, are only considered briefly.     

The upper Hawkesbury River, New South Wales, Australia: a Holocene example of an estuarine bayhead delta

Holocene deposits of the Hawkesbury River estuary, located immediately north of Sydney on the New South Wales coast, record the complex interplay between sediment supply and relative sea-level rise within a deeply incised bedrock-confined valley system. The present day Hawkesbury River is interpreted as a wave-dominated estuarine complex, divisible into two broad facies zones: (i) an outer marine-dominated zone extending 6 km upstream from the estuary mouth that is characterized by a large, subtidal sandy flood-tidal delta. Ocean wave energy is partially dissipated by this flood-tidal delta, so that tidal level fluctuations are the predominant marine mechanism operating further landward; (ii) a river-dominated zone that is 103 km long and characterized by a well developed progradational bayhead delta that includes distributary channels, levees, and overbank deposits. This reach of the Hawkesbury River undergoes minor tidal level fluctuations and low fluvial runoff during baseflow conditions, but experiences strong flood flows during major runoff events. Fluvial deposits of the Hawkesbury River occur upstream of this zone. The focus of this paper is the Hawkesbury River bayhead delta. History of deposition within this delta over the last c. 12 ka is interpreted from six continuous cores located along the upper reaches of the Hawkesbury River. Detailed sedimentological analysis of facies, whole-core X-ray analysis of burrow traces and a chronostratigraphic framework derived from 10 C-14 dates reveal four stages of incised-valley infilling in the study area: (1) before 17 ka BP, a 0–1 m thick deposit of coarse-grained fluvial sand and silt was laid down under falling-to-lowstand sea level conditions; (2) from 17 to 6·5 ka BP, a 5–10 m thick deposit composed of fine-grained fluvial sand and silt, muddy bayhead delta and muddy central-basin deposits developed as the incised valley was flooded during eustatic sea-level rise; (3) during early highstand, between 6·5 and 3 ka BP, a 3–8 m thick bed of interbedded muddy central-basin deposits and sandy river flood deposits, formed in association with maximum flooding and progradation of sandy distributary mouth-bar deposits commenced; (4) since 3 ka BP, fluvial deposits have prograded toward the estuary mouth in distributary mouth-bar, interdistributary-bay and bayhead-delta plain environments to produce a 5–15 m thick progradational to aggradational bayhead-delta deposit. At the mouth of the Hawkesbury estuary subaqueous fluvial sands interfinger with and overlie marine sands. The Hawkesbury River bayhead-delta depositional succession provides an example of the potential for significant variation of facies within the estuarine to fluvial segment of incised-valley systems.     

Architectural analysis of a volcaniclastic jökulhlaup deposit, southern Iceland: sedimentary evidence for supercritical flow

The 1918 eruption of the glacially capped Katla volcano, southern Iceland, generated a violent jökulhlaup, or glacial outburst flood, inundating a large area of Mýrdalssandur, the proglacial outwash plain, where it deposited ca 1 km³ of volcaniclastic sediment. The character of the 1918 jökulhlaup is contentious, having been variously categorized as a turbulent water flow, a hyperconcentrated flow or as a debris flow, based on localized outcrop analysis. In this study, outcrop‐based architectural analyses of the 1918 deposits reveal the presence of lenticular and tabular bedsets associated with deposition from quasi‐stationary antidunes and down‐current migrating antidunes, and from regular based bedsets, associated with transient chute‐and‐pool bedforms, all of which are associated with turbulent, transcritical to supercritical water flow conditions. Antidune wavelengths range from 24 to 96 m, corresponding to flow velocities of 6 to 12 m sec^?1 and average flow depths of 5 to 19 m. This range of calculated flow velocities is in good agreement with estimates made from eyewitness accounts. Architectural analysis of the 1918 jökulhlaup deposits has led to an improved estimation of flow parameters and flow hydraulics associated with the 1918 jökulhlaup that could not have been achieved through localized outcrop analysis. The observations presented here provide additional sedimentological and architectural criteria for the recognition of deposits associated with transcritical and supercritical water flow conditions. The physical scale of sedimentary architectures associated with the migration of bedforms is largely dependent on the magnitude of the formative flow events or processes; sedimentary analyses must therefore be undertaken at the appropriate physical scale if reliable interpretations, regarding modes of deposition and formative flow hydraulics, are to be made.     

Inversion of crater morphometric data to gain insight on the cratering process

Abstract— In recent years, morphometric data for Venus and several outer planet satellites have been collected, so we now have observational data of complex craters formed in a large range of target properties. We present general inversion techniques that can utilize the morphometric data to quantitatively test various models of complex crater formation. The morphometric data we use in this paper are depth of a complex crater, the diameter at which the depth-diameter ratio changes, and onset diameters for central peaks, terraces, and peak rings. We tested the roles of impactor velocities and hydrostatic pressure vs. crustal strength, and we tested the specific models of acoustic fluidization (Melosh, 1982) and nonproportional growth (Schultz, 1988). Neither the acoustic fluidization model nor the nonproportional growth in their published formulations are able to successfully reproduce the data. No dependence on impactor velocity is evident from our inversions. Most of the morphometric data is consistent with a linear dependence on the ratio of crustal strength to hydrostatic pressure on a planet, or the factor c/p g.