<Emphasis Type="Bold">Rhodophycean algae from the Lower Cretaceous of the Cauvery Basin,South India</Emphasis> by P.K. Misra,S. Kishore,S.K. Singh and A.K. Jauhri. Jour. Geol. Soc. India,v.73, 2009, pp.325–334

Discussion

Rhodophycean algae from the Lower Cretaceous of the Cauvery Basin, South India by P.K. Misra, S. Kishore, S.K. Singh and A.K. Jauhri. Jour. Geol. Soc. India, v.73, 2009, pp.325–334     

Limits to the age of the Lapstone monocline,N.S.W.—a palaeomagnetic study

The age of formation of the Lapstone Monocline has been long considered to be late Pliocene/early Pleistocene (the Kosciusko Uplift) but it is now generally thought to be much older. Palaeomagnetic data from haematite‐rich beds within the Hawkesbury Sandstone on and about the monocline indicate that it formed before the oldest haematite was introduced to these beds. The age of this oldest haematite is 15 ± 7 Ma. On the basis of these data, the age of the monocline is unlikely to be less than 8 Ma, probably exceeds 15 Ma, and could be older than 22 Ma.     

Strain and displacement across the Pinaleño Mountains shear zone,Arizona, U.S.A.

The 27 km long, 0.5 km thick mylonite zone of the Pinaleño Mountains metamorphic core complex is interpreted as a normal-displacement simple-shear zone on the basis of major, minor and microscopic-scale structures. Shear strains calculated as a function of the angle between the foliation and the lower boundary of the shear zone range from 2 to 19 and have a mean value of 3.5, corresponding to a mean angular shear of 74°. These values are the same as those calculated independently from rotations of sheared lithologic contacts. Integration of the calculated shear strains yields a minimum translation estimate of 2.9 km.     

The Global Boundary Stratotype Section and Point for the base of the Turonian Stage of the Cretaceous： Pueblo, Colorado, U.S.A.

Following the recommendation of the International Commission on Stratigraphy (16 votes Yes [94%], 1 abstention, 2 votes not received), the Global boundary Stratotype Section and Point (GSSP) for the base of the Turonian Stage of the Cretaceous System is defined as the base of bed 86 of the Bridge Creek Limestone Member of the Greenhorn Limestone Formation at the western end of the Denver and Rio Grande Railroad cut near the north boundary of the Pueblo Reservoir State Park Recreation Area, west of Pueblo, Colorado, USA. This GSSP horizon is also exposed and protected in the adjacent state recreation area. It coincides with the first occurrence of the ammonite Watinoceras devonense, is in the middle of a global positive excursion in Carbon-13 isotopes, and is bracketed by widespread bentonites that have yield edages of 93 to 93.5 Ma.     

On Liaoximordellidae Fam. Nov. (Coleoptera, Insecta) from the Jurassic of Western Liaoning, China

A new insect family Liaoximordellidae (fam. nov.) has been named for a well-preserved specimen which was collected from the Upper Jurassic outcropping west of Daxinfangzi Village, Lingyuan County, Liaoning Province. The specimen can not be put into Mordellidae or Praemordellidae because it possesses some original and intermediate characters in morphology. It serves the study of mordellid evolution. Besides, the fossil group associated with the insect is important for the stratigraphic division of the Mesozoic in western Liaoning.     

Pre-Cenozoic geology of the Latrobe Valley Area—onshore Gippsland Basin,S.E. Australia

In 2010–2011, a well on the uplifted northern edge of the Latrobe Valley (Yallourn North-1A) cored a 550 m section of mostly arenaceous sediments from the Lower Cretaceous Tyers River Subgroup. A follow-up core-hole (Yallourn Power-1) aimed at extending the Tyers River Subgroup section some 5 km south into the Latrobe Valley instead encountered Paleozoic basement rocks immediately below Cenozoic coal measures. From a re-examination of earlier coal and groundwater bore results, and new interpretations from gravity, seismic and magneto-telluric (MT) surveys, there is a significant area of Paleozoic basement rock that may underlie the whole northern Latrobe Valley area. The uplifted Yallourn North Lower Cretaceous sediments are a separate basin entity herein named the Monash trough. It appears they are separate from the main Lower Cretaceous Strzelecki Group Basin sediments on the southern side of the Latrobe Valley. Attributes of the Monash trough may underlie the main Strzelecki Basin, but this remains to be substantiated by further drilling. The intervening subcrop of Paleozoic basement rocks is herein named the Glengarry basement block. It shows characteristic gravity, MT and seismic features covering some 200 km² of the northern Latrobe Valley area. The boundary between the Glengarry basement block and Strzelecki Basin approximates to the Princes Highway. It is uncertain whether structural separation of the Monash trough from the main Strzelecki Basin always existed, or whether uplift and stripping of Cretaceous rocks over the Glengarry basement block occurred in post-Cretaceous but pre-Cenozoic times. Comparative rank and maturity indices indicate a greater depth of burial of the Glengarry basement block than what exists today, whereas less stripping and loss of section have occurred to the Monash trough. Cretaceous sediments of the Tyers River Subgroup (Rintouls Creek Formation, Tyers Conglomerate) in the Monash trough are dominated by mudstones, siltstones with lesser quartzose sandstones, conglomerates and thin coals. The sediments are over 300 m thick and are conformably overlain by 100 m of volcaniclastic sediments typical of the main Strzelecki Group, in turn overlain by nearly 100 m of Cenozoic coal measures. New detailed spore–pollen dating of Yallourn North-1A cores indicates that all Cretaceous sediments in the Monash trough are Barremian in age. This revises the traditional Neocomian age assigned to the formation. High total organic carbon levels in the 100 m-thick mudstones of the Locmany Member in the Rintouls Creek Formation constitute a mature petroleum source rock worthy of future hydrocarbon exploration.     

Alteration of uraninite from the Nopal I deposit,Pen˜a Blanca District,Chihuahua, Mexico,compared to degradation of spent nuclear fuel in the proposed U.S. high-level nuclear waste repository at Yucca Mountain,Nevada

At the Nopal I uranium deposit, primary uraninite (nominally UO_2+x) has altered almost completely to a suite of secondary uranyl minerals. The deposit is located in a Basin and Range horst composed of welded silicic tuff; uranium mineralization presently occurs in a chemically oxidizing and hydrologically unsaturated zone of the structural block. These characteristics are similar to those of the proposed U.S. high-level nuclear waste (HLW) repository at Yucca Mountain, Nevada. Petrographic analyses indicate that residual Nopal I uraninite is fine grained (5–10 μm) and has a low trace element content (average about 3 wt%). These characteristics compare well with spent nuclear fuel. The oxidation and formation of secondary minerals from the uraninite have occurred in an environment dominated by components common in host rocks of the Nopal I system (e.g. Si, Ca, K, Na and H₂O) and also common to Yucca Mountain. In contrast, secondary phases in most other uranium deposits form from elements largely absent from spent fuel and from the Yucca Mountain environment (e.g. Pb, P and V). The oxidation of Nopal I uraninite and the sequence of alteration products, their intergrowths and morphologies are remarkably similar to those observed in reported corrosion experiments using spent fuel and unirradiated UO₂ under conditions intended to approximate those anticipated for the proposed Yucca Mountain repository. The end products of these reported laboratory experiments and the natural alteration of Nopal I uraninite are dominated by uranophane [nominally Ca(UO₂)₂Si₂O₇·6H₂O] with lesser amounts of soddyite [nominally (UO₂)₂SiO₄·2H₂O] and other uranyl minerals. These similarities in reaction product occurrence developed despite the differences in time and physical—chemical environment between Yucca Mountain-approximate laboratory experiments and Yucca Mountain-approximate uraninite alteration at Nopal I, suggesting that the results may reasonably represent phases likely to form during long-term alteration of spent fuel in a Yucca Mountain repository. From this analogy, it may be concluded that the likely compositional ranges of dominant spent fuel alteration phases in the Yucca Mountain environment may be relatively limited and may be insensitive to small variations in system conditions.     

Estimation of the composition of the Earth’s primary aqueous phase. 2. Synthesis from the mantle and igneous rock material. Comparison with synthesis from the carbonaceous chondrite material

Simulation results of the equilibrium state of systems water-carbonaceous chondrite material, water-primary mantle material, water-ultramafic rock material, and water-mafic rock material open with respect to carbon dioxide and methane at 25°C, 1 bar indicate that highly alkaline reduced aqueous solutions with K/Na > 1 can be formed only if water is in equilibrium with compositions close to those of continental crust and primitive mantle. Yu.V. Natochin’s hypothesis that the living cell can be formed only in an aqueous environment with K/Na > 1 leads to the conclusion that terrestrial life could arise and further evolve on the Earth during the differentiation of primary chondritic material into the Earth’s core and mantle (during the first few million years of the planet’s lifetime) in an alkaline (pH 9–10) reduced (Eh = −400–500 mV) aqueous solution at a temperature of 50–60°C, in equilibrium with an N₂-bearing atmosphere, which also contained CH₄ (partial pressure from 10⁻² to 10⁻⁸ bar), CO₂ (partial pressure from 10⁻⁵ to 10⁻⁸ bar), NH₃, H₂, H₂S, CO, and other gases.     

Carbonate-sulfide equilibria and “stratabound” disseminated epigenetic gold mineralization: a proposal based on examples from Alleghany,California, U.S.A.

“Stratabound” disseminated pyritic Au ore bodies were produced by reactions between wall rocks and through-flowing fluids in Mesozoic epigenetic Au quartz vein systems in the Sierra Nevada metamorphic belt. Equilibrium relations among Fe-bearing carbonate and sulfide minerals were critical in determining which rock types were likely to host disseminated mineralization along portions of discordant veins. The compositions of metasomatic carbonates in hydrothermally altered wall rocks at Alleghany, California, U.S.A., were larely predetermined by the relative proportions of Fe, Mg and Ca in the unaltered wall rocks. Thus, coexisting solid solutions in the magnesite-siderite and dolomite-ankerite series from a variety of different wall rocks yield an empirical phase diagram for a large part of the Ca CO₃MgCO₃FeCO₃ system at the temperature of metasomatism (325 ± 50°C). Because Fe,Mg-silicates were unstable in alteration zones adjacent to the veins, wall rock Fe was partitioned between carbonates and sulfides. Pyritization and disseminated Au mineralization occur in a variety of igneous and metasedimentary wall rocks in which the initial molar Fe/(Fe + Mg) ≧ 0.5. In altered wall rocks with initial molar Fe/(Fe + Mg) ≦ 0.5, Fe was incorporated almost entirely within Mg-rich carbonates (X_FeCO3 ≦ 0.6 in magnesite-siderite solutions). It is proposed that the CO₂-rich vein fluid responsible for the alteration and mineralization was partially buffered with respect to H₂S/CO₂/H₂ ratios by equilibrium between pyrite and Mg_0.4Fe_0.6CO₃ (+graphite?) as it traversed and altered intermediate volcanic and sedimentary rocks. This fluid then locally reacted with lower Fe/(Fe + Mg) rocks to form Fe-bearing dolomite + magnesite assemblages, and reacted with higher Fe/(Fe + Mg) rocks to form ankerite + pyrite assemblages. Gold precipitated from saturated solutions of bisulfide complexes partly in response to fluid desulfidation and reduction caused by the pyritization reactions. In terranes dominated by intermediate metavolcanic and metasedimentary rocks, favorable host rocks for this type of mineralization need not have high Fe contents, but do require high Fe/(Fe + Mg) ratios. They may include felsic volcanic and plutonic rocks, Fe-rich tholeiitic differentiates, banded Fe formations, and a variety of siliceous and argillaceous sedimentary rocks. Rocks which tend not to be heavily sulfidized because they have low initial Fe/(Fe + Mg) ratios include ultramafic and mafic igneous rocks, and some argillaceous sedimentary rocks. Exploration guidelines based on these principles may be useful elsewhere in the Sierra Nevada and in other comparable heterogeneous metamorphic terranes, if modified to reflect the dominant buffering rock types in a given fluid flow path. Carbonate-sulfide equilibria are capable of approximately buffering the carbonate-sulfide ratios of CO₂-rich vein fluids (f_CO2≧ 10^2.8 at 325°C, 200MPa or 2000 bar). The Alleghany fluid (f_CO2 ≈ 10^3.2, or ∼ 10 mol % CO₂) had a molar CO₂/H₂S ratio of approximately 10³, assuming graphite saturation. At lower CO₂ fugacities, Fe-bearing silicates entered the buffering assemblages. Carbonatization reactions could potentially de-sulfidize some wall rocks, releasing S (and associated metals?) to the fluid. This would be most likely to occur in pyrite-bearing mafic and ultramafic rocks and some argillites.