Gravel antidunes in the tropical Burdekin River, Queensland, Australia

The geological record is punctuated by the deposits of extreme event phenomena, the identification and interpretation of which are hindered by a lack of data on contemporary examples. It is impossible to directly observe sedimentary bedforms and grain fabrics forming under natural particle-transporting, high-velocity currents, and therefore, their characteristics are poorly documented. The deposits of such flows are exposed however, in the dry bed of the Burdekin River, Queensland, Australia following tropical cyclone-induced floods. Long wave-length (up to 19 m) gravel antidunes develop during short (days) high-discharge flows in the upper Burdekin River (maximum recorded discharge near the study reach over 25 600 m³ s^?1 in February 1927). Flood water levels fall quickly (metres in a day) and flow is diverted away from raised areas of the river bed into subchannels, exposing many of the high-stage bedforms with little reworking by falling-stage currents. Gravel bedforms were observed on the dry river bed after the moderate flows of February 1994 (max. 7700 m³ s^?1) and January 1996 (max. 3200 m³ s^?1). The bedforms had wave-lengths in the range 8–19 m, amplitudes of up to 1 m with steeper stoss than lee faces and crest lines generally transverse to local peak-discharge flow direction. The gravel fabric and size sorting change systematically up the stoss and down the lee faces. The antidune deposits form erosive based lenses of sandy gravel with low-angle downstream dipping lamination and generally steep upstream dipping a-b planes. The internal form and fabric of the antidune gravel lenses are distinctly different from those of dune lee gravel lenses. The erosive based lenses of low-angle cross-bedded gravel with steep upstream dipping a-b planes are relatively easy to recognize and may be diagnostic of downstream migrating antidunes. The antidune gravel lenses are associated with thick (to 1 m) high-angle cross bed sets. Ancient antidune gravel lenses may be diagnostic of episodic high-discharge conditions and particularly when they are associated with high-angle cross-bedded gravelly sand they may be useful for palaeoenvironmental interpretation.     

Pb-Pb single-zircon ages of granitoid boulders from the Vendian tillite of Wahlenbergfjorden, Nordaustlandet, Svalbard

There are three areas in eastern Svalbard where Vendian tillites are exposed: eastern Ny Friesland, western Nordaustlandet (north and south of Murchisonfjorden) and further east in inner Wahlenbergfjorden, near Aldousbreen. Clasts within the massive unmetamorphosed clay-mica-carbonate matrix of the tillites include carbonates, sandstones, siltstones, metavolcanics, schists and different granitoids, the metamorphic and igneous rock types being more frequent in the upper levels of the formation. Large granite boulders, up to 1 m in diameter, are known from the easternmost outcrop at Aldousbreen. Three granitoid boulders from the Aldousbreen outcrop, differing in petrography and chemistry. have been dated by the Pb-Pb singlezircon method. They yield ages of 2830 ± 5 Ma, 1802 ± 4 Ma and 1497 ± 26 Ma. These clasts also differ in petrography, chemistry and age from all known granitic rocks on Nordaustlandet, which have recently yielded Grenvillian (950-960 Ma) and Caledonian (ca. 410 Ma) ages. The concentration of large granitic clasts in the easternmost known tillite outcrops suggests derivation from the east and/or south. Possible areas include those beneath the ice of Austfonna and below the Carboniferous strata of southeastem Nordaustlandet. The apparent lack of a significant Grenvillian overprint suggests the possibility of a more distant source.     

The lack of potassium‐isotopic fractionation in Bishunpur chondrules

Abstract— In a search for evidence of evaporation during chondrule formation, the mesostases of 11 Bishunpur chondrules and melt inclusions in olivine phenocrysts in 7 of them have been analyzed for their alkali element abundances and K‐isotopic compositions. Except for six points, all areas of the chondrules that were analyzed had δ⁴¹K compositions that were normal within error (typically ±3%, 2s?). The six “anomalous” points are probably all artifacts. Experiments have shown that free evaporation of K leads to large ⁴¹K enrichments in the evaporation residues, consistent with Rayleigh fractionation. Under Rayleigh conditions, a 3% enrichment in δ⁴¹K is produced by ～12% loss of K. The range of L‐chondrite‐normalized K/Al ratios (a measure of the K‐elemental fractionation) in the areas analyzed vary by almost three orders of magnitude. If all chondrules started out with L‐chondrite‐like K abundances and the K loss occurred via Rayleigh fractionation, the most K‐depleted chondrules would have had compositions of up to δ⁴¹K ? 200%. Clearly, K fractionation did not occur by evaporation under Rayleigh conditions. Yet experiments and modeling indicate that K should have been lost during chondrule formation under currently accepted formation conditions (peak temperature, cooling rate, etc.). Invoking precursors with variable alkali abundances to produce the range of K/Al fractionation in chondrules does not explain the K‐isotopic data because any K that was present should still have experienced sufficient loss during melting for there to have been a measurable isotopic fractionation. If K loss and isotopic fractionation was inevitable during chondrule formation, the absence of K‐isotopic fractionation in Bishunpur chondrules requires that they exchanged K with an isotopically normal reservoir during or after formation. There is evidence for alkali exchange between chondrules and rim‐matrix in all unequilibrated ordinary chondrites. However, melt inclusions can have alkali abundances that are much lower than the mesostases of the host chondrules, which suggests that they at least remained closed since formation. If it is correct that some or all melt inclusions remained closed since formation, the absence of K‐isotopic fractionation in them requires that the K‐isotopic exchange took place during chondrule formation, which would probably require gas‐chondrule exchange. Potassium evaporated from fine‐grained dust and chondrules during chondrule formation may have produced sufficient K‐vapor pressure for gas‐chondrule isotopic exchange to be complete on the timescales of chondrule formation. Alternatively, our understanding of chondrule formation conditions based on synthesis experiments needs some reevaluation.     

Estimation of Tsunami Risk for the Coasts of Peru and Northern Chile

Data for tsunamigenic earthquakes and observed tsunami run-up are used to estimate tsunami-risk for the coasts of Peru and northern Chile for zones bounded by 5–35° S latitude. Tsunamigenic earthquake estimates yield magnitudes of 8.52, 8.64, and 8.73 for recurrence periods of 50, 100, and 200 years, respectively. Based on three different empirical relations between earthquake magnitudes and tsunamis, we estimate expected tsunami wave heights for various return periods. The average heights were 11.2 m (50 years), 13.7 m (100 years), and 15.9 m (200 years), while the maximum height values (obtained by Iidas method) were: 13.9, 17.3, and 20.4 m, respectively. Both the averaged and maximum seismological estimates of tsunami wave heights for this region are significantly smaller than the actually observed tsunami run-up of 24–28 m, for the major events of 1586, 1724, 1746, 1835, and 1877. Based directly on tsunami run-up data, we estimate tsunami wave heights of 13 m for a 50-year return period and 25 m for a 100-year return period. According to the seismic gap theory, we can expect that the next strong earthquake and tsunami will occur between 19 and 28° S in the vicinity of northern Chile.     

Bleached chondrules: Evidence for widespread aqueous processes on the parent asteroids of ordinary chondrites

Abstract— We present the first detailed study of a population of texturally distinct chondrules previously described by Kurat (1969), Christophe Michel‐Lévy (1976), and Skinner et al. (1989) that are sharply depleted in alkalis and Al in their outer portions. These “bleached” chondrules, which are exclusively radial pyroxene and cryptocrystalline in texture, have porous outer zones where mesostasis has been lost. Bleached chondrules are present in all type 3 ordinary chondrites and are present in lower abundances in types 4–6. They are most abundant in the L and LL groups, apparently less common in H chondrites, and absent in enstatite chondrites. We used x‐ray mapping and traditional electron microprobe techniques to characterize bleached chondrules in a cross section of ordinary chondrites. We studied bleached chondrules from Semarkona by ion microprobe for trace elements and H isotopes, and by transmission electron microscopy. Chondrule bleaching was the result of low‐temperature alteration by aqueous fluids flowing through finegrained chondrite matrix prior to thermal metamorphism. During aqueous alteration, interstitial glass dissolved and was partially replaced by phyllosilicates, troilite was altered to pentlandite, but pyroxene was completely unaffected. Calcium‐rich zones formed at the inner margins of the bleached zones, either as the result of the early stages of metamorphism or because of fluid‐chondrule reaction. The mineralogy of bleached chondrules is extremely sensitive to thermal metamorphism in type 3 ordinary chondrites, and bleached zones provide a favorable location for the growth of metamorphic minerals in higher petrologic types. The ubiquitous presence of bleached chondrules in ordinary chondrites implies that they all experienced aqueous alteration early in their asteroidal histories, but there is no relationship between the degree of alteration and metamorphic grade. A correlation between the oxidation state of chondrite groups and their degree of aqueous alteration is consistent with the source of water being either accreted ices or water released during oxidation of organic matter. Ordinary chondrites were probably open systems after accretion, and aqueous fluids may have carried volatile elements with them during dehydration. Individual radial pyroxene and cryptocrystalline chondrules were certainly open systems in all chondrites that experienced aqueous alteration leading to bleaching.     

A molecular and isotopic study of the macromolecular organic matter of the ungrouped C2 WIS 91600 and its relationship to Tagish Lake and PCA 91008

Abstract– Insight into the chemical history of an ungrouped type 2 carbonaceous chondrite meteorite, Wisconsin Range (WIS) 91600, is gained through molecular analyses of insoluble organic matter (IOM) using solid‐state ¹³C nuclear magnetic resonance (NMR) spectroscopy, X‐ray absorption near edge structure spectroscopy (XANES), and pyrolysis‐gas chromatography coupled with mass spectrometry (pyr‐GC/MS), and our previous bulk elemental and isotopic data. The IOM from WIS 91600 exhibits similarities in its abundance and bulk δ¹⁵N value with IOM from another ungrouped carbonaceous chondrite Tagish Lake, while it exhibits H/C, δ¹³C, and δD values that are more similar to IOM from the heated CM, Pecora Escarpment (PCA) 91008. The ¹³C NMR spectra of IOM of WIS 91600 and Tagish Lake are similar, except for a greater abundance of CH_xO species in the latter and sharper carbonyl absorption in the former. Unusual cross‐polarization (CP) dynamics is observed for WIS 91600 that indicate the presence of two physically distinct organic domains, in which the degrees of aromatic condensation are distinctly different. The presence of two different organic domains in WIS 91600 is consistent with its brecciated nature. The formation of more condensed aromatics is the likely result of short duration thermal excursions during impacts. The fact that both WIS 91600 and PCA 91008 were subjected to short duration heating that is distinct from the thermal history of type 3 chondrites is confirmed by Carbon‐XANES. Finally, after being briefly heated (400 °C for 10 s), the pyrolysis behavior of Tagish Lake IOM is similar to that of WIS 91600 and PCA 91008. We conclude that WIS 91600 experienced very moderate, short duration heating at low temperatures (<500 °C) after an episode of aqueous alteration under conditions that were similar to those experienced by Tagish Lake.     

Presolar silicon carbide from the Indarch (EH4) meteorite: Comparison with silicon carbide populations from other meteorite classes

Abstract— Presolar SiC from the Indarch (EH4) meteorite was studied by scanning electron microscopy (SEM), by ion probe analysis for C and Si isotopic compositions, and by static source mass spectrometry for noble gas and C isotopic compositions. The data obtained are compared to SiC data from other meteorites, especially from Murchison (CM2), for which there is the most information available. The isotopic compositions of the major elements in SiC from Indarch and Murchison are similar. Stepped combustion data suggest a mean δ¹³C for SiC from both meteorites of ～+1430%o. Silicon isotopes in Indarch and Murchison SiC also compare well. In some other important respects, however, SiC in the two meteorites are different. Morphologically, SiC from Indarch appears finer grained than SiC from Murchison and is entirely composed of submicron grains. The finer-grained nature of Indarch SiC is confirmed by its noble gas characteristics. The mean Ne-E/Xe-S ratio for bulk Indarch SiC is significantly lower than the same ratio in Murchison (625 ± 47 vs. ～3500) but is similar to that of the finest grain-size fractions (<1 μm) in Murchison. A comparison of noble gas data from SiC from several different meteorites suggests that it might be Murchison SiC, rather than Indarch SiC, that is unusual. The grain-size disparities in SiC between meteorites are difficult to explain by residue processing differences or differing parent body processing. Instead, we speculate that a grain-size sorting mechanism for SiC may have operated in the solar nebula.     

Large-scale structure of Calamus River deposits (Nebraska, USA) revealed using ground-penetrating radar

The large-scale (i.e. bar-scale) structure of channel deposits of the braided, low-sinuosity Calamus River, Nebraska, is described using ground-penetrating radar (GPR) profiles combined with vibracores. Basal erosion surfaces are generally overlain by medium-scale, trough-cross-stratified (sets 3–25 cm thick), very coarse to medium sands, that are associated with relatively high amplitude, discontinuous GPR reflectors. Overlying deposits are bioturbated, small-scale cross-stratified (sets <3 cm thick) and vegetation-rich, fine to very-fine sands, that are associated with low-amplitude discontinuous reflectors. Near-surface peat and turf have no associated GPR reflectors. In along-stream profiles through braid and point bars, most GPR reflectors dip downstream at up to 2° relative to the basal erosion surface, but some reflectors in the upstream parts of bars are parallel to the basal erosion surface or dip upstream. In cross-stream profiles through bars, GPR reflectors are either approximately parallel to bar surfaces or have low-angle inclinations (up to 6°) towards cut banks of adjacent curved channels. Basal erosion surfaces become deeper towards cut banks of curved channels. These structures can be explained by lateral and downstream growth of bars combined with vertical accretion. Convex upwards forms up to 0·5 m high, several metres across and tens of metres long represent episodic accretion of unit bars (scroll bars and bar heads). Stratal patterns in channel fills record a complicated history of erosion and deposition during filling, including migration of relatively small bars. A revised facies model for this type of sandy, braided river has been constructed based on this new information on large-scale bedding structure.     

Application of MELTS to kinetic evaporation models of FeO‐bearing silicate melts

Abstract— Incorporation of the MELTS silicate melt solution model into models of evaporation successfully reproduces the evaporation behavior of alkali‐free, FeO‐bearing (≥2 mol%) chondritic melts at temperatures between 1700 and 2000 °C. In conjunction with the Berman CMAS melt solution model for FeO‐poor melts, evaporation of alkali‐free melts can now be modeled over a very wide range of conditions. MELTS‐based evaporation models can also quite successfully reproduce the evaporation behavior of K when Al/(Na + K) > 1. However, reproduction of Na evaporation experiments is much poorer.