Aragonite relic preservation in Jurassic calcite-replaced bivalves

Shells of the aragonite bivalve Neomiodon (Great Estuarine Group, Jurassic, Scotland) replaced by coarse neomorphic calcite contain oriented relics of the original aragonite ultrastructure. The presence of these relics in such old altered shells, as well as the high Sr content of the replacement calcite, indicate that the process of calcite replacement of aragonite is not a cumulative slow process involving repeated alteration events, but rather a rapid, one-step process. Aragonite relics, once encased in neomorphic spar, will survive as unequivocal evidence of original aragonite mineralogy, barring total remobilization of the enclosing stable calcite, a generally unlikely event. The retention of this residual aragonite and high-Sr calcite supports recent isotopic studies which suggest that the multiple phases of alteration (‘recrystallization’) invoked in earlier literature are unlikely events in the diagenesis of most undolomitized limestones. Retention of aragonite relics appears to be independent of whether alteration occurs in shallow meteoric or, as in the case of our Neomiodon material, deeper burial environments. Pseudopleochroism of the replaced Neomiodon shells appears to be due to organic, largely graphitic, relics, not to the aragonite relics.     

Mercury in Geologic Reference Samples

The mercury contents of 109 reference samples of 12 organizations have been determined by flameless atomic absorption spectrometry. The homogeneity of mercury in the six USGS samples issued in 1964 was tested twice, a year apart, using unopened bottles of samples. The analysis of variance of these data in the form of a design with a single variable of classification showed that the mercury contents of the 12 sets can be considered homogeneous at F _0.95 and that 10 of these sets can be considered homogeneous at F _0.90 , a more stringent test.
Data for other reference samples were generally obtained using Youden Squares. This experimental design was used so that we could determine the significance of the variation attributable to some cause on the days on which mercury was determined, to the order in which the determinations were made on the several days, and to the mercury contents of the samples in each design. For the latter, one may assume that the mercury contents of samples will differ and that a significant F ratio may be obtained. For the principal variable of classification, the variation attributable to some cause on the days on which mercury was determined was significant for only 1 of 9 analyses of variance of sets of data but we have not been able to explain the cause of the significance.
The mercury contents of the several types of samples, except for micas, dunites, and perhaps the granodiorites, depend on the origin of the samples. The mercury contents of ande-sites and basalts of continental origin are higher than those for similar samples originating in island arcs or the circum-Pacific belt. We conclude that the high and extremely variable mercury content of W-1 is probably due to mercury resulting from the use of mercury fulminate detonators in the quarry prior to the collection of the sample.     

Wave-generated structures in the Devonian lacustrine sediments of south-east Shetland and ancient wave conditions

The most common wave-generated structures in the nearshore lacustrine sediments of the south-east Shetland basin are cosets of undulatory and unidirectional ripple cross-lamination. The undulatory lamination was produced at relatively high oscillatory flow strengths by accretion of rolling grain (post-vortex) ripples, and the unidirectional cross-sets were formed by the migration of vortex (orbital) ripples at lower strengths. Unidirectional solitary lenses were generated under moderate but discontinuous wave activity on a partly sand-starved substrate. Some lenses were reworked during periods of more prolonged wave activity. The Inman-Komar plot of near-bottom orbital diameter versus ripple spacing (λ= 0.80d₀ for small d₀, or λ= 0.65d₀ as modified by Miller & Komar) may only be used in estimating ancient wave conditions for vortex ripples with low Vertical Form Indices and small wavelengths. This laboratory based relationship (minimum d₀ conditions) is utilized in this study since wave periods in lakes are small. The estimation of ancient wave conditions suggests that the ripples were produced in water depths of up to 10 m and in most cases less than 5 m. The formative waves possessed periods of up to 3.4 sec and suggest that the lake was relatively small, perhaps of the order of 20 km wide.