Alan C. Kendall

Origin of Fabrics in Speleothems Composed of Columnar Calcite Crystals

ABSTRACT Most calcite in speleothems is composed of columnar crystals (palisade calcite) and exhibits fabrics similar to those in some porefilling calcites interpreted to be replacive after acicular carbonate cements. The columnar crystals do not interfere with each others growth (suggesting that they are secondary features) and this, together with the occurrence of layers of acicular calcite in some speleothems, leads to a conclusion that columnar crystals have replaced acicular carbonate. The evidence, however, is misleading. The same crystal fabrics can be explained by normal, but somewhat complex, growth processes. Inclusions (and patterns made by them) constitute the most important clues to the origin of the columnar crystals. Most inclusions are fluid-filled cavities and six types of growth layering are distinguished on the basis of variations in inclusion abundance, size and pattern. Growth layers defined by parallel, linear inclusions are interpreted to have formed during the incomplete lateral coalescence of numerous syntaxial overgrowth crystallites which grow upon the speleothem surface. The linear inclusions represent remnants of the former inter-crystallite spaces. Complete crystallite coalescence generates inclusion-free calcite, whereas inhibition of the lateral coalescence of the overgrowth crystallites generates layers of acicular calcite. During episodes of cave-flooding, however, he crystallites merge and overgrow each other and precipitation eventually occurs upon large, planar crystal faces. It is believed that the distinctive fabrics of palisade calcite are formed because precipitation usually occurs from thin water films that flow over the growing speleothem surfaces. Large crystal terminations do not form on the speleothem surface because they form projections that disturb the water flow away from the projections which, as a consequence, are gradually eliminated. Small crystal terminations (crystallites), on the other hand, do not disturb the water-flow and thus come to dominate the growth surfaces. Petrographic distinction columnar calcite crystals in speleothems (and other vadose calcites with similar fabrics) and mosaics of columnar crystals that have replaced earlier, acicular-carbonate cements is commonly difficult. Such distinctions are attempts to distinguish between calcite crystals that have grown penecontemporaneously from numerous syntaxial overgrowths (calcites in speleothems) and other calcites in which replacement occurs at a much later date, possibly accompanied by replacement of a metastable phase (replacement of acicular cements).

The diagenesis and low-grade metamorphism of Devonian styliolinid-rich pelagic carbonates from West Germany; possible analogues of recent pteropod oozes

ABSTRACT The diagenesis and low-grade metamorphism of Devonian styliolinid-rich pelagic limestones (forming part of the Cephalopodenkalk) from West Germany has involved several stages and resulted in a rock, different in many respects from the original sediment. These limestones are considered to be ancient analogues of Recent pteropod oozes. Styliolina shells are surrounded by a calcite envelope which also replaces the shells themselves. Most envelopes are divided by non-planar intercrystalline boundaries into fibrous calcite crystals (between 14 and 18 per shell in transverse shell section) which may be terminated by scalenohedral faces. Some of the fibrous calcite crystals around styliolinids may partly fill bedding-parallel sheet-crack-type structures (considered to have formed by d watering of unconsolidated sediment between lithified styliolinid-rich layers) which occur in styliolinid microcoquinas. Other calcite envelopes are not subdivided into fibrous crystals and the optic axes of the calcite are radially disposed about the shell centre. Twin lamellae are prominent in the calcite envelopes and may be curved or straight, but extend across intercrystalline boundaries. Inclusions are present in all envelopes and define various patterns, some of which reveal the position of former crystal terminations. The envelopes are interpreted as being a replacement after an early diagenetic acicular cement, which grew epitaxially on the Styliolina shells at a depth of a few hundred metres on the sea floor. It is probable that this acicular cement replaced an original fine-grained carbonate matrix of the sediments. Fibrous calcite replacement of acicular carbonate probably took place a long time after deposition, when the sediments were uplifted (during the mid-Carboniferous) and came into contact with fresh groundwater. The replacement of acicular carbonate by fibrous calcite is subject to two growth tendencies which determine the type and fabrics of the envelope produced. These tendencies are (1) for the c- or optic axes of the replacement to be determined by the c-axis orientations of the host crystals, and (2) an opposing tendency for the replacement calcite to become crystallographically orientated with respect to its direction of growth. The pelagic limestones themselves are mostly composed of microspar, with areas of coarser and finer mosaic formed from the diagenetic alteration of skeletal material. The fibrous calcite envelopes around styliolinids have also been converted to microspar. Three periods of pressure solution affected the limestones, with the latest period producing solution-stringers or flasers, parallel to the direction of incipient fracture-plane cleavage.

Geochemical variations within a laminated evaporite deposit: evidence for brine composition during formation of the Permian Castile Formation, Texas and New Mexico, USA

Abstract The Upper Permian Castile Formation of the Delaware Basin in northwest Texas and New Mexico consists of up to 600 m of evaporites and is subdivided into units of anhydrite overlain by halite. The Castile Formation has commonly been interpreted as a deep-water, deep-basin deposit in which sediments were laid down in several hundred metres of water or brine. Recent textural observations within anhydrite units, in which the thick-bedded anhydrite horizons have been interpreted as being of shallow-water origin, have challenged this assumption. This geochemical study of the oldest anhydrite unit in the Castile Formation (the Anhydrite 1 Member) attempts to resolve some of the problems regarding brine depth and evolution in the basin. The Anhydrite 1 Member has been subdivided into five major cycles on the basis of the distribution of stratigraphic units of thick-bedded anhydrite. Stable isotopic analyses of sulphur from anhydrite, and oxygen and carbon from calcite show that the basin waters were chemically homogeneous during precipitation of anhydrite, and do not indicate any significant input of meteoric, continental-derived waters. Throughout the section studied progressive enrichment of 18 O upwards within cored intervals indicates continuous evaporation of the water body. Carbon isotopes appear to indicate fluctuations in organic activity within the cycles. Trace elemental analyses of Fe, Mg, Sr, Mn, Al, Ba, Zn, Pb and Cu from the sulphate fraction of the samples show a very high variability. There is a distinct increase in trace elemental abundances at the tops of cycles which may indicate variations in precipitation kinetics. Analyses of texturally defined cycles show that up-core trends for many of the trace elements correlate with changes in δ 18 O, indicating a progressive increase in the influence of evaporation. In addition, cyclical variations in trace elemental composition indicate changes in basin conditions with around a 350-year cyclicity. These changes are independent of δ 18 O values. The geochemical data do not provide conclusive proof of water depth during deposition of the Castile Formation. The data are interpreted as reflecting small-scale changes in conditions of deposition, despite the fact that water input remained essentially constant in terms of chemical composition.

Microbial crust with Frutexites(?) and iron staining in chalks: Albian–Cenomanian boundary, Hunstanton, UK

New petrographic observations and stable isotope data help reinterpret the complex sedimentology of the Albian–Cenomanian boundary exposed in the famous red and white chalks of the cliff section at Hunstanton in Norfolk, UK. Thin-section analysis of a prominent crust at the top of the Hunstanton Red Chalk Formation reveals fossil microstructures attributable to the microbe Frutexites. These Red Chalk microstructures are less bushy that Frutexites sensu stricto, and poor preservation, in part caused by later diagenetic iron migration, means they are identified only tentatively as Frutexites. Stable oxygen isotope values from the crust are similar to those from early diagenetic nodular chalks immediately below the crust, and to partially altered chalks elsewhere in Norfolk. The δ18O data are interpreted as Albian seafloor depositional values albeit slightly altered by subsequent meteoric diagenesis. The microbial affinities of Frutexites are not yet proven; thus, the presence of Frutexites alone is not diagnostic of either photic zone or deep-water sedimentation. However, the presence of Frutexites(?) suggests that the red colour of the Hunstanton Red Chalk is due, at least in part, to the mediation of iron-fixing microbes in the accumulating chalk sediment at a dysoxic–anoxic interface. Centimetre-scale columnar and nodular structures above the ‘Frutexites crust’ that project upwards into the basal Paradoxica Bed of the overlying Ferriby Chalk Formation are sites of localized syndepositional iron staining. These nodules are not stromatolitic or microbial and are not evidence for deposition in shallow-water or intertidal settings.