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 dewatering 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.

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