Relative Ages of Visibly Crystalline Calcite in Late Paleozoic Limestones

Abstract

Four types of visibly crystalline calcite occur in late Paleozoic limestones examined in this study. (A) Grain-growth calcite, characterized principally by irregular size and shape and random orientation of crystal grains, probably formed by solid-state recrystallization of microcrystalline calcite sediment. (B) Blade calcite, characterized by tapered, blade-shaped crystals bunched in flower-like aggregates, probably formed by recrystallization under mild shearing stresses. (C) Encrusting calcite, characterized by aggregates of fibrous to irregular crystals, probably formed by precipitation around algal tissues that subsequently decayed. (D) Void-filling calcite, characterized by orientation of crystals perpendicular to walls of filled voids and by general increase in crystal size toward center of filled voids, formed through inorganic precipitation.

Encrusting calcite probably formed during deposition of surrounding sediment and seems to be the earliest of the calcite types to form. Most void-filling calcite probably formed early in the limestone's history, during or soon after partial consolidation of fine sediment. Early filling of voids, reducing the porosity, probably inhibited compaction. Although its age relations are poorly known, grain-growth calcite is probably not older than most of the adjacent void-filling calcite. Blade calcite and void-filling calcite that occupies post-lithification fractures and other openings seem to be the latest to form.

Visibly crystalline calcite may be defined simply as calcite consisting of crystals that are large enough to be seen readily as individuals in thin section at low magnification. In this respect, visibly crystalline calcite contrasts with microcrystalline calcite, which consists of crystals either too small or too poorly defined to permit individual crystals to be distinguished. The distinction between visibly crystalline calcite and microcrystalline calcite is a semi-quantitative one. Limits between the two types, in terms of dimensions of individual crystals, cannot be precise, however, because size of crystals intergrades continuously.

This paper is based principally on the study of Pennsylvanian limestones, particularly those of the Upper Pennsylvanian Lansing Group in southeast Kansas and parts of the Magdalena Limestone (Pennsylvanian) in the Caballo Mountains of southwest-central New Mexico and the Hueco Mountains of West Texas. In addition, several hundred specimens from various Pennsylvanian and Permian limestone units in Kansas and the McCloud Limestone (Permian) of northern California have been examined for comparison. Conclusions drawn in this study are probable applicable to many late Paleozoic limestones, and perhaps to limestones of other ages as well.

Both thin sections and acetate peels have been examined in studying the limestones described here. Peels are superior in some respects because of the great fidelity with which they reproduce the microscopic details of the limestone. Methods of preparing and photographing peels have been described previously (Harbaugh, 1959, p. 295).