Practical Petrographic Classification of Limestones

Abstract

Limestones are divisible into eleven basic types, which are relatively easy to recognize both in the laboratory and in the field. These rocks are made up of three constituents: (1) allochems, evidently transported or otherwise differentiated carbonate bodies; (2) 1-4-micron microcrystalline calcite ooze matrix, and (3) coarser and clearer sparry calcite, which in most rocks forms as a simple pore-filling cement (like the calcite cement in a quartz sandstone), and only uncommonly forms by recrystallization. Only four types of allochems are volumetrically important in limestones: (a) intraclasts (reworked fragments of penecontemporaneous carbonate sediment), (b) oolites, (c) fossils, and (d) pellets (rounded aggregates of microcrystalline calcite averaging .04-.10 mm.). All chems provide the structural framework of limestones, just as sand grains provide the structural framework of sandstones; microcrystalline calcite and sparry calcite are analogous with the clay matrix and chemical cement of sandstones. A triangular diagram showing the relative proportions of allochems, calcite ooze matrix, and sparry calcite cement is used to define three major limestone families. Family I consists of abundant allochems cemented by sparry calcite; these are the cleanly washed limestones, analogous with well sorted, clay-free sandstones and similarly formed in loci of vigorous currents. Family II consists of variable amounts of allochems embedded in a microcrystalline ooze matrix; these are the poorly washed limestones that are analogous with clayey, poorly sorted sandstones, and form in loci of ineffective currents. Family III limestones consist almost entirely of calcite ooze, hence are analogous with terrigenous claystones. Just as clayey versus non-clayey sandstones can be divided mineralogically into orthoquartzites, arkoses, and graywackes, similarly the first two limestone families are subdivided by considering the nature of the allochems. Family I includes respectively intrasparite, oosparite, biosparite, and pelsparite; family II includes intramicrite and oomicrite (both rare), biomicrite, and pelmicrite. Family III includes homogeneous ooze (micrite), and disturbed ooze with irregular openings filled with spar (dismicrite). Rocks made up largely of organisms in growth position are considered as a separate family IV (biolithite). Properties and mode of formation of each of these types are discussed briefly. Content of admixed terrigenous material or dolomite is shown by additional symbols; pure dolomites are classified on allochem content and crystal size. Recrystallization in limestone is believed to be locally abundant but of over-all minor importance. Among several types of recrystallization, that in which a former microcrystalline ooze matrix recrystallizes to 5-15-micron microspar is considered most common. The term calclithite is suggested for the terrigenous carbonate rocks, e.g., limestone conglomerates End_Page 1------------------------------ or sandstones made up of material eroded from outcrops of considerably older lithified-carbonate formations exposed in an uplifted source land.