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On the basis of gross morphology, occurrence, abundance, and chemistry of relic organic tissues, four organomineral associations may be recognized. The mineral phase is low-magnesian calcite; its fabric occurs as micron-sized needles and rhombs. The organic phase includes four taxonomic groups: filamentous soil fungi (dominant); filamentous soil algae (rare); actinomycetes (common); and root hairs of vascular land plants (common). Filamentous soil fungi are generally 1 to 10µ in diameter, branch dichotomously and are nonseptate. Filamentous soil algae are 2 to 10µ in diameter, unbranched or show false ramifications, and are septate. Actinomycetes are less than 1µ in diameter, branch irregularly and are nonseptate. Root hairs of vascular land plants are 5 to 15µ in iameter, unbranched and nonseptate. All four groups are dominantly chasmolithic.
Morphology of calcified filaments depends on whether calcification is determined by physicochemical or biochemical processes, or both. The calcified product may be a hollow tube or a solid rod, depending on the condition of the organic substrate before, during, and after its calcification. Biochemical control of calcification produces filaments whose morphologies are related closely to those of the organic substrate; physicochemical control of calcification produces filaments whose morphologies may or may not be related to those of the organic substrate.
Calcretes containing calcified filaments indicate that they functioned, at some stage in their evolution, as biologic soils. Such calcretes are paleosols; they record the presence of a former land surface, colonization by terrestrial organisms, and subaerial vadose conditions in ancient successions.
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