Abstract

At Shiqiang, China, there are spring towers, up to 4 m high and 3 m in diameter, that are formed of porous, gray to orange to brown, bedded limestones that have a multistage diagenetic history. The walls of many cavities in these limestones are covered with hypocoatings and/or needle-fiber calcite (NFC) that formed during the last stage of diagenesis. Where present, the hypocoatings are ∼ 10 µm thick, appear homogeneous, and typically lie under the NFC mats. The NFC is divided into nanofibers (rods, < 1 µm long, 75–100 nm diameter), composite fibers (bundles of 2 to 16 rods, each rod 300–400 nm in diameter, up to 25 µm wide and 100 µm long), pyramid fibers (pyramid shape with 4 to 6 sides, up to 25 wide at base, tapering to point, up to 100 µm long), and ribbon fibers (blade shape, 2–4 µm wide, ∼ 1 µm thick, up to 100 µm long). Many crystals are highly ornate with epitaxial overgrowths that display elements of skeletal and/or dendritic growth. The wall-covering NFC mats are formed of one crystal type or any combination of different crystal types, with or without epitaxial overgrowths.

Precipitation of needle-fiber calcite, known from a diverse array of geological settings throughout the world, is commonly linked to fungi or other biogenic processes. In the Shiqiang spring towers, however, no fungi were found with the NFC and only scattered microbes and minor amounts of exopolysaccharides were associated with some of the NFC in some pores. The NFC crystals appear to have been precipitated from fluids that were highly saturated with respect to calcite as a result of evaporation. The calcium probably came from rainwater, groundwater (via capillary action), and/or dissolution of the calcite framework whereas evaporation was mediated by solar radiation and/or the wind. Available evidence indicates that the NFC found in the spring deposits at Shiqiang were a late diagenetic product that probably formed through abiogenic precipitation.