HERRINGBONE CALCITE - PETROGRAPHY AND ENVIRONMENTAL SIGNIFICANCE

Herringbone calcite is a previously undescribed carbonate cement and s ea-floor precipitate that is common in Archean carbonates but rare in Proterozoic and Phanerozoic rocks, It is abundant in the similar to 25 20 Ma Campbellrand-Malmani platform, South Africa, where field relatio nships, such as erosional truncation of layers of herringbone calcite and interbedding of herringbone calcite with grainstones, demonstrate that it precipitated from ambient marine water, This interpretation is supported by depositional relationships in the greater than or equal to 2.6 Ga Huntsman Limestone of the Bulawayo greenstone belt, Zimbabwe ; the 2.6 Ga Carawine Dolomite, Australia; the 1.90 Ga Rocknest Format ion and the 1.8-1.2 Ga Dismal Lakes Group, Canada; the Ordovician Port erfield carbonate buildup, Virginia; and various Silurian carbonate bu ildups in the Midcontinent, United States, Each of these occurrences i s associated with anaerobic depositional environments or organic-rich sediments, Herringbone calcite consists of alternating light and dark crenulated bands; each light-dark pair is 0.5-1.0 mm thick, Microscopi cally, each pair of bands consists of a row of elongate crystals with their long axes aligned perpendicular to banding and along the growth direction of the cement, The bases of the crystals are optically unori ented, but upwards in each crystal, the optical c axis rotates until i t is perpendicular to crystal elongation, The tops of the elongate cry stal are thus optically aligned and length slow, The light bands of he rringbone calcite correspond to the optically oriented parts of the el ongate crystals, whereas the dark bands correspond to the optically un oriented, lower parts of the elongate crystals, Microspar crystals are also present in some dark bands, A Mg-calcite precursor for herringbo ne calcite, now preserved as low-Mg calcite or dolomite, is supported by the presence of micro-dolomite inclusions and textural differences between herringbone calcite and textures interpreted as neomorphosed f ormer aragonite or low-Mg calcite, Precipitation of herringbone calcit e may be consistent with a diffusionally controlled growth model invol ving branching growth of fibrous crystals and diffusion of a precipita tion inhibitor away from the crystallization surface, Since herringbon e calcite is associated with anaerobic depositional environments, the inhibitor promoting precipitation of herringbone calcite may be presen t only in poorly oxygenated sea water, Thus, the stratigraphic distrib ution of herringbone calcite may be an important indicator of the abun dance of oxygen in carbonate depositional environments through time.