Abstract

Waterfall travertine deposits in the Arbuckle Mountains of Oklahoma contain laminated, coarsely crystalline, neomorphic crusts which retain virtually no evidence of their organic origin. The precursors of these neomorphic crusts are laminated, organic crusts composed of two basic alternating layers: 1) layers of filamentous cyanophyte "bushes" in which each bush is encased by one or more elongate spar crystals; and 2) layers in which equidimensional crystals surround individual cyanophyte filaments. As diagenesis commences, the elongate calcite spar crystals enclosing algal bushes grow into the overlying, more finely crystalline calcite layers by aggradational neomorphism, forming coarse, columnar crystals oriented perpendicularly to the laminae. As the crystals increase in size, they incorporate the inclusion-rich laminae of the surrounding material, thereby maintaining the laminated appearance. The end product of this diagenetic process is a dense, laminated crust composed almost entirely of columnar calcite crystals which commonly reach lengths of more than 1 cm. Such crusts display almost no evidence of their former constituents; however, after immersion in hydrochloric acid and application of an organic stain, remains of algal bushes are visible within thin sections. Many other nonmarine carbonate deposits include similar-looking crusts that appear to be solely inorganic in origin; in light of the present findings, caution certainly is warranted before automatically assuming an inorganic origin for such crusts.

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