About This Item
Share This Item
Literature on carbonate diagenesis has, in general, suggested that cements of virtually any geologic age and morphology were derivable from alteration of aragonite precursors similar to modern submarine aragonites. That interpretation requires for the fibrous fringe cements in many Jurassic and Mississippian oolites, for example, an inference of pseudomorphing of fibrous aragonite by fibrous calcite. Such an assumption of pseudomorphing is unsupported by any examples from calcitization of known aragonite cements (botryoids), ooids, or skeletons. Aragonite relics in calcitized botryoidal cements in the Pennsylvanian of Kansas are comparable to relics in calcitized ooids and skeletons. This underscores the similarity of calcitization behavior of aragonites of diverse origin .
Radiaxial-fibrous (RFC) and fascicular-optic (FOC) calcites are generally interpreted as replacement, by two different modes, of fibrous cement precursors. Those supposed precursors are often inferred to have been aragonite, based on the common fibrous habit in modern aragonite cements. Samples of the Pleistocene Ryukyu limestone (Japan) contain a cement fringe of randomly mixed RFC and FOC on aragonite skeletal substrates and sometimes as a second generation cement on a loose fringe of acicular aragonite. This indicates that RFC and FOC are not distinctive in genesis, and they cannot have originated by replacement of aragonite by proximal to distal migration of a thin film diagenetic front. Geometric considerations also indicate that that generally accepted model is untenable.
Ancient cements whose original aragonite mineralogy can be confidently recognized show a non-random, clumped distribution with respect to geologic time. Bases for such confident recognition of original aragonite include preservation as still aragonite or as replacement calcite irregularly crosscutting original structure and containing relic aragonite inclusions and/or elevated strontium content (with or without distinctive original external morphologies, such as botryoids or square-end rays). Such cement occur in rocks of the Lower Cambrian and Upper Mississippian to Upper Triassic and perhaps Lower Jurassic. Cenozoic cements have not been very closely investigated, and the Cenozoic picture may be rather complex. Nevertheless, the general pattern which emerges appears to parallel that I have found for ooids: aragonite restricted to the Cenozoic, Late Mississippian to Early Jurassic, and Early Cambrian (and late Precambrian).
End_of_Article - Last_Page 544------------