Pétrographie fine de calcaire d'Alcova représentant des débris de pelécypodes). le milieu les entourant est probablement une mitricisation du carbonate microcristallin calcitique ou aragonitique (ou les deux).
English: Title: Bulletin Identifier: bulletin44peab (find matches) Year: 1991 (1990s) Authors: Peabody Museum of Natural History Subjects: Natural history; Natuurlijke historie Publisher: New Haven : The Museum Contributing Library: Harvard University, Museum of Comparative Zoology, Ernst Mayr Library Digitizing Sponsor: Harvard University, Museum of Comparative Zoology, Ernst Mayr Library View Book Page: Book Viewer About This Book: Catalog Entry View All Images: All Images From Book Click here to view book online to see this illustration in context in a browseable online version of this book. Text Appearing Before Image: COROSAURUS ALCOVENSIS 99 Text Appearing After Image: Fig. 49. Petrographic thin-section of Alcova Limestone depicting pelecypod debris creating biosparitic texture. Field of view is 3.70 mm across. much or all of the primary carbonate component of the rock has undergone replacive dolomitization as noted. Because of these changes, much of the original microstructural fabric of the rock has been obscured. Nevertheless, certain char- acterizations may be made of localized rock types within the member. The orthochemical contribution to the Alcova, or that component that has been precipitated vv^ithin the depositional basin and is presumably relatively untrans- ported, has been most greatly disguised by diagenetic recrystallization. Of the several genetic types discussed above, carbonate produced by inorganic chemical precipitation, phytochemical precipitation, and by the disintegration of pelagic microorganisms is presumed to constitute the major orthochemical foundation of the rock. This foundation was probably a microcrystalline carbonate ooze (micrite) of calcitic and aragonitic composition or both. Following recrystallization, how- ever, this ooze was transformed into a calcitic microcrystalline sparite or micro- sparite with a typical crystal diameter of 5 microns. Also of an orthochemical nature but present in much smaller amounts is primarily deposited sparry calcite. Sparite may have formed some of the original matrix cement of the rock, but was certainly more notably present as penecon- temporaneous crystal growths in primary cavities of the sediment at the time of deposition. Most such cavities were those located in piles of molluscan skeletal debris. Original sparite cannot now, however, be reliably differentiated from aggraded recrystallized micrite (Folk 1965). Certain allochems, or elements of the rock which, although formed within the depositional basin, are not precipitates as discussed above and have been poten- tially or actually transported, have been less affected by Alcova recrystallization. These are principally molluscan remains which, while completely inverted to calcite, remain recognizable although the structure of the shells has been partially obscured (Figure 49). Also present are textural ghosts of dissolved bivalved mi- croorganisms, possibly ostracods. The stromatolites which form a major textural constituent of the Alcova Mem- ber, particularly in the lower part of the unit, are an in situ biohermal component Note About Images Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original work.