A Study of the Origin and Significance of Pyrite Ooids

点击翻译按钮获取中文摘要

9706178 Schieber Pyrite ooids are sedimentary grains of up to 4mm diameter that show concentric pyrite laminae around a core of variable composition. First mention of such grains was made by Hayes (1915), and since then they have sporadically been mentioned in the literature (e.g. Kalliokoski, 1966; Gorter, 1991; Taylor, 1949; Kearsley, 1989). In all these references, pyrite ooids are found in association with oolitic ironstones, are summarily described in a few sentences, and interpreted as diagenetic alterations of chamositic or goethitic ooids. Hayes (1915) and Carozzi (1972) are the lone exceptions in considering pyrite ooids as of primary origin. My interest in pyrite ooids stems from my first encounter with these enigmatic grains in the Chattanooga Shale of Tennessee. In a sedimentary sequence that was supposedly deposited under anoxic conditions in comparatively deep water (Potter et al., 1982; Ettensohn et al., 1988; Woodrow et al., 1988; Leventhal et al., 1987), these grains, with their implication of agitation and wave reworking, seemed to be oddly out of place. Since then I have become aware of other occurrences of pyrite ooids, and was able to acquire material from two of them. Samples and polished thin sections from the Chattanooga Shale, the Decorah Shale (Cambrian of Iowa), and the Powers Steps Formation (Wabana iron ores, Ordovician of Newfoundland) were examined with petrographic and electron microscopes (reflected/transmitted light; SEM and BEI mode; EDAX). This preliminary study showed features that suggest that pyrite ooids may indeed be primary intrabasinal grains, and also indicates that there are several types of pyrite ooids that may well be reflective of paleoceanographic conditions at the time of deposition. Funds are requested for a detailed comparative study of pyrite ooids from a variety of depositional settings and time periods. The aim of the study is (1) to verify that primary pyrite ooids do indeed exist, (2) to show that primary pyrite ooids ar e a more widespread phenomenon than typically appreciated, (3) to determine the conditions for their formation, (4) to understand the various pathways for pyrite ooid formation, and (5) to derive information about paleoceanographic parameters, such as presence or absence of anoxia, energy level of the environment, water depth, chemical conditions at the seafloor and in the surface sediment, and rate of deposition.