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Glauconitic minerals range from green smectite to 10Å glauconite, occur mainly in peloids (most of which were fecal pellets), and apparently were produced largely by neoformation replacing several kinds of substrate. Chamositic minerals, berthierine and chamosite, occur in Recent and ancient peloids, but mostly in ancient ooids, and they developed by alteration of a precursor Al-rich clay mineral. Ooid sheaths were built by rolling of cores on ue sea floor. Glauconitic greensands and chamositic oolitic ironstones are condensed sequences deposited as sandwaves during long periods of reduced influx of sediment. Some are associated with hardgrounds. 相似文献
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Glauconitic minerals constitute a family ranging from green smectite to a 10Ådioctahedral mica (glauconite). Chamositic minerals include a 7Åtrioctahedral serpentine (berthierine) and a 14Åtrioctahedral chlorite (chamosite). These green iron-rich, neoformed or transformed clay minerals are most commonly concentrated in sand-size granules.Recent berthierine and Recent and ancient glauconitic minerals occur mainly in structureless peloids, most of which are believed to have been fecal pellets. In contrast, most of the ancient chamositic minerals are in multi-coated ooids generally assumed to have been made by gentle rolling on the sea floor.Glauconitic and chamositic granules accumulated most commonly in marine shelf environments during episodes of reduced influx of sediment. In modern deposits chamositic peloids predominate on the inner shelf, whereas glauconitic peloids are most abundant on the middle and outer shelf. In general, ancient glauconitic and chamositic deposits had a rather similar environmental distribution; in detail, however, they reflect more varied and overlapping marine habitats.Glauconitic greensands and chamositic ironstones commonly occur above a coarsening- or shoaling-upward facies sequence. Many of them are cross-bedded and burrowed, and some are interbedded with a ferruginized or phosphatized hardground. Although differing in detail, their temporal distributions throughout Phanerozoic time were rather similar. Both attained a maximum when cratonic blocks were widely dispersed and sea level was high in Early Paleozoic and Late Mesozoic time. In addition, recurring development of chamositic ooids commonly coincided with repeated regional transgressions.This review of current information and differing interpretations leads to significant questions that are essential subjects for future research. Moreover, some of these relate to unsolved problems of phosphorite genesis. 相似文献
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Essentially isochemical thermal metamorphism of soda-rich Stockton, Lockatong and Brunswick formations of the Newark Group by diabase sills produced unusually varied and unique mineral assemblages, most of which are predominantly Na. feldspar and biotite. Within a meter of a sill Stockton arkose was altered to quartzo-feldspathic hornfels with common diopside and sphene. Within 50 m of a sill Lockatong calcitic and dolomitic mudstone formed calc-silicate hornfels with differing combinations of diopside-hedenbergite, andradite and grossular, prehnite, datolite, idocrase and wollastonite. Within a meter of a sill metamorphosed Lockatong calcareous feldspathic argillite contains sanidine-anorthoclase, aegirine, aegirine-augite, riebeckite and scapolite. Lockatong analcime-dolomite argillite was altered to unique feldspathoidal assemblages containing cancrinite, natrolite-thomsonite and rarely sodalite within 134 m, and nepheline within 30 m of the Byram Sill. Reddish-brown Brunswick mudstone produced spotted pelitic hornfels within a few 10's of meters of a sill.Response to thermal metamorphism varied directly with diminishing grain size. In both sandstone and mudstone Na. feldspar increases and K. feldspar decreases toward intrusions; quartz is rare or absent in highest-grade hornfels. Development of biotite was retarded by detrital clay minerals and hematite pigment, as well as by low temperature. Minor differences in composition among carbonate-rich and analcime-rich Lockatong deposits led to a diversity of closely associated assemblages. Aqueous solutions and relatively low temperature, probably in part during retrogressive metamorphism, produced hydrous minerals. Datolite, tourmaline, scapolite and fluorite suggest minor additions of volatiles, but the widespread feldspathoids were made from soda-rich sedimentary rocks without significant additions from an igneous source. 相似文献
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Huge residual boulders on and near the surface of the woodlands on the slopes of Coffman Hill probably are corestones of a for derived from the early Jurassic diabase sill. Accordingly, they were produced in two stages - first by chemical weathering along joints before Cenozoic regional uplift, then by exhumation of the blocks during Cenozoic dissection of the old lowland surface.The Ringing Rocks barren block field within the woodlands probably resulted from local mass transport of large boulders from a higher part of Coffman Hill by periglacial creep and solifluction during the Pleistocene epoch. Loss of forest cover and removal of matrix from the patch of boulders exposed the blocks to further mechanical and chemical weathering. Persistent shattering of the boulders along joints and minor movement of smaller derived blocks destroyed any inherited debris-flow pattern. Pitting on the bare joint surfaces has resulted from effective solution by rain in the absence of forest shelter and a mantle of soil. In contrast, exfoliation predominates in the surrounding wooded residual boulder field. 相似文献
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