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The late (?) Miocene non-marine to paralic Guayabo Group in northeastern Colombia and adjacent Venezuela contains as many as thirty-four thin (to 15 cm) goethitic and chamositic oolites and ooid-bearing sandstones in 1080 m of section. This eastward prograding deltaic complex consists mainly of chert-rich litharenite in fluvial, distributary channel, and shoreline facies, and of montmorillonite mudstone in floodplain and interdistributary embayment facies. Within this framework the oolites are restricted to a paralic association that developed during episodes of waning detrital sedimentation when distributary abandonment was followed by minor transgression across mudflats. Most of the multilayered and symmetrical ooids are composed of goethite, rarely with a small amount of chamosite. These are essentially spherical; many of the chamosite-rich ones have been plastically deformed. Goethitic ooids resemble those in a thin layer accumulating in brackish Lake Chad, central Africa. Chamositic ooids have affinities with those accumulating in a sea loch in western Scotland. Each of these examples is associated with detrital (silicate) sedimentation, apparently developed directly from colloidal ferric oxide and silicate precursors, and affords no evidence that primary aragonitic ooids were later placed by ironbearing oxide and clay. 相似文献
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