Abstract

Studies of alluvial deposits of Holocene and Pleistocene age in Puerto Rico and in The Orinoco basin suggest that red beds can form diagenetically in moist tropical climates by intrastratal alteration processes that are similar to those producing red beds diagenetically in deserts. The studies show that modern sediments from these tropical regions contain ample iron, in the form of both detrital oxides and unstable iron-bearing silicates, to produce bright red sediments if the interstitial chemical environment is favorable for the formation and preservation of iron oxide. That such favorable environments exist in some tropical regions is indicated by the fact that yellow and brown iron oxides commonly occur in Quaternary alluvial deposits in Puerto Rico many tens of feet below the water table. In addition, chemical analyses of ground-water samples from Puerto Rico show that the interstitial environment in the alluvium commonly lies in the stability field of hematite. These data indicate that sediments which may be precursors of red beds are accumulating in Puerto Rico today. Where favorable interstitial chemical conditions persist long enough for the yellow and brown oxides (both detrital and authigenic) to be converted to red oxides, these deposits should produce red beds. Such deposits may be modern analogues of ancient red beds associated with fauna, flora, or other evidence of warm, moist conditions at the time of deposition.

Chemical and mineralogic analyses of samples from Puerto Rico and the Orinoco basin, when compared with analogous data from the Sonoran Desert, do not reveal any criterion that can be confidently extrapolated to ancient sediments to differentiate red beds formed in moist climates from those formed in deserts. Other characteristics of ancient red beds, such as fauna, flora, and association with evaporite minerals or with aeolian sandstone, provide the most reliable evidence of the climate at the time of deposition.

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