Print this pageGenetic Characteristics of Glauconite and Siderite: Implications for the Origin of Ambiguous Isolated Marine Sand Bodies
STONECIPHER, SHARON, A.
Authigenic glauconite and siderite form under a limited range of well documented geological and geochemical conditions. For example, glauconite typically develops on the outer margins of continental shelves in areas of low sediment input. Glauconite precursors need to remain at or near the sediment water surface for long period of time in a setting where they can be repeatedly exhumed and shallowly buried. Glauconite generally occurs as widely dispersed pellets scattered throughout the host sediment. High concentrations of glauconite (approx. 15-50%) denote extensive winnowing and removal of the enclosing sediment, but not necessarily transport of the glauconite. Glauconite degrades and weathers rapidly on exposure to oxidizing conditions.
Similarly, the presence of siderite connotes precipitation either under fresh water reducing conditions or post-oxic marine conditions in sediments where the available iron content exceeds the amount of sulfate entrained in depositional pore waters. The difference between these two environments is reflected in the stable isotope and trace element geochemistry of the siderites. The occurrence of siderite in nodules or beds is an indication of the amount of iron in and the permeability of the sediment, not depositional environment.
These restrictions on environment of origin can be used as geochemical fingerprints to provide information on the hydrologic regime and, by inference, the depositional and sequence stratigraphic setting of the host sediment. Thus, by examining the genetic significance of minerals such as glauconite and siderite, as well as other very early authigenic cements, we are likely to find evidence that will help resolve the origin of ambiguous, controversial, isolated marine sand bodies such as the Shannon, Sussex, and Tocito.