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Replacement of Fossils by Length-Slow Chalcedony and Associated Dolomitization
Alonzo D. Jacka
The Middle Permian Getaway Limestone Member of the Cherry Canyon Formation was deposited as carbonate turbidites in the Delaware Basin. Invertebrate skeletal components and ooids exhibit selective replacement by length-slow chalcedony and megaquartz within the Getaway. Where silica has partially replaced skeletal components originally consisting of high magnesium calcite (fusulinids, bryozoans and echinoderm fragments) dolomite rhombs occur only within areas of silicification. This suggests that silica replacement occurred before high magnesium calcite stabilized to low magnesium calcite and that magnesium for dolomitization was locally derived. In silica-replaced areas of monocrystalline echinoderm components all dolomite rhombs are aligned in linear patterns and exhibit single cryst l extinction simultaneous with that of the unreplaced calcite monocrystal; thus orientation of dolomite crystals was controlled by that of the monocrystal. It is inferred that dolomite exsolved from high magnesium calcite at centers of magnesium enrichment during ionic mobilization that accompanied solution-precipitation replacement of magnesium calcite by silica. Silica nuclei then expanded, coalesced, engulfed and partially to completely replaced dolomite rhombs.
In formerly aragonitic shells, original structure and texture are commonly pseudomorphed by silica replacement; in portions of shells not replaced by silica original texture and structure have been obliterated when aragonite later inverted to calcite forming a coarse anhedral mosiac. Thus, silicification also preceded mineralogic stabilization of aragonite.
Where megaquartz partially replaced skeletal material it exhibits highly undulose extinction like that of most calcite which has inverted from aragonite.
Silicification is inferred to have occurred during relatively shallow burial when the carbonate was partially lithified. Silica nucleation seems to have been controlled by diagenesis of organic matter in skeletal components and ooids.
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