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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 64 (1980)

Issue: 5. (May)

First Page: 805

Last Page: 805

Title: Model of Autocementation of Quartz Sands as Suggested by SEM Study of St. Peter Sandstone: ABSTRACT

Author(s): J. R. Wood

Article Type: Meeting abstract

Abstract:

Autocementation of quartz sand commonly involves growth of ^agr-quartz in crystallographic continuity with the host grain (overgrowth). This mechanism is not well understood, but SEM examination of quartz grain surfaces from the St. Peter (Ordovician) sandstone suggests that overgrowths develop by recrystallization of a precursor matte or druse which covers much of the original grain surface. However, the matte is always absent along grain contact, and commonly exhibits foliation concentric with the host surface. A cyclic process is thereby suggested for the matte, but one which does not permit deposition between grain contacts. Alternating wetting and drying episodes in which dissolved material is deposited on the sand grain during each cycle is a possible mechanism. Num rous small (1 to 2 µm) euhedral kaolinite crystals scattered over and embedded in the matte are evidence of this mechanism. In the St. Peter sand, it appears that a pore fluid saturated with quartz and kaolinite, and containing numerous kaolinite crystallites, repeatedly came into contact with the sands and evaporated leaving a residue of SiO2 and kaolinite during each cycle. Whether these cycles were diurnal, annual, or reflect rises and falls in the water table is not known, but it is likely that the cementation involved stages requiring complete removal and reinjection of pore fluid, but not as an uninterrupted process in a permanent, continuous pore fluid.

In this process, the driving forces (e.g., evaporation and a changing water table) are easy to identify and quantify. However, a static situation involving a stationary pore fluid does not present any obvious driving force for either mass transfer or recrystallization.

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