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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 52 (1968)

Issue: 3. (March)

First Page: 523

Last Page: 524

Title: Selective Dispersal of Quartz: ABSTRACT

Author(s): David K. Davies

Article Type: Meeting abstract

Abstract:

Polycrystalline, undulatory, and nonundulatory quartz extinction varieties are characterized during transport by differing degrees of resistance to degradation. Polycrystalline quartz, the most unstable variety, is the first to be broken down and, in the examples discussed, is not modified significantly during transport. Undulatory quartz is being reduced continually in size during transport, and within a particular grain size there will be a concomitant enrichment in the more resistant nonundulatory variety.

Such a selectivity in the dispersal of quartz extinction types characterizes Pleistocene sediments of the Mississippi cone and Sigsbee Deep of the Gulf of Mexico. Proximal to input source (the Mississippi) the sediments have 8 percent less nonundulatory quartz than distal sediments on the western edge of the Sigsbee Deep. This variation is attributed to the differing degree of resistance to degradation of each of the monocrystalline quartz varieties. The westerly increase in nonundulatory quartz is not, however, a simple linear trend. Within the grain-size limits of this study (74-37ยต), there is no marked enrichment of nonundulatory quartz along the whole of the Mississippi

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cone. However, from east to west of the Sigsbee Deep, the degree of enrichment is marked. It is postulated that this difference is a result of contrasted flow regimes between the two physiographic provinces. The cone, because of its slope and high sedimentation rate (compared with the deep), was characterized by both slumping (low-velocity mass transfer of sediment) and high-velocity turbidity currents. The latter supplied the main source of sediment to the Sigsbee Deep. Once the break in slope was encountered, velocities diminished rapidly. Such a velocity diminution resulted in the Sigsbee Deep constantly being supplied by currents representative of the lower flow regime. Thus the selective and relatively rapid degradation of undulatory quartz is more marked in that physiographic pr vince dominated by traction currents.

Other sample traverses in fluvial, shallow-marine, and deltaic complexes confirm the relative instability of undulatory quartz. The total quartz extinction assemblage also is affected by variations in hydrodynamic conditions between environments within such a complex, yielding an additional tool for the reconstruction of paleoenvironments.

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Copyright 1997 American Association of Petroleum Geologists