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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 66 (1982)

Issue: 5. (May)

First Page: 573

Last Page: 573

Title: Dispersal and Provenance of Terrigenous Sand by Fourier Grain Shape Analysis, Northern Puerto Rico: ABSTRACT

Author(s): Emery D. Goodman, Robert Ehrlich

Article Type: Meeting abstract

Abstract:

The narrow, steep, north insular shelf of Puerto Rico contains terrigenous sands and silts derived from rivers originating in nearby mountains. This compact system provides an ideal setting for the study of sand sized sediment dispersal and provenance in coastal and nearshore environments using Fourier shape analysis.

Quartz sand shape on the north shelf reflects the lateral change in source-rock composition. The shelf can be divided into a western part containing dominantly angular quartz grains and an eastern part containing rounded abraded quartz sand. Rivers feeding the western shelf drain a significant body of granitic rock, whereas rivers to the east drain a quartz-poor, volcanic rock terrane. Rounded grains on the eastern shelf are mostly relict.

Quartz sand shapes do not change significantly with changing size either upstream or on the shelf. A size-shape dependence does exist in downstream, river mouth, and slope environments (fine sand is more angular than coarser sand) and on beaches (fine sand is more abraded than coarser sand).

It is significant that environments in which size-shape relationship exists are also sites of mixing between sands of different origins and transport histories. For example, samples studied from downstream and river mouth areas indicate mixing between fine-grained, angular fluvial sand, and coarser, smoother residual sand. Mixing occurs because fluvial sand is deposited on the shelf only when storm-induced flooding results in river-mouth bypassing.

This study suggests that investigations into the origin of size-shape relationships can aid in understanding climatically controlled, short-term changes in terrigenous output.

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