It has been shown previously that different source rocks produce quartz sand grains with unique gross shape characteristics. These gross shape characteristics cannot be altered by abrasion, and are thus an immutable record of sediment source. Therefore, sand grains in a sedimentary basin should contain in the gross shape a "fingerprint" of their source unaffected by the amount of reworking to which the sands have been subjected.

To test this idea, shapes of late Pleistocene-Holocene sand grains from two sources in the northwest Gulf of Mexico (the Rio Grande and western Gulf province) were analyzed with the Fourier grain-shape technique. The results indicate that two gross grain-shape assemblages are present in the sands of these two provinces. One assemblage is associated with a mineralogical suite typical of relatively mature coastal plain sediments, and thus it probably represents the contribution of sediment from such a "coastal plain source terrane." The other assemblage is associated with a relatively immature mineralogical suite more typical of sediments derived

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from crystalline sources, and thus it probably represents the contribution from such a "highland source terrane." Both assemblages are found in sands of both provinces, inasmuch as the major rivers in these provinces, which act as conduits for these sediments, drain both source terrane types. However, the amount of area of these terranes drained by the rivers differs between the two provinces; therefore the relative proportions of these assemblages in the sands of the two provinces also differ. Rio Grande province sands contain on the average 59% of the assemblage associated with the highland source terrane, whereas western Gulf province sands contain only 34% of this assemblage. Thus, it is the difference in the relative proportion of these two assemblages in the samples which can be used to identify and distinguish the sands from the two provinces.

The results of this pilot project indicate that the analysis of gross grain shape can indeed differentiate sediments from different sources in the Gulf of Mexico.

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