ABSTRACT

Inherited quartz overgrowths represent remnants of cement adhering to a grain's surface from a previous sedimentary cycle and can be recognized in petrographic studies on the basis of textural criteria. They may occur as rounded, double, or penetrating overgrowths. Since these features occupy only a part of a grain's surface and are randomly positioned, when studied in two-dimensional sections the likelihood of their being transected and observed is significantly less than their whole-rock abundance. This has resulted in an underestimation of their importance as a provenance indicator. Probability that a random cut will transect a cap of inherited cement on any particular grain depends upon proportion of the grain's surface covered by the cap, position of the cap on the grain's surfac , and orientation of the cut through the grain. An experiment was devised to analyze the relationship between size of a grain-coating cap and likelihood that it will be transected in a random cut. This involved using an eccentric stereonet, one in which the origin is not at the projection sphere's center, to project random planes. Results of the experiment were plotted as a curve and then derived mathematically. Since size of a coating cap is related to its projection angle measured from the sphere's center, a second experiment was devised to determine the relationship between projection angles and equivalent apparent projection angles seen when a cut does not pass through a sphere's center. It was found that the true projection angle equals the mode of apparent projection angles. Relati nships determined in these two experiments enable calculation of the whole-rock proportion of grains with inherited overgrowths from petrographic data. Although at the present time results must be considered as estimates, this capability leads to a more precise understanding of importance of inherited overgrowths as an indicator of sedimentary provenance. The procedure was applied to samples of Upper Precambrian quartz arenites from the Uinta Mountain Group, the Grand Canyon Supergroup, and the Belt Supergroup, resulting in calculated whole-rock proportions of grains with inherited overgrowths that are, in general, twice the values observed in thin section. This procedure may also be useful in a variety of other applications, including studies of sediment distribution, intrabasinal sedim nt maturation, lithostratigraphic correlation, and rates of grain surface abrasion.