ABSTRACT

Examination of a seif dune, an interdune area, and an adjoining serir near Sebhah Oasis in Libya indicates marked differences in texture and structure that should assist in the recognition of such deposits where they are preserved in the geologic record. The seif dune is largely composed of rounded, moderately well sorted, fine- to coarse-grained sand forming large-scale cross strata which dip at high angles in two nearly opposite directions. The interdune sand is consistently less well sorted and forms horizontal laminae or strata dipping at low angles. The serir characteristically contains clay, silt, and lag gravels, in addition to sand, giving the samples a double maxima in grade size analyses; it is poorly sorted and is comprised of low angle to horizontal strata, commonly contai ing carbon fragments or other impurities.

Comparison of stratification in the seif dune with that in barchan dunes suggests that it should be possible to differentiate between these two types, where they are represented in ancient rocks, through an analysis of dip directions. Distribution of the steeply dipping cross strata of seif dunes to form two groups, with essentially opposite dip directions, contrasts with the general unidirectional dips of the steeply dipping cross strata in barchans.