Stratigraphic evolution of heavy-mineral provenance signatures in the sandstones of the Wajid Group (Cambrian to Permian), southwestern Saudi Arabia

Abstract

ABSTRACT The Wajid Group of southwestern Saudi Arabia consists of a dominantly sandy succession of Cambrian to Permian age that spans several discrete phases in the tectonic evolution of the Arabian Peninsula. The principal aim of this study was to determine whether successive changes in the tectonic setting are reflected in changes in provenance-related mineralogy. Because of the relatively limited compositional range of the Wajid sandstones, heavy-mineral assemblages have been used as the primary tool for assessing changes in provenance signature. A comparison of heavy-mineral and petrological data has, however, also been carried out. Variation in the relative abundances of zircon, rutile, monazite, tourmaline and apatite has revealed significant changes in provenance signature between the Dibsiyah (Cambrian–Ordovician), Sanamah (Ordovician–Silurian), Khusayyayn (Devonian–Carboniferous) and Juwayl (Carboniferous–Permian) sandstones. Since previous studies have established that northward-flowing rivers deposited the fluvial sandstones of the Wajid Group, it appears that the source area lay to the south. In the absence of data from the region to the south, it is not possible to identify specific source areas. It is clear, however, that the successive changes in provenance signature must reflect exposure of new source rocks through progressive denudation, changes in the pattern of tectonic uplift or changes in the drainage system. It is also possible that some of the observed mineral variation is related to lateral influx of sands through long-shore drift during times of high sea level. Two distinct mineral compositions occur within the Dibsiyah sandstones, indicating that a major change in provenance took place during deposition of the Upper Dibsiyah sands. The boundary between the Dibsiyah and Sanamah formations is sharply defined, although the overall composition of the Sanamah sandstones is in many respects similar to that of the Dibsiyah sandstones. There is a relatively small difference in composition between the Sanamah sandstones and the associated diamictites. A major change in provenance is indicated at the base of the Khusayyayn Formation, with an increase in the proportion of monazite and staurolite. This change in composition persists into the Juwayl Formation although the greater variability displayed by the Juwayl heavy-mineral assemblages indicates contribution from several sources. Heavy-mineral assemblages in the Juwayl sandstones are comparable to those of the Unayzah C and B sandstones of central Saudi Arabia, but differences suggest mixing between a southern (Juwayl) and western (Shield) source for the Unayzah sandstones. Compositionally, Wajid sandstones range from quartz arenite to arkose. Comparison of the petrographic and heavy-mineral data is hampered by the different grain-size ranges studied. However, it would appear that samples with similar heavy-mineral provenance character do not necessarily possess similar feldspar percentages, even when the latter are corrected for in-situ kaolinization. The data set is too small to establish an explanation for this apparent discrepancy.