Diagenetic control on mineralogical suites in sand, silt, and mud (Cenozoic Nile Delta): Implications for provenance reconstructions

Abstract

This Nile Delta case study provides quantitative information on a process that we must understand and consider in full before attempting provenance interpretation of ancient clastic wedges. Petrographic and heavy-mineral data on partly lithified sand, silt, and mud samples cored from the up to 8.5 km-thick post-Eocene succession of the offshore Nile Delta document systematic unidirectional trends. With increasing age and burial depth, quartz increases at the expense of feldspars and especially of mafic volcanic rock fragments. Heavy-mineral concentration decreases drastically, transparent heavy minerals represent progressively lower percentages of the heavy fraction, and zircon, tourmaline, rutile, apatite, monazite, and Cr-spinel relatively increase at the expense mainly of amphibole in Pliocene sediments and of epidote in Miocene sediments. Recent studies have shown that the entire succession of the Nile Delta was deposited by a long drainage system connected with the Ethiopian volcanic highlands similar to the modern Nile since the lower Oligocene. The original mineralogy should thus have resembled that of modern Delta sand much more closely than the present quartzose residue containing only chemically durable heavy minerals. Stratigraphic compositional trends, although controlled by a complex interplay of different factors, document a selective exponential decay of non-durable species through the cored succession that explains up to 95% of the observed mineralogical variability. Our calculations suggest that heavy minerals may not represent >20% of the original assemblage in sediments buried less than ~1.5 km, >5% in sediments buried between 1.5 and 2.5 km, and >1% for sediments buried >4.5 km. No remarkable difference is detected in the intensity of mineral dissolution in mud, silt, and sand samples, which argues against the widely held idea that unstable minerals are prone to be preserved better in finer-grained and therefore presumably less permeable layers. Intrastratal dissolution, acting through long periods of time at the progressively higher temperatures reached during burial, can modify very drastically the relative abundance of detrital components in sedimentary rocks. Failure to recognize such a fundamental diagenetic bias leads to grossly mistaken paleogeographic reconstructions, as documented paradigmatically by previous provenance studies of ancient Nile sediments.