Iron-Stained Quartz to Distinguish Holocene Deltaic from Pleistocene Alluvial Deposits in Small Core Samples

Abstract

The recorded proportions of three sand-size quartz grain types- clear, partially stained and fully stained- are used to distinguish Holocene from Pleistocene sediment in small core cuttings and washings of borings recovered in the Nile and Ganges-Brahmaputra deltas. To verify the general applicability of the method, we selected for study 343 samples from 13 cores in these two deltas that formed in markedly different geographic and climatic settings: the Nile delta in the highly arid and warm to hot region of Egypt; and, in contrast, the Ganges-Brahmaputra delta in the humid hot region of India and Bangladesh. Analyses indicate that pigments are composed of iron oxides, likely including goethite. Samples of late Pleistocene alluvial sediment show consistently higher proportions of both partially and fully coated quartz types than in the overlying Holocene deltaic deposits. This finding prevails regardless of factors such as sediment process, transport distance, lithology of sediment section examined, and sand grain texture. On the basis of actual measurements made in the present investigation, it appears possible to recognize Holocene and Pleistocene strata by counting the 3 sand-sized quartz grain types (325-350 grains/sample) in as few as 10 to 20 small samples that are distributed downcore in a minimum of 3 to 6 borings. The method is simple, rapid, inexpensive, and particularly well suited for stratigraphic interpretation of sediment sections that are incomplete and/or poorly dated.