X-ray mineralogical discrimination of depositional environments of the Krishna Delta, Peninsular India

Abstract

ABSTRACT X-ray mineralogical analyses were applied to 107 surface and vibracore samples from the Krishna Delta of peninsular India to determine whether mineralogical assemblages produce distinct criteria for the recognition of modern deltaic subenvironments. Standard techniques were used to estimate the quantities of quartz, dolomite, K-feldspar, plagioclase, amphibole, calcite, clinoptilolite, and total clay mineral abundance in each sample. Clay mineral abundance in the m fraction was measured by a technique employing NEWMOD simulated patterns as standards. Discriminant function analysis was applied to 69 of the samples from eight subenvironments (lagoon, river mouth bar, mudflat, barrier island, mangrove swamp, foreshore, tidal creek, and channel) with 100% classification success when the whole-sample results included estimates of ideal clay-mineral layer types and a loss-on-ignition estimate of the organic content. The 38 additional samples were used as an overlay to provide a limited test of the canonical equations established by the first DFA analysis and also were 100% successful in the classification analysis. Most of the mineralogical changes can be attributed to grain-size sorting in the subenvironments of the delta. The major factors contributing to the success of the analysis are associated with quartz enrichment in the subenvironments falling within the marine-dominated part of the delta and smectite enrichment in the riverine-process-dominated subenvironments. The associations revealed by the study of this delta may be generally applicable to the discrimination of subenvironments in other modern and ancient deltas.