Macrotexture and grain size characteristics of lagoonal and riverine coastal deposits along the Southeastern coast of Sri Lanka: implication for paleoenvironment

Abstract

Beach placers are concentrations of economically potential minerals formed by surface earth processes and are identified as one of the most easily exploitable mineral deposits in the world. We studied the placer and non-placer sediments in lagoonal and riverine beaches of the southeastern part of Sri Lanka to unravel their specific sedimentary features, depositional settings and paleoenvironments. Swash zone deposits (105) were investigated for their surface microtextures, granulometry and their identification. Near the lagoonal outlets, black-coloured placers are reported with abundant ilmenite and accessory zircon, while red-coloured almandine placers are founded in the riverine areas. Placer deposits are composed of coarse-skewed leptokurtic to platykurtic fin sand and evidenced for dominant bottom suspension mechanisms on the weak wave-generated depositional agent. Quartz grains (240) from placer and siliciclastic sediments showed the presence of 25 pre-defend microtextures indicating the influences from sub-aqueous beach, fluvial, aeolian and chemical alteration processes. From siliciclastic to red placer to black placer sediments, decreasing order of source-sink distance and increasing order of fluvial influences are resulted. Cracks and solution pits are higher in the sediments of placer deposits revealing their deposition in the steady low energy environment. The crosscut relations indicate prevailed pre-aeolian processes and post-chemical alteration processes on placer deposits. Large conchoidal fractures, arcuate and straight steps microtextures, and mineralogical contents of the placer deposits infer the crystalline rock sources of granitic gneiss and garnet-bearing granulates of the Precambrian Vijayan Complex and Highland-Vijayan tectonic boundary zone.