Quartz grain assessment for reconstructing the coastal palaeoenvironment

This study proposes a combination of sedimentological techniques as a tool to understand depositional palaeoenvironments. Grain size, mineralogy, compositional data, stratigraphic framework, degree of rounding, optical appearance and microtextures of quartz grains were analyzed; sub-surface sediments were collected from 4 boreholes spaced across coastal settings, from the beach towards the continent, from locations on the south-central coast of the state of Espirito Santo, southeast Brazil. Five palaeoenvironments were identified: fluvial (characterized by gravelly sand fades, composed predominantly of sub-angular and sub-rounded dirty quartz grains with microtexures caused by sudden impact and grain surfaces modified by chemical action, as well as other non-quartz terrigenous minerals); continental deposit with marine influence, such as an estuary (characterized by muddy sand facies, composed of immature grains with natural glow and non-abraded grains with a fresh clean surface, little to no chemical change, a few bioclastic fragments, carbonate nodules and grains embedded with carbonate); modern estuary (characterized by sandy mud facies, composed of mixed mature and immature quartz grains, chemically frosted, bioclastic fragments, carbonate nodules, and high organic matter content); bay (characterized by sandy mud and mud fades, composed predominantly of mature grains, highly chemically frosted, with microtextures clearly associated with post-depositional alteration, many bioclastic fragments, and organic matter); and beach (characterized by gravelly sand facies, composed predominantly of sub-rounded dirty grains, followed by shiny grains, with smooth edges, signs of former impact, little chemical dissolution on the quartz grain, and bioclastic fragments). The association between the degree of rounding, optical aspect and microtextures of quartz grains was essential to estimate the extent and strength of seawater intrusion in filling of the sedimentation basin. The palaeoenergy of each deposition was not always clearly recognized using only statistical particle size distribution parameters and compositional data. (C) 2016 Elsevier Ltd. All rights reserved.