Application of Laser Diffraction Grain-Size Analysis to Reveal Depositional Processes in Tidally Influenced Systems

Summary

The differentiation of depositional environments using the grain-size distribution patterns of estuarine channel sediments is investigated in two parallel depositional environments of different ages: the Cretaceous McMurray Formation in the Western Canadian Sedimentary Basin; and the Miocene Pebas Formation in the Amazonia foreland basin. Channel sediments from fluvial-dominated coastal plain/inner estuary settings are compared with marine-influenced middle estuarine channel deposits. The upward decreasing grain-size trend and simultaneous change to more poorly sorted grain-size distributions of the coastal plain estuary depositional sequence indicate the transition from a fluvial-dominated regime to a mixed fluvial and tidal regime. The fluvial-dominated coastal plain/inner estuary sands have clear bimodal grain-size distributions with a minor mode in coarse sand. They are coarser grained, better sorted and more negative skewed than the inclined heterolithic stratified channel sands of the mixed fluvial and tidal regime of the middle estuary. X-ray diffraction results show that decreases in the proportion of kaolinite from fluvial to estuarine sediments and then a further decrease to shoreface sediments combined with a simultaneous increase in the proportions of smectite, illite and chlorite are indications of the change of the depositional environments from riverine to more marine during the ongoing transgression.