Landform Evolution in the Armidale-Uralla Region, New South Wales

Abstract

This thesis aims to explain the geomorphic history of the Armidale-Uralla region in New South Wales. A subsidiary aim is to assess the validity of six theories and concepts of landscape development; and to set the geomorphic history of the Armidale-Uralla region into the east Australian context. Detailed field mapping showed that the Tertiary basalts in the study area are mainly valley-fill basalts, now relief-inverted. Their minimum age range is 33-22 my, and they may have been extruded from regional vents in the Glen Innes area, 60-80 km to the north. Ferricrete occurs as both subsurface horizons and as surface lag deposits, and is usually associated with basaltic soil. It is often vesicular and nodular, with quartz sand. It is post-basaltic, and has formed by the mobilisation and concentration of iron minerals in basaltic soil profiles. There is no single 'ferricrete surface' in the region. The three main silcrete types are: silicified sorted clasts; silicified poorly sorted clasts; and silica-cemented brecciated chert. Silicified clasts appear to be mainly the result of post-basaltic silicification of pre-basalt stream sediments, many of which were probably covered by basalt flows before silicification. There is no evidence of a direct genetic relationship between the basalt flows and silicification of the sediments. Thin section examination of quartz grains in ferricrete and silcrete suggests grains are derived from both granitic and vein sources. Silica-cemented brecciated chert has developed by silicification of fractured chert bedrock.