[H-GM04] Geomorphology

[HGM04-05] High mountains entirely covered by granitic boulders and blocks in the northeastern part of Australia

★Invited Papers

*Masahiro Chigira¹, Makoto Kaneko¹ (1.Fukada Geological Institute)

Keywords:Granitoid, Colulmnar joints, Boulders, Black Mountain

Boulder fields are a typical feature of granitoid landscapes. Previously reported boulder fields are typically an accumulation of boulders on gently undulating surfaces; however, there are 300-500 m high mountains entirely covered by granitic boulders and blocks in the northeast of Australia. The Black and Melville Mountains are underlain by Permian granitoid rocks intruded into Silurian–Devonian strata. We performed field surveys, UAV observations, and satellite image analysis of these mountains using Digital Elevation Models (DEMs) obtained from satellite data. Melville Mountain has low-relief flat-top surfaces surrounded by steep slopes covered by rounded boulders with (sub)angular blocks. The rounded boulders are assumed to have been formed by subsurface spheroidal weathering of the rock columns below the low-relief surfaces. The low-relief surfaces, which are likely uplifted after their formation, are not vertically incised because of boulder armors that easily drain water but are eroded laterally by the sea and river to form steep slopes that surround the low-relief top surfaces. The steep slopes are covered by boulders originating from low-relief top surfaces and rock blocks from rock columns below the zone of spheroidal weathering. Black Mountain lacks low-relief top surfaces and comprises steep slopes covered with rock blocks. Rounded boulders on Black Mountain exist at the feet of steep slopes and are mixed with rock blocks. The boulders are probably from previous low-relief top surfaces that are now denudated. Columnar joints, spheroidal weathering near the ground surface, uplift, and lateral erosion are essential for the formation of high mountains covered by boulders and granitoid blocks.