Topography, Geology & Soils
The topography of DVCA is rugged, but mostly at moderate elevation, with over 90% of the land below 760m asl - the upper limit for lowland dipterocarp forest. There are three major summits within DVCA; Mount Danum (1093m) in the centre, Mount Nicola (917m) which is part of the Brassey Range forming the northern perimeter of the Conservation Area, and Mount Tribulation (861m) in the upper Segama highlands to the south west of DVCA. The local topography is undulating with short but steep slopes with angles generally 10-25º but locally up to 40º, especially on the higher hills.
As in many parts of the tropics, the dense vegetation cover and deeply weathered soils restrict the occurrence of rock outcrops and only the general geology of the area has been investigated and mapped. The three main geological formations found in the DVCA are Crystalline Basement, Chert-Spillite and Kuamut:
- Crystalline Basement includes the oldest rocks in the region - metamorphic and igneous rocks of the Lower Triassic age.
- The Chert-Spillite formation originates from the late Cretaceous to the early Tertiary and overlays or forms fault contact with the Crystaline Basement. This formation outcrops as a belt across the central region of DVCA, to within 2km of the field centre.
- The Kuamut formation (Lower to Upper Miocene) occupies regions of low relief in DVCA including the area close to the Field Centre. The Kuamut Formation consists of slumped sedimentary and volcanic rocks with interbedded sandstone, mudstone and tuffs, known collectively as slump breccia.
The complex geology of the area has led to the formation of locally very heterogeneous soils. The soils of DVCA have been surveyed to a 1:250,000 scale and at that level include four soil associations, although 85% of the area is covered by just two (Mentapok and Bang). Mentapok soils (found in the more rugged areas of DVCA) develop over the basic and igneous rocks of the Crystalline Basement. Bang soils (found near the field centre) develop over the Kuamut formation. These soils are classified in the USDA system as Ultisols (equivalent to the Acrisol-Alisol groups, in the FAO-UNESCO system). Close to the Field Centre loams and silt-loams are dominant with sandy loams are characteristic of steeper slope sites deep in the Conservation Areas. Soil depths are generally at least 1.5 m and often considerably deeper. In undisturbed primary forest bulk densities are generally 0.84-0.94 g/cc for surface soil and 1.03-1.28 g/cc at 20 cm depth, but in severely eroded or compacted terrain in logged forest (e.g. unsurfaced logging roads and landslide scars) bulk densities of 1.20-1.34 g/cc are typical.
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