GEOLOGICAL CONTROLS ON DEVELOPMENT OF EROSIONAL GULLIES (LAVAKA), CENTRAL MADAGASCAR

Lavaka (the Malagasy word means "hole") are characteristic, extremely conspicuous features in the recently-uplifted highlands in central Madagascar. They occur in deeply-lateritised and saprolitic Precambrian basement rocks, in steep hilly country, on convex slopes that are generally bare of outcrop. Lavaka formation is poorly understood, and has multiple causes; but field measurements from areas north and west of Antananarivo indicate that lavaka occurrence and geometry are controlled by a combination of slope steepness and the attitude of gneissic foliation in the bedrock. The saprolite preserves the gneissic foliation, so that strike and dip measurements were possible in the lavaka interiors. There is a complete continuum in lavaka geometry, but they can be subdivided into simple and complex. Simple lavaka are most common. They are bulbous in shape and strongly symmetric, with a broad head scarp, wide interior basin (with or without internal septa) and well-defined, narrow outflow channel. Complex lavaka include composite or dentritic varieties consisting of multiple coalesced simple lavaka, and asymmetric groups of gullies and ravines, usually with a broader, more valley-like outflow. Simple lavaka form readily in strongly convex slopes where the bedrock (saprolite) foliation dips into the hillside. The gradient in the head scarp region is usually 5-20°, becoming steeper (20-40°) downslope, where the narrow outflow channel is deeply incised; and the fall line of the lavaka is generally normal to the bedrock strike and opposite to the dip direction. To illustrate: if gneissic bedrock strikes north-south and dips west, east-facing hillsides will develop east-west oriented, east-draining simple lavaka, but west-facing (dip-slope) hillsides will not. Complex lavaka are associated with steeply-dipping (85-90°) gneissic foliation, and also tend to occur on slopes that are more gentle (15-25°) or less dramatically concave. Under these conditions, linear or irregular gullies may occur parallel to the foliation, or at an angle to it. Lobes with simple lavaka geometry may form as part of the complex where the local dip of foliation is into the hillside.