The character and occurrence of colluvium-filled bedrock depressions in the Wellington Region

Abstract

Bedrock depressions are convergent forms serving as places of regolith and water accumulation which may, if the material attains critical depth and develops critical pore water pressure, fail in the form of a landslide. The recognition of colluvium filled bedrock depressions (CBDs) as potential sites of natural hazards has prompted further investigation into the character and occurrence of these landforms in the Wellington region. Study areas established in strongly sloping greywacke terrain with a variable cover of regolith, provided cross-sections of CBDs along access roads traversing the hillslopes. Two hypotheses were investigated: one, that a variety of CBDs can be identified on the basis of their morphological, textural and stratigraphic characteristics; and two, that CBDs do not occur randomly within the terrain and that their location is related to specific topographic and geomorphic factors. Eight types of CBDs have been established based on process-related morphological characteristics. Six of these eight types can be conclusively recognised and are represented in the two field areas surveyed, suggesting that formative conditions producing variations in CBDs are relatively widespread. The strength of clast orientation, upslope imbrication, and nature of the colluvium indicate a solifluction genesis. However, comparison of fabric results from samples in the Wellington region with those from a present day solifluction environment, suggests that solifluction was not the only process operating and that several different processes and rates of activity were important. Sedimentology and fabric orientation indicate that the type of processes operating was strongly influenced by topography. CBDs are evenly distributed between side slopes and dells with a few occurring on noses (spurs). Two bedrock forms, V-shaped and broad-based, occur on side slopes and noses. Broad-based CBDs predominantly consist of clean clast fill which is the equivalent to talus material which appears to have entered the depressions. Consistently 'shaved' bedrock surfaces indicate active processes where bedrock has actually been abraded. This and the fact that different filling processes can occur within one depression form suggests that the bedrock depression and the colluvium were formed by separate processes. Air photo analysis, field measurement and detailed stereo plotting has been employed to elucidate the pattern of CBDs in the landscape. The topographic position clearly influences the size, plan shape and integrity of these features. Most important, however, are the clear indications that the CBD pattern represents a subsurface extension of the drainage channel network. Certain factors are established to aid in locating those features which have no topographic expression or where there are no exposed cross-sections. Vegetation differences and concavities in ridge tops prove to be the best indicators of the location of CBDs, although the use of both these factors may be limited by the nature of the vegetation cover. A CBD network is established by mapping dells in the hillslope and is confirmed by the distribution of landslide scars in similar terrain after an intense rainstorm. Explanations regarding formative processes are offered on the basis of the geomorphic history of the area which was, and continues to be, largely dictated by tectonic and climatic conditions.