Soil-landform relationships on Bullock Creek fan, North Canterbury

Abstract

Steep slopes prone to mass wasting processes such as debris flows cause extensive damage on hill country farms and adjacent lowlands. Fan activity is episodic and can be deciphered using soil stratigraphic techniques. Bullock creek fan is an active, debris flow dominated fan, covering 300 hectares. The fault shear zone on the eastern flank of Mount Thomas provides an abundant and easily eroded source of brecciated siltstones and highly fractured sandstones. The resultant fan sediments are mainly sandy matrix supported, gravelly to bouldery debris flow deposits and finer, massive or laminated sheet flood deposits. Five geomorphic surfaces have been recognised. The soils that characterise the three late Holocene surfaces, displayed increasing development from Fluvial Recent Soils (Entisols-USA), to Stony Brown Soils (Inceptisols-USA). Orthic Brown Soils (Aquepts-USA) formed in mixed loess and gravelly alluvium occurred on a mid to early Holocene surface, and Fragic Pallic Soils (Fragiaquepts-USA) were formed in 2.5 metres of loess overlying a Late Pleistocene surface. In areas of thinner, sheet flood deposits, on the younger geomorphic surfaces, multisequal soil profiles with evident buried horizons were more common than unisequal soil profiles. By comparison, the thicker, debris flow lobes displayed a gradient of multisequal to unisequal soil profiles, from the edge to the centre of the lobes. Much of the detailed history of recent fan activity could be interpreted from the stratigraphy and distribution of multisequal soil profiles. With increased soil development and surface age, multisequal soils evolved to take on the morphological characteristics of unisequal soils and the details of earlier fan activity could only be broadly interpreted. The current period of fan activity has been occurring since the early 1900's, producing a surface, dominated by Fluvial Recent Soils, which covered 60% of the fan. In excess of ten deposition events were recognisable from the stratigraphy in the multisequal Fluvial Recent Soils. There has been significant aggradation on the mid and lower segments of the fan. Sedimentation, since 1978 has been largely confined to the back-filling of a fan head trench and valley floor. Soil development and the re-establishment of pastures on the aggrading fan surface had been enhanced by the texture and natural fertility of the sediments. Sheet flood deposits produced more easily cultivated soils because of their finer and more uniform soil textural properties and better moisture regime. Further phases of fan aggradation, caused by erosion of stored sediments in the valley floor and fan head, are anticipated in coming decades.