Colluvium ‒ the ugly duckling of clastic sedimentology

Colluvium (or talus) is a general term for clastic slope-waste sediments, typically coarse grained and immature, deposited in the lower part and foot zone of a mountain slope or other topographic escarpment and brought there... [ view full abstract ]

Colluvium (or talus) is a general term for clastic slope-waste sediments, typically coarse grained and immature, deposited in the lower part and foot zone of a mountain slope or other topographic escarpment and brought there chiefly by sediment-gravity processes. Colluvial depositional systems abound in the modern Holocene landscape, but are a little-explored frontier of clastic sedimentology, hardly mentioned in textbooks and virtually ignored in the development of clastic facies models. Colluvial deposits have drawn little sedimentological interest by being generally discarded as nearly intractable ‘chaotic slope breccias’. At the lack of criteria for recognition, relatively few ancient cases of colluvium have been reported, albeit ranging from Pleistocene to Precambrian. The interpretive labels used vary from ‘basin-margin breccias’ and ‘proximal fanglomerates’ to ‘debris slopes’, ‘slope-waste aprons’ and – in connection with fault escarpments – ‘footwall fans/aprons’.
The present review of colluvial processes and facies is based on a detailed sedimentological study of modern and older Quaternary colluvial systems in the western Norway, central Greece, south-central Turkey, northern Croatia and north-western Spain. The main processes of colluvial sedimentation are ‘avalanches’, rapid mass movements including debris falls, various debris flows and debris-bearing snow flows, with a contributing role of debris creep, water sheetwash and minor channelized waterflow. Debris flows may be watery or slushy, typically cohesive, characterized by a low to high apparent viscosity; or may be cohesionless, dry or semi-dry. Common modifying processes include deposition of wind-blown silt or fine sand and the infiltration of gravel frameworks with fine-grained sediment by percolating water (illuviation). The relative role and time incidence of the various processes depend strongly upon the local weather (reflecting regional climate) and slope conditions (lithology and morphology). However, the time-stratigraphic local changes in sedimentation processes appear to be regionally correlative and can be attributed to climatic changes. Colluvial fans/aprons are closed-end systems, with a considerable import and negligible export of sediment and with relatively little contemporaneous reworking of deposits, and hence are capable of recording legible signatures of regional climatic changes. The sedimentation is highly episodic, merely ‘probing’ the local weather conditions and allowing long-term changes in processes to be recorded within relatively small thicknesses of deposits.
Colluvial systems may not necessarily react to the same main atmospheric factors that tend to be immediately recorded by plant ecosystems or lacustrine hydrological systems. This is because some climatic thresholds have to be reached, in terms of precipitation, runoff and air-temperature conditions, before a recognizable change in sedimentation processes will occur. Colluvial systems are thus more likely to respond to major climatic changes, rather than short-term minor climatic fluctuations. Furthermore, the local slopes – differing in their lithological and morphological conditions – may have different thresholds of response to a particular climatic change, and this variation will inevitably add ‘noise’ to a regional-scale record. The climate-change resolution of colluvial record may thus not be the same as that of palynological or limnological record, although the latter are prone to other kinds of ‘noise’ (local response irregularities) and are not necessarily always superior.