Differential features of alluvial fans controlled by tectonic or eustatic accommodation space. Examples from the Betic Cordillera, Spain

Abstract

The creation of accommodation space in subaerial sedimentary environments is closely linked to uplift the source area (along basin–margin faults) or to a rise in base level. Depending (amongst others) on the relative importance of these two factors, the margins of terrestrial sedimentary basins (where alluvial fans develop) produce aggradational, progradational or retrogradational stratigraphic architecture. We studied a total of 43 Quaternary alluvial fans in the Granada and Bajo Segura basins (Betic Cordillera, Spain). The analysis of their stratigraphic architecture and sedimentary facies has revealed the existence of three groups characterised by distinct stratal stacking patterns, which is the result of differences in eustasy and tectonics. The qualitative and quantitative analysis of these alluvial fans and their drainage basins has allowed us to define certain characteristic geomorphological patterns related, in each case, to the main mechanism operative in the creation of accommodation space. These patterns are described as follows. (1) Alluvial aggradation is the main result of high tectonic subsidence with the development of quite open, but small fans with a constant steep slope lacking incised channels and headward-eroding gullies. (2) In contrast, when the principal mechanism in creating accommodation space is low tectonic subsidence, progradation occurs and the fans are narrower and larger. Moreover, the slopes of such fans are less steep in relation to the drainage area than in the above case, being constant or even concave in longitudinal profile. They also have short, deep incised channels, culminating downfan in a depositional lobe. In inactive zones, long, deep headward-eroding gullies develop. (3) Areas of extremely low tectonic subsidence and base level rise produce very open fans in the sedimentary basin, although with a pronounced mountain embayment, which gives them a characteristic “mushroom” shape in plan view. In the latter case, retrogradation is the typical stratigraphic stacking pattern and the fans are quite extensive with respect to their drainage areas, usually showing a segmented longitudinal profile with a low gradient. The feeder channels do not surpass stream order 3, and various extremely long and shallow incised channels can appear. (4) Contrary to the case of fans developing under a regime of subsidence, when the accommodation is due to base level rise, the fan slopes bear no relation to the extent of the catchment basins. In such a case, the overall fan slope is strongly influenced by the extent of the mountain embayment because it determines the distributions of debris and sheet-flows over the alluvial surface.