Are Existing Facies Models Wrong?
Facies models for extensional basins imply that laterally-derived alluvial sediment forms an insignificant part of the basin-fill. In these models an early syn-rift phase is characterised by endorheic drainage with alluvial fans flanking the basin. Fans pass basinwards into central basin deposits which dominate the basin-fill. In the late syn-rift stage continued extension results in linking of isolated half-graben to develop an axial drainage system dominated by fluvial channel and associated floodplain deposits. Recent work in the Basin and Range (USA) suggests that this simplistic approach does not apply in drylands (arid/semi-arid) areas for the following reasons: • Many basins cannot support an axial fluvial system unless sourced from outside the drylands climate area; axial fluvial deposits therefore form a minor component of the basin-fill. • Laterally-derived alluvium (alluvial fan and bajada) dominates the latter stages of the extensional basin-fill as reworking by axial syste
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