THE END OF THE TERMINAL FAN (MODEL)

We propose the so-called 'terminal fan' facies model should be abandoned since it is flawed on several counts and it is leading to misunderstanding and miscommunication. Rivers in drylands may experience excessive downstream discharge reduction so that they terminate sub-aerially rather than reach the sea or a lake. This facies model predicts the distal reaches of such rivers form a network of relatively straight, bifurcating distributary channels producing a fan-shaped sediment body, with downstream thinning and fining of sedimentary units, ending in sand-filled ribbons encased in muds. The model is gaining favour despite being dependant solely on disjointed rock record interpretation for support.

Extensive review of modern rivers has failed to turn up convincing examples that fit the model. Rivers in drylands do not ubiquitously terminate in fans. Fan-shaped fluvial bodies are common wherever rivers are released from confinement and the discharge conditions promote frequent avulsion. Streams on such fans generally do not repeatedly bifurcate downstream. Where they are seen to do so, it can usually be shown they are relic deltas from wetter times. The term 'distributary' is being used carelessly.

The fluvial fan is a composite body resulting from frequent nodal avulsions in a setting with ample horizontal accommodation space. Streams on fans range in planform as much as any other river. The resultant sedimentary record will differ little from that expected from non-fan fluvial systems, including downstream fining, except having a regionally radiating orientation when viewed over geological time scales. There is no distinctive 'terminal fan' sedimentary succession.