Terminal fans-a review with reference to Devonian examples

Terminal fans occur where sediment-laden streams decrease in size and vanish as a result of evaporation and transmission losses. They tend to form in arid or semi-arid regions which are characterized by a moisture deficit. Distributary channel patterns are characteristic of terminal fans, and reflect both loss of stream power and spatially/temporally fluctuating discharge. In a number of Devonian basins, terminal fan sediments form conspicuous sequences with examples from Spitsbergen, England, Ireland and Greenland. Examples of terminal fan systems from the Northeast Greenland Basin and the Munster Basin in Ireland are presented in this paper. The ancient examples are used in combination with modern distributary systems to construct a simple facies model for terminal fans and their deposits. The model includes a tripartite zonation of terminal fans into feeder, distributary and basinal zones. The feeder zone is characterized by large channel bodies associated with interchannel fines. An increase in channel body frequency may occur at the transition from the feeder zone to the distributary zone, reflecting the downstream multifurcation of channels. The distributary zone is characterized by a downstream decrease in both the scale and frequency of channel deposits, which are mainly replaced by sheetflood deposits. This is the result of the decline of both water depth and stream power downslope. Further evidence of terminal fan systems is the downstream transition from distal to basinal zone deposits of floodbasin, playa mudflat or aeolian origin, reflecting the absence of a terminal base level in the form of a lake or the sea.