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 t o form in arid or semi-arid regions which are characterized by a moist ure deficit. Distributary channel patterns are characteristic of termi nal fans, and reflect both loss of stream power and spatially/temporal ly 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 pre sented in this paper. The ancient examples are used in combination wit h modern distributary systems to construct a simple facies model for t erminal fans and their deposits. The model includes a tripartite zonat ion of terminal fans into feeder, distributary and basinal zones. The feeder zone is characterized by large channel bodies associated with i nterchannel fines. An increase in channel body frequency may occur at the transition from the feeder zone to the distributary zone, reflecti ng the downstream multifurcation of channels. The distributary zone is characterized by a downstream decrease in both the scale and frequenc y of channel deposits, which are mainly replaced by sheetflood deposit s. This is the result of the decline of both water depth and stream po wer downslope. Further evidence of terminal fan systems is the downstr eam transition from distal to basinal zone deposits of floodbasin, pla ya mudflat or aeolian origin, reflecting the absence of a terminal bas e level in the form of a lake or the sea.