PRESERVATION OF PRE-VEGETATIONAL MIXED FLUVIO-AEOLIAN DEPOSITS IN A HUMID CLIMATIC SETTING - AN EXAMPLE FROM THE MIDDLE PROTEROZOIC ERIKSFJORD FORMATION, SOUTHWEST GREENLAND

Sedimentological studies of a 30 m thick coastal cliff section within the Middle Proterozoic Eriksfjord Formation in western South Greenland reveals three distinct types of fluvial sand sheet deposits that refl ect perennial streams (Type I), semi-perennial streams (Type II), and ephemeral flash floods (Type m). Perennial river sand sheets are chara cterised by co-sets of medium-scale trough cross-beds, interbedded wit h isolated medium- and large-scale, high-angle, tabular cross-beds. In dications of desiccation or subaerial exposure are absent. Semi-perenn ial fluvial sand sheets consist predominantly of low-angle cross-beds, interbedded with isolated sets of high-angle tabular cross-beds with common reactivation surfaces. Horizontal lamination and climbing rippl e lamination form subordinate structures. Associated with the sand she ets are adhesion structures and 0.05-0.4 m thick sets of wind ripple-l amination indicating periods of subaerial exposure and aeolian reworki ng. High-energy ephemeral flash flood sand sheets consist almost exclu sively of planar-parallel lamination and climbing ripple lamination wi th some isolated sets of low-angle cross-bedding. Scouring and interna l truncation surfaces are common. The three types of sand sheets are c onsidered to reflect deposition under changing climatic conditions, va rying from humid to arid or semi-arid. Aeolian deposits are preserved within the sand sheets showing characteristics of dominantly perennial flow punctuated by shorter periods of desiccation (Type II), while sa nd sheets showing features typical of arid and or semi-arid flow condi tions (Type m) contain no preserved aeolian deposits. This selective p reservation is interpreted to be a result of the combined effect of gr oundwater table level and fluvial style which in turn are inferred to have been controlled by the climatic regime. The deposits show that du ring pre-vegetational times the preservation of aeolian deposits, unde r certain conditions, may be more optimal in fluvial systems formed in a humid climate than in fluvial systems formed under semi-arid or ari d circumstances. The occurrence of aeolian deposits within a Precambri an succession of fluvial deposits therefore, need not be an indication of the most arid environmental conditions. (C) 1998 Elsevier Science B.V, All rights reserved.