Alluvial Fan and Fluvial Deposits
Subaerial and subaqueous debris flow, streamflood, sheetflood, sieve and low-energy shoreline deposits have been documented within six alluvial fan-fan delta bodies, representative of the various fanglomerate sequences fringing the fault margins of Hornelen Basin (Devonian), western Norway. Waterlaid beds are recognised largely by their internal stratification and sorting, sieve deposits by their geometry, sorting and ‘open’ or bimodal texture, while shoreline-modified conglomerates are well sorted and have a “fitted” fabric. Subaerial and subaqueous debris flows are distin-guished from each other on the basis of bed thickness/grain size ratio, texture, degree of grading and imbrication, and association with other facies.
Mapping and analysis of six representative fan bodies shows that: (1) In mixed fans the debris flows tend to give way distally to sheetflood and stream deposits. (2) In debris flow dominated fan deltas the subaqueous portion occurs as a distal facies of diamictite-like deposits generated by mixing with and resedimentation in the impinging lacustrine environment. (3) Coarseness (maximum particle size) and thickness of beds generally increase both proximally and upwards within the fan bodies, though this happens much more evenly in fluvial fans. (4) All fan bodies thin distally, though the geometry and manner of thinning varies, dependent primarily on the relative rates of aggradation of fan and adjacent alluvium.
Conglomerate beds and sequences of beds have been used to draw some tectonic inferences for the accumulation of Hornelen Basin's 25 km thick sediment pile. The all-pervasive, small-scale (5-25 m) C-U sequences reflect a general background of rapid basin subsidence. The larger scale (100-200 m), basinwide, onlapping C-U sequences are thought to record progradational intervals between periods of lateral migration of the basin and may therefore be an important characteristic of basins generated by strike-slip faulting. The proposed horizontal component of movement is recorded either by hiatuses (and occasionally a marked unconformity) between adjacent major C-U sequences or by a F-U capping to such sequences. Differences in fan facies, geometry and thickness between northern and southern margins of the basin indicate more rapid subsidence against the northern margin suggesting that this was the major tectonic zone.