Diagnostic Evidence
Seismic
Vertical seismic sections through alluvial fan complexes typically show discontinuous internal reflectors. (Figure 1, Seismic section through a probable alluvial fan that developed over structurally deformed basement.
Note the relatively poor internal seismic character to the deposit.) This should be expected, given the great lateral and vertical variation in lithology. Furthermore, the concave-upward ideal profile of an individual fan is more often subdued in the subsurface, due to postdepositional compaction and tectonic tilting. Bajadas are more extensive and sheet-like and will display only a vague fan-type profile over their width. Where fan development continued for considerable periods of time, the original mountain front may be worn down and completely buried in its own debris; in this case, fan sediments are likely to grade upward into braided stream alluvium, which is also characterized by poor internal reflections. Seismic profiles can be very useful in delineating old mountain fronts and often the extent of the immediately adjacent sediments they produced. Specific lithologic types within the fan itself, however, are not normally discernible.
Wireline Logs
Most geologists maintain that alluvial fan facies generate no important diagnostic response on wireline logs. The diverse processes acting to create and prograde the fan result in considerable vertical and lateral change, which means that log profiles will be correspondingly varied and therefore difficult to correlate. On the other hand, it should not be overlooked that this type of unpredictable log response showing substantial changes between even closely spaced wells, might itself be considered a characteristic of fan deposits.
Dipmeter logs (Figure 2, Idealized gamma ray and dipmeter logs for an alluvial fan sequence, showing both fanglomerate and channel development.
Note the three major patterns: lowest green dips, which represent shale breaks and correspond to spikes on the gamma ray, curve; random “bag o’nails” dips in fanglomerate; and dip clusters that show an upward-increasing blue pattern in channel sands.) will most often show a “bag o’ nails” pattern for fanglomerate. A green (“shale”, i.e., structural slope) pattern may be discernible, but depositional slopes are frequently high (5° to 25° ), and this could confuse correlations. Due to the braided stream flow on a surface of radiating geometry, dip switches are characteristic. Frequency diagram plots, ideally, should show approximately a 180° azimuth sweep with, perhaps, a subdued bisecting maximum that represents the influence of the feeding stream.
Figure 2 shows an idealized dipmeter profile through a fan. Several shale layers are indicated simultaneously by spikes of high radioactivity on the gamma ray log and low dips (green motif) on the dipmeter log. These shales separate three channels, whose tadpole patterns show a clustering at a higher magnitude of dip. This is due to cross-bedding. These channels overlie the random “bag o’nails” pattern of fanglomerate, and thus show the outward progradation of the fan.
Cores
The general coarseness, poor-sorting, clast-angularity, and immaturity of alluvial fan sediments are the conspicuous features which dominate most core samples of the upperfan. Finer-grained, cross-stratified or flat-bedded channel sandstones can also be prevalent, particularly from midfan sediments. It is therefore, the association of these – as well as that of thin shales (mudflows), sand-silt beds (sheet-floods), and gravel layers – which is most prescriptive in terms of what is normally seen in core samples. Such diversity might be found in cores taken from a single well, or from a number of nearby wells.
