They seem to be differentiated according to their morphology, Qof deposits being highly dissected and more consolidated than Qyf, thus older. From a random pamphlet of the map series:
Qyf Alluvial fan deposits (Holocene)—Unconsolidated, heterogeneous layers of sand, silt, and gravel; relatively undissected; deposited by streams emanating from canyons onto ...
This map should address at least part of your question
The here documented GIS map and dataset contains a collection of Last
Glacial Maximum (LGM, ~21k yBP) paleoenvironmental data. It is the
first result of a project that aims to acquire, produce and publish
GIS datasets from non-GIS based sources such as analogous maps,
textual informations or figures of ...