How are Quaternary alluvial deposits differentiated in subglacial latitudes?
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 alluvial plains; identified primarily by fan morphology and topographic expression. Internal contacts delineate individual alluvial fans
Qof Alluvial fan deposits (Pleistocene)—Discontinuous or highly dissected deposits of semiconsolidated, moderately to poorly sorted layers of silty clay, silt, sand, and gravel


The pamphlet also states that &quot;unit ages [...] reflect local stratigraphic relations&quot;, so they are relative ages, not absolute dating.