The volcano-sedimentary rocks of the Semenof block of central Yukon have not been closely studied in the past. Recent bedrock mapping in the southern Semenof Hills highlights the presence of possibly two exceptionally well preserved, undeformed and unmetamorphosed volcanosedimentary sequences of Late Paleozoic age. The main sequence is composed, from bottom to top, of (1) 2- to 3-km-thick thinly bedded fragmental volcanic rocks interbedded with few limy intervals, (2) thick massive plagioclase- and clinopyroxene-phyric basaltic lava fl ows with clastic intervals, and (3) a 1- to 3-km-thick volcanic conglomerate. The other sequence, of lesser extent, consists of
(1) a thin quartz-porphyritic felsic volcanic unit, less than 50 m thick, (2) 2- to 3-km of massive to pillowed fine-grained basaltic lavas, and (3) 100 m of fossiliferous Upper Carboniferous limestone. These two sequences sit in faulted contact on ~2 km of a deformed clastic sequence of unknown affinity.
