PETROLOGY AND PROVENANCE OF THE ARKOSE RIDGE FORMATION ADJACENT TO THE SOUTHERN TALKEETNA MOUNTAINS TERTIARY VOLCANIC FIELD, ALASKA

The Arkose Ridge Fm. in the Boulder and Billy Creek areas of the southern Talkeetna Mountains (just north of the Matanuska Valley) is located on the southern margin of a Tertiary volcanic field where it is interbedded with and overlain by basaltic lavas and felsic and mafic tuffs. Sandstone petrographic data and conglomerate clast counts reveal a progressive change in provenance for the Arkose Ridge Fm. from mostly Mesozoic metamorphic and altered volcanic-plutonic source rocks to Tertiary volcanic-dominated sources. The lower part of the Arkose Ridge Fm. contains an average Lv:Lm:Ls proportion of 48:37:15 where most of the volcanic grains are strongly altered (chloritized, abundant clay minerals). Paleoflow data for the lower part of the Arkose Ridge Fm. at Boulder Ck. shows westward-directed transport. The upper part of the Arkose Ridge Fm. contains an average Lv:Lm:Ls proportion of 83:9:8 where most of the volcanic grains are fresh (little to no alteration -- glassy, clear mineral grains, few clay minerals). Paleoflow data for conglomerate and pyroclastic surge deposits in the upper Arkose Ridge Fm. shows drainage away from the central part of the Tertiary volcanic field. We interpret that the lower Arkose Ridge Fm. at Boulder Ck was deposited in a westward axial drainage system north of the Chugach accretionary complex with detritus shed predominantly from uplifted Triassic-Jurassic volcanic rocks of the Talkeetna Fm. (an arc assemblage that underlies the Arkose Ridge Fm. to the south and east of the study area) and a diverse assemblage of Mesozoic metamorphic and mafic to intermediate plutonic rocks of the Chugach Terrane located south of the study area. The upper Arkose Ridge Fm. reflects the sedimentary response to the onset of Tertiary volcanism in the southern Talkeetna Mountains when new volcanic drainages were established and existing drainage systems were inundated with volcanic detritus.