Heavy mineral analyses to reconstruct basin evolution, an insight from the Yukon-Koyukuk basin sandstones, Alaska.

We present a heavy minerals study of several sedimentary units deposited within the Yukon-Koyukuk basin (YKB) in Alaska. The YKB started to form in middle to late Jurassic after the collision between an intraoceanic volcanic arc and the Arctic Alaska margin. The collision led to thrusting of the seafloor (mafic and ultramafic rocks of the Angayucham Terrane), over the future Brooks Range and Ruby Terrane. The basin is flanked on three sides by metamorphic rocks of the Seaward Peninsula to the west, the Ruby terrane to the east and the Brooks Range to the north. The remnants of the volcanic arc (Koyukuk Arc Terrane, KAT) divide the basin into a northern Kobuk-Koyukuk basin (KKB) and a southern Lower-Yukon basin (LYB).

We present results from the Kv, Kvg, Ks, Kms, Kmc, and Kqc units (after Patton et al.2009), collected along the Koyukuk and the Yukon rivers. The units are as follows:

• Kv: formed by basaltic and andesitic lava flows interbedded with volcanogenic conglomerate to mudstone rocks. K-Ar ages vary from 134 Ma and 118 Ma with a U-Pb age obtained through a tuff of about 138 Ma.
• Kvg: mainly consists of volcaniclastic greywacke and mudstone interbedded with tuffaceous layers which gave U-Pb ages comprised between 112 and 110 Ma (Albian). Molluscs of the same time have been reported throughout the entire unit.
• Kms: mainly fine to coarse sandstone interbedded with shaly layers. Interpreted to be the marine tongue of the Ks deposits.
• Ks: late Cretaceous in age, this unit consists of alternations of sandstone and shale layers deposited in fluvial to shallow marine environments.
• Kmc: mafic igneous clasts conglomerate with mafic and calcareous greywacke and mudstone. Marine molluscs of Cretaceous age have been found.
• Kqc: overall a quartz rich unit composed of conglomerate, sandstone and mudstone. Plant fossils date the unit to the Cretaceous.

We use Quantitative Evaluation of Minerals by Scanning Electron Microscopy (QEMSCAN®) for heavy mineral (HM) analysis to establish a clear relationship between sediments and source regions and build a model for the basin formation and evolution. Kv and Kmc samples reflect a volcanic source, while Kvg, Kms, Ks, and Kqc display the progressive unroofing of deeper and higher grade metamorphic rocks. Combining these data with DZ and U-Pb absolute ages, we interpret the YKB to be formed prior to 138 Ma as the forearc basin of the intraoceanic arc. It evolved into a hinterland setting at about 110 Ma when the first deposition of metamorphic detritus is recorded by the Kvg unit, mainly derived from erosion of the Brooks Range.  Limited paleocurrent data along with the novel HM data attribute the Ks, Kms, Kmc and the Kqc to the erosion of the Ruby terrane as it uplifted during middle to late Cretaceous time.