Abstract

Neogene basins exposed along the north-central flank of the Alaska Range record the geologic development of the highest mountain range in North America. Strata from these basins consist of the Miocene Usibelli Group ( approximately 600 m thick) and the Pliocene Nenana Gravel ( approximately 1000 m thick). Petrographic analyses of 54 sandstone thin sections and 36 conglomerate clast counts from one of these basins, the Healy Creek basin, delineate two distinct petrofacies. A stratigraphically older, quartz-rich petrofacies defines the Healy Creek, Suntrana, and Lignite Creek Formations of the Usibelli Group. A stratigraphically younger, lithic-rich petrofacies defines the Nenana Gravel. Sandstones of the lower petrofacies are characterized by a dominance of quartz (Q 60 F 27 L 12 ; Qm 33 F 28 Lt 39 ) and a high proportion of polycrystalline quartz (Qp 68 Lvm 21 Lsm 11 ). Schist and quartz clasts are common in conglomerates from this petrofacies. Imbricated conglomerate clasts and planar cross-stratification indicate southward to southwestward paleoflow for this quartz-rich petrofacies. Relative to the lower petrofacies, sandstones of the upper petrofacies are defined by a dominance of lithic fragments (Q 39 F 19 L 43 ; Qm 16 F 21 Lt 63 ), a lower proportion of polycrystalline quartz, and a higher proportion of volcanic lithic fragments (Qp 37 Lvm 55 Lsm 8 ). Conglomerate clast compositions in this petrofacies consist mainly of sedimentary and igneous clasts. This lithic-rich petrofacies is dominated by northward paleocurrent indicators. The compositional and paleocurrent data of the Usibelli Group and Nenana Gravel suggest a regional paleodrainage reversal from southward-flowing fluvial systems transporting sediment from the quartz-rich, metamorphic Yukon-Tanana terrane, to northward-flowing fluvial systems transporting sedimentary and igneous detritus from the growing Alaska Range. Superimposed on the first-order tectonic signal are second-order compositional differences. These differences may be related to a combination of factors but appear to be consistent with well documented changes in paleoclimate. For example, sandstones deposited during the warmest and wettest paleoclimates are characterized by a relative enrichment in quartz, especially polycrystalline quartz (Healy Creek Formation, Q 81 F 16 L 3 . Qp 93 Lvm 3 Lsm 4 ; Suntrana Formation, Q 61 F 25 L 14 , Qp 66 Lvm 25 Lsm 9 ). Sandstones deposited during cooler and drier climates, in contrast, have lower proportions of quartz and higher proportions of feldspar and lithic fragments (Lignite Creek Formation, Q 45 F 33 L 14 , Qp 56 Lvm 28 Lsm 17 ). Our analysis cannot account for all the possible influences on sandstone compositions during deposition of the Usibelli Group, but we contend that the apparent correlation between sandstone composition and changes in paleoclimate suggest that climatic modification of sandstone composition may have been significant.

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