Petrology and provenance of the Neogene Usibelli Group and Nenana Gravel; implications for the denudation history of the central Alaska Range

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 (ϳ 600 m thick) and the Pliocene Nenana Gravel (ϳ 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 lithicrich 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 quartzrich, 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.

Provenance signature of changing plate boundary conditions along a convergent margin: Detrital record of spreading-ridge and flat-slab subduction processes, Cenozoic forearc basins, Alaska

“…3 and 9A) from north of the Alaska Range reveals similar major populations of Late Cretaceous, Paleozoic, and Precambrian age zircons (Brennan and Ridgway, 2015). Detrital zircon ages, compositional data, and southwestward paleoflow indicators from the Tanana basin strata are interpreted to represent probable sediment derivation from the Yukon-Tanana terrane (Ridgway et al, 1999;Brennan and Ridgway, 2015). Similarities in the major detrital zircon age populations between the Cook Inlet and Tanana strata, the likely location of sediment sources located in the Yukon composite terrane for both sets of strata, and southwestward paleocurrent flow indicators in the Tanana strata suggest that the forearc may have been depositionally linked to areas north of the present-day Alaska Range.…”

The Quaternary thrust system of the northern Alaska Range

“…The general bedrock geology and Quaternary stratigraphy are documented in a series of eight geologic maps (Wahr haftig, 1970a and a number of related papers (Wahrhaftig, 1958(Wahrhaftig, , 1968Wahrhaftig et al, 1969). Subsequent workers have revised and refi ned the understanding of the Neogene stratigraphic record (e.g., Ridgway et al, 1999Ridgway et al, , 2007Thoms, 2000) and recently developed the framework of faults that accommodates the Quaternary uplift and deformation of this region (Bemis and Wallace, 2007;Bemis, 2010).…”

“…Deformation related to the modern Alaska Range was probably under way by the early Miocene as indicated by the formations of the Oligocene-Miocene Usibelli Group (Wahrhaftig et al, 1969;Ridgway et al, 1999Ridgway et al, , 2007 and thermochronologic exhumation ages Benowitz et al, 2009). Stratigraphy, paleofl ow directions, and unconformities within the Usibelli Group indicate minor syndepositional foreland deformation .…”

Sedimentary petrology and geochemistry of siliciclastic rocks from the upper Jurassic Tordillo Formation (Neuquén Basin, western Argentina): Implications for provenance and tectonic setting

“…Conglomerate and sandstone petrography is a powerful tool for determining the tectonic setting of ancient terrigenous deposits (e.g., Ingersoll, 1978;Dickinson and Suczek, 1979;Ingersoll and Suczek, 1979;Mack et al, 1981;Valloni and Maynard, 1981;Dickinson et al, 1983;Dickinson, 1985;Spalletti et al, 1986;Spalletti and Matheos, 1987; Neuquén CHILE Study area A t l a n t i c O c e a n South America Ridgway et al, 1999;Lawton et al, 2003). The clay mineral assemblage may be used for provenance studies, though data must be considered with caution because postdepositional processes may vary significantly from original compositions (Arribas et al, 2003).…”