A sequence of metasedimentary rocks in Denali National Park (Mt. McKinley and Healy quadrangles), previously mapped by Csejtey and others (1992) as unit DOs (Ordovician to Middle Devonian metasedimentary sequence) and correlated with rocks of the Nixon Fork terrane, contains both deep- and shallow-water facies that correlate best with rocks of the Dillinger and Mystic sequences (Farewell terrane), respectively, exposed to the southwest in the McGrath quadrangle and adjacent areas.
New conodont collections indicate that the deep-water facies are at least in part of Silurian age, and can be grouped into three broad subunits. Subunit A is chiefly very fine grained, thinly interbedded calcareous, siliceous, and siliciclastic strata formed mostly as hemipelagic deposits. Subunit B is characterized by abundant calcareous siliciclastic turbidites and may correlate with the Terra Cotta Mountains Sandstone in the McGrath quadrangle. Subunit C contains thin-bedded to massive calcareous turbidites and debris flows, locally intercalated with calcareous siliciclastic turbidites. Sedimentary features suggest that subunits B and C accumulated in a fan and (or) slope apron setting. All three subunits contain subordinate layers of altered tuff and tuffaceous sediment. Turbidites were derived chiefly from a quartz-rich continent or continental fragment and a carbonate platform or shelf, with subordinate input from volcanic and (possibly) subduction complex (accretionary prism) sources. Limited paleocurrent data from subunit B turbidites show generally southward transport. Stratigraphic relations between the three subunits are uncertain, although we believe that subunit A is probably the oldest. Shallow-water facies, at least in part of earliest Late Devonian (early Frasnian) age, are exposed locally and were deposited in intertidal to deeper subtidal settings.
Reconnaissance structural studies indicate that the most significant of two generations of folds have northerly vergence and presumably are the product of Mesozoic plate convergence.
Deep-water rocks of Silurian age have been recognized in six Alaskan terranes outside the Farewell terrane. Comparison of unit DOs with coeval strata in these terranes reveals closest sedimentologic and biostratigraphic similarities with rocks of east-central Alaska (Livengood terrane) and western Alaska (Seward terrane) and less striking similarities with rocks in southeastern Alaska (Alexander terrane) and northern Alaska (Hammond subterrane of Arctic Alaska terrane). Coeval sequences in easternmost Alaska (Porcupine and Tatonduk terranes) correlate least well with DOs because they lack both Silurian siliciclastic turbidites and Upper Devonian platform carbonate rocks. Our correlations permit the interpretation that all Alaskan terranes with Silurian deep-water strata originated along or adjacent to the North American continental margin, but imply a gradient in Silurian turbidite distribution along this margin. Volcanic material preserved in DOs and related rocks may have been derived from the island arc represented by the Alexander terrane.
|Title||Sedimentology, conodonts, structure, and correlation of Silurian and Devonian metasedimentary rocks in Denali National Park, Alaska: A section in Geologic studies in Alaska by the U.S. Geological Survey, 1996|
|Authors||Julie A. Dumoulin, Dwight C. Bradley, Anita G. Harris|
|Publication Subtype||USGS Numbered Series|
|Series Title||Professional Paper|
|Record Source||USGS Publications Warehouse|
|USGS Organization||Alaska Science Center; Alaska Science Center Geology Minerals; Central Mineral and Environmental Resources Science Center|