Preliminary Bedrock Geologic Map of the Seward Peninsula, Alaska, and Accompanying Conodont Data By Alison B. Till1, Julie A. Dumoulin1, Melanie B. Werdon2, and Heather A. Bleick1 1U.S. Geological Survey, Anchorage, Alaska 2Alaska Division of Geological and Geophysical Surveys, Fairbanks, Alaska Pamphlet to accompany Open-File Report 2009–1254 2010 U.S. Department of the Interior U.S. Geological Survey Contents Introduction ..........................................................................................................................................................................................................1 Sources of Data.............................................................................................................................................................................................1 Components of the Map and Accompanying Materials.........................................................................................................................1 Geologic Summary ..............................................................................................................................................................................................2 Major Geologic Components.......................................................................................................................................................................2 York Terrane ............................................................................................................................................................................................2 Grantley Harbor Fault Zone and Contact Between the York Terrane and the Nome Complex.................................................3 Nome Complex ........................................................................................................................................................................................3 High-grade Metamorphic and Associated Igneous Rocks .............................................................................................................5 Yukon-Koyukuk Basin, Angayucham Terrane, and the Kugruk Fault Zone ..................................................................................6 Igneous Rocks.........................................................................................................................................................................................7 Tertiary and Active Deformation ................................................................................................................................................................7 Acknowledgments...............................................................................................................................................................................................8 Description of Map Units....................................................................................................................................................................................8 Surficial Deposits ..........................................................................................................................................................................................8 Mesozoic and Cenozoic Igneous Rocks....................................................................................................................................................8 York Terrane.................................................................................................................................................................................................13 York Mountains Succession ...............................................................................................................................................................13 Units with Uncertain Affinities............................................................................................................................................................16 Grantley Harbor Fault Zone .......................................................................................................................................................................18 Nome Complex.............................................................................................................................................................................................19 Layered Sequence................................................................................................................................................................................19 Scattered Metacarbonate Rocks.......................................................................................................................................................26 Metaturbidites.......................................................................................................................................................................................28 High-grade Metamorphic and Associated Igneous Rocks..................................................................................................................30 Kugruk Fault Zone .......................................................................................................................................................................................32 References Cited ...............................................................................................................................................................................................35 Appendix. Conodont data from the Seward Peninsula ..............................................................................................................................44 Introduction..................................................................................................................................................................................................44 Sources of Data...........................................................................................................................................................................................44 Geologic Implications.................................................................................................................................................................................45 Nome Complex and Kugruk Fault Zone.............................................................................................................................................45 York Terrane and Grantley Harbor Fault Zone.................................................................................................................................46 Figures Figure 1. Map of northern Alaska showing major geographic area, geological provinces, and the outline of the map area. ......48 Figure 2. Map of the Seward Peninsula, Alaska showing locations and names of Quaternary, Tertiary, and Cretaceous igneous rocks. Cenozoic volcanic rocks, pale yellow; Late Cretaceous tin granites, orange; compositionally diverse Early and Late Cretaceous rocks, red. ..............................................................................................................................................49 ii Tables Table 1. Distribution of geologic units by 1:250,000-scale quadrangle maps in the map area.............................................................50 Table 2. Selected K-Ar, 40Ar/39Ar and Rb-Sr age determinations ...........................................................................................................51 Table 3. Selected U-Pb zircon analyses........................................................................................................................................................52 Table A-1. Conodont data from the Seward Peninsula..............................................................................................................................53 Table A-2. Temperature ranges for Color Alteration Indices (CAIs) of conodonts................................................................................57 iii Conversion Factors Multiply By To obtain Length centimeter (cm) 0.3937 inch (in.) millimeter (mm) 0.03937 inch (in.) meter (m) 3.281 foot (ft) kilometer (km) 0.6214 mile (mi) kilometer (km) 0.5400 mile, nautical (nmi) meter (m) 1.094 yard (yd) Area square meter (m2) 0.0002471 acre square kilometer (km2) 247.1 acre square meter (m2) 10.76 square foot (ft2) square kilometer (km2) 0.3861 square mile (mi2) Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows: °C=(°F-32)/1.8 Horizontal coordinate information is referenced to North American Datum of 1927 (NAD 27) iv Introduction This 1:500,000-scale geologic map depicts the bedrock geology of Seward Peninsula, western Alaska, on the North American side of the Bering Strait. The map encompasses all of the Teller, Nome, Solomon, and Bendeleben 1:250,000- scale quadrangles, and parts of the Shishmaref, Kotzebue, Candle, and Norton Bay 1:250,000-scale quadrangles (sh. 1; sh. 2). Rolling, tundra-covered hills cover most of Seward Peninsula; trees and shrubs are present only in limited areas in the southern and eastern parts of the peninsula. Broad lowlands underlie large areas in central and northern Seward Peninsula. Part of the central area and large areas in northernmost and easternmost Seward Peninsula are an extensive, unvegetated lava field. Four glacially carved mountain ranges, the York, Kigluaik, Bendeleben, and