Division of Geological & Geophysical Surveys PRELIMINARY INTERPRETIVE REPORT 2009-5 GEOLOGY OF THE COBBLESTONE CREEK–MAY CREEK AREA, EAST-CENTRAL BROOKS RANGE FOOTHILLS, ALASKA by Charles G. Mull, Ellen E. Harris, Paige R. Delaney, and Robert F. Swenson June 2009 Released by STATE OF ALASKA DEPARTMENT OF NATURAL RESOURCES Division of Geological & Geophysical Surveys 3354 College Rd. Fairbanks, Alaska 99709-3707 $17.00 CONTENTS Geology of the Cobblestone Creek–May Creek area, east-central Brooks Range foothills, Alaska by Charles G. Mull, Ellen E. Harris, Paige R. Delaney, and Robert F. Swenson Introduction ..................................................................................................................................................... 1 History of exploration ....................................................................................................................................... 1 Acknowledgments ............................................................................................................................................. 2 General geology and structural evolution of the Cobblestone Creek–May Creek area .................................... 3 Regional setting ......................................................................................................................................... 3 Generalized stratigraphy ........................................................................................................................... 3 Mountain front—allochthonous rocks ............................................................................................... 3 Disturbed belt—allochthonous rocks ................................................................................................. 6 Disturbed belt—parautochthonous rocks ........................................................................................... 6 Disturbed belt—Cobblestone parautochthonous sequence ................................................................ 8 Foothills foldbelt—foreland basin deposits ....................................................................................... 9 Generalized structure ................................................................................................................................. 12 Northern fl ank of the Brooks Range .................................................................................................. 12 Endicott Mountains allochthon ................................................................................................... 12 Disturbed belt .............................................................................................................................. 13 Picnic Creek or Ipnavik River allochthon ................................................................................... 13 Mélange/olistostrome belt ........................................................................................................... 13 Cobblestone parautochthonous sequence .................................................................................... 13 Triangle zone ............................................................................................................................... 14 Foothills foldbelt ......................................................................................................................... 14 High-angle faults ......................................................................................................................... 15 Summary of structural history ............................................................................................................ 16 Depositional and structural evolution of the Cobblestone Creek–May Creek area ........................... 16 Hydrocarbon source rock potential and thermal maturity anomalies ............................................................... 17 Triassic–Jurassic source rocks ................................................................................................................... 17 Cretaceous source rocks ............................................................................................................................ 19 Preliminary apatite fi ssion-track data from the Cobblestone Creek–May Creek area by Paul B. O’Sullivan Introduction ..................................................................................................................................................... 20 Results and interpretation ................................................................................................................................. 20 Endicott Mountains allochthon at mountain front ..................................................................................... 20 Disturbed belt—Picnic Creek or Ipnavik River allochthon ...................................................................... 20 Disturbed belt—Unnamed mélange/olistostrome ..................................................................................... 21 Disturbed Belt—Cobblestone parautochthonous sequence ...................................................................... 22 Foothills foldbelt ....................................................................................................................................... 22 Description of map units Surfi cial deposits ....................................................................................................................................... 23 Deposits of the Colville Basin foldbelt ..................................................................................................... 23 Deposits of the disturbed belt .................................................................................................................... 25 Cobblestone parautochthonous sequence ........................................................................................... 25 Allochthonous Rocks ................................................................................................................................ 28 Syntectonic turbidites, mélange, and olistostromal deposits ............................................................. 28 Picnic Creek or Ipnavik River allochthon .......................................................................................... 29 Deposits of the Brooks Range Mountain front .......................................................................................... 29 Endicott Mountains allochthon .......................................................................................................... 29 References cited ................................................................................................................................................ 31 FIGURES FIGURES Figure 1. Generalized geologic map and index map of Cobblestone Creek–May Creek area, central Brooks Range foothills ................................................................................................................... 4 2. Diagrammatic section showing generalized stratigraphy and structure of Colville Basin ............. 5 3. View westward along Endicott Mountains front at Cobblestone Creek, showing frontal thrust fault in Lisburne Group carbonates and overturned exposures of Siksikpuk and Otuk Formations in west bank of Cobblestone Creek ............................................................................. 5 4. View eastward along Endicott Mountains front at Nanushuk Lake showing two thrust sheets of Lisburne Group limestone and dolomite ......................................................................................... 6 5. Structurally contorted Otuk Formation on east side of the west fork of May Creek at Endicott Mountains front ............................................................................................................................... 7 6. View of blocks of Buchia limestone coquina in sheared black shale groundmass of mélange of southern mélange/olistostrome belt along east fork of May Creek just north of mountain front ... 8 7. View of a structurally complicated bluff on east side of Cobblestone Creek ................................. 9 8. View eastward of unnamed phosphatic–manganiferous shale on north side of Cobblestone ridges near western tributary of Cobblestone Creek ....................................................................... 10 9. Eastward view of Cobblestone ridges west of Cobblestone Creek showing tightly folded beds of Cobblestone Member of Fortress Mountain Formation in axis of tight anticlinal fold deformed by overlying thrust sheet of Cobblestone Member and Torok Formation ...................................... 10 10. View west along crest of Cobblestone ridges east of May Creek showing north-vergent, thrust- imbricated sheets of Cobblestone Member of Fortress Mountain Formation ................................ 11 11. View east to north-dipping lower Nanushuk Formation on Tuktu Escarpment at Cobblestone Creek .............................................................................................................................................. 12 12. View east to Arc Mountain anticline at Nanushuk River ............................................................... 15 13. Diagram showing correlation of various thermal maturation indices for organic material, and zones of petroleum generation and destruction .............................................................................. 18 14. Cross section A–A’ showing generalized