Geology of the Nanushuk Group and Related Rocks, North Slope, Alaska U.S. GEOLOGICAL SURVEY BULLETIN 1614 Geology of the Nanushuk Group and Related Rocks, North Slope, Alaska By A. Curtis Huffman, Jr., Editor U.S. GEOLOGICAL SURVEY BULLETIN 1614 DEPARTMENT OF THE INTERIOR DONALD PAUL HODEL, Secretary U.S. GEOLOGICAL SURVEY Dallas L. Peck, Director UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1985 For sale by the Branch of Distribution U.S. Geological Survey 604 South Pickett Street Alexandria, VA 22304 Library of Congress Cataloging in Publication Data Geology of the Nanushuk Group and related rocks, North Slope, Alaska. (United States Geological Survey bulletin ; 1641) v, 129 p. Bibliography: p. 124-129 Supt. of Docs. No.: 119.3:1614 1. Sandstone-Alaska-North Slope. 2. Geology, Stratigraphic-Cretaceous. I. Huffman, A. C. II. Series. QE75.B9 no. 1614 557.3s [557.98'7] 84-600055 [QE471.15.S25] CONTENTS Introduction to the geology of the Nanushuk Group and related rocks, North Slope, Alaska, by A. C. Huffman, Jr. 1 Cretaceous tectonics, depositional cycles, and the Nanushuk Group, Brooks Range and Arctic Slope, Alaska, by C. G. Mull 7 Subsurface correlations and depositional history of the Nanushuk Group and related strata, North Slope, Alaska, by C. M. Molenaar 37 Depositional and sedimentologic factors affecting the reservoir potential of the Creta ceous Nanushuk Group, central North Slope, Alaska, by A. C. Huffman, Jr., T. S. Ahlbrandt, Ira Pasternack, G.D. Stricker, and J.E. Fox 61 Petrography of sandstones of the Nanushuk Group from four measured sections, central North Slope, Alaska, by Susan Bartsch-Winkler 75 An analysis of the Umiat delta using palynologic and other data, North Slope, Alaska, by F. E. May and J. D. Shane 97 Uranium potential of the Cretaceous Nanushuk Group, North Slope, Alaska, by A. C. Huffman, Jr. 121 FIGURES 1. General index map of the North Slope 2 2. Measured sections from Corwin and Umiat deltas 4 3. Generalized nomenclature and stratigraphic relationships of the N anushuk Group and related rocks, western and central North Slope 6 4. Stratigraphic relationships of lower Cretaceous deposits from the Brooks Range to the Beaufort Sea 8 5. Diagrammatic cross section from the Brooks Range to the Beaufort Sea 9 6. Generalized map showing the distribution of the Nanushuk and Colville Groups, and the major allochthons and tectonic features of the Brooks Range 10 7. Photograph showing the Nanushuk Group and Torok Formation in the Pit megea River area 12 8. Table showing the vertical sequence of major allochthons in the Brooks Range 13 9. Photograph showing the Fortress Mountain Formation at Ekakevik Mountain 14 10. Chart showing global Cretaceous eustatic cycles compared to generalized Arc- tic Slope Cretaceous stratigraphy 15 11. Photograph showing the Okpikruak Formation 16 12. Photograph showing olistostrome, De Long Mountains 17 13. Chart describing the pebble shale unit and adjacent rock units in Kugrua and Inigok test wells 20 14. Diagrammatic maps of major tectonic elements during Neocomian and Aptian times 26 15. Photograph showing Mount Kelly Graywacke Tongue of the Fortress Moun- tain Formation 29 16. Diagrammatic map of major tectonic elements at the end of Aptian time 32 17. Index map of part of the North Slope 38 18. Diagram showing stratigraphic cross sections of Nanushuk Group and related rocks, National Petroleum Reserve in Alaska 42 19. Seismic cross section showing the relationship of the Nanushuk Group and Torok Formation 52 20. Seismic cross section showing disturbed zone in the Torok Formation 53 Contents Ill FIGURES 2 I . Isopach map of N anushuk Group 54 22. Isopach map of Torok Formation 55 23. Plot of directional data in the Nanushuk Group and Torok Formation 56 24. Paleogeographic maps, mid-Nanushuk and latest Nanushuk time 57 25. Isopach map of seismic velocities in the lower Nanushuk Group 58 26. Map showing percentage of sandstone, Nanushuk Group 62 27. Generalized stratigraphic column of Nanushuk Group, central North Slope 66 28. Diagram showing terminology and major depositional environments 67 29. Plots showing porosity vs. permeability for Nanushuk Group rocks 69 30. Plots showing distribution of sandstone within the Nanushuk Group by facies 70 3 I. Paleogeographic maps showing depositional history of Nanushuk Group and related rocks 72 32. Location map of four measured sections 76 33. Compositional diagrams of Nanushuk Group sandstones 76 34. Triangular plots of Nanushuk Group sandstones 77 35-38. Triangular diagrams showing modal analyses plotted according to grain size: 35. Kurupa anticline section 81 36. Tuktu Bluff section 82 37. Arc Mountain section 83 38. Marmot syncline section 84 39. Diagrams of vectoral analyses showing compositional trends according to grain size 85 40. Histograms of textural characteristics 86 4 I-5 I. Photographs showing: 4 I . Intergrain boundaries in sandstone 87 42. Tightly appressed sandstone showing grain deformation 88 43. Replacement by calcite cement in sandstone 88 44. Kaolinite pore-space filling in sandstone 89 45. Original porosity and minor secondary porosity 90 46. Chlorite coating on quartz grains in sandstone 90 47. Quartz overgrowths on sandstone after clay formation 91 48. Abundant quartz overgrowths on sandstone 92 49. Secondary porosity features in sandstone (scanning electron micro scope) 93 50. Secondary porosity features in sandstone (photomicrograph) 94 51. Possible hydrocarbons in Nanushuk Group rocks 95 52. Index map of Umiat delta area 98 53. Diagrams showing locations of invertebrate megafossils observed in Nanushuk Group rocks 106 54-62. Relative frequency diagrams of main palynomorph groups: 54. Knifeblade-I well 108 55. Titaluk-I well 109 56. Oumalik-I well 111 57. East Oumalik well 113 58. Wolf Creek-2 and -3 wells 114 59. Square Lake-I well 116 60. Umiat-I well 118 61. Umiat-11 well 119 62. Grandstand-I well 120 IV Contents TABLES 1. Microfaunal and palynological data from Lower Cretaceous rocks 22 2. Megafossil data from Kemik Sandstone Member 24 3. Location of North Slope test wells shown in figure 17 39 4. Location of Nanushuk Group measured sections shown in figure 17 40 5. Resource-related data from sections measured in Nan~shuk Group rocks 64 6. Compositional data from four sections measured in Nanushuk Group rocks 78 7. Sedimentologic data from transitional and nonmarine parts of sections measured in N anushuk Group rocks 122 Contents V Introduction to the Geology of the Nanushuk Group and Related Rocks, North Slope, Alaska By A. Curtis Huffman, Jr. Abstract A detailed model of the Umiat delta and related deposits may be constructed through a combination of The Cretaceous Nanushuk Group has long been sedimentology, paleontology, and seismology studies. considered one of the major hydrocarbon plays on the This model suggests that a belt of high-energy coastal North Slope and as such has been the focus of a number barrier deposits trending northwest from the northern of investigations. The primary purpose of this four-year edge of the Umiat delta may contain reservoir sand study was to evaluate the reservoir potential of the Cre stones and stratigraphic traps. taceous Nanushuk Group in and adjacent to the National Petroleum Reserve in Alaska (NPRA). This study sought to apply modern stratigraphic concepts as well as up-to INTRODUCTION date geochemical, petrographic, and paleontologic methods in evaluating the hydrocarbon, coal, and ura nium potential of the Nanushuk and related rocks. Pre liminary results were published in Ahlbrandt (1979), Much of the outcrop and subsurface work by the which concentrated on the western North Slope. This U.S. Geological Survey and the U.S. Navy in the 1940's report is primarily concerned with the central North and 1950's focused on rocks of the Nanushuk Group be Slope, although several preliminary regional syntheses cause the most significant oil field on the North Slope are presented as well. prior to the discovery of the Prudhoe Bay field in 1967 The results of this and previous investigations indi was the Nanushuk field at Umiat (fig. 1), estimated by cate that the Nanushuk Group was deposited as two Espach (1951) to contain as much as 122 million barrels. delta systems onto a shelf of marine shale of the Torok The transfer of jurisdiction of Naval Petroleum Reserve Formation, which prograded into the Colville basin gen No. 4 from the U.S. Department of the Navy to the U.S. erally from west to east. Deposition of the western, or Department of the Interior by the Naval Petroleum Re Corwin, delta began in early Albian or perhaps late Ap serves Production Act of 1976 changed the name to Na tian time, and the delta prograded northeastward from the vicinity of the De Long Mountains, Tigara Uplift, and tional Petroleum Reserve in Alaska (NPRA) and called Herald Arch. The deltaic and shelf topsets can be traced for a reevaluation of the resource potential of the reserve. basinward into large foresets and coeval bottomsets of As a part of this evaluation, the Nanushuk Group the Torok Formation. Petrographic analyses confirm that and related rocks were studied by a team of U.S. Geologi sediment of the Torok Formation is closely related to cal Survey scientists from 1977 to 1981. Fieldwork for that of the Nanushuk Group in the Corwin delta and this investigation was begun in the western part of the is quite different from the Nanushuk of the Umiat delta. North Slope in 1977, and preliminary results were pub The Umiat delta was an elongate to lobate, river lished in Ahlbrandt ( 1979). The 1978 and 1980 fieldwork dominated delta, which prograded northward from the concentrated on the central and eastern regions, and many vicinity of the Endicott Mountains, beginning in mid-Al of the results are presented in this report. Detailed meas bian time. Sandstones of the Umiat delta are dominantly phyllarenite, as compared with the sedimentary lithare ured sections from the western North Slope are described nite of the Corwin delta. The differences in sediment in Huffman and others (198la) and from the central North type between the two depocenters reflect the sequence Slope in Huffman and others (1981b). of lithologies exposed in the various thrust slices of the Emphasis during this study has been on the interpre Brooks Range. The Endicott Group of the central Brooks tation of depositional environments and their relationship Range contains large thicknesses of Upper Devonian and to one another and to reservoir potential. This regional Mississippian quartzitic conglomerate and phyllitic shale, approach was necessitated by several factors: (1) The whereas the sequences exposed in the western Brooks Nanushuk Group was deposited in two large depocenters, Range are dominantly shale, limestone, and chert. Be the Corwin delta of the western North Slope and the cause the Umiat delta was not subjected to high temper Umiat delta of the central North Slope, with a region of atures and deep burial as was the Corwin delta, micro paleontological and palynological analyses have proven low sandstone content and meager exposures in between. valuable in the interpretation of its depositional environ (2) No unit above the Torok Formation can be physically ments. traced the length of the outcrop belt. (3) Very few data Introduction 1 168° 16fl0 164° 162° 160° 158° ~-~----~--------·-------------------------------------------, -------...,.....---------·-.-----1------Ir 710) EXPLANATION : i ! i jl 3(,01, 09)~4!t PALEOTRANSPORT DIRECTIONS DETERMINED FROM PLUNGE ' I ! . ALONG AXIS OF SYMMETRICALLY FILLED NONMARINE Percent of CHANNELS OF THE NANUSHUK GROUP~Number of reading measurements indicated by number in parentheses; arrow indicates vector mean transport direction; percent of readings in each segment - !'' - : ---------------+ of rose is to scale shown ~-----r--- ---+ ! ----+- LINE OF MEASURED SECTION iski : Tunalik 1• WELL LOCATION AND NAME -+- ANTICLINE-Dashed where approximately located :-+- SYNCLINE-Dashed where approximately located 700i I~ APPROXIMATE OUTLINE OF BASAL PART OF •0 NANUSHUK GROUP-Kukpowruk and Tuktu Formations OIL FIELD GAS FIELD c ff Figure 1. Index map showing the study area of Nanushuk Group rocks, locations of sections measured during 1977 and 1978, and locations of wells, western and central North Slope, Alaska. have been found with which to precisely date or correlate volved in trying to apply formal nomenclature to these the units. The cross sections in figure 2 illustrate these units on a regional basis. The correlations implied on this problems. Very few outcrops exist in the 150-km interval chart are approximate at best. separating the Carbon Creek and Kurupa anticline meas During this study, a total of 25,000 m of section ured sections (fig. 1). The widely separated exposures that at 33 localities (fig. 1) were measured, described, and are present in this region are predominantly shale and sampled in detail. Samples were collected for petro mudstone. graphic, geochemical, and paleontologic studies. Relation By concentrating on genetic units, equivalent envi ships observed in the surface exposures were combined ronments can be compared between the two depocenters with subsurface and geophysical data to build a deposi without regard to time or formal nomenclature. A strati tional model that was tested and perhaps modified by sub graphic nomenclature chart of the Nanushuk Group and sequent drilling. Similar methods were used to project res related units (fig. 3) illustrates some of the problems m- ervoir properties into the subsurface. 2 Geology of the Nanushuk Group and Related Rocks, Alaska