AN ABSTPCT OF THE THESIS OF BEVERLY EDLYN FAHLSTROM for the degree of MASTER OF SCIENCE in GEOLOGY presented on December 1, 1981 Title: STRATIGPMHY AND DEPOSITIONAL HISTORY OF THE CRETACEOUS NANAIMO GROUP CF THE CHEMAINUS AREA, BRITISH COLUMBIA Abstract approved: Redacted for privacy ith F. Oles Rocks of the Nanaimo Group were deposited in the Nanaimo basin, Late Cretaceous basin which occupied part of the southeastern margin of Vancouver Island and which extended almost 200 miles north- west from the San Juan Islands to Campbell River. In the thesis area, low grade metamorphic and volcanic basement rock of the Paleo- zoic Sicker Group is overlain by three formations of the Nanaimo Group: the Haslam, Extension-Protection, and Cedar District Forma- tion. Deposition of the Cretaceous sediments occurred during two depositional cycles, each marked by a sea level rise. The Haslam Formation comprises the upDer fine-grained rocks deposited during the first depositional cycle in a nearshore to offshore marine environment. The Hasiam is believed to overlie the lower coarse-grained rocks of the cycle, the Comox Formation, which is not exposed in the thesis area because of faulting. The second depositional cycle resulted in a transition from nonmarine to nearshore marine sediments of the Extension-Protection Formation to offshore marine deposition of the Cedar District Formation. Coarser clastics of the Extension-Protection Formation were deposited as deltas of short-headed streamswhich projected into a shallow marine environment. Deposition of the Cedar District Formation was in the delta front or pro-delta environment where pen- odic influxes of coarser material, brought in by turbidity currents, resulted in rhythmically bedded sequences. Paleocurrent and petrographic data indicate that the Sicker Group was the major source of sediments for all the formations in the thesis area, with a nearby source located somewhere in the west to southwest. Diagenetic processes affecting the sandstones include cementa- tion by calcite, quartz and feldspar overgrowths, and minor laumon- tite. Formation of clays and alteration of volcanic rock fragments created abundant matrix in some of the rocks. Post-Cretaceous faulting is in two sets: one strikes northwest and one strikes north to northeast. Faulting is responsible for the present day configuration of rock units in the thesis area and preservation of some of the less resistant rock units, such as Haslam mudstone. Erosion was severe during the Pleistocene when continental and piedmont glaciers scoured Vancouver Island. Stratigraphy and Depositional History of the Cretaceous Nanaimo Group of the Chemainus Area, British Columbia by Beverly Edlyn Fahistrom A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Master of Science Completed December 1, 1981 Commencement June 1982 APPROVED: Redacted for privacy Profsor of Ge9'logy in charge of major Redacted for privacy i5Chairman of Depa t of Geology Redacted for privacy Dean of Graduat.TSchool Date thesis is presented December 1, 1981 Typed by Opal Grossnicklaus for Beverly Edlyn Fahistrom ACKNOWLEDGEMENTS My sincere appreciation goes to the Amoco Production Company for their support of this thesis. I would also like to thank Dr. Yeats for granting a research assistantship during the winter and spring terms of 1980. Dr. Oles has certainly earned my respect and admiration and I am proud to have been one of his graduate students. Thanks are due to the committee members, Dr. Johnson, Dr. Enlows, and Joel Davis for their reading and review of the thesis. It is a pleasure to be able to thank Jan Muller, Mary Akehurst and A. C. Riccardi, all of the Canadian Geological Survey, for their kind assistance. Special thanks are extended to Eleanor Pounds, Ed Howes, Opal Grossnicklaus, Kurt Andrews, Reed Glassrnan, and last, but not least, Julius Dasch. I thank my family for their love and unending support during this endeavor and would like to dedicate this thesis to my nephew, Austin Edward Bothwell. TABLE OF CONTENTS Page INTRODUCTION Purposes of Investigation 1 Investigative Methods 2 Location and Accessibility 2 Geomorphology 4 Climate and Vegetation 6 Previous Work 8 REGIONAL GEOLOGIC SETTING 11 Basement Complex 11 Sicker Group 11 Nitinat Formation 12 Myra Formation 15 Age of the Sicker Group 17 Nanaimo Group 17 Glacial Drift 19 STRATIGRAPHY AND DEPOSITIONAL HISTORY OF THE LOWER NANAIMO GROUP 21 Introduction 21 Fiaslam Formation 21 Distribution and Thickness 24 Basal Contacts 24 Lithology and Stratification 25 Petrography 34 Environments of Deposition and Provenance 37 Extension-Protection Formation 41 Distribution and Thickness 41 Basal Contacts 42 Lithology and Stratification 42 Lower Conglomerate Member 43 Middle Member 48 Upper Sandstone Member 50 Petrography 55 Paleocurrent Data 55 Environments of Deposition and Provenance 56 Cedar District Formation 60 Distribution and Thickness 60 Basal Contacts 60 Lithology and stratification 61 Environments of Deposition and Provenance 68 Page PETROLOGY OF CRETACEOUS ROCKS 70 Sandstones 70 Framework Grains 70 Quartz 72 Feldspar 72 Rock Fragments 75 Matrix 80 Diagenesis 80 Conglomerate Pebble Counts 91 Clay Mineralogy of the Haslam Formation 93 Lab Procedure 93 Results 93 STRUCTURE 97 ECONOMIC GEOLOGY 100 Porphyry Copper Deposits 100 Petroleum 101 GEOLOGICAL HISTORY 102 BIBLIOGRAPHY 105 APPENDICES 110 Appendix I. Modal Analyses of Sandstones 110 LIST OF FIGURES Figure Page 1 Index map of the thesis area 3 2 Sicker volcanics along Chemainus River 5 3 Glaciofluvial drift along the Checnainus River 7 4 Lahar-type breccia of the Nitinat Formation, along Bannon Creek 14 5 Massive Sicker volcanics of the Myra Formation, Chemainus River 16 6 Epidotized breccia fragments stretched into boudins by fault movement, Highway #1 18 7 Stratigraphy of the thesis area 22 8 Calcarecus concretions in the Haslam Formation, station 4 28 9 Lenticular conglomerate interbed in Haslam siltstone, station 7 29 10 Fine-grained sandstone channel overlying and cutting into finer grained Haslam rock. Also, mudstone ripup in fine sandstone channel fill, station 40 30 11 Spheroidal weathering features in Haslam siltstone, Highway #1, station 40 32 12 Lenticular fine-grained sandstone in Haslam mudstone, Ctiemainus River 33 13 Concretions in Haslam mudstone, Chemainus River 35 14 Classification of Haslam Formation sandstones 36 15 Normal graded bedding in conglomerate of Extension- Protection Formation, station 33 44 16 Conglomerate channel on Bare Point 45 17 Conglomerate with discontinuous sandstone interbed, Bare Point 46 18 Interlamination of coarse sandstone and pebbles, Bare Point 47 Figure Page 19 Classification of the Extension-Protection sandstones 49 20 Upper part of the Extension-Protection Formation, Shoal Islands 51 21 Planar cross-laminations in the sandstone of the upper Extension-Protection, station 19 52 22 Exposure of three different ripple covered intervals preserved in the sandstone of the upper Extension- Protection Formation 53 23 Convolute bedding in the sandstone of the upper Extension-Protection Formation 54 24 Paleocurrent directions of the Extension-Protection Formation 57 25 Flame structures in a sandstone interbed of the Cedar District Formation, station 60 62 26 Loading features in Cedar District Formation, station 60 63 27 Mudcracks preserved in siltstone of lower Cedar District Formation, station 60 64 28 Rhythmically bedded sequences of the Cedar District Formation, station 62 65 29 Classification of the Cedar District sandstones 67 30 Collection localities for sandstone samples analyzed petrographically 71 31 Scanning electron photomicrographs of chert grains, sample 72 73 32 Photomicrograph of upper Extension-Protection feldspathic arenite, sample 15 74 33 Photomicrograph of upper Extension-Protection feldspathic arenite, sample 29 76 34 Photomicrographs of lower Extension-Protection sandstone, sample 33 77 35 Photomicrographs of lower Extension-Protection coarse sandstone, sample 64 78 Figure Page 36 Photomicrographs of Cedar District sandstone, sample 28 79 37 Scanning electron photomicrograph showing mica grain set in a fine-grained matrix. Extension-Protection Formation lithic wacke, sample 72 81 38 Scanning electron photomicrograph of lower Extension- Protection medium-grained sandstone, sample 72 83 39 Scanning electron photomicrograph showing interface between grain boundary and matrix, sample 72 84 40 Photomicrographs of upper Extension-Protection calcite-cemented sandstone, sample 16 85 41 Photomicrograph of upper Extension-Protection sandstone, sample 15 86 42 Photomicrographs of upper Extension-Protection Formation feldspathic arenite, sample 30 87 43 Photomicrographs of upper Extension-Protection Formation sandstone, sample 15 89 44 Photomicrograph of Cedar District feldspathic arenite, sample 31 90 45 Pebble count data of Extension-Protection Formation 92 46 X-ray diffraction patterns for Haslam mudstone, sample 54 95 47 X-ray diffraction patterns for Haslam mudstone, sample 4 96 48 Structure of the thesis area 98 STRATIGRAPHY AND DEPOSITIONAL HISTORY OF THE CRETACEOUS NANAIMO GROUP OF THE CHEMAINUS AREA, BRITISH COLUMBIA INTRODUCTION Purposes of Investigations This thesis grows out of some two and a half months spent in geologic activities in a southeast coastal section of Vancouver Island in the summer of 1980. The thesis area is centered around the town of Chemainus and the adjacent Shoal Islands and extends along the coast northwest from Crofton