BULLETIN 69 - GEOLOGY OF THE SOUTHWESTERN OREGON COAST WEST OF THE 124!!!. MERIDIAN STATE OF OREGON DEPARTMENT 01-' GEOLOGY AND MINERAL INDUSTRIES 1971 BULLETIN 69 STATE OF OREGON DEPA RTMENT OF GEOLOGY AND MINERAL INDUSTRIES 1069 Stote OHiee Building Portland, Oregon 97201 BULLETIN 69 GEOLOGY OF THE SOUTHWESTERN OREGON COAST WEST OF THE 124th MERIDIAN By R. H. Oou, Jr. University of Wisconsin, Modison, Wisconsin 1971 GOVERNING BOARD Foyelte 1. Bristol, Rogue River, Choirmon R. W. deWeese, Portlond Harold 6onto, Boker STATE GEOLOGIST R. E. Corcoran FOREWORD The Klamath Mountains Province of southwestern Oregon is probably one of the mOre geologically inter .. esting regions in the state. Historically1 it was in this pcwt of Oregon, near the present town of Jocks:on­ ville, that gold was discovered in 1850. Since that time, several million dollars in precious metals, copper, mercury, chromium, and nickel hove been mined from this highly mineralized region. Mineral exploration is still being carried on in the Klamath Mountains byprivate COR1>onies in the hope of finding new- deposits. One of the primory functions of the State of Oregon Deportment of Geology and Mineral Industries is to encouroge development of our mineral resources in OC"der to enhance the economy of the state and to provide the row mote.-iols required by our technological society. At the present time, yearly per-c:opito demand for minerals is obovt SISO. The U.S. Bureau of Mines believes that by the yeor 2000ovr require­ ment will be approximately S420. In order to corry out o mining-exploration program in the most efficient monnet, it is necessory to utilize all ovailable geologic mopping. For the post several years, Dt. R. H. Don and his groduote stu­ dents from the University of Wisconsin hove been investi9oting the extremely complex geology that under­ lies the Klomoth Mountoin.s Province, and their preliminary reports have been very useful to economic geologists in outlining the most favorable oreo.s for mineral exploration. This bulletin summorixes the Held studies carried out by the University of Wisconsinpersonnel in southwest Oregon, supplemented by oil ovoiloble published and some unpublished mapping. The lnformotion presented in this report will pro­ vide much new geologicdote for the use of exploration companies. If the mining industry is to meet the future demands for mineral products, it will need to make use of oil of the mostrecent geologic mapping in those mineralized regions showing the greatest promise. SouthwesternOregon is on oreo that warrants further investigation. R. E. Corcoran Oregon State Geologist ii CONTENTS Fortword . ii List of illustrations v vii Introduction . Purpose , Previous mopping along the coost I Previous stratigraphic studies . 2 lnvettigotions in northern California 4 Offshore geologic end geophysical studies 4 University of Wisconsin studies 4 At knowl edgments 6 Tectonic setting 6 Tectonic subprovinces of the mop area 9 Mesozoic rocks of the central region of the mop aeo 9 Galice Formation (Late Jurossie) 9 Definition 9 Lit hology. 11 AQe and stratigraphic relotiomhips 11 Colebrooke Schist (Jurassic) . 11 Definition 11 Lithology. 13 Metamorphic grade 13 Structure . 13 .AQe of metamorphism . 13 Simi lor' schists in northwestern California 16 Probable precursors and structural relationships of Colebrooke . 16 Gloucophone schisrs and omphibolit es 17 Gloucophone-beoring blueschists . 17 Amphibolites 17 Ultromoric roeks ossocioted with the Colebrooke Schist 19 Dioritic ond gobbroic intrusive rocks 19 Oi1tribution . 19 Pearse Peak diorite 19 Other diorite moues 21 A9•• . 21 Humbvg N\ovntoin Conglomerate (Early Cretaceous) 22 Definition 22 Bosol (Nevodon) unconformity. 23 Uthology 23 Aee ond correlation . 23 Rocky Point Formation ( Early Cretaceous). 26 Definition ond lithology . 26 AQeand correlation . 26 iii Mesozoic rocks of the central region of the mop Ofeo, continued: Undifferentiated Late Jurassic and Eody Cretaceous strcto (Myrtle Group) of the east-central region 26 Mesozoic rocks along the coost. 27 Otter Point Formotion or complex (lot¢st Jurassic). 27 Definition 27 Conglomerate . 29 Sandstone 29 Volcanic rocksond bedded chert 29 Age end carrelorion . 31 Cope Sebastien Sandstone (new nome) (Late Cretaceous). 31 Occurrencel of Upper Creta ceous strata . 31 Definition of Cope Sebostion Sandstone 33 Lower conglomerate portion . 35 Upper mossive sandstone . 35 Hunters Cove Formotion (new nome) (late Cretaceous) 38 Definition 38 Hunters Cove Formation north of Bloekloek Point . 40 Aee end regional cOrrelations of Late Cretaceous strata . 41 Dothan (or Fra nc::iscan? ) rocks of the southern region 43 Oothon Formation (J urassi c) . 43 Definition and distribution 43 Regional subdivisions 43 44 Mocklyn Member • Winchuck Member . 46 Age and rclotion�ipswith the Franciscan end Otter Point ·:omplcxes 46 Ceno%oic rocks. 48 Eocene strata (undifferentiated) 48 Distribution and lithology 48 Structural ood stratigraphic r·elationships 49 MountEmily doeitic intrusives (medici Cenozoic) 50 Mafic dikes be!Ween Wholeheod ond Crook Point (medial Cenozoic) 50 Miocene and Pliocene sediments (undiffer entiated ). 50 Quoterncwy deposits. 52 lmpli cotions of the MW global t(!etonics to southwestern Oregon 54 Lolc Jurassic ond Eody Cretoceous of the Klomoth Province 54 Late Juranic of the coosral province 54 Cretaceous thrust faulting . 55 lote Cretoceous and eorly Cenozoic tectonic quiescence . 56 late Cenozoic tectonism 56 Economic geology . 58 8;bl;ogropily. 59 I LLUSTRAT 10NS P l ates I. Geologic compilation mop of western C urr y County, Oregon · Pocket II. Geologic mop of Cope Sebostion · Pocket F ig ures I. Humbug h\ountoin os seen looking southeast fr om near Port O rford 3 2. Tectonic setting of the Pacific Northwes t of\d odjoeent oceanic: crust 8 3. Intensely sheared mudstone and sandstone typical of the Otter Point Formation, exp0$ed north of Sisters Rock (o typical broken formation) 9 4. Geologic mop of the Barklow tv\ountoin-JohnsonCreek-upper E!lk River oreo. 12 5. Geologic mop of the Collier Butte-Quosotona Butte region 14 6. Intensely sheared soncktone and mucktone of the Otter Point complex along U.S. Highway 101 ot mouth of RogvoRivo• 20 7. Photomicrograph of typical Pearse Peak quortz diorite 20 8. Cross section through the southeast end of Barklow N!ountoin 22 9. Coarse Humbug Mountain Conglomerate along Elk River obove bosol unconformity 24 10. Gro<led conglomerate units typical of upper Humbvg Mountain Conglomerate exposed in heodlond north of Myrtle Creek 24 11. Comparison of sondst0ne compositions for the Otter Pointand Lower Cretaceous formations . 25 12. Contorted zone within alternating graded sandstone and mudstone units typical of Rocky Point F·ormotion · 25 13. Otter Point, 3 miles north of the movth of Rogue River· 28 14. Flysch-like sequence of thick, grodcd sandstone unitsa lter nating with mudstone in Otter Point at uBelemnite Cove"· 28 15. Unstratified pebbly mudstone in Otter Point rocks on we�t face of Cope Blanco 30 16. Otter Point pillow b<nOitwith conspicuous coor se feldspar phen1ocrysts exposed on Pistol River . 30 17. Volcanic breccia closely associated with pillow lovos along Pi!;tol River 32 18. Cope Sebastian and Hunters Cove from the lOuth . 32 19. Upper Cr etaceou s strata exposed at low tide northeast of Blacklock Point . 33 v 20. Oiogrommotic. portroyolor type sectionsof Hunten Cove Fonnotion end Cope- SebostionSandstone • 34 21. Trough-type cron bedding ond box""Work weathering in CopeSebostion Sandstone 36 22. Typic.ol Hunters Cove Formation dilhormonicolly folded. 36 23. Stratigraphic diogrom of inferred lateral relotio.uhips among major lithic types in CopeSebastian ond Hunters Cove formations 37 thi F ma 24. Unu�ual conglomefotic sandstone moss wi n Hunters Cove or tion south or Crook Point 38 25. Convolute lamination within o thick sond,tone mou interstrotifted with more normolHunte" Cove strata 39 26. Hunton Cove FO<motionoxpom jut! -th ol 81ocklock Point. 39 27. Diagnostic fossils from lowe.rpc...-t or Huntert Cove Farmotion • 42 28. Rainbow Rock, a Iorgemoss of bedded ch.,t In the Ootho.n Formation 42 29. Flu to structures on sole olo Dothan groywocke bed • •5 30. Massive heterogeneous conglomerate assigned to the Dothan Formotion ot Point St. George, Crescent City .. California . 45 31. Paleocene or lower Eocene rlysch s�uence exposed in 11Moson•s Wall" neor Agness . 48 32. 8o�o�lder or Cretoceous (?) eonglo!Ufottwithin e�ene conglomerate 49 33. Sh ol low-morine Miocene sond> ond grovels nO<thol Blocklock syncline. 51 34. Oblique air photographof Cope Bloneo, ihowing flot Pleistocene marine terroce 6epositl unc.onformoble uponTertiary ondJurOlSic. rocb 52 35. View n«thwe.t towordCrook Point ondMock Rool lrom Deer Point • S6 36. Simplifiedtectonic mcpor s.ovthwtit«n Oregon • 57 T obl e s Toblo 1. Inferred correlations or stroto amongthe mop subprovinces 10 Tobie 2. Selected chemicol analyse$ of igneous rocks 15 Tobie 3. l$0topic age data . 18 Tobie -4. Chc:roGteristics of southwesternOregon beach sonc:h 53 vi Abstract Unt il 1969, the southwestern eoost of Oregon hod not been studied in detail since the pioneer mappir19 of J. S. Oilier ot the turn of the century. From 1959 through 1970, University of Wisc:ons.in g.alogists con­ ducted o variety of studies, bvt with emphaais upon sedimentology ond its relation to tectonic htstOt'y. Beginning about 1965, U.S. Geologico! Survey and University of Oregon geologist$ oiJO began working in tho oreo. In the late 1960's, great advances