(a-0g) R ago (na. 96-53 14. U.S. DEPARTMENT OF THE INTERIOR , U.S. GEOLOGICAL SURVEY Alatzi2/6 (Of (c,c) - R qo rite 6/6-53y Geologic Map of the Cascade Head Area, Northwestern Oregon Coast Range (Neskowin, Nestucca Bay, Hebo, and Dolph 7.5 minute Quadrangles) by Parke D. Snavely, Jr.', Alan Niem 2 , Florence L. Wong', Norman S. MacLeod 3, and Tracy K. Calhoun 4 with major contributions by Diane L. Minasian' and Wendy Niem2 Open File Report 96-0534 1996 This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or with the North American stratigraphic code. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. 1/ U.S. Geological Survey, Menlo Park, CA 94025 2/ Oregon State University, Corvallis, OR 97403 3/ Consultant, Vancouver, WA 98664 4/ U.S. Forest Service, Corvallis, OR 97339 TABLE OF CONTENTS INTRODUCTION 1 GEOLOGIC SKETCH 2 DESCRIPTION OF MAP UNITS SURFICIAL DEPOSITS 7 BEDROCK UNITS Sedimentary and Volcanic Rocks 8 Intrusive Rocks 14 ACKNOWLEDGMENTS 15 REFERENCES CITED 15 MAP SHEETS Geologic Map of the Cascade Head Area, Northwestern Oregon Coast Range, scale 1:24,000, 2 sheets. Geologic Map of the Cascade Head Area, Northwest Oregon Coast Range (Neskowin, Nestucca Bay, Hebo, and Dolph 7.5 minute Quadrangles) by Parke D. Snavely, Jr., Alan Niem, Florence L. Wong, Norman S. MacLeod, and Tracy K. Calhoun with major contributions by Diane L. Minasian and Wendy Niem INTRODUCTION The geology of the Cascade Head (W.W. Rau, personal communication, 1978 area bridges the geology in the Tillamook to 1988; Bukry and Snavely, 1988). Highlands to the north (Wells and others, Many important stratigraphic and 1994; 1995) with that of the Newport igneous relationships are displayed within Embayment on the south (Snavely and the Casacde Head area: others, 1976 a,b,c). The four 7.5-minute (1) turbidite sandstone of the middle quadrangles (Neskowin, Nestucca Bay, Eocene Tyee Formation, which is Hebo, and Dolph) which comprise the widespread in the central and southern part Cascade Head area include significant of the Oregon Coast Range (Snavely and stratigraphic, structural, and igneous data others, 1964), was not deposited in the that are essential in unraveling the geology western part of the Cascade Head, and is of of the northern and central part of the limited extent north of the map area (Wells Oregon Coast Range and of the adjacent and others, 1994); continental shelf. (2) the late middle Eocene Yamhill Earlier studies (Snavely and Vokes, Formation, which crops out along the west 1949) were of a broad reconnaissance nature and east flank of the Oregon Coast Range, because of limited access in this rugged, overlaps older strata and overlies an densely forested part of the Siuslaw National erosional unconformity on the lower Eocene Forest. Also, numerous thick sills of late Siletz River Volcanics (Snavely and others, middle Eocene diabase and middle Miocene 1990; 1991); basalt mask the Eocene stratigraphic (3) thick sills of late middle Eocene relationships. Previous mapping was diabase (43 Ma) are widespread in the hampered by a lack of precise Cascade Head area and also form much of biostratigraphic data. However, recent the eastern flank of the Tillamook Highlands advances in biostratigraphy and radiometric (Wells and others, 1994), but are rare south age dating and geochemistry have provided of the map area; the necessary tools to decipher stratigraphic (4) Cascade Head is the and structural relationships in the Eocene northernmost eruptive center of late Eocene sedimentary and volcanic rock sequences alkalic basalts--85 km north of the eruptive center of correlative alkalic flows of the USGS Open-File Report 96-0534 1 Yachats Basalt in the Newport Embayment sedimentary sequences mapped in the (Snavely and Vokes, 1949; Snavely and Newport Embayment and in the Tillamook others, 1990; Barnes and Barnes, 1992; Highlands as well as in western Washington. Davis and others, 1995); New 7.5-minute topographic maps (5) early Oligocene (33 Ma) sills and and aerial photographs which became dikes of nepheline syenite and camptonite available in the late 1980's provided detailed present in the Newport Embayment (Snavely topography which can be related to the and Wagner, 1961) are not found in the distribution of thick sills and broad landslide Cascade Head area; areas, as well as a precise geographic (6) extensive middle Oligocene (30 relationship of geologic observations in this Ma) granophyric gabbro sills that are densely forested and brush-covered terrain. widespread in the central part of the Oregon New geographic information systems Coast Range (Snavely and Wagner, 1961; (GIS) technology has produced a digitized MacLeod, 1969) are not present in the color map of the Cascade Head area that Cascade Head area. combines the four 7.5-minute quadrangles The Cascade Head area is the last that previously were open-filed as separate segment of the Oregon Coast to receive black and white 7.5-minute quadrangles detailed geologic mapping. Increased (Snavely and others, 1990; 1990a; 1991; logging operations in the 1970's and 1980's 1993). created numerous new roadcut exposures The tectonic framework and and access to exposures in stream beds. stratigraphic architecture presented on the More importantly, microfossil map of the Cascade Head area was obtained biostratigraphic control, available since by classic geologic field methods. This 1970, based upon foraminifer determinations information could have been obtained only by W.W. Rau and nannofossil through detailed observation and sampling determinations by David Bukry provided along stream beds, road cuts, and outcrops. critical information on stratigraphic Remote sensing techniques were of minor succession as well as on depositional help in unraveling the geology in this poorly environments of the deep water (bathyal) exposed and complex terrain, a terrain that siltstone units present in much of the characterizes much of the Oregon and Cascade Head area. These paleontologic Washington Coast Ranges. data also permitted correlations with other GEOLOGIC SKETCH The Cascade Head area lies astride (Snavely, 1991a), and is overlain the north-trending faulted asymmetric unconformably by lower and middle Eocene Salmon River arch that is cored by lower marine sedimentary rocks. The steep west Eocene oceanic basalt and volcaniclastic flank of the arch consists of marine rocks which include the Siletz River sedimentary rocks that range in age from Volcanics (Tsr) (Snavely and others, 1968) upper Eocene to middle Miocene with a and the Salmon River Formation (Tsar) thick prism of upper Eocene alkalic basalt flows and breccia (Snavely and others, Cross-hatched stripes on the geologic map show the 1990a; 1991). The east flank consists chiefly inferred crest of the Salmon River arch. USGS Open-File Report 96-0534 2 of late middle Eocene siltstone of the movement occurred along these northwest Yamhill Formation (Ty) intruded by faults. extensive diabase sills (Tid) near the base of The structural style differs in the the siltstone sequence. In the northern part of south part of the Cascade Head area, and the Cascade Head area thick, widespread extends further southward into the adjacent sills of middle Miocene basalt (Tidb) cap the Euchre Mountain quadrangle (Snavely and arch and form the high upland areas. others, 1976). Here northeast-trending A family of northwest-trending normal and reverse faults interleave Siletz sinistral high-angle faults, linked by River Volcanics and intensely deformed northeast-trending normal and oblique slip basaltic sedimentary rocks of the lower dextral faults, form a conjugate strike-slip Eocene Salmon River Formation (Bukry and pattern in the northern and central parts of Snavely, 1988). This family of faults is the map area. Both the northwest- and overlapped by the early middle Eocene northeast-trending faults are intruded in (CP12) basaltic sandstone beds of Otis places by diabase dikes (Tid) (43 + 1.8 Ma), Junction (Tobs) (Snavely and others, 1990), indicating that the faulting occurred soon strata correlative with the Tyee Formation. after the deposition of the late middle Although the major offset on these Eocene Yamhill Formation. A pop-up block northeast-trending reverse faults was in the of Tyee Formation (Tt) within the Yamhill early Eocene, renewed movement occurred Formation in the north central part of the in the late Eocene as these faults offset strata Dolph quadrangle is bounded by two major in the Nestucca Formation in the Euchre northwest trending sinistral faults. The Tyee Mountain quadrangle south of the map area crustal block is segmented into (Snavely and others, 1976). rhombohedral blocks by the northeast The oldest rocks in the Cascade normal and oblique slip dextral faults. The Head area, the Siletz River Volcanics uplift of this Tyee pop-up is most likely the (Snavely and others, 1968), are seamounts result of underplating and uplift due to the which are largely pillow lava and breccia emplacement of thick diabase sill or sills with minor basaltic sandstone and siltstone related to the diabase dikes that occur along interbeds. Some basalt formed oceanic the margin of the pop-up block. islands in the northern part of the Euchre Since faulting, igneous activity, and Mountain quadrangle at Ball Mountain, just the regional unconformity at the base of the south of the area. In the map area, coccolith Yamhill Formation are essentially flora from siltstone interbeds in the Siletz contemporaneous, these events probably are River Volcanics indicate an early Eocene related to the major plate reorganization in age (zones CP10 and 11, Bukry and Snavely, the Cascade forearc at about 43 Ma 1988). (Snavely, 1987). Intrusion of diabase along The overlying lower Eocene (CP11) the northwest sinistral faults indicates a pre- Salmon River Formation (Snavely, 1991a) 43 Ma period of major faulting; however, in consists of basaltic sandstone and the western part of the Cascade Head area conglomerate derived from erosion of the some northwest-trending faults offset strata Siletz River volcanic islands.