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Stratigraphy and Sedimentology of Upper Cretaceous Rocks in Coastal Southwest Oregon: Evidence for Wrench-Fault Tectonics in a Postulated Accretionary Terrane

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Stratigraphy and Sedimentology of Upper Cretaceous Rocks in Coastal Southwest Oregon: Evidence for Wrench-Fault Tectonics in a Postulated Accretionary Terrane Downloaded from gsabulletin.gsapubs.org on November 13, 2015 Stratigraphy and sedimentology of Upper Cretaceous rocks in coastal southwest Oregon: Evidence for wrench-fault tectonics in a postulated accretionary terrane JOANNE BOURGEOIS Department of Geological Sciences, University of Washington, Seattle, Washington 98195 R. H. DOTT, JR. Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706 ABSTRACT on the north side of Cape Sebastian (Fig. 2), and of the Neogene Humboldt microplate (Fig. 3) more detailed sedimentologic and stratigraphic (Dott, 1979; Herd, 1978), but some of the same Southwest Oregon is a region of complex field work (Bourgeois, 1980a) established that faults may be older, fundamental boundaries be- juxtaposition of tectonostratigraphic ter- the sequence beneath the Cape Sebastian Sand- tween tectonostratigraphic terranes (Blake and ranes. In southwest Oregon, three Upper stone also occurs at the southern end of the field Jayko, 1980; Roure, 1979). The restricted Cretaceous (Campanian-?Maastrichtian) for- area (Fig. 2). This sequence has been named the Upper Jurassic Otter Point complex (see below) mations occur in fault and depositional con- "Houstenaden Creek Formation." also occurs only west of the high-angle faults tact with the Upper Jurassic Otter Point The three Upper Cretaceous formations occur (Fig. 1), and, with the Upper Cretaceous rocks, complex, an oceanic assemblage. These four only west of high-angle, north-northwest-trend- it forms a distinct terrane unlike anything east of units occur only west of high-angle, north- ing faults. The faults mark the ragged boundary the faults. The Otter Point complex and Upper northwest-trending faults, and they make up a terrane (Gold Beach terrane) unlike any ter- rane east of these faults. 124° 15' DOTHAN The patterns of sedimentation and the stra- COMPLEX? tigraphie relationships of the three Upper Cretaceous formations indicate that they were deposited in a tectonically active setting influenced by vertical tectonics. Source areas ALLOCHTHONOUS for clasts in the Cretaceous conglomerates JURASSIC A LOWER cannot be found in the adjacent Klamath Figure 1. Tectonostrati- .CRETACEOUS ROCKS Mountains or other nearby terranes. We pos- graphic terranes of south- tulate that they were deposited in a border- west Oregon (Dott and land-type basin with a sediment source at Bourgeois, 1980) based on least as far south as southern California. They Medaris and Dott (1970); re- were translated north during latest Creta- vision of the extent of the ceous to early Paleogene time and were then Otter Point complex and Do- accreted to the Oregon margin. than terranes is based on Blake and Jayko (1980) and INTRODUCTION Roure (1979). Some of the (?thrust) fault boundaries in Southwest Oregon is a region of complex the allochthonous terrane are juxtaposition of tectonostratigraphic terranes not completely understood. (Fig. 1) (Dott and Bourgeois, 1980; Blake and They include Pearse Peak Jayko, 1980). It has been the site for numerous Diorite and overlying sedi- studies of structure, stratigraphy, and tectonics ments and Colebrooke Schist and sedimentation by University of Wisconsin and associated ultramafic personnel (summarized in Dott, 1971) and oth- rocks. ers (for example, Phillips and Clifton, 1974; Walker, 1977; Garcia, 1982). Two Upper Cre- taceous formations—Cape Sebastian Sandstone and Hunters Cove Formation—were mapped by Howard (1961) and named by Dott (1971). Hunter and others (1970) recognized an uncon- formity beneath the Cape Sebastian Sandstone Geological Society of America Bulletin, v. 96, p. 1007-1019, 13 figs., 1 table, August 1985. 1007 Downloaded from gsabulletin.gsapubs.org on November 13, 2015 420 15' BURNT HILL COVE MACK ARCH oj 3 KM I HOUSTENADEN CREEK <5> 1261 Downloaded from gsabulletin.gsapubs.org on November 13, 2015 EVIDENCE FOR WRENCH-FAULT TECTONICS, OREGON 1009 or melange, by some definitions) by Dott mation by Dott (1971). Although broken by Figure 2. Detailed map of Upper Creta- (1971). many faults, the measured thickness of this se- ceous rocks, and the Otter Point complex, Of the entire Otter Point complex, black quence is at least 500 m; it lies unconformably from Cape Sebastian to the southern limit of mudstone and thin-bedded, fine-grained sand- beneath the Cape Sebastian Sandstone (Fig. 4) the Upper Cretaceous rock outcrops (see stone constitute -30% to 40%; sandstone and and is lithologically distinct from it. The repre- Fig. 1). Revision of Dott (1971), with conglomerate, 20% to 30% each; and volcanic sentative section of the Houstenaden Creek additional information from Hunter and oth- rocks, the remainder. The volcanic rocks include Formation is a composite (Fig. 6) based on mea- ers (1970) and Bourgeois (1980a); Dott pillow lavas and volcanic breccias, and the sed- surements of all accessible sections, as well as (1971) and Roure (1979) mapped more de- iments are rich in cherty, volcanic, and dioritic visual inspection of sequences in vertical cliffs. tailed structure. clasts. Compositions of volcanic rocks vary from The most complete section is exposed south of silicic, vitric porphyry pebbles to mafic pillow Burnt Hill Cove (Fig. 2); the entire mapped area basalt. Much of the chert detritus is considered is considered a type area. A 350-m-thick section to be devitrified glass. Coarse-grained diorite at Blacklock Point (Fig. 1) is considered equiv- Cretaceous rocks together will be referred to pebbles and fine-grained dioritic dikes cutting alent to it (Bourgeois, 1980a). here as the "Gold Beach terrane," a name sug- Otter Point rocks are thought to be evidence of Lithology. The coarsening-upward section is gested by Blake and Jayko (1980). exhumed subarc intrusive phases. Serpentine probably thicker than 500 m—the base of the and serpentine breccia occur along shear zones. formation is not exposed, and in construction of Otter Point Complex Despite the intensely sheared nature of much of the composite section, some lateral equivalence the Otter Point complex, some coherent, well- was assumed from Burnt Hill Cove to the north The Otter Point complex is a structurally exposed sections have been studied sedimento- side of Cape Sebastian (a separation of -10 km; complicated assemblage of turbidites, mud- logically (Aalto, 1968; Aalto and Dott, 1970; see Fig. 5), where several hundred metres of stones, volcanic rocks, and lenses of bedded Walker, 1977). These studies indicated that the conglomerate and sandstone crop out in a verti- chert; there are no known exotic blocks. There is Otter Point sediments were principally deep- cal cliff. If little lateral equivalence were as- little stratigraphic continuity within the intensely water, submarine slope, fan, and fan-channel sumed, the formation would constitute at least sheared complex, named the "Otter Point For- deposits. The Otter Point complex, as a whole, 700 m of section. mation" by Koch (1966) but identified as a tec- appears to be an island-arc-derived assemblage The lowermost part of the sequence consists tonostratigraphic complex (broken formation, of Tithonian age (Dott, 1971). Paleocurrent of poorly exposed mudstone with calcareous analysis of sole marks (Aalto, 1968) and of clast concretions and thin siltstone and fine-grained imbrication (Walker, 1977) indicated derivation from the present offshore direction. CAPE SEBASTIAN UPPER CRETACEOUS ROCKS SEA CLIFFS In a coastal strip of southwest Oregon from Cape Sebastian to Houstenaden Creek, three distinct sedimentary sequences of Campanian- ?Maastrichtian age are present (Figs. 2, 4, and MYERS CREEK 5). Stratigraphic, sedimentologic, and structural QUARRY relationships suggest that, within a relatively short time, active faulting, as well as sea-level changes, induced rapid sedimentation and signif- icant changes in depositional environment. The sedimentology of the three formations and their stratigraphic and structural relations are described first, followed by a discussion of petrography and provenance. Readers are re- ferred to papers by Bourgeois (1980a and 1980b) for a more detailed treatment of the sed- imentology and petrology of these formations. Houstenaden Creek Formation Figure 4. Generalized stratigraphic section Figure 3. Humboldt intracontinental plate An unconformity beneath the Cape Sebastian of Upper Cretaceous rocks in southwest of Herd (1978), modified by Dott (1979) to Sandstone was first recognized by Hunter and Oregon (Bourgeois, 1980a) (see Fig. 2 for lo- show its probable northward extension to others (1970), who called the underlying se- calities). At Cape Sebastian, Cape Sebastian coastal and offshore southwest Oregon. Fault quence "lower Cape Sebastian Sandstone." Sandstone was deposited on Houstenaden zones: POF = Port Orford; PRF = Pistol Bourgeois (1980a) recognized that this same Creek Formation (which is in fault contact River; LMF = Lake Montain; HF = Hay ward; lower sequence occurred in a thrust sheet of with Otter Point complex, presumably); at CF = Calaveras; SAF = San Andreas; SGF = overturned strata south of Burnt Hill Cove (see Myers Creek, Cape Sebastian Sandstone San Gregorio. Fig. 2), formerly mapped as Hunters Cove For- rests on Otter Point complex (also see Fig. S). Downloaded from gsabulletin.gsapubs.org on November 13, 2015 1010 BOURGEOIS AND DOTT NORTH SOUTH HUNTERS COVE FORMATION 800-1 Figure 5. Stratigraphie dia- gram of Upper Cretaceous rocks, Cape Sebastian to Hous- tenaden Creek (revision of Dott, 600- 1971) (see Fig. 2 for localities).
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