Downloaded from gsabulletin.gsapubs.org on November 13, 2015

Stratigraphy and sedimentology of Upper 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 (-?Maastrichtian) for- area (Fig. 2). This sequence has been named the Upper 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). At Myers Creek and Pistol SEBASTIAN SANDSTONE River, Tithonian Otter Point complex has been upfaulted and 400- ESS S3 truncated by pre-Cape Sebas- tian erosion. Cross-hachured OTTER HOUST. CK.FM. . POINT ^ bodies in Hunters Cove Forma- COMPLEX tion are postulated channel

200- sandstone(s); the slump at the south end of Hunters Cove con- tains blocks of basal Cape Se- bastian Sandstone (CSS).

CAPE HUNTERSCOVE MYERS PISTOL MACK BURNT HOUSTENADEN SEBASTIAN NORTH SOUTH CHEEK RIVER POINT HILL CREEK AREA AREA COVE AREA sandstone beds, overlain by alternating beds of consist primarily of turbidites, amalgamated Dott (1971). The ~200-m-thick sequence sandstone and mudstone. The sandstone beds sandstone, and channelized conglomerate. The (Fig. 7) is best exposed in sea cliffs north of and are 5- to 50-cm-thick, Bouma Tbcd turbidites abrupt transition from shale and thin-bedded on Cape Sebastian; the type section was mea- that appear to be laterally persistent. The transi- turbidites to amalgamated units presumably rep- sured in Salal Cove (see Fig. 2). tion to thick, amalgamated, medium- to coarse- resents rapid introduction of a prograding supra- Lithology. The Cape Sebastian Sandstone grained sandstone (Fig. 6, at 100 m on the fan to inner-fan, submarine-channel system has been divided into four facies (Fig. 7). The composite section) appears to be abrupt. The into what was a basin-plain or outer-fan conglomeratic facies is composed of a basal remainder of the section exposed south of Burnt environment. conglomerate overlain by through cross-bedded, Hill Cove consists primarily of this sandstone, Abundant burrow structures in midsection plane-bedded, and pebbly, coarse-grained sand- with granule, pebble, and rare cobble conglom- indicate that the environment was well oxygen- stone. The lower hummocky-bedded facies con- erate present as channelized bodies. At least one ated and nutrient-rich; beds rich in plant debris sists exclusively of hummocky-bedded sand- bed of pebbly mudstone and one bed of slump also attest to the latter. Rarity of burrows high in stone with scattered pebble lenses that are less deposits occur within the sequence. the section is probably due to rapid rates of sed- abundant higher in the section. The upper On the north side of Cape Sebastian, the imentation and erosion. The shell and plant de- hummocky-bedded and burrowed facies is Houstenaden Creek Formation consists of bris and well-rounded clasts could have come composed of alternating hummocky-bedded coarse sandstone and abundant channelized from a fluvial/littoral system (?fan delta). The fine-grained sandstone and burrowed sandy silt- conglomerates of varied clast size and composi- calcareous intraclasts were probably ripped stone; other features such as symmetrical ripples tion. Conglomerate clasts are well rounded, from channel walls on the outer shelf or the and layers rich in plant debris are present in this resistant lithologies; shell debris and conspicuous slope. The slump and pebbly mudstone(s) could facies. The uppermost parallel-laminated and calcareous-siltstone intraclasts also are common. also have come from a slope above the fan sur- burrowed sandstone and siltstone facies consists Representative sedimentary structures are illus- face or from a feeder canyon. of zones of very low-angle, hummocky-bedded trated in a composite section (Fig. 6). Age of the Formation. The Houstenaden to horizontally laminated very fine-grained Blacklock Point Strata. Blacklock Point Creek Formation is Campanian in age, based on sandstone alternating with burrowed sandy silt- strata, exposed at Blacklock Point (see Fig. 1), dinoflagellates and angiosperm pollen (W. R. stone beds, which increase in thickness upward sedimentologically resemble the Houstenaden Evitt, 1979, written commun.) and on redepos- in the section. Creek Formation (Bourgeois, 1979). Although ited pelecypods (L. Saul, 1978, written com- Facies Interpretation. The four facies are in- there are no conglomerates at Blacklock Point, mun.). It is possible that the lowermost shales terpreted to represent a progression from near- sandstone petrography of the two sequences is are Albian in age, based on poorly preserved shore to outer-shelf sedimentation (Bourgeois, undistinguishable, and scant paleontological ev- palynomorphs (W. R. Evitt, 1979, written 1980b). The conglomeratic facies was deposited idence suggests that the sequences are correla- commun.) and foraminifera (R. E. Olsen, 1961, in a high-energy nearshore where lunate meg- tive. As Blacklock Point strata are isolated written commun. to R. H. Dott, Jr.). aripples dominated, but with interspersed structurally and do not bear on the interpreta- graded storm layers, pebble lenses, and lami- tions in this paper, they are not discussed in Cape Sebastian Sandstone nated coarse sand. Hummocky bedding has detail (see Bourgeois, 1980a). been interpreted as a shelf-storm deposit (see Facies Interpretation. Strata of the Hous- The Cape Sebastian Sandstone was named Dott and Bourgeois, 1982). In shallow water, tenaden Creek Formation coarsen upward and informally by Howard (1961) and formally by storm waves that affected the bottom were fre- Downloaded from gsabulletin.gsapubs.org on November 13, 2015

EVIDENCE FOR WRENCH-FAULT TECTONICS, OREGON 1011

t) (geographic break)

BOTTOM

Figure 6. Composite section of the Houstenaden Creek Formation (Bourgeois, 1980a). The lower 100 m and upper 100 m are estimated thicknesses; it is probable that the Formation is significantly thicker (see Fig. 5 and text). Many fault-bounded sections were measured in the field, and these sections were (approximately) lithologi- cally correlated. The Formation becomes generally younger from south to north; a large geographic break occurs at -350 m—from Burnt Hill Cove to the north side of Cape Sebastian (see Figs. 2 and 5). quent enough to preclude, or destroy evidence deeper, quieter water, and burrowing could have cause the shelf-wave regime was high-energy of, biogenic activity (lower hummocky-bedded destroyed most physical sedimentary structures. and because the sediment source was a tectoni- facies). Farther offshore, burrowing intervals oc- The Cape Sebastian Sandstone is interpreted cally active, high-relief region (Bourgeois, curred between significant storm events (see to represent sedimentation during a period of 1980b; Bourgeois and Leithold, 1983). Bourgeois, 1980b; Hunter and Clifton, 1982). In transgression of at least 250 m, on the basis of Age of the Formation. The Cape Sebastian the uppermost facies, storm events produced the 200-m thickness of the section plus a 50-m Sandstone is middle or late Campanian in age sand layers but could not have sculpted the sea shelf-water depth. Continuous sedimentation on the basis of an Inoceramus fauna and other bottom; sedimentation rates were slower in the during a relative sea-level rise was possible be- bivalves. Downloaded from gsabulletin.gsapubs.org on November 13, 2015

1012 BOURGEOIS AND DOTT

CAPE SEBASTIAN SANDSTONE Other structures include ball-and-pillow struc- environment. Later (?Eocene), a compressional ture and small-scale penecontemporaneous setting produced thrust faults. The lensoid map faults and web structure in patterns indicative of pattern (Fig. 2) may have been produced during PARALLEL-LAMINATED an extensional regime (Bourgeois, 1982). these earlier events; it has certainly been modi- ANO BURROWED SANOY SILTSTONE The upper half of the sequence is chiefly mud- fied since the thrusting event, and, at present, a FACIES stone, with silty or very fine sandstone beds and transcurrent fault system prevails (Dott, 1979). rare Ta_e turbidites. Also present are broad, low- angle channels and a thick unit of slump breccia, Stratigraphy outer shelf as well as smaller-scale slumped units (see Fig. 8). The slump breccia is 20 m thick in the type There is no depositional contact between the section but thickens laterally to >100 m (Phil- Houstenaden Creek Formation and the Otter lips and Clifton, 1974); significantly, it contains Point complex, and no recognized Otter Point clasts of basal Cape Sebastian Sandstone. detritus occurs in the Houstenaden Creek con- Facies Interpretation. The Hunters Cove glomerates. By middle Campanian time, how-

UPPER Formation appears to reflect deposition in a ever, the Houstenaden Creek Formation was HUMMOCKY-BEDDED basin that deepened with time, probably in a juxtaposed with the Otter Point complex. Basal AND BURROWED FACIES retrograding submarine-slope basin that was in- Cape Sebastian Sandstone was deposited upon fluenced by contemporaneous vertical (oblique both within a few kilometres's distance (Figs. 4 slip?) faulting. Turbidity currents dominated and 5); additionally, basal Cape Sebastian Sand- during deposition of the lower half; small-scale stone contains Otter Point clasts even where it channels cut across this slope, and slumps intro- lies upon Houstenaden Creek Formation. jrmer_ shelf duced upslope material. The thick sandstones The basal contact of the Cape Sebastian lower shoreface may have been deposited at the mouths of small Sandstone with the Otter Point complex (see I LOWER canyons cut into fault scarps and fed by long- Fig. 2) is exposed in Myers Creek (Dott, 1971), HUMMOCKY-SEDOED shore drift; these canyons are similar to canyons in a fault sliver within Otter Point terrane FACIES now present in the California borderland. (Roure, 1979), and probably also north of the The upper half of the formation represents Pistol River, ~3 km from its mouth. On the nearshore i more pelagic sedimentation, with some turbi- north side of Cape Sebastian, in Salal Cove -Sr^-"--! CONGLOMERATIC dites deposited in broad, shallow channels. The (Fig. 2), a low-angle unconformity is present .y. y...-.-:'.-.-•••Vu V.I FACIES large-scale slump deposit indicates that a pene- between the Cape Sebastian Sandstone and the contemporaneous fault scarp existed nearby, Houstenaden Creek Formation. Both of these which exposed lowermost Cape Sebastian sedi- formations are Campanian in age, the Hous- Figure 7. Composite section of the Cape ments (Phillips and Clifton, 1974) and thus had tenaden Creek being a deep-water deposit and Sebastian Sandstone (after Bourgeois, a vertical offset of as much as 250 m (Fig. 5). the basal Cape Sebastian, a shallow-water 1980b), measured in Salal Cove and on the The progressive deepening-upward of Cape deposit. south side of Cape Sebastian (see Fig. 2). Sebastian and Hunters Cove facies almost cer- The contact between the Cape Sebastian tainly reflects world-wide sea- Sandstone and Hunters Cove Formation is be- level rise (Bond, 1978) but also may reflect lieved to be gradational (Howard, 1961; Dott, Hunters Cove Formation tectonic activity. 1971), although it is obscured by small-scale Age of the Formation. The Hunters Cove faults. The facts that these two formations are The Hunters Cove Formation (Howard, Formation is late Campanian possibly to early conformable, nearly the same age (middle to late 1961; Dott, 1971) is best exposed in Hunters Maastrichtian in age on the basis of ammonites Campanian to ?early Maastrichtian), and petro- Cove (Fig. 2), where an -300-m-thick, fining- (Dott, 1971; D. L. Jones, 1963, written com- graphically similar, as well as the indication of a upward sequence of turbidites, shale, and slump mun.) and poorly preserved dinoflagellates and continuously deepening-upward environment, deposits is exposed (Fig. 8). The Hunters Cove- angiosperm pollen (W. R. Evitt, 1979, written all argue for a transitional contact. The transi- Cape Sebastian contact is assumed to be grada- commun.). tion, however, is from shelf (wave- and current- tional (Fig. 4) but is obscured by small-scale dominated) to slope and base-of-slope (gravity- faults; the top of the formation is not exposed. STRATIGRAPHIC AND STRUCTURAL dominated) deposition, and its exact nature is Lithology. The lower Hunters Cove Forma- RELATIONSHIPS obscure. There are some slumps and very coarse tion consists primarily of Bouma TbC[i turbidites sandstones near the base of the Hunters Cove interbedded with finely laminated and rippled The stratigraphic relationships of the three Formation, whereas the uppermost Cape Sebas- siltstone and mudstone. Coarse-grained, chan- Upper Cretaceous formations, plus the Otter tian Sandstone is finely laminated or burrowed neled sandstone and small-scale slumps are pres- Point complex, are illustrated in Figure 5. The silty sandstone. The initiation of slope sedimen- ent; burrowing is common throughout. The relationships among the three Campanian to tation may have been fault-controlled. most distinctive lithology is thick (1-14 m), fine- (?)lower Maastrichtian sequences indicate that There are blocks of basal Cape Sebastian grained sandstone beds with abundant climbing they were deposited in a tectonically active, ex- Sandstone within the large slump in the Hunters ripples, convolution, and fluid-escape features. tensionally, and possibly transcurrently, faulted Cove Formation, first recognized by Phillips and Downloaded from gsabulletin.gsapubs.org on November 13, 2015

EVIDENCE FOR WRENCH-FAULT TECTONICS, OREGON 1013

TOP Figure 8. Generalized section of the Hunt- 300-1 ers Cove Formation, made primarily on the basis of detailed measurement of the type sec- tion at the north end of Hunters Cove (Bour- low-angle geois, 1980a). At the south end of the cove, channel the slump (at 170-190 m) thickens to >100 m fissile (Phillips and Clifton, 1974). --1 mudstone

Clifton (1974). These blocks indicate that there was possibly 250 m of vertical fault offset during — siltstone Hunters Cove deposition; if the two formations are in part contemporaneous, this offset may have been less. Deformed boundaries of the Cape Sebastian clasts indicate that they were only semiconsolidated at the time of faulting; some slump breccia consists of sharply broken clasts, which were probably derived from older units (?Houstenaden Creek Formation). Perva- sive small-scale extensional faults in the Cape Sebastian Sandstone and in Hunters Cove Sand- stone bodies also attest to penecontemporaneous faulting. In the Hunters Cove Formation, this faulting could be attributed to extension caused by compaction or movement downslope, but low-angle these processes would not have been effective in channel the Cape Sebastian Sandstone. In the Cape Sebastian area (Fig. 2), only Qua- ternary terrace sediments overlie rocks of the Gold Beach terrane. At Cape Blanco, however (Fig. 1), an early to middle Miocene shelf se- quence (Addicott, 1980; Leithold and Bour- geois, 1983) unconformably overlies the Otter Point complex and Cretaceous strata at Black- lock Point. If Blake and Jayko (1980) were cor- rect in their restriction of the Otter Point complex to a fault-bounded coastal strip (Fig. 1), then no Eocene is known to rest on the Gold Beach terrane. There are, however, low- ermost Eocene marine shales in fault contact with the Otter Point complex on Cape Blanco. In summary, salient stratigraphic characteris- tics of the Upper Cretaceous rocks include: — 0) n W <0 (1) relatively thin (tens to hundreds of metres) sequences that are difficult to correlate laterally; (2) relatively rapid vertical facies changes, in- cluding major shifts from prograding deep- to transgressive shallow- to deep-water sedimenta- tion; (3) at least one unconformity within the section; and (4) stratigraphic, sedimentologic, and structural evidence for repeated periods of vertical uplift and local extensional faulting dur- ing deposition. These features are all characteris- very coarse- tic of sedimentation in wrench-tectonic settings gr. sandstone (Ballance and Reading, 1980) that are asso- ciated with either oblique subduction or strike- BASE slip faulting. One unusual aspect of the Oregon Downloaded from gsabulletin.gsapubs.org on November 13, 2015

1014 BOURGEOIS AND DOTT

sequence is that it preserves a 200-m-thick HUNTERS COVE transgression • transgressive shelf sequence (Bourgeois, 1980b). DEPOSITION Howell and others (1980a) suggested that shelf sediments are rare in wrench-tectonic settings, LATE CAMPANIAN which tend to have rugged coastlines and nar- OR ? EARLY ~ OR© Ö P C^^Tfs row shelves where fluvio-deltaic systems may MAESTRICHTI AN ? debouch directly into deep-water environments. Nevertheless, the Cretaceous rocks in southwest Oregon contrast markedly with typical forearc- CAPE SEBASTION (CSS) MID transgression—». DEPOSITION basin sequences that are characterized by CAMPANIAN (1) lateral uniformity measured in hundreds to thousands of kilometres; (2) stratigraphie, litho- logie, and pétrographie uniformity measured in hundreds to thousands of metres of thickness; and (3) sequences reflecting relative tectonic stability for periods of 5 m.y. or more. HOUSTENADEN CREEK (HCK) DEPOSITION

Structure and Tectonics EARLY

CAMPANIAN ?ra(ted Structures that have affected the coastal se- continental quences are discussed chronologically, with im- block? plications for the tectonic setting. A postulated tectonosedimentologic history of the Upper Cre- EARLY taceous rocks is illustrated in Figure 9. The structure of the southwest Oregon coast is cov- CRETACEOUS ered in more detail in Dott (1971) and Roure OTTER (1979). POINT COMPLEX (OPC) Pre-Late Cretaceous. The Otter Point com- plex is intensely sheared, and much of this de- j OTTER POINT formation took place before the mildly de- LATE ' DEPOSITION formed Upper Cretaceous rocks were deposited JURASSIC + ?older volcano-plutonic upon it. The trend of many of the structures in (TITHONIAN) and basement the Otter Point complex is northwest, parallel to complex? existing faults and shear zones (Koch, 1966), but much of this orientation may be related to post- ©= MOVEMENT INTO PAGE 0 = MOVEMENT OUT OF PAGE Cretaceous compression and faulting. Serpen- tine in shear zones appears to have been Figure 9. Possible tectono-sedimentologic scenario that could produce the Gold Beach intruded along faults. It may have originated terrane of southwest Oregon. Late Jurassic until early Campanian was a time of compressional from oceanic basement beneath the Otter Point tectonics. Early Campanian to early Maastrichtian time was a period of extensional and/or complex (L. G. Medaris, 1980, oral commun.). strike-slip tectonics (wrench tectonics?). A general west-east orientation was assumed (based Intense shearing of the Otter Point complex on present map orientation), and paleocurrent directions were considered. Contemporaneous suggests that it was accreted to a continental and postdepositional deformation and rotation could belie these constraints. margin during a period of compression ^sub- duction). The age of this deformation is not known precisely; it may have begun in the Late tive Otter Point clasts in the Houstenaden Creek land-type basins probably provided the tectono- Jurassic and continued into the Cretaceous Formation. By the time that Cape Sebastian sed- sedimentologic framework. (Dott, 1971). Laumontite in the Otter Point imentation commenced, however, the Otter Post-Late Cretaceous. Sometime between probably was developed during this event, which Point complex was exposed in a fault block ad- Maastrichtian and Miocene time, the Gold Roure (1979) suggested culminated in the Al- jacent to the Houstenaden Creek Formation Beach terrane of southwest Oregon was sub- bian. The Otter Point complex was further de- (Fig. 9). jected to a compressional, perhaps subduction- formed in later events. Hunters Cove deposition (late Campanian to related, regime. The Houstenaden Creek Forma- Late Cretaceous. The Houstenaden Creek ?early Maastrichtian) also was influenced by ac- tion was thrust over Hunters Cove Formation, Formation was mildly deformed, uplifted, and tive vertical faulting. It appears that the envi- and the Otter Point complex over the Hous- truncated within Campanian time, before shal- ronment represented by Campanian to (?) Maa- tenaden Creek Formation (see Fig. 2) and prob- low-water Cape Sebastian Sandstone was de- strichtian rocks in southwest Oregon was ably also over the other two Cretaceous posited upon it. The Otter Point complex, which subject to periods of relatively rapid uplift and formations (Roure, 1979). The Upper Creta- now surrounds the Houstenaden Creek Forma- subsidence, perhaps the result of several phases ceous rocks may have been tectonically tion, has not been documented as basement for of compression and extension. High-angle faults, kneaded into the Otter Point complex (see the Houstenaden Creek Formation. There are with vertical and possibly transcurrent motion, Scholl and others, 1980), thus producing the no known depositional contacts and no distinc- were active during deposition. Small, border- lensoidal map pattern. The Upper Cretaceous Downloaded from gsabulletin.gsapubs.org on November 13, 2015

EVIDENCE FOR WRENCH-FAULT TECTONICS, OREGON 1015

HOUSTENADEN CREEK cation of Dott, 1964). The sand grains are BLACKLOCK POINT Figure 10. Q (quartz, poly- angular to subrounded, and many of the unsta- crystalline quartz, chert), F ble or deformable grains, such as lithic volcanic (total feldspar), L (aphanitic fragments and biotite, have been squeezed into lithic fragments) diagram pore spaces, producing "pseudomatrix" (Dick- comparing Upper Cretaceous inson, 1970) and a resultant close-packed sandstones of southwest framework. Unstable lithic grains and feldspars Oregon with Upper Creta- are moderately to highly altered; sericitization ceous Great Valley sequence and calcite cementation and replacement are (Ingersoll, 1978) and Salinian most typical, and diagenetic chlorite, zeolite and Nacimiento block sand- minerals, chalcedony, and clay are also com- stones (Lee-Wong and How- mon. Despite this alteration, the most probable ell, 1977). For complete pét- original detrital-grain composition was noted for rographie data, see Bour- statistical analysis. geois (1980a). A number of parameters have been developed to aid in defining sandstone petrography and in determining provenance and basin types (Dick- inson, 1970; Dickinson and Rich, 1972; Dickin- son and others, 1979a; Dickinson and Suczek, 1979). These parameters were calculated for the rocks have not intensely sheared, however, and tonostratigraphic boundaries (Figs. 1 and 2) that Upper Cretaceous sandstones in southwest they remain relatively mildly deformed, al- were reactivated in late Cenozoic time. Oregon and compared with sandstones of sim- though pervasively faulted. ilar age from California (Figs. 10 and 11; see Perhaps as early as Miocene time (Dott, PETROGRAPHY AND PROVENANCE Fig. 12 for locations). Overall, the mineralogy of 1979) and continuing to the present, strike-slip many of the Upper Cretaceous sandstones on tectonics have affected the coastal Sandstones the West Coast is strikingly similar (Lee-Wong rocks in a zone of decoupling between the North and Howell, 1977), and the southwest Oregon American and the Pacific and Gorda plates— Sandstone petrographies of the three Upper sandstones do not fall easily into any petrofacies the Humboldt microplate (Fig. 3) (Herd, 1978). Cretaceous formations in southwest Oregon are classification. They are most like the "late Late In northern California, this recent faulting is very similar (Figs. 10 and 11) and are treated Cretaceous" of the Great Valley sequence (In- parallel to the Hayward branch of the San together. Well-sorted, fine- to medium-grained gersoll, 1978) and the uppermost Cretaceous Andreas fault system (see Kelsey and Cashman, sandstones were studied petrographically, using sandstones in the California Coast Ranges (Bai- 1983). In southwest Oregon, however, it ap- Dickinson's (1970) recommended technique for ley and others, 1964), particularly those of the pears that at least some of the north-north- point counting. The sandstones are volcano- Nacimiento block (Lee-Wong and Howell, west-trending faults are older, fundamental tec- lithic, feldspathic arenites (Fig. 10) (using classifi- 1977) (Fig. 12). In some ternary diagrams, however, the sandstones fall between fields for the Great Valley sequence and the Salinian block. In order to characterize sandstones from var- ious tectonic settings, Dickinson and Suczek (1979) plotted numerous sandstone composi- tions on various ternary diagrams, defining fields for continental-block, magmatic-arc, and re- cycled-orogen provenances. Ingersoll and Suc- zek (1979) produced some refinements, using slightly different criteria. The Salinian block (Fig. 12) would exemplify derivation from an uplifted continental basement provenance. Great Valley sequence sandstones indicate a magmat- ic-arc provenance—the Sierra Nevada. Selected Nacimiento block (Fig. 12) sandstones had a subduction-complex (recycled-orogen) prov- Figure 11. Three ternary diagrams comparing petrographic characteristics of Upper Cre- enance (Lee-Wong and Howell, 1977). The taceous rocks of southwest Oregon (X and O) and California on more refined bases than in Upper Cretaceous of southwest Oregon appears Figure 10. GVS = Great Valley sequence, and SMB = Sierra Madre Basin (both from Dickin- to represent a hybrid of these provenances, plus son and others, 1979b); NB = Nacimiento block, and SB = Salinian block (both from Lee-Wong oceanic volcanic components from the Otter and Howell, 1977). Qm = monocrystalline quartz; F = total feldspar; Lt = total lithic grains, Point complex. Unlike some California Coast including polycrystalline quartz; Qp = polycrystalline quartz; Ls = lithic sedimentary clasts; Lv = Range sandstones with subduction-complex lithic volcanic clasts; P = plagioclase feldspar; K = potassium feldspar. For complete petro- components, no low-temperature, high-pressure graphic data for the Oregon samples, see Bourgeois (1980a). (blueschist) minerals have been positively identi- Downloaded from gsabulletin.gsapubs.org on November 13, 2015

1016 BOURGEOIS AND DOTT fied in the southwest Oregon rocks, in spite of in the field and in the laboratory (Table 1). Dis- (Table 1). The Cape Sebastian conglomerates search in some heavy-mineral separates (Dott, tinctive clast lithologies include fine-grained si- contain slightly more quartzite than do the 1971). licic to intermediate volcanic rocks, feldspathic Houstenaden Creek, but durability of quartzite porphyries, welded tuffs, quartzite, and felsic during reworking could account for its increased Conglomerates plutonic rocks. The basal Cape Sebastian Sand- concentration. stone contains large sandstone boulders that are Provenance of the Houstenaden Creek con- The upper part of the Houstenaden Creek probably concretions derived from the Hous- glomerates is problematical, for no distinctive Formation and the lower 20 m of Cape Sebas- tenaden Creek Formation, and distinctive Otter Otter Point lithologies have been recognized in tian Sandstone contain conglomerates with Point lithologies (chert-pebble conglomerate, the conglomerate. The next most likely apparent pebble- to cobble-sized clasts that were counted cherty sandstone) are common; the large pro- source, based upon present-day proximity, portion of sedimentary clasts in the Cape Sebas- would be the Dothan and Klamath terranes tian Sandstone is primarily from the Otter Point (Figs. 1 and 12), but two major problems argue complex. The remainder of the clasts in the against them as sources for the Houstenaden Cape Sebastian Sandstone could have been de- Creek Formation. First, many of the Hous- rived from Houstenaden Creek conglomerates tenaden Creek clast lithologies are not known, at present, to occur to the east; lithologies very dif- ferent from those present in the Houstenaden Creek Formation are found on gravel bars of the modern Rouge River, which dissects the Dothan and Klamath terranes. None of the distinctive Cape Blanco vein quartz so prevalent in the adjacent Cole- Cape Mendocino brooke Schist has been found in the Upper Cre- San Francisco taceous rocks. Second, with respect to the Klamaths, it has been postulated that the Kla- Los Angeles math Mountains did not rotate into their present San Diego location until Eocene to Oligocene time (Simp- San Andreas Fault son and Cox, 1977; Cox and Magill, 1980). Al- though the Upper Cretaceous rocks could have Sur Nacimiento Fault been attached to the Klamath block at this time, Garlock Fault there is no evidence that they were. There are no depositional contacts, as there are for the Great Valley sequence where it overlaps the Sierra Nevada and southern Klamaths. Instead, the Gold Beach terrane is in high-angle fault contact with the Dothan and Klamath terranes. Apparently, its final emplacement was after ro- tation of the Klamaths. The Houstenaden Creek conglomerates are markedly rich in volcanic clasts (Fig. 13), nota- bly fine-grained silicic to intermediate volcanic

200 km CALIFORNIA

Figure 12. Map of general physiographic and tec- tonic provinces of California and southwest Oregon (after Nilsen and Clarke, 1975; Howell and others, 1977; Irwin, 1977). In some recent papers, the Salinian block has been referred to as the "Salinia composite terrane" and the Nacimiento block renamed the "Sur Obispo terrane" (see Vedder and others, 1983). Downloaded from gsabulletin.gsapubs.org on November 13, 2015

EVIDENCE FOR WRENCH-FAULT TECTONICS, OREGON 1017

TABLE 1. CLAST COMPOSITIONS, HOUSTENADEN CREEK was adjacent to a Houstenaden Creek basin in California (Fig. 13) shows that Oregon con- AND CAPE SEBASTIAN CONGLOMERATES southwest Oregon when the conglomerate was glomerates are generally similar to some, but

HCF CSS deposited, and later this block was rafted away; not all, of the California conglomerates. The or the Houstenaden Creek Formation was adja- Oregon conglomerates contain fewer plutonic Volcanic rocks cent to a sediment-source terrane elsewhere and and metamorphic components than do those in Fine-grained, silicic-intermediate 35% to 55% 5% to 25% Feldspathic porphyries 12% to 20% 1% to 6% has since been moved and emplaced in south- California, perhaps suggesting a shallower Welded tuff 3% to 10% 0% to 2% Other 0% to 5% 12% to 26%* west Oregon. The former scenario would still source in a magmatic-arc complex, or longer not explain the absence of Klamath-derived transport distances, which could cause the de- Chert 6% to 10% 8% to 13% clasts, and the latter hypothesis seems to be struction of less durable clasts (Abbott and Siltstone and shale 3% to 10% 7% to 8« Sandstone 3% to 10% 17% to 45% much more likely. The tectonic history of the Peterson, 1978). Some Oregon conglomerates west coast of North America would suggest that contain abundant sedimentary clasts, particu- Silicic plutonic rocks 2% to 6% l%to 2% the Gold Beach terrane may have been trans- larly those from Cape Sebastian Sandstone, be- Quartzite l%to 6% 1% to 10% Vein quartz and schist 0% to 3% 2% to 3% lated from the south along strike-slip faults, as cause it had local Houstenaden Creek Forma- have been many other terranes (see, for exam- tion and Otter Point complex as sources. Other 0% to 5% 2% to 4% ple, Beck, 1980; Champion and others, 1980; The Oregon conglomerates resemble selected Note: HCF = Houstenaden Creek; CSS = Cape Sebastian Conglomerates; Vedder and others, 1983; Dickinson, 1983; conglomerates in the Nacimiento block where -200 pebbles per count, calculated in percent, excluding intraclasts. 'Mainly Otter Point volcanic clasts. Jones and others, 1982). they occur in fault contact with Franciscan mé- The geology of southwest Oregon has been lange; they are believed to have been originally largely ignored in over-all tectono-stratigraphic in depositional contact with the Franciscan, as studies of the west coast; northernmost Califor- recycled Franciscan material is found within rocks, including red and gray rhyolites, plagio- nia and southwest Oregon are often left blank them (Cowan and Page, 1975; Howell and oth- clase and potassium-feldspar porphyries, and on paleogeographic reconstructions. Generally, ers, 1977; Underwood, 1977). No blueschist or very distinctive gray, pink, and red welded tuffs. it has been assumed that the Upper Cretaceous serpentine has been identified in Houstenaden Other lithologies include intermediate-volcanic rocks in southwest Oregon are remnants of Creek or Cape Sebastian conglomerates, how- breccia; mafic igneous rocks; granodiorite; gran- Great Valley-type strata, that is, preserved ever, although varied chert clasts and some ite; red, gray, and green chert; radiolarian-rich pieces of a forearc basin (Dickinson, 1976). If mafic igneous debris are present. The Oregon siltstone; arkosic sandstone; orthoquartzite; vein the Cretaceous sequence in Oregon were, at one conglomerates resemble some Salinian-block quartz; and schist. This conglomerate assem- time, part of a forearc basin, however, it was conglomerates that are poor in granitic or blage suggests a predominant shallow-depth deposited farther south, because there is no evi- metamorphic detritus; these Salinian-block con- magmatic-arc provenance (volcanic and granitic dence of a Late Cretaceous arc in Oregon. Also, glomerates are rich in durable silicic-volcanic rocks) of indeterminate age, with more unusual the rapid, fluctuating compressional and exten- and quartzite clasts and contain no blueschist or lithologies derived from country rock or possi- sional tectonics indicated in the Upper Creta- serpentine. The Oregon conglomerates also re- bly recycled from older conglomerates. The ceous sequence would argue for a wrench-tec- semble some Great Valley sequence conglomer- mafic rocks, chert, and radiolarian siltstones tonic setting rather than a strictly compressional ates, but, as already noted, the stratigraphic, may indicate an oceanic-crust contribution. one (Moody and Hill, 1956; Wilcox and others, sedimentologic, and structural evidence argues If one excludes the Klamath terrane, two pos- 1973; Ballance and Reading, 1980). against the Oregon strata being part of a major sible sources for the Houstenaden Creek con- A comparison of Upper Cretaceous conglom- forearc-basin sequence. glomerate remain. Either a continental fragment erates in southwest Oregon and in coastal Franciscan-related Nacimiento-block con- glomerates had multiple sources: a magmatic arc (including metamorphosed ?Paleozoic country S rock), a forearc basin, and an inner trench wall. One fundamental difference between Naci- miento-block rocks and Upper Cretaceous strata Figure 13. Comparison of of southwest Oregon is that the latter are asso- conglomerate compositions, ciated with the Otter Point complex, a relatively Upper Cretaceous of southwest coherent Jurassic island-arc association, rather Oregon (see Table 1) and Cre- than with a more diverse subduction complex. It taceous and Eocene conglomer- may be that the Otter Point complex was an ates of central and southern unusually large block within a Franciscan-type California (see Fig. 12). S = sed- terrane. Except for rare granitic clasts in the Cre- imentary clasts; V + C = volcanic taceous rocks, there is no evidence of a Salinian- clasts plus chert; M + P = meta- type basement beneath the Cape Sebastian morphic and plutonic clasts. terrane. As is the case with the Nacimiento Sources for E: Woodford and block, but contrasting with Coastal Belt Francis- others (1968); for NB, SB: '-•E^' SM SB E CD co SM SB can, the Oregon sequence contains no Paleocene Howell and others (1977), Un- SM SB SB strata; on the Salinian block, there are some Pa- derwood (1977); for SM: Carey V+C M+P leocene rocks, and a pronounced unconformity and Colburn (1978). • - HOUSTENADEN CREEK FM CS - COPE SEBASTIAN SS., SW OREGON is seen in the late Paleocene rocks. E - EOCENE, S CALIFORNIA NB - NACIMIENTO BLOCK, SW CAL I FOR N I Upper Cretaceous conglomerates in south- SB - SALINIAN BLOCK SM - SANTA MONICA MOUNTAINS west Oregon are rich in silicic and intermediate- Downloaded from gsabulletin.gsapubs.org on November 13, 2015

1018 BOURGEOIS AND DOTT

volcanic clasts, and they have no known nearby ited in association with southern California or son, 1984). A major key to this problem is the source. Their composition indicates that they Salinian-Nacimiento sequences, they must have determination of the late Mesozoic to early Ter- were derived from a terrigenous source, and been translated north 1,000 km or more and tiary position of the North America-Kula- they broadly resemble other Upper Cretaceous emplaced in southwest Oregon. Farallon triple junction, which is not known conglomerates in California. It appears most There were two post-Campanian periods of (Beck, 1984). likely that their source was at least hundreds of volcanic quiescence in western North America kilometres, if not a thousand or more, to the (see Armstrong, 1978) that could indicate times south, along the continental margin. of transcurrent faulting rather than orthogonal CONCLUSIONS subduction. Other explanations for these quies- SUMMARY cent periods include changing dips of the sub- Upper Cretaceous rocks in southwest Oregon ducting plate (Coney and Reynolds, 1977, and record rapid sedimentation and rapid changes in Origin of Upper Cretaceous Rocks in others), oblique subduction (Dickinson, 1976, sedimentary environment during a relatively Southwest Oregon 1979), and other more complex explanations short period of time. These conditions and the (see Nilsen and McKee, 1979; Page and Enge- evidence for active vertical faulting during depo- Setting aside speculative models, what do we bretson, 1984). Nevertheless, there are the two sition suggest that the Upper Cretaceous rocks in know about the Upper Cretaceous rocks in periods of quiescence when the Oregon rocks southwest Oregon were deposited in a border- southwest Oregon, and how can their geology could have moved relatively rapidly north: land-basin, wrench-fault-influenced setting. best be explained? The rocks were deposited 65-53.5 Ma and 8-18 Ma (Armstrong, 1978). The Gold Beach terrane appears to be al- almost entirely during the Campanian, possibly How do these observations mesh with current lochthonous. It is bounded on the east by major into the early Maastrichtian. They were depos- plate models? If the rocks moved during the high-angle faults, it is unlike any of the terranes ited in a tectonically active environment where earlier period, consistent with Atwater's (1970) to the east, and many of the conglomerate clasts it appears that significant vertical faulting oc- model, they would have been emplaced by mid- within the coastal terrane have no known source curred at least twice—before Cape Sebastian dle Eocene time. If the rocks moved during in now-adjacent terranes. This Upper Creta- deposition and during Hunters Cove deposition. Oligocene to Miocene time, and they originated ceous sequence bears striking resemblance to A wrench-tectonic setting, rather than a simply from anywhere south of Cape Mendocino (see sequences of similar age in coastal-central and compressional one, seems indicated. Fig. 12), it is not clear how they could have southern California. Their tectonostratigraphic The Upper Cretaceous rocks in southwest passed the triple junction at Cape Mendocino. setting and proposed southerly source are con- Oregon appear to be allochthonous, because Until Beck's (1980) compilation of published sistent with recent data from many parts of the there is no known adjacent source for many of paleomagnetic data for the western edge of western border of North America (for example, their conglomerate clasts. Paleocurrent data are North America, there had been no published see Beck, 1984; Vedder and others, 1983; Jones scattered and therefore inconclusive (Table 1). evidence for northward transport of terranes be- and others, 1982; Beck, 1980; Champion and Complex borderland depositional geometries tween Cape Mendocino and Vancouver Island, others, 1980; Howell and others, 1980b; Jones may have produced the scatter, or, less likely, only evidence for clockwise rotation. The rocks and others, 1977). rotations such as have occurred in the modern of the Gold Beach terrane, however, almost There is, at present, no accepted model that California borderland (Kamerling and Luden- surely come from the south, and preliminary can fully explain the tectonostratigraphy of the dyk, 1979) may have confused the data. A con- paleomagnetic results (M. C. Blake, Jr., 1980, southwest Oregon rocks. Refined models incor- glomerate source may have rafted by, or the written commun.; D. Engebretson, 1981, verbal porating long-distance transport and microplate Otter Point terrane may have been carried in as commun.) suggest major northward movement tectonics must, in the future, accommodate the a microplate from the west, but it seems most of the Gold Beach terrane. Gold Beach terrane in southwest Oregon. Posi- reasonable to compare southwest Oregon strata Early Tertiary, long-distance transport of the tioning the late Mesozoic North America-Kula- with markedly similar strata of the same age in Oregon rocks along the continental margin is Farallon triple junction at a latitude of ~15°N central and southern California. Evidence for a inconsistent with the oblique-subduction models (as in Page and Engebretson, 1984, Fig. 4, op- Cretaceous to Paleogene history of strike-slip of Coney (1978) and Dickinson (1979). Given tion 2) would better explain the tectonic and transport along this coastline is becoming more the above facts and constraints, however, we sedimentologic history of southwestern Oregon and more conclusive (for example, Vedder and postulate that the southwest Oregon, Upper Cre- rocks than would a position of 50°N (Page and others, 1983; Dickinson, 1983; Beck, 1984). taceous sequence was deposited off southern Engebretson, 1984, Fig. 4, option 1). Paleomag- The Oregon sequence may have been depos- California or farther south, was moved north netic studies should be especially useful in refin- ited only a few hundred kilometres south of its with the associated Otter Point complex during ing the models. More detailed petrographic and present position, with a northern Sierran sedi- Maastrichtian to Paleocene time, and was em- geochemical comparisons of clasts in coastal ment source that has been completely eroded placed in Oregon in middle Eocene time, at the Mesozoic sequences of the North American away, but the conglomerates would then neces- outset of renewed volcanism, probably marking West Coast also could shed more light on the sarily have been transported across the Great a change from a principally transcurrent to a provenance problems. The sedimentology and Valley forearc basin. Proximal facies in the subduction regime. This interpretation is in basic stratigraphy of the Upper Cretaceous rocks in Oregon Cretaceous rocks are evidence against agreement with one possible reconstruction of southwest Oregon have provided important in- this possibility. Alternatively, the Oregon strata the positions of, and relative motions between, formation bearing on their tectonostratigraphy. may have been deposited in proximity to Sono- North America and adjacent oceanic plates in It will be a challenge of the future to fit them ran or other similar volcanic sequences even Late Cretaceous to early Tertiary time (Atwater, into the over-all tectonic history of the west farther south. If the rocks were originally depos- 1970; Engebretson, 1982; Page and Engebret- coast of North America. Downloaded from gsabulletin.gsapubs.org on November 13, 2015

EVIDENCE FOR WRENCH-FAULT TECTONICS, OREGON 1019

Geology, v. 6, p. 247-250. California margin: A plate tectonic margin of subduction, oblique sub- ACKNOWLEDGMENTS Bourgeois, J., 1979, Sedimentology of two progradational submarine-channel duction and transform tectonics: International Association of Sedimen- deposits on an active continental margin, Cretaceous, southwestern tologists Special Publication 4, p. 43-62. Oregon: Geological Society of America Abstracts with Programs, Howell, D. G., McLean, H„ and Vedder, J. G., 1980b, Late Cretaceous sutur- The basis for this study was part of a Ph.D. v. 11(7), p. 392. ing and translation of the Salinian and Nacimiento blocks, California 1980a, Sedimentology and tectonics of Upper Cretaceous rocks, [abs.]: EOS (American Geophysical Union Transactions), v. 61(46), dissertation by Bourgeois (1980a) at the Univer- southwest Oregon [Ph.D. dissert.]: Madison, Wisconsin, University of p. 948. sity of Wisconsin-Madison, under the supervi- Wisconsin, 298 p. Hunter, R. E., and Clifton, H. E., 1982, Cyclic deposits and hummocky cross- — 1980b, A transgressive shelf sequence exhibiting hummocky stratifica- stratification of probable storm origin in Upper Cretaceous rocks of the sion of Dott. We have benefited from innu- tion: The Cape Sebastian Sandstone (Upper Cretaceous), southwestern Cape Sebastian area, southwestern Oregon: Journal of Sedimentary Oregon: Journal of Sedimentary Petrology, v. 50, p. 681-702. Petrology, v. 52, p. 127-144. merable discussions with colleagues in the field 1982, Deposition and penecontemporaneous deformation of sediments Hunter, R. E„ Clifton, H. E„ and Phillips, R. L„ 1970, Geology of the stacks and at meetings during this time of rapidly on a faulted submarine slope: Hunters Cove formation (Campanian- and reefs off the southern Oregon coast: Ore Bin, v. 32, p. 185-204. Maestrichtian) SW Oregon: American Association of Petroleum Geol- Ingersoll, R. V., 1978, Petrofacies and petrologic evolution of the Late Creta- changing tectonic models for the West Coast. ogists Bulletin, v. 66, p. 551. ceous forearc basin, northern and central California: Journal of Geology, Bourgeois, J., and Leithold, E. L., 1983, Sedimentation, tectonics and sea-level v. 86, p. 335-352. Among those we would especially like to thank change as reflected in four wave-dominated shelf sequences in Oregon Ingersoll, R. V., and Suczek, C A., 1979, Petrology and provenance of Neo- are M. E. Beck, Jr.; M. C. Blake, Jr.; H. E. and California, in Larue, D. K., and Steel, R. J., eds., Cenozoic marine gene sands from Nicob&r and Bengal fans, DSDP Sites 211 and 218: sedimentation, Pacific Margin, U.S.A.: Society of Economic Paleontol- Journal of Sedimentary Petrology, v. 49, p. 1217-1228. Clifton; D. S. Cowan; W. R. Dickinson; D. En- ogists and Mineralogists, Pacific Section, p. 1-17. Irwin, W. P., 1977, Review of Paleozoic rocks of the Klamath Mountains, in Carey, S. McD., and Colburn, I. P., 1978, Late Cretaceous sedimentation in the Stewart, J. H., and others, eds., Paleozoic paleogeography of the west- gebretson; D. G. Howell; R. E. Hunter; D. G. Santa Monica Mountains, California, in Howell, D. G-, and ern United States: Society of Economic Paleontologists and Mineralo- Jones; J. Magill; L. G. Medaris; R. L. Phillips; McDougall, K. A., eds., Mesozoic paleogeography of the western gists, Pacific Section, Pacific Coast Paleogeography Symposium 1, United States: Society of Economic Paleontologists and Mineralogists, p. 441-450. and F. Roure. Pacific Section, p. 547-558. Jones, D. L„ Silberling, N. J., and Hillhouse, J., 1977, Wrangellia—A dis- Champion, D., Grommi, S., and Howell, D. G., 1980, Paleomagnetism of the placed terrane in northwestern North America: Canadian Journal of The advice and critical review by M. C. Cretaceous Pigeon Point Formation and inferred northward displace- Earth Sciences, v. 14, p. 2565-2577. ment of 2500 km for the Salinian Block, California [abs.]: EOS (Ameri- Jones, D. L., Cox, A., Coney, P., and Beck, M., 1982, The growth of western Blake, Jr., D. S. Cowan, and particularly of can Geophysical Union Transactions), v. 61(46), p. 948. North America: Scientific American, November, p. 70-84. D. G. Howell are gratefully acknowledged. Coney, P. J., 1978, Mesozoic-Cenozoic Cordilleran plate tectonics: Geological Kamerling, M. J., and Ludendyk, B. P., 1979, A model for Neogene tectonics Society of America Memoir 152, p. 33-50. of the inner southern California borderland constrained by paleomag- H. E. Clifton and P. D. Snavely reviewed the Coney, P. J., and Reynolds, S. J., 1977, Cordilleran Benioff zones: Nature, netic data: Geological Society of America Abstracts with Programs, v. 270, p. 403-406. v. 11(7), p. 453. submitted manuscript, and their careful, con- Coney, P. J., Jones, D. L., and Monger, J.W.H., 1980, Cordilleran suspect Kelsey, H. M., and Cashman, S. M., 1983, Wrench faulting in northern Cali- structive criticism is appreciated. terranes: Nature, v. 288, p. 329-333. fornia and its tectonic implications: Tectonics, v. 2, p. 565-576. Cowan, D. S., and Page, B. M., 1975, Recycled Franciscan material in Francis- Koch, J. G., 1966, Late Mesozoic stratigraphy and tectonic history, Port Support for this project was provided princi- can melange west of Paso Robles, California: Geological Society of Orford-Gold Beach area, southwestern Oregon coast: American America Bulletin, v. 86, p. 1089-1095. Association of Petroleum Geologists Bulletin, v. 50, p. 25-71. pally by National Science Foundation Grant Cox, A., and Magill, J., 1980, Tectonic rotation of the Oregon western Cas- Lee-Wong, F., and Howell, D. G., 1977, Petrography of Upper Cretaceous EAR77-13132 to R. H. Dott, Jr., and also by an cades: Oregon Department of Geology and Mineral Industries Special sandstone in the Coast Ranges of central California, in Howell, D. G., Paper 10,67 p. and others. Cretaceous geology of the California Coast Ranges, west of American Association of Petroleum Geologists Dickinson, W. R., 1970, interpreting detrital modes of graywacke and arkose: the San Andreas fault: Society of Economic Paleontologists and Miner- Journal of Sedimentary Petrology, v. 40, p. 695-707. alogists, Pacific Section, Pacific Coast Paleogeography Field Guide 2, Grant-in-Aid (1977) and a Geological Society 1976, Sedimentary basins developed during evolution of Mesozoic- p. 47-55. of America Research Grant (1977-1978), both Cenozoic arc-trench system in western North America: Canadian Jour- Leithold, E. L., and Bourgeois, J., 1983, Sedimentology of the sandstone of nal of Earth Sciences, v. 13, p. 1268-1287. Floras Lake (Miocene) transgressive, high-energy shelf deposition, SW to Bourgeois. A National Science Foundation 1979, Cenozoic plate tectonic setting of the Cordilleran region in the Oregon, in Larue, D. K., and Steel, R. J., eds., Cenozoic marine sedi- United States, in Armentrout, J. M., and others, eds., Cenozoic paleo- mentation, Pacific Margin, U.S.A.: Society of Economic Paleontologists Science Faculty Fellowship to Dott in 1978 al- geography of the western United States: Society of Economic Paleon- and Mineralogists, Pacific Section, p. 17-28. lowed an extended stay at the U.S. Geological tologists and Mineralogists, Pacific Section, p. 1-13. Medaris, L. G., and Dott, R. H., Jr., 1970, Mantle-derived peridotites in south- 1983, Cretaceous sinistral strike slip along Nacimiento fault in coastal western Oregon: Relation to plate tectonics: Science, v. 169, Survey, Menlo Park. This study was completed California: American Association of Petroleum Geologists Bulletin, p. 971-974 v. 67, p. 624-645. Moody, J. D., and Hill, M. J., 1956, Wrench-fault tectonics: Geological Society with support from the Graduate School Re- Dickinson, W. R., and Rich, E. I., 1972, Petrologic intervals and petrofacies in of America Bulletin, v. 67, p. 1207-1246. search Fund, University of Washington. the Great Valley Sequence, Sacramento Valley, California: Geological Nilsen, T. H., and Clarke, S. H„ Jr., 1975, Sedimentation and tectonics in the Society of America Bulletin, v. 83, p. 3007-3024. early Tertiary continental borderland of central California: U.S. Geolog- Dickinson, W. R., and Suczek, C. A., 1979, Plate tectonics and sandstone ical Survey Professional Paper 925,64 p. compositions: American Association of Petroleum Geologists Bulletin, Nilsen, T. H., and McKee, E. H., 1979, Paleogene paleogeography of the REFERENCES CITED v. 63, p. 2164-2182. western United States: Society of Economic Paleontologists and Miner- Dickinson, W. R., Ingersoll, R. V., and Graham, S. A., 1979a, Paleogene alogists, Pacific Section, p. 257-276. Aalto, K. R„ 1968, The sedimeniology of the Late Jurassic (Portlandian) Otter sediment dispersal and paleotectonics in northern California: Summary: Page, B. M., and Engebretson, D. C., 1984, Correlation between the geologic Point Formation of southwestern Oregon [M.S. thesis]: Madison, Wis- Geological Society of America Bulletin, Part 1, v. 90, p. 897-898. record and computed plate motions for central California: Tectonics, consin, University of Wisconsin, 60 p. Dickinson, W. R., Helmold, K. P., and Stein, J. A., 1979b, Mesozoic lithic v. 3, p. 113-155. Aalto, K. R„ and Dott, R. H„ Jr., 1970, Late Mesozoic conglomeratic flysch in sandstones in central Oregon: Journal of Sedimentary Petrology, v. 49, Phillips, R. L., and Clifton, H. E., 1974, Late Cretaceous submarine slump southwestern Oregon, and the problem of transport of coarse gravel in p. 501-516. breccia, southwest Oregon: Geological Society of America Abstracts deep water: Geological Association of Canada Special Paper 7, Dott, R. H., Jr., 1964, Wacke, graywacke and matrix: Journal of Sedimentary with Programs, v. 6, p. 235. p. 53-65. Petrology, v. 34, p. 625-632. Roure, F., 1979, Un profil géologique dans les chaînes del l'ouest américain Abbott, P. L„ and Peterson, G. L., 1978, Effects of abrasion durability on 1971, Geology of the southwestern Oregon coast west of the 124th (Californie et sud-ouest Orégon): Discussion d'un modèle de paléosub- conglomerate clast populations: Examples from Cretaceous and Eocene meridian: Oregon Department of Geology and Mineral Industries duction [Thesis]: Paris, Université Pierre-et-Marie-Curie, 148 + XLV p. conglomerates of the San Diego, California area: Journal of Sedimen- Bulletin 69, 63 p. Scholl, D. W., von Huene, R., Vallier, T. L., and Howell, D. G., 1980, Sedi- tary Petrology, v. 48, p. 31-42. 1979, Comment on 'Intracontinental plate boundary east of Cape mentary masses and concepts about tectonic processes at underthrust Addicott, W. O., 1980, Miocene stratigraphy and fossils, Cape Blanco, Oregon: Mendocino': Geology, v. 7, p. 322-323. ocean margins: Geology, v. 8, p. 564-568. Oregon Geology, v. 42, p. 87-98. Dott, R. H., Jr., and Bourgeois, J., 1980, Late Mesozoic tectonics and sedimen- Simpson, R. W., and Cox, A., 1977, Paleomagnetic evidence for tectonic Armstrong, R. L„ 1978, Cenozoic igneous history of the U.S. Cordillera from tation along the southwestern Oregon coast: Geological Society of rotation of the Oregon Coast Range: Geology, v. 5, p. 585-589. lat 42° to 49°N, in Smith, R. B., and Eaton, G. P., eds., Cenozoic America Abstracts with Programs, v. 12(3), p. 104. Underwood, M. B„ 1977, The Pfeiffer Beach slab deposits, Monterey County, tectonics and regional geophysics of the Western Cordillera: Geological 1982, Hummocky stratification—Significance of its variable bedding California: Possible trench-slope basin, in Howell, D. G., and others, Society of America Memoir 152, p. 265-282. sequences: Geological Society of America Bulletin, v. 93, p. 663-680. Cretaceous geology of the California Coast Ranges, west of the San Atwater, T., 1970, Implications of plate tectonics for the Cenozoic tectonic Engebretson, D. C., 1982, Relative motions between oceanic and continental Andreas fault: Society of Economic Paleontologists and Mineralogists, evolution of western North America: Geological Society of America plates in the Pacific basin [Ph.D. dissert.]: Stanford, California, Stanford Pacific Section, Pacific Coast Paleogeography Field Guide 2, p. 57-69. Bulletin, v. 81, p. 3513-3536. University, 211 p. Vedder, J. G., Howell, D. G„ and McLean, H., 1983, Stratigraphy, sedimenta- Bailey, E. H., Irwin, W. P., and Jones, D. L., 1964, Franciscan and related Garcia, M. O., 1982, Petrology of the Rogue River island-arc complex, tion, and tectonic accretion of exotic terranes, southern coastal ranges, rocks and their significance in the geology of western California: Cali- southwest Oregon: American Journal of Science, v. 282, p. 783-807. fornia Division of Mines and Geology Bulletin 183, 177 p. California: American Association of Petroleum Geologists Memoir 34, Herd, D. G., 1978, Intracontinental plate boundary east of Cape Mendocino, Ballance, P. F., and Reading, H. G., eds., 1980, Sedimentation in oblique-slip p. 471-496. California: Geology, v. 6, p. 721-725. mobile zones: International Association of Sedimentologists Special Walker, R. G., 1977, Deposition of late Mesozoic resedimented conglomerates Howard, J. K., 1961, Stratigraphy and structure of the Cape Sebastian-Crook Publication 4, 165 p. and associated turbidites in southwestern Oregon: Geological Society of Point area, southwestern Oregon [M.S. thesis]: Madison, Wisconsin, Beck, M. E., Jr., 1980, Paleomagnetic record of plate-margin tectonic processes America Bulletin, v. 88, p. 273-285. University of Wisconsin, 52 p. along the western edge of North America: Journal of Geophysical Wilcox, R. E„ Harding, T. P., and Seely, D. R., 1973, Basic wrench tectonics: Howell, D. G., Vedder, J. G., McLean, H., Joyce, J. M., Clarke, S. N., Jr„ and Research, v. 85 (B12), p. 7115-7131. American Association of Petroleum Geologists Bulletin, v. 57, p. 74-96. Smith, G., 1977, Review of Cretaceous geology, Salinian and Naci- Woodford, A. O., Welday, E. E., and Merriam, R., 1968, Siliceous tuff clasts in 1984, Introduction to the special issue on correlations between plate miento blocks, Coast Ranges of California, in Howell, D. G., and oth- the upper Paleogene of southern California: Geological Society of motions and Cordilleran tectonics: Tectonics, v. 3, p. 103-105. ers, eds., Cretaceous geology of the California Coast Ranges, west of America Bulletin, v. 79, p. 1461-1486. Blake, M. C., Jr., and Jayko, A., 1980, Late Mesozoic tectonostratigraphic the San Andreas fault: Society of Economic Paleontologists and terranes (microplates) in southwest Oregon: Geological Society of Mineralogists, Pacific Section, Pacific Coast Paleogeography Field America Abstracts with Programs, v. 12(3), p. 98. Guide 2, p. 1-46. MANUSCRIPT RECEIVED BY THF. SOCIETY MARCH 17, 1982 Bond, G. C., 1978, Speculations on real sea-level changes and vertical motions Howell, D. G., Crouch, J. K., Greene, H. G., McCulloch, D. S., and Vedder, REVISED MANUSCRIPT RECEIVED AUGUST 2,1984 of continents at selected times in the Cretaceous and Tertiary periods: J. G., 1980a, Basin development along the late Mesozoic and Cainozoic MANUSCRIPT ACCEPTED AUGUST 17, 1984

Printed in U.S.A. Downloaded from gsabulletin.gsapubs.org on November 13, 2015 Geological Society of America Bulletin

Stratigraphy and sedimentology of Upper Cretaceous rocks in coastal southwest Oregon: Evidence for wrench-fault tectonics in a postulated accretionary terrane

JOANNE BOURGEOIS and R. H. DOTT, JR.

Geological Society of America Bulletin 1985;96, no. 8;1007-1019 doi: 10.1130/0016-7606(1985)96<1007:SASOUC>2.0.CO;2

Email alerting services click www.gsapubs.org/cgi/alerts to receive free e-mail alerts when new articles cite this article Subscribe click www.gsapubs.org/subscriptions/ to subscribe to Geological Society of America Bulletin Permission request click http://www.geosociety.org/pubs/copyrt.htm#gsa to contact GSA Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. Individual scientists are hereby granted permission, without fees or further requests to GSA, to use a single figure, a single table, and/or a brief paragraph of text in subsequent works and to make unlimited copies of items in GSA's journals for noncommercial use in classrooms to further education and science. This file may not be posted to any Web site, but authors may post the abstracts only of their articles on their own or their organization's Web site providing the posting includes a reference to the article's full citation. GSA provides this and other forums for the presentation of diverse opinions and positions by scientists worldwide, regardless of their race, citizenship, gender, religion, or political viewpoint. Opinions presented in this publication do not reflect official positions of the Society.

Notes

Geological Society of America