Stratigraphy and sedimentology of Upper …...EVIDENCE FOR WRENCH-FAULT TECTONICS, OREGON 1009...

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

Southwest Oregon is a region of complex juxtaposition of tectonostratigraphic ter-ranes. In southwest Oregon, three Upper Cretaceous (Campanian-?Maastrichtian) for-mations occur in fault and depositional con-tact with the Upper Jurassic Otter Point complex, an oceanic assemblage. These four units occur only west of high-angle, north-northwest-trending faults, and they make up a terrane (Gold Beach terrane) unlike any ter-rane east of these faults.

The patterns of sedimentation and the stra-tigraphie relationships of the three Upper Cretaceous formations indicate that they were deposited in a tectonically active setting influenced by vertical tectonics. Source areas for clasts in the Cretaceous conglomerates cannot be found in the adjacent Klamath Mountains or other nearby terranes. We pos-tulate that they were deposited in a border-land-type basin with a sediment source at least as far south as southern California. They were translated north during latest Creta-ceous to early Paleogene time and were then accreted to the Oregon margin.

INTRODUCTION

Southwest Oregon is a region of complex juxtaposition of tectonostratigraphic terranes (Fig. 1) (Dott and Bourgeois, 1980; Blake and Jayko, 1980). It has been the site for numerous studies of structure, stratigraphy, and tectonics and sedimentation by University of Wisconsin personnel (summarized in Dott, 1971) and oth-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

on the north side of Cape Sebastian (Fig. 2), and more detailed sedimentologic and stratigraphic field work (Bourgeois, 1980a) established that the sequence beneath the Cape Sebastian Sand-stone also occurs at the southern end of the field area (Fig. 2). This sequence has been named the "Houstenaden Creek Formation."

The three Upper Cretaceous formations occur only west of high-angle, north-northwest-trend-ing faults. The faults mark the ragged boundary

of the Neogene Humboldt microplate (Fig. 3) (Dott, 1979; Herd, 1978), but some of the same faults may be older, fundamental boundaries be-tween tectonostratigraphic terranes (Blake and Jayko, 1980; Roure, 1979). The restricted Upper Jurassic Otter Point complex (see below) also occurs only west of the high-angle faults (Fig. 1), and, with the Upper Cretaceous rocks, it forms a distinct terrane unlike anything east of the faults. The Otter Point complex and Upper

Figure 1. Tectonostrati-graphic terranes of south-west Oregon (Dott and Bourgeois, 1980) based on Medaris and Dott (1970); re-vision of the extent of the Otter Point complex and Do-than terranes is based on Blake and Jayko (1980) and Roure (1979). Some of the (?thrust) fault boundaries in the allochthonous terrane are not completely understood. They include Pearse Peak Diorite and overlying sedi-ments and Colebrooke Schist and associated ultramafic rocks.

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Figure 2. Detailed map of Upper Creta-ceous rocks, and the Otter Point complex, from Cape Sebastian to the southern limit of the Upper Cretaceous rock outcrops (see Fig. 1). Revision of Dott (1971), with additional information from Hunter and oth-ers (1970) and Bourgeois (1980a); Dott (1971) and Roure (1979) mapped more de-tailed structure.

Cretaceous rocks together will be referred to here as the "Gold Beach terrane," a name sug-gested by Blake and Jayko (1980).

Otter Point Complex

The Otter Point complex is a structurally complicated assemblage of turbidites, mud-stones, volcanic rocks, and lenses of bedded chert; there are no known exotic blocks. There is little stratigraphic continuity within the intensely sheared complex, named the "Otter Point For-mation" by Koch (1966) but identified as a tec-tonostratigraphic complex (broken formation,

Figure 3. Humboldt intracontinental plate of Herd (1978), modified by Dott (1979) to show its probable northward extension to coastal and offshore southwest Oregon. Fault zones: POF = Port Orford; PRF = Pistol River; LMF = Lake Montain; HF = Hay ward; CF = Calaveras; SAF = San Andreas; SGF = San Gregorio.

or melange, by some definitions) by Dott (1971).

Of the entire Otter Point complex, black mudstone and thin-bedded, fine-grained sand-stone constitute -30% to 40%; sandstone and conglomerate, 20% to 30% each; and volcanic rocks, the remainder. The volcanic rocks include pillow lavas and volcanic breccias, and the sed-iments are rich in cherty, volcanic, and dioritic clasts. Compositions of volcanic rocks vary from silicic, vitric porphyry pebbles to mafic pillow basalt. Much of the chert detritus is considered to be devitrified glass. Coarse-grained diorite pebbles and fine-grained dioritic dikes cutting Otter Point rocks are thought to be evidence of exhumed subarc intrusive phases. Serpentine and serpentine breccia occur along shear zones. Despite the intensely sheared nature of much of the Otter Point complex, some coherent, well-exposed sections have been studied sedimento-logically (Aalto, 1968; Aalto and Dott, 1970; Walker, 1977). These studies indicated that the Otter Point sediments were principally deep-water, submarine slope, fan, and fan-channel deposits. The Otter Point complex, as a whole, appears to be an island-arc-derived assemblage of Tithonian age (Dott, 1971). Paleocurrent analysis of sole marks (Aalto, 1968) and of clast imbrication (Walker, 1977) indicated derivation from the present offshore direction.

UPPER CRETACEOUS ROCKS

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 5). Stratigraphic, sedimentologic, and structural 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

An unconformity beneath the Cape Sebastian Sandstone was first recognized by Hunter and others (1970), who called the underlying se-quence "lower Cape Sebastian Sandstone." Bourgeois (1980a) recognized that this same lower sequence occurred in a thrust sheet of overturned strata south of Burnt Hill Cove (see Fig. 2), formerly mapped as Hunters Cove For-

mation by Dott (1971). Although broken by many faults, the measured thickness of this se-quence is at least 500 m; it lies unconformably beneath the Cape Sebastian Sandstone (Fig. 4) and is lithologically distinct from it. The repre-sentative section of the Houstenaden Creek Formation is a composite (Fig. 6) based on mea-surements of all accessible sections, as well as visual inspection of sequences in vertical cliffs. The most complete section is exposed south of Burnt Hill Cove (Fig. 2); the entire mapped area is considered a type area. A 350-m-thick section at Blacklock Point (Fig. 1) is considered equiv-alent to it (Bourgeois, 1980a).

Lithology. The coarsening-upward section is probably thicker than 500 m—the base of the formation is not exposed, and in construction of the composite section, some lateral equivalence was assumed from Burnt Hill Cove to the north side of Cape Sebastian (a separation of - 1 0 km; see Fig. 5), where several hundred metres of conglomerate and sandstone crop out in a verti-cal cliff. If little lateral equivalence were as-sumed, the formation would constitute at least 700 m of section.

The lowermost part of the sequence consists of poorly exposed mudstone with calcareous concretions and thin siltstone and fine-grained

CAPE SEBASTIAN SEA CLIFFS

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Figure 4. Generalized stratigraphic section of Upper Cretaceous rocks in southwest Oregon (Bourgeois, 1980a) (see Fig. 2 for lo-calities). At Cape Sebastian, Cape Sebastian Sandstone was deposited on Houstenaden Creek Formation (which is in fault contact with Otter Point complex, presumably); at Myers Creek, Cape Sebastian Sandstone rests on Otter Point complex (also see Fig. S).



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Figure 5. Stratigraphie dia-gram of Upper Cretaceous rocks, Cape Sebastian to Hous-tenaden Creek (revision of Dott, 1971) (see Fig. 2 for localities). At Myers Creek and Pistol River, Tithonian Otter Point complex has been upfaulted and truncated by pre-Cape Sebas-tian erosion. Cross-hachured bodies in Hunters Cove Forma-tion are postulated channel sandstone(s); the slump at the south end of Hunters Cove con-tains blocks of basal Cape Se-bastian Sandstone (CSS).

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sandstone beds, overlain by alternating beds of sandstone and mudstone. The sandstone beds are 5- to 50-cm-thick, Bouma Tbcd turbidites that appear to be laterally persistent. The transi-tion to thick, amalgamated, medium- to coarse-grained sandstone (Fig. 6, at 100 m on the composite section) appears to be abrupt. The remainder of the section exposed south of Burnt Hill Cove consists primarily of this sandstone, with granule, pebble, and rare cobble conglom-erate present as channelized bodies. At least one bed of pebbly mudstone and one bed of slump deposits occur within the sequence.

On the north side of Cape Sebastian, the Houstenaden Creek Formation consists of coarse sandstone and abundant channelized conglomerates of varied clast size and composi-tion. Conglomerate clasts are well rounded, resistant lithologies; shell debris and conspicuous calcareous-siltstone intraclasts also are common. Representative sedimentary structures are illus-trated in a composite section (Fig. 6).

Blacklock Point Strata. Blacklock Point strata, exposed at Blacklock Point (see Fig. 1), sedimentologically resemble the Houstenaden Creek Formation (Bourgeois, 1979). Although there are no conglomerates at Blacklock Point, sandstone petrography of the two sequences is undistinguishable, and scant paleontological ev-idence suggests that the sequences are correla-tive. As Blacklock Point strata are isolated structurally and do not bear on the interpreta-tions in this paper, they are not discussed in detail (see Bourgeois, 1980a).

Facies Interpretation. Strata of the Hous-tenaden Creek Formation coarsen upward and

consist primarily of turbidites, amalgamated sandstone, and channelized conglomerate. The abrupt transition from shale and thin-bedded turbidites to amalgamated units presumably rep-resents rapid introduction of a prograding supra-fan to inner-fan, submarine-channel system into what was a basin-plain or outer-fan environment.

Abundant burrow structures in midsection indicate that the environment was well oxygen-ated and nutrient-rich; beds rich in plant debris also attest to the latter. Rarity of burrows high in the section is probably due to rapid rates of sed-imentation and erosion. The shell and plant de-bris and well-rounded clasts could have come from a fluvial/littoral system (?fan delta). The calcareous intraclasts were probably ripped from channel walls on the outer shelf or the slope. The slump and pebbly mudstone(s) could also have come from a slope above the fan sur-face or from a feeder canyon.

Age of the Formation. The Houstenaden Creek Formation is Campanian in age, based on dinoflagellates and angiosperm pollen (W. R. Evitt, 1979, written commun.) and on redepos-ited pelecypods (L. Saul, 1978, written com-mun.). It is possible that the lowermost shales are Albian in age, based on poorly preserved palynomorphs (W. R. Evitt, 1979, written commun.) and foraminifera (R. E. Olsen, 1961, written commun. to R. H. Dott, Jr.).

Cape Sebastian Sandstone

The Cape Sebastian Sandstone was named informally by Howard (1961) and formally by

Dott (1971). The ~200-m-thick sequence (Fig. 7) is best exposed in sea cliffs north of and on Cape Sebastian; the type section was mea-sured in Salal Cove (see Fig. 2).

Lithology. The Cape Sebastian Sandstone has been divided into four facies (Fig. 7). The conglomeratic facies is composed of a basal conglomerate overlain by through cross-bedded, plane-bedded, and pebbly, coarse-grained sand-stone. The lower hummocky-bedded facies con-sists exclusively of hummocky-bedded sand-stone with scattered pebble lenses that are less abundant higher in the section. The upper hummocky-bedded and burrowed facies is composed of alternating hummocky-bedded fine-grained sandstone and burrowed sandy silt-stone; other features such as symmetrical ripples and layers rich in plant debris are present in this facies. The uppermost parallel-laminated and burrowed sandstone and siltstone facies consists of zones of very low-angle, hummocky-bedded to horizontally laminated very fine-grained sandstone alternating with burrowed sandy silt-stone beds, which increase in thickness upward in the section.

Facies Interpretation. The four facies are in-terpreted to represent a progression from near-shore to outer-shelf sedimentation (Bourgeois, 1980b). The conglomeratic facies was deposited in a high-energy nearshore where lunate meg-aripples dominated, but with interspersed graded storm layers, pebble lenses, and lami-nated coarse sand. Hummocky bedding has been interpreted as a shelf-storm deposit (see Dott and Bourgeois, 1982). In shallow water, storm waves that affected the bottom were fre-




t ) (geographic break )

B O T T O M

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 - 3 5 0 m—from Burnt Hill Cove to the north side of Cape Sebastian (see Figs. 2 and 5).

quent enough to preclude, or destroy evidence of, biogenic activity (lower hummocky-bedded facies). Farther offshore, burrowing intervals oc-curred between significant storm events (see Bourgeois, 1980b; Hunter and Clifton, 1982). In the uppermost facies, storm events produced sand layers but could not have sculpted the sea bottom; sedimentation rates were slower in the

deeper, quieter water, and burrowing could have destroyed most physical sedimentary structures.

The Cape Sebastian Sandstone is interpreted to represent sedimentation during a period of transgression of at least 250 m, on the basis of the 200-m thickness of the section plus a 50-m shelf-water depth. Continuous sedimentation during a relative sea-level rise was possible be-

cause the shelf-wave regime was high-energy and because the sediment source was a tectoni-cally active, high-relief region (Bourgeois, 1980b; Bourgeois and Leithold, 1983).

Age of the Formation. The Cape Sebastian Sandstone is middle or late Campanian in age on the basis of an Inoceramus fauna and other bivalves.




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Figure 7. Composite section of the Cape Sebastian Sandstone (after Bourgeois, 1980b), measured in Salal Cove and on the south side of Cape Sebastian (see Fig. 2).

Hunters Cove Formation

The Hunters Cove Formation (Howard, 1961; Dott, 1971) is best exposed in Hunters Cove (Fig. 2), where an -300-m-thick, fining-upward sequence of turbidites, shale, and slump deposits is exposed (Fig. 8). The Hunters Cove-Cape Sebastian contact is assumed to be grada-tional (Fig. 4) but is obscured by small-scale faults; the top of the formation is not exposed.

Lithology. The lower Hunters Cove Forma-tion consists primarily of Bouma TbC[i turbidites interbedded with finely laminated and rippled siltstone and mudstone. Coarse-grained, chan-neled sandstone and small-scale slumps are pres-ent; burrowing is common throughout. The most distinctive lithology is thick (1-14 m), fine-grained sandstone beds with abundant climbing ripples, convolution, and fluid-escape features.

Other structures include ball-and-pillow struc-ture and small-scale penecontemporaneous faults and web structure in patterns indicative of an extensional regime (Bourgeois, 1982).

The upper half of the sequence is chiefly mud-stone, with silty or very fine sandstone beds and rare Ta_e turbidites. Also present are broad, low-angle channels and a thick unit of slump breccia, as well as smaller-scale slumped units (see Fig. 8). The slump breccia is 20 m thick in the type section but thickens laterally to >100 m (Phil-lips and Clifton, 1974); significantly, it contains clasts of basal Cape Sebastian Sandstone.

Facies Interpretation. The Hunters Cove Formation appears to reflect deposition in a basin that deepened with time, probably in a retrograding submarine-slope basin that was in-fluenced by contemporaneous vertical (oblique slip?) faulting. Turbidity currents dominated during deposition of the lower half; small-scale channels cut across this slope, and slumps intro-duced upslope material. The thick sandstones may have been deposited at the mouths of small canyons cut into fault scarps and fed by long-shore drift; these canyons are similar to canyons now present in the California borderland.

The upper half of the formation represents more pelagic sedimentation, with some turbi-dites deposited in broad, shallow channels. The large-scale slump deposit indicates that a pene-contemporaneous fault scarp existed nearby, which exposed lowermost Cape Sebastian sedi-ments (Phillips and Clifton, 1974) and thus had a vertical offset of as much as 250 m (Fig. 5). The progressive deepening-upward of Cape Sebastian and Hunters Cove facies almost cer-tainly reflects Late Cretaceous world-wide sea-level rise (Bond, 1978) but also may reflect tectonic activity.

Age of the Formation. The Hunters Cove Formation is late Campanian possibly to early Maastrichtian in age on the basis of ammonites (Dott, 1971; D. L. Jones, 1963, written com-mun.) and poorly preserved dinoflagellates and angiosperm pollen (W. R. Evitt, 1979, written commun.).

STRATIGRAPHIC AND STRUCTURAL RELATIONSHIPS

The stratigraphic relationships of the three Upper Cretaceous formations, plus the Otter Point complex, are illustrated in Figure 5. The relationships among the three Campanian to (?)lower Maastrichtian sequences indicate that they were deposited in a tectonically active, ex-tensionally, and possibly transcurrently, faulted

environment. Later (?Eocene), a compressional setting produced thrust faults. The lensoid map pattern (Fig. 2) may have been produced during these earlier events; it has certainly been modi-fied since the thrusting event, and, at present, a transcurrent fault system prevails (Dott, 1979).

Stratigraphy

There is no depositional contact between the Houstenaden Creek Formation and the Otter Point complex, and no recognized Otter Point detritus occurs in the Houstenaden Creek con-glomerates. By middle Campanian time, how-ever, the Houstenaden Creek Formation was juxtaposed with the Otter Point complex. Basal Cape Sebastian Sandstone was deposited upon both within a few kilometres's distance (Figs. 4 and 5); additionally, basal Cape Sebastian Sand-stone contains Otter Point clasts even where it lies upon Houstenaden Creek Formation.

The basal contact of the Cape Sebastian Sandstone with the Otter Point complex (see Fig. 2) is exposed in Myers Creek (Dott, 1971), in a fault sliver within Otter Point terrane (Roure, 1979), and probably also north of the Pistol River, ~3 km from its mouth. On the north side of Cape Sebastian, in Salal Cove (Fig. 2), a low-angle unconformity is present between the Cape Sebastian Sandstone and the Houstenaden Creek Formation. Both of these formations are Campanian in age, the Hous-tenaden Creek being a deep-water deposit and the basal Cape Sebastian, a shallow-water deposit.

The contact between the Cape Sebastian Sandstone and Hunters Cove Formation is be-lieved to be gradational (Howard, 1961; Dott, 1971), although it is obscured by small-scale faults. The facts that these two formations are conformable, nearly the same age (middle to late Campanian to ?early Maastrichtian), and petro-graphically similar, as well as the indication of a continuously deepening-upward environment, all argue for a transitional contact. The transi-tion, however, is from shelf (wave- and current-dominated) to slope and base-of-slope (gravity-dominated) deposition, and its exact nature is obscure. There are some slumps and very coarse sandstones near the base of the Hunters Cove Formation, whereas the uppermost Cape Sebas-tian Sandstone is finely laminated or burrowed silty sandstone. The initiation of slope sedimen-tation may have been fault-controlled.

There are blocks of basal Cape Sebastian Sandstone within the large slump in the Hunters Cove Formation, first recognized by Phillips and




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

Clifton (1974). These blocks indicate that there was possibly 250 m of vertical fault offset during 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 these processes would not have been effective in 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: (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-tic of sedimentation in wrench-tectonic settings (Ballance and Reading, 1980) that are asso-ciated with either oblique subduction or strike-slip faulting. One unusual aspect of the Oregon



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Figure 9. Possible tectono-sedimentologic scenario that could produce the Gold Beach terrane of southwest Oregon. Late Jurassic until early Campanian was a time of compressional tectonics. Early Campanian to early Maastrichtian time was a period of extensional and/or strike-slip tectonics (wrench tectonics?). A general west-east orientation was assumed (based on present map orientation), and paleocurrent directions were considered. Contemporaneous and postdepositional deformation and rotation could belie these constraints.

sequence is that it preserves a 200-m-thick transgressive shelf sequence (Bourgeois, 1980b). Howell and others (1980a) suggested that shelf sediments are rare in wrench-tectonic settings, which tend to have rugged coastlines and nar-row shelves where fluvio-deltaic systems may debouch directly into deep-water environments. Nevertheless, the Cretaceous rocks in southwest Oregon contrast markedly with typical forearc-basin sequences that are characterized by (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.

Structure and Tectonics

Structures that have affected the coastal se-quences are discussed chronologically, with im-plications for the tectonic setting. A postulated tectonosedimentologic history of the Upper Cre-taceous rocks is illustrated in Figure 9. The structure of the southwest Oregon coast is cov-ered in more detail in Dott (1971) and Roure (1979).

Pre-Late Cretaceous. The Otter Point com-plex is intensely sheared, and much of this de-formation took place before the mildly de-formed Upper Cretaceous rocks were deposited upon it. The trend of many of the structures in the Otter Point complex is northwest, parallel to existing faults and shear zones (Koch, 1966), but much of this orientation may be related to post-Cretaceous compression and faulting. Serpen-tine in shear zones appears to have been intruded along faults. It may have originated from oceanic basement beneath the Otter Point complex (L. G. Medaris, 1980, oral commun.).

Intense shearing of the Otter Point complex suggests that it was accreted to a continental margin during a period of compression ^sub-duction). The age of this deformation is not known precisely; it may have begun in the Late Jurassic and continued into the Cretaceous (Dott, 1971). Laumontite in the Otter Point probably was developed during this event, which Roure (1979) suggested culminated in the Al-bian. The Otter Point complex was further de-formed in later events.

Late Cretaceous. The Houstenaden Creek Formation was mildly deformed, uplifted, and truncated within Campanian time, before shal-low-water Cape Sebastian Sandstone was de-posited upon it. The Otter Point complex, which now surrounds the Houstenaden Creek Forma-tion, has not been documented as basement for the Houstenaden Creek Formation. There are no known depositional contacts and no distinc-

tive Otter Point clasts in the Houstenaden Creek Formation. By the time that Cape Sebastian sed-imentation commenced, however, the Otter Point complex was exposed in a fault block ad-jacent to the Houstenaden Creek Formation (Fig. 9).

Hunters Cove deposition (late Campanian to ?early Maastrichtian) also was influenced by ac-tive vertical faulting. It appears that the envi-ronment represented by Campanian to (?) Maa-strichtian rocks in southwest Oregon was subject to periods of relatively rapid uplift and subsidence, perhaps the result of several phases of compression and extension. High-angle faults, with vertical and possibly transcurrent motion, were active during deposition. Small, border-

land-type basins probably provided the tectono-sedimentologic framework.

Post-Late Cretaceous. Sometime between Maastrichtian and Miocene time, the Gold Beach terrane of southwest Oregon was sub-jected to a compressional, perhaps subduction-related, regime. The Houstenaden Creek Forma-tion was thrust over Hunters Cove Formation, and the Otter Point complex over the Hous-tenaden Creek Formation (see Fig. 2) and prob-ably also over the other two Cretaceous formations (Roure, 1979). The Upper Creta-ceous rocks may have been tectonically kneaded into the Otter Point complex (see Scholl and others, 1980), thus producing the lensoidal map pattern. The Upper Cretaceous




Figure 10. Q (quartz, poly-crystalline quartz, chert), F (total feldspar), L (aphanitic lithic fragments) diagram comparing Upper Cretaceous sandstones of southwest Oregon with Upper Creta-ceous Great Valley sequence (Ingersoll, 1978) and Salinian and Nacimiento block sand-stones (Lee-Wong and How-ell, 1977). For complete pét-rographie data, see Bour-geois (1980a).

H O U S T E N A D E N C R E E K B L A C K L O C K P O I N T

rocks have not intensely sheared, however, and they remain relatively mildly deformed, al-though pervasively faulted.

Perhaps as early as Miocene time (Dott, 1979) and continuing to the present, strike-slip tectonics have affected the coastal Mesozoic rocks in a zone of decoupling between the North American and the Pacific and Gorda plates— the Humboldt microplate (Fig. 3) (Herd, 1978). In northern California, this recent faulting is parallel to the Hayward branch of the San Andreas fault system (see Kelsey and Cashman, 1983). In southwest Oregon, however, it ap-pears that at least some of the north-north-west-trending faults are older, fundamental tec-

tonostratigraphic boundaries (Figs. 1 and 2) that were reactivated in late Cenozoic time.

PETROGRAPHY AND PROVENANCE

Sandstones

Sandstone petrographies of the three Upper Cretaceous formations in southwest Oregon are very similar (Figs. 10 and 11) and are treated together. Well-sorted, fine- to medium-grained sandstones were studied petrographically, using Dickinson's (1970) recommended technique for point counting. The sandstones are volcano-lithic, feldspathic arenites (Fig. 10) (using classifi-

Figure 11. Three ternary diagrams comparing petrographic characteristics of Upper Cre-taceous rocks of southwest Oregon (X and O) and California on more refined bases than in Figure 10. GVS = Great Valley sequence, and SMB = Sierra Madre Basin (both from Dickin-son and others, 1979b); NB = Nacimiento block, and SB = Salinian block (both from Lee-Wong and Howell, 1977). Qm = monocrystalline quartz; F = total feldspar; Lt = total lithic grains, including polycrystalline quartz; Qp = polycrystalline quartz; Ls = lithic sedimentary clasts; Lv = lithic volcanic clasts; P = plagioclase feldspar; K = potassium feldspar. For complete petro-graphic data for the Oregon samples, see Bourgeois (1980a).

cation of Dott, 1964). The sand grains are angular to subrounded, and many of the unsta-ble or deformable grains, such as lithic volcanic fragments and biotite, have been squeezed into pore spaces, producing "pseudomatrix" (Dick-inson, 1970) and a resultant close-packed framework. Unstable lithic grains and feldspars are moderately to highly altered; sericitization and calcite cementation and replacement are most typical, and diagenetic chlorite, zeolite minerals, chalcedony, and clay are also com-mon. Despite this alteration, the most probable original detrital-grain composition was noted for statistical analysis.

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 Upper Cretaceous sandstones in southwest Oregon and compared with sandstones of sim-ilar age from California (Figs. 10 and 11; see Fig. 12 for locations). Overall, the mineralogy of many of the Upper Cretaceous sandstones on the West Coast is strikingly similar (Lee-Wong and Howell, 1977), and the southwest Oregon sandstones do not fall easily into any petrofacies classification. They are most like the "late Late Cretaceous" of the Great Valley sequence (In-gersoll, 1978) and the uppermost Cretaceous sandstones in the California Coast Ranges (Bai-ley and others, 1964), particularly those of the Nacimiento block (Lee-Wong and Howell, 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-enance (Lee-Wong and Howell, 1977). The Upper Cretaceous of southwest Oregon appears to represent a hybrid of these provenances, plus oceanic volcanic components from the Otter Point complex. Unlike some California Coast Range sandstones with subduction-complex components, no low-temperature, high-pressure (blueschist) minerals have been positively identi-




fied in the southwest Oregon rocks, in spite of search in some heavy-mineral separates (Dott, 1971).

Conglomerates

The upper part of the Houstenaden Creek Formation and the lower 20 m of Cape Sebas-tian Sandstone contain conglomerates with pebble- to cobble-sized clasts that were counted

in the field and in the laboratory (Table 1). Dis-tinctive clast lithologies include fine-grained si-licic to intermediate volcanic rocks, feldspathic porphyries, welded tuffs, quartzite, and felsic plutonic rocks. The basal Cape Sebastian Sand-stone contains large sandstone boulders that are probably concretions derived from the Hous-tenaden Creek Formation, and distinctive Otter Point lithologies (chert-pebble conglomerate, cherty sandstone) are common; the large pro-portion of sedimentary clasts in the Cape Sebas-tian Sandstone is primarily from the Otter Point complex. The remainder of the clasts in the Cape Sebastian Sandstone could have been de-rived from Houstenaden Creek conglomerates

Cape Blanco Cape Mendocino San Francisco Los Angeles San Diego

San Andreas Fault Sur Nacimiento Fault Garlock Fault

(Table 1). The Cape Sebastian conglomerates contain slightly more quartzite than do the Houstenaden Creek, but durability of quartzite during reworking could account for its increased concentration.

Provenance of the Houstenaden Creek con-glomerates is problematical, for no distinctive Otter Point lithologies have been recognized in the conglomerate. The next most likely apparent source, based upon present-day proximity, would be the Dothan and Klamath terranes (Figs. 1 and 12), but two major problems argue against them as sources for the Houstenaden Creek Formation. First, many of the Hous-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 vein quartz so prevalent in the adjacent Cole-brooke Schist has been found in the Upper Cre-taceous rocks. Second, with respect to the Klamaths, it has been postulated that the Kla-math Mountains did not rotate into their present location until Eocene to Oligocene time (Simp-son and Cox, 1977; Cox and Magill, 1980). Al-though the Upper Cretaceous rocks could have been attached to the Klamath block at this time, 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 C A L I F O R N I A <J

B O R D E R L A N D

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).




TABLE 1. CLAST C O M P O S I T I O N S , H O U S T E N A D E N CREEK A N D C A P E SEBASTIAN C O N G L O M E R A T E S

H C F CSS

Volcanic rocks Fine-grained, silicic-intermediate Feldspathic porphyries Welded tuff Other

35% to 55% 12% to 20%

3% to 10% 0% to 5%

5% to 25% 1% to 6% 0% to 2%

12% to 26%*

Chert 6% to 10% 8% to 13%

Siltstone and shale Sandstone

3% to 10% 3% to 10%

7% to 8 « 17% to 45%

Silicic plutonic rocks 2% to 6% l % t o 2%

Quartzite Vein quartz and schist

l % t o 6% 0% to 3%

1% to 10% 2% to 3%

Other 0% to 5% 2% to 4%

Note: H C F = Houstenaden Creek; CSS = Cape Sebastian Conglomerates; - 2 0 0 pebbles per count, calculated in percent, excluding intraclasts.

'Mainly Otter Point volcanic clasts.

rocks, including red and gray rhyolites, plagio-clase and potassium-feldspar porphyries, and very distinctive gray, pink, and red welded tuffs. Other lithologies include intermediate-volcanic breccia; mafic igneous rocks; granodiorite; gran-ite; red, gray, and green chert; radiolarian-rich siltstone; arkosic sandstone; orthoquartzite; vein quartz; and schist. This conglomerate assem-blage suggests a predominant shallow-depth magmatic-arc provenance (volcanic and granitic rocks) of indeterminate age, with more unusual lithologies derived from country rock or possi-bly recycled from older conglomerates. The mafic rocks, chert, and radiolarian siltstones may indicate an oceanic-crust contribution.

If one excludes the Klamath terrane, two pos-sible sources for the Houstenaden Creek con-glomerate remain. Either a continental fragment

was adjacent to a Houstenaden Creek basin in southwest Oregon when the conglomerate was deposited, and later this block was rafted away; or the Houstenaden Creek Formation was adja-cent to a sediment-source terrane elsewhere and has since been moved and emplaced in south-west Oregon. The former scenario would still not explain the absence of Klamath-derived clasts, and the latter hypothesis seems to be much more likely. The tectonic history of the west coast of North America would suggest that the Gold Beach terrane may have been trans-lated from the south along strike-slip faults, as have been many other terranes (see, for exam-ple, Beck, 1980; Champion and others, 1980; Vedder and others, 1983; Dickinson, 1983; Jones and others, 1982).

The geology of southwest Oregon has been largely ignored in over-all tectono-stratigraphic studies of the west coast; northernmost Califor-nia and southwest Oregon are often left blank on paleogeographic reconstructions. Generally, it has been assumed that the Upper Cretaceous rocks in southwest Oregon are remnants of Great Valley-type strata, that is, preserved pieces of a forearc basin (Dickinson, 1976). If the Cretaceous sequence in Oregon were, at one time, part of a forearc basin, however, it was deposited farther south, because there is no evi-dence of a Late Cretaceous arc in Oregon. Also, the rapid, fluctuating compressional and exten-sional tectonics indicated in the Upper Creta-ceous sequence would argue for a wrench-tec-tonic setting rather than a strictly compressional one (Moody and Hill, 1956; Wilcox and others, 1973; Ballance and Reading, 1980).

A comparison of Upper Cretaceous conglom-erates in southwest Oregon and in coastal

S

California (Fig. 13) shows that Oregon con-glomerates are generally similar to some, but not all, of the California conglomerates. The Oregon conglomerates contain fewer plutonic and metamorphic components than do those in California, perhaps suggesting a shallower source in a magmatic-arc complex, or longer transport distances, which could cause the de-struction of less durable clasts (Abbott and Peterson, 1978). Some Oregon conglomerates contain abundant sedimentary clasts, particu-larly those from Cape Sebastian Sandstone, be-cause it had local Houstenaden Creek Forma-tion and Otter Point complex as sources.

The Oregon conglomerates resemble selected conglomerates in the Nacimiento block where they occur in fault contact with Franciscan mé-lange; they are believed to have been originally in depositional contact with the Franciscan, as recycled Franciscan material is found within them (Cowan and Page, 1975; Howell and oth-ers, 1977; Underwood, 1977). No blueschist or serpentine has been identified in Houstenaden Creek or Cape Sebastian conglomerates, how-ever, although varied chert clasts and some mafic igneous debris are present. The Oregon conglomerates resemble some Salinian-block conglomerates that are poor in granitic or metamorphic detritus; these Salinian-block con-glomerates are rich in durable silicic-volcanic and quartzite clasts and contain no blueschist or serpentine. The Oregon conglomerates also re-semble some Great Valley sequence conglomer-ates, but, as already noted, the stratigraphic, sedimentologic, and structural evidence argues against the Oregon strata being part of a major forearc-basin sequence.

Franciscan-related Nacimiento-block con-glomerates had multiple sources: a magmatic arc (including metamorphosed ?Paleozoic country rock), a forearc basin, and an inner trench wall. One fundamental difference between Naci-miento-block rocks and Upper Cretaceous strata of southwest Oregon is that the latter are asso-ciated with the Otter Point complex, a relatively coherent Jurassic island-arc association, rather than with a more diverse subduction complex. It may be that the Otter Point complex was an unusually large block within a Franciscan-type terrane. Except for rare granitic clasts in the Cre-taceous rocks, there is no evidence of a Salinian-type basement beneath the Cape Sebastian terrane. As is the case with the Nacimiento block, but contrasting with Coastal Belt Francis-can, the Oregon sequence contains no Paleocene strata; on the Salinian block, there are some Pa-leocene rocks, and a pronounced unconformity is seen in the late Paleocene rocks.

Upper Cretaceous conglomerates in south-west Oregon are rich in silicic and intermediate-

Figure 13. Comparison of conglomerate compositions, Upper Cretaceous of southwest Oregon (see Table 1) and Cre-taceous and Eocene conglomer-ates of central and southern California (see Fig. 12). S = sed-imentary clasts; V + C = volcanic clasts plus chert; M + P = meta-morphic and plutonic clasts. Sources for E: Woodford and others (1968); for NB, SB: Howell and others (1977), Un-derwood (1977); for SM: Carey and Colburn (1978).

' - • E ^ ' S M S B E C D c o S M SM S B S B S B

V + C M + P

• - HOUSTENADEN CREEK FM C S - COPE S E B A S T I A N SS. , SW OREGON

E - EOCENE, S C A L I F O R N I A N B - N A C I M I E N T O BLOCK, SW CAL I FOR N I S B - S A L I N I A N BLOCK SM - SANTA MONICA MOUNTAINS




volcanic clasts, and they have no known nearby source. Their composition indicates that they were derived from a terrigenous source, and they broadly resemble other Upper Cretaceous conglomerates in California. It appears most likely that their source was at least hundreds of kilometres, if not a thousand or more, to the south, along the continental margin.

SUMMARY

Origin of Upper Cretaceous Rocks in Southwest Oregon

Setting aside speculative models, what do we know about the Upper Cretaceous rocks in southwest Oregon, and how can their geology best be explained? The rocks were deposited almost entirely during the Campanian, possibly into the early Maastrichtian. They were depos-ited in a tectonically active environment where it appears that significant vertical faulting oc-curred at least twice—before Cape Sebastian deposition and during Hunters Cove deposition. A wrench-tectonic setting, rather than a simply compressional one, seems indicated.

The Upper Cretaceous rocks in southwest Oregon appear to be allochthonous, because there is no known adjacent source for many of their conglomerate clasts. Paleocurrent data are scattered and therefore inconclusive (Table 1). Complex borderland depositional geometries may have produced the scatter, or, less likely, rotations such as have occurred in the modern California borderland (Kamerling and Luden-dyk, 1979) may have confused the data. A con-glomerate source may have rafted by, or the Otter Point terrane may have been carried in as a microplate from the west, but it seems most reasonable to compare southwest Oregon strata with markedly similar strata of the same age in central and southern California. Evidence for a Cretaceous to Paleogene history of strike-slip transport along this coastline is becoming more and more conclusive (for example, Vedder and others, 1983; Dickinson, 1983; Beck, 1984).

The Oregon sequence may have been depos-ited only a few hundred kilometres south of its present position, with a northern Sierran sedi-ment source that has been completely eroded away, but the conglomerates would then neces-sarily have been transported across the Great Valley forearc basin. Proximal facies in the Oregon Cretaceous rocks are evidence against this possibility. Alternatively, the Oregon strata may have been deposited in proximity to Sono-ran or other similar volcanic sequences even farther south. If the rocks were originally depos-

ited in association with southern California or Salinian-Nacimiento sequences, they must have been translated north 1,000 km or more and emplaced in southwest Oregon.

There were two post-Campanian periods of volcanic quiescence in western North America (see Armstrong, 1978) that could indicate times of transcurrent faulting rather than orthogonal subduction. Other explanations for these quies-cent periods include changing dips of the sub-ducting plate (Coney and Reynolds, 1977, and others), oblique subduction (Dickinson, 1976, 1979), and other more complex explanations (see Nilsen and McKee, 1979; Page and Enge-bretson, 1984). Nevertheless, there are the two periods of quiescence when the Oregon rocks could have moved relatively rapidly north: 65-53.5 Ma and 8-18 Ma (Armstrong, 1978).

How do these observations mesh with current plate models? If the rocks moved during the earlier period, consistent with Atwater's (1970) model, they would have been emplaced by mid-dle Eocene time. If the rocks moved during Oligocene to Miocene time, and they originated from anywhere south of Cape Mendocino (see Fig. 12), it is not clear how they could have passed the triple junction at Cape Mendocino. Until Beck's (1980) compilation of published paleomagnetic data for the western edge of North America, there had been no published evidence for northward transport of terranes be-tween Cape Mendocino and Vancouver Island, only evidence for clockwise rotation. The rocks of the Gold Beach terrane, however, almost surely come from the south, and preliminary paleomagnetic results (M. C. Blake, Jr., 1980, written commun.; D. Engebretson, 1981, verbal commun.) suggest major northward movement of the Gold Beach terrane.

Early Tertiary, long-distance transport of the Oregon rocks along the continental margin is inconsistent with the oblique-subduction models of Coney (1978) and Dickinson (1979). Given the above facts and constraints, however, we postulate that the southwest Oregon, Upper Cre-taceous sequence was deposited off southern California or farther south, was moved north with the associated Otter Point complex during Maastrichtian to Paleocene time, and was em-placed in Oregon in middle Eocene time, at the outset of renewed volcanism, probably marking a change from a principally transcurrent to a subduction regime. This interpretation is in basic agreement with one possible reconstruction of the positions of, and relative motions between, North America and adjacent oceanic plates in Late Cretaceous to early Tertiary time (Atwater, 1970; Engebretson, 1982; Page and Engebret-

son, 1984). A major key to this problem is the determination of the late Mesozoic to early Ter-tiary position of the North America-Kula-Farallon triple junction, which is not known (Beck, 1984).

CONCLUSIONS

Upper Cretaceous rocks in southwest Oregon record rapid sedimentation and rapid changes in sedimentary environment during a relatively short period of time. These conditions and the evidence for active vertical faulting during depo-sition suggest that the Upper Cretaceous rocks in southwest Oregon were deposited in a border-land-basin, wrench-fault-influenced setting.

The Gold Beach terrane appears to be al-lochthonous. It is bounded on the east by major high-angle faults, it is unlike any of the terranes to the east, and many of the conglomerate clasts within the coastal terrane have no known source in now-adjacent terranes. This Upper Creta-ceous sequence bears striking resemblance to sequences of similar age in coastal-central and southern California. Their tectonostratigraphic setting and proposed southerly source are con-sistent with recent data from many parts of the western border of North America (for example, see Beck, 1984; Vedder and others, 1983; Jones and others, 1982; Beck, 1980; Champion and others, 1980; Howell and others, 1980b; Jones and others, 1977).

There is, at present, no accepted model that can fully explain the tectonostratigraphy of the southwest Oregon rocks. Refined models incor-porating long-distance transport and microplate tectonics must, in the future, accommodate the Gold Beach terrane in southwest Oregon. Posi-tioning the late Mesozoic North America-Kula-Farallon triple junction at a latitude of ~15°N (as in Page and Engebretson, 1984, Fig. 4, op-tion 2) would better explain the tectonic and sedimentologic history of southwestern Oregon rocks than would a position of 50°N (Page and Engebretson, 1984, Fig. 4, option 1). Paleomag-netic studies should be especially useful in refin-ing the models. More detailed petrographic and geochemical comparisons of clasts in coastal Mesozoic sequences of the North American West Coast also could shed more light on the provenance problems. The sedimentology and stratigraphy of the Upper Cretaceous rocks in southwest Oregon have provided important in-formation bearing on their tectonostratigraphy. It will be a challenge of the future to fit them into the over-all tectonic history of the west coast of North America.




ACKNOWLEDGMENTS

The basis for this study was part of a Ph.D. dissertation by Bourgeois (1980a) at the Univer-sity of Wisconsin-Madison, under the supervi-sion of Dott. We have benefited from innu-merable discussions with colleagues in the field and at meetings during this time of rapidly changing tectonic models for the West Coast. Among those we would especially like to thank are M. E. Beck, Jr.; M. C. Blake, Jr.; H. E. Clifton; D. S. Cowan; W. R. Dickinson; D. En-gebretson; D. G. Howell; R. E. Hunter; D. G. Jones; J. Magill; L. G. Medaris; R. L. Phillips; and F. Roure.

The advice and critical review by M. C. Blake, Jr., D. S. Cowan, and particularly of D. G. Howell are gratefully acknowledged. H. E. Clifton and P. D. Snavely reviewed the submitted manuscript, and their careful, con-structive criticism is appreciated.

Support for this project was provided princi-pally by National Science Foundation Grant EAR77-13132 to R. H. Dott, Jr., and also by an American Association of Petroleum Geologists Grant-in-Aid (1977) and a Geological Society of America Research Grant (1977-1978), both to Bourgeois. A National Science Foundation Science Faculty Fellowship to Dott in 1978 al-lowed an extended stay at the U.S. Geological Survey, Menlo Park. This study was completed with support from the Graduate School Re-search Fund, University of Washington.

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Geological Society of America Bulletin

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

JOANNE BOURGEOIS and R. H. DOTT, JR. Oregon: Evidence for wrench-fault tectonics in a postulated accretionary terraneStratigraphy and sedimentology of Upper Cretaceous rocks in coastal southwest

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