Geological Society of America Centennial Field Guide—Cordilleran Section, 1987

Shelf and deep-marine deposits of Late Cretaceus age, CapeSebastian area, southwest Oregon

Ralph E. Hunter and H. Edward Clifton, U.S. Geological Survey, Menlo Park, California 94025

LOCATION

The outcrops described in this report are in the vicinity ofCape Sebastian, Curry County, Oregon (Fig. 1). The principaloutcrop (Point A, Fig. 2) is in the Cape Sebastian Sandstone asrestricted by Bourgeois (1980), at the south tip of the Cape Sebas-tian headland. Visitors can most easily reach the outcrop by a1.2-mi-long (2 km) foot trail that leads down to the south tip ofthe headland from the parking area at the south end of the roadthrough Cape Sebastian State Park (Point B, Fig. 2). The en-trance to the state park is on U.S. 101, 6 mi (10 km) south ofGold Beach, Oregon. The outcrops of the Hunters Cove Forma-tion of Dott (1971) are located along the shore of Hunters Coveand can most easily be reached by walking northward along thebeach from the parking area alongside U.S. 101 just south of thecove (Point C, Fig. 2).

Figure 1. Index map of southern Oregon coast.

SIGNIFICANCE OF LOCALITY

The Cape Sebastian Sandstone as restricted by Bourgeois(1980) is a shallow-marine sandstone of Late Cretaceus age thatcontains well-exposed cyclic deposits interpreted to have formedduring alternating storms and periods of relative calm. A com-plete cycle (average thickness 5.2 ft; 1.6 m) consists of a lower,storm-deposited sandstone unit characterized by hummockycross-stratification, an upper bioturbated sandstone unit thatformed during relatively calm conditions, and an intermediate

unit composed of planar- to ripple-bedded sandstone and shalethat formed during the waning phase of the storm and at thebeginning of the following calm period. Changes in grain size andsedimentary structures vertically through the section suggest thatthe formation accumulated in gradually deepening water.

The Cape Sebastian Sandstone unconformably overliesthick-bedded turbidite sandstones and pebbly sandstones of theLate Cretaceus Houstenaden Creek Formation of Bourgeois andDott (1985) and underlies, probably conformably, thin-beddedturbidite sandstones and shales of the Late Cretaceus HuntersCove Formation of Dott (1971). The Hunters Cove Formationincludes a submarine-slump breccia and several intervals of tur-bidite sandstone beds without interbedded shale.

GEOLOGIC SETTING

The small area of Upper Cretaceus rocks in the vicinity ofCape Sebastian, Oregon, is remarkable for the range of deposi-tional environments represented. Although the beds have beenmoderately folded and faulted, as shown by the detailed mappingof Howard and Dott (1961), the structural complexity is muchless than that of the unconformably underlying Otter Point For-mation, of Late Jurassic age (Koch, 1966). The Upper Cretace-us rocks were divided by Dott (1971) into the Cape SebastianSandstone and the overlying Hunters Cove Formation, consistingof sandstone and shale. The Cape Sebastian Sandstone was sub-sequently restricted by Bourgeois (1980) to beds that overlie anangular unconformity recognized by Hunter and others (1970)within the formation as defined by Dott (1971). Bourgeois andDott (1985) have named the sequence that underlies the uncon-formity the Houstenaden Creek Formation.

Apart from their occurrence in the Cape Sebastian area,Upper Cretaceus rocks are of very limited occurrence in coastalsouthwestern Oregon. They are known from the Rogue RiverReef, from Blacklock Point, and less certainly from a few otherlocalities (Fig. 1; Hunter and others, 1970; Dott, 1971; Bourgeoisand Dott, 1985). The paleogeography of the depositional basin isunknown, as is the source of the light-colored, quartz- andfeldspar-rich sand and of many of the pebble types (Bourgeoisand Dott, 1985). Fossils indicate that the Upper Cretaceus bedsin the Cape Sebastian area range from Campanian (or possibly asold as Albian) to possibly Maestrichtian in age (Dott, 1971;Bourgeois, 1980; Bourgeois and Dott, 1985).

HOUSTENADEN CREEK FORMATION

About 660 ft (200 m) of the Houstenaden Creek Formationof Bourgeois and Dott (1985) is exposed on the west face of the

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296 R. E. Hunter and H. E. Clifton

Figure 2. Geologic map of Cape Sebastian area. Base modified from Gold Beach 15-minute Quadrangle.Geology modified from Howard and Dott (1961), Hunter and others (1970), Dott (1971), -Bourgeois(1980), Hunter and Clifton (1982), and Bourgeois and Dott (1985). Principal outcrop described in thisreport is labeled A; parking areas are labeled B and C.

Cape Sebastian headland (Fig. 3),and the upper few ft (m) canbe seen at the tips of two small points that bound the cove,informally named Salal Cove by Bourgeois (1980), just north ofthe headland (Fig. 2). All these localities are difficult to reach.The formation in the Cape Sebastian area consists of sandstone,pebbly sandstone, and conglomerate in thick, normally gradedturbidity beds (Hunter and others, 1970). Bourgeois and Dott(1985) consider this facies to overlie a less conglomeratic, moreshaly facies of the formation; this more shaly facies is exposed atthe type locality of the formation near the mouth of HoustenadenCreek, 7.4 mi (12 km) south of Cape Sebastian. The exclusivelysandy and gravelly nature of the turbidites in the Cape Sebastianarea suggest an origin in the proximal (inner) part of a submarinefan or fan channel.

The Houstenaden Creek Formation was deformed andpartly eroded before deposition of the Cape Sebastian Sandstone.At a site on the west face of the Cape Sebastian headland that isprobably not accessible by foot, the Cape Sebastian Sandstoneoverlies the Houstenaden Creek Formation with angular uncon-formity (Fig. 4). Near the mouth of Myers Creek, 1.9 mi (3 km)southeast of Cape Sebastian, the Houstenaden Creek Formationis apparently absent, for beds assigned to the Cape SebastianSandstone unconformably overlie the Otter Point Formation atthat locality (Bourgeois, 1980).

CAPE SEBASTIAN SANDSTONE

The Cape Sebastian Sandstone as restricted by Bourgeois(1980) is well exposed on the Cape Sebastian headland and inSalal Cove, just north of the headland (Fig. 2). In this area theformation is about 660 ft (200 m) thick and has been divided intofour facies (Bourgeois, 1980; Fig. 3). The formation is not richlyfossiliferous, but enough fossils have been found to indicate amarine origin for all facies.

Conglomeratic Facies

The basal 33 ft (10 m) of the Cape Sebastian Sandstoneconsists of conglomerate and crossbedded to planar-bedded peb-bly sandstone. This facies is exposed on the west face of the CapeSebastian headland (Fig. 4) and in Salal Cove, but it is difficult toreach at both places. The facies is also found at several pointsoutside the area shown in Figure 2 (Bourgeois, 1980). The basalconglomerate bed contains sandstone boulders that apparentlyare concretions eroded from the underlying Houstenaden CreekFormation of Bourgeois and Dott (1985). The crossbedding ismostly high-angle (original dip angles 15°–300) and of mediumscale (set thicknesses 8 to 12 in; 20–30 cm). Bivalve shell frag-ments and trace fossils are common.

Late Cretaceus deposits, Cape Sebastian, Oregon 297

Figure 3. Stratigraphic section of rocks exposed in Cape Sebastian ares.Modified from Bourgeois (1980) and Bourgeois and Dott (1985).

The coarse texture and high-angle crossbedding of the con-glomeratic facies suggest deposition in a very shallow shorefaceenvironment. Wave-generated nearshore currents probablyformed the megaripples whose migration was responsible for theformation of the crossbedding.

Lower Hummocky Cross-Stratified Facies

The conglomeratic facies is overlain by about 165 ft (50 m)of medium- to fine-grained sandstone and pebbly sandstone inwhich hummocky cross-stratification is almost the only sedimen-tary structure. Shells and shell fragments are present but notcommon. This hummocky cross-stratified facies is well exposedand accessible along a 1,300 ft-long (400 m) stretch of rocky

Figure 4. Angular unconformity (marked by arrows) between conglom-eratic facies of Cape Sebastian Sandstone and underlying, more steeplydipping beds of Houstenaden Creek Formation, west side of Cape Sebas-tian headland.

shore extending north (downsection) from the south tip of theCape Sebastian headland.

The cross-stratification in this and the overlying facies (Figs.5 and 6) has most of the features listed by Harms and others(1975) as typical of hummocky cross-stratification (Bourgeois,1980). The sets of cross-strata have low-angle (original dip anglesgenerally less than 15°) erosional bounding surfaces. Crosslami-nae immediately above the lower bounding surfaces are approx-imately parallel to these surfaces. Dip angles of the crosslaminaetend to decrease upward within a set, because of a gradual fillingof the scour depressions defined by the wavy lower boundingsurface. Dip directions of the crosslaminae are widely scatteredand show little or no preferred orientation. The cross-stratification in this facies differs somewhat from typical hum-mocky cross-stratification in having few convex-up segments ofbounding surfaces or of the crosslaminae, evidently because suchsegments were preferentially eroded (Bourgeois, 1980; Dott andBourgeois, 1982, 1983).

Although the details of origin are still controversial, hum-mocky cross-stratification probably forms in relatively shallowwater during storms, when oscillatory wave-induced currents arecombined with a superimposed net current (Harms and others,1975, 1982; Bourgeois, 1980; Hunter and Clifton, 1982; Dottand Bourgeois, 1982, 1983; Walker and others, 1983). In thisfacies any nonstorm deposits were completely eroded during sub-sequent storms, leaving a sequence of amalgamated storm depos-its. The water depth is inferred, on the basis of the sandy textureand the absence of high-angle crossbedding, to have been some-what greater than that in which the conglomeratic facies wasdeposited.

Upper Hummocky Cross-Stratified and Burrowed Facies

Above the lower hummocky cross-stratified facies is an ap-proximately 300-ft-thick (90 m) interval containing cyclic alter-

298 R. E. Hunter and H. E. Clifton

Figure 5. Hummocky cross-stratification in Cape Sebastian Sandstonenear south tip of Cape Sebastian headland. The hummocky cross-stratified sandstone overlies burrowed sandstone.

nations of hummocky cross-stratified sandstone and intenselybioturbated sandstone (Bourgeois, 1980; Hunter and Clifton,1982). Shells are rare, but trace fossils are common. The facies iswell exposed at the south tip of the Cape Sebastian headland.

The cycles have erosional bounding surfaces and average 5ft (1.6 m) in thickness. A complete cycle contains three intervals(Fig. 6) (1) a lower interval of fine-grained sandstone character-ized by hummocky cross-stratification; (2) a middle interval offine- to very tine-grained sandstone characterized by planar- andripple-bedding and with a thin shale bed in its middle part; and(3) an upper interval of fine- to very fine-grained silty sandstonein which almost all of the original stratification was destroyed byburrowing. The shale bed in the middle interval occurs in only asmall fraction of the cycles. The hummocky cross-stratification inthis facies differs from that in the underlying facies in havingbetter preservation of convex-up segments of bounding surfacesand of crosslaminae.

The cycles in the upper hummocky cross-stratified and bur-rowed facies resemble cycles in hummocky cross-stratified depos-its elsewhere (Dott and Bourgeois, 1982). The cycles areinterpreted to have originated by the alternation of storm andrelatively calm conditions (Hunter and Clifton, 1982). Erosionduring the intensifying phase of a storm produced the lowerbounding surface of a cycle, according to this interpretation. Thelower, stratified, fining-upward part of a cycle, up to the top ofthe shale bed in the middle interval of the cycle, was depositedduring the gradually waning phase of a storm. The part of theplanar- and ripple-bedded sandstone above the shale bed wasprobably deposited during relatively calm conditions after thestorm but before reestablishment of a normal benthic fauna. Thebioturbated sandstone interval was deposited during periods ofrelative calm, which may have included minor storms. A some-what deeper environment for this facies than for the underlyingfacies is suggested by the somewhat finer texture and by thepreservation of bioturbated deposits.

Parallel-Laminated and Burrowed Sandy Siltstone Facies

The upper approximately 165 ft (50 m) of the Cape Sebas-

Figure 6. Complete cycle of the type occurring in the upper hummockycross-stratified and burrowed facies of the Cape Sebastian Sandstonenear south tip of Cape Sebastian headland. Hummocky cross-stratifiedsandstone sharply overlies burrowed sandstone of underlying cycle (con-tact labeled A) and is overlain (contact B) by planar- and rippled-beddedsandstone containing a thin shale bed (C). Burrowed sandstone grada-tionally overlies planar-bedded sandstone (contact D); thin shale bed (E)within burrowed interval is atypical. Top of cycle is erosional contact (F)between burrowed sandstone and hummocky cross-stratified sandstoneof overlying cycle.

tian Sandstone consists of alternating parallel-laminated verytine-grained sandstone and intensely bioturbated sandy siltstonerich in organic matter (Bourgeois, 1980). As in the underlyingfacies, cyclic bedding is apparent, particularly because the basesof the sandstone beds are sharp and slightly erosional whereas thetops are gradational into the overlying siltstone beds. Trace fossilsare common, and some shells are present. This uppermost faciesof the formation is exposed in Salal Cove but is difficult to reach.

The cycles in the uppermost facies of the Cape SebastianSandstone are interpreted to have originated, like those in theunderlying facies, by deposition during alternating storms andperiods of relative calm. The decrease in average grain size fromthe underlying facies to this facies, together with the disappear-ance of hummocky cross-stratification, suggests that this facieswas deposited in deeper water than the underlying facies. Outer-shelf depths may have been reached (Bourgeois, 1980).

HUNTERS COVE FORMATION

The Hunters Cove Formation of Dott (1971) is well ex-posed along the shore of Hunters Cove. It consists largely ofthinly and evenly interbedded sandstone, siltstone, and olive grayshale (Fig. 7), and is at least 990 ft (300 m) thick (Fig. 3). Thecontact between the Cape Sebastian Sandstone and the HuntersCove Formation is not exposed in Hunters Cove, where the twoformations are in fault contact, but is thought to be gradational onthe basis of poor exposures in Salal Cove (Dott, 1971; Bourgeois,1980). The formation is overlain locally by Pleistocene terracedeposits and modem sediments.

Late Cretaceus deposits, Cape Sebastian Oregon 299

Figure 7. Thinly and evenly interbedded sandstone turbidites and shale Figure 8. Submarine-slump breccia in the Hunters Cove Formation atbeds of the Hunters Cove Formation at Hunters Cove. Hunters Cove, Note indistinct bedding.

The sandstone beds of the Hunters Cove Formation aretypically less than 12 in (30 cm) thick and are characterized bysedimentary structures that indicate deposition by turbidity cur-rents (Phillips and Clifton, 1974; Bourgeois and Dott, 1985).Among such structures are sole marks, normal grain-size gradingwithin beds, and Bouma sequences (i.e., the vertical sequence ofsedimentary structures within abed recognized by Bouma [1962]to be characteristic of turbidites). Most of the Bouma sequencesare incomplete, being without the lower unlaminated interval(Ta) of a complete sequence; most commonly, the lowermostinterval of the Bouma sequence represented in the Hunters Coveturbidites is the interval characterized by small-scale crosslamina-tion (Tc). Normal grading within the turbidites is common, butthe overall upward decrease in grain size within a typical bed issuperimposed on the alternations in grain size that define thelamination and crosslamination.

In addition to the thinly interbedded sandstone and shale,the Hunters Cove Formation contains a thick breccia interpretedto be a submarine-slump deposit (Phillips and Clifton, 1974;Bourgeois and Dott, 1985). The breccia crops out near both thenorth and south ends of Hunters Cove, but near the south endrecent landsliding has obscured the original character of theslump deposit and has produced modem breccia difficult to dis-tinguish from the original breccia. Nevertheless, the presence ofbreccia within the Hunters Cove Formation is indicated by theintact sequence near the north end of the cove and by featuressuch as sand injection structures and calcite-filled veinlets cuttingacross the breccia. The breccia is composed largely of intraforma-tional clasts of sandstone and mudstone, some of boulder size,derived from the Hunters Cove Formation (Fig. 8). Also presentin the breccia are large blocks apparently derived from theboulder conglomerate at the base of the Cape Sebastian Sand-stone; these blocks form sea stacks on the present beach. Thebreccia suggests the presence of steep scarps, probably faultscarps, during the deposition of the Hunters Cove Formation.

Another rock type in the Hunters Cove Formation is well-sorted, light-colored sandstone in intervals several feet (m) thickwithout interbedded shale. One of these sandstone intervals is

well exposed near the south end of Hunters Cove, and similarsandstone bodies occur in and near Burnt Hill cove, 6 mi (10 km)south of Cape Sebastian. The sandstone is characterized by alter-nating intervals of planar lamination and climbing current ripples,convolute lamination (Dott and Howard, 1962), fluid-escape.structures, and load structures at the contact of the sandstone andunderlying shale (Fig. 9). Each sandstone interval seems to represent the deposits of several separate turbidity currents.

The Hunters Cove Formation is interpreted to be asubmarine-fan deposit (Bourgeois and Dott, 1985). The thinnessof the turbidites might be taken to suggest deposition on the distalpart of a fan, but a more proximal depositional site is possible ifthe turbidity currents were of small size and low velocity. Theslump breccia and the relatively thick sandstone bodies are morecompatible with a proximal depositional site than with a distalone,

CONCLUSIONS

The Upper Cretaceus rocks in the Cape Sebastian areawere deposited in a tectonically active area. Deformation anduplift occurred between the time of deposition of the Houst-enaden Creek Formation and that of the Cape Sebastian Sand-

Figure 9. Sandstone interval characterized by alternating zones of planarlamination and climbing-ripple structures. Hunters Cove Formation atHunters Cove.

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stone, and faulting probably took place whileFormation was being deposited. During the

R. E. Hunter and H.

the Hunters Covedeposition of the

Cape Sebastian Sandstone the water depth is inferred to haveincreased gradually, probably due in part to subsidence and inpart to eustatic sea level rise (Bourgeois, 1980; Bourgeois andDott, 1985). The inferred gradualness of the deepening suggests aclose approach to long-term balance between the rate of deposi-tion and the rate of relative rise of sea level. On a shorter timescale (probably from a few decades to a few centuries), the rate ofdeposition was extremely variable due to cycles of major stormsand relatively calm conditions.

REFERENCES CITED

Bouma, A. H., 1962, Sedimentology of some Flysch deposits: Amsterdam,Elsevier Publishing Company, 168p.

Bourgeois, J., 1980, A transgressive shelf sequence exhibiting hummocky stratifi-cation; The Cape Sebastian Sandstone (Upper Cretaceus), southwesternOregon: Journal of Sedimentary Petrology, v. 50, p. 681-702.

Bourgeois, J., and Dott, R. H., Jr., 1985, Stratigraphy and sedimentology ofUpper Cretaceus rocks in coastal southwest Oregon; Evidence for wrench-fault tectonics in a postulated accretionary terrane: Geological Society ofAmerica Bulletin, v. 96, p. 1007-1019.

Dott, R. H., Jr., 1971, Geology of the southwestern Oregon coast west of the124th meridian: Oregon Department of Geology and Mineral IndustriesBulletin 69, 63p.

Dott, R. H., Jr., and Bourgeois, J., 1982, Hummocky stratification; Significance of

E. Clifton

its variable bedding sequences: Geological Society of America Bulletin, v. 93,p. 663-680.

9 1983, Hummocky stratification: Significance of its variable beddingsequences: Reply: Geological Society of America Bulletin, v. 94,p. 1249-1251.

Dott, R. H., and Howard, J. K., 1962, Convolute lamination in non-gradedsequences: Journal of Geology, v. 70, p. 114-121.

Harms, J. C., Southard, J. B., Spearing, D. R., and Walker, R. G., 1975,Depositional environments as interpreted from primary sedimentary struc-tures and stratification sequences: Society of Economic Paleontologists andMineralogists, Short Course No. 2 Lecture Notes, 161 p.

Harms, J. C., Southard, J. B., and Walker, R. G., 1982, Structures and sequencesin elastic rocks: Society of Economic Paleontologists and Mineralogists,Short Course No. 9, Lecture Notes, variously paged.

Howard, J. K., and Dott, R. H., Jr., 1961, Geology of Cape Sebastian State Parkand its regional relationships: Ore Bin, v. 23, p. 75-81.

Hunter, R. E., and Clifton, H. E., 1982, Cyclic deposits and hummocky cross-stratification of probable storm origin in Upper Cretaceus rocks of the CapeSebastian area, southwestern Oregon: Journal of Sedimentary Petrology,V. 52, p. 127-143.

Hunter, R. E., Clifton, H. E., and Phillips, R. L., 1970, Geology of the stacks andreefs off the southern Oregon coast: Ore Bin, v. 32, p. 185-201.

Koch, J. G., 1966, Late Mesozoic stratigraphy and tectonic history, Port Orford--Gold Beach area, southwestern Oregon coast: American Association ofPetroleum Geologists Bulletin, v. 50, p. 25-71.

Phillips, R. L., and Clifton, H. E., 1974, Late Cretaceus submarine slump-breccia, southwest Oregon: Geological Society of America Abstracts withPrograms, v. 6, p. 235.

Walker, R. G., Duke, W. L., and Leckie, D. A., 1983, Hummocky stratification;Significance of its variable bedding sequences: Discussion: Geological So-ciety of America Bulletin, v. 94, p. 1245-1249.