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Article Information Journal Title: Journal of Sedimentary Petrology
Volume: 50 Issue: 3 S Month/Year: 1980Pages: 681-702 n ~ Article Author: ~
~ Article Title: A transgressive shelf sequence exhibiting hummocky stratification; The Cape Q) Sebastian Sandstone (Upper Cretaceous), southwestern Oregon ;:j O" Q) Loan Information ~ Loan Title:
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Customer Information A TRANSGRESSIVE SHELF SEQUENCE EXHIBITING HUMMOCKY STRATIFICATION: THE CAPE SEBASTIAN SANDSTONE (UPPER CRETACEOUS), SOUTHWESTERN OREGON 1
JOANNE BOURGEOIS Department of Geology and Geophy sics University of Wisconsin, Madison , Wisconsin 53706
ABSTRA CT: In the tectonically active Circum-Pacific Belt, thick transgressive sequences are not uncom mon, in contrast with their rarity in the Cretaceous of the Western Interior. Thick transgressive sediment packages reflect rapid sedimentation rates but even more rapid rates of relative sea-level rise . A well-exposed example is the Cape Sebastian Sandstone, a 200-m thick , fining-upward sequence representing foreshore to offshore deposition. Progressively increasing depth of deposition is indicated by both physical and biogenic sedimentary structures in the Cape Sebastian Sandstone. Four facies make up the Cape Sebastian Sandstone. The lowest unit is a basal, shelly, boulder conglomerate overlain by trough cross-bedded pebbly sandstone, plane-laminated coarse sandstone, and crudely graded conglomerates. A single type of subvertical trace fo ssil is locally abundant. These sediments represent beach to nearshore deposition. The middle and thickest part of the formation comprises hummocky-bedded sandstone, divided into a lower hummocky-bedded facies and an upper hummocky-bedded and burrowed facies. Grain size, frequency of pebble lenses, and thickness of hummocky laminae all decrease upward through this part of the sequence. Conversely, burrowed zones, diversity of burrows, plane-laminated zones, plant debris, and symmetrical-ripple preservation increase upward. These sediments record storm inf1uenced, inner-shelf sedimentation. The uppermost part of the formation consists of alternating laminated, very fine sandstone and progressively thicker, burrowed sandy siltstone. Increased trace-fossil size and number and abundant plant debris characterize these sediments, which represent outer-shelf deposition. Modern examples of the structures described above have been observed on the Oregon and California shelves, supporting the hypothesis that the Cape Sebastian Sandstone represents a transgressive shelf sequence. The same structures have also been described in progradational (" regressive") sequences in the Cretaceous of the Western Interior, where thick transgressive sequences are rare or absent. Evidence for Late Cretaceous faulting in southwestern Oregon supports the proposition that thick transgressive sequences may be deposited in tectonically active regions.
INTRODUCTION Harms and others, 1975; Ryer, 1977). This In western North America, the combina model appears to be widely applicable, but tion of tectonics, high rates of sediment examination of Cretaceous sedimentary supply, and sea-level changes during Creta rocks on the west coast of North America ceous time produced abundant paralic sedi (the Cretaceous of the Western Exterior) mentary sequences. Numerous studies of the indicates that whereas the same facies are Cretaceous of the Western Interior have led present as in the Interior, their thickness and to the establishment of a "model" prograda tional, or regressive 2 shelf sequence (e.g., (usually accompanied by geographic shifts of the shore line). The term progradation is usually employed to describe a seaward shift in shoreline location caused 1 Manuscript received January 11 , 1980; revised by deposition (a " depositional regression"), producing February 11 , 1980. a vertical sedimentary sequence of shallowing-upward 2 In my review of the literature and in communication facies (progradational sequence). The term retrograda with reviewers and others, I have found no standard tion, however, cannot refer to a "depositional trans usage of the terms regression/ transgression and pro gression," which is nonsensical; retrogradation is a gradation / retrogradation; the A. G. I. Glossary of landward retreat of the shoreline, caused by erosion. Geology is surprisingly terse and not in agreement with Hence I use the term transgressive sequence to describe many working geologists. In this paper I use the terms a vertical sedimentary sequence of deepening-upward transgression and regression to refer to sea-level changes facies.
JO URNAL OF SEDIMENTARY PETROLOGY, VoL. 50, No. 3, SEPTEMB ER, 1980, P. 0681 - 0702. Copyright © 1980, The Society of Economic Paleontologists and Mineralogists 0022-4472 / 80 / 0050- 0681 / $03.00 .- -· 682 JOANNE BOURGEOIS
vertical sequence differ significantly. coarse conglomerate overlying (?) Campa The Cape Sebastian Sandstone is a 200-m nian submarine-fan-channel sediments (Bour thick, fining-upward shelf sequence. Out geois, in prep.). At localities slightly inland, crops are limited (Fig. l), but well-exposed the Cape Sebastian Sandstone overlies the sea cliffs on Cape Sebastian, southwestern (?) Upper Jurassic Otter Point Formation, Oregon, permit a detailed study of sedimen a melange complex. The upper contact is tary structures (Fig. 2) . Studies of nearshore believed to be gradational with the lower and shelf sedimentation in Oregon (Clifton Maestrichtian Hunters Cove Formation (J . and others, l 97 l; Komar and others, l 972; K . Howard, 1961; Dott, 1971), which is a Kulm and others, 1975; Hunter and others, submarine slope to fan deposit, but the con 1979), Washington (Creager and Sternberg, tact is obscured by minor faults and poor 1972; Smith and Hopkins, 1972), and Califor exposure. nia (Drake and others, 1972), and of animal Coastal exposures of the Cape Sebastian sediment relationships off Oregon and Cali Sandstone are nearly continuous, but there fornia (Carey, 1972; Howard and Reineck, are small-scale faults in repetitious parts of 1979) provide modern data to compare with the section. The total thickness shown in the Cape Sebastian Sandstone. the composite section, therefore, is approxi This study is part of a broader investigation mate (see Fig. 2). of the sedimentology and tectonics of Upper Cretaceous rocks in southwestern Oregon LITHOLOGY AND SEDIMENTARY STRUCTURES (Bourgeois, in prep.). University of Wiscon Figure 2 shows the generalized strati sin students and staff have worked in this graphic sequence of the Cape Sebastian Sand area since 1959 (summarized in Dott, 1971). stone. Based on lithology and on physical In particular, J. K . Howard (1961) mapped and biogenic sedimentary structures, the the Upper Cretaceous rocks of coastal formation can be divided roughly into four southwestern Oregon (Fig. I). Stratigraphic facies. The cong/omeratic f acies comprises and sedimentologic investigations of these a basal conglomerate overlain by trough rocks have also been carried out by workers cross-bedded, plane-bedded, and pebbly from the U .S. Geological Survey (Hunter coarse sandstone. The lower hummocky-bed and others, 1970; Phillips and Clifton, 1974; ded f acies consists exclusively of hum Hunter and Clifton, in prep.). mocky-bedded sandstone with scattered The Cape Sebastian Sandstone is rich in pebble lenses that decrease upward. The K-feldspar (10- 15 %) and relatively poor in upper hummocky-bedded and burrowed volcanic material, like the uppermost Creta f acies comprises alternating hummocky-bed ceous rocks of the Coastal Belt Franciscan ded fine sandstone and burrowed sandy silt (Bailey et al., 1964) and unlike most other stones; other features such as symmetrical Mesozoic sandstones on the Pacific coast. ripples and plant-debris-rich layers appear Detailed petrography and tectonics will be in this facies. The uppermost parallel-lami presented in another paper. nated and burrowed sand- and siltstonefacies AG E AND STRATIGRAPHY consists of zones of very low-angle, hum mocky-bedded to horizontally laminated, The Cape Sebastian Sandstone was named very fine sandstone alternating with bur by Dott ( 1971 ). It is probably late Campanian rowed sandy siltstone, which increases up (mid Campanian to early Maestrichtian) in ward in thickness. age, based primarily on an Inoceramus fauna These four facies are interpreted as repre and other bivalves (Popenoe and others, senting a progression from foreshore to 1960; Dott, 1971 ; L. E. Saul, written comm.). outer-shelf sedimentation. Terminology used Fossils are not common in the Cape Sebastian to describe dynamic zones of the beach to Sandstone and occur as molds except in the outer shelf are illustrated in Figure 3. basal conglomeratic facies. Paucity of shell material may be the result of post-deposi tional leaching. Cong/omeratic Facies On Cape Sebastian and at other coastal The lowermost Cape Sebastian is a basal localities (Fig. l), the lower contact is a conglomerate, in places containing rounded HUMMOCKY-BEDDED SHELF SEQUENCE, SW OREGON 683
:. ; . Jo HU NTERS COVE B F O RMATION SAL AL CAPE S EBASTIAN lE£] FORMATION
" LOWER SEQUENCE"
HUNTERS OTTER POI NT COMPLEX
FAULT S "l> () ------THRUST FAULT • • • • • CONTACTS ()
Jo
0 () l"T\ l> z
OREGON
Joo INDEX MAP CRO OK POINT;~ Kh
Kh o:i"' Jo MA CK POINT
0 ' ~ ' BURNT HILL
MACK ARCH 01' 0 2 ~ KM '
HOUS TENA DEN CREE K
F1G. !.- Map of Upper Cretaceous rocks in southwestern Oregon, modifi ed from J. K. Howard ( 196 1). The best and most accessible exposures of Cape Sebastian Sandstone are on the south side of Cape Sebastian; the type sectio n (Fig. 2) was measured in Sala! Cove. Double asterisks (**) mark well-established basal contacts of Cape Sebastian conglomerate on old er fo rmati ons. Single asterisks (*) are basal conglomerate localities in fa ult slivers. 684 JOANNE BOURGEOIS .·
CAPE SEBASTIAN SANDSTONE sandstone boulders up to 0.5 m in diameter
200 (Figs. 4, 5); these boulders appear to be 1 i\ 11, concretions eroded from underlying sand stone (Fig. 6). At some localities this con ., ~ PARA LL EL-LAMINATED ~1 ·==:--- J > 't. ;..._ AND BURROWED glomerate is merely pebbly to cobbly with SANDY SILTSTONE FACIES a varied clast composition, including silicic, 1 s ~I , intermediate and basic volcanics, chert, Ii ·,.._I "I diorite, quartzite, mudstone, calcilutite, 160 :}
UPPER degrees; the predominant dip direction is SE (/) er HUMMOCKY-BEDDED (Fig. 7, left). Planar-bedded sandstones, also ,_UJ AND BURROWED in sets averaging 20- 30 cm thick, increase UJ FACIES :!: upward as pebble abundance decreases. Clast sizes in the conglomeratic facies range from 80 IO-cm cobbles to medium sand; coarse sand predominates. Paleontology. - Broken pieces of thick, probable-oyster shells occurring within the basal conglomerate and smaller shell frag LOWER ments in the pebbly sandstones are the only 40 HUMMOCKY-BEDDED recognizable fossil debris in the lowest facies. F AC IE S Escape traces occur in the graded conglom-
CONGLOMERATIC F1 G. 2. - Cape Sebastian Sandstone composite sec FACIES tion, based on measured sections in Sala! Cove and on the south side of Cape Sebastian (see Fig. l). Facies boundaries are approximate.
....(/) (/) (/) OSCILLATORY WAVES WAVES OF TRANSLATION .... I u STORM WAVES I WAVE 0 er NOT CAPABLE STORM WAVES CAPABLE 1 BUILD- Q. Of EROSION Of EROSION UP BREAKER SURF SWASH
STORM WAVE LIMIT
MHW
MLW MLLW NORMAL WAVE BASE
(PLANAR-BEDDED SANDS) U BURROWED BURROWED GRADED CONGLOMERATES erates, and there is some bioturbation within this facies; Scolicia is present near the top. By far the most abundant trace, however, is a subvertical V-form burrow that tapers downward in vertical cross-section; in the horizontal dimension it commonly has a curtain-like form (Fig. 5). This trace occurs in coarse, trough cross-bedded sands, but it is most abundant in the planar-bedded facies where it appears that the trace-maker thrived. Sand in the burrow itself is better sorted and poorer in heavy minerals and biotite than the surrounding sediment. The organism that made this burrow and the ethology of the trace are speculative because no definite modern counterpart has yet been recognized. The morphology in some ways resembles smaller-scale poly chaete worm burrows (see e.g., Hill and Hunter, 1976, Fig. 16). The burrow probably represents a dwelling or combined dwelling feeding burrow in a moderately unstable substrate. Lower Hummocky-Bedded Facies Conglomeratic facies persist upward for only approximately IO m, succeeded by hummocky-bedded, medium- to fine-grained sandstone with no burrowed zones (Fig. 2). Pockets of pebbly conglomerate and conglom eratic sandstone decrease upward in fre quency, but rare, scattered pebbles are found well into the upper hummocky-bedded and burrowed facies. Pebbles average l-2 cm in diameter but may be up to 5 cm across. Hummocky Bedding.- The term hummocky bedding (strictly, hummocky cross-strati fication) was first introduced by Harms and others ( 1975), who outlined its essential characteristics (Fig. 8): l) sets have low F1G. 4.-Physical sedimentary features of the angle, erosional lower surfaces; 2) imme conglomeratic facies (see also Figs. 5, 6). A) trough diately overlying laminae are parallel to these cross-bedded and planar-bedded pebbly sands, Mack surfaces; 3) laminae can systematically Point. B) planar-bedded sands and crudely graded conglomerate, Sala! Cove. C) basal conglomerate, Mack thicken laterally in a set, so that their traces Point. Scale is in centimeters. on a vertical surface are fan-like, and dip diminishes regularly upward; and 4) dip F1G . 3. - The dynamic zones of the beach to outer shelf, as used in this paper (after Brenninkmeyer, 1978; Clifton and others, 1971), and the associated facies, as seen in the Cape Sebastian Sandstone. Beachface planar-bedded sands (Clifton and others, 1971) are believed to be replaced by conglomerates in the Cape Sebastian Sandstone. The inner shelf/ outer shelf boundary is a practical . definition based on sedimentary structures in the Cape Sebastian Sandstone (see text). 686 JOANNE BOURGEOIS directions of the laminae are scattered. All of these characteristics are present in the majority of the bedded sandstone in the Cape Sebastian Formation (Fig. 2). The first three characteristics were confirmed by inspec tion; the low dip angles and scatter of dip direction were confirmed by numerous mea surements (Fig. 7, right). Harms and others (1975) attributed hum- · {) mocky bedding to the oscillatory motion of 1 storm waves, which swept and deposited material over a hummock-and-swale surface . , produced by an erosional storm event. The hummocks would have been generally 10-50 cm high and one to several meters across; laminae continue over the tops of preserved hummocks. The hydraulic regime of hum mocky bedding is speculative (Harms and others, 1975) because the structure has not been produced experimentally, nor has it been definitely observed forming in natural environments. Hummocky stratification was attributed to a flow greater than that needed to produce ripples (Harms and others, 1975), implying flows transitional to plane bed conditions. This suggestion is supported by the occur rence of ripples at the tops of some sets of hummocky bedding (see next section). For medium sand in a depth of tO m, minimum near-bottom velocities required to produce plane beds would be on the order of 50 cm/ sec. Most Cape Sebastian sand is fine grained (O . l- 0.2 mm) and would require lower velocities in l 0 m of water. Velocities of this order of magnitude are well within ob servational and calculated velocities for pres ent Pacific coast shelf hydraulic conditions. Further hydraulic interpretation is presented in a subsequent section. High suspended-sediment concentration would also be required in order to produce laminae that repeat (parallel) the hummock and-swale topography as deposition pro ceeds, in the manner of sinusoidal climbing ripples (Jopling and Walker, 1968; Harms and Fie. 5.-Problematical trace fossil from the con glomeratic facies, Sala! Cove (see also Fig. 6). The trace is preferentially cemented and weathers out in relief. A) Vertical face showing trace in plane- and trough-cross-bedded pebbly sands; B) close-up of a vertical face; C) oblique view of a weathered-out bed ding plane. Scale is in centimeters. HUMMOCKY-BEDDED SHELF SEQUENCE, SW OREGON 687 CAPE SEBASTIAN FORMATION CONGLOMERATIC FACIES SALAL COVE_ KM-SALAL COVE 05 NORTH . SOUTH (INACCESSIBLE) TROUGH X-BEDS HUMMOCKY BEDS FI G. 7. - Comparison of dip direction (arrows) and dip angle of trough cross-beds and hummocky beds in the Cape Sebastian Sandstone. Dots are poles to cross-bedding; dots within the small circle represent dips less than 15 degrees. others, 1975). Modern observations also con firm the ability of flood-storm events to CAPE SE BAST I AN produce shelf waters with high concentra FORMATION tions of suspended fine sand (Smith and Hopkins, 1972; Drake and others, 1972). The origin of the parallel lamination may be related to the oscillatory motion of storm waves, which at the time of maximum flow "'a: would suspend sediment above the bed and "'.... then allow it to settle out (Harms and others, "'::f 1975). Under these conditions it might be expected that individual laminae would be graded, but the uniformity of sediment size in Cape Sebastian hummocky laminae makes it difficult to detect grading. Weathering characteristics of the hummocky bedding (Fig. 8), emphasizing the laminae, suggest that there may be a subtle change in grain-size distribution within each lamina, for example, an upward increase in clay content or an increase in organic material. In the lower hummocky-bedded facies, grading of an entire hummocky bedset is not "LOWER SEQUENCE" apparent as it is in the overlying facies. Erosional truncations in this facies probably ...... ~, : ...... ;:.· .. •• represent multiple storm events (see section on interpretation). In some cases they may also represent fluctuations of intensity in a single storm event, producing alternating periods of erosion and deposition. Hum mocks, or positive bed forms with laminae FIG. 6. - Comparison of two measured sections of continuing over the crests, are present but the conglomeratic facies, Sala! Cove (see Fig. I for not abundant, presumably because they localities). The top of the northern section ends in a would be selectively eroded by successive vertical cliff. Hummocky bedding continues upward in the south section. Y-shaped symbols are trace fossils storm events (see Fig. 8) . (see Fig. 5). Access to these sections is limited by tides The characteristics of hummocky bedding (consult author). were first described by Campbell (1966, 688 JOANNE BOURGEOIS A FI G. 8.-Hummocky bedding in the Cape Sebastian Sandstone, south side of Cape Sebastian (see also Fig. 10) . A) field sketch of hummocky bedsets; note positive-relief bedding. B) photo of the bedset upon which the lower sketch is based; the zone below the hummocky bedding is burrowed; scale in centimeters. C and D) two examples from the hummocky-bedded facies; note pebbles in upper photograph. 1971), who recognized this form of bedding to-fluvial coarse sands encouraged the orig in the Cretaceous of New Mexico and else inal interpretation of hummocky stratifica where in shoreface facies. He called it " trun tion as a shelf storm deposit (Howard, 1972; cated wave-ripple laminae" but did not at Goldring and Bridges, 1973 ; Harms and tribute its genesis specifically to shelf others, 1975). Its recognition on modern storm waves. Hummocky bedding apparently shelves, however, is hampered by sampling was first attributed to shelf storms in Creta and observation difficulties. Because of its ceous progradational sequences of the West scale, complete sets of hummocky bedding ern Interior (Howard, 1972; Harms and cannot be recognized in vibracores or even others, 1975). Goldring and Bridges (1973) box cores. Parallel stratification with low cited other examples from the Paleozoic and angle truncations has been described, Mesozoic. Dott and Bourgeois (in prep.) have however, in the shoreface-to-offshore of the also recognized hummocky bedding in Gulf of Gaeta (Reineck and Singh, 1971 ); Eocene and Mio-Pliocene shelf sequences in the North Sea (Reineck and Singh, 1972); southwestern Oregon and probably in the seaward of Sapelo Island, Georgia (Howard Cambrian Jordan Formation in Wisconsin. and Reineck, 1972); on the Oregon shelf Numerous other descriptions under vari (Roush, 1970, in photographs; Clifton and ous names probably exist (e.g., the Upper Cre others, 1971 ); and off southern California taceous of California, Howell and others, (Howard and Reineck, 1979). 1977 , p. 12 ; see Hamblin and others, 1979). Parallel Lamination.-Hummocky bed The stratigraphic position of hummocky ding appears to be a form of parallel lamina bedded facies between offshore laminated tion, the genesis of which is problematic. and burrowed sandy siltstone and shoreface- If it represents hydraulic conditions that HUMMOCKY-BEDDED SHELF SEQUENCE, SW OREGON 689 produce flat beds, it could be produced during ( 1971 ), and float specimens of Wil/imactra upper-regime plane-bed phase, upper-to (aff. W. popenoei), Cymbophora sp. , and lower regime transition phase, or, for coarse Meekia (Mygallia) be/la (?) (identification by sands, during initial movement (lower-regime Lou Ella Saul) were collected by the author flat-bed phase). The latter can be eliminated on the west side of Cape Sebastian. The for the fine-grained Cape Sebastian Sand fossils are typically preserved as external stone. Parallel lamination has not, however, molds; many of the Jnoceramus shells are been reproduced experimentally under these articulated and unbroken. No biogenic sedi conditions (McBride and others, 1975). mentary structures were observed in this Rather it has been observed to form from facies (see discussion of amalgamation migration of in-phase wave bed forms in very below). shallow water (N. D. Smith, 1971 ; McBride and others, 1975) and from migration of low-relief ripples in water less than 5 cm Upper Hummocky-Bedded and Burrowed deep (see McBride and others, 1975). Ex Facies trapolating these studies to deeper-water Approximately 60 m above the basal conditions is not clearly valid, however, conglomerate, burrowing first appears in because both Smith ( 1971) and McBride and association with the hummocky bedding. others (1975) cite shallow depths as critical Typically, a set of hummocky-bedded, fine to their observations. Alternatively, Bridge grained sandstone grades up into very fine ( 1978) has suggested that the quasicyclic grained, finely laminated sandstone, suc phenomenon of bursting in turbulent bound ceeded by burrowed silty sandstone. The ary layers may sort sediment and produce presence of these burrowed zones, which a horizontal lamina during each cycle; Bridge are terminated by the erosional bases of (1978) also reviews other studies of parallel overlying hummocky bedsets, is evidence for lamination. All these studies refer to unidi hydrodynamically quieter periods separating rectional flows, which also makes extrap the events that produced hummocky bedding. olation to shelf environments questionable. This alternation of hummocky bedding and Another possible mechanism to produce burrowed zones continues for approximately the parallel laminae in the Cape Sebastian 80 m, within which Hunter and Clifton (in Sandstone would be by periodic oscillatory prep.) believe they can recognize storm wave pulses, which could cause sediment cycles. to be placed in temporary suspension and Amalgamation.-Detailed inspection of the then to settle out after each pulse passed. hummocky-bedded zones reveals evidence As mentioned previously, one would then that they are amalgamated; that is, successive expect individual laminae to be graded, a storm events eroded away intervening bur condition difficult to detect in the very-well rowed zones, leaving an "amalgamated" sorted Cape Sebastian Sandstone. In finely record of storm events (see Fig. 9). For laminated zones, very-fine-sand laminae al example, a small uneroded pocket of bur ternate with laminae rich in biotite and plant rowed sand and plant-rich laminae within a debris, but grading is not apparent. thick, hummocky-bedded sequence is evi In conclusion, the origin of parallel lam dence that the sediment representing a quies ination remains unclear, even under unidi cent period was eroded by the following rectional flow conditions (Bridge, 1978). storm event. The amount of burrowing in Its origin in the Cape Sebastian Sandstone creases upward in this facies not only because is problematic but is surely related to the the bottom was quiescent more often (bur periodic passage of high-energy storm-wave rowing periods) but also because storms pulses. could erode less of the burrowed sediment. Paleonto/ogy. - An /noceramus locality on The burrowed zones thicken upward in this Myers Creek (Fig. I; Popenoe and others, facies until some are more than 2 m thick, 1960) probably includes this facies as well certainly thicker than the depth of normal as the basal facies. Leaf impressions have burrowing (20-30 cm; Schafer, 1972). There also been found here. Other clam, oyster, is evidence that these burrowed zones also and plant fragments were found by Dott are amalgamated. In this case laminated .- 690 JOANNE BOURGEOIS EROSION. DEPOSITION 8 EROSION 8 of biotite and plant debris. Some of these BURROWING DEPOSITION fine laminae are burrowed (Fig. IO). -----,), )sts/ <, ----.;-',l:1:','.1t.. ==--~ ~l~ - ~-~-L Several hummocky sequences in this facies ~~ ~- ----=--=-= include symmetrical ripples at the top of the -='--- :::::.-- -:-- -::::::=::::::.- "AMALGAMATED" hummocky-to-finely-laminated zone (Fig. HUMMOCKY BEDDING IO). These symmetrical ripples are mostly intermediate to long-crested, with rounded troughs and sharp to rounded crests; some interference ripples are also present. The .._g:....--. presence of tracks and trails on these rippled " AMALGAMATED"' BURROWED ZONE surfaces (Fig. l lF), as well as observations of modern ripples offshore (Komar and FIG. 9.-Alternating erosional, depositional and bur rowing processes leading to amalgamated hummocky others, 1972), suggest that all such ripples bedding (in a higher-energy setting) and amalgamated may originally be sharp-crested but are burrowed zones (in a lower-energy setting). The upper rounded and eventually destroyed by biologic example begins with a hummocky-to-burrowed sequence activity. If so, a hydraulic regime conducive (!st frame). The burrowed zone is eroded away, hum mocky laminae are deposited, and burrowing com to rippling may have prevailed more often mences again (2nd frame). Once more the burrowed than the sedimentary record indicates. zone is eroded away and more hummocky laminae In several sequences, 5-to- IO-cm thick deposited, leaving no evidence of burrowing (3rd frame). layers very rich in plant debris occur above The lower example begins with a burrowed zone (lst the rippled zone. These layers may represent frame), which is then eroded by a storm event that deposits hummocky laminae (2nd frame). This hum deposits of rafts of plant material produced mocky zone is thin enough, however, to be obliterated during floods and coastal storm attack and by further burrowing, leaving no evidence of hummocky transported seaward as the storm surge bedding (3rd frame). waned. Paleontology.- One set of articulated reptil ian vertebrae discovered by U.S. Geological sands were deposited between burrowing Survey paleontologists occurs at the top of periods, but the laminated zones were thin a hummocky bedset near the base of this enough that burrowers could destroy nearly facies at the south end of Cape Sebastian. all the primary lamination, leaving an Although not further identified (R. Hunter, "amalgamated" record of burrowing events verbal comm.), the bones could be either (see Fig. 9) . a marine animal or a nonmarine one that A composite, typical hummocky-bedded washed into the sea. Articulation of the bones to-burrowed sequence would include the fol suggests that a carcass decayed in place on lowing features from base to top (Dott and the ~ea floor, which would indicate rapid Bourgeois, in prep.): 1) a sharp, erosional burial or a reducing environment (Schafer, base with 5- 20 cm relief; 2) a zone of light 1972), the latter being very unlikely for the gray, hummocky-bedded fine sandstone in Cape Sebastian Sandstone. which lamina thickness and sediment grain Inoceramus and other unidentified bivalve size decrease upward, whereas horizontality shell debris have been found in this facies of laminae and organic content increase up but are not common. Poor preservation and ward; 3) one or more sets of symmetrically vertical outcrop orientation may be partly rippled fine to very-fine laminated sandstone; responsible. 4) a zone very rich in plant fragments; and Most of the burrowed silty sandstone does 5) burrowed, medium-gray, very fine silty not exhibit distinctive trace fossils. Ophio sandstone. morpha (Fig. l lA,B) is the most common Characteristics of these hummocky beds recognizable trace and was usually identified are similar to those in the lower hummocky in vertical cross-section as a 1-2 cm diameter bedded facies, but laminae are typically thin ellipse lined by dark plant debris and biotite, ner ( -0. 5 vs. I. 0 cm) and the sets grade presumably a cross-section of an oblique upward into very fine (1 - 2 mm), essentially burrow. One vertical, branching form of horizontal laminae defined by very thin layers Ophiomorpha was found within the upper, HUMMOCKY-BEDDED SHELF SEQUENCE, SW OREGON 691 CAPE SEBASTIAN FORMATION HUMMOCKY- BEDDED AND BURROWED FACIES F1G. 10.-Sketch of part of the hummocky-bedded and burrowed facies, and two photographs of alternating burrowed and hummocky zones, south side of Cape Sebastian. Scale is in centimeters. finely laminated part of a hummocky bedset; Parallel-Laminated and Burrowed Sandy one example of boxwork (hexagonal) Ophio Siltstone Facies morpha, with distinctly nodular lining, was found on a rippled surface. The uppermost part of the Cape Sebastian Also occurring within the bioturbated Sandstone (Fig. 2) consists of alternating zones are vertical cross-sections of burrows parallel-laminated very fine sandstone and resembling the V-form burrows of the lower burrowed, organic-rich sandy siltstone. conglomeratic facies. In the hummocky-and Fine-grained material, organic debris, and burrowed facies, these burrows are recog thickness of burrowed zones all increase nized by better cementation and a bleached upward, the top 5-10 m of the section being white appearance due to absence of plant completely burrowed. Rippled bedding was debris within the burrow (Fig. l lB,C). The not detected in this facies. burrows, 5-15 cm long and 0.5- 1 cm in Finely laminated (0.5- 2 mm), medium-gray diameter, may occur at any level within the sandstone zones are 10-150 cm (average burrowed horizons; they are typically trun 20-40 cm) thick; they generally have a sharp cated where they occur within a burrowed but low-relief base and a gradational upper zone, providing evidence for multiple ero contact with burrowed sediments. Increased sional events (Fig. 10). quantities of plant debris and biotite make Trace fossils in the finely laminated beds the laminae more distinct than in lower facies; include horizontal Planolites (Fig. llE), burrows are locally present within these ? Curvolithos, ? Gyrochorte and vertical, lined laminated sediments. The burrowed sandy burrows (Fig. I ID). Scolicia and other graz siltstone is medium to dark gray, rich in ing and crawling traces occur on the upper, organic material, and contains some clay. sometimes-rippled surfaces of these laminat Paleontology.- An ammonite, Anapachy ed zones (Fig. l IF). Trace fossils in this discus cf. A. peninsu/aris, was found by J. facies represent a wide range of ethologies K. Howard (1961) near the top of the Cape dwelling, dwelling-feeding, grazing and Sebastian section. Several large fragments crawling. of Pinna (? P. ca/amitoides; Packard and 692 JOANNE BOURGEOIS F10. I I .- Trace fossils in the hummocky-bedded and burrowed facies of the Cape Sebastian Sandstone, south side of Cape Sebastian; scale in centimeters. A) Vertical face - Ophiomorpha. B) Vertical face- V-shaped trace, P/anolites. C) Vertical face-V-shaped trace. D) Vertical face- lined trace in finely laminated sand . E) Bedding plane- P/anolites. F) Rippled surface with varied grazing traces, illuminated from the left. HUMMOCKY-BEDDED SHELF SEQUENCE, SW OREGON 693 Jones, 1965) were recovered by the author CAPE SEBASTIAN FO RMATION from apparently whole specimens in a thick, PAR ALL EL- L AM I NATED burrowed sandy siltstone bed; they appeared AND BU RROWE D FACI E S to be in life position. An articulated small (?juvenile) bivalve was also found in the same bed. Modern Pinna are filter feeders that live vertically embedded in soft substrates with part of their valves projecting above the surface (Yonge, 1953). Thus they tend to live in relatively quiet water, either " at some depth" (Yonge, 1953) or in protected areas. Some modern Pinna attach byssus threads to underlying gravel (Yonge, 1953), but on the Georgia shelf they are found in muddy sand (J . D. Howard, written comm.). All the trace-fossil genera described in the previous facies also occur in the uppermost facies: Ophiomorpha (oblique cross-sec tions); V-form (?polychaete) burrows; Planolites and Sco/icia. In addition plant lined, one-cm wide horizontal burrows, Sa F1G. 12 .- Sketch and photograph of the upper Cape bellarites, and 5-cm wide (?deposit-feeding) Sebastian Sandstone: parallel-laminated, very-fine burrows are common. A star-shaped Thalas sandstone and burro wed sandy siltstone facie s, Salal sinoides and a 4-cm diameter hexagonal bur Cove. Scale is in centimeters. row were also found. These traces represent a large proportion of dwelling and dwelling f eeding ethologies; in addition, grazing and (between wave base and the swash zone- see deposit-feeding traces are present. Fig. 3) of a high-energy nearshore environ ment. Clifton and others ( 1971) described INTERPRETATION OF THE CAPE SEBASTIAN FACIES lunate megaripples, planar beds, and asymmetric ripples in this zone (see Fig. 13); Cong/omeratic Facies Howard and Reineck ( 1979) also described The conglomeratic facies is interpreted as cross-bedded and planar-bedded pebbly representing beach to shoreface deposition sands, as well as rippled sands, in the near along an irregular, somewhat rocky shore shore zone. The absence of asymmetric rip line . Shell debris supports a marine origin. ples in the Cape Sebastian Sandstone may Where the basal IO m is exposed (Fig. 1) , be attributed to the coarse nature of the grain size, thickness of the conglomeratic sands; for sands coarser than about 0.8 mm, facies, and relative amounts of the different a lower flat bed state exists rather than ripples sedimentary structures varies (Fig. 6) . The at velocities just above threshold (Boguchwal variation suggests a coastline of rocky head and Southard, 1978). Planar bedsets, com lands and relatively quiet coves, similar to monly with basal and internal erosional Pacific-coast shorelines today. The coarse truncations, also occur seaward of the ness of the basal conglomerate, represent megaripple zone (Howard and Reineck, ing the beach or a basal lag deposit, would 1979; see Fig. 14). depend on proximity to cliffs of appropriate Whereas the planar bedsets represent es material. Pebbles and cobbles may have been sentially vertical deposition, trough cross both eroded by waves from local formations beds reflect lateral migration of bed forms and transported by rivers to the beach; exotic in the nearshore zone. Longshore currents, lithologies such as metaquartzite pebbles onshore transport by wave translation, and support a dual provenance. offshore transport by rip currents may pro The planar-bedded and trough cross-bed duce lunate megaripples. Figure 6 is a plot ded sands represent everyday sedimentation of trough cross-beds measured at one locality in the build-up, breaker, and surf zones in the lower Cape Sebastian Sandstone. Of 694 JOANNE BOURGEOIS GUL F OF GAE TA CA LI FO RNI A S HELF OR EGON SHELF <• l ( & ) <• l -200 -200 (NOT - 200 -r------. l2 t. .J' """ SA MPLED) -, &. '\ 'i ~ ... ; I J \ l SI \. ""-~~ i ',,.. 11_ \ { ) -" '.: J ~ "J ~S"" •"\"'-"1,C."" '., -ee !i. ' "" " '1 OFFSHORE OFFSHORE q ;"-'1.,. l ,1 f OFFSHORE V> ... '' ,,.._ .... ,. , ... a: -50 ,,,, , , -50 -50 w - - >- TI:=f ;5 , .... , t:l:1 ~=-\ \- w ~, ,,._ '•' ... ::!O -15 - -15 - ~ I "I. .. l \ 11 >1' -15 - e- - r I .J \ I \ l. ~ t =;,"I;.. - >- TRANSITION ll. \"\.~ l.l, l --"".=!J ,~J-1 - w ' TRANSITION 0 -- (NOT '-\ '\ I i t l I f' '\ SAMPLED) "- ' l'> "'" >'l _J ::111__ w r -6 - "l'~'"' -6 - ...... -..,.. V> ==--1 • .:>t=- _J ) 'lo~" \~ ... -·u F1G. 13 .-Sedimentary structures sampled in modern-shelf transects compared with measured sections from the Cape Sebastian Sandstone, SW Oregon, and the Upper Cretaceous Gallup Sandstone, New Mexico. Note that the Gallup section is progradational and is inverted here to compare with the transgressive Cape Sebastian Sandstone. The three diagrams on the left are hypo1he1ical transgressive sequences constructed by sketching structures found in depths of water from +2 to - 200 meters; note approximate log scale . I hypothesize that all measured cross-beds (n = 130) more than Cape Sebastian Sandstone, I) there are no 50 percent occur in the SE quadrant with angular boulders; 2) the largest rounded significant SSW and ENE components. boulders are almost certainly rounded Symmetrical ripples in the upper hummocky concretions from the underlying "Lower bedded and burrowed facies (see below) Sequence"; and 3) the limited size of other suggest an E-W trending shoreline (Fig. 6); rounded cobbles may be only a function of thus, lunate megaripple migration appears to the maximum size of resistant clasts available have been principally onshore with some for reworking from the underlying formation. longshore components. Nevertheless, a minimum breaker height The graded conglomerates are interpreted necessary to move 50 cm boulders can be as shoreface storm deposits produced be calculated. The threshold velocity (V.) nec tween wave base and the breaker zone (see essary to move a 50 cm boulder is about e.g., Kumar and Sanders, 1976). Seaward 300- 370 cm / sec (see Novak, 1972; Dott, of normal wave base, presumably, storms 1974). That represents the maximum horizon produced hummocky-bedded deposits (see tal water-particle velocity along the bed (U) below). beneath a solitary wave, which in turn is Calculation of Wave Heights.-Dott (1974) one-half the surface velocity or celerity of estimated maximum storm-breaker heights of the wave, C (Inman, 1963). Hence C = 2U Cambrian seas based on relative sizes of = 2V, = 600- 740 cm/sec. Surface velocity rounded and angular quartzite boulders in of a solitary; wave at the point of breaking some Cambrian rocks in Wisconsin. In the is Cb = 8~ (in ft/sec), where H b is the HUMMOCKY-BEDDED SHELF SEQUENCE, SW OREGON 695 CAPE SEBASTIAN SANDSTONE GALLUP SANDSTONE CHARACTERISTIC GRAIN SIZE (0) (X) (BOTTOM ) 200 30 - fl ,, r /l ~"- - ~·,12_'~\ OFFSHORE -\- '1 I t-:_ 25 OFFSHORE - .!:'.!!!..~..!.. \ -- OH SH.ORE/ •~·~ r.... OUTER SHELF 50 \1 (Bernard and others, 1962), the Gulf of Gaeta where d 0 is the orbital diameter and L is HUMMOCKY-BEDDED SHELF SEQUENCE, SW OREGON 697 the (water) wavelength. L is related to the In this case (H = 6 m, T = 15 sec, h = 2 : deep-water wavelength L00 = (g/2-rr)T 50 m, L = 290 m) , u, = 94 cm/sec. If H 12 were 3 m, u, would be 47 cm/sec. The 6-m L = L ltanh (2-rrh / L )l 1 (2) 00 00 condition could move 8-mm grains, the 3-m The maximum horizontal near-bottom veloc condition 2-mm grains. These calculations ity (um) associated with oscillatory motion are for maximum, sustained storm conditions on the sea bottom beneath a deep-water wave but are also for maximum depths (50 m) so is : that shallower depths and less violent storms (shorter wave periods) should produce results -rrdo of the same order of magnitude. u = - (3) m T In 50 m depth of water under normal summer wave conditions (T = 12 sec, H Taking 12 .7 cm/sec as the threshold velocity = 2 m) , near-bottom velocities would be (u m) for the movement of fine (. 125 mm) about 15 cm / sec, just capable of moving sands (Komar and Miller, 1973) and um equal 0. 1 mm sand (Komar and Miller, 1973); average winter waves (T = 12 sec, H = 3 to the threshold velocity, d 0 would be ap proximately 50 cm; longer (15 sec) storm m) could move 0.3 mm sand. Hence, we would expect the bottom to be always rippled wave periods would give a d 0 of 60 cm. For 7-cm wavelength ripples these figures are except during high-energy events; bioturba consistent with observed ripple length and tion, however, might easily destroy ripple orbital diameters of shelf-depth waves forms during periods of low accretion rates. (Inman, 1957 ; Komar, 1974). Komar' s These calculations are for wave-induced , bottom currents, which, ideally, would be graphical solutions for h/ L 00 A. of 7 cm, H of 3 m, and T of 12 seconds indicate a water purely oscillatory and could put sediment into depth of 50 meters. This depth should be suspension but could not transport it. The considered to approach a maximum for the observations of net northwesterly current upper mid-Cape Sebastian Sandstone be velocity vectors off the Washington shelf cause the longest ripple wavelengths were (Smith and Hopkins, 1972), however, have used. led several workers (e .g., Creager and Stern Shelf storm-wave (T = 15 sec) heights of berg, 1972) to suggest that wind-induced 1.5 mare necessary to set fine sand in motion, currents are responsible for net shelf that is , to develop a near-bottom velocity sediment transport. These currents on the of 12 .7 cm / sec in 50 m depth of water (Komar shelf bottom need not be parallel to surface and Miller, 1973). Average winter waves on wind directions because of the Coriolis ef the present Oregon shelf (T = 12 sec) are fect, producing an Ekman Layer, and factors 3 m high, and large storm waves commonly such as shoreline and bottom configuration, are twice that height, i.e., 6 m. During extreme wind speed, and water stratification. wave conditions on the Pacific coast, wave Ripple Orientations.- Symmetrica1 ripple heights of 15 .2, 17.7 , and even 29 m have crest, orientations in the Cape Sebastian been recorded (see Komar and others, 1972) . Sandstone are primarily oriented in an E-W Waves of 6-m height and 15-second period direction (Fig. 7). Present-day deep-water in 50 m of water would produce conditions shelf ripples produced by storm waves tend intermediate between deep- and shallow to be parallel to the coastline (Komar and water waves (Komar, 1976) . For these condi others, 1972) . It is not clear whether the tions we can use equation (2) to calculate Cretaceous ripple orientations reflect an , east-west-trending paleocoastline, a north the wavelength L. L 00 the wavelength in deep water, is in this case 345 m (from gT 2 / 2-rr) , south storm track, or a 90-degree tectonic and Lis approximately 290 m. The near-bot reorientation of the Cape Sebastian Sand tom orbital velocity (u,) can then be calculat stone. ed (Komar and Miller, 1973): Para/lei-Laminated and Burrowed Sandy Siltstone Facies H 4 This facies is believed to have been depos u, = Tsinh (2-rrh/L) ( ) ited on the outer shelf, seaward of the 698 JOANNE BOURGEOIS erosional influence of nearly all storm waves succession (perhaps in one storm season), (Fig. 3). To compute a minimum depth for or burrowing would have destroyed the pri the threshold of grain motion we may use mary lamination. the minimum near-bottom orbital velocity u, Creager and Sternberg ( 1972) argued that of 10 cm/sec for 0.4 mm sand (Komar and the currents measured by Smith and Hopkins Miller, 1973). For two possible maximum ( 1972) were produced by wind-drift. Alterna storm conditions (T = 12 sec, H = 12 m; tively, sediment may initially be brought to and T = 15 sec, H = 9 m), Komar and the nearshore by river floods (see e.g., Drake others ( 1972) calculated depths of 149 m and and others, 1972) and then redistributed to 204 m, respectively, as the threshold depths the outer shelf when a storm wanes, as of rippling; even for average winter waves piled-up, sediment-charged water moves (T = 12 sec, H = 3 m) the depth of rippling across the shelf; hurricanes may produce would be 99 m. similar effects (Hayes, 1967). This process It seems unlikely that the uppermost facies would be a combined type of hydraulic- and was deposited below these depths because density-driven mechanism. Howard and calculations of water depth based on ripple Reineck (in prep.) suggested this mechanism wavelengths indicated a maximum depth of could have produced the 28 storm sequences 50 meters. The absence of symmetrical rip they recognized and were able to correlate ples in this facies may be explained by several in most of their 23 vibracores over the entire factors: l) ripples were not recognized in Ventura-Por,t Hueneme shelf area. the field; 2) most deposition took place under Kulm and others (1975) recognized three a non-oscillatory regime; or 3) the ripples zones of seaward sediment transport in Ore were destroyed by bioturbation. It is likely gon shelf waters. First, in the swface turbid that all three factors are significant. First, layer, at the level of the seasonal thermocline, this facies occurs in the least-well-exposed sediment moves seaward during times of part of Cape Sebastian, above the zone of large surface waves or high river discharge; modern wave attack. Second, measurements this layer persists across the shelf, but its at depths of 50 and 80 m during storms on intensity decreases relatively rapidly away the Washington shelf indicated non-oscilla from the source. Second, the mid-water layer tory currents capable of moving bottom sedi occurs at the level of the permanent thermo ment (fine sand and silt) with a net north cline (also coincident with the permanent westerly vector (Smith and Hopkins, 1972), pycnocline). Sediment received from the surf but asymmetric ripples have not been ob zone and also from the water column above served in the Cape Sebastian Sandstone. moves slowly across the shelf on the gently Third, rippled surfaces might be produced sloping pycnocline; the mid-water turbid during quieter periods, but preservation po layer becomes more diffuse as it moves tential could be low due to bioturbation seaward, but west of the Columbia River (Komar and others, 1972). the layer maintains its intensity to the outer Sediment Transport A cross the Shelf. - shelf. The bottom turbid layer is most pro Sediment may be supplied to the outer shelf nounced over muddy areas but in places is by several mechanisms, which may be in nonexistent; it receives material from the surf terrelated and whose relative significance is zone, from the water column, and from the difficult to evaluate with present knowledge bottom itself. This bottom layer may develop (see discussions in Swift and others, 1972). into a low-density bottom current near the Smith and Hopkins (1972) estimated that the middle or outer part of the shelf (Komar average particle on the Washington shelf (fine and others, 1974). sand and silt) moves 40 km/yr alongshore Swift and others ( 1971) suggested that and 7 km/yr offshore. Only suspended oscillatory storm surges could cause fine sand sediment transport was deemed significant, to be suspended in the mid-depth seaward and only severe storm conditions could return flow (mid-water layer of Kulm and transport fine sand, which makes up the others) on the shelf. This process was not laminated zones of the Cretaceous rocks. confirmed by Cook ( 1970) and Cook and These lamina-sets must have been deposited Gorsline (1972), who emphasized the action in one event, or several events in rapid of rip currents on the shelf off California. HUMMOCKY-BEDDED SHELF SEQUENCE, SW OREGON 699 Goldring and Bridges (1973) endorsed storm in tidal flats (van Straaten and Kuenen, 1958). I· action as a major mechanism for producing Hence the shelves of these passive continen shelf sheet sandstones but also suggested that tal margins are starved for sediment, but such rip currents and tsunamis were possible observations cannot be extrapolated to the processes that could supply sediment to the high-relief, high-energy Pacific coast. Final shelf. Tsunamis certainly might play a signif ly , arguments against symmetrical transgres icant role in sedimentation on a tectonically sive-regressive sequences and for "punc active shelf, but sedimentologic effects sug- tuated aggradational cycles" (rapid, non ~ gested for tsunamis are very speculative depositional transgressions followed by slow, (Coleman, 1978). progradational regressions; Anderson and Cape Sebastian deposition. - The laminat Goodwin, 1978) are currently in vogue. ed fine sands of the Cape Sebastian Sand Asymmetric cycles have been argued in par stone probably accumulated relatively rapid ticular for the Cretaceous sequences of the ly during or following storms, by combined Western Interior (e.g., Ryer, 1977) and else . wave, wind-drift and post-storm-surge den where in Paleozoic sequences (e .g., Ander sity-driven mechanisms. The burrowed son and Goodwin, 1978 ; Goodwin and An sandy silts could represent periods of slower derson, 1978; Byers, 1978). deposition, perhaps by settling from the tur None of the above arguments applies bid layers recognized by Kulm and others strongly to the Pacific coast of North Ameri (1975) ; burrowing during these times kept ca. On an active continental margin, high pace with accumulation rates. The decrease rates of sediment supply and tectonic activity ' upward of laminated zones suggests a may easily produce deposition during a deepening of shelf waters through time, transgression (Curray, 1964; Swift, 1968). based also on the evidence in facies below Also, a high-energy shelf environment exists .· it for progressively deepening waters. on the Pacific coast, with strong, seaward However, a decrease in sediment supply, as shelf currents confirmed by direct observa by the shifting of a delta distributary, could tions. The differences between shelf sedi also account for the decrease in laminated ments on the Georgia and California shelves zones. (Howard and Reineck, 1979), for example, may well be explained by differences in CONCLUSION: CAPE SEBASTIAN SANDSTON E- A tectonic and climatic-oceanographic settings. TRANSGRESSIVE SEQUENCE The deepening water in the Late Creta There are a number of trends (Fig. 13) ceous of southwestern Oregon may have from the base to the top of the Cape Sebastian been the result of a Cretaceous transgression Sandstone that indicate it is a transgressive or of shelf subsidence- or both. There is sequence; that is, it represents deposition worldwide evidence for a transgression beneath a progressively deepening shelf sea. (Bond, 1978) and local evidence for Creta In addition, it is a very thick (200 m) trans ceous faulting (Bourgeois, in prep.). In addi gressive sequence. Such sequences have re- tion, rapid sediment accumulation on the l~ cently received a stigma for several reasons. shelf itself and on the adjacent slope could First, sampling of modern shelf sediments, reinforce subsidence. particularly off eastern North America, has Trends that best establish the Cape Sebas )"l revealed coarse, relict sediments on the outer tian Sandstone as transgressive are con shelf, evidence that little sediment accumu- firmed by studies of recent sediments and lated on the shelf during or since the Holo modern environments off the west coast of cene transgression. Relict sediments also North America (Fig. 13). These trends in occur off the Pacific coast in patches, but clude a seaward decrease in grain size they are not prevalent (Emery, 1960; Kulm (Emery, 1960; Clifton and others, 1971 ; Kulm and others, 1975). Second, studies such as and others, 1975; Howard and Reineck, 1979; those by Meade (1968) have shown that most many others) and a seaward increase in river-borne sediment on the Atlantic coast organic content (Gross and others, 1972; of North America is being trapped in es Carey, 1972) and amount of bioturbation tuaries, and studies of the North Sea coast (Carey, 1972; Kulm and others, 1975; suggest that most sediment there is trapped Howard and Reineck, 1979). The sequence 700 JOANNE BOURGEOIS of sedimentary structures from the beach to Devonian of New York: Geol. Soc. America, Abs. nearshore is confirmed in particular by Clif with Programs, v. 10, p. 30. ton and others (1971) and from the nearshore BAILEY, E. H ., IRWI N, W. P., AND JO NES, D. L. , 1964, Franciscan and related rocks, and their significance to offshore by Howard and Reineck (1979). in the geology of western California: California Div. The entire sequence is an inverted and much Mines Geology Bull. 183, 177 p. thicker version of Harms and others (1975) BERNARD, H. A., LEBLA NC, R. J., AND MAJOR, c. F., ideal progradational sequence, as illustrated 1962, Recent and Pleistocene geology of southeast Texas, in Geology of Gulf Coast and Central Texas by the Cretaceous of the Western Interior Guidebook: Houston, Houston Geol. Soc., p. 175- 224 . (Fig. 13) . BoGuCHWAL, L.A., AND SOUT HARD, J.B., 1978 , Dynamic Other transgressive sequences in the Cir scale modeling of bed configurations: Am . Assoc. cum-Pacific zone have recently been men Petroleum Geologists Bull., v. 62, p. 497. tioned by Fritsche (1977), Hirayama and Bo ND, G. C., 1978, Speculations on real sea-level changes and vertical motions of continents at selected Nakajima (1977), Howell and others (1977), times in the Cretaceous and Tertiary periods: Geology, and Fisher and Magoon (1978). Thus it v. 6, p. 247- 250. appears that thick, transgressive shelf se BREN NINKM EYER , B. M., 1978 , Littoral sedimentation, quences may be deposited on tectonically in Fairbridge, R. W., and Bourgeois, J., eds., Ency clopedia of Sedimentology: Stroudsburg, Pa., Dow active continental margins, whereas in more den, Hutchinson and Ross, p. 448-457 . stable regions they are less likely. The Cape BY ERS, C. W ., 1978, Enigmas in the Wisconsin Cambrian Sebastian Sandstone, southwestern Oregon, and a new depositional model for the type St. Croixian: is a good, well-exposed example. Am. Assoc. Petroleum Geologists Bull., v. 62 , p. 502 . CAMPBELL, C. V ., 1966, Truncated wave-ripple laminae: Jour. Sed. Petrology, v. 36, p. 825- 828. ACKNOWLEDGMENTS ---1971 , Depositional model- Upper Cretaceous I am grateful to the many people who have Gallup Beach shoreline, Ship Rock area, New Mexico: Jour. Sed. Petrology, v. 41 , p. 395-409. discussed parts of this study with me and CAREY , A. G., JR., 1965, Preliminary studies of animal have provided advice and information. sediment interrelationships off the central Oregon R. H. Dott, Jr. was an invaluable and coast: Ocean Science Ocean Engineering 1965, v. I , intrepid field advisor; he also improved the p. 100- 110 . manuscript with careful reading and sugges ---, 1972, Ecological observations on the benthic invertebrates from the central Oregon continental tions. R. E. Hunter and H. E. Clifton also shelf, in Pruter, A. T ., and Anderson, D. L. , eds., provided field advice. Other field assistance The Columbia River Estuary and Adjacent Ocean was supplied by D. W. Larson, P. Blanchard, Waters: Seattle, Univ. Washington Press, p. 422-443. and E. Domack. CLIFTON, H. E ., 1976, Wave-formed sedimentary struc tures: a conceptual model: Soc. Econ. Paleontologists R. E. Hunter, H. E. Clifton, and J. D. Mineralogists Spec. Pub. No. 24, p. 126- 148. Howard kindly supplied me with early drafts CLIFTON, H. E., H UNTE R, R. E., AND PHILLIPS , R. L., of their manuscripts. L. D. Kulm graciously 1971 , Depositional structures and processes in the made unpublished Oregon State thesis work nonbarred high-energy nearshore: Jour. Sed. Pe L. trology, v. 41 , p. 651 -670. available to me. D. G. Jones and E. Saul COLEMAN, P. G., 1978, Tsunami sedimentation, in Fair made paleontological identifications. bridge, R. W., and Bourgeois, J ., eds., Encyclopedia Others who reviewed parts or all of the of Sedimentology: Stroudsburg, Pa., Dowden, manuscript and whose comments are grate Hutchinson and Ross, p. 828-832. fully acknowledged are: C. W. Byers, J. C. CooK, D. 0., AND GORSLIN E, D. S., 1972 , Field observa tions of sand transport by shoaling waves: Marine Harms, J. D. Howard, R. E. Hunter, L. D. Geology, v. 13 , p. 31 - 55 . Kulm, and J . B. Southard. CREAGER, J. S., AND STERNBERG, R. W., 1972, Some This study was funded by National Science specific problems in understanding bottom sediment Foundation Grant No. EAR77-l3132 to R. distribution and dispersal on the continental shelf, H . Dott, Jr., Geological Society of America in Swift, D. J. P., Duane, D. B., and Pilkey, 0. H ., eds., Shelf Sediment Transport: Process and Grant 2169-77 (Ashland Exploration Pattern: Stroudsburg, Pa., Dowden, Hutchinson and Company Gift), and an American Association Ross, p. 347-362. of Petroleum Geologists Grant-in-Aid. CuRRAY, J . R., 1964 , Transgressions and regressions, in Miller, R. L. , ed., Papers in Marine Geology: New REFER ENCES York, Macmillan Co., p. 175-203. DoTT , R. H., JR ., 1971, Geology of the southwestern ANDERSON, E. J ., AND GOODWIN, P. W., 1978, Punctuated Oregon coast west of the !24th Meridian: Oregon aggradational cycles: The Helderberg Group Lower Dept. Geol. Mineral Industries Bull. 69, 63 p. HUMMOCKY-BEDDED SHELF SEQUENCE, SW OREGON 701 ---, 1974, Cambrian tropical storm waves in Wis recognizing shorelines in the stratigraphic record: Soc. consin : Geology, v . 2, p. 243 - 246. Econ. Paleontologists Mineralogists Spec. Pub. No. Don, R.H ., JR ., AND BouRGEOIS , J ., in prep, Hummocky 16, p. 215- 225 . cross stratification- importance of variable bedding HowARD, J . D., AND REINEC K, H . E., 1972, Physical sequences analogous to the Bouma sequence. and biogenic sedimentary structures of the nearshore DR AKE, D. E., KoLPACK, R. L., AND FISCHER, P. J ., shelf: in Georgia coastal region, Sapelo Island, 1972 , Sediment transport on the Santa Barbara-Ox U.S.A.: sedimentology and biology: Senckenberg. nard shelf, Santa Barbara C hannel, California, in Maril. v. 4, p. 81- 123. Swift, D. J. P., Duane, D. B., and Pilkey, 0. 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