Hummocky stratification: Significance of its variable bedding sequences

R. H. DOTT, JR. Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706 JOANNE BOURGEOIS Department of Geological Sciences, University of Washington, Seattle, Washington 98195

ABSTRACT mudstone separates hummocky beds. An account for such differences. Further doc- idealized hummocky stratification se- umentation of variations in hummocky Hummocky cross-stratification is an im- quence, which can serve a purpose similar stratification should reveal important de- portant structure formed on the shoreface to the Bouma sequence for graded beds, is tails about these factors. and shelf by waves. It characterizes a wave- as follows (bottom to top): first-order dominated facies. Attention to its variabil- scoured base (± sole marks); characteristic INTRODUCTION ity can reveal much about sedimentary hummocky zone with several second-order history and paleogeography. Diagnostic truncation surfaces separating individual An important primary structure first traits are antiformal hummocks and syn- undulating lamina sets; a zone of flat lami- called "truncated wave-ripple laminae" formal swales defined by randomly orient- nae; a zone with well-oriented ripple cross- (Campbell, 1966; 1971), and later renamed ed, even lamination with dip angles and laminae and symmetrical ripple forms; all "hummocky cross-stratification" (Harms truncation angles of < 15°. Hummocky stra- overlain by a more or less burrowed mud- and others, 1975; Harms, 1979), has attract- tification forms primarily in silt to fine stone or siltstone. This sequence reflects ed much attention (Fig. 1). It was recog- sand. Although size grading of individual waning of storm waves followed by fair- nized and defined from the ancient record, laminae is not characteristic, concentrations weather sedimentation and burrowing. Var- where it typically occurs in vertical sequence of mica and plant detritus in the tops of iations from this idealized conceptual se- between nearshore and outer-shelf facies. It many laminae indicate a shape sorting. quence involve omissions and/or expan- also has been reported from lacustrine, Parting lineation is common. Hummocky sions of one or more of the zones. The most intertidal, and estuarine deposits (Campbell beds vary in thickness from a few centime- common variant is amalgamation either by and Oakes, 1973). Cores of Holocene tres to 5 or 6 m; bed sets may be tens of the stacking of successive hummocky zones marine sediments are generally not large metres thick. Hummocky stratification ap- or by intense bioturbation that obliterates enough for certain identification of hum- parently is formed most commonly by rede- original boundaries between depositional mocky stratification, but we believe that position below normal fair-weather wave units. Other variations include units com- fine sands with "parallel lamination," low- base of fine sand delivered offshore by mencing with flat-lamination; units with angle truncations, and burrowed zones, flooding rivers and scour of the shoreface or predominant cross-lamination; and lenticu- reported from shelf areas, represent hum- shoals by large waves. Deposition involves lar micro-hummocky lenses within shale. mocky stratification (Howard and Reineck, both fallout from suspension and lateral Combinations of relative sand supply, rela- 1981). Scuba divers have reported hum- tractive flow due to wave oscillation. There tive depth, tidal range, frequency, duration mocky bed surfaces immediately following is evidence that, under intense oscillatory and magnitude of storms, and relative pro- winter storms (Hunter and Clifton, 1982; flow, large waves drape sand over an irregu- ductivity for a burrowing benthos must Greg Geehan, 1979, oral commun.). At lar scoured surface and also mold sand into roughly circular, unoriented hummocks and swales. We postulate that these circumstan- ces are analogous to the transition to upper flat-bed conditions in unidirectional flow. Hummocky stratification shows impor- tant variability. It occurs in both regressive (progradational) and transgressive strata in intervals a few centimetres to 175 m thick 0.5 m and may be interstratified with mudstone, sandstone, or conglomerate. Hummocky stratification commonly occurs in repetitive successions with the products of individual Figure 1. Characteristics of hummocky cross-stratification, published by Harms and depositional events being clearest where others (1975, p. 88).

Geological Society of America Bulletin, v. 93, p. 663-680, 24 figs., 2 tables, August 1982.

663

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/93/8/663/3419266/i0016-7606-93-8-663.pdf by guest on 24 September 2021 664 DOTT AND BOURGEOIS

least one experimental study also has pro- Devonian examples, but later Goldring and evidence of intense and rapid processes. duced a hummocky bed surface (Carstens Bridges (1973) referred to "low-angle trun- Rapid deposition is also indicated by zones and others, 1969). All of the original cated ripple lamination's characteristic of of contorted or ball and pillow structures. authors attributed hummocky stratification sublittoral sheet sandstones, which they These, together with evidence of episodicity to wave processes, but they differed in defined as relatively thin, shallow-marine provided by the distribution of bioturbation whether usual or unusual waves were re- deposits characterized by fine-grained, and analogies with Holocene shelf sedi- quired. plane-laminated, or low-angle cross-lami- ments, persuade us that most preserved Campbell defined truncated wave-ripple nated sandstones interstratified with fossi- examples of hummocky stratification itself, laminae from examples in the Cretaceous liferous marine shales; gutter casts were as well as the scours, were formed by unus- Gallup Sandstone, New Mexico, as "sets of commonly associated. Because of the epi- ual waves beyond the fair-weather breaker parallel laminae that conform to the surface sodic nature of deposition revealed by trun- zone. We believe that the structure also may form of wave (oscillation) ripples. The cations and by burrowed and nonburrowed be formed by modest waves in shallower laminae peak upward or curve over the crest intervals, they considered storm waves, tidal environments, but we regard preservation of the ripple, continue downward into the influences, turbidity currents, rip currents, potential as low in those circumstances. In adjacent trough, and again curve upward to and tsunamis as possible agents for deposi- any case, hummocky stratification indicates the next crest. . . [Most of the laminae] tion of sheet sandstones. a wave-dominated setting. were truncated prior to deposition of the In the progradational sequences where Hummocky stratification was unrecog- overlying set of wave-ripple laminae." the structure was defined, hummocky strati- nized for so long because it was easily con- (Campbell, 1966, p. 826-827; brackets fication overlies marine mudstone and un- fused with more familiar structures. The low- added). By parallel, Campbell meant "never derlies shallow-marine or fluvial sandstones angle parallel laminae and low-angle trun- intersecting" rather than "everywhere equi- with well-oriented trough cross-stratifica- cations closely resemble swash-zone and distant." The feature was observed only in tion. This position, association with wave some fluvial lamination. Abundant mica coarse silt and fine sandstone. Maximum ripples, lack of orientation, sharp basal con- and plant flakes help distinguish hummocky set thickness reported was 60 cm; crest spac- tact, and mantling relationships of overly- from swash-zone laminae, which are purged ing ranged from 10 cm to 10 m, and ripple ing laminae suggested to Harms deposition of such grains by surf agitation. The syn- heights from 1 cm to 1 m. by strong oscillations of greater intensity formal swales resemble both trough cross- Harms and others (1975, p. 87) character- than required to form ordinary wave rip- stratification and ordinary scour-and-fill ized hummocky cross-stratification as fol- ples; peak velocities of at least 1 m/sec and structures; there is even evidence that hum- lows: "(1) lower bounding surfaces of sets variable directions were envisioned. First, mocky stratification may grade to ordinary are erosional and commonly slope at angles the bed is scoured into unoriented hum- cross-stratification. The synforms are less less than 10°, though dips can reach 15°; (2) mocks and swales and then is mantled by regularly spaced and less oriented than laminae above these erosional set bounda- laminae of fine particles swept over these trough sets, and dip angles are statistically ries are parallel to that surface or nearly so, irregularities. If wave intensity increases smaller. Although both structures have ero- (3) laminae can systematically thicken later- again, partial scouring recurs and the whole sional bases, upward-diminishing inclina- ally in a set, so that their traces on a vertical process repeats. If, however, wave intensity tions are more characteristic of hummocky surface are fanlike and dip diminishes regu- continues to wane, ordinary wave ripples stratification. Hummocky units also tend to larly, and (4) the dip directions of erosional may form briefly, before deposition reverts be tabular and laterally persistent at normal set boundaries and of the overlying laminae to very fine-grained fair-weather sediments outcrop scale. Sequences with many tabular are scattered. . . The bases of hummocky typified by a burrowing fauna. Thus, a beds separated by mudstones superficially cross-stratified layers are commonly sharp complete bed with hummocky lamina sets resemble turbidite sequences, but internal and may have drag and prod marks at con- succeeded by wave ripples was inferred by details are different. Vertical cross sections tacts with underlying clay-rich beds. The Harms to be the product of a single storm look the same regardless of orientation, tops may have wave ripples." Hummocky event. Harms (1979) estimated that hum- unlike any other type of cross-stratification, stratified fine sandstones and coarse silt- mocky stratification may form in water 5 to which is particularly helpful in recognizing stones typically contain abundant mica and 30 m deep; C. V. Campbell (1981, written hummocky stratification in drill cores (J. C. carbonaceous detritus and are generally commun.) suggested 0 to 80 m. Hunter and Harms, 1980, written commun.). To us, the interstratified with shale or siltstone with an Clifton (1981) suggested a depth as shallow single most compelling feature is the hum- abundant marine fauna. as 2 m for an Eocene example in Texas. mock or antiform (Fig. 2A), but this obviously has a lower preservation potential Others who recognized the uniqueness of Many workers have accepted the inter- than the synform. As a result, many hum- what has come to be called "hummocky pretation of Harms that hummocky stratifi- mocky beds consist mainly of preserved cross-stratification" by many included J. D. cation is formed by unusually large waves, synforms with few if any obvious antiforms Howard, who wrote of "truncated megarip- but others such as C. V. Campbell (1981, and so are easily mistaken for trough cross- ples" and "ripple and trough laminations" written commun.) believe that the structure stratification (Fig. 2B). This inhibits certain inferred to be shoreface to offshore deposits is so common that it was deposited by usual identification of hummocky cross-stratifica- (1972). Repeated truncation of burrowed waves with only the associated truncation tion, but familiarity with other characteris- zones led him to propose "episodic storms surfaces being formed by unusual ones. tics helps one make a proper identification. or tidal action" as the mechanism of forma- Presence of rare pebbles, local laminae Besides antiforms, low-angle laminae, lack tion; Cotter (1975) adopted a similar expla- inclined above the angle of repose, and mul- of orderly spacing and orientation of syn- nation for nearby older Cretaceous strata. tiple scour surfaces within hummocky units forms, association of wave-formed ripples, Goldring (1971) coined "Reynard facies" for are taken by Hunter and Clifton (1982) as

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/93/8/663/3419266/i0016-7606-93-8-663.pdf by guest on 24 September 2021 HUMMOCKY STRATIFICATION 665

Figure 2A. Hummocky stratification in the Cretaceous Ferron Figure 2B. Thick, lenticular hummocky unit with slightly convex Sandstone of central Utah showing well-defined lamination and top and trough-like second-order scour surfaces. Abrupt thinning is hummock or antiform, the single most distinctive feature of the shown in upper right corner of photo. Thinner hummocky beds structure. separated by mudstones occur below and above. [Lower Coaledo Formation (Eocene), Sunset Bay, Oregon; courtesy D. B. and characteristic position within a vertical McCubbin.] sequence have helped us identify hum- mocky stratification in nine different forma- 1979; Hamblin and Walker, 1979; Wright Hummocky stratification deserves fuller tions scattered among Oregon, Wisconsin, and Walker, 1981; Leckie and Walker, discussion because of its great sedimento- Texas, and the Rocky Mountains. On sev- 1982); the Lower Cretaceous of Texas logical significance and potential for mis- eral occasions, we successfully predicted (Hobday and Morton, 1981); the Upper identification. It is significant for the fol- both its presence and stratigraphie position. Cretaceous, Eocene, and Miocene in south- lowing reasons: (1) potential bathymétrie Since 1975, occurrences of hummocky western Oregon (Bourgeois, 1979, 1980; guide for the inner-shelf to lower shoreface stratification have been recorded in the Dott and Bourgeois, 1979, 1981; Hunter transition; (2) apparent evidence for large Cambrian of Wisconsin (Dott and Byers, and Clifton, 1982); and in Cretaceous sand- storm waves or tsunamis; (3) evidence of 1981) and California (Mount, 1981); the stones in Colorado (Boyles and Scott, relative event frequency and rates of scour- Ordovician of Norway (Brenchley and oth- 1981a, 1981b) and Montana (Dudley Rice, ing, deposition, and burrowing. To maxi- ers, 1974) and of Virginia (Kreisa, 1981); 1980, oral commun.). It can be inferred less mize its interpretive potential, however, the the lower Carboniferous of Ireland (DeRaaf confidently from other publications. variability of hummocky stratification and others, 1977); the late Miocene in Spain (Roep and others, 1979); the Silurian in Nova Scotia (Cant, 1980); the Jurassic and Cretaceous in western Canada (Walker,

IDEALIZED HUMMOCKY SEQUENCE

UNBORROWED BURROWED M MUDSTONE Mb X CROSS LAMINAE F FLAT LAMINAE

H HUMMOCKY ZONE

(iLAG tSOLES) Figure 4. Example of an ideal hummocky unit (upper). Note sharp base, thick H zone with several second-order truncations, thinner F zone, Xb zone with symmetrical ripple profiles, and an Figure 3. The idealized hummocky sequence showing suggested Mb zone. Lower bed shows amalgamation by cut-out of mudstone notation for its component zones. Note distinction of hummocky (center) and much burrowing in top. Intercalation of thin mudstone beds from lamina sets and a hierarchical order of surfaces (numbers) and cross-laminated sandstone at tops of units produces a complex explained in text. Burrowing may overprint the top or even the X-M transition. See Figure 3 for shorthand notations. [Upper entire sequence. (After Dott and Bourgeois, 1979, 1980.) Coaledo Formation (Eocene), Yokam Point, Oregon.]

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/93/8/663/3419266/i0016-7606-93-8-663.pdf by guest on 24 September 2021 666 DOTT AND BOURGEOIS

COMMON VARIATIONS present, even if very thin (Figs. 2B, 4), the differentiation of individual hummocky beds—therefore also of major depositional events—is clear. Where there is no such mudstone, sandy units are juxtaposed or amalgamated (Fig. 4, lower) so that the dis- tinction of a first-order from a second-order truncation surface may be difficult. Basal first-order boundaries may show as much as 30 or 40 cm of relief but commonly are nearly flat; current sole marks are observed M ZONE DEEPLY X ZONE X THICK on some (Harms and others, 1975; Hamb- MISSING BIOTURBATED MISSING F MISSING lin and Walker, 1979). Load structures are TOP very rare, suggesting that initial scour cut down to a moderately compacted, cohesive Figure 5. Common variations from the ideal hummocky unit. Two left examples are substrate. Pebble, shell, and even mudstone most common. intraclast lags may occur along basal con- tacts, less commonly along second-order truncations, and rarely even within lamina sets. needs full documentation, and we have had ally useful standard for comparison by the fortune to encounter the structure in showing all features that would be expected A complete hummocky bed is interpreted very diverse settings. In building upon the if every change of process left a record. As to represent a single tsunami or major storm work of Harms, Campbell, Clifton and such, it can provide guidance and prediction event or storm season, which might span Hunter, Goldring and Bridges, Howard and. for new studies as well as a basis for genetic many days or weeks. Assuming that first- Reineck, and Walker, we describe first an interpretation. order surfaces reflect onset of major storm idealized descriptive sequence for hum- The basal contact of a hummocky se- events, they would be separated in time by mocky beds, and then we document the quence is sharp and erosional. It is here weeks, months, or years. Conversely, if variability of such beds. By comparing spe- designated a "first-order boundary" to dis- second-order surfaces represent pulses with- cies of the structure with the idealized tinguish it from internal second-order trun- in a single storm event or season, as we sequence, we then suggest a continuum of cations within hummocky units (Fig. 3). believe, they would be separated by hours, variations that may be interpretable in Thus, hummocky beds are defined by first- days, or weeks. Individual laminae seem- terms of factors such as proximity to shore- order boundaries, which also may be term- ingly would represent seconds or minutes, line, depth of water, magnitude and dura- ed "bedding surfaces," whereas hummocky that is, the periods of individual wave oscil- tion of waves, and magnitude of sand lamina sets are defined by second-order lations or pulsations of wave trains as sug- supply. boundaries (Fig. 3); individual laminae are gested by Harms (Harms and others, 1975; defined by third-order laminar surfaces Harms, 1979). AN IDEALIZED SEQUENCE FOR (heirarchy patterned after Campbell, 1967). In our experience, thickness of hum- HUMMOCKY STRATIFICATION Typically, beds contain several second- mocky beds varies from a few centimetres order truncations, but these may be subtle to ~ 1-1.5 m, the most common range being We have proposed already (Dott and due to a very low angle of truncation and 20-80 cm. Most are tabular and laterally Bourgeois, 1979, 1980) an idealized se- the tendency for truncations to merge later- persistent at least at the scale of outcrops, quence or descriptive model for hummocky ally with concordant laminar surfaces (Fig. but a few are lenticular (Fig. 2B). A possible stratification which we feel could serve a 4, lower left). This lateral change suggests end member illustrated below has very fine role analogous to that of the Bouma only local scouring within a nearly continu- sandstone lenses only a few centimetres sequence for turbidites. Figures 3 and 4 ous episode of deposition, and such intri- thick and 10+ cm wide within shale. Indi- illustrate that sequence. Deviations from cate associations also suggest that the vidual antiforms as seen in cross section in this ideal hummocky sequence involving hummock bed form is produced by a com- the hummocky (H) zone may be as high as omissions or expansions of some of the bination of scour and deposition operating 50-80 cm. Both hummocks and swales may zones are common (Fig. 5), as with base- within a brief span of time. The boundary be either symmetrical or asymmetrical in cut-out or top-truncated Bouma sequences. between the hummocky (or flat) zone and cross section. Rarely, topographic form is Our ideal hummocky sequence was in- the cross-laminated zone, although distinct preserved at the top of a bed (Fig. 6A). spired initially by Eocene examples at Coos and somewhat erosive, neither produces a From favorable exposures and orientation Bay, Oregon, but supplemented by many bedding surface when weathered nor shows measurements (see Fig. 11 and Bourgeois, other examples. We emphasize that it is not evidence of cessation of deposition, but only 1980), it is clear that hummocks and swales claimed to be the most common sequence; a change of process. Therefore, we regard it are essentially circular. Individual laminae as with the Bouma model, single beds con- as second-order. Inclusion of the top silt- vary from about 1 mm to 1 or 2 cm in thick- taining all of the prescribed zones are less stone-mudstone zone in the ideal sequence, ness. Even though generally curved, they common than incomplete variants. Instead, rather than treating it as a separate bed, is remain parallel (nonintersecting) in spite of it is a composite generalization such as the discussed below. lateral thinning over hummocks (antiforms) Bouma. sequence that provides a conceptu- In cases having mudstone or siltstone and thickening within swales (synforms). As

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/93/8/663/3419266/i0016-7606-93-8-663.pdf by guest on 24 September 2021 HUMMOCKY STRATIFICATION 667

Figure 6A. Unusual preservation of hummock (center) and swale (upper left) profiles. Note second-order truncations within hum- Figure 6B. Thin hummocky beds with prominent mudstone mock. Sharp truncation of sandstone laminae at top indicates final intercalations between sandstones. Conspicuous mud-filled scours shaping of this hummock by scour. Fair-weather muddy sediment on top of one HS unit (arrow). (Same locality as in Fig. 2B.) then draped the bed forms. Upper right: - - more normal hum- mocky units; Lower left: - - alternating, thin cross-laminated sand- lensing (Fig. 4). Although common in some stones and mudstones (XM beds) produced by wave rippling. stratigraphie sequences, this zone is com- (Same locality as in Fig. 2B.) pletely missing in others (Fig. 5). In favora- ble exposures, symmetrical ripple crests can a result of such thickness changes, the lami- cation is the result of a combination of be seen in the zone (Fig. 4). Internally, most nae tend to flatten upward to produce a flat- fallout and the molding of stationary hum- cross-laminae show bipolar orientations laminated (F) zone, which may not be very mocks and swales by flow induced by characteristic of symmetrical wave oscilla- distinct (Fig. 4). Sedimentation tends to vigorous but waning wave oscillation as tion, but asymmetrical cross-laminae also smooth the depositional surface by filling in sand falls onto the smooth but undulating occur, and in some cases show clear unidi- swales. bed. We envision it as the analogue for rectional orientations. Many of the ripples There is some controversy about whether oscillatory flow of the upper flat-bed condi- apparently are the combined flow type of hummocky stratification is strictly a scour- tion of unidirectional flow. Harms (1969, 1979); see also DeRaaf and and-drape feature, or whether it reflects a Grain size, as already noted, typically others (1977) and Clifton (1976). growing bed form. D. B. McCubbin stresses falls within the fine-sand to coarse-silt The upper mudstone or siltstone (M) that common parting lineation requires range, but we have seen a few examples with zone is extremely important but difficult to some tractive transport along the deposi- medium sand. Sorting within laminae is treat. It can represent both waning-storm tional surface (1981, oral commun.). Sand- moderate to good. Mica and macerated and normal fair-weather sedimentation, and grain-orientation data reported by Gold- plant flakes tend to be concentrated in the so it presents both a descriptive and inter- ring and Bridges (1973) from their "sheet tops of many laminae, especially high in a pretive problem analogous to that of the d sandstones" also suggest traction. Hamblin bed set. Size grading is not conspicuous and e zones of the Bouma sequence. In both and Walker (1979) reported flat lamination except where skeletal grains are present cases, because weathering produces a reces- at the bases of many hummocky units, (Kreisa, 1981), apparently due to over-all sive profile (Fig. 6), one might separate this which changed upward into typical undulat- fine grain size, good sorting, and final trac- zone descriptively as a distinct bed. Gold- ing laminae, implying progressive upward tive transport. A subtle color gradation ring and Bridges (1973) argued that the growth of hummocks as bed forms. We find within laminae commonly reflects greater lower boundary of such fine zones in shelf in rare cases that some laminae thicken over concentration of mica and plant flakes at deposits is sharper than for turbidites, but the crests of antiforms, which also indicates the tops, which indicates some shape and this difference is subtle except where scours such growth. We also find basal laminae density sorting during deposition. on the top of a sandstone are filled with overstepping one another laterally at a very The zone of rippled and cross-laminated mud (Fig. 6B). In such obvious cases, we low angle across a flat basal, first-order sur- fine sand or silt, if present at the top of would treat the zone as a separate bed and face, indicating some simultaneous lateral hummocky beds, represents temporary re- the boundary as a first-order bedding sur- and vertical growth. In our experience, working at low flow intensity of the top of face. For other cases, however, we choose however, many examples that appear to the bed by wave oscillation as a storm operationally to include the mudstone- show upward growth actually contain very waned; fair-weather conditions followed. siltstone zone within the ideal hummocky subtle, low-angle, second-order truncations This zone (X) averages between 1 and 5 cm sequence because of uncertainty about any that produce an illusion of increase of con- in thickness, but may be variable even break in deposition and the near impossibi- vexity. We believe that hummocky stratifi- within one hummocky unit due to lateral lity of separating waning-phase from true

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/93/8/663/3419266/i0016-7606-93-8-663.pdf by guest on 24 September 2021 668 DOTT AND BOURGEOIS

pelagic deposition. An alternate approach emphasizing every minor change of process might exclude not only the mudstone- siltstone but also the ripple-laminated zones from the ideal sequence. We wished to emphasize major-event distinction, how- ever, and to us the fine zone represents part of the complete history recorded between successive first-order truncation surfaces; our individual zones and bounding surfaces recognize changes of process during one. event. The mudstone-siltstone zone commonly is massive, but it may show parallel lamina- tion defined by silt or very fine sand streaks and fine cross-laminae (see DeRaaf and others, 1977). Very commonly, however, it is characterized by animal burrowing that has more or less obliterated original internal structure and blurred the lower contact. For our idealized sequence, we indicate only partial burrowing of the upper zones, the most common situation in our experience, Figure 7. Two intensely burrowed hummocky beds with almost complete obliteration of but greater or lesser density and thickness of original lamination; more moderate burrowing density below and above these beds. Upper bioturbation is one of the most important burrowed bed has convex-up shells near base. (Same locality as Fig. 4.) variations observed in hummocky cross- stratified deposits (Fig. 7, see also Fig. 16). ly, within lamina sets. If no clue is present, Pockets of relict laminae and truncated bur- As indicated in Figure 3, burrowing can be or if bioturbation was extreme, it may be rows within an otherwise homogeneous- an overprint on any of the zones of the ideal impossible to discriminate the products of looking amalgamated burrowed sequence sequence. It is of great importance in defin- individual major events. provide clues to multiple sedimentary ing boundaries descriptively and in estab- Amalgamated burrowed sequences may events (see Bourgeois, 1980). lishing relative rates of deposition interpre- also occur in association with hummocky tively. stratification. In such a case, storm-de- VARIABILITY OF HUMMOCKY Amalgamated Hummocky sequences that posited hummocky zones were thin enough STRATIFICATION WITHIN are devoid of a mudstone zone, of rippled and periods between storms long enough STRATIGRAPHIC SEQUENCES surfaces, and of burrowed intervals between that burrowing destroyed the laminae (Fig. individual bed sets are common (Fig. 8). In 8). In this way, burrowed intervals more Genera) such sequences, one hummocky zone suc- than 2 m thick can be produced—much ceeds another for as much as 40 m vertically greater than 20- to 30-cm average maximum We shall document a broad range of vari- (Bourgeois, 1980). Such amalgamation ren- depth of modern burrowing (Schafer, 1972). ability of hummocky stratification from ders it very difficult to discriminate indivi- dual depositional events. That is, the dis- tinction of first-order from second-order AMALGAMATED SEQUENCES truncation surfaces (Fig. 3) is not obvious, just as is true for the discrimination of indi- H vidual Bouma sequences in amalgamated turbidites. Yet such distinction is important PEBBLES because of the disparate time values of these two types of surfaces. Several clues may help one to discriminate first-order trunca- tions from among associated second-order ones. These include lateral cutting-out of SHELLS mudstone layers from between hummocky H units, truncated contorted zones, and trun- cation of burrows within an otherwise homogeneous bioturbated sequence (Fig. M-CUTOUT BIOTURBATED LAG TRUNCATED TYPE TYPE TYPES CONTORTIONS 8). Lags of pebbles, shells, or exceptionally rich plant and mica detritus may distinguish Figure 8. Several types of amalgamation ("welding together") of successive hummocky first-order boundaries, but they may also beds. The two left examples are most common. (See Bourgeois, 1980, for further illustra- occur along second-order surfaces or, rare- tions of amalgamation;)

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/93/8/663/3419266/i0016-7606-93-8-663.pdf by guest on 24 September 2021 HUMMOCKY STRATIFICATION 669

diverse stratigraphic settings (Table 1) to Eocene at Coos Bay, Oregon two types. Both have similar lower and provide a basis for generalizations about the middle portions with alternating marine structure in a concluding synthesis. Com- Setting. The Upper Eocene at Coos Bay is shale and fine sandstone strata containing mon variations include differences of thick- a lithologically varied deltaic complex. many hummocky units; only thicknesses ness of bed-set intervals, thickness of Northwestward progradation brought del- and tops of the successions differ. A first individual hummocky beds, ratio of hum- taic facies over a narrow shelf-slope facies type is capped by very coarse, cross-bedded mocky-stratified sandstone to associated (Elkton Siltstone of Fig. 9; Dott, 1966; Dott sandstone containing considerable petrified shale, absence of one or more of the zones and Bird, 1979). The Lower Coaledo For- wood but devoid of fauna except Teredo of the ideal sequence, and degree of biotur- mation (400 m) records eight to ten coarsen- borings in logs, these are constructional bation of the hummocky bed sets. We have ing-upward progradational episodes, and deposits of delta distributaries. A second studied several examples of hummocky the Upper Coaledo Formation (300 m) type is capped by very coarse sandstone stratification in the Cretaceous of the shows six or eight upward-coarsening se- showing intense bioturbation and contain- Rocky Mountain region, but we stress quences (Ryberg, 1978; Dott and Bird, ing shells of burrowing clams; wood is lack- Oregon examples because the former have 1979). Faunal evidence suggests total depth ing. This type represents distributary-mouth been described already and the latter illus- fluctuations of < 100 m (Rooth, 1974; Dott bars and destructional delta-margin de- trate greater variability. Superb sea-cliff and Bird, 1979). Presence of a large Pacific posits. exposures and very thick hummocky-bear- basin to the west, a narrow shelf, and Hummocky Stratification in the Coaledo ing intervals combine to provide excep- abundant symmetrical ripples and hum- Formation. Hummocky stratification oc- tional insights; abundant mudstone inter- mocky stratification suggest a wave-domi- curs in all of the progradational successions calations at Coos Bay produce an especially nated shoreline. and constitutes as much as 50% or 60% of instructive succession. Coarsening-upward successions are of the total thickness (Fig. 10). The hummocky stratification has essentially random dip TABLE 1. PRINCIPAL EXAMPLES OF VARIABILITY OF orientations, wheras associated ripple-crest HUMMOCKY STRATIFICATION trends and ripple-cross-lamination are strongly oriented (Fig. 11). The few current Locale Stratigraphic setting Nature of hummocky sole marks observed show the same north- stratification* northwestward dispersal pattern typical for current features in the entire formation Miocene Transgressive shallow-marine Mostly HF amalgamated units (Dott, 1966). Blacklock Point and Cape sandstone and conglomerate intercalated with thin peb- Figure 10 shows some variations of Blanco, Oregon ble and cobble conglomerate layers hummocky stratification within the Coa- ("Sandstone of Floras No mudstone ledo. The typical succession is from very Lake") Little bioturbation thin hummocky to cross-laminated (HX) and hummocky-only (H) units interstrati- Eocene Thick delta-margin complex with Mostly HFX units typically sepa- fied with mudstone and with thin cross- Coos Bay, Oregon many progradational cycles rated by mudstone laminated (X) units (Figs. 4, 6, 7), upward Bioturbation prominent through thicker and more amalgamated, (Elkton and Coaledo Much plant and mica detritus light tan hummocky units with little or no Formations) Some truncated contorted laminae intercalated shale, finally to either a coarse distributary or delta-front sandstone. Hum- Upper Cretaceous Transgressive sandstone succes- HF amalgamated units (lower) Cape Sebastian, Oregon sion 200 m thick composed HFX units with prominent mocky lamination is in all cases sharply mostly of hummocky stratifica- bioturbation (upper) defined by dark laminae that are rich in tion (see Bourgeois, 1980) Mudstone very minor (top only) plants and mica. Hummocky bed sets vary (Cape Sebastian Sandstone) Some plant and mica detritus from 10 cm to 4 m thick, but single hum- mocky beds are 5 cm to 1 m thick. Anti- Cretaceous Progradational deltaic and barrier HFX and HF units, some with forms and synforms are from about 20 cm Colorado and Utahf mudstone and some amalga- complexes (Ferron and Price to 3 m across and as much as 0.5 m in verti- River) and transgressive succes- mated (Dakota, Ferron, and Price sions (Dakota) with hummocky Bioturbation variable cal dimension (Fig. 6A). Penecontempo- River Formations) stratification between shelf Some plant and mica detritus raneous deformation is illustrated well in shales and littoral sandstones Small lenticular hummocky strati- several of the thicker amalgamated bed sets fication units in shale (Dakota) (Fig. 10). A few dark gray zones homogen- ized by almost complete liquefaction are Cambrian Fine-sand shelf deposits with H, HF and HFX thin units represented as well (see Doe and Dott, 1980, Wisconsin hummocky stratification inter- No mudstone calated with medium-grained, Some units seem to grade laterally Fig. 14). (Tunnel City and Norwalk trough cross-bedded sandstones to trough cross-bedding There is much variation of association of Formations) hummocky and cross-lamination in the Coaledo. Many hummocky units are cap- •See Figure 3 for key to shorthand letter designations. ped either by clear symmetrical or com- fNot described here (see Campbell, 1966, 1971; Harms and others, 1975; Howard, 1972; Cotter, 1975; Goldring and Bridges, 1973). bined-flow ripples (Fig. 4), but asymme- trical ripple cross-lamination and climb-

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/93/8/663/3419266/i0016-7606-93-8-663.pdf by guest on 24 September 2021 670 DOTT AND BOURGEOIS

PR06R AD AT I ON AL SEQUENCES IN paucity of internal truncations. Abundant THE LOWER COALEDO FORMATION mica argues against a swash-zone origin. Variations of flat-laminated, cross-lami- nated, and combined flat to cross-laminated O units associated with hummocky stratifica- lu a tion attest to considerable variation of flow oÀ wI conditions, sometimes strictly wave oscilla- tion but at other times unidirectional flow 9 < RELATIVE DEPTH: and fluctuations of flow intensity between 5 (SHOALING)— 8 rippled and flat-bed conditions. | / I SHOAL ORIENTATION DEP0SIT DATA Hummocky Stratification in the Elkton -HUMMOCKY 7 Siltstone. Unusual hummocky stratification INTERVAL(H)^-, SHOAL ® occurs in a laterally persistent zone at the top of shelf-slope deposits beneath the 200- Coaledo Formation (Fig. 9). The structure in the upper Elkton is notable for (I) very lenticular geometry, (2) abundant mudstone SHOAL SUBSEA and Singh (1972) with associated clear CHANNELS symmetrical ripples and, at some localities, ÜJ d to unusual vertically climbing symmetrical STORM- ripple lamination (see Dott and Bird, 1979, WAVE Fig. 9). Both peculiar types of lamination BASE are gradational above and below with either Figure 9. Generalized columnar section for lower upper Eocene strata at Coos Bay, parallel lamination or small-scale asymmet- Oregon, showing position of hummocky stratification in a series of shoaling-upward (pro- rical cross-lamination. gradational) successions. For orientation data, see Figure 11. Depositional Regime. Dott and Bird ing-ripple lamination also are associated tops on many hummocky beds (Fig. 12, (1979) interpret upper Elkton strata to have (Fig. 12). In a few cases, hummocky stratifi- upper right). Hummocky stratification also been deposited in front of the advancing cation can even be seen to pass laterally into occurs here with sandstones showing only Coaledo delta distributary system and sub- such cross-lamination. Moreover, thin beds flat lamination or flat lamination with very ject both to great influxes of terrigenous of entirely asymmetrical or climbing cross- rare, shallow, localized swales and very low- sediment and to vigorous wave and tidal lamination are commonly interstratified angle truncations (Fig. 12, bottom). The influences; water depth was several tens of with mudstone and hummocky strata. The exact origin of the latter beds is not clear, metres. Large volumes of mud, silt, and Coos Bay rocks tend to have exceptionally but their association suggests that they are a sand apparently were always available. We thick and rather lenticular cross-laminated hybrid of hummocky stratification with a infer that storms scoured the bottom and

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/93/8/663/3419266/i0016-7606-93-8-663.pdf by guest on 24 September 2021 HUMMOCKY STRATIFICATION 671

THIRD PROGRADATIONAL SUCCESSION BELOW HS FINE SS TOP OF LOWER COALEDO FM.(SUNSET BAY) CONTORTED HS;FINE MICACEOUS SS

AMALGAMATED THICK HS;FINE SS

TOP OF SUCCESSION HS FINE SS

COARSE SHOAL-WATER DEPOSIT SS XENOLITHS LARGE BURROWING CLAMS SYMM. RIPPLE X-LAM.

'mi HS 4- SYMM. RIPPLES \=I0-I5CM,A ICM 22- HS "" SKOLITHOS TYPE BURROWS HS 6"» >• FINE SS HS FINE-MEDIUM SS HS J 8- SYMM.RIPPLES X=I2-I5CM,A ICM DISARTICULATED MOLLUSCS 26- CONVOLUTED CBLS UP TO 5 CM HS ? FINE SS HS FINE SS ASYMMETRICAL RIPPLE X-LAM. HS IO-; SYMMETRICAL RIPPLE X-LAM. HS VERY FINE SS AMALGAMATED HS HS " - FINE SS HS MICACEOUS VERY FINE SS HS , 32-

DARK MUDSTONE CONTORTED HS; FINE MICACEOUS SS 34-, AMALGAMATED THICK HS; FINE SS 16-5 BASE OF SUCCESSION

HS FINE SS COARSE SS, SHELLS + BURROWS

Figure 10. Detailed columnar section of part of one typical progradational succession near top of the Lower Coaledo Formation at Sunset Bay, Oregon. Note relatively thin, more or less ideal units of hummocky stratification in lower part separated by mudstone and thin cross-laminated sandstone beds. Upward, hummocky units became thicker, more amalgamated, and have little or no intercalated mud- stone. "Xenoliths" were illustrated in Dott (1966, PI. 4C) and Doe and Dott (1980, Fig. 14). (A = spacing; A = amplitude of ripples.)

created isolated hummocks of large dimen- due to more uniform bottom topography astian, on the southwest Oregon coast. It is sions surrounded by normal, thinly lami- and more stable sea level. Wave magnitudes a 200-m-thick, Campanian transgressive nated shelf muds and fine sands. An are assumed to have been comparable to shelf sequence (Bourgeois, 1980), uncon- unusual balance of fluvial sediment input those of today. River flooding as well as formably overlying Campanian submarine during floods with waves and tides perhaps storm surges provided the sand to the near- fan-channel deposits. Its upper contact led to deposition of the graded rhythmites shore, which was then molded into hum- appears gradational with Campanian-Maes- and rhythmically climbing symmetrical rip- mocky stratification. While subsidence ap- trichtian(?) submarine slope to base-of- ple laminae between hummocks by simple parently was more or less continuous, slope deposits. The Cape Sebastian was fallout of fine sand and mud from suspen- deposition near active distributaries pro- deposited on a tectonically active, high- sion. The apparent rhythmicity may reflect duced shoaling successions with hummocky relief continental margin where sedimenta- influence of tidal cycles superimposed upon stratification. Hummocky units became tion rates almost kept pace with relative a continuing but waning river-flood dis- thicker and more amalgamated as the delta sea-level change. Hummocky sequences charge. margin prograded. characterize 80% to 90% of the formation, Hummocky stratification in the Coaledo and amalgamated units are very common. Formation also formed on the periphery of Upper Cretaceous of Oregon The Cape Sebastian Sandstone is a con- the delta, but the hummocky units are more tinuously fining-upward succession divisible laterally extensive and more numerous in Setting. The Cape Sebastian Sandstone is into four facies (Bourgeois, 1980, and our vertical sections than in the Elkton, perhaps best exposed on the south side of Cape Seb- Fig. 14). The lowest is a basal, shelly,

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/93/8/663/3419266/i0016-7606-93-8-663.pdf by guest on 24 September 2021 672 DOTT AND BOURGEOIS

HUMMOCKS

N Figure 11. Orientatioin diagrams for three different prograda- tional successions in the Lower Coaledo Formation. Black arcs = ripple-crest trends; central histograms - ripple cross-lamina- tion dip directions; dots = poles to associated hummocky stratifica- tion plotted in lower-hemisphere stereographic projections. (A and B from Sunset Bay; C near Shore Acres, Oregon.)

N=70

VARIATIONS

SYMM.RIPPLE X-LAM.n CLIMBING ASYMM.RIPPLE X-LAM.> ' AR

HS-»X

AR

40- SR

60-

N = 55 FLAT LAM.

120- CM 0 100 COALEDO FM. 200 CM

Figure 12. Composite diagram showing other stratification types commonly associated with hummocky beds at Coos Bay. Upper part shows variety of occurrences of symmetrical and asymmetrical N = I3 cross lamination. Lower part shows flat, parallel lamination with very rare low-angle truncations. Gray is mudstone; AR = asymmet- N=34 N=7I rical ripples; SR = symmetrical ripples.

boulder conglomerate overlain by trough Hummocky Stratification in Cape Sebas- bined with a high-energy storm regime, fair- cross-bedded pebbly sandstone, plane-lam- tian Sandstone. Hummocky stratification is weather deposition either did not occur, or inated coarse sandstone, and crudely graded spectacularly exposed in wave-washed cliffs all evidence of it was eroded by frequent conglomerate (25 m). The middle and thick- on the south end of Cape Sebastian. The storms, resulting in a 40-m-thick amalga- est part (125 m) is made up of hummocky- laminae show no preferred orientation mated hummocky sequence. stratified sandstone divided into a lower (Bourgeois, 1980). The lower hummocky- Of all of the formations described here, hummocky-stratified facies and an upper stratified facies (Fig. 14) consists entirely of the Cape Sebastian Sandstone contains in hummocky-stratified and burrowed facies amalgamated, hummocky-laminated (0.5-2 the hummocky-stratified and burrowed fa- (50 m). Grain size, frequency of pebble cm), medium- to fine-grained sandstones cies some of the best examples of "classic" lenses, and thickness of hummocky laminae (Fig. 15), with pockets of pebbly conglom- hummocky sequences (Fig. 5, leftmost), all decrease upward through this part of the erate that decrease upward in frequency. which lack the mudstone zone (Figs. 14, 16; formation, whereas frequency of burrowed Pebbles average 1 to 2 cm in diameter and see also Bourgeois, 1980; Hunter and Clif- zones, diversity of burrows, plane-lami- may be as much as 5 cm across. Although ton, 1982). Typically, several sets of hum- nated zones, plant debris, and symmetrical- pebbles are more common at the base of a mocky-laminated (0.2-1 cm) fine-grained ripple preservation increase upward. Grain- lamina set, they may occur scattered along sandstone grade up into very fine-grained, size and ripple dimensions are documented laminae 10 to 20 cm or more above an ero- nearly flat-laminated (1-2 mm) sandstone, in Bourgeois (1980). These trends suggest a sional truncation. Because there are no bur- succeeded by burrowed sandy siltstone, decrease in energy, indicating a progres- rowed intervals in this facies and no flat- or capped by the first-order (basal) erosional sively increasing depositional shelf depth. cross-laminated or mudstone zones, most surface of the next hummocky set (Fig. 16). The uppermost part of the formation con- first- and second-order truncation surfaces Symmetrical ripples and plant-rich zones sists of alternating laminated, very fine cannot be distinguished. Antiformal lami- may cap the finely laminated zone, where- sandstones with very low-angle truncations, nae are very rare, but erosional truncations in laminae are outlined by biotite and fine and progressively thicker burrowed sandy are innumerable. Presumably because of plant debris. Rare convoluted bedding also siltstones. shallow depositional depths, probably com- occurs in this facies. Near the base of the

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/93/8/663/3419266/i0016-7606-93-8-663.pdf by guest on 24 September 2021 HUMMOCKY STRATIFICATION 673

CAPE SEBASTIAN SANDSTONE

PARALLEL-LAMINATED AND BURROWED SANDY SILTSTONE. FACIES

UPPER HUMMOCKY-BEDDED AND BURROWED FACIES

Figure 13. Graded rhythmite lamination closely associated with hummocky stratifica- tion. Note both' thinning- and thickening-upward sequences. (Upper Elkton Siltstone, South Cove, Oregon.)

LOWER- HUMMOCKY-BEDDED facies, amalgamated hummocky zones are also may contain burrows; increased FACIES common, locally with uneroded pockets of amounts of plant debris and biotite make burrowed sediment preserved; higher in the the laminae more distinct here than in lower facies, amalgamated burrowed- zones in- facies. The burrowed sandy siltstone is CONGLOMERATIC crease in frequency and thickness (Fig. 14). medium to dark gray, rich in organic mate- FACIES The uppermost part of the Cape Sebas- rial, and contains some clay. tian Sandstone (Fig. 14) consists of alternat- Depositional Regime. We interpret the ing finely laminated (1-3 mm) medium gray Cape Sebastian Sandstone as an open-shelf sandstone and burrowed, organic-rich deposit on a wave-dominated coastline. Figure 14. Columnar section . of Cape sandy siltstone. The laminated zones aver- Boulder size in the basal conglomerate indi- Sebastian Sandstone on Cape Sebastian, age 20 to 40 cm thick, and they have sharp cates breaker-wave heights of at least 3 m Oregon. Note that hummocky stratification erosional bases and gradational upper con- (Bourgeois, 1980), and application of wave composes most of the upper 180 m of this tacts with the burrowed zones. Subtle, very theory to the symmetrical ripples and peb- transgressive succession. (From Bourgeois, low-angle, second-order truncations are still bles within hummocky sets indicates waves 1980.) common within flat-laminated zones, which at least 3 m high (Hunter and Clifton, 1982).

Figure 15. Amalgamated hummocky stratification with many Figure 16. Hummocky stratification in middle Cape Sebastian pebble lags (P) and a few shells (S) along truncation surfaces. It is Sandstone showing exceptionally clear antiform (center) with two virtually impossible to distinguish first-order from second-order associated second-order truncation surfaces and very sharp basal truncations here. (Lower Cape Sebastian Sandstone, southwest side first-order surface. Unit beneath shows intense burrowing with only of Cape Sebastian, Oregon.) relict lamination surviving. (South end of Cape Sebastian, Oregon.)

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/93/8/663/3419266/i0016-7606-93-8-663.pdf by guest on 24 September 2021 674 DOTT AND BOURGEOIS

NORTH BLACKLOCK POINT,OREGON The average winter deep-water wave height cross-stratification with hummocky stratifi- in the open Pacific today is 3 m, and aver- cation, as well as for a complete lack of age storm waves there are about twice as mudstone and rippled zones. All of the high. Using these modern storm-wave data hummocky units are amalgamated types and observed ripple lengths on the modern with only the hummocky (H) zone repres- HUMMOCKY (H) Oregon shelf (Komar and others, 1972; ented (Fig. 18).

COALY Komar, 1974), a maximum depositional A coarse basal conglomerate overlies an SHELLS shelf depth of 50 m was suggested for the angular unconformity (Fig. 1.7) and con- BURROWS middle Cape Sebastian Sandstone (Bour- tains rounded sandstone boulders as much -— HUMMOCKY (H) geois, 1980). Hunter and Clifton (1982) as 1.5 m in diameter (some with pholad H estimated it at 15 to 40 m. The thickness of clam borings). These would require break- the Cape Sebastian Sandstone, combined HI-ANGLE CROSS SETS ers at least 7 to 8 m high to move them. The with such a depth, indicates a 250-m relative basal conglomerate is succeeded by 15 to 20 H sea-level rise during Cape Sebastian deposi- tn of interbedded pebble to cobble .conglo- HI-ANGLE CROSS SETS tion. Nevertheless, sediment supply and merate with clasts as much as 15 cm long, WOOD FRAGMENTS storm-wave energy were sufficient to pro- sandstone with high-angle cross-bedding, Í duce sediment transport and deposition and numerous molluscan and barnacle shell across the shelf. lag coquinas. Such shells occur sporadically to the top of the column (Fig. 17), but they Miocene of Cape Blanco: are very rare in the upper three-quarters due ooo o° Blacklock Point Area, Oregon mainly to leaching. Trace fossils are rather 80" rare, but some large burrows occur in the — TROUGH SETS Steep sea cliffs 60 km south of Coos Bay lower one-third. Cobble conglomerate oc- SOQMP UPPERMIOCEN CRETACEOUE S and just north of Blacklock Point as well as curs in relatively thin, tabular and lenticular around Cape Blanco expose more than 300 layers; finer pebbles typically occur along Figure 17. Columnar section of lower, m of middle Miocene shallow-marine, near- high-angle laminae intercalated with coarse trasgressive half of Miocene strata near shore sandstone and conglomerate (infor- sandstone. Blacklock Point showing association of mally known as "Sandstone of Floras In the middle 60 to 70 m, hummocky hummocky stratification with conglomer- Lake"; see Addicott, 1980). Like the Cape stratification is the predominant structure. ates, high-angle cross-stratification (note Sebastian Sandstone, the lower half of the It occurs in beds as much as 1 m thick with orientations), and complete lack of mud- Miocene sequence is transgressive and con- swales (synforms) as .much as 2 m wide. stone. White areas are sandstone. (Sea cliffs tains considerable hummocky stratification Antiforms are rare, and therefore the identi- halfway between Blacklock Point and Flo- (Fig. 17). It is most notable for the associa- fication of hummocky stratification here is ras Lake, southwestern Oregon.) tion of much conglomerate and high-angle based more upon low-angle inclinations of most cross-strata in fine sandstones and irregular spacing of synforms. Conglomer-

HI6HWAY 23-3 MILES NORTH OF SPRING GREEN,WIS.

BIOTURBATED

SHALE

0.75 CRACK ? FILLINGS

a UJ H UJ 2

TRUNCATION 0.25- BIOTURBATION

Figure 19. Hummocky stratification in very fine sandstone of the Norwalk Figure 18. Hummocky stratification in fine sandstone Member, Jordan Formation (Cambrian) of Wisconsin. Note variable interstratified with conglomerate and coarse sandstone bioturbation, two thin units with flat lamination (near top), and sand-filled layers. Note hummock or antiform in middle. (North of polygonal cracks in thin shale (upper right). (From 1980 guidebook for Blacklock Point, Oregon, near 60-m level shown in Fig. SEPM Research Conference on Modern Shelf and Ancient Cratonic 17.) Sedimentation.)

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/93/8/663/3419266/i0016-7606-93-8-663.pdf by guest on 24 September 2021 HUMMOCKY STRATIFICATION 675

ate persists through this, interval as thin hummocky units are relatively thin, being scour more shoreward and simple gravity- tabular and lenticular layers, some of which typically only 5 to 20 cm thick. Some thin, deposited silt laminae more seaward. preserve convex tops interpreted to reflect flat, parallel-laminated sandstones without Our scenario for the formation of hum- original surfaces of gravel bars. Most of the obvious hummocky undulations are inter- mocky stratification is as follows. Either conglomerate layers have sharp boundaries stratified. Mudstone is almost totally lack- flooding rivers or scour of the bottom by with sandstones. Conglomerate diminishes ing. A few layers of symmetrical ripples are wave surges and wind-driven currents de- upward (Fig. 17): Thin layers so rich in associated, as are rare polygonal crack fil- liver much sand to the offshore. R. charcoal as to be coaly are prominent and lings. Intense burrowing of much of this Schwartz (1980, oral coramun.) reported as are interpreted as accumulations of water- facies attests to generally slow deposition in much as 4 m of shoreface scour by storms logged wood fragments brought to the sea a relatively tranquil setting, whereas spo- on the Atlantic coast. Pebbles and shells by river floods. radic occurrence of less-burrowed^ low- occasionally concentrated along scoured Both faunal and sedimentological evi- angle stratification is consistent with episod- surfaces associated with hummocky stratifi- dence indicate a very shallow, wave-domi- ic vigorous stirring as by storm waves. The cation attest to very large flow velocities of nated, nearshore marine environment for all polygonal sand-filled cracks are problemat- the order of a few hundred centimetres per of the sequence shown in Figure 17. Judging ical; Possibly they are syneresis cracks second, and thus great competence, during from abundance of conglomerate, sparsity formed under water. If they reflect exposure scouring events. The almost universal fine of burrowing, and complete lack of mud- and desiccation, then depths less than grain size of hummocky stratification must stone, the environment was even more expected for deposits affected only by rare reflect efficient selective sorting of sand by vigorous than that of the Cape Sebastian storms are suggested. At a 1980 conference, suspension transport to the site of deposi- Sandstone. Both the conglomerate units however, R. N. Ginsburg suggested that tion. As intensity of water motion dimin- and coquinas are interpreted as material polygonal cracks may reflect rare exposure ishes, fallout from suspension begins. Seem- swept offshore beyond the breaker zone by of large areas that were normally subtidal ingly actual deposition of the hummocky retreating storm surges and then winnowed due to very strong storm winds tilting the laminae involves major lateral components and stranded. Associated high-angle cross- water surface for several days (in Dott and of oscillatory flow at the bed associated bedding was probably produced by lunate Byers, 1981). with fallout from the water column as was megaripples as reported from the modern implied by Harms and others (1975) and argued by Hunter and Clifton (1982) as well shoreface by Clifton and others (1971). SYNTHESIS Practically all of the finer sands were depos- as by ourselves. Near the bed, falling grains encounter a zone of intense oscillatory sheet ited in hummocky units; thus we infer that Depositional Mechanics the majority of the section represents storm flow, which has molded the bed into undu- deposition. Strata above the interval shown, Hummocky stratification is important lating hummocks and swales. The result is a however, are progradational, and they ter- not only as an indicator of episodic large smooth but not perfectly flat bed with part minate in fluvial and deltaic conglomerates. waves, but also for determining relative of its irregularity inherited from slightly ear- depth or position relative to shore (or a lier scour and part due to the flow itself. Sand grains are deposited in sharply defined Cambrian of Wisconsin shoal). Although it probably forms across a wide depth range, its greatest preservation undulating laminae. Each lamina is thought to represent deposition from a single wave Two Upper Cambrian formations in potential seems to be between fair-weather or wave train. If the water disturbance fluc- western Wisconsin contain examples of wave base and storm wave base, that is, tuates somewhat in intensity, as is the rule, hummocky stratification. Those of the Tun- from a few to several tens of metres. In a then second-order scour surfaces form lo- nel City Formation are poorly developed, spectrum of storm-produced features, hum- cally within sequences of hummocky lami- low-angle, slightly hummocky laminae as- mocky stratification falls between complete sociated, with much more abundant trough, cross-bedding. A very fine sandstone.mem- ber (Norwalk) of the Jordan Formation 600 contains the best and most numerous examples (Fig. 19). These epeiric sea depos- its are neither simple transgressive nor Figure 20. Maximum or- regressive ones, and there is no demonstra- bital velocity thresholds for ble paleoshoreline within the area of out- initiation of grain movement crop. Instead, hummocky units seem to for four different wave peri- have formed, during over-all slow, but epi- ods as well as change from sodic, deposition in less frequently agitated rippled to flat bed for quartz (deeper?) areas than were associated with sand in oscillatory water flow trough cross-bedded sandstones. (after Clifton, 1976, and A fine sandstone facies has nearly flat Dingier, 1974). We have stratification, but intense bioturbation com- added inferred position of monly has obliterated most of it. Some less- hummocky stratification. burrowed' zones show very low-angle lami- nation, scattered' shallow synforms, and 2.00 0.50 0.25 0.125 0:062

rare antiforms (Fig. 19). These Cambrian DIAMETER (MM)

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/93/8/663/3419266/i0016-7606-93-8-663.pdf by guest on 24 September 2021 676 DOTT AND BOURGEOIS

nae, and they are then draped by more laminae. Overall, however, deposition is essentially continuous. As intensity of water disturbance wanes .below a critical thres- hold, ordinary wave and wind-current rip- pling occurs until the unusual water-dis- turbing event ceases, whereupon silt and mud begin to settle. If the disturbance ceases abruptly, rippling may not occur. Slow, fair-weather deposition of finer sedi- ment and burrowing by organisms generally follow. In some cases, the .fine sediment is gradational with the top of the rippled sand (Fig. 4); in others, , it may .rest upon a very sharply scoured surface (Fig.-6B). What eiffect. tidal currents might, have upon hum- mocky stratification is not clear, but we pre- sume that they would modify or destroy it. Bourgeois (1980) and Hunter and Clifton (1982) have used wave theory to estimate water depth and wave heights from wave ripples associated with hummocky stratifi- . cation. By way of empirical illustration, in water as deep as 50 m, normal Pacific shelf summer waves with an average period of 12 sec, average heights , of 2 m, and producing an orbital velocity of ~20 cm/sec are capa- ble.of moving 0.1-mm sand grains; average winter waves with the same period but aver- age heights of 3 m and producing orbital velocities of~25 cm/sec could move 0.3- mm sand grains (Komar and Miller, 1973; see our Fig. 20). Under conditions such, as these, a fine sandy or silty bottom should always.be rippled except during high-energy storm events. The oscillatory (orbital) veloc- ity necessary to produce, a smooth bed by sheet flow on 0.1 mm sand is of the order of 70 cm/sec (Fig. 20). For comparison, the unidirectional upper flat bed condition for 0.1-mm sand requires a minimum shear velocity (U«) at the bed of the. order of 100 cm/sec. All of these velocities are more than mm competent to move fine sand, the Stokesian settling velocity of which is ~ 1 cm/sec. Figure 21. Topographic map (contour interval and scale in millimetres) of-three- Oscillatory flow is, of course, complex,' and dimensional bed forms with hummock and swale topography developed experimentally in bed-form thresholds, cannot be expressed an oscillatory water tunnel on fine sand (0.19 mm). Arrows'indicate water motion. Ratio of completely by one simple parameter such as water motion amplitude to grain size was 1,247; wave period,T = 3.53 sec (Fig. 9, Run 111 velocity, which fluctuates greatly in magni- of Carstens and'others, 1969, p. A24.) tude as well as direction. Komar and Miller (1975) used dimensionless relative stress result from the combined effects of fallout hummocky stratification with (and rare . derived from the Shields parameter as a cri- and lateral sheet flow due to the passage of transitions laterally to) symmetrical and terion for both appearance and disappear- individual waves or wave trains. We pre- even asymmetrical ripple cross-lamination ance of wave ripples, but it does not contain sume, further, that the lateral-flow compo- (for example, Fig. 12). There is also some velocity as a term and so is hard to visualize. nent is roughly analogous to vigorous experimental evidence to support this belief. Dingier (1974) and Clifton (1976) employed unidirectional flow in producing the "upper . Carstens and others (1969) and Komar. and velocity and wave period (Fig! 20). flat bed." Clifton (1976) has emphasized a Miller (1975) have documented with oscil- As already noted, we believe that the close analogy between oscillatory, and uni- latory-flow experiments. using U-shaped sharply defined individual Jaminae charac- directional bed forms, which is supported water tunnels that an evolutionary progres- teristic of hummocky stratification must • empirically by the intimate intercalation of sion exists with increasing flow intensity

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/93/8/663/3419266/i0016-7606-93-8-663.pdf by guest on 24 September 2021 HUMMOCKY STRATIFICATION 677

TABLE 2. SOURCES OF SEDIMENT FOR VARIOUS LOCI OF DEPOSITION OF undulating. An undulatory surface seems HUMMOCKY STRATIFICATION intuitively more plausible for oscillatory flow than a perfectly planar surface. Locus of deposition Chief sediment source Source and Introduction of Sediment Estuary and tidal flats Wave erosion of sand flats and channels Delta margin River flooding and wave erosion Considerable sand and rare gravel or Lower shoreface and inner shelf Wave erosion of barrier front or other beach shells must be introduced to the lower Outer shelf Wave erosion of offshore shoals or shore zone with shoreface and nearshore in order to pre- transport by storm-driven currents serve hummocky beds several tens of cen- timetres in thickness. Observations of mo- dern shorelines show that both river floods and stormy seas produce high concentra- from flat bed with no movement to straight- intense oscillatory sheet flow must be typi- tions of suspended fine sand and silt in shelf crested symmetrical ripples, then to ran- cal of stormy seas at all shelf depths less waters (Creager and Sternberg, 1972; Drake domly disposed, three-dimensional "dunes" than several tens of metres. and others, 1972; Smith and Hopkins, (Fig. 21), and finally to a smooth, nearly flat Because experimentalists report that a 1972). Whether sediment is introduced by bed after all order has given way to chaotic flat bed succeeds large ripples under increas- river floods or wave erosion (Table 2), sand motions. Carstens and others established ingly intense oscillatory flow, as with unidi- can be dispersed by a variety of mecha- empirically that the most critical factor is rectional flow, why is hummocky stratifi- nisms. Seaward return of storm surges, the ratio of the amplitude of vertical water cation not perfectly planar? We suggest two storm rip currents, tsunamis, wind drift cur- motion above the bed forms to mean parti- possible but untested explanations. First, rents, and density currents have all been cle diameter. In their experiments, two- hummocky stratification may reflect a tran- suggested (Swift and others, 1971; Cook dimensional oriented ripples were charac- sitional bed form between oriented linear and Gorsline, 1972; Hayes, 1967; Kulm and teristic if this ratio was less than 775, while ripples and a truly flat bed (Fig. 20). There others, 1975; Coleman, 1978; Reineck and three-dimensional unoriented forms occur- is a superficial resemblance of the hum- Singh, 1972). D. J. Swift (1982, personal red if the ratio was between 775 and 1,700 mocks to three-dimensional antidunes stu- commun.) and Morton (1979) found that (Fig. 21). Bed-form amplitude decreased died in flumes by Kennedy (1961). He found substantial unidirectional wind-driven flow almost linearly with further increase of that such antidunes could form at transi- is superimposed upon oscillatory flow dur- water motion until the ratio exceeded 1,700, tional Froude numbers between ~0.8 and 1.2 ing storms. at which a flat bed developed. If the on fine sand beneath complex "rooster tail" Hummocky stratification, symmetrical amplitude-to-diameter ratio exceeded water-surface waves. Thin laminae accumu- ripples, and coarse shell or gravel lag depos- 1,700, the bed was flat regardless of its lated over the crests of stationary bed its are the most common storm deposits, initial condition. To illustrate, for fine sand forms, but on either side of migrating ones, but others exist as well. Thin sands with with a mean diameter of 0.2 mm, a total which is reminiscent of the geometric varia- graded bedding found within some muddy water-motion amplitude above the bed in tions of hummocky laminae. Is it possible shelf sediments seem to represent an end excess of only 34 cm resulted in a plane bed that there is an oscillatory-flow hummocky member mechanically distinct from, but under oscillatory flow. If this modest exper- bed form analogous to unidirectional-flow related to, hummocky stratification. Hayes imental value for water-motion amplitude three-dimensional antidunes? Alternatively, (1967) argued for important transport of can be extrapolated to nature, where water perhaps the restricted scale of experiments such sands to the offshore shelf by turbidity motion would be much greater beneath gives an erroneous impression of a perfectly currents initiated by storms. Reineck and large waves, then it implies to us that flat bed that in nature is actually gently Singh (1972), however, doubted Hayes' mechanism for deposition of laminated and graded layers within shelf muds. They sug- STORM SURGE gested instead deposition from simple sus- pension; first, parallel-laminated sand is deposited, but as storm wave action sub- sides, mud laminae alternate with sand. "Generally, separation into sand and mud layers is well-developed. The sand laminae become thinner upwards, whereas mud laminae become thicker. Thus, a graded GRADED RHYTHMITE DEPOSITION rhythmite is produced" (Reineck and Singh, (SIMPLE FALLOUT) 1972, p. 127). More recently, Hamblin and Figure 22. Diagram showing inferred storm origin of hummocky stratification and others (1979) (see also Hamblin and graded sand laminae on shelves. Storm surge erodes sand at shore; hummocky stratifica- Walker, 1979; Walker, 1979; Wright and tion is deposited and preserved in stormy seas between fair-weather and storm-wave bases; Walker, 1981) have proposed that retreating graded laminae may be deposited and preserved at greater depths by simple settling from sand-laden storm surges first deposit hum- suspension and/or from turbidity currents. (Modified from Walker, 1979, Fig. 15, and Dott mocky stratification down to storm wave and Bourgeois, 1980, Fig. 3.) base and then turbidites at greater depths

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/93/8/663/3419266/i0016-7606-93-8-663.pdf by guest on 24 September 2021 678 DOTT AND BOURGEOIS

The most common deviations from the ideal sequence are hummocky—flat—cross- laminated (HFX) and hummocky—cross- laminated (HX) units with the mudstone- siltstone (M) zone missing, hummocky—flat —mudstone (HFM) or hummocky—mud- stone (HM) units with the cross-laminated zone missing, and units with unusually thick cross-laminated (X) zones. In addition, there are strata that lack any definite inclined (H) laminae or show only a few scattered, very low-angle truncations. Such units include flat—cross-laminated—mud- stones (FXM), flat—mudstone (FM), and cross-laminated—mudstone (XM) types. Lat- eral gradations and intercalation with hum- mocky units suggest a common origin. Beds commencing with flat parallel lamination may represent storm-wave stirring that was not severe enough to scour the bottom but was still capable of suspending considerable fine sediment; deposition simply draped a Figure 23. Very fine sandstone lenses and silt streaks in shale. Lenses show internal flat rather than hummocky bottom surface. curving laminae and low-angle truncations suggesting micro-hummocky stratification. Alternatively, perhaps these were formed by (Cretaceous Skull Creek Shale, Dakota Group, Spring Canyon Dam, Fort Collins, unidirectional storm-rip or wind-driven Colorado.) currents under unidirectional upper flat-bed flow; such units could be difficult to distin- guish from Tb or Tbc turbidites. Units that farther seaward (Fig. 22). Swagor and oth- The Ideal Sequence and Its Variations begin with ripple cross-lamination may ers (1976) and Wright and Walker (1981) represent erosion of nearby hummocky- postulated storm-generated currents with As with other sedimentary models, such stratified units and tractive transport of velocities of at least 160 cm/sec to form as the very successful Bouma sequence for sand to an adjacent area. The problem pre- hummocky beds and pebble conglomerates turbidites, our idealized hummocky stratifi- sented by this type is analogous to that of tens of kilometres from shore during Late cation sequence (HFXM) provides a con- interpreting entirely cross-laminated strata Cretaceous (Cardium) time in Alberta. ceptual standard to which variations can be interstratified with graded turbidite units— Morton (1979), however, argued that surge referred and interpreted (Figs. 3, 5). Bur- in which case, they could be either very dis- retreat has been overemphasized as a dis- rowing is a common overprint, and it tal turbidites (Tce units) or strictly tractive persing mechanism. extends to varying depths in the sequence. deposits such as contourites.

A POSSIBLE CONTINUUM AND CAUSAL FACTORS Finally, some lenses of fine sandstone in shale showing curving laminae and internal truncations suggestive of hummocky zones separated by second-order boundaries (Fig. 23) seem to be wave-formed analogues of starved current-ripple structures. If they are a kind of micro-hummocky unit, as we believe, then they represent an extreme end member wherein sand supply was barely adequate to form a diminutive hummocky deposit (Fig. 24). Amalgamation between successive beds, which is very common in sequences contain- ing many units with hummocky stratifica- tion, provides evidence either of relatively frequent storm events or events so vigorous MORE SANO LESS SAND SHALLOWER DEEPER that all fair-weather deposits were scoured LARGER WAVES WEAKER EFFECTS before deposition of a new hummocky bed MORE FREQUENT LESS FREQUENT commenced. Convoluted lamination occurs MORE PROXIMAL MORE DISTAL in some amalgamated intervals (for exam- Figure 24. Proposed continuum through different hummocky stratification types to ple, Fig. 10), but it is not so common as in graded laminae and graded rhythmites in shelf deposits. See text for discussion. many Bouma (turbidite) sequences.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/93/8/663/3419266/i0016-7606-93-8-663.pdf by guest on 24 September 2021 HUMMOCKY STRATIFICATION 679

While it is tempting to draw facile infer- cally all examples described previously in REFERENCES CITED ences from the above variations in hum- the literature occur within intervals only 10 mocky-stratified units, such as relative prox- to 15 m thick in progradational sections, Addicott, W. O., 1980, Miocene stratigraphy and fossils, Cape Blanco, Oregon: Oregon Geol- imality, this could be as misleading as with our Oregon examples are in intervals as ogy, v. 42, p. 87-97. Bouma sequences in the past. Greater pro- much as ten times as thick, and they occur Bourgeois, J., 1979, Retrogradational shelf se- portion of the upper fine mudstone zone in transgressive as well as progradational quence—Upper Cretaceous (Campanian- consistently associated with many thin hum- successions. Maestrichtian Cape Sebastian Sandstone, mocky units may indicate a more distal set- One of the most important results of the southwestern Oregon [abs.]: American As- sociation of Petroleum Geologists Bulletin, ting, whereas, conversely, many thick amal- increasing recognition of hummocky cross- v. 63, p. 422-423. gamated hummocky-stratified units may stratification and abundant wave-formed 1980, A transgressive shelf sequence exhibit- indicate relative proximity to source. A lat- ripple cross-lamination is the implication ing hummocky stratification: The Cape Se- eral change of this sort within one thin stra- that many ancient shallow seas were wave bastian Sandstone (Upper Cretaceous), tigraphic interval probably would be cor- dominated—specifically, storm-wave domi- southwestern Oregon: Journal of Sedimen- tary Petrology, v. 50, p. 681-702. rectly interpreted in this manner. However, nated—at least in terms of the preserved Boyles, M. J., and Scott, A. J., 1981, Upper Cre- other possibilities also must be considered. record. This is in contradistinction to the taceous shelf sandstones, northwestern Colo- The local rate of influx of terrigenous sedi- more familiar emphasis upon tide-domi- rado: Migrating sand-ridge model [abs.]: ment to the sea, the relative mix of sand nated nearshore deposits. Mere preserva- American Association of Petroleum Geolo- gists Bulletin, v. 65, p. 905. versus mud, the frequency, duration and tion of hummocky stratification or any 1981, Depositional systems Upper Cretace- magnitude of storms, the tidal range, and other wave-formed feature implies wave ous Mancos Shale and Mesaverde Group timing of the tidal cycle relative to storm dominance, because other, more frequently northwestern Colorado, Part I: Society of waves are some of the other important fac- acting vigorous processes, such as unidirec- Economic Paleontologists and Mineralo- tors that must also influence the character tional tidal or rip currents, would tend to gists, Rocky Mountain Section, Field Trip Guidebook, 82 p. of hummocky cross-stratified deposits. The destroy wave-formed features. Are these Brenchley, P. J., Newall, G., and Stanistreet, I. thick Pacific coast examples described types perhaps more or less exclusive in the G., 1979, A storm surge origin for sandstone above formed in a regime of relatively rapid sense that there are wave-dominated, tide- beds in an epicontinental platform sequence, influx of sediment and large waves. dominated, and also current-dominated fa- Ordovician, Norway: Sedimentary Geology, ciès that must be sorted out? More careful v. 22, p. 185-217. The Coos Bay deltaic setting, although Campbell, C. V., 1966, Truncated wave-ripple proximal, received abundant mud. In con- documentation of hummocky stratification laminae: Journal of Sedimentary Petrology, trast, the thinner Cambrian cratonic exam- in terms of its own variability as well as its v. 36, p. 825-828. ples formed in epeiric seas under conditions variable associations holds great promise of 1967, Lamina, laminaset, bed and bedset: Sedimentology, v. 8, p. 7-26. of slower subsidence, relatively slower rates interpretive value in terms of relative wave magnitude, frequency and duration, relative 1971, Depositional model—Upper Cretace- of sediment influx, lesser average wave ous Gallup beach shoreline, Ship Rock area, magnitude, and practically no mud. Cre- depth, and directions of wave motions. northwestern New Mexico: Journal of Sed- taceous examples were formed under inter- Such insight in turn would greatly enhance imentary Petrology, v. 41, p. 395-409. mediate conditions. our understanding of physical processes of Campbell, C. V., and Oaks, R. Q., Jr., 1973, shelf sedimentation. Estaurine sandstone filling tidal scours, Figure 24 is a conceptualization of the Lower Cretaceous Fall River Formation, major variations of hummocky stratifica- Wyoming: Journal of Sedimentary Petrol- tion plus graded planar laminae arranged in ACKNOWLEDGMENTS ogy, v. 43, p. 765-778. a continuum. Categorizing examples dis- Cant, D. J., 1980, Storm-dominated shallow marine sediments of the Arisaig Group cussed above, Blacklock Point (Miocene) We are deeply indebted to Ralph E. Hun- (Silurian-Devonian) of Nova Scotia: Cana- and lower Cape Sebastian (Cretaceous) ex- ter and H. Edward Clifton for first suggest- dian Journal of Earth Sciences, v. 17, emplify the amalgamated cases; upper Cape ing that the Cape Sebastian Sandstone in p. 120-131. Sebastian and Coos Bay (Eocene), many of Oregon was composed largely of hum- Carstens, M. R., Neilson, F. M., and Altinbilek, the Rocky Mountain (Cretaceous), and the mocky stratification. Subsequent discus- H. D., 1969, Bed forms generated in the laboratory under an oscillatory flow: Analy- Wisconsin (Cambrian) cases exemplify the sions with them as well as with John C. tical and experimental study: U.S. Army normal and flat and cross-laminated Harms, James D. Howard, Donald G. Coastal Engineering Research Center Tech- (FXMb) columns; and the Dakota Group at McCubbin, Dudley Rice, Donald J. Swift, nical Memorandum no. 28, 39 p. (plus 2 Fort Collins and Price River Formation at and Roger G. Walker have been very appendices). Grand Junction, both in Colorado, fit the important to the development of our own Clifton, H. E., 1976, Wave-formed sedimentary structures—A conceptual model, in Davis, fourth case. Overall, it has been our expe- ideas. Howard and McCubbin visited our R. A., Jr., and Ethington, R. L., eds., Beach rience that the normal and amalgamated Oregon localities with us in 1981 and made and nearshore sedimentation: Society of types are most common and roughly equal valuable suggestions. John Harms, C. V. Economic Paleontologists and Mineralogists in abundance, whereas the flat and cross- Campbell, Douglas Cant, Ralph Hunter, R. Special Publication no. 24, p. 126-148. laminated (FXMb) types are intermediate, Goldring, and George deVries Klein pro- Clifton, H. E„ Hunter, R. E„ and Phillips, R. L., 1971, Depositional structures and processes and the micro-hummocky lenses and graded vided valuable criticisms of the manuscript. in the non-barred high energy nearshore: laminae are least frequent. Paul Dombrowski and James Gallagher Journal of Sedimentary Petrology, v. 41, Besides the variability among individual prepared the line drawings. Our research on p. 651-670. hummocky stratification has been support- Coleman, P. J., 1968, Tsunamis as geologic hummocky-stratified bed sets, there is also agents: Journal of the Geological Society of significant variability of occurrence within ed by Grant No. EAR 77-13132 from the Australia, v. 15, p. 267-273. stratigraphic successions. Whereas practi- National Science Foundation. Cook, D. O., and Gorsline, D. S., 1972, Field

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/93/8/663/3419266/i0016-7606-93-8-663.pdf by guest on 24 September 2021 680 DOTT AND BOURGEOIS

observations of sand transport by shoaling cator of storm dominated shallow-marine and tide-dominated shorelines in Cretaceous waves: Marine Geology, v. 13, p. 31-55. environments [abs.]: American Association Moosebar-Lower Gates interval: American Cotter, E., 1975, Late Cretaceous sedimentation of Petroleum Geologists Bulletin, v. 63, Association of Petroleum Geologists Bul- in a low-energy coastal zone: The Ferron p. 460-461. letin, v. 66, p. 138-157. Sandstone of Utah: Journal of Sedimentary Harms, J. C., 1969, Hydraulic significance of Morton, R. A., 1979, Re-evaluation of storm Petrology, v. 45, p. 669-685. some sand ripples: Geological Society of surge processes as agents in the formation of Creager, J. S., and Sternberg, R. W., 1972, Some America Bulletin, v. 80, p. 363-396. graded storm deposits [abs.]: International specific problems in understanding bottom 1979, Primary sedimentary structures: An- Association of Sedimentologists, Interna- sediment distribution and dispersal on the nual Reviews of Earth and Planetary Sci- tional Meeting on Holocene Marine Sedi- continental shelf, in Swift, D.J.P., Duane, ences, v. 7, p. 227-248. mentation in the North Sea Basin, p. 87-88. D. B„ and Pilkey, O. H„ eds., Shelf sedi- Harms, J. C., Southard, J. B., Spearing, D. R., Mount, J. F., 1981, Storm-surge ebb deposits of ment transport: Process and pattern: and Walker, R. G., 1975, Depositional Early Cambrian shelf: Andrews Mountain Stroudsburg, Pennsylvania, Dowden, Hutch- environments as interpreted from primary Member, Campito Formation, eastern Cali- inson and Ross, p. 347-362. sedimentary structures and stratification se- fornia [abs.]: American Association of Pe- DeRaaf, J.F.M., Boersma, J. R., and Van quences: Society of Economic Paleontolo- troleum Geologists Bulletin, v. 65, p. 965. Gelder, A., 1977, Wave-generated structures gists and Mineralogists Short Course Notes Reineck, H. E., and Singh, I. B., 1972, Genesis of and sequences from a shallow marine succes- no. 2, 161 p. laminated sand and graded rhythmites in sion, Lower Carboniferous, County Cork, Hayes, M. O., 1967, Hurricanes as geological storm-sand layers of shelf mud: Sedimentol- Ireland: Sedimentology, v. 24, p. 451-483. agents, south Texas coast: American Associ- ogy, v. 18, p. 123-128. Dingier, J. R., 1974, Wave-formed ripples in ation of Petroleum Geologists Bulletin, Roep, Th. B., Beets, D. J., Dronkert, H., and nearshore sands [Ph.D. thesis]: San Diego, v. 51, p. 937-942. Pagnier, H., 1979, A prograding coastal California, University of California, 136 p. Hobday, D. K., and Morton, R. A., 1981, Lower sequence of wave-built structures of Messi- Doe, T. W., and Dott, R. H., Jr., 1980, Genetic Cretaceous shelf storm deposits [abs.]: Amer- nian age, Sorbas, Almeria, Spain: Sedimen- significance of deformed cross bedding— ican Association of Petroleum Geologists tary Geology, v. 22, p. 135-163. With examples from the Navajo and Weber Bulletin, v. 64, p. 723. Rooth, Guy, 1974, Biostratigraphy and paleo- Sandstones of Utah: Journal of Sedimentary Howard, J. D., 1972, Trace fossils as criteria for ecology of the Coaledo and Bastendorff Petrology, v. 50, p. 793-812. recognizing shorelines in the stratigraphie Formations, southwestern Oregon [Ph.D. Dott, R. H., Jr., 1966, Eocene deltaic sedimenta- record, in Rigby, J. K., and Hamblin, W. K., thesis]: Corvallis, Oregon, Oregon State tion at Coos Bay, Oregon: Journal of Geol- eds., Recognition of ancient sedimentary University. ogy, v. 74, p. 373-420. environments: Society of Economic Paleon- Ryberg, P. T., 1978, Lithofacies and depositional Dott, R. H., Jr., and Bird, K. J., 1979, Sand tologists and Mineralogists Special Publica- environments of the Coaledo Formation, transport through channels across an Eocene tion no. 16, p. 215-255. Coos County, Oregon [M.S. thesis]: Eugene, shelf and slope in southwestern Oregon, Howard, J. D., and Reineck, H., 1981, Deposi- Oregon, University of Oregon, 159 p. U.S.A., in Doyle, L. J., and Pilkey, O. H., tional facies of high-energy beach- to-off- Schafer, W., 1972, Ecology and paleoecology of eds., Geology of continental slopes: Society shore sequence: Comparison with low-en- marine environments: Edinburgh, Scotland, of Economic Paleontologists and Mineralo- ergy sequence: American Association of Oliver and Boyd, 568 p. gists Special Publication no. 27, p. 327-342. Petroleum Geologists Bulletin, v. 65, Smith, J. D„ and Hopkins, T. S., 1972, Sediment Dott, R. H„ Jr., and Bourgeois, J., 1979, Hum- p. 807-830. transport on the continental shelf off Wash- mocky cross stratification—Importance of Hunter, R. E„ and Clifton, H. E., 1982, Cyclic ington and Oregon in light of recent current variable bedding sequences analogous to the deposits and hummocky cross-stratification measurements, in Swift, D.J.P., Duane, D. Bouma sequence [abs.]: Geological Society of probable storm origin in Upper Cretace- B., and Pilkey, O. H., eds., Shelf sediment of America Abstracts with Programs, v. 11, ous rocks of the Cape Sebastian area, transport: Process and pattern: Stroudsburg, p. 414. southwestern Oregon: Journal of Sedimen- Pennsylvania, Dowden, Hutchinson and 1981, Hummocky cross stratification: New tary Petrology, v. 52, p. 127-144. Ross, p. 143-180. York, McGraw-Hill, Yearbook of Science Kennedy, J. F., 1961, Stationary waves and Swagor, N. S., Oliver, T. A., and Johnson, B. A., and Technology, p. 349-351. antidunes in alluvial channels: Pasadena, 1976, Carrot Creek Field, central Alberta, in Dott, R. H., Jr., and Byers, C. W., 1981, SEPM California, W. M. Keck Lab, California Lerand, M. M., ed., The sedimentology research conference on modern shelf and Institute of Technology, Report KH-R-2, of selected clastic oil and gas reservoirs ancient cratonic sedimentation—The ortho- 146 p. in Alberta: Canadian Society of Petro- quartzite-carbonate suite revisited: Journal Komar, P. D., 1974, Oscillatory ripple marks and leum Geologists Core Conference, Calgary, of Sedimentary Petrology, v. 51, p. 330-347. the evaluation of ancient wave conditions p. 78-95. Drake, D. E., Kolpack, R., and Fischer, P. J., and environments: Journal of Sedimentary Swift, D. J., Holliday, B., Avignorne, N., and 1972, Sediment transport on the Santa Petrology, v. 50, p. 681-702. Shideler, G., 1971, Anatomy of a shoreface Barbara-Oxnard shelf, Santa Barbara Chan- Komar, P. D„ and Miller, M. C., 1973, The ridge system: False Cape, Virginia: Marine nel, California, in Swift, D.J.P., Duane, D. threshold of sediment movement under oscil- Geology, v. 12, p. 59-84. B., and Pilkey, O. H., eds., Shelf sediment latory water waves: Journal of Sedimentary Walker, R. G., 1979, Shallow marine sands, in transport: Process and pattern: Stroudsburg, Petrology, v. 43, p. 1101-1110. Walker, R. G., ed., Facies models: Geo- Pennsylvania, Dowden, Hutchinson and 1975, The initiation of oscillatory ripple science Canada Reprint Series 1, p. 75-89. Ross, p. 307-331. marks and the development of plane-bed at Wright, M. E„ and Walker, R. G„ 1981, Car- Goldring, R., 1971, Shallow-water sedimentation high shear stresses under waves: Journal of dium Formation (U. Cretaceous) at Seebe, as illustrated in the Upper Devonian Baggy Sedimentary Petrology, v. 45, p. 697-703. Alberta—Storm-transported sandstones and Beds: Geological Society of London Mem- Kreisa, Ronald D., 1981, Storm-generated struc- conglomerates in shallow marine deposi- oir no. 5, 80 p. ture in sub-tidal marine facies with examples tional environments below fair-weather Goldring, R., and Bridges, P., 1973, Sublittoral from the middle and upper Ordovician of wave base: Canadian Journal of Earth sheet sandstones: Journal of Sedimentary southwestern Virginia: Journal of Sedimen- Sciences, v. 18, p. 795-809. Petrology, v. 43, p. 736-747. tary Petrology, v. 51, p. 823-848. Hamblin, A. P., and Walker, R. G., 1979, storm- Kulm, L. D., Roush, R. C„ Harlett, J. C„ Neu- dominated shallow marine deposits: The deck, R. H., Chambers, D. M., and Runge, Fernie-Kootenay (Jurassic) transition, south- E. J., 1975, Oregon continental shelf sedi- MANUSCRIPT RECEIVED BY THE SOCIETY ern Rocky Mountains: Canadian Journal of mentation: Interrelationships of facies dis- MARCH 11, 1981 Earth Sciences, v. 16, p. 1673-1690. tribution and sedimentary processes: Jour- REVISED MANUSCRIPT RECEIVED Hamblin, A. P., Duke, W. L., and Walker, R. G„ nal of Geology, v. 83, p. 145-175. AUGUST 21, 1981 1979, Hummocky cross stratification—Indi- Leckie, D. A., and Walker, R. G., 1982, Storm- MANUSCRIPT ACCEPTED AUGUST 27, 1981

Printed in U.S.A.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/93/8/663/3419266/i0016-7606-93-8-663.pdf by guest on 24 September 2021