H. Edward Clifton

Depositional Structures and Processes in the Non-barred High-energy Nearshore

ABSTRACT The marginal marine environment consists of an offshore where the wave form is approximately sinusoidal and a nearshore where the wave form is either solitary or that of a bore. Within the nearshore, shoaling waves become progressively higher and steeper until they break. After breaking, the waves progress as bores through a surf zone; these bores ultimately terminate within a swash zone on the beach itself. In a high-energy coastal environment where long-period swell enters a nearshore uncomplicated by offshore bars, sedimentary structures develop on the seafloor in facies that trend parallel to the zones of different wave activity. In the offshore, small sand ripples are the most common depositional structure, but in the nearshore larger bed forms predominate. Seaward from the line of breakers, in the zone of wave build-up, are landward-oriented lunate megaripples. Near the outer portion of the surf zone the bed form is planar (outer planar facies), but, in the inner portion of the zone, a area of large-scale bed roughness (inner rough facies) commonly is present. Within the swash zone, the bed form is again planar (inner planar facies). The boundaries of the facies shift in response to changes in waves or tide, and certain of the zones are sometimes missing. The relative position of the zones, however, is invariable. The major features of the bed forms can be interpreted in terms of flow regime. The stronger of the two opposing transient currents caused by passing waves produces structures analogous to those produced by continuous, unidirectional currents. Landward wave surge is dominant in the outer three structural facies, whereas seaward surge predominates in the innermost (inner planar) facies. Wave surge over the intermediate inner rough facies is more complex, and the direction of strongest surge may be variable. In the outer three facies, the landward sequence from small asymmetric ripples to lunate megaripples to plane bed suggests an increase in flow regime from the lower part of the lower regime to the upper regime; this shoreward increase in flow regime is associated with a shoreward increase in orbital velocity at the bottom. The inner planar facies is produced by flow in the upper regime. The inner rough facies, situated between two zones of flow in the upper regime, is apparently a product of flow in the upper part of the lower regime. Within each of the structural facies a distinctive set of internal structures is produced. Internal structure of the asymmetric ripple facies consists of shoreward-inclined ripple cross-lamination and gently inclined cross-stratification. The lunate megaripples produce medium-scale landward-dipping foresets. Within the outer planar facies bedding is nearly horizontal. Structures in the inner rough facies produce medium-scale foresets that mostly dip directly or obliquely seaward, although landward-dipping foresets also occur. Within the inner planar facies bedding is gently inclined seaward. Migration of the facies in response to changes in waves or tide produces distinctive assemblages of structures where the facies overlap. These assemblages provide criteria for paleoenvironmental i terpretation, particularly where interrelated assemblages occur in a meaningful spatial distribution.

Beach lamination : Nature and origin

Abstract A distinctive two-fold sedimentation unit characterizes lamination in the upper swash zone of beaches. Within the unit a fine and/or a heavy mineral rich layer at the base grades upward into a coarser and/or a heavy mineral poor layer at the top. This distinctive type of lamination results from grain segregation within bed flow during wave backwash.

Wave-formed structures and paleoenvironmental reconstruction

Clifton, H.E. and Dingier, J.R., 1984. Wave-formed structures and paleoenvironmental reconstruction. In: B. Greenwood and R.A. Davis, Jr. (Editors), Hydrodynamics and Sedimentation in Wave-Dominated Coastal Environments. Mar. Geol., 60: 165--198. Wave-formed sedimentary structures can be powerful interpretive tools because they reflect not only the velocity and direction of the oscillatory currents, but also the length of the horizontal component of orbital motion and the presence of velocity asymmetry within the flow. Several of these aspects can be related through standard wave theories to combinations of wave dimensions and water depth that have definable natural limits. For a particular grain size, threshold of particle movement and that of conversion from a rippled to fiat bed indicate flow-velocity limits. The ratio of ripple spacing to grain size provides an estimate of the length of the near-bottom orbital motion. The degree of velocity asymmetry is related to the asymmetry of the bedforms, though it presently cannot be estimated with confidence. A plot of water depth versus wave height (h--H diagram) provides a convenient approach for showing the combination of wave parameters and water depths capable of generating any particular structure in sand of a given grain size. Natural limits on wave height and inferences or assumptions regarding either water depth or wave period based on geologic evidence allow refinement of the paleoenvironmental reconstruction. The assumptions and the degree of approximation involved in the different techniques impose significant constraints. Inferences based on wave-formed structures are most reliable when they are drawn in the context of other evidence such as the association of sedimentary features or progradational sequences.

Progradational Sequences in Miocene Shoreline Deposits, Southeastern Caliente Range, California

ABSTRACT An exceptionally well exposed marine-nonmarine transition in middle Miocene strata exists in the southeastern Caliente Range, California. About 50 individual progradational sequences form a succession that ranges in thickness from approximately 1000 m (where predominantly nonmarine) to more than 2500 m (where predominantly marine). Paleogeographic evidence in basalt flows near the top of the succession and in overlying fluvial deposits indicates that these middle Miocene strata were deposited across a north-northwest trending shoreline. A complete progradational sequence typically is several meters to a few tens of meters thick and includes strata that represent three intertonguing stratigraphic units. Individual sequences generally rest on a thin gravel deposit interpreted as a transgressive lag on an erosional surface. The gravel is overlain by structureless siltstone or fine-grained sandstone deposited at water depths where the rate of faunal mixing exceeded that of production of structures by physical processes. These rocks grade upward into bedded fine sandstone deposited closer to shore where physical processes exceeded bioturbation. Crossbedded lenses of coarse sand or fine gravel in the upper part of this facies suggest the presence of fairly long-period surface waves. The bedded fine sandstone is sharply ove lain by a cross bedded coarse sandstone facies that is interpreted as a combined offshore bar-rip channel-surf zone assemblage. Cross-strata dip dominantly offshore, suggesting substantial deposition from rip currents. A secondary, shoreparallel mode of cross-strata direction suggests longshore currents produced by surface waves from the northwest. The crossbedded coarse-grained sandstone grades upward into planar-bedded medium-grained sandstone that is interpreted as a beach foreshore. This facies grades upward through structureless medium-grained sandstone into nonmarine or lagoonal red and green mudstone of the Caliente Formation. The middle Miocene succession was deposited in a subsiding basin that was otherwise remarkably stable tectonically; the position of the strand line differed no more than a few kilometers through a period of 1 to 3 m.y. The average duration of the transgressive-regressive cycles, a few tens of thousands of years, together with their distribution in groups of three or four in the lower two-thirds of the succession, is consistent with the pattern of long-term climatic cycles produced by periodicity of the earths solar orbit and may be related to eustatic sea level changes attendant to the development of the Antarctic ice cap. Changes in the pattern of progradation in the upper part of the succession and nearby basaltic eruptions may have been precursors to the onset of movement along th San Andreas fault in this area 12-14 m.y. ago.

Discrimination between subtidal and intertidal facies in Pleistocene deposits, Willapa Bay, Washington

ABSTRACT Sea cliffs on the north, west, and south sides of Willapa Bay, Washington, present excellent exposures of Pleistocene estuary deposits. Analysis of these deposits and comparison of them with modern facies in Willapa Bay indicate that a primary key to their interpretation is the delineation of stillstand units--that is, shallowing-upwards bodies of sediment deposited at a particular stand of the sea. Once identified, these units provide a basis for studying both vertical sequences and lateral trends as well as identifying the effects of tectonic deformation. To recognize a stillstand unit, one must infer the relative water depth represented by successive facies or deposits. This inference in turn requires a distinction between intertidal and subtidal facies. In the analysis of the Willapa Bay deposits, a set of criteria for this distinction, derived largely from characteristics of modern facies in the bay, proved useful. Criteria for identifying deposits as subtidal include the presence of 1) abundant Ostrea lurida in growth position; 2) units of inclined strata more than 2 m thick; 3) laterally persistent lag deposits; 4) laterally persistent thin layers of mud; 5) medium- to large-scale crossbedding; 6) directionally uniform crossbedding and ripple lamination; and 7) assemblages of predominantly concave-up shells and shell fragments. Of these criteria the first is diagnostic of subtidal facies; the others are characteristic but not necessarily diagnostic of this environment. In association, however, the characteristic features provide generally adequate evidence for the indicated facies. Criteria for recognizing intertidal deposits include the presence of 1) root or rhyzome structures; 2) evidence of runoff channels; 3) regular lamination characteristic of supratidal deposits; 4) supratidal bluff breccia; and 5) vertical sequences in which upper accretionary bank deposits underlie those of the tide flat, which in turn underlie supratidal deposits. Some of these (root or rhyzome structures) are probably diagnostic of an intertidal facies; the others are characteristic of such a facies.

Amino-acid racemizarion in Quaternary shell deposits at Willapa Bay, Washington

Extents of racemization (dl ratios) of amino acids in fossil Saxidomus giganteus (Deshayes) and Ostrea lurida Carpenter were measured on shell deposits exposed at 21 sites on the east side of Willapa Bay, Washington. Amino acids from Saxidomus show less variability in dSpl ratios and, therefore, are of greater use in correlation and age estimation than are amino acids from Ostrea. Shells of two different ages, about 120,000 ± 40,000 yr old and about 190,000 ± 40,000 yr old, are present. These ages correspond to Stages 5 and 7 of the marine isotope record defined by Shackleton and Opdyke in 1973 and hence the shell deposits likely formed during two different high stands of sea level. The stratigraphic record at Willapa Bay is consistent with this interpretation.

Analysis of Eustatic, Tectonic, and Sedimentologic Influences on Transgressive and Regressive Cycles in the Upper Cenozoic Merced Formation, San Francisco, California

The Merced Formation consists of approximately 2,000 m of shallow marine and coastal nonmarine sediment of late Cenozoic age that accumulated in a structural trough south of the city of San Francisco. About 1,750 m of these deposits crop out in a well exposed tilted sequence in sea cliffs on the north side of the San Andreas fault. The part of the section north of the fault appears to be of Pleistocene age.

Seismic stratigraphy and late Quaternary shelf history, south-central Monterey Bay, California

Abstract The south-central Monterey Bay shelf is a high-energy, wave-dominated, tectonically active coastal region on the central California continental margin. A prominent feature of this shelf is a sediment lobe off the mouth of the Salinas River that has surface expression. High-resolution seismic-reflection profiles reveal that an angular unconformity (Quaternary?) underlies the entire shelf and separates undeformed strata above it from deformed strata below it. The Salinas River lobe is a convex bulge on the shelf covering an area of approximately 72 km 2 in water depths from 10 to 90 m. It reaches a maximum thickness of 35 m about 2.5 km seaward of the river mouth and thins in all directions away from this point. Adjacent shelf areas are characterized by only a thin (2 to 5 m thick) and uniform veneer of sediment. Acoustic stratigraphy of the lobe is complex and is characterized by at least three unconformity-bounded depositional sequences. Acoustically, these sequences are relatively well bedded. Acoustic foresets occur within the intermediate sequence and dip seaward at 0.7° to 2.0°. Comparison with sedimentary sequences in uplifted onshore Pleistocene marine-terrace deposits of the Monterey Bay area, which were presumably formed in a similar setting under similar processes, suggests that a general interpretation can be formulated for seismic stratigraphic patterns. Depositional sequences are interpreted to represent shallowing-upwards progradational sequences of marine to nonmarine coastal deposits formed during interglacial highstands and/or during early stages of falling sea level. Acoustic foresets within the intermediate sequence are evidence of seaward progradation. Acoustic unconformities that separate depositional sequences are interpreted as having formed largely by shoreface planation and may be the only record of the intervening transgressions. The internal stratigraphy of the Salinas River lobe thus suggests that at least several late Quaternary regressions and transgressions may be recorded under the present shelf. This record may represent the last major eustatic cycle of sea level, an interval not observed in uplifted onshore Pleistocene marine terraces.

Orientation of Empty Pelecypod Shells and Shell Fragments in Quiet Water

ABSTRACT Participation in man-in-the-sea experiments Tektite I and II permitted an examination of the orientation of pelecypod shells and shell fragments on sediment unaffected by waves or currents. Whether on vegetated or unvegetated unrippled sand or on solid coral, the particles lie predominantly with concave sides up. This orientation occurs both for open articulated empty valves and for isolated disarticulated valves. The degrees of orientation differs with shell size; the smaller shells are less preferentially oriented concave-up than are the larger shells. The concave-up orientation results from the activity of predators and scavengers, from bioturbation, or from a combination of the two mechanisms. An experiment that exposed nearly 1200 shells to 40 days of bioturbation demonstrated that disturbance by organisms is an effective agent for producing a dominantly concave-up orientation. Shell size and shell shape are significant factors in bioturbational rotation. In general, the tendency to be rotated to a concave-up position increases with increasing shell size to the point where shells are too large to be readily overturned by most organisms. The percent of concave-up shells also depends on the angle of balance of the shell; shells rotated by bioturbation tend to behave like loaded dice. Shells that became buried during the experiment howed a more random orientation than did those exposed on the surface after 40 days. The orientation of shells in quiet water differs markedly from those orientations produced under most conditions of waves or currents. Although concave-up assemblages can be produced by deposition from turbidity currents or from the transport of shells across small ripples, analysis of orientation as it relates to shell size and shape may provide a means of identifying specific depositional mechanisms or conditions.

Heavy-mineral distribution in modern and ancient bay deposits, Willapa Bay, Washington, U.S.A.

Abstract Analysis of heavy-mineral distribution in modern sediments of Willapa Bay, Washington, indicates a dominance of two mineralogic assemblages, one with approximately equivalent amounts of hornblende, orthopyroxene and clinopyroxene, the other dominated by clinopyroxene. The hornblende-orthopyroxene-clinopyroxene suite is derived from the Columbia River, which discharges into the ocean a short distance south of the bay. The clinopyroxene suite is restricted in modern sediments to sands in rivers flowing into the bay from the east. The heavy-mineral distributions suggest that sand discharged from the Columbia River, borne north by longshore transport, and carried into the bay by tidal currents accounts for most of the sand within the interior of Willapa Bay. Three heavy-mineral assemblages are present in the surrounding Pleistocene deposits; two of these are identical to the modern assemblages described above. These heavy-mineral assemblages reflect the relative influence of tidal and fluvial processes on the Late Pleistocene deposits; their relative influences are consistent with those inferred on the basis of sedimentary structures and stratigraphic relations in about two-thirds of the samples examined. The anomalies can be explained by recycling of sand from older deposits. The persistence of the two heavy-mineral assemblages suggests that the pattern of estuarine sedimentation in Late Pleistocene deposits closely resembled that of the modern bay. The third heavy-mineral suite, dominated by epidote, occurs in a few older Pleistocene units. On the north side of the bay, the association of this suite with southwest-directed foresets in crossbedded gravel indicates derivation from the northeast, perhaps from an area of glacial outwash. The presence of this suite in ancient estuarine sands exposed on the east side of the bay suggests that input from this northerly source may have intermittently dominated bay deposition in the past.