Ralph E. Hunter

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.

Bedform Alignment in Directionally Varying Flows

Many kinds of sediment bedforms are presumed to trend either normal or parallel to the direction of sediment transport. For this reason, the trend of bedforms observed by remote sensing or by field observations is commonly used as an indicator of the direction of sediment transport. Such presumptions regarding bedform trend were tested experimentally in bidirectional flows by rotating a sand-covered board in steady winds. Transverse, oblique, and longitudinal bedforms were created by changing only two parameters: the angle between the two winds and the proportions of sand transported in the two directions. Regardless of whether the experimental bedforms were transverse, oblique, or longitudinal (as defined by the bedform trend relative to the resultant transport direction), they all had trends that yielded the maximum gross transport across the bedforms. The fact that many of the experimental bedforms were neither transverse nor parallel to the resultant transport direction suggests that transport directions cannot be accurately determined by presuming such alignment.

Storm-controlled oblique dunes of the Oregon coast

The large (mean height 25 m, spacing 300 m), relatively straight-crested dunes of the central Oregon coast migrate an average of 3.8 m/yr toward an azimuth of 26°. The dunes are transverse to the strong, south-southwesterly winter storm winds that are responsible for their basic form, orientation, and migration. The dry, moderate, north-northwesterly summer winds modify the dune form but not the dune trend. Comparison of the sand transport calculated from wind data and the transport measured from dune migration indicates that the actual transport by the wet southerly winds is only one-third of the amount calculated assuming dry conditions. The resultant (vector-mean) transport rate, as recalculated by comparison of the measured and initially calculated rates, is 34 m 3 /m·yr toward an azimuth of 45°. The dunes are thus oblique by our definition of an oblique dune (angle between dune trend and resultant transport direction between 15° and 75°). The internal structures of the dunes confirm northward migration during wet conditions. Evidence for deposition during wet conditions includes slipface deposits deformed mostly by sliding and various structures formed by the adhesion of sand grains to wet surfaces. Most summer deposits are not preserved, but those on the basal apron (the gentle north slope at the base of the winter slipface) have a high preservation potential. A depositional model based on dune climbing predicts that the preserved record of oblique dunes formed by an obtuse-bimodal wind regime would consist of tabular sets of crossbeds in which the dip angles increase upward from the base of each set.

Cyclic deposits and hummocky cross-stratification of probable storm origin in Upper Cretaceous rocks of the Cape Sebastian area, southwestern Oregon

ABSTRACT Cyclic deposits containing hummocky cross-stratification occur in the upper part of the Cape Sebastian Sandstone of Bourgeois (1980), a shallow marine transgressive sandstone of Late Cetaceous age on the southern Oregon coast. The cycles average 1.6 m in thickness and consist, where complete, of a lower hummocky cross-stratified sandstone, a middle planar and ripple bedded sandstone with a shale bed in its middle part, and an upper bioturbated sandstone. Noteworthy features of the hummocky cross-stratification include the presence of depositional domes in addition to scoured depressions, the absence of significant bedform migration, and the presence of a small proportion of dip angles greater than the angle of repose (>34°) in addition to the large proportion of low (<15° ) dip angles. The lower, stratified, fining-upward part of the cycle (up to the top of the shale bed) is interpreted as having accumulated under conditions of initially great but gradually decreasing current velocity and deposition rate. The currents probably had a strong oscillatory component, and the depositional event is inferred to have been a storm. The part of the planar- and ripple-bedded sandstone above the shale bed was probably deposited during relatively fair weather after the storm but before re-establishment of a normal benthic fauna. The bioturbated sandstone is interpreted to have been deposited during fair weather or during minor storms separated by long intervals of fair weather.

Interpreting Cyclic Crossbedding, with An Example from the Navajo Sandstone

Publisher Summary This chapter examines the interpretation of cyclic crossbedding with an example from the Navajo sandstone. Cyclic crossbedding of the fluctuating-flow type can be produced by periodic fluctuations in flow direction, flow velocity, depth of flow in aqueous environments, or other parameters affecting sediment transport rates and depositional mechanisms. The cycles may or may not be bounded by erosional surfaces. In the absence of any erosional surfaces, the cycles are concordant and are made evident by variations in texture or sedimentary structure. Fluctuating-flow cycles bounded by surfaces of erosion generally imply major changes in flow direction or, in aqueous flows, changes in water depth. Erosion surfaces produced by reversals of the normal-to-bedform component of flow tend to be best developed on the upper lee slope and to become nonerosional hiatal surfaces downward. Superimposed-bedform cycles differ considerably, depending on whether the superimposed bedforms exist on both the stoss and lee slopes of the main bedform or whether they exist only on the stoss slope, moving up it until they reach the crest, whereupon their sand avalanches down the slipface of the main bedform.

Burrows of the ghost crab Ocypode quadrata (Fabricius) on the barrier islands, south-central Texas coast

ABSTRACT Burrows of the grapsoid crab Ocypode quadrata (Fabricius) are important biogenic sedimentary structures on Texas barrier islands. Variation in the shape, diameter, length, orientation, and areal density of the burrows can be used to define subenvironments of the beach and foredune ridge. An increase in diameter, length, and morphological complexity of the burrows together with a decrease in burrow abundance from the upper foreshore to the back edge of the beach differentiate beach subzones. Greater burrow size and abundance and a simpler shape distinguish O. quadrata burrows in the dune mounds from burrows in the interdune flats of the foredune ridge. A preferred burrow orientation descending to the northwest in the backshore contrasts with a random orientation of burrow in the foredune ridge. Although the preferred orientation is away from the shoreline, factors not directly related to shoreline trend may be responsible.

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.

Reconstructing Bedform Assemblages from Compound Crossbedding

Publisher Summary This chapter discusses the reconstruction of bedform assemblages from compound crossbedding. Sedimentologists have observed that small bedforms are commonly superimposed on large bedforms, and they have realized that many complicated cosets of crossbeds in eolian and subaqueous sandstones can be readily explained by the migration of small bedforms on the lee surfaces of other large bedforms. The critical factor that determines whether the crossbedding deposited by a bedform is simple or compound is the absence of or the occurrence of intermittent erosion on the lee slope. It is found that where no erosion occurs, beds deposited on the lee slope are not truncated, bounding surfaces are not generated within the set, and crossbedding is simple. In contrast, where parts of a lee slope occasionally undergo erosion, layers on the lee slope are truncated, bounding surfaces are produced within the set, and the resulting crossbedding is compound. The key to reconstructing bedform geometry from compound crossbedding is visualizing the structures generated when hypothetical lee slopes are translated through space and later exposed in variably oriented outcrop planes.

An experimental study of subaquaeous slipface deposition

ABSTRACT A flume study indicates that grainflow on slipfaces accounts for most cross-strata formed in unidirectional, shallow-water flows. The slipfaces studied were on small megaripples and delta-like steps (0.06-0.28 m high). During intermittent avalanching, at relatively low flow velocities, periods between avalanches were marked by grainfall onto the slipface, the intensity of which was greatest near the brink of the slipface and increased with current velocity. Nearly all grainfall deposits, however, were incorporated into subsequent grainflows. Grain flow cross-strata were made up of relatively distinct layers, at least near the base of the slipface. Continuous avalanching at high flow velocity was marked by a steady stream of grains forming more poorly defined cross-strata. Although the fundamental cause of grain flow is the gradual buildup of sediment on the upper slipface to the angle of initial yield, four other processes were recognized as promoting avalanching: 1) migration of superimposed bedforms to the brink, 2) generation of turbulent pulses upstream of the brink, 3) lee-eddy impingement on the lower slipface, and 4) extension of the lee eddy above the brink. The lee eddy proved very significant in slipface processes by redistributing grainfall sediments and both promoting and impeding grainflow. Regression analyses showed that the slipface advance per avalanche, Sa, is strongly correlated with the slipface height, H, expressed approximately by Sa = 0.060H. In addition, Sa is a direct function of the rate of slipface advance, Vb. The relationship among Sa, H, and Vb can be expressed as Sa/H = 0.0385[1 - 0.134 (min/cm) Vb]-1. Cross-strata dip angles between 28° and 34° show no systematic relation to H and Vb, but dip angles greater than 34° occurred only when both H and Vb were small, and dip angles less than 28° occurred only when both H and Vb were large.

Subaqueous sand-flow cross strata

ABSTRACT Subaqueous sand-flow cross strata are formed by the flowage of sand down the slipfaces of submerged bedforms and isolated steps. Such cross strata make up the great bulk of the foreset deposits of shallow-stream megaripples and bars. Characteristics of sand-flow cross strata that form in fine to medium sand on subaqueous slipfaces less than 2 m high include steep dip angles (generally 27-34°), little if any curvature in dip cross sections, distinct segregation of grain types, downward coarsening along the cross stratum and generally upward coarsening (inverse grading) across the cross stratum, loose gram packing, absence of interbedded grainfall or other deposits, small range of cross-stratum thickness within a set, increasing cross-stratum thickness with increasing slipface heigh , and great width (lateral extent) relative to cross-stratum thickness and slipface height. In coarse, subaqueous sand, the sand-flow cross strata tend to be narrower (more lenticular in horizontal sections). Flume experiments suggest that each cross stratum is produced by one in a series of intermittent sand flows and that the avalanching is generally not closely controlled by bedforms that spill over the brink of the slipface. Subaqueous cross strata that formed in fine to medium sand on relatively low slipfaces typically differ from eolian cross strata that formed under similar conditions. Grainfall deposits are less common on subaqueous slipfaces, probably because avalanching tends to be more nearly continuous there than on eolian slipfaces. Another difference is that subaqueous sand-flow cross strata tend to be wider (more extensive laterally) than eolian ones, in part because avalanching tends to be more nearly continuous on subaqueous slipfaces and in part because submerged sand is even less cohesive than dry eolian sand.