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Geology topics

R. E. Hunter

Publications and source records attributed to R. E. Hunter.

At least 19 recordsLinked to original sources
USGS2008

Application of sedimentary-structure interpretation to geoarchaeological investigations in the Colorado River Corridor, Grand Canyon, Arizona, USA

We present a detailed geoarchaeological study of landscape processes that affected prehistoric formation and modern preservation of archaeological sites in three areas of the Colorado River corridor in Grand Canyon, Arizona, USA. The methods used in this case study can be applied to any locality containing unaltered, non-pedogenic sediments and, thus, are particularly relevant to geoarchaeology in arid regions. Resolving the interaction of fluvial, aeolian, and local runoff processes in an arid-land river corridor is important because the archaeological record in arid lands tends to be concentrated along river corridors. This study uses sedimentary structures and particle-size distributions to interpret landscape processes; these methods are commonplace in sedimentology but prove also to be valuable, though less utilized, in geoarchaeology and geomorphology. In this bedrock canyon, the proportion of fluvial sediment generally decreases with distance away from the river as aeolian, slope-wash, colluvial, and debris-flow sediments become more dominant. We describe a new facies consisting of 'flood couplets' that include a lower, fine-grained fluvial component and an upper, coarser, unit that reflects subaerial reworking at the land surface between flood events. Grain-size distributions of strata that lack original sedimentary structures are useful within this river corridor to distinguish aeolian deposits from finer-grained fluvial deposits that pre-date the influence of the upstream Glen Canyon Dam on the Colorado River. Identification of past geomorphic settings is critical for understanding the history and preservation of archaeologically significant areas, and for determining the sensitivity of archaeological sites to dam operations. Most archaeological sites in the areas studied were formed on fluvial deposits, with aeolian deposition acting as an important preservation agent during the past millennium. Therefore, the absence of sediment-rich floods in this regulated river, which formerly deposited large fluvial sandbars from which aeolian sediment was derived, has substantially altered processes by which the prehistoric, inhabited landscape formed, and has also reduced the preservation potential of many significant cultural sites.

Geomorphology↗
USGS2006

Bedform climbing in theory and nature

Where bedforms migrate during deposition, they move upward (climb) with respect to the generalized sediment surface. Sediment deposited on each lee slope and not eroded during the passage of a following trough is left behind as a cross-stratified bed. Because sediment is thus transferred from bedforms to underlying strata, bedforms must decrease in cross-sectional area or in number, or both, unless sediment lost from bedforms during deposition is replaced with sediment transported from outside the depositional area. Where sediment is transported solely by downcurrent migration of two-dimensional bedforms, the mean thickness of cross-stratified beds is equal to the decrease in bedform cross-sectional area divided by the migration distance over which that size decrease occurs; where bedforms migrate more than one spacing while depositing cross-strata, bed thickness is only a fraction of bedform height. Equations that describe this depositional process explain the downcurrent decrease in size of tidal sand waves in St Andrew Bay, Florida, and the downwind decrease in size of transverse aeolian dunes on the Oregon coast. Using the same concepts, dunes that deposited the Navajo, De Chelly, and Entrada Sandstones are calculated to have had mean heights between several tens and several hundreds of metres.

Sedimentology↗
USGS1988

Coarse-sediment bands on the inner shelf of southern Monterey Bay, California

Bands of coarse sand that trend parallel to the shore, unlike the approximately shore-normal bands found in many inner shelf areas, occur in southern Monterey Bay at water depths of 10–20 m, less than 1 km from the shore. The bands are 20–100 m wide and alternate with bands of fine sand that are of similar width. The coarse-sand bands are as much as 1 m lower than the adjacent fine-sand bands, which have margins inclined at angles of about 20°. The mean grain sizes of the coarse and fine sand are in the range of 0.354–1.0 mm and 0.125–0.354 mm, respectively. Wave ripples that average about 1 m in spacing always occur in the coarse-sand bands.

Marine Geology↗
USGS1988

Daily cycles in coastal dunes

Daily cycles of summer sea breezes produce distinctive cyclic foreset deposits in dune sands of the Texas and Oregon coasts. In both areas the winds are strong enough to transport sand only during part of the day, reach a peak during the afternoon, and vary little in direction during the period of sand transport. Cyclicity in the foreset deposits is made evident by variations in the type of sedimentary structure, the texture, and the heavy-mineral content of the sand. Some of the cyclic deposits are made up entirely of one basic type of structure, in which the character of the structure varies cyclically; for example, the angle of climb in a climbing-wind-ripple structure may vary cyclically. Other cyclic deposits are characterized by alternations of two or more structural types. Variations in the concentration of fine-grained heavy minerals, which account for the most striking cyclicity, arise mainly because of segregation on wind-rippled depositional surfaces: where the ripples climb at low angles, the coarsegrained light minerals, which accumulate preferentially on ripple crests, tend to be excluded from the local deposit. Daily cyclic deposits are thickest and best developed on small dunes and are least recognizable near the bases of large dunes.

California, Oregon, Texas↗
USGS1987

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.

Science↗
USGS1986

An experimental study of subaqueous slipface deposition

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, S a , is strongly correlated with the slipface height, H, expressed approximately by S a = 0.060H. In addition, S a is a direct function of the rate of slipface advance, V b . The relationship among S a , H, and V b can be expressed as S a /H = 0.0385[1 - 0.134 (min/cm) V b ] (super -1) . Cross-strata dip angles between 28 degrees and 34 degrees show no systematic relation to H and V b , but dip angles greater than 34 degrees occurred only when both H and V b were small, and dip angles less than 28 degrees occurred only when both H and V b were large.

Journal of Sedimentary Petrology↗
USGS1985

Alaska peregrine falcon productivity in 1984 and the role of organochlorine residues

Twenty Peregrine Falcon (Falco peregrinus) eggs (one per eyrie) were collected at random in Alaska in 1984 for organochlorine residue analysis. Prior to this study we had obtained only addled eggs which were possibly not representative of the egg-laying population if addled eggs contained different levels of organochlorines than viable eggs that hatched. Ten eggs were collected from the American (anatum) population (8 upper Yukon River, 2 Tanana River) and 10 eggs from the Arctic (tundrius) popUlation were later collected in the study area. Productivity at specific eyries and eggshell thickness were evaluated in relation to residue burdens in sample eggs collected. Residue levels and residue profiles for the two subspecies were compared. The tentative 'critical' residue level in Peregrine eggs of 15-20 ppm DDE (wet weight) was compared to results obtained from this study. We also present some data on residue levels in prey species from the study area.

Book chapter↗
USGS1982

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

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 degrees ) in addition to the large proportion of low (<15 degrees ) 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.

Journal of Sedimentary Petrology↗
USGS1981

Depositional and erosional features of the inner shelf, northeastern Bering Sea.

Sonographs and bathymetric profiles from water depths less than 15m in the Nome-Solomon, Port Clarence, and Yukon Delta areas of the Alaskan Bering Sea coast show features generated by waves, currents, and drifting ice. The surficial sediments in the Nome-Solomon and Port Clarence areas range in grain size from sand to boulder gravel and have many surface features visible on sonographs, whereas the sediments off the Yukon Delta are fine sands and silts that have few such features. Materials in the Nome-Solomon and Port Clarence areas have been seggregated by grain size into ribbons and irregular, elongate, and lobate patches. Coarse sand and fine gravel patches and ribbons are characterized by symmetrical ripples generated by storm waves. Ice-gouged furrows occur in all the nearshore areas studied.-from Authors

Geologie en Mijnbouw↗
USGS1980

Nearshore current pattern off south Texas: an interpretation from aerial photographs.

Current patterns in a 4-km-wide zone along the south Texas coast were interpreted from patterns of water turbidity visible in aerial photographs taken during a winter day of moderate northerly winds. Features of the turbidity pattern remained recognizable on photographs taken 25 min apart. Currents measured from the movements of these features were southward and nearly parallel to shore, increasing from about 17 cm/sec in an offshore zone to about 40 cm/sec at the line of breaking waves. - from Authors

Remote Sensing of Environment↗
USGS1980

Depositional environments of some Pleistocene coastal terrace deposits, southwestern Oregon - case history of progradational beach and dune sequence.

These deposits comprise a basal gravelly unit and 3 overlying sandy units, each with mud beds, a paleosol, or the modern soil in its uppermost part. The gravelly unit is interpreted as a progradational deposit. The main parts of the sandy units are made up of 1) a crossbedded sand facies, the dominant structure in which is medium-scale crossbedding (interpreted as the product of small eolian dunes), and 2) an irregularly bedded sand facies, which is locally pebbly and is dominated by scour-and-fill structures, interpreted as deposits of interdune ephemeral streams, ephemeral ponds, and wet to dry subaerial flats. The mud beds and paleosols represent times of temporary stabilization of the dune field.- from Author

Sedimentary Geology↗