INTERPRETATION OF DEPOSITIONAL ENVIRONMENTS
The White Rim Sandstone Member in Canyonlands National Park was deposited in an eolian system. Unlike many other depositional environments, no single facies model exists for eolian systems, resulting in no preferred vertical sequences nor any consistent lateral changes (Walker, 1979). Ancient eolian deposits may best be recognized by the presence of features that are characteristic of, or at least compatible with, a modern eolian environment (McKee and Bigarella, 1979a). Glennie (1970), Bigarella (1972), McKee and Bigarella (1979a, b), Hunter (1977), Kocurek and Dott (1981), and Ahlbrandt and Fryberger (1982) gave good general discussions of different features of eolian deposits. Some of the more important characteristic features are:
1. Sets of medium- to large-scale tabular planar or wedge planar crossbeds that dip down wind.
2. Bounding surfaces between crossbed sets that are either horizontal or inclined at low angles.
3. High-index ripples, R.I.>15, oriented parallel or sub-parallel to the dip direction of the foresets.
4. Inverse graded laminations within crossbed sets.
5. Sand flow toes.
6. Coarse-grained lags between crossbed sets that are often bimodal.
7. Signs of exposure to subaerial conditions such as raindrop impressions, root structures, or animal tracks and trails.
Detailed examination of the White Rim reveals the presence of many of these features. Dune, interdune, and sabkha deposits are recognized. Each of these deposits has distinctive sedimentary structures and petrologic features.
The most prominent feature of the dune deposits is the conspicuous tabular planar crossbedding. Thicknesses range from less than 0.5 m to 6 m, averaging 1.5 m. Laminations near the base of each set are commonly concave upward, forming long, sweeping tangential bases that become nearly horizontal (fig. 8). Small-scale stratification features observed within the dune sets are tongue-shaped sand flow toes near the base of crossbed sets, related to avalanching of noncohesive sand on the dune slip faces (fig. 9); normally graded strata related to grainfall processes; and inversely graded strata related to ripple migration (fig. 10). All of these features are consistent with eolian depositional processes (Hunter, 1977; Ahlbrandt and Fryberger, 1982).
Crossbedding dip directions are strongly unidirectional (fig. 11), having a vector mean dip direction of S. 47 E., and an average dip angle of 22°. This strongly unidirectional, northwest to southeast trend of the crossbeds suggests the dune form was most likely either barchanoid or transverse ridges (McKee, 1979a). These dune types are commonly oriented at approximately right angles to the dominant effective wind direction and are the result of moderate winds and fairly abundant sand supply (McKee, 1979b).
High-index ripples, R.I.>15, oriented parallel to the dip direction of the foresets, are commonly observed on exposed slip faces (fig. 12). High-index ripples on slip faces are formed either by deflection of wind over the dune surface or by a temporary change in wind direction (Bigarella, 1972). Strata produced by migrating wind ripples are distinctly laminated and inversely graded (Fryberger and Schenk, 1981), and are characterized by a "pin-striped" appearance (fig. 13).
Rare raindrop impressions with raised rims and depressed centers are found preserved on exposed slip face surfaces. Raindrop impressions, although not definitive proof of eolian deposition, do indicate subaerial exposure.
Colors of the White Rim dune deposits are commonly white, very light gray, or yellowish gray. These dune sands are generally well to moderately well sorted, generally fine grained but ranging from fine to coarse grained, and subrounded to rounded. The dominant mineral constituent is monocrystalline quartz, and most of the sands would be classified as quartz arenites (Folk, 1974). Many of the coarser quartz grains are frosted. Other major mineral constituents are microcline, polycrystalline quartz of unknown origin, and chert. Trace amounts of zircon, tourmaline, apatite, glauconite, mica, altered iron-titanium oxides, and gamet are also present. Little or no clay was observed in the samples examined. These sandstones are commonly calcite cemented. Silica cement, in the form of quartz overgrowths, is also present and has altered the original roundness of some quartz grains.
An interdune is defined as an area either enclosed or partially enclosed by dunes. Interdune deposits are sediments that occur in the interdune areas and may record either deflationary or depositional processes. Recent work by Ahlbrandt and Fryberger (1981, 1982) and Kocurek (1981) on modern and ancient eolian systems has recognized the relationship of water content to the types of sedimentary structures formed in interdunes. They have informally subdivided depositional and deflationary interdunes into dry, damp, wet, and evaporite on this basis. Fryberger and others (1983) and Kocurek (1981) listed the sedimentary structures common to each type of interdune setting. Also, like all depositional environments, interdune environments are not static, and any single interdune deposit may reflect a variety of conditions. Deflationary and depositional interdunes that formed under varying degrees of wetness are recognized in the White Rim.
Generally, White Rim interdune deposits are thin and lenticular, a geometry that may be related to the unimodal wind regime in which the White Rim sand was deposited. According to Ahlbrandt and Fryberger (1981), interdune geometries appear to be closely related to wind regime, and interdunes from unimodal wind regimes are thought to be thin and lenticular.
Dry deflationary interdune deposits are common in the White Rim and occur as thin erosional lags between crossbed sets (fig. 14). Their average thickness is less than 0.5 m; they are lenticular and appear to be roughly horizontally stratified. The basic mineralogy of these deposits is similar to the dune deposits, but in general they are darker colored, coarser grained, and more poorly sorted than the surrounding dune sands. Samples taken from these lag deposits commonly have a bimodal grain size distribution and show textural inversion (fig. 15). Folk (1968) described textural inversion as the occurrence of large, well-rounded quartz grains "floating" within finer, more angular quartz grains, and considered the phenomenon to be a product of deflationary processes.
Other White Rim interdune deposits appear to be closely related to water table fluctuations and are most likely the result of a combination of damp or evaporitic depositional processes. These interdune deposits commonly have wavy, discontinuous, horizontal laminations that have been disrupted by either biological activity or diagenetic changes (fig. 16). Primary sedimentary structures in the interdune deposits are commonly discernible only on bedding surfaces. Two distinctive interdune sedimentary structures recognized in the White Rim are adhesion ripples and desiccation polygons. Adhesion ripples form when sand is blown across a damp surface, commonly grow into the wind, and are strongly asymmetrical (Reineck and Singh, 1975). Even minor shifts in wind direction affect these ripples, creating an irregular, bumpy bedding surface (fig. 17). Generally, sets of adhesion ripple strata are less than 0.5 m thick in the White Rim, but can reach thicknesses of over 1 m. According to Ahlbrandt and Fryberger (1981), a rising water table may account for the thicker accumulations of adhesion ripple strata.
Some White Rim interdune surfaces are covered with 5-sided polygons (fig. 18) that have widths of up to 0.7 m and occur in almost totally sand-sized material. These polygons may represent relict salt-ridge structures and indicate occasional evaporitic conditions in the interdune areas. Initially, capillary evaporation of saline groundwater creates a salt crust, and the polygons later form as the result of alternating periods of desiccation and deposition (Glennie, 1970; Ericksen and Stoertz, 1978). The sand that forms these polygons often is stained reddish brown. This staining may be a result of the exposure of salt-encrusted sand to the sun during salt-ridge formation (Fryberger and others, 1983).
In many ancient and modern interdune areas, both plant and animal burrows are common (Fryberger and others, 1983). No distinct burrows of any kind were observed in the White Rim interdunes. Bioturbation is occasionally present (fig. 19), which gives the sediment a characteristic mottled, homogeneous appearance (Friedman and Sanders, 1978).
The basic mineralogy and texture of these interdune deposits are similar to the dune deposits, but there are some differences. Their color is normally darker than the dune sands, ranging from yellowish orange to brownish orange, and they commonly have a banded appearance (fig. 16). Concentrations of heavy mineral grains in the laminations are common. Dolomite is present in the interdune sands and occurs as individual, euhedral to subhedral rhombohedrons that are heavily iron stained. The rhombohedrons commonly form distinct layers, often appear abraded (fig. 20), and are probably of secondary origin (Scholle, 1978).
Few biological constituents were observed in the White Rim. A crinoid ossicle was identified from a coarse-grained zone near the base of the White Rim in the White Crack section (fig. 3). The ossicle is well preserved, showing single-crystal extinction, which is typical of echinoderm grains. Fecal pellets were also observed in a number of interdune samples and are commonly en cased in dolomite rhombohedrons.
In the eastern part of the study area, the entire basal White Rim consists of flat-bedded sandstone that has most of the characteristics of depositional interdune deposits. This may represent an ancient sabkha deposit. Sabkhas, although similar to interdunes, are more aerially extensive than interdunes and are related to a stable and continued rise in the water table (Fryberger and others, 1983). Johnson and others (1978) defined inland sabkhas as evaporitic, commonly salt-encrusted, saline flats underlain by clay, silt, and sand. The development and distribution of inland sabkhas are controlled by topography and fluctuations in the local water table. Also, sabkhas can be either detrital dominant or evaporite dominant. The White Rim sabkha consists largely of clastic material, and no evidence of extensive evaporite minerals is present; thus the sabkha would be classified as detrital dominant (Fryberger and others, 1983). One feature observed in the White Rim that has been described in modern detrital-dominant sabkhas (Fryberger and others, 1983) is bedding with a banded appearance (fig. 16). This light-dark color banding may result from the intercalation of light-colored sand of ripple-produced strata and reddish-brown stained sand of salt-ridge structures.
Several factors contributed to the location and development of this sabkha. The sabkha location coincides with the White Rim's eastern pinchout, which Baars (1979) considered to be related to early movement of the Monument upwarp. Thus, the Monument upwarp created a topographic barrier to the east. The sabkha was also protected to the west by the main White Rim dune complex. In some modern eolian systems, sabkhas commonly occur down wind of the major dune complex (Fryberger and others, 1983). Growth of the sabkha most likely resulted from a slow but steadily rising water table and continued accumulation of sediment.
Figure 21 is a schematic fence diagram compiled from the measured sections, showing the regional strati graphic relationships and distribution of dune, interdune, and sabkha deposits. In the eastern part of the study area, the entire basal White Rim may be a sabkha deposit, reaching a thickness of almost 6 m. Depositional interdunes occur as distinct, thin, lenticular units within the dune deposits (fig. 22). To the northwest, in sections QAB and BB (fig. 3), no depositional interdunes were observed. Sabkha and depositional interdune deposits both require damp or wet conditions during their formation. Distribution of these deposits in the White Rim was probably closely related to the water table variations. The location of the sabkha deposit and the prevalence of the depositional interdunes in the eastern part of the study area suggest the water table was shallower in this area. Dry deflationary interdune deposits, on the other hand, occur throughout the study area, but are more common in the western part. These deposits were most likely independent of the water table fluctuations and were related to dune migration during White Rim deposition.
Last Updated: 09-Nov-2009