Clay dunes (lunettes) occur downwind of deflated pans in hypersaline
environments around the world. In San Luis Obispo County, California, active
clay dunes exist in the area surrounding Soda Lake, the sink for runoff
from the 1,230 km2 Carrizo Plain drainage, the only closed basin in the
Southern Coast Ranges. Internal drainage of the Carrizo Plain began during
Plio-Pleistocene time when tectonic deformation associated with the San
Andreas Fault defeated a stream that once drained the valley. An originally
fresh to brackish water lake probably persisted through much of the Pleistocene
during which coastal California was wetter and cooler than now. Diminished
Holocene precipitation and a higher evaporation rate led to shrinkage of
the ancestral lake and associated increased salinity which set the stage
for clay dune formation.
The Soda Lake complex consists of two large basins and at least 130 smaller pans. Water levels in the basins rise and fall seasonally. Following exceptionally wet winters (typically El Nino years) the large North and South Basins never dry completely, although the water retreats toward the center of the basin leaving a salt crust up to 20 cm thick. Most of the large and small pans are fringed by clay dunes. The largest dune bounds the eastern and southern edges of the North Basin which has a surface area of ~10.5 km2. This dune is up to 470 m wide, 16.7 m high, and nearly 9.5 km in length. The southern portion of the dune is active, receiving sediment from the mud flat exposed between the dune and the salt pan. Most of the eastern (north-south) leg of the dune, which is lower and narrower, is currently inactive. Westward retreat of the shoreline exposed the former lake flat to colonization and stabilization by salt-resistant plants. For this eastern leg of the dune to have formed, the lake level must have been about 3 m higher than at present; tectonic warping of the basin may account for the abandonment of the former shoreline.
The Carrizo Plain is located 90 km west of Bakersfield and 85 km north of Santa Barbara. The Plain is about 15 km wide (NE-SW) and 75 km long (NW-SE). Elevations range from 1,908 ft (580 m) in the alkali wetland of the Soda Lake basin to 5,105 ft (1,556 m) at Caliente Mountain, giving it a maximum relief of 3,200 ft (975 m). With elevations up to 4,332 ft (1,320 m), the ~100 km long Temblor Range separates the Carrizo Plain from the southern San Joaquin Valley on the northeast. On its southwest side, the Carrizo Plain is separated from the Cuyama Valley by the ~80 km long Caliente Range. The Carrizo Plain is a perched basin, about 1,000 ft higher than the Cuyama Valley to the southwest, and about 1,500 ft above the Southern San Joaquin Valley. Dibblee (1962) suggested that the region has been lifted about 1,000 ft since the late Pliocene.
The Carrizo Plainís climate is controlled by the East Pacific High
and is, therefore, strongly seasonal. Normally the High shifts southward
during the winter months drawing the jet stream with it and allowing Pacific
storms to cross southern California. If other conditions are in place,
this is an ideal climate for the formation of clay dunes. The figure
below summarizes 1995 climatic data for a RAWS installation located about
20 km south of Soda Lake.
Precipitation: The long term average annual rainfall in the Carrizo Plain is ~ 23 cm. During El Niño winters, rainfall may be three times the average. Virtually all rainfall occurs between October and March. Once the rains begin, aeolian activity essentially ceases. During this period sediment added to the dune is stabilized. Summer rainfall is quite rare and when it does occur is little more than a trace.
Temperature: From May through September the Carrizo is typically hot and dry. Soda Lake shrinks in size and after winters with below average rainfall, may evaporate completely.
Wind Speed: Winds show some
seasonality. The highest mean wind speeds and the largest average
gusts between March and May but these winds are largely inaffective in
transporting sediment because it is usually still damp from the winterís
rains. Aeolian transport is most active between June and early October.
|
|
(mm) |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Clay dune sediment is produced by salt efflorescence which aggregates
clay particles. The aggregates are fine-sand size and easily transported
by the wind. When the particles are deposited above lake level the
salt is readily leached. The resulting mass is approximately 76%
by weight clay and silt and 24% fine sand. The sand fraction consists
of rounded gypsum grains, quartz, and plant debris.
X-ray analysis of the clay dune materials as raw powder mounts produced strong peaks for quartz, halite, gypsum and lesser peaks for muscovite and albite. Bassanite, blodite, and illite are present in some samples.
Ephemeral lake basins and the dunes that accompany them present
a variety of forms. Bowler (1986) specified the controlling variables
as:
1) Catchment area to lake-surface area ratio (Ac/Al)
Basins with relatively large catchment areas maintain surface water
coverage for longer periods of time.
2) Climatic setting
The ratio of potential evaporation to precipitation is critical in
determining the hydrologic balance of the system. The magnitude and
seasonality of winds determine the period of effective aeolian transport.
3) Tectonic-physiographic setting
Relief strongly influences sediment and salt supply.
4) Hydrochemistry
The composition and final mineralogy of evaporite deposits is strongly
influenced by the chemistry of surface and ground waters.
The Index of Disequilibrium (Del) combines the area ratio and climatic setting and is defined as the "difference between the climatic setting necessary to maintain the basin in steady-state lake-full conditions and that which prevails today" (Bowler, 1986, p. 25). Plotting Del against the percent of time the basin has a water cover provides a means of characterizing playa basins and the dunes that form adjacent to them.
The main clay dune or "Big Berm" that bounds the northern and eastern limits of the North Basin is 9.5 km long. The berm is largest at its south end and gradually, although not consistently decreases in size to the north.
Cross Section 1 shows the dune at nearly its maximum size. At this location, the dune is ~ 480 m wide and reaches a maximum height of 16.8 m above the lake basins. The dune appears to be a compound form with a lower and smaller crest ~ 10 m high. On the South Basin side of the dune, waves attack the base of the slope and have produced a scarp ~ 6 m high.
Cross Section 2 crosses the dune where aeolian deposition is most active. Accretion on the western side is sufficiently rapid to prevent dissection of the face.
Cross Section 3 traverses the dune at the Pig Pond, one of the secondary pans. The dune is ~170 m wide and stands 8.9 m above Pig Pond, but only ~4 m above the alluvial surface.
Cross Section 4 shows the most obvious compound form. The higher, and presumably older, dune crest stands 11.3 m above the Round Pond and ~180 m from the present shoreline. This distance is unusually large. At Cross Section 1, where the dune is its largest, the crest is only 140 m from the edge of the North Basin. The lower crest is 6.8 m above the basin and probably represents a recent period of active deflation of the pan, perhaps because of a lower water table.
The gross morphology of the Big Berm and indicates a dominant summer wind direction from the west. The variable dimensions of the dune may indicate differences in:
1) ageAll three probably play a role. The dune is largest where it is still active along the southeastern end of the North Basin. The remainder of the dune is mostly inactive and undergoing dissection except where small pans provide a sediment source.
2) wind direction
3) sediment supply.
The most active part of the main dune is located at the southeastern corner of the North Basin. This area, the stoss side of the dune facing the North Basin, is outlined in the photograph above.
In contrast to most of the dune, this face is almost ungullied because the surface receives a steady supply of aeolian sediment. The lighter tone of the photograph results from relatively sparse vegetation in this area of active deposition.
The large sediment supply to this portion of the dune results from its location relative to the salt crust. Sediment can be swept from the basin only after the evaporation has dried the lake bed. However, the salt crust that forms over most of the basin prevents deflation. Only that part of the pan not covered by salt during the dry season produces sediment. Along most of the southeastern end of the North Basin the salt crust extends to within 80-90 m of the base of the dune. In contrast, where aeolian deposition is most active more than 220 m of lake bed is exposed to the wind.
The pits shown in this photograph are examples of one important source
of sediment for the active dune. Although a continuous salt crust
does not form on this portion of the lake bed, enough salt is present to
form domal ìblistersî during the final stages of evaporation. When
these structures are breached, the dry sediment beneath them is exposed
to the wind and deflation commences. Most of the pits are elliptical
in form. The long axis presumably parallels the dominant wind direction.
Sand-size aggregate particles may be transported individually or as part of a ripple sheet. Upon reaching the base of the dune, the material is transported up the stoss face, moving around the vegetation, as shown in the photograph below. The new layer of clay aggregate is stabilized during the next wet season as rain leaches the salt binding the aggregated clay particles. This clay layer is incorporated into the dune and is unlikely to be reactivated during subsequent dry seasons.
Seven cores of varying length have been removed from Soda Lake. Radiocarbon dates have been obtained from core SLC-SB-10/3/97-1. Organic material obtained from near the bottom of the core yielded a age of ~4850 radiocarbon years. This date is a minimum age for Soda Lake. The mean accumulation rate for the entire core is ~3.5 mm/yr.
A dark black organic-rich layer lies below the salt crust at the top of the core (on the left in this photo). The remainder of the core consists of gray clay interrupted by relatively thin layers of brown clay. These bands may represent decades of dessication or events during which oxidized sediment was washed into the lake.
Two explanations have been proposed to explain the Carrizo Plain's
internal drainage: 1) an originally northwest flowing tributary of
the Salinas River was defeated by regional uplift of the area (Diblee,
1962) or 2) the closed system is a relic of an originally south and
east flowing drainage to the southern San Joaquin Valley that was defeated
by uplift of the Temblor Range (Galehouse, 1967).
In Diblee's (1962) view the Carrizo Plain is the result of erosion and deposition by a ìmajor streamî that flowed northwestward during Paso Robles time. Regional uplift exceeding 300 m defeated this stream and initiated a period of dissection of the Carrizo Plain by tributaries of San Juan Creek that continues today. Presumably this occurred near the end of the late Pliocene to Pleistocene time, the age Diblee (1973) attributed to the Paso Robles Formation.
Galehouse (1967) provides the most complete, although also dated, investigation of the problem. He applied heavy-mineral provenance and paleocurrent analysis to the Paso Robles Formation. Deposition of the Paso Robles commenced with the uplift of the Santa Lucia and La Panza ranges in the early Pliocene. Drainage from these highlands crossed the present location of the Temblor Range to reach a marine basin in what is now the southern San Joaquin Valley. Channel and floodplain deposits spread from theses main sources now crop out over more than 2,500 km2. Paso Robles deposition continued until near the end of the Pliocene when uplift of the Temblor Range defeated the stream system and initiated a period of internal drainage. Headward advance by tributaries of the Salinas River gradually captured much of the drainage, reversing its direction from south and east to north and west. Galehouse (1967, p. 973) considered the Carrizo Plain to be "a relic of this period of interior drainage, not yet captured by the Salinas River and its tributaries."
Both Diblee's and Galehouse's interpretations place the loss of external drainage near the beginning of the Pleistocene. This presents a problem. Several lines of evidence obtained from Clear Lake (located in the Coast Ranges about 500 km north of Soda Lake) indicate that the Pleistocene climate in coastal California was cooler and wetter than today (Bradbury, 1988; Forester, 1988). Because it has external drainage, Clear Lake did not have a recognizably higher Pleistocene lake level (Forester, 1988). However, closed basins elsewhere in California expanded greatly during the Pleistocene leaving a remarkable record of shoreline features. No features associated with a notably higher Pleistocene lake level have been recognized in the Carrizo Plain.
If Soda Lake existed for any substantial part of the Pleistocene
it should have been considerably larger than it is today. However,
unlike many of California's pluvial lakes, easily recognizable strand
lines do not exist. Two features, the clay dunes and "slickspots"
may provide evidence of higher lake levels.
"Slickspots" are barren shallow depressions common to sodic soils (Reid et al., 1993). On older aerial photography, produced before much of the Plain had been tilled, slickspots were obvious in their light tones and lack of vegetation. Using 1936 photos, the slickspots were mapped onto a 7.5-minute quadrangle base. From this mapping we learned that the slickspots encircle the modern lake (except along the northwest where fluvial deposition is most active). The slickspots on the northeast side of the basin and the southern quarter of the southwest side are consistently at 1,940 ft (591 m). Along the northern 75% of the southwest side the slickspots are mostly at ~1,930 ft (588 m). The remarkable coincidence of elevations on the northeast side of the valley lead us to speculate that the slickspots might record a former lake level. Small fluctuations in water level would cause alternate wetting and drying along the shoreline. Strong evaporation from this zone could concentrate salts and initiate the formation of the slickspots.
The Big Berm provides definite evidence of a former shoreline. The northern two-thirds of the dune is separated from the modern lake (at 1,908 ft) by an exposed former lake-bed surface which is up to 2 km wide. The clay dune could not have formed under present conditions. Without fluctuating water levels nearby, no sediment source would have been available. When this portion of the dune was active, water must have intermittently inundated the adjacent surface. In the North Basin the base of the dune is consistently located at an elevation of 1,920 ft (585 m). Since the dune was formed, the average level of the lake must have changed by ~12 ft (4 m).
Comparisons of both the 1,940-ft and 1,920-ft surfaces with the modern lake level show that shrinkage of the shoreline was not symmetrical. This asymmetry can be explained by tilting Carrizo Plain along an axis parallel to the valley with the northeast side up. Slickspots on the southwest side of the basin are consistently ~10 ft (3 m) lower elevation than those on the northeast side. The base of the inactive portion of the Big Berm stands about 12 ft above the modern lake, which itself is located hard against the southwest side of the valley. Tectonic warping of the Carrizo Plain associated with the San Andreas Fault system may account for these differences.
The Soda Lake Interdisciplinary Project is a cooperative research
effort devoted to understanding the record of climatic, geomorphic, and
tectonic change preserved in and around Soda Lake, San Luis Obispo County,
California. Participants in the project to date include:
Mora, German, Pratt, L. M., Eigenbrode, J., Rhodes, D.D., 1998, Biogeochemical
dynamics and evolution of gypsiferous soils in the Carrizo Plain, California:
Geological Society of America Abstracts with Programs, v. 30, no. 7, p.
A359.
Bowler, J. M., 1973, Clay dunes: their occurrence, formation and environmental significance: Earth Science Reviews, v. 9, p. 315-338.
Bowler, J. M., 1986, Spatial variability and hydrologic evolution of Australian lake basins: analogue for Pleistocene hydrologic change and evaporite formation: Paleogeography, Paleoclimatology, Paleoecology, v. 54, p. 21-41
Dibblee, T. W., Jr., 1962, Displacements on the San Andreas rift zone and related structures in Carrizo Plain and vicinity; in Hackell, O., ed., Geology of Carrizo Plains and San Andreas fault: San Joaquin Geological Society and Pacific Section, American Association of Petroleum Geologists and Society of Economic Paleontologists and Mineralogists Guidebook, p. 5-12.
Forester, R.M., 1988, The Clear Lake, California, ostracode record; in Sims, J.D., editor, Late Quaternary climate, tectonism, and sedimentation in Clear Lake, northern California Coast Ranges: Geological Society of America Special Paper 214, p.131-139.
Galehouse, J.S., 1967, Provenance and paleocurrents of the Paso Robles Formation, California: Geological Society of America Bulletin, v. 78, p. 951-978.
Ried, D. A., Graham, R.C., Southard, R.J., and Amrhein, C., 1993, Slickspot
soil genesis in the Carrizo Plain, California: Soil Science Society
of America Journal, v. 57, p. 162-168.