Late Quaternary Paleohydrology of the Mojave Desert

Lake Mojave and its upstream precursor, Lake Manix, have not previously received the detailed study that is needed to understand their response to local factors, such as changing water inputs from the Mojave River due to basin integration events. In addition to external forcing by climate, we need to better understand the history of drainage integration along the Mojave River to interpret the sediment record properly. For example, previously published paleolake interpretations of Lake Manix sediments have not recognized that part of the apparent changes in sedimentology are due to events that changed the configuration of the lake basin rather than to changes in paleoclimate. Mojave lakes are supported by long-distance river systems (Fig. 4), each of which was integrated through time in ways that are not well understood. We cannot accurately compare environmental histories in different parts of the desert without understanding the integration histories for those rivers. New interpretations of the integration history of the Mojave and Amargosa Rivers may also help improve our understanding of the history of migration and evolution of aquatic species in the Desert Southwest, many of which are both endemic and endangered (for example, the Mojave River tui chub, whose only present refuge is a spring on the edge of the former Lake Mojave).

Surficial geologic mapping and dating in selected critical areas, particularly the Manix and Afton Canyon areas, are being used to understand the fluctuation in size and depth of pluvial lakes, the history of drainage integration, and the interaction of lakes and adjacent alluvial and eolian systems. Mapping is critical to the accurate interpretation of core analytical data. Reconnaissance mapping has demonstrated that the Mojave River has had complex interconnections with several different basins, including Harper Lake, the subbasins of Lake Manix, the Soda and Silver Lake basins, and the Cronese Lakes. Recent mapping indicates that Afton subbasin was separate from Lake Manix farther upstream for part of the history represented by the 45-m core; thus the sedimentary record of the core partly reflects a drainage integration event. Stratigraphic sections and soils in critical outcrops show environmental changes and interactions between lakes, rivers, and dune sands.

Early history of Lake Manix and the catastrophic flood

For much of its history during the middle Pleistocene, Lake Manix was confined to the Cady, Troy Lake, and probably Coyote Lake subbasins to the west of Buwalda Ridge (Fig. 4). During this long period, probably at least 200,000 years, the Afton subbasin was an internally drained basin separate from the lake. Outcrops along the Mojave River show that the Afton subbasin was fringed by alluvial fans that had sources in the surrounding hills and mountains, and supported a playa, but not a lake, during this time (Fig. 5, photo of playa beds). We have found that most lake sediments in the Manix Formation exposed in outcrops near the core site were deposited in shallow lacustrine to mudflat environments punctuated by short episodes of deeper water. Although some units can be traced for kilometers and are relatively uniform in character, other units undergo facies changes over short distances. For example, unit Upper B of Jefferson (2003) is a sandy gravel unit along the Manix Wash bluffs, a mud unit in the core several hundred meters away, and a widespread sand bed for several kilometers to the west of the core site. Tufa-coated gravel beds mark episodes of lake deepening or lake transgression, and are found capping both lacustrine and alluvial gravels. Buried soil profiles or soil horizons within the lacustrine section can be identified in the core and outcrops, and suggest multiple periods of non-deposition on the low-gradient floor of the Cady subbasin during the Pleistocene.

At about the time of deposition of the Manix ash bed, ~185,000 years ago (Jefferson, 2003), or a little earlier, our new studies show that the old Lake Manix overtopped a threshold or natural dam located near the east end of Buwalda Ridge and entered the Afton subbasin in a catastrophic flood event (Reheis and others, 2007). In this way, the Afton subbasin suddenly became part of Lake Manix, and for a long period of time was the deepest part of the lake. This event has profound implications for the interpretation of younger sediments deposited in the Afton and Cady subbasins.

The flooding event into the Afton subbasin is revealed by outcrops that show dramatically deformed lacustrine muds (Fig. 6) that represent large, reworked but semi-intact blocks of Lake Manix sediment derived from the central Cady subbasin upstream and, in turn, these muds overlie the playa and distal-fan deposits of the formerly closed basin. These muds are overlain by fluvial gravel and sand (Fig. 7) that represent the initial establishment of stream drainage from the central subbasin. A rapidly upward-fining sequence of sand (Fig. 8), silt, and mud follows, which marks the flooding of the newly incorporated subbasin with Lake Manix (Mojave River) water.

Figure 6. Ripped-up blocks of deformed Lake Manix clay, silt, and sand locally overlie the playa beds, and are capped by volcanic-clast fluvial gravel. Closeup shows some of the deformed blocks. Click on image for larger view.

Figure 7. Fluvial gravel and sand overlie the zone of deformed lake sediment and represent a temporary establishment of an eastward-flowing river before the rising lake in the Afton subbasin backflooded this site. Shovel (indicated by yellow arrow) below the geologist marks a location where the gravels include reworked gravel-sized clasts of green mud. Click on image for larger view.

Figure 8. Photographs of shallow-lake sands representing first lake to fill Afton subbasin. Railroad cut exposes about 2.5 m of granitic sand compositionally different from the underlying distal fan deposits (marked by vertical runnels). Granitic sand characterized by sweeping, east-directed crossbeds. Close-up shows cross-bedding within the lower sand beds. Click on image for larger view.

Later history of Lake Manix after the flood

The ~7 m of sediment above the tephra in the core from the Manix area had previously been interpreted as perennial-lake deposits (Jefferson, 2003), but new mapping and core analysis (Figs. 13 and 14) suggest that these sediments were deposited in a fluctuating environment of shallow lakes and mudflats. Deposits in the Afton subbasin interpreted as correlative with the 7-m interval consist of perennial-lake muds (Fig. 9) intercalated with sands that thicken and coarsen shoreward. These relations indicate that lake depth and distance from fluvial inputs were greater in the newly incorporated Afton subbasin than in the correlative Cady subbasin. By this time the Manix area, having been the depocenter of Lake Manix for possibly hundreds of thousands of years, had lost much of its sediment storage capacity; thus, relatively small changes in runoff and lake level could easily cover or expose previously deposited sediment. Sediment patterns in the core suggest that millennial- to centennial-scale cycles of wet and dry periods may be recorded by alternating soils and fluvial, shallow-lake, and mudflat beds.

After the subbasins were integrated, Lake Manix rose to a level as much as 10-15 m higher than its previously recognized highstands at about 543 m in altitude, probably during and (or) after marine oxygen-isotope stage 6 (180,000-130,000 years ago), and episodically discharged eastward into the Soda Lake basin. These older highstands at higher levels are very eroded and usually quite difficult to recognize, but are preserved in a few places (Fig. 10; Reheis and Redwine, 2008). During these eastward discharges, the threshold at the head of the present-day Afton Canyon probably was gradually incised to about 543 m in altitude. Fluvial sand and gravel deposited by a discharge stream lie high above the rim of the present-day canyon and are buried beneath many beds of colluvial gravel derived from the adjacent slope (Fig. 3). The timing of these events is not well constrained because they are older than the practical age of radiocarbon dating. However, some tufa (calcium carbonate) that was precipitated onto gravels during lake rises (Fig. 11) has been dated using uranium-series techniques. Ages of a few tufa samples from these lake-rise periods range from about 150,000 to 70,000 years (Reheis and others, 2007).

During subsequent lake highstands of OIS 4?, 3, and 2, the lake repeatedly rose to about 543 m in altitude, maintained there in part by an internal shallow threshold between the Manix and Coyote Lake subbasins (Fig. 2)(Meek, 2004) and perhaps in part by the eastern threshold at the head of Afton Canyon. These lake phases built large beach barriers in many locations; perhaps the most prominent is the north Afton beach ridge, which lies north of Afton Canyon along the east side of the lake basin (Fig. 12). These highstands have been dated using radiocarbon ages of Anodonta shells, which are commonly preserved in nearshore sediments, from all of the subbasins of Lake Manix (most dated shell deposits were from the Afton and Cady subbasins). The near-543-m highstands occurred during episodes at about 45,000-35,000, 33,000-30,000, and 27,000-25,000 years ago (Reheis and Redwine, 2008). In the Cady subbasin along the bluffs near the coresite, sediment deposition was mainly fluvial during these periods of time. Sedimentary facies and structures mimic those of the present-day Mojave River, including cross-bedded channel-fill deposits, crevasse-splay deposits, marsh, and floodplain deposits. We conclude that these sediments represent progradation of fluvial deposits over lacustrine units, with only brief lake inundations. The radiocarbon ages (and the older ages indicated by uranium-series dating) suggest that Lake Manix achieved high levels during several episodes corresponding to OIS 6, 5, 4(?), 3, and 2, in contrast to pluvial lakes farther north in the Great Basin whose highstands correspond mainly with OIS 6 and 2. These ages suggest that high levels of winter precipitation necessary to sustain Lake Manix may be influenced by ENSO-like conditions and southward positions of the Aleutian Low, with or without southward depression of the polar jet stream.

Figure 12. Aerial view of the north Afton beach ridge, which is the long linear ridge in photo center. The ridge is about 2 km long. North is to the right. The Mojave River and the upper part of Afton Canyon are to the left. Click on image for larger view.

The last highstand of Lake Manix was terminated when the lake overtopped its eastern threshold and Afton Canyon was incised, probably at or just after 25,000 years ago. But it took a long time--thousands of years--for the newly formed drainage to erode westward through the former lakebeds, and during this time the Mojave River sometimes changed direction and discharged north into the Coyote Lake subbasin of the former Lake Manix. LIDAR data was obtained over apparent inverted channels that fed Mojave River into Coyote Lake after Afton Canyon was cut. These data clearly show several sets of channels that graded to shorelines within the Coyote subbasin during different periods dated by Anodonta shells found in barrier and shallow lake platform deposits. Dating suggests river-fed lake events at roughly 15 and 13 ka, each period associated with a stable shoreline that was 1-2 m lower than the highstand of full Lake Manix.

More project studies

References

Jefferson, G.T., 2003, Stratigraphy and paleontology of the middle to late Pleistocene Manix Formation, and paleoenvironments of the central Mojave River, southern California, in Enzel, Y., Wells, S.G., and Lancaster, N., eds., Paleoenvironments and paleohydrology of the Mojave and southern Great Basin deserts: Boulder, Colo., Geological Society of America Special Paper 368, p. 43-60.

Meek, N., 2004, Mojave River history from an upstream perspective, in Reynolds, R.E., ed., Breaking up--the 2004 Desert Symposium Field Trip and Abstracts: Fullerton, Calif., University of California Fullerton, Desert Studies Consortium, p. 41-49.

Reheis, M.C., and Redwine, J.L., in press 2008, Lake Manix shorelines and Afton Canyon terraces: Implications for incision of Afton Canyon, in Reheis, M.C., Hershler, R., and Miller, D., eds., Late Cenozoic Drainage History of the Southwestern Great Basin and Lower Colorado River Region: Geologic and Biotic Perspectives: Geological Society of America Special Paper 439.

Reheis, M.C., Miller, D.M., and Redwine, J.L., 2007, Quaternary stratigraphy, drainage-basin development, and geomorphology of the Lake Manix basin, Mojave Desert--Guidebook for fall field trip, Friends of the Pleistocene, Pacific Cell, October 4-7, 2007: U.S. Geological Survey Open-File Report 2007-1281, 31 p.

Late Quaternary Paleohydrology of the Mojave Desert

Drainage-Basin History Through Geologic Mapping

Early history of Lake Manix and the catastrophic flood

Later history of Lake Manix after the flood

More project studies

References