DEFLATION AND METEORITE EXPOSURE ON PLAYA LAKES IN THE SOUTHWESTERN UNITED STATES: UNPAIRED METEORITES AT LUCERNE DRY LAKE, CALIFORNIA. Robert S. Verish1, Alan E. Rubin2, Carleton B. Moore3, and Ronald A. Oriti4. 1P.O. Box 237, Sunland, CA 91040, USA; 2Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095-1567, USA; 3Center for Meteorite Studies, Arizona State University, Tempe, AZ 85287-1604, USA, 4Department of Earth and Space Sciences, Santa Rosa Jr. College, 1501 Mendocino Ave., Santa Rosa, CA 95401, USA.

Numerous dry lakes (playa lakes) dot the Mojave Desert in Southern California and adjacent desert regions in Nevada and Arizona. Some of these lakes last held permanent water at the end of the Pleistocene, ~11,000 years ago; others contained shallow water for periods less than a century during the middle and late Holocene (3500-4000 and ~400 years ago, respectively) [1]. Since that time, many of the lakes partly fill with water each year during the rainy season and then dry out again; this may happen many times in a single season. Most of the dry lakes have been significantly affected by deflation processes which involve the lifting and removal of unconsolidated clay- and silt-size particles from the lake surface by wind erosion. Deflation is aided by the lack of protective vegetation and occurrence of fine-grained sediments at dry lakes [2]. That deflation is a significant erosive process at dry lakes is consistent with the occurrence of clay dunes near many of the dry lakes, ventifacts (sharp-angled rocks sculpted by wind erosion) on many dry lakes (N. Gessler, pers. commun., 1999), and yardangs (wind-carved landforms of weakly cemented sediments) near a few of the dry lakes [3,4].

Meteorites have been found on several of these dry lakes including Dale Dry Lake, Lucerne Dry Lake, Muroc Dry Lake, Rosamond Dry Lake, Roach Dry Lake, Alkali Lake, and an unnamed dry lake near the town of Bonnie Claire in Nye County, Nevada. In several cases, unpaired meteorites have been found near one another on the same dry lake. Two unpaired L chondrites (Muroc and Muroc Dry Lake) were found within 5 m of each other on Muroc Dry Lake in 1936 [5]; two probably unpaired H5 chondrites (Bonnie Claire 001 and 002) were found within 1 m of each other on the unnamed dry lake in Nye County, Nevada in 1998; and an H5 (Primm) and an LL6 chondrite (Roach) were found on Roach Dry Lake in 1997-1998 [6]. A few meteorites from dry lakes have sharp-angled surfaces and appear to be ventifacts.

Since 1963, 17 meteorite specimens (1.2-37.4 g), collectively called Lucerne Valley, have been found on Lucerne Dry Lake. The collection of meteorites on this lake is aided by the paucity of terrestrial rocks coarser than small pebbles; this is unusual for dry lakes in the region. Lucerne Dry Lake is ~3´ 6 km in size and is located in the southern Mojave Desert, ~17 km north of the San Bernardino Mountains. Sand dunes occur east of the lake. Thirteen of the meteorite specimens from Lucerne Dry Lake were available for analysis. We analyzed olivine by electron microprobe and classified the meteorites by chondrite group, petrologic type, shock stage and weathering stage. We used these data along with the meteorites’ detailed petrographic characteristics to determine likely pairings.

Out of the 13 analyzed stones, there are seven separate meteorites. The meteorites have retained the name Lucerne Valley (abbreviated LV); they are numbered in the order that they were found. The names have been approved by the Nomenclature Committee of the Meteoritical Society.

meteorite

group/ type

shock stage

weather-ing stage

olivine

paired specimens

LV001

L6

S2

W3

Fa24.3

LV004,005

LV003

H6

S3

W3

Fa18.0

none

LV006

H4

S2

W3

Fa18.4

LV008,009,010

LV011

L6

S4

W3

Fa24.5

none

LV012

H6

S2

W3

Fa19.4

none

LV014

L5

S2

W3

Fa25.3

LV016

LV015

LL6

S3

W2

Fa30.9

none

The two L6 chondrites (LV001 and LV011) are distinguishable because they differ significantly in shock stage (S2 and S4, respectively). The two H6 chondrites (LV003 and LV012) are distinguishable on the basis of their non-overlapping olivine compositional distributions (Fa18.0± 0.4 and 19.4± 0.3 mol%, respectively). The L5 chondrite (LV014) is significantly less recrystallized than either of the L6 chondrites; similarly, the H4 chondrite (LV006) is appreciably less recrystallized than the two H6 chondrites.

Upon falling, small meteorites do not penetrate the dry lake surface. When the lakes are partly filled with water during the rainy season, they contain some suspended clay particles, but not enough to cause the meteorites to be deeply buried after the clay settles. Only rare catastrophic floods carrying large sediment loads would be able to bury the meteorites to depths >1 m. However, there are two principal factors which render this scenario unlikely [2]: (1) Because discharge decreases downstream in arid channels, playa lakes are likely to receive only fine-grained sediments. (2) Alluvial fans located between playa lakes and nearby mountains trap much of the sediment before it reaches the playa lakes. Both of these factors are important for Lucerne Dry Lake, which is located near the drainage terminus of an arroyo and is separated from nearby mountains by alluvial fans. Furthermore, playa lakes that experience very infrequent inundation may develop uneven surfaces due to the growth of evaporites or the development of dunes [2]. Lucerne Dry Lake is flat. It thus seems likely that the meteorites on the lake represent generally unburied remnants of strewn fields from small overlapping meteorite showers of different ages.

The ratio of separate meteorites to total number of specimens (7/13 = ~0.5) is among the highest in the world. It results from two effects: (1) No large meteorite showers such as that of the L/LL6 Holbrook fall in 1912 (which produced ~14,000 individual specimens [5]) are represented at Lucerne Dry Lake. (2) Lack of deep burial and significant deflation have exposed remnants of distinct meteorite strewn fields. Although the occurrence of some Lucerne Dry Lake specimens atop ~3-cm-high clay pedestals suggests that water runoff may be responsible for minor, localized erosion of the lake surface, deflation is the predominant erosive mechanism.

The surface of Lucerne Dry Lake is analogous to meteorite-rich collecting areas from the Sahara Desert, Nullarbor Plain in Australia and Roosevelt County, New Mexico where meteorites are concentrated by deflation. Lucerne Dry Lake affords us an opportunity to collect a wide variety of meteorites in the 1-40-g range that could be found on any other similarly sized area of the Mojave Desert that has a surface of comparable age. Planned studies of the 14C terrestrial ages of these meteorites should help quantify their rate of accumulation. Because other dry lakes in the Mojave Desert have experienced a similar geologic history (and a few have already yielded two or more unpaired meteorites), they also are excellent candidates for being high-yield meteorite stranding surfaces.

References:

[1] Enzel Y., Brown W.J., Anderson R.Y., McFadden L.D. and Wells S.G. (1992) Short-duration Holocene lakes in the Mojave River drainage basin, Southern California. Quaternary Res. 38, 60-73; [2] Shaw P.A. and Thomas D.S.G. (1997) Pans, playas and salt lakes, In: Arid Zone Geomorphology: Process, Form and Change in Drylands, 2nd edition, (D.S.G. Thomas, editor), Wiley & Sons, Chichester; [3] Ward A.W. and Greeley E.R. (1984) Evolution of yardangs at Rogers Lake, California. Geol. Soc. Am. Bull. 95, 829-837; [4] Meek N. (1994) The stratigraphy and geomorphology of Coyote Basin, central Mojave Desert, California. San Bernardino Co. Mus. Assoc. Quarterly 41, 5-13; [5] Graham A.L., Bevan A.W.R. and Hutchison R. (1985) Catalogue of Meteorites, Univ. Arizona Press, Tucson; [6] Meteoritical Bulletin, No. 83, 1999 July, Meteorit. Planet. Sci. 34, in press.



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