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, most of these 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].

Meteorites have been found on several of these dry lakes including 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 all of the above cases, unpaired meteorites have been found near one another on the same dry lake.

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. Deflation has removed fine-grained sediments from the surface of this dry lake to form dunes along the margin of the lake. These dunes are best developed where the sediment laden winds blowing off of the lake bed surface encounter the greatest rise in topography.

Since 1963, 17 meteorite specimens (1.2 to 37.4 g), collectively called Lucerne Valley, have been found on Lucerne Dry Lake. Most appear to be completely covered with fusion crust, suggesting that their small size is due to fragmentation in the atmosphere and not to terrestrial weathering. 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. Fifteen 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 15 analyzed stones, there are nine 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.

The three L6 chondrites (LV001/004/005 and LV011) are distinguishable because they differ significantly in shock stage (S2, S4, and S3 respectively). The LL4 chondrite (LV002) is significantly less recrystallized (and much less oxidized) than the LL6 chondrite (LV015). 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 (LV013/014/016) 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 reach a terminal velocity of ~300 m ´ s^-1 and are unlikely to 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 (9/16 = ~0.6) at Lucerne Dry Lake is among the highest in the world, and is comparable to that of well-characterized samples from Roosevelt County, New Mexico (~0.7). It results from two effects: First, no large meteorite showers such as that of the Holbrook fall in 1912, which produced ~14,000 individual specimens [5], are represented at Lucerne Dry Lake. Second, lack of deep burial coupled with significant deflation have exposed remnants of distinct meteorite strewn fields. (Although the rare occurrence of some terrestrial rocks atop ~3 cm high clay pedestals suggests that there may be some minor, localized erosion of the lake surface by rain-water runoff, deflation is still the predominant erosive mechanism.)

The surface of Lucerne Dry Lake is analogous to other meteorite-rich collecting areas where meteorites are concentrated by deflation. In addition, Lucerne Dry Lake is conducive to collecting meteorites in the 1 to 40 gram range, because the lake surface lacks terrestrial rocks even as small as fine-pebble. It has been suggested that future studies include the determination of the ¹⁴C terrestrial ages of these meteorites. This should help quantify their rate of accumulation. Since no evidence has been found of any special process which is concentrating meteorites from outside of Lucerne Dry Lake onto its surface, it can be concluded that a comparable number of falls has occurred on any other similarly sized area of the Mojave Desert. On any other similarly sized area of the Mojave Desert, and with a comparable rate and style of erosion as Lucerne Dry Lake, it can be concluded that a similar number of unpaired meteorite specimens could be found as well. Because other dry lakes in the Mojave Desert have experienced a similar geologic history, they also are excellent candidates for being high-yield meteorite stranding surfaces. This study is on-going as we continue to recover more unpaired meteorites from other dry lakes in the Mojave Desert.

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.