For much of the last 2.5 million years Searles Valley was covered by water supplied largely from the Owens River. About half of this time water overflowed eastward into Panamint Valley. Old shore lines on the alluvial slopes of the Slate Range to the east show several lake levels including the prominent overflow level. These periods of high lake level coincided with the high precipitation epochs when large glaciers were common in the Sierra Nevada Range of California.
The major rocks on the eastern flanks of the Sierra Nevada Range of California are granites and volcanic debris. The adjacent White, Inyo and Coso ranges contributed more volcanic rock, but also sedimentary rock. Together, these rocks contain virtually all of the non-radioactive elements in the periodic table and even uranium which is radioactive but has a very long half-life.
Glaciers exert massive forces on the earth and underlying rocks. As they slowly move down hill, they grind away the rock, reducing much of it to a very fine powder called glacial flour. This fine powder makes leaching (dissolving) chemicals from the rock much easier. Gradually, over tens of thousands of years, water from the melting glaciers dissolved many chemicals from the rocks. In fact, everything has some solubility in water. So whatever was in these rocks, some of it was dissolved. Contributing to the chemical load of the runoff water was water supplied by the many volcanic hot springs in the drainage area. The water in these springs rises from deep underground, and being very hot, it was especially effective in dissolving minerals from the rocks on its way to the surface.
The main cations leached were sodium, potassium, and calcium while the main anions were chloride, carbonate, bicarbonate, sulfate, and borate. Also leach to a lessor degree were lithium, magnesium, fluoride, bromide, iodide, arsenic, tungsten, and molybdenum. In addition to chemicals, virtually all other non-ratioactive elements would have been leached into these waters, although many were at very low concentrations.
Because of the heavy rainfall and snowfall during the glacial periods, the runoff area would also have had lush vegetation, teeming with wildlife. And this vegetation would have contributed a heavy load of dissolved organic matter to the runnoff waters. This organic matter would have come from decaying vegetation and animals, and from animal body waste. So, in addition to the many minerals leached by the runoff and geothermal spring waters, these waters would have also carried a heavy load of organic matter.
The Owens River collected all of the runoff water on the eastern side of the Sierra Nevada Range from as far north as Mono Lake. As the Owens River flowed southward it went through a chain of large lakes: Owens, China, Searles, Panamint, and Manly Lake in present day Death Valley. The first lakes settled coarser sediments and started the evaporative concentration of mineral and organic matter. Being late in the sequence, Searles Lake received only minor amounts of very fine particles and these settled to make up the mud beds found in and around the lake. Very fine mud would also form when calcium carbonate precipitated as the water concentrated. Generally, muds were deposited very slowly in Searles Valley, at a rate between 6 and 12 inches per 1,000 years.
During most periods with lower precipitation Searles Dry Lake was the terminus of the Owens River and the mineral and organic matter carried by the River accumulated in the lake waters. During very dry periods Searles Dry Lake would desiccate and the soluble minerals would crystallize to deposit the saline minerals accumulated over thousands of years. During final desiccation these minerals could be deposited as fast as 6 to 12 inches per year. Any organic matter would also concentrate, but it would largely remain in the brine or on the surface of the crystallizing minerals. The fate of the organic matter was most likely to feed certain aneorobic bacteria that draw oxygen from sulfate and carbon from the organic matter to create the sulfide present in Searles Dry Lake brine. (This is the origin of the rotten egg smell that is so often noticed by visitor.)
The minerals to crystallize first on desiccation were sodium carbonate decahydrate (Na2CO3.10H2O) and Borax (Na2B4O7.10H20), and this would occur in the winter. During the following summer the Na2CO3.10H2O would melt, and carbon dioxide from the atmosphere and from biological action would react with the Na2CO3 to cause Na2CO3.NaHCO3.2H2O, also known as Trona, to replace the Na2CO3.10H2O. The next major mineral to crystallize as desiccation continued was sodium sulfate decahydrate (Na2SO4.10H2O). This would also crystallize in the winter but during the following summer it too would melt and the released Na2SO4 would recrystallize as Thenardite, Na2SO4 without the hydrating water. When there was a sufficient amount of potassium, it would crystallize as Glaserite (K3Na(SO4)2) at the same time as sodium sulfate decahydrate. Over time much of the Glaserite has recrystallized as the rare mineral Hanksite. The final major mineral to crystallize was Halite (NaCl) and this occurred near the end of desiccation. This depositional sequence is easily observed in the vertical stratification found today in the highest saline mineral bed, the Upper Salt, where Halite makes up the top 20 to 25 feet, Borax and Trona are next to the mud at the bottom of the salt bed, and the sulfates (Hanksite and Thenardite) are in between.
Once deposited, later flooding had little immediate effect on the soluble deposits because the water near the salts would become saturated by the salts making the water's density very high. This dense water would stay on the bottom of a lake to shield the salts from the lower density, unsaturated waters floating above. Over time, the salts would become covered by a new mud layer, providing an even tighter barrier to leaching.
But Searles Valley is not a water tight basin. Were water to fill the basin today the hydrostatic pressure against the Slate Range would be over 600 feet, and when the bottom mud was being deposited, it would have been over 900 feet. This pressure was enough to cause a slow flow through the Slate Range mountains into Panamint Valley. At least some of this water permeated downward through the muds covering the bottom of the lake. At around 300 feet below the present surface this water encountered very permeable, soluble mineral beds through which it traveled laterally toward the Slate Range. When this happened the saline minerals were subjected to leaching which gradually altered them. Sodium chloride would dissolve first since it would have been on top (having crystallized last) where it would see the water first. Sodium chloride is also very soluble. This would be followed in order by sodium sulfate, the sodium carbonate fraction of trona, and Borax. The final mineral to dissolve would be the sodium bicarbonate left when sodium carbonate was leached from the Trona. The effect of this leaching is that the minerals in the saline beds 250 to 350 feet below the surface are present today in the opposite order in which they are found in the Upper Salt. That is, the mineral on top is Nahcolite (NaHCO3) with Trona beneath the Nahcolite. The Thenardite and Halite are usually only present beneath about 350 feet, the exit level for the leaching waters. And this order is backwards from the order in which they had to have been deposited. It also means that probably well over half of the original beds deposited have been washed away.
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