Borate formation in a perennial lacustrine setting: Miocene– Pliocene Furnace Creek Formation, Death Valley, California, USA Lawrence H. Tanner * Department of Geography and Geoscience, Bloomsburg University, Bloomsburg, PA 17815, USA Accepted 1 August 2001 Abstract The Miocene – Pliocene Furnace Creek Formation was deposited in marginal lacustrine and lacustrine environments in an extensional setting. Laminated facies comprising marlstone/limestone and mudstone/marlstone laminites lack features of surficial or shallow water deposition, indicating that sediment accumulated on the floor of a deep, perennial lake. Meter-scale mounds of porous limestone with a clotted-micritic fabric are spring-apron tufas that formed where groundwater discharged into more saline lake-bottom waters. The bedded borate facies comprises meter-scale bodies of borate, mainly colemanite, interbedded with laminated mudstone. The massive borate facies comprises thick, nonbedded borate layers, locally containing host rock clasts with sharp to diffuse boundaries. Borate formation took place at or below the sediment/water interface at the bottom of a permanently stratified carbonate lake. The composition of the borate deposits varies from mainly colemanite to zoned, with colemanite surrounding a core of probertite and ulexite. High Ca 2+ /Na + ratios favored primary deposition of hydrated Ca-borates, which subsequently transformed by diagenetic dehydration to colemanite. Gradual depletion of Ca 2+ and decreasing Ca 2+ /Na + ratios resulted in NaCa-borate precipitation. Finally, renewed onset of elevated Ca 2+ /Na + ratios again caused Ca-borate deposition. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Borate; Colemanite; Tufa; Lacustrine 1. Introduction Since the discovery of economic deposits of borate minerals in the United States in the 19th century, considerable research has been devoted to the environ- ments and mechanisms of borate formation. It is now known that large accumulations of the hydrated borate minerals occur principally in volcanically active non- marine basins of Cenozoic age, particularly playa and shallow lake environments (Smith, 1985; Smith and Medrano, 1996; Swihart et al., 1996; Helvaci and Ortı ´, 1998). Most models of borate formation are heavily influenced by the modern occurrences of borates in salt marshes and playas where surface evaporation causes concentration of boron-rich brines (Muessig, 1958; Papke, 1976; Kistler and Smith, 1983; Smith and Medrano, 1996; Swihart et al., 1996). But assumptions of the requirement for evaporative con- centration of saline waters have complicated under- standing of the genesis of both Ca-borates and stratigraphically zoned deposits containing both Ca- and Na – Ca-borates, typically attributed to diagenetic 0037-0738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII:S0037-0738(01)00221-4 * Tel.: +1-570-389-4142; fax: +1-570-389-3028. E-mail address: lhtann@planetx.bloomu.edu (L.H. Tanner). www.elsevier.com/locate/sedgeo Sedimentary Geology 148 (2002) 259 – 273