Revisiting silicate authigenesis in the Pliocene–Pleistocene Lake Tecopa beds, southeastern California: Depositional and hydrological controls Daniel Larsen* Department of Earth Sciences, University of Memphis, Memphis, Tennessee 38152, USA ABSTRACT Keywords: saline, alkaline lake, Pliocene– (especially marker beds) and three-dimensional Pleistocene, zeolite, depositional facies, authi- analysis of lateral facies variations. The hyper- The Pliocene–Pleistocene Lake Tecopa genic minerals. arid climate of the southwestern Great Basin has beds present a well-documented example of preserved much of the authigenic mineralogy, authigenic silicate diagenesis in an ancient INTRODUCTION although the most soluble salts, such as halite saline, alkaline lake environment. Controls and trona, are largely absent from surface expo- on authigenic mineral formation and dis- The Pliocene–Pleistocene Lake Tecopa beds sures due to dissolution during weathering. The tributions were investigated in nine strati- represent a classic example of concentric distri- stratigraphic and mineralogical results are used graphic sections aligned along a north-south butions of authigenic silicate minerals (Fig. 1) to (1) construct a conceptual model emphasiz- transect in the Tecopa basin. Specifi cally, attributed to diagenetic reactions in saline, alka- ing controls on authigenic silicate distributions potential depositional and hydrologic con- line lake deposits (Hay, 1966; Sheppard and in saline, alkaline lake deposits, and (2) discuss trols on mineral assemblages and distribu- Gude, 1968, 1969, 1973; Surdam and Parker, the paleohydrologic and paleoclimatic informa- tions were addressed by correlating detailed 1972). Past studies of silicate authigenesis tion recorded by authigenic silicate distributions sedimentological data and basin hydrology in saline, alkaline lake deposits have largely in such deposits. with authigenic mineral facies distributions. focused on the types and distributions of zeo- Deposition occurred within the Lake lites (Sheppard and Gude, 1968, 1973; Surdam AUTHIGENIC SILICATE MINERAL Tecopa basin in environments ranging from and Parker, 1972; Ratterman and Surdam, 1981; DISTRIBUTIONS IN SALINE, alluvial and eolian around the basin margin Sheppard, 1994) and pore-water chemistry ALKALINE LAKES to lake margin, mudfl at, and shallow and (Jones, 1966; Eugster, 1970; Surdam and Eug- perennial lacustrine in the basin center. The ster, 1976; Eugster and Hardie, 1978; Taylor and In most cases, saline lake basins are partially authigenic silicate minerals include trioc- Surdam, 1981). Several studies, mainly from or completely hydrologically closed all or most tahedral smectite, phillipsite, clinoptilolite, East Africa, have attempted to establish relation- of the time, so that evaporation leads to forma- opal C-T (cristobalite-tridymite), potas- ships, if present, between specifi c sedimentary tion of concentrated water compositions that sium feldspar, illite, albite, and searlesite, as facies in basins containing saline, alkaline lake may ultimately become brines (Jones, 1966; well as many other minor or less commonly deposits and their authigenic silicate mineralogy for a review, see Jones and Deocampo, 2003). observed phases. Authigenic mineral distri- (Hay, 1976; Bellanca et al., 1992; Renaut and The chemical evolution of evaporating waters butions along the margin of the basin are Tiercelin, 1994; Ingles et al., 1998; Deocampo is most sensitive to the balance of Ca2+ and 2+ strongly controlled by sediment composition and Ashley, 1999; Ashley and Driese, 2000; bicarbonate contents (Ca -HCO3–chemical (primarily tuffaceous component) and lake- Hay and Kyser, 2001; Hover and Ashley, 2003). divide; Hardie and Eugster, 1970). Water com- level variations. Authigenic mineral composi- Because distributions and types of sedimentary positions in which the bicarbonate equivalents tions in the center of the basin are dominated facies in lake basins are fundamentally related exceed the calcium equivalents (2m – > HCO3 by feldspar, illite, and searlesite, and are less to basin hydrology and climate (Smoot and mCa2+) lead to calcium-poor, bicarbonate-rich infl uenced by sediment composition or short- Lowenstein, 1991; Rosen, 1994), relationships water compositions during evaporation and term changes in lake level. The authigenic between sedimentary facies and authigenic min- associated precipitation of calcium carbon- silicate mineral composition in the central eral distributions provide information regarding ate. Because Mg carbonates do not precipitate part of the basin is interpreted to be a result hydrological processes in saline, alkaline lake readily, bicarbonate concentrations increase of chemical interaction with a saline, alkaline basins and, indirectly, paleoclimatic conditions. with evaporation until oversaturation with brine that moved in accord with lake-level In this study, the stratigraphy and authigenic alkali–alkali earth carbonates and bicarbon- changes and induced density-driven circula- mineralogy of the Pliocene–Pleistocene Lake ates (e.g., gaylussite, trona) is reached (Eug- tion. The results suggest that distributions of Tecopa Beds were examined to assess the vary- ster and Hardie, 1978). Assuming that the pH authigenic silicate minerals in saline, alkaline ing roles of sedimentary, chemical, and hydro- is controlled by the carbonate system, the pH lake deposits are complexly related to depo- logical processes in controlling the types and increases through evaporative concentration of sitional and hydrologic processes and may distributions of authigenic minerals. The incised such waters until bicarbonate mineral precipi- be of limited utility in resolving lake-level badlands exposures in the Tecopa basin pro- tation and hydrolysate complexation (e.g., sili- changes in ancient lacustrine systems. vide relatively easy lateral correlation of beds cic acid) buffer the pH (Eugster, 1980). *[email protected] Geosphere; June 2008; v. 4; no. 3; p. 612–639; doi: 10.1130/GES00152.1; 11 fi gures; 2 tables; 2 plates. 612 For permission to copy, contact [email protected] © 2008 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/4/3/612/3339499/i1553-040X-4-3-612.pdf by guest on 25 September 2021 Revisiting silicate authigenesis in the Pliocene–Pleistocene Lake Tecopa beds Figure 1. (A) Map of region surrounding the Tecopa basin, illustrating mountains, streams, and general groundwater fl ow directions. (B) Map of the Tecopa basin illustrating the locations of measured stratigraphic sections and correlated sections (line A-A' in Plates 1 and 2, line B-B' in Figs. 3 and 9, and line C-C' in Fig. 10). Note that line A-A' in Plate 2 includes section LT-1 rather than LT-8. Also shown are the authigenic mineral facies identifi ed in the B tuff by Sheppard and Gude (1968). Geosphere, June 2008 613 Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/4/3/612/3339499/i1553-040X-4-3-612.pdf by guest on 25 September 2021 Larsen The unique combination of evaporative con- tors of paleoclimatic change (Street-Perrott METHODS centration and bicarbonate-rich water compo- and Harrison, 1985; Cohen, 2003). Based on sitions in saline, alkaline lakes leads to condi- this reasoning, variations in lake level, or more Stratigraphic sections were measured in tions favoring rapid hydrolysis of labile silicate specifi cally lake surface area, should also affect areas with moderate to good exposure and minerals and volcanic glass (Hay, 1966). The in a systematic manner clay mineral and zeolite were trenched or scraped to reveal stratigraphic resulting high pH water (pH commonly >9) distributions through time, assuming that all detail. Samples for mineralogical analysis were leads to solubilization of silica and aluminum other factors are equal. collected on a meter to submeter scale in the from relatively unstable mineral phases, which sections, depending on the degree of lithologic subsequently drives chemical oversaturation of STUDY AREA variability. The samples were split visually into the water with stable minerals such as quartz, representative fractions for X-ray diffraction alkali feldspar, and muscovite. In saline, alka- The Pliocene–Pleistocene Lake Tecopa Beds (XRD), petrographic, and scanning electron line lakes, the stable minerals cannot precipitate (Allogroup of Morrison, 1999) of southeastern microscope (SEM) analysis. quickly enough to keep pace with evaporation California (Fig. 1) are a Pliocene–Pleistocene Samples for XRD analysis were initially and hydrolysis; thus, precipitation of metastable basin-fi ll sequence that has been extensively crushed to disaggregate the sample and then zeolite and clay mineral phases from gels par- dissected by late Pleistocene incision of the ground to a fi ne powder using a mechanical tially decreases the chemical energy of the sys- Amargosa River (Morrison, 1991, 1999). Prior mortar and pestle. Bulk sample powder was tem (Dibble and Tiller, 1981). Ultimately, the to middle or late Pleistocene breaching of the quantitatively mixed with crystalline CeO2 stable minerals, such as quartz, alkali feldspar, Tecopa basin, Lake Tecopa was the terminal mineral standard using a 9:1 ratio of sample and muscovite, are believed to replace the ear- lake for discharge of the ancestral Amargosa to standard. Mineral standards for most phases lier formed zeolites and clays. Zeolites are not River during Quaternary and late Tertiary time present in the Lake Tecopa