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We describe the depositional environments of the Cache, Lower Lake, and Kelseyville Formations in light of habitat preferences of recovered mollusks, ostracodes, and diatoms. Our reconstruction of paleoenvironments for these late Cenozoic deposits provides a framework for an understanding of basin evolution and deposition in the Clear Lake region. The Pliocene and Pleistocene Cache Formation was deposited primarily in stream and debris flow environments; fossils from fine-grained deposits indicate shallow, fresh-water environments with locally abundant aquatic vegetation. The fine-grained sediments (mudstone and siltstone) were probably deposited in ponds in abandoned channels or shallow basins behind natural levees. The abandoned channels and shallow basins were associated with the fluvial systems responsible for deposition of the bulk of the technically controlled Cache Formation. The Pleistocene Lower Lake Formation was deposited in a water mass large enough to contain a variety of local environments and current regimes. The recovered fossils imply a lake with water depths of 1 to 5 m. However, there is strong support from habitat preferences of the recovered fossils for inferring a wide range of water depths during deposition of the Lower Lake Formation; they indicate a progressively shallowing system and the culmination of a desiccating lacustrine system. The Pleistocene Kelseyville Formation represents primarily lacustrine deposition with only minor fluvial deposits around the margins of the basin. Local conglomerate beds and fossil tree stumps in growth position within the basin indicate occasional widespread fluvial incursions and depositional hiatuses. The Kelseyville strata represent a large water mass with a muddy and especially fluid substrate having permanent or sporadic periods of anoxia. Central-lake anoxia, whether permanent or at irregular intervals, is the simplest way to account for the low numbers of benthic organisms recovered from the Kelseyville Formation. Similar low-oxygen conditions for benthic life are represented throughout the sedimentary history of Clear Lake. Water depths for the Kelseyville Formation of 10 to 30 m and 12 m near the margins of the basin are inferred both before and after fluvial incursions. These water-depth fluctuations cannot be correlated with major climatic changes as indicated by pollen and fossil leaves and cones; they may be due to faulting in this technically active region.

Modern-day Clear Lake is a turbulent, turbid, permanent, polymictic lake. It is also a highly productive fresh-water lake whose dominant solutes are Mg 2+ + Ca 2+ – HCO 3 − . The lake has a diverse and abundant limnetic community, yet has a depauperate benthic community. The benthic community structure appears to be ecologically simple as the result of turbulence-induced substrate instability coupled with unpredictable periods of anoxia induced by the oxygen-consuming organic matter. Ostracodes, which are ubiquitous, largely benthic, environmentally sensitive, and diverse organisms, are represented in Clear Lake only by the nektic species Cypria ophtalmica . The substrate conditions provide adequate reason for the absence of most ostracodes from the modern lake, but their absence in the fossil record suggests that the modern lacustrine environment existed in the past despite known climate changes. This seeming paradox can be explained by considering the influence of various types of climatic change on the lacustrine environment; certain types of climate-environmental changes would maintain a lacustrine environment unsuited to ostracodes. These hypothesized climate-lacustrine environmental changes would favor the modern (Holocene) and other oak-dominated periods to be the warmest and driest in Clear Lake history, whereas the pine-TCT (Taxodiaceae, Cupressaceae, and Taxaceae) periods would be cooler and wetter than today. The largely barren ostracode record, coupled with rare ostracode occurrences, would support a glacial-interglacial Clear Lake climatic history characterized primarily by changes in the annual precipitation-evaporation budget.

Abstract The fossil record of continental organisms is an important source of paleoclimatic information. Extant taxa provide a way to transfer both qualitative and quantitative climatic information to the fossil record (Delorme and others, 1977; Bartlein and others, 1984). Terrestrial taxa such as insects and especially plants are traditional sources of paleoclimatic information, because their life cycles are directly related to climate (see chapters in The Biological Record on the Continent, this volume). By contrast, aquatic organisms are not traditional sources of paleoclimatic information, because their ecology necessarily is determined in part by hydroenvironmental parameters and because their life cycles must therefore be related to parameters at least one step removed from climate. Thus lacustrine sediments, which typically have the greatest stratigraphic continuity of all nonmarine sediments, are largely sought out only as repositories of terrestrial fossils. Unfortunately this approach ignores the climatic record of the lake itself. Lakes, especially those in hydrologically closed basins, are a valuable source of paleoclimate records. Paleoshore deposits define paleolake levels that reflect changes in the local paleohydrologic budget. This approach dates from the classic study of Gilbert (1890) and is used today over large geographic areas (Street-Perrott and Harrison, 1985). Lake-level variation is an important tool for interpreting paleohydrography, especially when it is applied to many lakes so as to identify local nonclimatic variability. This form of paleolake-level information, however, depends on the ability to date discrete paleoshore deposits, is incremental in nature, usually omits lower-than-modem levels, and can often only be applied to the last lake cycle. Recent studies (e.g., Lerman, 1978) demonstrate that lakes are complex chemical and physical systems in which parameters such as water temperature and hydrochemistry are often coupled with local climate. When a record of these parameters is preserved in lake sediments, it provides a continuous record of climate. Because hydroenvironmental parameters are important limiting factors for aquatic organisms, their stratigraphic record in lake sediments becomes an important source of paleoclimatic information.