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.

Fossil diatoms from a 177-m core (CL-80-1) taken near the center of the main basin (Upper Arm) of Clear Lake, California, provide evidence about the stratigraphie relationships, age, and environmental history of these lacustrine deposits. In general, diatom assemblages from the core are dominated by planktonic genera such as Stephanodiscus , Cyclotella , and Melosira . Shallow-water species of Fragilaria and Amphora are common and sometimes abundant. Several planktonic diatoms from the core are also found in the Kelseyville Formation, which is exposed on the southern margin of Clear Lake and is inferred to underlie the modern lacustrine deposits. The presence of these taxa in the same stratigraphie order in both the Clear Lake core and the Kelseyville Formation suggests partial correlation between the two and implies a relationship between the Kelseyville Formation and the lacustrine sediments beneath Clear Lake. In the upper 50 m of core CL-80-1, diatom assemblages apparently reflect late Pleistocene and Holocene paleoenvironmental changes, although their environmental significance may be obscured by reworking of diatoms from older sediments, by tectonically caused changes in patterns and rates of sedimentation, and by the impact of volcanism. Nevertheless, the diatoms indicate that lacustrine environments have been characterized by fresh, moderately deep, nutrient-rich water throughout much of their sedimentary history. Cooler climatic and lacustrine environments of the late Pleistocene were characterized by a codominance of Stephanodiscus and Melosira species, implying a mesotrophic to eutrophic, stratified lake. After the change from Pleistocene to Holocene climates, Clear Lake became yet more eutrophic and turbid. Stratification was short-term and irregular, and warm-water conditions extended throughout a greater portion of the growing season although there is evidence for a middle Holocene return to cooler and moister conditions. The modern limnology of Clear Lake, which is characterized by massive blooms of blue-green algae and by the abundance of Melosira granulata , apparently began about 15,000 years ago.

Clear Lake, California, lies in a volcano-tectonic depression that received nearly continuous lacustrine deposition for the past 500,000 yr and probably longer. The lake has been shallow (<30 m) and eutrophic throughout its history. Sediments beneath the floor of the lake are fine grained (chiefly >7.0φ) and contain fossils of a large lacustrine biota, as well as a pollen record of land plants that lived in the basin. The sediments also contain tephra units of local and regional extent. The ages of the sediments in Clear Lake are determined from radiocarbon dates on the sediments, from correlation of regionally distrbuted tephra units, and from inferred correlation of oak-pollen spectra with the marine oxygen-isotope record. From the chronology of events recorded in the cores from Clear Lake, the late Quaternary history of the lake can be deciphered and the sediments correlated with other basins in northern California. Comparison of cores from Clear Lake with strata of the Kelseyville Formation, exposed south of the main basin, suggests a general northward migration of lacustrine sedimentation, which in turn suggests a northward tilt of the basin. Migration of the lake was a response to volcanism and tectonism. Volcanic rocks erupted from Mt. Konocti, on the southern margin of the lake, and displaced the shoreline to the west and north. Clear Lake is bounded by faults that are part of the San Andreas fault system. These faults strongly influenced the position, depth, and longevity of Clear Lake. Movement on these boundary faults deepened the Highlands and Oaks arms of the lake about 10 ka. Tectonic movement accompanying faulting was probably also largely responsible for hiatuses in the deposits beneath Clear Lake about 17 and 350 ka that have been inferred from the subbottom stratigraphy of the lake. Climate change, although responsible for large variation in the composition of the terrestrial flora of the Clear Lake drainage basin, has not influenced the areal extent, depth, or position of the lake.

Abstract In this paper, we consider the primary controls on gas and liquid geochemistry at The Geysers geothermal field (California) prior to reservoir exploitation and reinjection programs. Well discharges vary considerably in steam/gas ratio, gas composition, and dD and δ 18 O of steam. Many of the variations can be linked to the degree of liquid saturation or steam fraction (Y) within the reservoir. Discharged fluids from the central Northwest Geysers have low molar steam/gas (<200) and are produced from reservoir vapor because little condensed liquid water appears to exist in that part of the system (i.e., they are high Y fluids). The gas is relatively uniform in composition, typically with ~60 mol percent CO 2 and around 10 mol percent NH 3 + CH 4 on an H 2 O-free basis. N 2 /Ar ranges to values >500. Discharges from the central Northwest Geysers are interpreted to contain a mixture of connate and metamorphic gases derived from high-temperature breakdown of carbon- and nitrogen-bearing metasediments, either within or below the geothermal reservoir. Input of volcanic gas from underlying intrusions appears to be present but minor. The gas-rich end member is less evident in the Southeast and Central Geysers where discharged fluids consist primarily of steam boiled from condensed reservoir liquid (i.e., they are low Y fluids). Molar steam/gas in these parts of the field commonly exceeds 3,000; N 2 /Ar approaches that of airsaturated meteoric H 2 O (~38). Isotopes within reservoir steam (dD and δ 18 O) are only slightly shifted from local meteoric waters. Reservoir gases in the Southeast and Central Geysers are thus diluted by the dominant input of meteoric water, which disguises the connate and/or metamorphic signature of the gas. The resulting small proportion of gas is highly variable in composition.