Paleolimnology and paleontology of the Miocene Quincy Diatomite deposit.

摘要

The Quincy Diatomite deposit, central Washington, USA, is a middle Miocene (∼15 Ma) freshwater lacustrine deposit located between flows of the Columbia River Basalt Group. Three localities along the western margin of the deposit are examined and the diatom flora are described at the species level, with 84 species recognized. Of these species, one is new, Fragilariforma intortus, and one is elevated in rank to the species level, Tetracyclus williamsensii. One species, Pseudostaurosira brevistriata var. subcapitata, is also is transferred to a new genus for taxonomic clarification. Although rare, three species of the genus Sellaphora occur in the Quincy diatomite deposit, making this occurrence the oldest for the genus in the fossil record. The northwestern United States endemic fossil species Ellerbeckia baileyi is also observed in the deposit, and is abundant in particular stratigraphic intervals.;Stratigraphic and geographic variation within the deposit is also examined. The three lithologic units recognized in the deposit, the Four Crude, Bottom Crude, and Top Crude, are distinguished by subtle lithologic differences and contain distinct diatom floras. Textural properties are largely a function of floral differences between units. Diatom assemblages also vary geographically along the western margin of the basin particularly between localities of Top Crude in northern outcrops of the Ancient Lakes Park versus more southerly localities in the Frenchman Hills and Gorge Amphitheater. Distinctions between these assemblages are confirmed through analysis of point count data using nonMetric Multidimensional Scaling and cluster analyses.;Some species within the flora, primarily Aulacoseira granulata and Staurosira construens var. venter, are used as ecologic proxies to interpret the paleolimnology of the Ancient Quincy Lake as an alkaline, eutrophic to hypereutrophic lake with high Total Phosphorus (TP), and sufficient dissolved Silica (Si) levels capable of sustaining a large phytoplankton population.;Three stages of development in lake ecology, directly influenced by the regional geology and geography, are recognized. The earliest stage, represented by the Four Crude, was deposited in a series of small-interconnected pocket lakes that were monomictic, eutrophic, slightly alkaline, and had a depth of greater than 15 meters. This lake stage was succeeded by Bottom Crude deposition and is interpreted to represent the period of maximum shallowing, creating a polymictic lake of less than 10 meters depth and dominated by benthic species. The latest stage of the lake, represented by the Top Crude, represents the deepest and broadest expansion of the lake and shows variable development of lake stratification through its deposition. Tectonic uplift of the western Frenchman Hills Anticline combined with overall subsidence of the Quincy Basin during intermediate and late stages of lake development is interpreted to be the driver for changes in lake bathymetry.