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Open-File Report 2005–1205
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Sedimentary Features of Cores BL96-1, -2, -3, Bear Lake, Utah−Idaho

By Joseph P. Smoot

thumbnail image of figure 1 in report: map of Bear Lake

Sedimentary analysis of cores from Bear Lake emphasizes textural characteristics that can be observed continuously, rather than by sample-dependent analytical techniques. The textural observations emphasize characteristics such as layering, bioturbation features, and distribution of grain sizes and grain types (Smoot, 1998; Smoot and Benson, 1998; Kemp and others, 2001; Last, 2001). The sedimentary analysis not only constrains the variability of the depositional environment, but also provides important information about the nature of the materials analyzed biologically and chemically. The origin or significance of the sedimentary features observed may be inferred by comparison to features from other lakes or from basic mechanical, chemical, or biological principles (Lomando and Schreiber, 1994; Smoot and Benson, 1998). More often, the sedimentary features need additional constraints from chemical or biological analyses in order to distinguish among several possible origins.

Three Kullenburg piston cores (Glew and others, 2001) were collected along an east-to-west transect in 1996. The cores were cut into meter-long segments and each segment was split in half lengthwise for sampling and archival purposes. The archive half was cleaned with a straight razor, described with a 10-power hand lens, logged, and photographed with 35-millimeter slide film in 14-centimeter (14 cm) overlapping segments. The photographs for each core segment were scanned at 3,000 dpi (dots per inch), stored on compact discs (CDs), and then combined into a composite core photograph using Adobe Photoshop v.4.2. The composite photographs and log descriptions were used to produce detailed graphical core descriptions. A 1-cm-thick slab was cut from each archive core segment for x-radiography in a Picker Minishot 1 using 17-inch Agfa Structurix D4DW film. The x-radiographs were described and logged with a 10-power hand lens to produce the sections presented here.

Smear slides were collected at 10-cm intervals and at visible changes in sediment character. A subsequent set of smear slides was collected where changes in fabric are visible in the x-radiographs. The smear slides were examined using a Zeiss binocular petrographic scope with 10-power occular and 40-power objective lens. Several smear slides were counted along a ten-micron grid for variability in diatoms and grain types. Three hundred diatoms were counted on each slide, whereas grain types were estimated by using a shadow diagram (Terry and Chilingar, 1955). A few intervals were washed in a wet sieve to examine sand-sized material using a Nikon Zoom binocular scope at 40-power resolution. No attempt was made to identify genera or species of shells observed in this manner. Polished thin sections were made at select intervals from chips imbedded with Spurr resin as described in Kemp and others (2001). These were examined by petrographic microscope and Scanning Electron Microscope.

Version 1.0

Posted August 2005

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