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Sediment Maps Introduction
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Other Quarternary Strata

 In addition to the two main Quaternary units summarized above, a variety of less widespread Quaternary units are found in the GWMA. These strata consist of a mix of silt, sand, and gravel deposited adjacent to slopes and on alluvial fans by debris flows, by fluvial processes along stream courses, and by wind before and after the Pleistocene Cataclysmic Floods. The main localized Quaternary units found within the GWMA are summarized below:

Alluvium: This unit predominantly consists of (1) river deposited silt, sand, and gravel adjacent to modern streams including, but not limited to, the Columbia, Snake, and Palouse Rivers and (2) debris flow deposits on alluvial fans adjacent to topographic highlands like the Saddle Mountains. Alluvium commonly consists of basalt and reworked Pleistocene Cataclysmic Flood deposits which commonly are basaltic. Given this, alluvial deposits may be difficult to differentiate from cataclysmic flood deposits and they may have similar overall physical characteristics. Alluvium can range from matrix rich to matrix poor.

Colluvium: This unit generally consists of rocky and blocky debris found at the base of slopes, including the steep sides of the large hill systems (Saddle Mountains, Beezely Hills, etc.) and coulee walls found in the GWMA. Colluvium generally is derived from weathering and mass wasting of the bedrock comprising the higher slopes of these landform features. Given that basalt forms the predominant bedrock of the region, colluvium in the GWMA is overwhelmingly composed of basalt debris. Except in outcrops where bedforms and its blocky nature can be readily identified, basalt colluvium may be very difficult to differentiate from basaltic cataclysmic flood deposits.

Landslide deposits:  Landscapes marked by hummocky topography are present locally along the slopes of the Saddle Mountains and Frenchman Hills and adjacent to the Columbia River. These landscapes are the products of large landslides off the adjacent uplands. Consequently, these deposits consist of a mix of material derived from the adjacent uplands, most commonly basalt debris mixed with loess and flood deposits.

Quaternary sand dunes: This unit typically consists of unconsolidated and uncemented, Holocene-aged (post-Cataclysmic Flood), very fine- to medium-grained sand that forms both active and stabilized sand dunes. Quaternary dune sand generally is much coarser than Palouse Formation loess. Quaternary sand dunes are common around Moses Lake, Potholes Reservoir, and in east-central Franklin County.  Small localized accumulations (covering a few acres) and thin (<3 feet) surface layers of dune sand (both active and inactive) can be found almost anywhere in the GWMA. The other Quaternary deposits typically are localized and generally thin, and except where outcrops are present they may be difficult to differentiate from the two predominant Quaternary units summarized previously.

Plio-Pleistocene Strata

Plio-Pleistocene StrataPlio-Pleistocene strata consist of a range of lithologies similar to the younger Quaternary strata, but generally more cemented and indurated. Regionally, Plio-Pleistocene strata can be divided into four basic types (or facies): 1) calcium carbonate‑rich strata (also referred to as caliche), 2) basaltic alluvial deposits, 3) stratified silt, and 4) massive silt. Of these, the later three usually can be distinguished from younger deposits if outcrops are available for examination. However, without good maps or outcrop descriptions it is very difficult to identify these strata. For this report they are not identified. Only caliche was found to be easily recognizable in the primary source of information used for subsurface mapping for this project, water well drillers logs.

Caliche, generally a pedogenic calcium carbonate deposit, is a common lithology found across many of the upland areas of the GWMA that were not deeply eroded by Pleistocene Cataclysmic Floods. However, caliche generally has not been mapped by previous investigators and it is not shown on existing geologic maps. For this project, caliche was mapped because it forms a generally easily recognized marker horizon on most water well drillers well logs.

Caliche found in the GWMA usually consists of a thin, less than 10 foot (<3 m) thick, sequence of laterally discontinuous calcium carbonate (Figures 10 and 11) developed on, and incorporating material derived from, older fluvial, alluvial, eolian, colluvial, and lacustrine deposits and bedrock (basalt) units. Where present (e.g., where not removed by Pleistocene Cataclysmic Flood erosion) the unit is most commonly developed on either the Ringold Formation or Columbia River basalt. Individual horizons within the unit are laterally discontinuous, and alternating carbonate-rich and carbonate-poor intervals are common. The unit itself can be very widespread, being found across upland areas beneath thin veneers of wind blown sand and in some lowland areas where flood erosion has not deeply incised into underlying strata, most notably the Quincy Basin.

Last Updated ( Thursday, 19 March 2009 )
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