Sediment Maps Introduction PDF Print E-mail
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Sediment Maps Introduction
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This section presents a summary of the primary sedimentary units mapped during this part of the GWMA subsurface mapping effort.. This summary is subdivided into sections describing various Quaternary units, Plio-Pleistocene strata, and the Mio-Pliocene Ringold Formation.

During the Quternary Period (approximately 2 million years ago to the present) the GWMA region experienced a variety of geologic events. These included the formation of the modern river drainages (commonly in pre-existing channels), cutting of the unique coulee landscape that characterizes so much of the region, formation of the caliche hardpan common to many upland areas (commonly superimposed on older caliche), accumulation of vast fields of wind-blown silt and sand (loess and sand dunes), and the continued slow uplift of the major east-west ridges that dominate the skylines of the western part of the GWMA. As one might surmise from this history, the strata formed in association with these events can have complex relationships, and in many cases are difficult to differentiate. Nevertheless, it is possible to reliably identify strata associated with these different events, group them into map units defined on the basis of unique physical characteristics, and use this information to interpret the subsurface distribution of these strata.

Publicly available geologic literature provides the information needed to identify and map Quaternary units throughout the GWMA. Using this information, the two most widespread Quaternary units are the silt, sand, and gravel of the Pleistocene Cataclysmic Flood deposits and the silt and fine sand (loess) of the Palouse Formation.

Largely uncemented, typically poorly indurated, well-stratified interbedded silt and sand, sand, gravelly sand, and pebble to boulder gravel is present across much of the region. Published geologic maps and reports show these materials form the uppermost geologic unit beneath many of the flat lowland areas common in the western part of the GWMA and they partially fill many of the coulees which cut across the eastern part of the GWMA. These strata have been interpreted as having been deposited by the Pleistocene Cataclysmic Floods that were released from glacial Lake Missoula periodically between approximately 1,000,000 and 12,000 years ago.

Where present, Pleistocene Cataclysmic Flood deposits range from a few feet (<1 m) thick to more than 200 feet (+60 m) thick.  Pleistocene Cataclysmic Flood deposits, also known informally as the Hanford formation in the southern part of the GWMA in the Pasco Basin, are commonly divided into three basic sediment types (facies) which commonly form the basis for subdividing the flood deposits into map units.

Intercalated, well stratified silt and fine‑ to coarse‑grained sand forming normally graded (fining upwards) beds that range from inches (few centimeters) to several feet (tens of centimeters) thick. This facies, also known as Touchet beds, is not widespread in the GWMA, being found predominantly on highland surfaces (ridges) and localized in small valleys tributary to the larger coulees.

Light layers correspond to silty strata

Laminated to massive, uncemented, felsic to basaltic, fine- to very coarse-grained sand. These sands can contain thin, lenticular silt to fine gravel interbeds. Where the silt content is low, a well‑sorted and open-framework texture is common. Where the basalt content in these deposits is high, they are often referred to as “black sands” because of the dark gray to black color caused by the high basalt content. This facies forms thick sheets beneath many of the broad, flat, lowland land surfaces in parts of Grant County (near Mattawa and the western Quincy Basin) and Franklin County (north of Pasco).

Well-stratified to massive, uncemented, unweathered, mixed lithology (although basalt content is usually high) pebble to boulder gravel. Interstitial matrix in this facies generally ranges from absent to predominantly coarse sand and granules. Where these strata contain little or no matrix sand they commonly have an open‑framework texture where open intergranular pores are readily apparent to the unaided eye. This facies may be locally muddy, although such fine content does not appear to be widespread. Flood-deposited gravel commonly is referred to and mapped as the “Pasco gravel”. Flood gravel facies are found most commonly on the floors of Scabland coulees (including Cow Creek, Lind, Esquatzel, and Washtucna Coulees, and Crab Creek), the mouths of coulees (such as the Banks Coulee), comprising large terraces and flatlands (e.g., in the Mattawa, Quincy/Ephrata, Adams Dryland, and Moses Lake areas), and along the tracts of the Columbia and Snake Rivers. Also, this facies commonly is found interstratified with the sand facies.


 Clastic dike




Another feature common to the flood deposits is clastic dikes.  Clastic dikes usually consist of alternating vertical to subvertical layers of silt, sand, and granule gravel less than 0.5 in (~1 cm) up to 6 ft (2 m) thick.

Clastic dikes typically cross‑cut bedding, although they do locally parallel bedding.  Where dikes intersect the ground surface, a polygonal feature visible from the air and known as “patterned ground” is observed.

Clastic dikes are generally best developed in the interstratified silt and sand facies and to a lesser extent in the sand-dominated facies.






Palouse Formations

 Palouse formations

The rolling hills adjacent to the coulees cross-cutting northern Grant County, eastern Franklin County, and the majority of Adams County are composed of a sequence of massive to poorly stratified, light colored, silt and very fine sand (Figure 9). These strata commonly are pedogenically altered (e.g., display evidence of soil forming processes, including animal burrow and root casts), in some areas contain air fall ash, and display evidence of multiple, stacked and superimposed soil horizons reflecting subtle changes in climate and erosion conditions in the region during the Quaternary. Caliche can be present in these strata. These strata, commonly referred to as loess, comprise the Palouse Formation or Palouse loess.

The fine-grained material comprising the Palouse Formation generally is thought to consist of glacial “rock flour”. The source of this rock flour is thought to be the Cordilleran and Continental ice sheets. This rock flour was transported and deposited in the GWMA region by glacial melt waters (possibly including cataclysmic floods). Following deposition, the rock flour was reworked (transported) and deposited by wind across much of the GWMA region. Airfall ash found intermittently within the loess came from volcanic eruptions in the Cascade Range. Caliche, where found in the loess, suggests semi-arid conditions periodically occurred throughout the GWMA in the Quaternary.

Across the GWMA, loess deposits generally are absent in scabland coulees and modern active river valleys. Because scabland coulees incise through the loess, and commonly into underlying basalt bedrock, much of the loess found in the Columbia Basin predates the later stages of the Pleistocene Cataclysmic Floods and resulting scabland erosion. Loess deposits adjacent to eroded scabland features may range from less than 1 ft (<0.3 m) to more than 225 feet (+75 m) thick in many of the dryland farming areas of the eastern GWMA. In the Columbia Basin, Baker and others (1991) identify multiple loess units that range in age from more than 1 million years in age to less than 10,000 years in age.

Last Updated ( Thursday, 19 March 2009 )
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