|Sediment Maps Introduction|
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A widespread sequence of indurated clay, silt, sand, and gravel (conglomerate) is found in the lowland areas common to the western and southern portion of the GWMA, especially in Franklin and Grant Counties. These indurated sediments are mainly found in the Pasco Basin south of the Saddle Mountains, between the Saddle Mountains and Frenchman Hills (in a structural low designated here the Othello Basin), and in the Quincy Basin between the Beezley Hills and Frenchman Hills (Figure 1). These sediments typically consist of well-stratified siltstone/claystone, pedogenically altered mudstone and sand, felsic fine‑ to coarse‑grained sand, and multi‑lithologic, variably indurated granule‑to‑cobble. These strata are assigned to the approximately 10.5 to 3 million year old Ringold Formation.
The Ringold Formation is divided into three informal members, or map units (Figure 2), each dominated by different facies associations. These members record the evolution of the ancestral Columbia River system in the GWMA region, and are referred to as the:
Member of Savage Island: This member consists predominantly of planar-tabular, lacustrine sand and silt with diatomaceous intervals. This member records deposition in a series of lakes that formed near the end of Ringold deposition between 5 and 3 million years ago. The member of Savage Island is inferred to overlie both other Ringold members throughout the GWMA.
Member of Taylor Flat: This member consists predominantly of sand found in elongated channel deposits intercalated with large silt paleosol intervals. It is interpreted to record deposition predominantly in sand-transporting river channels migrating across muddy flood plains. This member may be widespread across the GWMA region and it may be as old as 10 million years (or more) to as young as 3 to 4 million years old. Much of the Ringold Formation mapped in Grant County and Adams County probably belongs to this member.
Member of Wooded Island: This member consists predominantly of gravelly strata with intercalated sand and silt paleosol intervals. The member is interpreted to reflect deposition in gravel-transporting river systems and adjacent flood plains and overbank areas. These rivers are thought to have formed broad, braided channels migrating across a gravelly plain. The member occurs in western Franklin and southern Grant Counties, forms the majority of the lower half of the Ringold Formation in Franklin County, and comprises much of the oldest part of the Ringold, generally being more than approximately 5.5 million years old.
Generally, Ringold Formation units thin and become more clay- and silt-rich to the northeast and east across the GWMA. This trend suggests the river system(s) that deposited the sand and gravel portions of the Ringold Formation become more common from the eastern to the western and central part of the GWMA region.
|Last Updated ( Thursday, 19 March 2009 )|