|Basalt Maps Introduction|
To date, the GWMA subsurface geologic mapping effort has focused on the uppermost CRBG units underlying the GWMA, the Saddle Mountains and Wanapum Basalts. Ongoing, and soon to be completed efforts are mapping the next deeperst unit, the Grande Ronde Basalt. Saddle Mountains Basalt and Wanapum Basalt maps are currently included on the website. Grande Ronde Maps will be added as that work is completed. At this time only the top of the Sentinel Bluffs Member of the Grande Ronde Basalt was mapped for this study.
Stratigraphic nomenclature of the CRBG
Saddle Mountains Basalt
Saddle Mountains Basalt members mapped within the GWMA to-date are, from youngest to oldest, the:
The Saddle Mountains Basalt is not found everywhere beneath the GWMA. It is most widespread in the southwestern portion of the GWMA (Franklin and Grant Counties) – generally south of the Frenchman Hills. In Adams and eastern Franklin Counties, Saddle Mountains Basalt members occur as intracanyon flows that filled paleoriver channels incised into underlying CRBG units.
Wanapum BasaltWanapum Basalt members mapped within the GWMA to-date are, from youngest to oldest, the:
Originally flows of the Wanapum Basalt covered nearly the entire GWMA, except in northern Grant County area where pre-CRBG rocks formed local highlands that were not buried by these flows.
The Eckler Mountain Member of the Wanapum Basalt is known to be present in eastern Franklin County, but its distribution has not been mapped. In this area the Eckler Mountain Member is described as consisting of up to 2 flows of the Basalt of Dodge. Due to the lack of available mapping, and the difficulty in distinguishing Eckler Mountain flows from Frenchman Springs Member flows on driller’s logs, these 2 flows have been included into the Frenchman Springs Member for this study.
The maps produced for this study were built from information found in a variety of data sources describing both surface and subsurface geologic conditions. The largest data source used for the study consists of drillers’ descriptions of geologic materials encountered during water well drilling. This information is recorded on Water Well Reports (drillers’ logs). Additional data sources included borehole geologic logs compiled by geologists for various projects, borehole geophysical logs which have been collected for a number of municipal water supply wells and large private irrigation wells, geologic maps compiled for the region and for specific areas, and regional and local geologic cross-sections. Specific entities and agencies from which these materials were collected include:
In addition to the data sources listed above, personal files and knowledge provided an important source of information. Mr. Tolan and Mr. Campbell’s private data files were used extensively during the project. Dr. Stephen P. Reidel, L.H.G., of Benton City, Washington also contributed geologic logs and information from his personal files that
|Last Updated ( Sunday, 08 March 2009 )|