Are age dates on carbonate cements in Pasco Basin regionally relevant?
February 29, 2020
Uranium-thorium is a radiometric technique for dating carbonates. Paces (2014) estimated the ages of secondary carbonate cements in the southern Pasco Basin using U-series methods as part of a probabilistic seismic hazard assessment of the U.S. Department of Energy's Hanford Site. Ages on calcic paleosols (marker beds) help geologists constrain Quaternary deformation rates in the Yakima Fold Thrust Belt.
The material sampled for analysis was carbonate-silica rinds on cobbles and cemented fault gouge collected from five sites near Richland, WA. Minimum ages for the 39 samples fell between 17 ka and >500 ka, dates consistent with Late Pleistocene cataclysmic flooding and local Middle Pleistocene alluvial fan activity.
Thick calcium carbonate rinds on cobbles. These examples are from Idaho.
Uranium–thorium dating has a reliable upper limit of somewhat over 500,000 years, which some of the samples exceeded (Steptoe Quarry and Finley Quarry sites). The Th230/U234 method is typically used to date materials that retain U and Th (i.e., exhibit closed system behavior) such as teeth or stalagmites, which build in concentric layers. In this investigation, the innermost layer of the CaCO3 rinds was targeted (earliest cemented layers) as were vertically-layered cements overprinting gouge sediment in a fault (earliest layers not obvious). Gouge zones in the basalt commonly show calcium carbonate overprint (i.e., Galster, 1987, p.3).
Results from the Paces (2014) investigation indicate that secondary (pedogenic) carbonates formed in the southern Pasco Basin during the Late Pleistocene, Middle Pleistocene, and probably earlier. Secondary carbonate continued to be added during the Late Pleistocene, though the authors note their limited study "was not designed to address the history of pedogenesis or climate change at any of these sites". The study also found evidence for repeated faulting at Finley Quarry during the Middle Pleistocene.
The study provides important new age information and, therefore, has value beyond the operational needs of the Hanford Site.
My question is: Are these ages representative of calcic paleosols found throughout the larger region?
Samples collected for the study were from 5 sites clustered in one small area, though caliche layers and calcrete-bearing intervals are numerous and widespread across tens of thousands of square kilometers. Also, these 5 sites are the same used by Hanford geoscientists in many, many, many publications over the decades. While I agree it can be difficult to leave the lovely, influential, and trendsetting confines of Richland, WA for reasons involving mere field geology, it might prove illuminating (if scary) once in a while.
Are sample sites near Richland representative of flood deposits in Eastern Washington?
Many of the study samples were collected from Missoula flood deposits near Richland, WA. Blue areas of the map shows the larger Ice Age floodway where such deposits are found. The blue region is >25,000 km2.
Do the sample sites capture the multi-story nature of calcic paleosols in Pasco Basin?
Calcic paleosols are common in Plio-Pleistocene age sediments that unconformably overly the Ringold. Caliche horizons number in the dozens on the Palouse. Colorful areas show the mapped extent of the Ringold Formation in Grant, Franklin, and Adams Counties. Base image by Triangle Associates, Inc.
Do sample locations adequately characterize Quaternary faults in the region?
Map showing Quaternary faults (black lines) including those of the Yakima Fold Belt, CLEW, Hite Fault, and the Arlington-Shutler system. Blue areas highlight major valleys of the region. It would take more than 3 hours to drive east to west across the map area.
The answer is, of course, probably not. While Paces was not commissioned to conduct a regional study, the authors (or the client) do have a responsibility to provide appropriate geographic context to their work (i.e., a map), especially given the widespread nature of the "paleosurfaces" they investigated. The maps above took me 30 minutes to produce and provide the spatial context missing from the article. Certainly, it is not too much to ask. Walk down the hall and talk to your GIS person. Or just call me.
This review of Paces (2014) article is illustrative of a recurring complaint from geologists who work in Eastern Washington. Hanford geoscience staff have an annoying habit of conducting investigations in one small portion of Eastern Washington and implying their results have regional relevance. Sometimes they do, but many, many, many times they do not. Eastern Washington has two geologic histories, one written inside the Hanford fenceline in service of operational priorities and one written outside it by geologists in service of the public.