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Detrital zircon assemblages in sandstones indicate revised late Early to Late Cretaceous age for the Dothan Formation in southwestern Oregon
Wiley, T.J. 1, McClaughry, J.D. 2 (speaker), Rivas, J.A.3, and Schwartz, J.J4.
1 Oregon Department of Geology and Mineral Industries, Retired
2 Oregon Department of Geology and Mineral Industries, Baker City Field Office
1995 3rd Street, Suite 130, Baker City, OR 97814
3 Department of Geology and Geography, University of North Carolina Wilmington
601 South College Road, Wilmington, NC 28403-5944
4 Department of Geological Sciences, University of California, Northridge
Northridge, CA 91330-8266
The Dothan Fm. is a deformed flysch sequence that is exposed as broken formation in southwestern Oregon. Max-depositional ages recently determined from 206Pb/U238 LA-ICPMS dating and statistical analysis of 537 detrital zircon grains from Dothan Fm. sandstone range from 102 to 74 Ma (Early-Late Cretaceous), significantly younger than sparse Late Jurassic-Early Cretaceous fossils previously reported. Zircon separates from Dothan Fm. sandstone were prepared for five widely dispersed inland and coastal sites. Inland exposures within the Dothan "type" area along West Fork Cow Creek return max-depositional ages of 74 Ma and 78 Ma respectively (2 SCJ 13, UTM NAD83 443879E, 4739240N; 1 SCJ 13, 449210E, 4739076N). West of the “type” area, in the Oregon Coastal Range, Dothan Fm. sandstone returns a slightly older max-depositional age of 95 Ma (11 SCJ 13, 436831E, 4716884N). Coastal exposures of Dothan Fm. sandstone has similar max-depositional ages to those of inland exposures returning ages of 89 Ma at the Port Orford Marina (53 SCJ 13, 377221E, 4732986N) and 102 Ma near Brookings (215-11-1, 395965E, 4658639N). The contrast between previously reported Late Jurassic-Early Cretaceous fossil and Early-Late Cretaceous zircon ages likely relates to 1) inclusion of older, adjacent sections of Buchia bearing flysch in the Dothan Fm., such as those in the Sixes River terrane and those collectively referred to as "Galice" Fm. in nearby Western Jurassic terranes; 2) inclusion of older blocks of chert and flysch in olistostromes; and 3) inclusion of older blocks in tectonic shear zones that cut the Dothan Fm.
Widespread development of broken formation in the Dothan contrasts with broad folding developed in early Eocene sedimentary rocks that overlie it and in late Paleocene Siletz River Volcanics exposed north of the Canyonville Fault. Intense deformation of the Dothan Fm. is likely pre-late Paleocene and post-Late Cretaceous. This, and the absence of Paleocene rocks elsewhere in southern Oregon, suggests a Late Cretaceous minimum age for the Dothan Fm. Other late Early to Late Cretaceous submarine fan sequences in southern Oregon include the Hornbrook Formation that crops out inland in the Medford and Grave Creek areas and the upper part of the Days Creek Formation (Myrtle Group) that crops out north of the Canyonville Fault.
Jason McClaughry is the Eastern Oregon Regional Geologist, Earth Science Section Supervisor, and coordinates the statewide geologic mapping program for the Oregon Department of Geology and Mineral Industries (DOGAMI). He has worked extensively across the state mapping and researching a number of volcanic provinces. Jason holds a M.S. degree in Geology from Washington State University and a B.S. degree in Geology from the University of Puget Sound.
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