A NEW AGE CONSTRAINT ON LATE JURASSIC MAGMATISM AND SEDIMENTATION IN THE COON HOLLOW FORMATION (WALLOWA TERRANE)

The Coon Hollow Formation (CHF) is exposed in the Pittsburg Landing area of the lower Snake River canyon north of Riggins, Idaho. These fluvial to marine clastic sedimentary strata are deposited onto volcanic and volcaniclastic units of the Late Permian-Middle Jurassic Wallowa accreted terrane, which stretches across the Blue Mountains of eastern Oregon and western Idaho. A sample was collected from a prominent member of the hypabyssal andesite dike and sill complex intruding the lower fluvial and upper marine units of the CHF, in order to provide a minimum age constraint on the deposition of these sedimentary rocks, and a maximum age for subsequent faulting. The age of crystallization of this andesite was obtained from magmatic titanite and apatite using U-Pb isotope dilution thermal ionization mass spectrometry.

A five-point isochron calculated from multi-grain fractions of low-uranium titanite and apatite yielded a crystallization age of 152 +/- 2 Ma (2σ). The new result places this intrusion as younger than depositional ages of 196.6 Ma for the red tuff unit of the lowermost CHF and 159.9 Ma for a lapilli tuff in the fluvial unit of the CHF [1]. This new minimum depositional age constraint is irresolvable from detrital zircon maximum depositional ages of 154 ± 2 Ma from the uppermost fluvial unit and 150 ± 2 Ma from the uppermost marine beds of the CHF at Pittsburg Landing. Our intrusive age thus constrains the deposition of the fluvial and marine sediments of the CHF to a restricted interval between 160 and 150 Ma.

The age of this shallow intrusive complex in the CHF is at the younger end of the range of 162 to 154 Ma ages for mafic-intermediate plutonism within the adjacent Baker terrane [2]. CHF deposition is also coeval with deformation bracketed between 160 and 150 Ma throughout the Blue Mountains [3]. We thus ascribe to previous models linking the development of this basin with contractional tectonics and magmatism during Late Jurassic arc-arc collision.

[1] LaMaskin et al. (2015) J Geol 123:233–267. [2] Schwartz et al. (2011) Lithos 126:22-41. [3] Schwartz et al. (2011) GSA Bulletin 123:2083-2011.