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The John Day Formation of north-central Oregon preserves a succession of speciose, superposed Oligocene through early Miocene mammalian faunas that establish the sequence of mid-Cenozoic mammalian evolution within the Pacific Northwest. Upper John Day rock units initially described by Merriam (1900, 1901) in the Kimberly and Haystack Valley areas were later divided into lower (Kimberly) and upper (Haystack Valley) members by Fisher and Rensberger (1972). We focused our study on the lithostratigraphic succession within the Haystack Valley Member. Rocks previously included in the Haystack Valley Member can be subdivided into four unconformity-bounded, genetic lithostratigraphic units that range in age from ∼24 to ∼18 Ma, three of the units incorporating age-diagnostic mammalian faunas.
We have identified two principal depositional units within the Haystack Valley Member of Fisher and Rensberger south of Kimberly: (1) Johnson Canyon Member—late or latest Arikareean tuffaceous siltstones and fine sandstones (∼?19–22.6 Ma) with fluvial monomictic intraformational pebble gravels, well exposed along the west wall of the John Day valley; (2) Rose Creek Member—coarse polymictic welded tuff-bearing gravels, debris flows, coarse obsidian-shard tuffs, and fine-grained tuffaceous units, yielding early Hemingfordian mammals (∼18.2–18.8 Ma), deposited in angular unconformity on lower units of the John Day Formation along the east wall of the John Day valley.
At Balm Creek in the type area of the Haystack Valley Member, the southern limb of the Balm Creek syncline exhibits the most complete local section of upper John Day rocks, here comprising three members: (1) a revised Haystack Valley Member made up of early late Arikareean ribbed tuffs (∼23.5–23.8 Ma) with monomictic welded tuff conglomerate channels, overlain by a gray massive airfall marker tuff (GMAT); (2) Balm Creek Member—tuffaceous late Arikareean siltstones and fine sandstones interbedded with lacustrine tuffs, overlain by stacked fluvial fining-upward sequences and gray airfall tuffs; (3) Rose Creek Member—coarse polymictic welded tuff-bearing gravels, debris flows, lacustrine units, and fine-grained tuffaceous sediments, believed to correlate to the fossiliferous early Hemingfordian unit south of Kimberly.
The complexity of upper John Day rocks (evidenced by marked lithofacies variation within multiple unconformity-bounded subunits, punctuated by numerous paleosols) suggests an early Miocene depositional regime with more varied local environments and pronounced episodic sedimentation relative to the more uniform Oligocene environments documented by lower John Day strata. Whereas the lower John Day Formation consists of fine-grained volcaniclastic sediments that were deposited in basins with minimal topographic relief, the upper John Day Formation is characterized by a succession of increasingly coarse fluvial channel fills, as well as massive airfall and coarse-shard tuffs, well-developed paleosols, and relict topography. Regional compression appears to have triggered fluvial incision and valley filling from ∼24 Ma to at least ∼19 Ma. Extension in latest John Day times resulted in the development of half-grabens or grabens both in Haystack Valley and south of Kimberly, ensuring the preservation of upper John Day sediments. Significantly, a final episode of normal faulting appears to have immediately preceded the earliest eruption of Picture Gorge Basalt, as evidenced by flows of the Twickenham Member (PGBS) abutting against vertical fault scarps south of Kimberly. The faunas of the upper John Day units thus play a critical role in dating a complex sequence of tectonic events which preceded the onset of Columbia