Late Eocene–early Oligocene tectonism, volcanism, and floristic change near Gray Butte, central Oregon

Abstract

Mid-Tertiary strata near Gray Butte, central Oregon, record volcanism and sedimentation on the margin of a west-tilted basin that was subsequently disrupted by a northeaststriking fault system. Compositional characteristics of the volcanic rocks support correlation of the section to the upper Eocene‐Oligocene part of the John Day Formation. The ~1.2-kmthick section contains five fossil floras documenting climatic change in late Eocene‐early Oligocene time and a progression between better known Eocene and Oligocene floras in the region. The presence of the transitional floras is a consequence of the subsidence of the Gray Butte basin to accommodate a section that is nearly four times thicker than better studied correlative strata ~50 km to the northeast that consist almost entirely of volcanic rocks. The lowest flora is within flood-plain facies, whereas the other four are hosted in lacustrine strata. Although alteration of volcanic rocks most closely associated with the floras precludes development of a precise isotopic-age chronology, regional correlations and several accurate isotopic-age determinations indicate that the principal interval of climatic cooling may have been in late Eocene time (ca. 38‐39 Ma) rather than at the Eocene-Oligocene boundary. The paleoclimate interpretation is tempered, however, by the low diversity of the floras (12‐24 species) and possible taphonomic biases in comparing flood-plain and lacustrine environments. Mapping established the presence of the Cyrus Springs fault zone, a large (1.2 km vertical displacement, possibly >7 km dextral offset) shear zone that is possibly a surface expression of the Klamath‐Blue Mountain gravity-anomaly lineament. The orientations of this fault zone, subsidiary sinistral structures, dikes, and fold axes suggest that the presumed Mesozoic structure marked by the lineament was reactivated as a dextralnormal fault by east-northeast‐west-southwest‐oriented compressive stress. This stress is consistent with early Oligocene North America‐Farallon convergence but is inconsistent with northwest-southeast to northsouth compression suggested by structures farther east. Stress either varied temporally or was partitioned into complex local strain domains. The Cyrus Springs fault zone may have become active at about 28‐30 Ma,resulting in uplift and southward tilting of part of the Gray Butte basin fill, and ceased activity before deposition of the horizontal upper Miocene‐Pliocene Deschutes Formation. The Gray Butte area was also an eruptive center for rhyolitic and alkaline-mafic lava and tuff both before and after initiation of movement on the Cyrus Springs fault. Mid-Tertiary volcanism and sedimentation near the western end of the Blue Mountains, heretofore not clearly related to active structures, may have taken place within a regional transtensional regime associated with stress orientations different from those of Neogene time. Local basins with higher subsidence rates accumulated relatively thick sequences of lacustrine tuffaceous strata, and their fossil floras show progressive climate change through the Eocene-Oligocene boundary interval.