Geologic map of the Stemler Ridge Quadrangle, Malheur County, southeastern Oregon

Abstract

INTRODUCTIONThe Stemler Ridge quadrangle was selected for a comprehensive geochemical study to expand our regional understanding of the relationship, if any, between north-and northwest-striking faults and stratigraphy. In addition, the thick sequence of basalt of Malheur Gorge may provide clues to an association between the coeval Columbia River Basalt Group to the north, and Steens Basalt to the south.Geology was mapped on l:24,000-scale topographic maps. Aerial photographs were used to interpret the er*ent of faults. Petrographic descriptions are based on Meld examination of hand samples. For additional mineralogic details, see Kittleman and others (1965) or Evans (1990).Major and some trace element analyses were obtained using X-ray fluorescence spectroscopy (XRF) on fuse'! glass beads on an automated Rigaku 3370 spectrometer at the GeoAnalytical Laboratory, Washington State University, Pillman, Washington. Analyses were conducted by Ben Binger and Diane Johnson (Hooper and others, 1993). STRUCTURENormal faults are the dominant structural feature in the Stemler Ridge quadrangle. The faults occur chiefly in two discrete domains: northwest-striking normal faults prevail south of the Grasshopper Flat syncline, and north-striking normal faults are found to the north. The northwest-striking faults are the larger; their offset is at least 180 m, ani strata deformed by them dip northeast. This pattern continues for 15 km south of the map area (Greene and others, 1977). The north-striking faults have offset less than 120 m, and beds dip uniformly west No evidence was found for strike-slip faulting.Grasshopper Flat occupies a west-trending asymmetric syncline (fig. 4). Strata on the north limb dip 12-20° southward, whereas the south limb dips 5-12°. The syncline changes eastward into a normal fault near Dishrag Onyon, with fault offset increasing to the east-southeast. Offset is 350 m near the east margin of the map and increases to 750 m 6 km east of the map area (Evans, 1990). The syncline and fault were buried by younger tuff and tuffaceous sedimentary deposits (Tyts) in middle and late Miocene time.Extensive talus obscures all fault traces. Therefore, we have chosen to show no offset at fault intersections rail ~,r than guess at relative ages. Northwest-and north-striking faults may have been coeval or alternated over time. Synclinal folding appears to have predated the north-striking faults that disrupt the plateau north of Grasshopper Flat. Eas4 of the map area west-northwest-striking faults appear to be older (Evans, 1990).The structure appears more complicated in Dishrag Canyon because units dip both west and south, draping the broad ridge directly north of Dishrag Spring. All older units exposed east of Dishrag Canyon form a monocline as they dip westward beneath the sedimentary strata of the older tuffaceous and sedimentary rocks (Tots) along the canyon floor. Older rocks exposed in eroded monoclines create irregular map contacts that at first glance suggest faulted relations (for erample, see unit The in c...

Genesis of flood basalts and Basin and Range volcanic rocks from Steens Mountain to the Malheur River Gorge, Oregon

“…The Dinner Creek tuff (Greene et al, 1972) is a densely welded ignimbrite that may have erupted from a caldera near the vicinity of Castle Rock and/or Westfall Butte, ϳ30 km north of the map area (Rytuba and Vander Meulen, 1991;Evans and Binger, 1997). This location is consistent with the overall distribution and thinning of the Dinner Creek tuff from the Castle Rock area southward into the map area (Johnson et al, 1998b). Although it forms a spectacular marker bed throughout the Juntura Quadrangle ( Fig.…”

“…3B). Farther to the northwest, the upper part of this volcaniclastic succession contains abundant white diatomite and lacustrine sediments with plant and vertebrate fossils of late Miocene to early Pliocene age (Greene et al, 1972;Johnson et al, 1998b). Although most of the pyroclastic air-fall deposits are likely to have had local sources, it is not inconceivable that some of the finer ash-fall deposits were derived from explosive, Plinian-type eruptions associated with the Cascade volcanic arc.…”

“…Binger (1997), however, demonstrated that the Tims Peak basalt can be subdivided into at least four similar, but chemically distinct flow types. Two of these chemical types, the Juniper Gulch and Grasshopper Flat basalts, are ubiquitous in the Stemler Ridge Quadrangle, immediately north of the map area (Johnson et al, 1998b). In the Juntura Quadrangle, however, Juniper Gulch basalt is absent, and Grasshopper Flat basalt is restricted to a few outcrops along the eastern margin of the map sheet.…”

“…The identification of thick lake sediments containing pillows and invasive lava flows of Tims Peak basalt in the Stemler Ridge Quadrangle led Johnson et al (1998b) to suggest that a significant amount of deformation occurred after eruption of Hunter Creek basalt at ca. 15.3 Ma, but before eruption of Tims Peak basalt at ca.…”

Field Mapping Investigation and Geochemical Analysis of Volcanic Units within the Dinner Creek Tuff Eruptive Center, Malheur County, Eastern Oregon

“…This volcanic unit is believed to be derived from CRBG magma mixed with accreted terrain (Steiner, 2015). Finally, data from the Juniper Gulch Basalt was taken from Johnson et al (1998). This basalt occurs directly south of the study area and is likely late Miocene in age.…”