Geology topics
Publications and source records attributed to Robert Ahlberg Loney.
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The alpine-type peridotite in the area of Vulcan Peak, Oregon, is part of the larger Josephine ultramafic complex in the Klamath Mountains geologic province. Partially serpentinized, foliated harzburgite with 15 to 30 percent orthopyroxene makes up approximately 90 percent of the body. The remaining 10 percent is dunite that occurs in the harzburgite as concordant and discordant layers and as irregular bodies. In general, the peridotite at Vulcan Peak is similar in structure, texture, mineralogy, and chemistry to the peridotite at Burro Mountain, California. Structures, textures, and compositions of coexisting phases are consistent with high-temperature (1,000° to 1,200°C) deformation and recrystallization in the upper mantle, and tectonic emplacement into its present crustal position. Evidence to indicate whether the peridotite originated as a refractory residue during partial fusion processes that produced mafic melt or by crystallization from an ultramafic or picritic magma remains inconclusive; poikilitic clinopyroxene enclosing olivine in some dunites, and certain chromitite textures, may represent relict igneous features suggesting a magmatic stage in the history of the peridotite.
The northwest-striking Nacimiento fault, in the southern Coast Ranges of California, has generally been regarded as the boundary between two major structural blocks: the Nacimiento block to the southwest, in which the basement rocks belong to the Franciscan Formation (Upper Jurassic to Upper Cretaceous), and the Salinian block to the northeast, in which the basement rocks are granitic and high-grade metamorphic. It has been found, however, that in the Burro Mountain area of the southern Santa Lucia Range, the “Nacimiento” fault of Jennings (1959) is nearly vertical and is within the Nacimiento block. In this area, the Franciscan Formation crops out northeast of the “Nacimiento” fault through windows in an older, low-angle thrust fault that brings the Asuncion Group of Taliaferro (1943) (Upper Cretaceous) over the Franciscan Formation. The fault boundary between the Nacimiento and the Salinian blocks must therefore lie farther to the northeast, where it may be buried beneath the Asuncion Group and younger strata. This conclusion is supported by Hanna's recent aeromagnetic work (1969).
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The Pybus-Gambier area comprises about 215 square miles of uninhabited land on the southeastern coast of Admiralty Island, southeastern Alaska. The section consists of more than 20,000 feet of intensely folded sedimentary, volcanic, and metamorphic rocks, all probably of marine origin, ranging in age from Silurian(?) to Early Cretaceous, unconformably overlain by more than 10,000 feet of gently dipping nonmarine, coarse-grained sedimentary rocks, and basalt and andesite flows of Eocene age. Diorite plutons and associated contact metamorphic rocks occur in the little-know northwestern part of the area. The section here is subdivided into nine formations, eight of which are named for the first time, as follows: Gambier Bay formation of Middle(?) Devonian age, composed of greenschist, phyllite, marble, and metachert; Hood Bay formation or Silurian and Devonian age, composed of dark, carbonaceous, thin-bedded chert, argillite, limestone, and graywacke; Cannery formation of Permian age, composed of thin-bedded chert, argillite, and graywacke; Pybus dolomite of Permian age, composed of fossiliferous, cherty dolomite; Hyd formation of Late Triassic age, composed of a basal chert breccia, a limestone member, a thin-bedded argillite member, and a spilitic volcanic member; Seymour Canal formation of Late Cretaceous and Early Cretaceous age, composed of argillite, graywacke, and conglomerate; Brothers volcanic of Early Cretaceous age, composed of andesitic flows and breccia; unnamed conglomerate and sandstone of Eocene age, and Admiralty Island volcanics of Eocene age, composed of basaltic and andesitic flows. A marked angular unconformity occurs at the base of the Tertiary section, and unconformities of less angularity occur as follows: at the base of the Cannery formation; at the base of the Hyd formation, and at the base of the Seymour Canal formation. In general, pre-Seymour Canal deformation seems to have been mild, but the intensity of the post-Gambier Bay - pre-Cannery deformation is uncertain. The complex structure of the pre-Tertiary rocks seem to be the product chiefly of the post-Seymour Canal - pre-Eocene formation. The structure of the pre-Tertiary rocks is studied by graphical statistical analysis of the preferred orientation of the planar and linear structural elements. This analysis indicates that the post-Seymour Canal - pre-Eocene deformation consisted chiefly of three episodes of folding, all of which appear to have resulted from sub-horizontal, northeast-southwest compression. The first episode produced isoclinal folds in bedding, and in places associated axial plane foliation, that had north-northwesterly striking axial planes in eastern Pybus-Gambier area, but generally northeasterly striking axial planes west of False Point Pybus. Compression during the second episode deformed the first folds in the west where their axial planes were oriented about parallel to the compression into complex second folds in both bedding and axial plane foliation, having northwesterly striking axial planes. The second folds are absent in the east where the axial planes and limbs of the first folds were oriented about normal to the second compression. The divergence of the axial planes of the first folds in the west from those in the east may have been caused by the crowding of the first folds in the west against the irregular western margin of the geosyncline during the first compression. Northweststriking thrust and reverse faults occur between domains in which the axial planes of the first folds are strongly divergent. The third episode produced kink folds, having axial planes with subvertical dips but widely varying strikes, that are confined to the thinly fissile schists and phyllites of the Gambier Bay formation. The episodes of folding in the Gambier Bay formation are accompanied by metamorphic recrystallization not found in either younger or possibly coeval (Hood Bay formation) pre-Tertiary rocks. If these episodes are contemporaneous with those in nonmetamorphic pre-Tertiary rocks, then the metamorphism dies out both vertically and laterally, and may be part of the regional decrease in metamorphism westward away from the Coast Range batholith, located about 25 miles to the east. The gently dipping Tertiary strata have been broken into fault blocks by subvertical, north-and north-east striking, normal and reverse faults.