Abstracts of the Geological Society of America December Meeting at Austin Andaltjsite in Californian Eocene Sediments Bt Victor T
Total Page：16
File Type：pdf, Size：1020Kb
ABSTRACTS OF THE GEOLOGICAL SOCIETY OF AMERICA DECEMBER MEETING AT AUSTIN ANDALTJSITE IN CALIFORNIAN EOCENE SEDIMENTS BT VICTOR T. ALLEN Californian Chico Cretaceous sandstones near Folsom and Pentz and at the type locality on Chico Creek lack pink andalusite. The overlying Eocene lone sandstones and correlated auriferous gravels contain pink andalusite derived from the bedrock series of the Sierra Nevada. If these Cretaceous and Eocene sediments were both derived from the same source, it appears that contact aureoles around Sierran intrusions with a grade of metamorphism sufficiently high to contain andalusite were not exposed by erosion until Eocene time. Under the Great Valley at several local ities a similar limitation of andalusite to the Eocene and younger sediments has been found. It is suggested that careful watch be kept by all investigators to check the possibility of using andalusite to separate Cretaceous from Eocene sediments in California. (Geological Society project.) JOINT PATTERNS IN HIGHLY FOLDED AND CRYSTALLINE ROCKS OF THE NORTHERN NEW JERSEY HIGHLANDS AND THEIR RELATION TO APPALACHIAN OROGENY BY ALFRED N . APPLEBY* Pre-Cambrian rocks predominate in the Highlands of northern New Jersey. Asso ciated with them are Paleozoic formations, the youngest of which are Devonian. All are highly folded and metamorphosed. The mantle is largely resultant from Pleisto cene glaciation. The rocks are well jointed. Most are shear joints caused by com pressive stresses. The observations were made in an area of approximately 200 square miles, between Dover and the New York State line and between Hamburg and Pompton Lakes. For purposes of comparison, some Triassic rocks to the east of this area were examined. Readings were taken with a Brunton compass at 259 outcrops. More than 800 strikes and dips of joint systems were recorded. The figures were then tabulated, and their regional distribution and dispositions analyzed. Descriptive geometry was employed to determine the angles between sets of intersecting major joints and to determine the directions of application of stress. A mathematical investigation, suggested by analysis of the tables, was undertaken. The arrangement of force lines indicates that regional rather than local stresses caused the jointing. The patterns show that the jointing was not controlled by folia tion or bedding planes and that the jointing now in evidence occurred later than the magmatic invasions and folding. The jointing is closely related to the Appalachian orogeny. DISTRIBUTION AND ORIGIN OF TEKTITES BY VIRGIL E. BARNESt Tektites are naturally etched pebbles of acidic glass mostly black and lustrous in reflected light and bottle green, smoky, or brown in transmitted light. They are found in siliceous gravels of Miocene to Pleistocene age in Czechoslovakia, Australia, * Introduced by Ernest R. Lilley. t Introduced by E. H. Sellards. (1919) Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/51/12_2/1919/3415717/BUL51_12B-1919.pdf by guest on 30 September 2021 1920 ABSTRACTS OF THE GEOLOGICAL SOCIETY OP AMERICA Indo-China, Philippine Islands, Dutch East Indies, Siam, China, Ivory Coast, and possibly in South America. In North America tektites were recognized first in 1936. Excellent flow structure in tektites is outlined by adjacent schlieren of differ ent chemical compositions which etch differentially and produce the distinctive weathered surface. Flow structure in obsidian is outlined by crystallites and vesicules, and consequently etched obsidian is free of fine flow lines and resembles tektites only in the coarser etch markings. Tektites definitely are not of igneous origin. The prevalent assumption is that they are meteorites. Tektites correspond closely in chemical composition to clastic sedi mentary rocks ranging from shale to sandstone. The writer identified small included glass particles in tektites as lechatelierite, which must have been originally grains of quartz or other nonhydrous silica. They were melted during the process of tektite formation, and their presence probably excludes a meteoritic origin. Lightning may produce tektites. Glass formed by power line short circuits through soil contains lechatelierite particles. This glass very closely resembles tektite glass and when artificially etched develops a typical tektitdike surface. Observations are needed on the effect of lightning upon clastic sedimentary rocks and their soils. TKIASSIC OF ANSON COUNTY, NORTH CAROLINA BY WILLARD BERRY The Triassic sediments of Anson County, North Carolina, can be tentatively divided on the basis of mechanical analysis into two units that are equivalent to the Pekin and Sanford of the Deep River area. Coal has been reported but has not been found by the author. If it is present it should represent the coal measures of the Deep River area. Two inliers of Carolina slate are present in the Triassic near the west side of the basin; they are apparently islands of slate, not large fragments. The fault boundaries are not sharply defined because of subsequent erosion. No fossils have been found except microscopic fragments of coal. ORGANIC CONTENT OF CORES FROM GULF OF MEXICO OFF MISSISSIPPI DELTA BY H . J. BISSELL* More than 200 samples from 24 Piggot cores have been analyzed for organic content. Reducing power was determined by the chromic acid method for all samples, and 15 to 20 per cent were analyzed for organic carbon by wet combustion, and for nitrogen by the Kjeldahl method. Excluding a few thin beds of coarse silt and very fine sand, the samples show a minimum of 2 per cent organic matter, with many beds containing more than 4 per cent. Near shore samples from the subaqueous part of the delta are notably darker than those from deep water, probably due to an abundance of terrigenous humus. They are, however, only slightly higher in organic content than the predominantly light-colored deep-water sediments. Cores from deep water show a slight decrease in organic content with depth below the surface, while in those from near shore no such relationship is evident. All cores, however, show a decrease in nitrogen with depth; a rapid decrease for the first 3 or 4 feet, followed by a slow decrease. Thus the C/N ratio increases toward the bottom of the core. (Geological Society project.) * Introduced by R. Dana Russell. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/51/12_2/1919/3415717/BUL51_12B-1919.pdf by guest on 30 September 2021 ABSTRACTS 1921 BLACK BEACH SANDS OF GUATEMALA, CENTRAL AMERICA BY MARGARET FULLER BOOS The black beach sands of Pacific Central America extend from southern Mexico to northern Nicaragua. At San José, Guatemala, 66 per cent of the sand consists of minutely pustulate, highly magnetic, rounded grains of gleaming black magnetite 1-2 millimeters in diameter. The amount of magnetite decreases markedly close to mouths of streams. Chemical analyses show traces of tungsten, titanium, and 20-30 per cent of silica. Rare yellow topaz, jagged clear glassy quartz, rounded sphene, chips of black obsidian, minute dull red jaspery quartz and numerous smooth rounded grains of gray andesite dotted with minute octahedrons of magnetite occur with the magnetite. Eruptions of volcanoes along the Sierra Madre sprayed the region with andesite fragments loaded with magnetite. Wave action pounded out the magnetite grains and concentrated them in the strand areas. SPECTROSCOPIC DISTRIBUTION OF MINOR ELEMENTS IN IGNEOUS ROCKS FROM JAMESTOWN, COLORADO BY JOSEPH M . BRAY* Spectrographic analyses of a group of igneous rocks and their constituent minerals from Jamestown, Boulder County, Colorado, revealed: Qualitative Analyses Rocks: Each, regardless of age, is distinguishable by variations in content of certain elements (Sc, La, Ce, Nd, Zr, V, Cr, Co, Ni, Pb, and Zn). Comparison with several Massachusetts rocks emphasized the province relationship. Minerals : Minor elements occur systematically. Sr is highest in K- and Ca-minerals (particularly feldspars) ; Ba in K-minerals (potash feldspar and hornblende). All quartz and sphenes contain Al. Sc is most abundant in Mg- and Fen-minerals (micas and hornblende). Sphenes contain a remarkable proportion of rare earths. Ti, Mn, V, and Cr are concentrated in the dark minerals and muscovite. Ni and Co occur almost exclusively in biotite. Quantitative Analyses Quantitative analyses substantiated all conclusions based on qualitative work. Minerals: Characteristic minor elements of each mineral vary from rock to rock. The SrO:BaO ratios decrease numerically in the order of plagioclases, biotites, K-feldspars, and muscovites. Pegmatitic K-feldspars have much larger SrO :BaO ratios than parent rock samples. This is explained by temperature-crystal structure rela tions. Qualitatively and quantitatively each pegmatitic mineral contains less of almost every minor element than the same mineral in the parent rock. This also is explained by temperature-crystal structure relations. The minor elements are probably present in solid solution. CORRELATION OF EARLY PALEOZOIC SECTIONS IN CENTRAL AND WESTERN TEXAS BY JOSIAH BRIDGE Early Paleozoic rocks crop out in the Central Hill Country, the Marathon Basin, the Van Horn area, and the El Paso area. Upper Cambrian strata are well repre sented in the Central Hill Country, are present in the Marathon region, and are * Introduced by W. H. Newhouse. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/51/12_2/1919/3415717/BUL51_12B-1919.pdf by guest