Sam Boggs

Provenance interpretation of quartz by scanning electron microscope–cathodoluminescence fabric analysis

We used a cathodoluminescence (CL) detector attached to a scanning electron microscope (SEM) to study patterns of variable-intensity CL in quartz grains from a variety of igneous, metamorphic, sedimentary, and shock-deformed (meteorite-impact) rocks. Distinctive fabrics in quartz grains revealed by SEM-imaged differential CL include zoning, healed fractures, complex shears, planar features (shocked quartz), dark CL streaks and patches, indistinct, mottled texture, and nondifferential (low-contrast) CL. Zoning is common in volcanic quartz and some plutonic quartz. Zoned plutonic quartz is distinguished from volcanic quartz by the presence of closed fractures and dark CL streaks and patches. Metamorphic quartz displays either an indistinct, mottled texture, or nearly uniform (nondifferential) CL. Quartz from rocks severely deformed by tectonism displays a complex pattern of multiple, small-scale shears. Quartz from meteorite-impact sites and some system boundaries is characterized by intricate patterns of planar features, presumably created by shock metamorphism. Thus, the SEM-CL fabric-analysis technique provides a rapid method for distinguishing quartz from a variety of source rocks.

Is Quartz Cathodoluminescence Color a Reliable Provenance Tool? A Quantitative Examination

ABSTRACT We examined cathodoluminescence (CL) colors of quartz by using red (590-780 nm), green (515-590 nm), and blue (380-515 nm) optical filters interfaced with a cathodoluminescence (CL) detector attached to a scanning electron microscope (SEM). SEM/CL images taken through these filters were captured digitally and transferred to a computer. Luminescence intensities (luminosities) of the images were measured by using available commercial software. Measured luminosities of these CL images are directly related to relative intensities of red, green, and blue CL emissions. Luminosity data were then used to construct plots that display relative luminosities of the CL images acquired through the red, green, and blue filters. An unfiltered CL image of each quartz grain, generated by photons with wavelengths ranging from 200-700 nm, was also acquired. By subtracting the numerical luminosity values of the images acquired through the red, green, and blue filters from the luminosity value of the unfiltered image, the contribution to total luminosity provided by CL emission in the near ultraviolet (UV) was calculated. The CL colors of quartz from a variety of volcanic, plutonic, and metamorphic rocks and hydrothermal deposits were examined. Volcanic quartz phenocrysts have the most restricted CL color range, with strongest emission intensity in the blue wavelength band. CL colors of plutonic quartz overlap those of volcanic phenocrysts but extend over a broader range to include quartz that displays higher intensity of red emission. CL emission in hydrothermal (vein) quartz is similar to that in plutonic quartz, although some hydrothermal quartz exhibits stronger green-CL emission than does plutonic quartz. The CL colors of metamorphic quartz exhibit the widest variation, overlapping the color fields of both volcanic and plutonic quartz and extending further into the red. CL emission in the near UV makes a significant contribution ( 5-85 percent) to the total luminosity of SEM/CL images, particularly images of plutonic quartz. Because of overlap in the CL color ranges of volcanic, plutonic, metamorphic, and hydrothermal quartz, unambiguous identification of quartz provenance on the basis of CL color alone is problematic. It is difficult to distinguish between volcanic and some plutonic quartz, and between some plutonic and hydrothermal quartz, or to distinguish magmatic quartz from metamorphic quartz that exhibits blue CL color. Only metamorphic quartz that exhibits moderately strong red emission appears distinguishable (on the basis of color) from quartz of other origins. Our work thus suggests that CL color is not a reliable indicator of quartz provenance.

Provenance interpretation of Tertiary sandstones from the Cheju Basin (NE East China Sea): a comparison of conventional petrographic and scanning cathodoluminescence techniques

Abstract Cheju Basin is a Tertiary intracratonic basin filled with largely nonmarine sediments and is located SW of the Korean Peninsula beneath the NE East China Sea. Commercial petroleum companies drilled 10 deep wells in this basin between the late 1970s and the 1990s. We obtained samples from four representative wells for provenance analysis, using both conventional petrographic techniques and scanning cathodoluminescence (SEM-CL) imaging of quartz. This paper examines differences in provenance interpretation arising from the use of two different analytical techniques. On the basis of framework mineralogy, quartz undulosity and polycrystallinity, and feldspar compositions, we interpret that Cheju Basin sediments were derived from a mixed source rock terrane. Petrographic data suggest that plutonic source rocks were predominant, metamorphic rocks less abundant, and volcanic and sedimentary least important. SEM-CL imaging of quartz from nine of the same samples examined petrographically yielded a different provenance interpretation. SEM-CL results indicate that metamorphic quartz greatly predominates over plutonic quartz in all samples. This discrepancy in interpretation probably stems from the difficulty in distinguishing between plutonic and metamorphic quartz by petrographic methods. SEM-CL analysis shows that volcanic quartz is absent in many samples, even in samples that contain modest amounts of volcanic rock fragments, probably reflecting the generally low content of quartz in many volcanic source rocks. The SEM-CL method cannot directly detect sedimentary (recycled) quartz. Thus, neither petrographic analysis of sandstones nor SEM-CL analysis of quartz may provide completely accurate interpretation of provenance, implying that an integrated approach to provenance analysis is most reliable. Our data suggest that the Cheju Basin areas may have been connected by land to a sediment source on the Korean Peninsula during Oligocene to Miocene time. Alternatively, or additionally, a land source may have existed to the northwest along a narrow massif or suture belt now inundated by the East China Sea. Regional subsidence in Pliocene time initiated the onset of marine conditions in the Cheju Basin; however, one or more of the same land sources continued to furnish sediments to the basin.

Sand-Wave Fields in Taiwan Strait

During 1973, sand waves were detected by echo sounding in several areas of the shallow shelf northwest and west of Taiwan. They occur in fields 5 to 30 km in width, at water depths ranging from 20 to 90 m. The height of the sand waves primarily ranges from 5 to 8 m, and their average length ranges from about 100 to 500 m. The asymmetrical shape of most of the waves suggests that they migrate in response to present-day hydraulic conditions. Orientation of the steepest faces of the sand waves is quite variable, indicating that the pattern of bottom current movement in Taiwan Strait is probably quite complex.

Quaternary sedimentation in the Japan arc-trench system

Quaternary sedimentation around the Japanese Islands took place in a dynamic morphotectonic setting that included seismically active trenches, a large back-arc basin, and a tectonically complex slope environment characterized by en echelon folds and fault-bounded ridges with small intervening basins. These structural ridges created barriers to sediment transport by bottom currents and acted as dams on the slope behind which Quaternary sediments ponded. Sea levels higher than at present throughout most of the Pleistocene impeded escape of coarse sediments from the shelf, except through a few major submarine canyons that extended completely across the shelf and slope; thus, deeper-water sediments in both the Japan fore-arc and back-arc are dominantly hemipelagic muds and subordinate mud turbidites. A drop in sea level to about 130 m below the present level at approximately 18,000 yr B.P. resulted in deposition of widespread fluviomarine sands and gravels on the continental shelf and an increase in sand-silt turbidites in the latest Pleistocene record in both the fore-arc and the back-arc. Sandy turbidites interbedded with hemipelagic mud or mud turbidites occur particularly in the trench fill of Nankai Trough (Southwest Japan Trench), in some fore-arc basins on the South-west Japan slope, and in the western part of the Japan Sea back-arc basin. They are sparsely represented in the Japan Trench and the Northeast Japan slope, apparently owing to the poor development of throughgoing submarine canyons on the Northeast slope. Diatomaceous and calcareous muds and oozes occur in some parts of both the fore-arc and the back-arc, and volcanic ash and pumice are virtually ubiquitous throughout the Quaternary sediment record in nearly all parts of the Japan arc-trench system. Quaternary sedimentation rates are less than about 30 m/m.y. in the open Pacific deep-sea basins seaward of the trenches, but they increase toward the fore-arc and reach values as high as 230 m/m.y. in the Japan Trench inner slope and 300–900 m/m.y. in the trench fill of Nankai Trough. Back-arc sedimentation rates for the total Quaternary range from 20–140 m/m.y., and latest Pleistocene-Holocene rates range from 65–230 mm/1,000 yr.

Petrology of late Paleozoic-early Mesozoic Pyeongan Group sandstones, Gohan area, South Korea and its provenance and tectonic implications

Abstract Carboniferous to Triassic strata of the Pyeongan Group crop out in the Gohan area, northeastern South Korea. These strata consist, in ascending order, of the Hongjeom, Sadong, Gobangsan, and Nogam formations. Sandstones in these formations comprise three distinct framework-mineral suites. Suite 1 sandstones (Hongjeom Formation through the Middle Member of Gobangsan Formation) are quartz and quartzose arenites/wackes that contain abundant quartz, little or no feldspar, and few acid volcanic clasts. Heavy minerals are mainly tourmaline, rutile, and zircon. Suite 2 sandstones (Upper Member of Gobangsan Formation) are lithic arenites/wackes characterized by moderate quartz content, low amounts of feldspar, and moderate to abundant volcanic clasts. Sphene is the predominant heavy mineral. Suite 3 sandstones (Nogam Formation) are feldspathic arenites/wackes distinguished by low quartz content, moderate to abundant feldspar, and low amounts of acid volcanic clasts. The heavy-mineral assemblage of the Nogam Formation is characterized particularly by epidote and sphene. Feldspar-poor Suite 1 sandstones were derived from a source terrain rich in quartzite and quartzose sandstone. Abundant quartz and stable heavy minerals were supplied initially during Late Carboniferous time to a shallow-marine shelf, which changed gradually near the end of Carboniferous time to a nonmarine, paralic platform setting. Electron probe microanalysis shows that Suite 1 tourmaline was derived originally from Li-poor granitoids, pegmatites, and Ca-poor aplite and metapsammite. Petrographic evidence demonstrates that the scarcity of feldspars in Suite 1 sandstones is not the result of diagenetic intrastratal solution. Major changes in mineralogy from Suite 1 into Suite 2 and Suite 3 sandstone indicate changes in provenance. Suite 2 sandstones were derived from diorite, granodiorite, acid volcanic rocks, and possibly some metamorphic rocks, which furnished feldspars and sphene. The appearance of feldspars and volcanic rock fragments in these sandstones reflects orogenic changes and uplift resulting from incipient collision of the Asian mainland and the Honshu Block of southwest Japan. Compositionally immature Suite 3 sandstones were derived mainly from granite, granodiorite and granitic gneiss, indicating continuing uplift. These source rocks provided quartz, K-feldspar, plagioclase, epidote, and sphene. Acid volcanic rocks supplied minor amounts of rock fragments.

Experimental Study of Rock Fragments

An experimental study was conducted on a suite of igneous, metamorphic, and sedimentary rocks which had been artificially fragmented to five size fractions: granule, very coarse sand, coarse sand, medium sand, and fine sand. These fractions were studied to determine changes in textural properties with progressive size reduction and the feasibility of identifying parent rock textures in fragments ranging from granule to fine sand size. The diagnostic textures of all the rocks studied are preserved to essentially the same degree in granule-sized fragments as in thin sections of parent rocks. Identification is less reliable in smaller sizes and depends upon the mean grain size and sorting of the parent rock.

Taiwan: Geology, Geophysics, and Marine Sediments

The ocean margin island of Taiwan is a geodynamic body of young and complex build. Occupying an area of 36,000 km2 between the Chinese part of the Eurasian continent capping one lithospheric plate and the north¬western part of the Philippine Sea floored by another (Figs. 1, 7), this island, as a compression-plus-shear product, is elevated to a maximal height of almost 4000 m, higher than any other fold mountains on the northwest coast of the Pacific Ocean. It is an arcuate island extending its shorter arm eastward Open image in new window Fig. 1 Map of Taiwan and offshore areas. Isobaths (depth in 1000 m) are based on Chase et al. (1971). Heavy lines are locations of seismic reflection profiles. to the Ryukyus and its longer arm southward to the Philippines. The crestal zone of this mountainous island is the Central Range, which is fringed on its west side by the Foothill Zone and separated on its east from the Coastal Range by the corridor known as the Longitudinal Valley (Fig. 2). West of the Foothill Zone is a vast coastal plain with the very shallow Taiwan Strait farther west; east of the Coastal Ranage is the deep Philippine Sea whose floor exhibits arcs and trenches like the North Luzon Ridge and Trough. The dependent islands of Taiwan include the Penghu Islands in the Taiwan Strait and the islands of Lutao and Lanhsu off the southeast coast. It must be added that, in the less tightly compressed northeastern and southwestern parts of the mountain complex of the Central Range and Foothill Zone, there are, respectively, the Ilan Plain and the Pingtung Valley, each in the from of an intramontane trough wedging from the sea into the island.

Seasonal reversal of flood-tide dominant sediment transport in a small Oregon estuary

The Sixes River in southwestern Oregon has a summer discharge of only about 2 m 3 /sec. During these low-discharge conditions, a flood-dominated system of bottom tidal currents develops in the estuary and a deltalike sill, as much as 1.5 m in height, builds across the mouth of the estuary by upstream progradation. Flood-tide currents move across this sill at velocities of as much as 90 cm/sec 15 cm above the bottom, but the velocity of ebb-tide currents usually does not exceed about 40 cm/sec. Dispersal patterns of dyed sediment injected at the river mouth during low river discharge show that flood-tide currents transport sand across the sill and up the estuary as far as 0.8 km (about one-fourth the length of the estuary) in a single flood-tide phase. During ebb tide, the sill impedes movement of salt water along the estuary bottom, producing a sharply stratified two-layer water system. Although tracer experiments show that some fine sand is removed from the estuary during the ebb phase, primary sedimentary structures and the mineral composition of the sand indicate that flood-tide dominance of the bottom tidal currents causes a net gain of marine sediment in the estuary while the sill is in place. River discharge after winter storms may increase to more than 400 m 3 /sec, and large quantities of detritus, including gravel, are transported downstream into and through the estuary. High river discharge also causes erosion of the sill, greatly reducing the sediment-trapping capacity of the estuary. The finer fluvial detritus, together with fine marine sediment deposited during the summer, is swept from the estuary, leaving it floored largely by gravel. Thus, the hydraulic sediment-trapping mechanisms observed in the estuary of the Sixes River appear to be effective only on a seasonal basis under present hydrologic conditions.

Albitization of feldspars in sandstones from the Gohan (Permian) and Donggo (Permo-Triassic) formations, Gohan area, Kangwondo, Korea

Sandstones in the Gohan and Donggo formations in the Gohan area, Korea, contain considerable amounts of feldspars. Electron probe microanalysis (EPMA), backscattered electron microscopy (BSE), cathodoluminescence (CL), and microscopic petrography show that many of these feldspars consist of albite. Most of these albites are characterized by moderately high chemical purity (Ab 97–100) and do not display cathodoluminescence. These characteristics suggest that the albite is probably authigenic, formed during burial diagenesis by alteration of plagioclase and K-feldspar. The presence of abundant albite suggests that the sediments were buried to depths corresponding to the burial temperature that probably exceeds 90°C.