Strongly undersaturated Tertiary volcanic rocks from the Kangerdlugssuaq area, east Greenland     

C.K. Brooks  J.C. Rucklidge 《Lithos》1974,7(4):239-248

Erratic blocks from the interior of Kangerdlugssuaq fjord are described with chemical analyses and microprobe data for their constituent minerals. A strongly peralkaline phonolite, unlike any previously known from the province, shows that some magmas which precipitated alkali feldspar, nepheline and amphibole (magnesio-katophorite) differentiated along a trend towards increasing peralkalinity, although this trend is unknown in the nearby plutons. Feldspar-free nephelinite and less undersaturated limburgite are also encountered, which apparently provided a petrographic link between the strongly undersatrated ultramafic rocks recently discovered in the area and the alkali basalts of Prinsen af Wales Bjerge.

Recognition of these rock types provides further important analogies with the plume-generated triple junctions of the African Rift and affords samples approximating closely to the compositions of the more unusual magmas of the area.  相似文献   

Clinopyroxene-matrix partitioning of K,Rb, Cs,Sr and Ba     

S.E. Hart  C. Brooks 《Geochimica et cosmochimica acta》1974,38(12):1799-1806

Extremely pure samples of clinopyroxene phenocrysts from two volcanic rocks have been analyzed for K, Rb, Cs, Sr and Ba. In conjunction with matrix concentrations, partition coefficients are obtained which are in the range 0.001–0.004 for K, Rb, Cs and Ba. These values are lower than those in the literature by factors of 6–100 but are in good agreement with values determined experimentally at pressures of 15–30 kb by Shimizu (Geochim. Cosmochim. Acta38, 1974). Values for partition coefficients measured on separates of impure or cloudy pyroxenes from these same rocks were higher and similar to those in the literature. We suggest this effect is related to ‘trapping,’ during crystal growth, of liquid which is enriched in the larger ions (such as Rb and Cs) due to lack of diffusion equilibrium in the liquid. Partition coefficient values for olivine and plagioclase from one of these same rocks were also determined.  相似文献   

Comparative Accuracy in Four Civil War Maps of the Shenandoah Valley: A GIS Analysis*     

Brooks C. Pearson 《The Professional geographer》2005,57(3):376-394

This study compares the accuracy of four maps available to Union and Confederate officers during the 1862 Shenandoah Valley campaign of the American Civil War. It examines historical maps of the Valley of Virginia by the following cartographers: James W. Abert, Hermann Böÿe and Lewis von Buchholtz, Jedediah Hotchkiss, and Franz Kappner. Both simple and three‐tiered sinuosity measures are derived for reference points along study map representations of the Shenandoah river system. These data are then statistically compared to corresponding sinuosity data from USGS topographic quadrangles to identify the relative accuracies of the historical maps. This article offers evidence to refute the common historical assumption that Hotchkiss provided Maj. Gen. Thomas “Stonewall” Jackson with terrain intelligence that was far superior to that available to his Union opponents. Evidence indicates that maps by Union cartographers as well as the prewar Böÿe‐Buchholtz map were at least as accurate, or superior to, the work of Hotchkiss.  相似文献   

Overbank deposition along the concave side of the Red River meanders,Manitoba, and its geomorphic significance     

Gregory R. Brooks 《地球表面变化过程与地形》2005,30(13):1617-1632

Slow earth sliding is pervasive along the concave side of Red River meanders that impinge on Lake Agassiz glaciolacustrine deposits. These failures form elongated, low‐angled (c. 6 to 10°) landslide zones along the valleysides. Silty overbank deposits that accumulated during the 1999 spring freshet extend continuously along the landslide zones over hundreds of metres and aggraded the lower slopes over a distance 50 to 80 m from the channel margin. The aggradation is not obviously related to meander curvature or location within a meander. Along seven slope profiles surveyed in 1999 near Letellier, Manitoba, the deposits locally are up to 21 cm thick and generally thin with increasing distance from, and height above, the river. Local deposit thickness relates to distance from the channel, duration of inundation of the landslide surface, mesotopography, and variations in vegetation cover. Immediately adjacent to the river, accumulated overbank deposits are up to 4 m thick. The 1999 overbank deposits also were present along the moderately sloped (c. 23 to 27°) concave banks eroding into the floodplain, but the deposits are thinner (locally up to c. 7 cm thick) and cover a narrower area (10 to 30 m wide) than the deposits within the landslide zones. Concave overbank deposition is part of a sediment reworking process that consists of overbank aggradation on the landslide zones, subsequent gradual downslope displacement from earth sliding, and eventually reworking by the river at the toe of the landslide. The presence of the deposits dampens the outward migration of the meanders and contributes to a low rate of contemporary lateral channel migration. Concave overbank sedimentation occurs along most Red River meanders between at least Emerson and St. Adolphe, Manitoba. © Her Majesty the Queen in right of Canada.  相似文献   

The Muralla Pircada—an ancient Andean debris flow retention dam, Santa Rita B archaeological site, Chao Valley, Northern Perú     

William E. Brooks  Jason C. Willett  Jonathan D. Kent  Victor Vasquez  Teresa Rosales 《Landslides》2005,2(2):117-123

Debris flows caused by El Niño events, earthquakes, and glacial releases have affected northern Perú for centuries. The Muralla Pircada, a northeast-trending, 2.5 km long stone wall east of the Santa Rita B archaeological site (Moche-Chimú) in the Chao Valley, is field evidence that ancient Andeans recognized and, more importantly, attempted to mitigate the effects of debris flows. The Muralla is upstream from the site and is perpendicular to local drainages. It is 1–2 m high, up to 5 m wide, and is comprised of intentionally-placed, well-sorted, well-rounded, 20–30 cm cobbles and boulders from nearby streams. Long axes of the stones are gently inclined and parallel local drainage. Case-and-fill construction was used with smaller cobbles and pebbles used as fill. Pre-Muralla debris flows are indicated by meter-sized, angular boulders that were incorporated in-place into construction of the dam and are now exposed in breeches in the dam. Post-Muralla debris flows in the Chao Valley are indicated by meter-sized, angular boulders that now abut the retention dam.  相似文献   

Quantitative assessment of benthic food resources for juvenile Gulf sturgeon,Acipenser oxyrinchus desotoi in the Suwannee River estuary, Florida, USA     

R. Allen Brooks  Kenneth J. Sulak 《Estuaries and Coasts》2005,28(5):767-775

Gulf sturgeon,Acipenser oxyrinchus desotoi, forage extensively in the Suwannee River estuary following emigration out of the Suwannee River, Florida. While in the estuary, juvenile Gulf sturgeon primarily feed on benthic infauna. In June–July 2002 and February–April 2003, random sites within the estuary were sampled for benthic macrofauna (2002 n = 156; 2003 n = 103). A mean abundance of 2,562 ind m⁻² (SE ± 204) was found in the summer, with significantly reduced macrofaunal abundance in the winter (mean density of 1,044 ind m⁻², SE ± 117). Benthic biomass was significantly higher in the summer with an average summer sample dry weight of 5.92 g m⁻² (SE ± 0.82) compared to 3.91 g m⁻² (SE ± 0.67) in the winter. Amphipods and polychaetes were the dominant taxa collected during both sampling periods. Three different estimates of food availability were examined taking into account principal food item information and biomass estimates. All three estimates provided a slightly different view of potential resources but were consistent in indicating that food resource values for juvenile Gulf sturgeon are spatially heterogeneous within the Suwannee River estuary.  相似文献   

Two melting regimes during Paleogene flood basalt generation in East Greenland: combined REE and PGE modelling     

Peter Momme  Christian Tegner  C. Kent Brooks  Reid R. Keays 《Contributions to Mineralogy and Petrology》2006,151(1):88-100

Previously published platinum group element (PGE) and rare earth element data (REE) from a sample suite of the Palaeogene flood basalts of the East Greenland rifted margin are used to approximate primary magma compositions by numerical models of mantle melting. Both high-Ti and low-Ti basalts are found intercalated in the coastal section “the Sortebre Profile” in central East Greenland, and the apparent lack of mixing between the two series indicates coexistence of two geographically separated melting regions and plumbing systems during continental breakup above the Palaeogene Iceland plume. The lavas show little or no sign of crustal contamination and the limited variation in La/Sm and Cu/Pd ratios can be interpreted to reflect mantle source composition and melting processes. Numerical modelling indicate that the low-Ti series formed by F~20% melting in a columnar melting regime from a slightly depleted upper mantle source with a relatively normal S-content (~180 ppm S). In contrast, the high-Ti series formed by much lower degrees of melting (F~6%) in a spreading-related, triangular melting regime from a relatively S-poor (~100 ppm S) source. The low-Ti suite was S-undersaturated at the stage of melt segregation from a shallow mantle source due to the high degree of melting. In contrast, the high-Ti suite probably formed from a S-poor source where some low degree melt batches were S-saturated at the stage of deep segregation in distal parts of the triangular melting regime. This suite shows a geochemical high pressure garnet-signature and adiabatic decompression could therefore have played a role in keeping the mantle-derived S in solution before Fe-enrichment related to fractional crystallisation also increased the S-capacity of these melts. An erratum to this article can be found at  相似文献   

Two melting regimes during Paleogene flood basalt generation in East Greenland: combined REE and PGE modelling     

Peter Momme  Christian Tegner  C. Kent Brooks  Reid R. Keays 《Contributions to Mineralogy and Petrology》2006,151(5):631-632

  相似文献   

Stratigraphy of the Middle Devonian boundary: Formal definition of the susceptibility magnetostratotype in Germany with comparisons to sections in the Czech Republic, Morocco and Spain     

Brooks B. Ellwood  Jenaro L. García-Alcalde  Ahmed El Hassani  Jindrich Hladil  Francisco M. Soto  Montserrat Truyls-Massoni  Karsten Weddige  Leona Koptikova 《Tectonophysics》2006,418(1-2):31

One major difficulty in geology is high-resolution correlation among widely separated sections, especially in the Paleozoic where magnetostratigraphy polarity is not well established because rocks are often remagnetized, where critical biostratigraphic zonation may be poor or lacking, or where structural complexities make correlations very difficult. To address this problem, we have been using magnetostratigraphy susceptibility measurements. Here, we report our work from the Middle Devonian in Europe and North Africa. The Middle Devonian (Emsian–Eifelian) global boundary stratotype section and point (GSSP), located in the Eifel Hills, western Germany, was ratified by the International Subcommission on Stratigraphy in 1985, after careful evaluation of the biostratigraphy for this and many other sections. The boundary interval has been characterized using biostratigraphy, and the beginning of the Eifelian stage has been specifically defined by the first occurrence of the conodont Polygnathus costatus partitus. We have collected the Eifel Hills section for magnetic susceptibility (MS) measurement and here we establish it as the magnetostratotype for the Emsian–Eifelian stage boundary, by formally defining the magnetostratigraphy susceptibility for the section. We then collected, measured and compared the magnetostratotype to four other sections for which conodont biostratigraphy has been studied and where P. costatus partitus is present; two Emsian–Eifelian sections in Morocco and two sections in the Czech Republic (including the Emsian–Eifelian parastratotype). Finally, we have measured the MS for the El Puerto Creek section in the Cantabrian Mountains of Spain and identified the location of the Emsian–Eifelian boundary within the section based on MS comparison to the GSSP in conjunction with excellent biostratigraphic indicators, primarily brachiopods. While the conodont zonation in the El Puerto Creek section is poorly defined, we believe that the correspondence between the MS and biostratigraphy in the section allows the identification of the Emsian–Eifelian boundary. These results indicate that this method can be successfully applied to marine sequences where ambiguities in correlation exist.  相似文献   

Galveston Bay: Temporal changes in the concentrations of trace organic contaminants in national status and trends oysters (1986–1994)     

Thomas J. Jackson  Terry L. Wade  José L. Sericano  James M. Brooks  Jennifer M. Wong  Bernardo Garcia-Romero  Thomas J. McDonald 《Estuaries and Coasts》1998,21(4):718-730

The temporal distributions for six classes of trace organic contaminants (chlordanes, DDTs, dieldrin, PAHs, PCBs, and butyltins) in oysters from six Galveston Bay sites from National Oceanic and Atmospheric Administration’s National Status and Trends (NS&T) Mussel Watch Program are compared with other NS&T sites from the Gulf of Mexico as well as all NS&T sites of the United States (East Coast, West Coast, and Gulf of Mexico). Decreases in the median for the Gulf-wide concentration of chlordanes, dieldrin, and butyltins occurred during 1986–1994. The Gulfwide median concentrations of DDTs, PAHs, and PCBs exhibited a strong cyclic distribution with time. For Galveston Bay oysters, “high” concentration is defined as the concentration greater than the median plus one standard deviation for all Gulf of Mexico sites. The percentage of sites having high concentrations during 1986–1994 for Galveston Bay oysters are 49% for dieldrin, 45% for butyltins, 40% for chlordanes, 38% for PCBs, 30% for PAHs, and 21% for DDTs. For PCBs, 43% of Galveston Bay oyster samples analyzed over the first 9 yr have concentrations high enough for potential biological effects to be observed in oysters. The percentages in other agents were chlordanes (22%), butyltins (22%), dieldrin (5%), and PAHs (4%). National Academy of Science-proposed regulatory limits for oysters were exceeded in only 2% of Galveston Bay samples for DDTs and 1% for PCBs. All other contaminants were below proposed NAS limits.  相似文献