A laboratory flume investigation of the formation of fossil stem fills

The mechanism of formation of fossil plant ‘pith casts’ has been investigated experimentally using a small laboratory flume tank. The extent to which filling occurs is dependent on stem length and diameter, and on the current velocity. Stem fills produced by sediment carried in suspension show a distinct structure of two wedges of sediment deposited at either end of the stem cavity. These wedges are deposited from small flow vortices which form when the flow around the stem separates, as the flow through the stem becomes restricted. Stems filled by bedload currents do not show such a uniform structure. The experimentally produced stem fills are comparable with fossil pith casts, e.g. Catamites. The formation of cortical casts of Stigmaria is not simply explained in terms of the processes operating in open-ended stem segments. This investigation of the mechanics of filling of hollow plant organs, and the structure of the fill produced, offers a basis for interpreting the sedimentary environment in which the fossil fills have formed, and leads to a fuller understanding of the mode of formation of such fossils.     

Proto-basement in Svalbard

Candidates for possible Precambrian proto-basement throughout Svalbard are considered and evaluated. The paper provides a tectono-stratigraphic framework for consideration of new data. Proto-basement, in the sense of basement in Proterozoic time, implies the occurrence of a Precambrian tectonic event. Vendian rocks are widespread in Svalbard and, because there is no known Vendian major tectonic event, the search is narrowed to pre-Vendian rocks. The approach is primarily tectono-stratigraphic, to identify a significant break by unconformity, contrast in metamorphic facies and isotopic age determinations.
The paper concludes that the following rock units are proto-basement: Isbjømhamna and (newly defined) Eimfjellet groups, Nordbukta Group, Magnethøgda Group, the Richarddalen Complex; the Atomfjella Complex, the Brennevinsfjorden Group and the granitoids related to the Kapp Hansteen Group and the Duvefjorden Complex. While considered unlikely to be proto-basement the following cannot be ruled out: the Mefonntoppane and Kistefjellet rocks and the Sigfredbogen unit, the blue-schist Vestgötabreen rocks, the Pinkie Formation and parts of the Kongsvegen Group.
Terrane implications are consistent with the three terrane model of Harland & Wright (1979), modified by Harland et al. (1993); but the data presented here hardly constrain such a model. In so far as the conclusions here are valid the effect is to subtract from, rather than add to, the outcrops commonly mapped as likely proto-basement.     

RELIGION IN SIN CITY*

Religion is an important, yet overlooked, aspect of local life in Las Vegas, Nevada. Many residents attend worship services and practice faith in an environment that often promotes the opposite of their religious beliefs. Using qualitative data from interviews with clergy across the spectrum of faith traditions, I analyze the adaptation of institutional religion to local circumstances in order to understand how religious belief reflects local sense of place. In a city bifurcated into tourist and local parts, such analysis provides a glimpse into the insider/outsider place dichotomy of long interest to geographers. It further illuminates how religion and place interact. Religious believers, in Las Vegas and elsewhere, often negotiate between their spiritual beliefs and their surrounding cultural environment.     

Plume-Ridge Interaction: a Geochemical Perspective from the Reykjanes Ridge

Plume–ridge interaction in the Reykjanes Ridge and Icelandregion is graphically demonstrated by several V-shaped ridgessurrounding the spreading axis, indicating mantle flow awayfrom Iceland. It also has significant geochemical effects. Regionally,incompatible element concentrations increase northwards coincidingwith decreasing depth and increasing crustal thickness, depthof melting and proximity to Iceland. Major and trace elementdata show that isolated magma bodies feed individual volcanicsystems along the ridge. Fractionation within these systemsincreases towards 60–61°N, where it coincides withthe intersection of a V-shaped ridge, thicker crust and moreabundant seamounts. Trace element, Nd and Sr isotopic data revealdynamic melting and mixing within a southward-thinning, heterogeneousmantle wedge beneath the Reykjanes Ridge. Melting is variableand locally enhanced at 58°N, 59°N, 60°N and 61°N.A total of six mantle components are identified. Some are specificto Iceland whereas others are found only beneath the ridge axis.The geographical distribution of these components reflects theirorigin within the deep upper and lower mantle and subsequenttranslation by plume outflow along the entire length of theridge. KEY WORDS: plume–ridge interaction; Iceland; Reykjanes Ridge; dynamic mantle mixing and melting     

Geochronological and Petrogenetic Studies of High-Grade Metamorphic Rocks and Intrusives in Namaqualand, South Africa

The metamorphic sequence in the Nababeep district of Namaqualandis considered in terms of major recumbent folding accompaniedby hornblende granulite subfacies metamorphism. Rb-Sr isotopicdata are presented for thirty-one whole-rock samples of metamorphicrocks and associated intrusives, together with U-Pb isotopicdata for nineteen fractions of the constituent zircon and apatite.The metamorphic suite has yielded a Rb-Sr whole-rock isochronage of 1213?22 m.y., with an initial Sr⁸⁷/Sr⁸⁸ ratio (R_o) of0?7191?0?0021 suggesting reworking of pre-existing crustal rocks;data from certain distinctive rock-types indicate that Sr-isotopichomogenization was incomplete during this event. Cross-cuttinggranites and related rocks give a Rb-Sr isochron age of 1166?26m.y. (R_o, 0?708?0?001). These events were post-dated by theintrusion of largely dioritic bodies of the cupriferous NoritoidSuite; zircon recrystallization 1070 m.y. ago is interpretedas the age of emplacement of these bodies. On Sr-isotopic andchemical grounds, a source-bed model is tentatively suggestedfor the Noritoid Suite, involving derivation from gneisses andgranulites of intermediate-basic composition within the metamorphicsuccession. Isotopic events at 1000 and 900 m.y. ago, recordedby zircon and apatite, are believed to reflect the imprint ofregional pegmatite emplacement.