ALBERT

Notes: Coexisting Ca-poor and Ca-rich pyroxenes in granulites at Cape Riche, in the Precambrian Albany-Fraser Province, Western Australia, are dominantly chemically homogeneous within individual samples, suggesting a major episode of equilibration. However, occasional grains in a few samples contain exsolved domains interpreted as relics of an earlier, higher-T assemblage. Pyroxene pairs in ten, presumably isothermal, samples from a restricted area are used to (i) assess the suitability of several versions of the two-pyroxene thermometer for application to metamorphic rocks, and (ii) determine the thermal history of the Cape Riche pyroxenes.The various versions of the two-pyroxene thermometer applied to the well-equilibrated homogeneous pyroxene grains show poor to good precision and yield mean temperatures varying widely from 683° to 893°C, in the following order of increasing T: Lindsley (1983; opx version), 683°± 11°C; Kretz (1982; KD version), 705°± 19°C; Ross & Huebner (1975), 709°± 30°C; Kretz (1982; solvus version), 735°± 24°C; Fonarev & Graphchikov (1982; opx version), 〈750°C; Lindsley (1983; cpx version), 784°± 40°C; Fonarev & Graphchikov (1982; cpx version), ~820°± 30°C; Wood & Banno (1973), 849°± 16°C; Powell (1978), 854°± 23°C; Wells (1977), 893°± 10°C. Independent T estimates, based on mafic assemblages and garnet-biotite thermometry, suggest that the major episode of metamorphism occurred at 700-800°C (P ~ 5 kbar). Therefore the Wells, Powell, Wood & Banno and Fonarev & Graphchikov (cpx) temperatures are almost certainly too high. In the absence of a more precise independent T estimate it is difficult to assess the relative merits of the results obtained from the remaining versions of the two-pyroxene thermometer, none of which can be unequivocally demonstrated to be seriously in error, though the Lindsley (opx) T is probably too low. Other significant shortcomings evident in the results include the relatively poor precision obtained from the three methods based on purely graphical representation of the augite limb of the solvus (i.e., the Ross & Huebner, Fonarev & Graphchikov (cpx) and Lindsley (cpx) versions), and the apparent dependence of derived T on Mg/Fe2+ ratio for the Powell, Wood & Banno and Lindsley (cpx) methods.For the bulk compositions of exsolved domains, the different versions of the two-pyroxene thermometer yield mean temperatures 23° to 82°C (overall mean, 65°C) higher than for homogeneous grains in the same samples. These exsolved domains are interpreted as relics of a higher-T (peak?) metamorphic assemblage, rather than an igneous precursor.

Notes: Seismic reflection methods are being developed at the University of Manitoba to aid in determining fine crustal structure in the Precambrian of Manitoba and northwestern Ontario. Present-day environmental concern as well as mineshaft conditions necessitate the detonation of several smaller charges repeated, say, I times and followed by ‘vertical’ stacking. To obtain the familiar √I improvement in signal-to-noise (S:N) amplitude ratio applying the straight-sum (SS) method, one assumes, among other things, that both S:N ratio and signal variance are the same on all traces. Dropping these assumptions, as we must for our data, it becomes necessary to apply weighting coefficients to optimize the S:N ratio of the stacked trace. We still assume the signal shapes to be the same for repeated shots, so for the jth trace on the record of the ith shot we model the time series as: tij=ai (sj+nij); where ai is a scaling factor. The proper weights wi are then shown to be proportional to σsi/σ2ni where σ2 is variance, or to γi/ai where γi is S:N power ratio.Applying the weighted-stack (WS) method gives S:N amplitude ratios which are, on average, 55% of the optimal ratios expected from WS theory compared with only 24% for the SS method. The 45% shortfall in WS performance is ascribed mainly to trace-alignment (or time-delay) errors. Varying noise levels on individual traces, slight dissimilarity of signal shape, and correlated noise may also contribute to a lesser extent (in decreasing order of significance). This WS method appears to strike a good practical balance between S:N improvement and processing efficiency.

Notes: Contemporary use patterns of land and water resources create conflicts between natural and human environments. These conflicts are generally not discussed as part of a classical approach to geologic education. This paper addresses the need to increase non-earth science training simultaneous with traditional training. The benefits to be gained will be welcomed by both students and their potential employers. One approach to suggested interdisciplinary water-resource education is discussed.

Notes: Biological damage to plants is commonly found from the Devonian but occurs most commonly from the Cretaceous. Damage inflicted on plants whilst they were alive may trigger a pathological response involving the growth of abnormal tissues. Much of the damage is caused by arthropods, particularly insects. Whilst some damage is non-taxon specific, such as simple feeding traces, other damage, such as leaf mines, galls or bark boring, may reveal the co-evolution of host specific taxa and the timing of such interactions. Damaged plants, particularly from the Cretaceous and Tertiary are described and illustrated. The geological history of the evolution of insect-related plant damage is briefly reviewed. Increased variety in the pathological response of plants is seen from the Cretaceous with the evolution of the angiosperms and diversification of numerous insect groups.

Notes: The growing number of ground-water research projects that are based primarily on potential distribution data has stimulated the development of several new types of potential measuring, instruments. The primary requirements for these instruments are longevity, precision, sensitivity, and short timelag. Ease of installation, minimum maintenance, low capital investment, and provision of a continuous record are equally desirable. Generally, available instruments will satisfy two or three, but not all of these requirements.The need for a piezometer possessing all these characteristics was encountered during a flow-system study beneath an irrigated field. A piezometer subsequently was designed which combines the sensitivity of a Kecke electrical water-level sensing device with the continuous record provided by a Stevens Type F, graphic recorder. Replacement of the standard Keck sensing “Bob” with a probe specifically designed for small diameter pipes (5/16-inch I.D.),* provides instrumentation with a short timelag. The absence of a diaphragm and strain gauges enhances longevity. The device is inexpensive, simple and easily installed. Samples of well water for chemical analysis may also be obtained from the piezometers.

Notes: A finite-element method which incorporates mesh adaptation is used to calculate ground-water flow and pollutant transport. The formulation is based on the equations for conservation of mass, Darcy's law for an anisotropic medium, and the time-dependent species transport equation. Modifications have been implemented to the finite-element formulation to enhance computational speed and reduce storage; Petrov-Galerkin weighting of the advection terms provides numerical stability. An explicit time marching scheme is used to solve the transient equations. By utilizing unstructured adaptive meshing, species concentration and location of steep fronts are accurately resolved, even though one begins with a coarse mesh. The algorithm currently runs on PC and workstation class computers.

Notes: The Devonian of East Greenland comprises a thick sequence of continental clastic sediments infilling an extensional basin. West of the main basin bounding fault (Western Fault Zone) are scattered outliers of Devonian conglomerate which accumulated in small basins such as found on the island, Ella Ø. The Ella Ø Basin formed by extensional movement along the Narhval Sund Fault accompanied by the formation of a prominent bedding parallel detachment surface which was subsequently modified by sub-aerial exposure to become the unconformity surface. Mapping of this unconformity surface shows major vertical relief. A thick sequence of conglomerate occurs on Ella Ø, which, close to its exposed basal unconformity, has three lacustrine beds intercalated within it. Detailed analysis of one lake unit shows it to have significant lateral variation. At proximal localities it largely comprises lacustrine turbidites, whereas more distal locations were within a stratified lake. The interpreted sequence of events on Ella Ø is an interval of fluvial sedimentation followed by rapid drowning of the topography with lacustrine sediments onlapping onto basement. After an interval of deeper lacustrine sedimentation including laminites, the lake shallowed, the lithology changed to limestone and the lake dried out. Conglomerate deposition then recommenced. Maximum lake water depth of 100 m is estimated following correction (7°) for post-Devonian rotation, both determined using the distribution of lacustrine sediments on the unconformity surface. The preferred mechanism for flooding of the Ella Ø Basin is episodic flooding of the entire basin system. Lacustrine sediment preservation results purely from its deposition amongst topography at the edge of the active fluvial system. In such fluvial systems lacustrine sediments may be part of the normal sedimentary cycle but were almost always removed by reworking.

Notes: Many interesting inverse problems in geophysics are non-linear and multimodal. Parametrization of these problems leads to an objective function, or measure of agreement between data and model predictions, that has a complex topography with many local minima. Optimization algorithms that rely on local gradients in the objective function or that search the model space locally may become trapped in these local minima. By combining simulated annealing with the downhill simplex method, a hybrid global search algorithm is presented in this paper for non-linear, multimodal, inverse problems. The hybrid algorithm shares the advantages of both local search methods that perform well if the local model is suitable, and global methods that are able to explore efficiently the full model space. The hybrid algorithm also utilizes a larger and more complex memory to store information on the objective function than simulated annealing algorithms. The effectiveness of this new scheme is evaluated in three problems: minimization of the multidimensional Rosenbrock function, non-linear, 1-D, acoustic waveform inversion, and residual statics. The performance of the hybrid algorithm is compared with simulated annealing and genetic algorithms and is shown to converge more rapidly and to have a higher success rate of locating the global minimum for the cases investigated.