Analysis of bimodal orientation data

Statistical models underlying the analysis of orientation data commonly assume a unimodal symmetric population, such as the circular normal distribution. If the sample distribution is skewed or bimodal, standard procedures usually produce misleading results. Where such situations occur, a mixture of two or more circular normal distributions may be used as the population model. The parameters describing each mode and the mixing proportion may be estimated by the method of maximum likelihood using numerical techniques. This approach is applied to a distinctly bimodal set of cross-bedding data from the Mississippian Salem Limestone of central Indiana.     

Salt dissolution features in saline lakes of the northern Great Plains, western Canada

Large-scale salt dissolution is an important process affecting the sediments of many of the saline lakes in the northern Great Plains region of western Canada. The most easily recognized features of this salt karst are water-filled chimneys, vertical shafts, and collapse structures. The largest individual chimneys can be up to 20 m deep and 50 m in diameter, with volumes exceeding 25,000 m³. Large, mud-filled chimneys and cavities, and salt-filled chimneys have also been identified in both the modern lakes and in the Quaternary sediments of the basins, which can adversely affect the salt mining potential of the basins. Because these salt karst features can affect large vertical sections of the sediment fill in the lakes, their recognition is of fundamental importance in attempting to use the stratigraphic records of the basins for paleoenvironmental research.     

A Method for Assessing the Planimetric Accuracy of Historical Maps: The Case of the Colorado-Green River System*

The position of linear elements on historical maps can be digitized and numerically compared with similar features on modern maps. This allows for the evaluation of amounts and rates of improvement in cartographic accuracy and of the primary sources used for the generation of subsequent maps. A case study of the Colorado-Green River system illustrates the method and documents the pace of improvement in the cartographic depiction of the river system as a function primarily of the historical pressures of westward expansion.     

Ion balance,acid-base regulation,and chloride cell function in the common killifish,Fundulus heteroclitus—a euryhaline estuarine teleost

The common killifish,Fundulus heteroclitus, is a euryhaline teleost common throughout estuaries of eastern North America. This symposium paper reviews the important contributions of the killifish to our present understanding of ionoregulation in seawater (SW) fish and their mechanisms of euryhalinity, and presents new data developing the killifish as a freshwater (FW) model system. Experiments on killifish have characterized (i) drinking in SW and its reduction in FW; (ii) the adaptive roles of the kidney to SW and FW conditions; (iii) the instantaneous (Phase I) and delayed (Phase II) reductions in Na⁺ outflux that occur upon transfer from SW to FW; (iv) the importance of prolactin secretion in the Phase II effect; (v) the cortisol-stimulated induction of branchial Na⁺, K⁺-ATPase that occurs upon transfer from FW to SW; (vi) the accompanying changes in morphology of the mitochondria-rich (MR) or “chloride cells” on the gills; (vii) the localization of this Na⁺, K⁺-ATPase activity to the basolateral membrane of chloride cells; and (viii) the NaCl-secretory function of these cells in SW. The opercular epithelium, which is rich in chloride cells, has been used as an in vitro model to characterize the mechanisms and control of NaCl secretion in SW fish. Much less is known about gill function in fresh water (inward NaCl transport), primarily due to the absence of a comparable freshwater model. Here we show that killifish acclimated to dilute FW ([NaCl] = 1 mmol I^?1) possess large numbers of MR cells on the opercular epithelium. When mounted in vitro with FW on the outside, the preparation develops a large inside negative transepithelial potential (TEP) that is a Na⁺ diffusion potential. By the Ussing flux ratio criterion, Na⁺ fluxes are passive, but a small active influx of Cl^? occurs, an observation that supports the involvement of MR cells in active Cl^? uptake. This FW opercular epithelium if bathed with isotonic saline on both sides does not secrete Cl^?, indicating that the MR cells indeed are of the FW type. In vivo, the fish exhibits a high rate of Na⁺ influx and outflux; Cl^? outflux is much lower, and there is no detectable Cl^? influx. Experimental variation of FW [NaCl] reveals a saturable, low affinity Na⁺ uptake mechanism, a Cl^? influx mechanism that is activated only at much higher concentrations, and no evidence of exchange diffusion. Acid-base disturbance appears to be corrected by differential regulation of the outflux components only. Hence, the FW killifish ionoregulates somewhat differently from the few other FW teleosts that have been examined, and its opercular epithelium will serve as a very useful model system.     

A model for the three-dimensional evolution of continental rift basins,north-east Africa

Large areas of north-east Africa were dominated by regional extension in the Late Phanerozoic. Widespread rifting occurred in the Late Jurassic, with regional extension culminating in the Cretaceous and resulting in the greatest areal extent and degree of interconnection of the west, central and north African rift systems. Basin reactivation continued in the Paleocene and Eocene and new rifts probably formed in the Red Sea and western Kenya. In the Oligocene and Early Miocene, rifts in Kenya, Ethiopia and the Red Sea linked and expanded to form the new east African rift system.This complex history of rifting resulted in failed rift basins with low to high strain geometries, a range of associated volcanism and varying degrees of interaction with older structures. One system, the Red Sea rift, has partially attained active seafloor spreading. From a comparison of these basins, a general model of three-dimensional rift evolution is proposed. Asymmetrical crustal geometries dominated the early phases of these basins, accompanied by low angle normal faulting that has been observed at least locally in outcrop. As rifting progressed, the original fault and basin forms were modified to produce larger, more through-going structures. Some basins were abandoned, others experienced reversals in regional dip and, in general, extension and subsidence became focused along narrower zones near the rift axes. The final transition to oceanic spreading was accomplished in the Red Sea by a change to high angle, planar normal faulting and diffuse dike injection, followed by the organization of an axial magma chamber.     

Inversion of reflection seismograms by differential semblance analysis: algorithm structure and synthetic examples1

Seismograms predicted from acoustic or elastic earth models depend very non-linearly on the long wavelength components of velocity. This sensitive dependence demands the use of special variational principles in waveform-based inversion algorithms. The differential semblance variational principle is well-suited to velocity inversion by gradient methods, since its objective function is smooth and convex over a large range of velocity models. An extension of the adjoint state technique yields an accurate estimate of the differential semblance gradient. Non-linear conjugate gradient iteration is quite successful in locating the global differential semblance minimum, which is near the ordinary least-squares global minimum when coherent data noise is small. Several examples, based on the 2D primaries-only acoustic model, illustrate features of the method and its performance.