The feasibility of a land information system for Belize,Central America

Abstract

Limitations of the current land records systems in Belize, Central America, led leaders of the Ministry of Natural Resources to initiate this feasibility study for a computer-aided land information system (LIS). Land records in Belize have been based on the British Torrens land tenure system, which maintains five sets of records along with informative maps. One paper file is maintained for each title application and records are continually circulated between seven locations throughout the country, resulting in up to 3 years to complete the process of title registration. A LIS will permit quick access to land records and eliminate most of the current problems, and will merge with a geographic information system being developed for Belize.     

An inductive modelling procedure based on Bayes' theorem for analysis of pattern in spatial data

Abstract

This paper describes an inductive modelling procedure integrated with a geographical information system for analysis of pattern within spatial data. The aim of the modelling procedure is to predict the distribution within one data set by combining a number of other data sets. Data set combination is carried out using Bayes’ theorem. Inputs to the theorem, in the form of conditional probabilities, are derived from an inductive learning process in which attributes of the data set to be modelled are compared with attributes of a variety of predictor data sets. This process is carried out on random subsets of the data to generate error bounds on inputs for analysis of error propagation associated with the use of Bayes’ theorem to combine data sets in the GIS. The statistical significance of model inputs is calculated as part of the inductive learning process. Use of the modelling procedure is illustrated through the analysis of the winter habitat relationships of red deer in Grampian Region, north-east Scotland. The distribution of red deer in Deer Management Group areas in Gordon and in Kincardine and Deeside Districts is used to develop a model which predicts the distribution throughout Grampian Region; this is tested against red deer distribution in Moray District. Habitat data sets used for constructing the model are accumulated frost and altitude, obtained from maps, and land cover, derived from satellite imagery. Errors resulting from the use of Bayes’ theorem to combine data sets within the GIS and introduced in generalizing output from 50 m pixel to 1 km grid squares resolution are analysed and presented in a series of maps. This analysis of error trains is an integral part of the implemented analytical procedure and provides support to the interpretation of the results of modelling. Potential applications of the modelling procedure are discussed.     

Calcretes,fluviolacustrine sediments and subsidence patterns in Permo‐Triassic salt‐walled minibasins of the south Urals,Russia

The south Uralian foreland basin forms part of the giant, yet sparsely documented, PreCaspian salt tectonic province. The basin can potentially add much to the understanding of fluviolacustrine sedimentation within salt‐walled minibasins, where the literature has been highly reliant on only a few examples (such as the Paradox Basin of Utah). This paper describes the Late Permian terrestrial fill of the Kul’chumovo salt minibasin near Orenburg in the south Urals in which sediments were deposited in a range of channel, overbank and lacustrine environments. Palaeomagnetic stratigraphy shows that, during the Late Permian, the basin had a relatively slow and uniform subsidence pattern with widespread pedogenesis and calcrete development. Angular unconformities or halokinetic sequence boundaries cannot be recognized within the relatively fine‐grained fill, and stratigraphic and spatial variations in facies are therefore critical to understanding the subsidence history of the salt minibasin. Coarse‐grained channel belts show evidence for lateral relocation within the minibasin while the development of a thick stack of calcrete hardpans indicates that opposing parts of the minibasin became largely inactive for prolonged periods (possibly in the order of one million years). The regular vertical stacking of calcrete hardpans within floodplain mudstones provides further evidence that halokinetic minibasin growth is inherently episodic and cyclical.     

Rinded iron‐oxide concretions: hallmarks of altered siderite masses of both early and late diagenetic origin

Iron‐bearing concretions are valuable records of oxidation states of subsurface waters, but the first concretions to form can be altered drastically during later diagenetic events. Distinctive concretions composed of heavy rinds of iron oxide that surround iron‐poor, mud‐rich cores are common along bases of fluvial cross‐bed sets of the Cretaceous Dakota Formation, Nebraska, USA. Concretion rinds thicken inward and cores contain 46 to 89% void space. Millimetre‐scale spherosiderites are abundant in palaeosols that developed in floodplain facies. Evolution of rinded concretions began when intraformational clasts were eroded from sideritic soils, transported, abraded and deposited in river channels. Alteration of siderite and formation of rinds occurred much later, perhaps in the Quaternary when sandstone pore waters became oxic. Dakota concretions are analogous to ‘rattlestones’ in Pleistocene fluvial channels of The Netherlands, and their rinded structure is analogous to that of iron‐rich concretions in the aeolian Navajo Sandstone of Utah. In all three deposits, rinded concretions formed when pre‐existing, siderite‐cemented concretions were oxidized within a sand matrix. Unlike fluvial examples, siderite in the Navajo Sandstone was autochthonous and of late diagenetic origin, having precipitated from carbon dioxide and methane‐enriched waters moving through folded and jointed strata. Iron‐rich rinds formed in all these strata because concretion interiors remained anaerobic, even as oxygen accumulated in the pore waters of their surrounding, permeable matrix. Iron oxide first precipitated at redox boundaries at concretion perimeters and formed an inward‐thickening rind. Acid generated by the oxidation reaction drove siderite dissolution to completion, creating the iron‐poor core. Iron‐oxide rinds are indicators of the former presence of siderite, a mineral that forms only under reducing conditions, during either early or late diagenesis. Siderite is vulnerable to complete oxidation upon exposure, so the distinctive rinded concretions are valuable clues that aid in deciphering diagenetic histories and for recognizing methanic floodplain palaeoenvironments and wet palaeoclimate.     

Morphology and evolution of an anastomosed channel network where saline underflow enters the Black Sea

The Bosphorus Strait accommodates two‐way flow between the Aegean and Black Seas. The Aegean (Mediterranean) inflow has speeds of 5 to 15 cm sec^?1 in the strait and a salinity contrast of ～12‰ to 16‰ with the Black Sea surface waters on the shelf. An anastomosed channel network crosses the shelf and in water deeper than 70 m is characterized by first‐order channels 5 to 10 m deep, local lateral accretion bedding, muddy in‐channel barforms, and a variety of sediment waves both on channel floors and bar crests, crevasse channels entering the overbank area and levée/overbank deposits which are radiocarbon‐dated in cores to be younger than ～7·5 to 8·0 ka. This channel network accommodates the saline density current formed by the Mediterranean inflow. The density contrast between the density underflow and the ambient water mass is ～0·01 g cm^?3, similar to the density contrast ascribed to low‐concentration turbidity currents in the deep sea. Channel‐floor deposits are sandy to gravelly with local shell concentrations. Low‐relief bedforms on the channel floor have relatively straight crests, upflow‐dipping cross‐stratification, heights 1 to 1·5 m and wavelengths 85 to 155 m. Bankfull flows are subcritical, so these probably are not antidunes. Bar tops are ornamented locally with mudwaves having heights 1 to 2 m and wavelengths ～20 to 100 m; these are potentially antidunes formed under shallow overbank flows. Towards the shelf edge, the degree of channel bifurcation increases dramatically and bar tops are dissected locally by secondary channels, some of which terminate in hanging valleys. Conical mounds on the shelf (possibly mud volcanoes or sites of fluid seepage) interact with the channel network by promoting accretion of muddy streamlined macroforms in their lee. This channel network may be one of the largest and most accessible natural laboratories on Earth for the study of continuously flowing density currents. Although the driver is salinity contrast, the underflow transports sufficient sediment to form levée wedges and large streamlined barforms, and presumably transports sediment into deep water.     

Long-Term Evolution of Fluid-Rock Interactions in Magmatic Arcs: Evidence from the Ritter Range Pendant, Sierra Nevada, California, and Numerical Modeling

A record of > 100 million years of fluid flow, alteration,and metamorphism in the evolving Sierra Nevada magmatic areis preserved in metavolcanic rocks of the Ritter Range pendantand surrounding granitoids. The metavolcanic rocks consist of:(1) a lower section of mostly marine volcaniclastic rocks, lavas,and intercalated carbonate rocks that is Triassic to Jurassicin age, and (2) an upper section comprising a subaerial caldera-fillcomplex of mid-Cretaceous age. Late Cretaceous high-temperaturecontact metamorphism (2 kbar, >450–500C) occurredafter renewed normal faulting along the caldera-bounding faultsystem juxtaposed the two sections. The style and degree of alteration and ¹⁸O values differ amongthe rocks of the upper and lower sections and the granitoids.Rocks of the lower section show pervasive lithologically controlledalkali alteration, local Mn and Mg enrichment, and oxidation.Some ash flow tuffs now contain up to 10% K²O by weight. Therocks of the upper section show lesser extents of alkali alteration.Granitoids that cut both sections are generally unaltered. Mostmetavolcanic rocks of the lower section have high ¹⁸O values(+ 11 to + 16%; whole rock and quartz phenocrysts); however,lower-section rocks within the caldera-bounding fault systemhave low ¹⁸O values of + 4 to +7. The metavolcanic rocks ofthe upper section also have low ¹⁸O values of + 2 to + 7. Granitoidshave ¹⁸O values of + 7 to + 10, typical of unaltered Sierrangranitoids. The lower section contains discontinuous veins ofhigh-temperature (450–500C) calc-silicate minerals. Theseveins are typically <5 m long, do not cross intrusive contacts,and postdate the pervasive alkali alteration. Late veins aretypically > 10 m long, formed at temperatures of less than450–500C, and cross intrusive contacts. Veins have similar¹⁸O values to those of the local host rocks. The nature of the alteration and the high oxygen isotopic valuesof the rocks of the lower section indicate that these rocksinteracted extensively with seawater at temperatures <300C,probably in superposed marine hydrothermal systems associatedwith coeval volcanic centers. Metavolcanic rocks of the uppersection evidently interacted with meteorie waters, probablyin a hydrothermal system associated with the Cretaceous caldera;rocks of the lower section that were adjacent to the calderawere also affected by this alteration. The preservation of thesignatures of these earlier events, the nature of the earlyveins, and results from numerical models of hydrothermal flowthat include fluid production indicate that during progradecontact metamorphism, the rocks of the pendant primarily interactedwith locally derived fluids. Fluid flow was predominantly upwardand away from intrusive contacts and down-temperature. Permeabilitiesare estimated to have been between 0•1 and 1µD, whichis that necessary for maintenance of lithostatic fluid pressures.In hydrothermal models with such permeabilities, large-scalecirculation of meteoric fluids develops after prograde metamorphismceases. The nature of the late veins in the Ritter Range pendantsuggests that such a flow pattern evolved only after the pendantand granitoids had cooled below 450–500C. The long-termhistory of alteration documented in the Ritter Range pendantis probably typical of wall rocks in most batholiths *Present address: Department of Geosciences, University of Arizona, Tucson, Arizona 85721     

The effect of ductile deformation on the kinetics and mechanisms of the aragonite-calcite transformation

Abstract The effect of ductile deformation (dislocation creep) on the kinetics of the aragonite-calcite transformation has been studied at 1 atm (330° C and 360° C) and 900-1500 MPa (500° C) using undeformed and either previously or simultaneously deformed samples (500° C and a strain rate of 10^-6 s). Deformation enhances the rate of the transformation of calcite to aragonite, but decreases the rate of transformation of aragonite to calcite. The difference results from a dependence of transformation rate on grain size, coupled with a difference in the accommodation mechanisms, climb versus recry-stallization, of these minerals during dislocation creep. Dislocation climb is relatively easy in calcite and thus plastic strain results in high dislocation densities without significant grain size reduction. The rate of transformation to aragonite is enhanced primarily because of the increase in nucleation sites at dislocations and subgrain boundaries. In aragonite, on the other hand, dislocation climb is difficult and thus plastic strain produces extensive dynamic recry-stallization resulting in a substantial grain size reduction. The transformation of aragonite is inhibited because the increase in calcite nucleation sites at dislocations and/or new grain boundaries is more than offset by the inability of calcite to grow across high angle grain boundaries. Thus the net effect of ductile deformation by dislocation creep on the kinetics of polymorphic phase transformations depends on the details of the accommodation mechanism.     

Styles of ice-marginal deformation at Hagafellsjökull-Eystri, Iceland during the 1998/99 winter-spring surge

This paper describes the internal architecture of a push moraine formed by a winter-spring surge of Hagafellsjökull-Eystri (Iceland) in 1998/99. The sedimentary architecture of this push moraine consists of a multilayered slab of glaciofluvial sediments with a monoclinal structure that has been displaced laterally by the advancing ice margin. The crest and ice-distal face of the moraine consist of subhorizontal sediment sheets, while the ice-proximal face dips steeply (45° to 90°) towards the ice margin. The core of the moraine consists of frozen sediment and thin slabs of glacier ice are embedded in its proximal face. The sediment slabs are characterized by both brittle and ductile styles of deformation. We argue that the observed variation in deformation style is dependent on whether the glacial foreland was frozen or unfrozen at the time of displacement. Frozen foreland would behave in a brittle fashion, while unfrozen foreland is likely to have deformed in a more ductile manner. The associated spatial variations in the degree of foreland freezing could be explained by variation in ice-marginal snow cover. We conclude that the thermal regime of the foreland, and the timing of the ice advance, is of importance to the style of internal deformation found within ice-marginal push moraines.