The response of geomorphic systems to disturbance has been a major focus of geographic research. Nevertheless, because the sensitivity of geomorphic systems to external perturbation is complex, the response of those systems is still poorly understood for many agricultural systems in various geographic settings. This study investigates geomorphic sensitivity and soil erosion under traditional slash-and-burn cultivation. The response of soil erosion to this agricultural practice was investigated in selected plots at different stages of cultivation, representing a chronosequence of slash-and-burn cultivation for the study site. Selected physical and hydrological properties were measured in the field or determined in the laboratory from soil samples obtained from the selected plots. Soil erosion was monitored for the selected plots using bounded runoff plots. Finally, the response of soil erosion to slash-and- burn was assessed at the watershed scale by adapting the Revised Universal Soil Loss Equation to local field conditions. The study results showed that soil’s selected physical and hydrological properties differed according to the age of cultivation. In general, soil properties, such as organic matter, aggregate stability, and infiltration, showed signs of deterioration during the cultivation phase and improvement during the fallow stage of slash-and-burn cultivation. These differences in turn resulted in differences in the erodibility of the soil and the response of soil erosion at the plot scale. The soil erosion rate was observed to be higher during the cultivation stage of slash-and-burn cultivation and lower during the fallow stage. The lowest rate of erosion was recorded in natural forest plots. Overall, soil erosion rates were low considering the study site’s mountainous nature. The results of this study suggest that the response of soil erosion under the practice of slash-and burn cultivation could be minimal in a potentially sensitive humid tropical mountainous environment depending on the specific cover produced, the environmental factors, and the specific cultural management, such as cropping and tilling practices. Maintaining adequate ground cover through cropping and fallow management is the key to keeping soil erosion minimal under the practice of slash-and-burn cultivation in the study area. / text

Décollements are important tectonic elements in thin-skinned fold-thrust belts. However, few studies have addressed the internal structure of décollements because most are deeply buried and internal features typically cannot be resolved in seismic reflection images. Upper Jurassic evaporite exposures in the Potosí uplift of northeastern Mexico provide a unique tectonic window into the décollement of the Laramide-age Sierra Madre Oriental fold belt. In order to constrain the three-dimensional geometry of décollement structures, I mapped a ~20 km2 portion of the décollement at a scale of 1:10,000. I created a new stratigraphy for the décollement interval during mapping, and made detailed structural observations at targeted sites. The ~900 m thick décollement interval consists of gypsum with five carbonate members (up to 120 m thick) and numerous thin (<5 m) carbonate interbeds. These carbonate units delineate map-scale structural patterns and define two structural domains. The middle and upper parts of the décollement in the western domain contain map-scale folds with local map-scale boudinage and thrust faults. The eastern domain exposes the lower part of the décollement, and contains thrust repetitions of carbonate members and a regionally-persistent basal shear zone. These map relationships indicate a stratigraphic variation in structural style. Western domain folds and eastern domain thrust sheets both appear to be related kinematically to overburden folding. In contrast, the basal shear zone accommodated décollement-parallel shear strain in response to overburden translation. Folding and faulting of carbonate members and intervening gypsum units drove localization of simple shear into the basal shear zone, because only the lowermost gypsum interval maintained a favorable orientation sub-parallel to the regional transport direction throughout deformation. This investigation demonstrates that décollements have complex internal structural patterns that are below typical seismic resolution and lateral variations in structural style that cannot be reconstructed from single well cores or small outcrops. Décollement stratigraphy controls variations in strain magnitude within the décollement interval, so that previous models that invoke homogeneous strain within the décollement are incorrect. Complex, laterally-variable structural style and stratigraphic control of strain distribution could be general characteristics of décollements where the décollement interval contains significant contrasts in bed rheology. / text

Many low matrix-porosity hydrocarbon reservoirs are productive because permeability is controlled by natural fractures. The understanding of basic fracture properties is critical in reducing geological risk and therefore reducing well costs and increasing well recovery. Unfortunately, neither geophysics nor borehole methods are, so far, accurate in the acquisition of key fracture attributes, such as density, porosity, spacing and conductivity. This study proposes a new protocol to predict key fracture characteristics of subsurface carbonate rocks and describes how using a relatively low-cost but rock-based method it is possible to obtain accurate geological information from rock samples to predict fracture attributes in nearby but unsampled areas. This methodology is based on the integration of observations of diagenetic fabrics and fracture analyses of carbonate rocks, using outcrops from the Lower Cretaceous Cupido Formation in the Monterrey Salient of the Sierra Madre Oriental, northeastern Mexico. Field observations and petrographic studies of crosscutting relations and fracture-fill mineralogy and texture distinguish six principal coupled fracturing-cementation events. Two fracture events named F1 and F2 are characterized by synkinematic calcite cement that predates D2 regional dolomitization. A third fracture event (F3) is characterized by synkinematic dolomite fill, contemporaneous with D2 dolomitization of host strata. The fourth event (F4) is characterized by synkinematic D3 baroque dolomite; this event postdates D2. The fifth fracture event (F5) is characterized by C3 synkinematic calcite, and postdates D3 dolomite. Finally, flexural slip faulting (F6) is characterized by C3t calcite, and postdates D3 dolomite. Carbon and oxygen stable isotopes were used to validate the paragenetic sequences proposed for the Cupido Formation rocks. The dolomite isotopic signatures are consistent with increasing precipitation temperatures for the various fracture cements, as is expected if fractures grew during progressive burial conditions. Three main groups of calcite cement can be differentiated isotopically. Late calcite cement may have precipitated from cool waters under shallow burial conditions, possibly during exhumation of the SMO. The development of the Structural Diagenetic Petrographic Study protocol, and its integration with geological, geophysical and engineering data, can be applied to oil fields in fractured carbonates such as those located in Mexico, to validate its applicability. / text

Thesis--George-August-Universität. / Spanish summary. Includes bibliographical references (p. 137-142).

No description available.

Le livre 1 décrit le socle pré-jurassique, la couverture, la structure "alpine" de la région étudiée résultant de la superposition de diverses phases tectoniques, l'évolution paléo-géographique et structurale au cours du cycle mexicain. Le livre 2 est consacré à la stratigraphie et structure d'ensemble de la Sierra Madre, tente un essai sur l'évolution du nord du Mexique durant le mésozoïque et le cénozoïque, et met en évidence une ouverture tethysienne jurassique dans la partie sud-est du continent nord-americain.