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Abstract: Based on the study of the Madison Limestone at Sheep Mountain and Rattlesnake Mountain, a unique outcrop dataset including (1) facies and diagenetic analyses, (2) vertical persistence and cement stratigraphy of vein sets and (3) fluid inclusions thermometry are used to demonstrate the impact of folding and fracturing on paleo-hydrogeology. Quantification of the vertical persistence of fractures shows that Sheep Mountain and Rattlesnake Mountain differ by the vertical persistence of the pre-folding Laramide vein sets, which are strictly bed-confined in Sheep Mountain but cut across bedding at Rattlesnake Mountain, whereas the syn-folding veins are through-going in both. The emplacement chronology and the various sources of the fluids responsible for the paragenetic sequence are based on isotope chemistry and fluid inclusions analysis of the matrix and vein cements. At Sheep Mountain and Rattlesnake Mountain, the cements related to the burial are characterized by isotopic signatures of marine formation waters that were diluted during the karstification of the Madison Platform at the end of Mississippian. Meteoric fluids, presumably migrating during the Cenomanian from Wind River Range and Teton Range, recharge zones located in the south-west of the Bighorn Basin, were remobilized in the early bed-confined and through-going syn-folding veins of the Sheep Mountain Anticline. The former vein set drained only local fluids whose isotopic signature relates to an increase of temperature of the meteoric fluids during their migration, whereas the latter set allowed quick drainage of basinal fluids. [Copyright &y& Elsevier]

Abstract: The Rocky Mountains in western US provide among the best examples of thick-skinned tectonics: following a period of thin-skinned tectonics related to the Sevier orogeny, the compressional reactivation of basement faults gave birth to the so-called Laramide uplifts/arches. The Bighorn basin, located in Wyoming, is therefore a key place to study the transition from thin- to thick-skinned tectonics in orogenic forelands, especially in terms of microstructural and stress/strain evolution. Our study focuses on a classic Laramide structure: the Rattlesnake Mountain Anticline (RMA, Wyoming, USA), a basement-cored anticline located in the western part of the Bighorn basin. Stress and strain evolution analysis in folded sedimentary layers and underlying faulted basement rocks were performed on the basis of combined analyses of fractures, fault-slip data and calcite twinning paleopiezometry. Most of the fractures are related to three main tectonic events: the Sevier thin-skinned contraction, the Laramide thick-skinned contraction, and the Basin and Range extension. Serial balanced cross-sections of RMA and displacement profiles suggest that all thrust faults were coeval, evidencing strain distribution in the basement during faulting. The comparison of RMA with another structure located in the eastern edge of the Bighorn basin, i.e. the Sheep Mountain Anticline (SMA), allows to propose a conceptual model for the geometric and kinematic evolution of Laramide-related basement-cored anticlines. Finally, the stress evolution is reconstructed at both the fold scale and the basin scale. We show that the evolution of stress trends and magnitudes was quite similar in both structures (RMA and SMA) during Laramide times (thick-skinned tectonics), in spite of different stress regimes. During Sevier (thin-skinned tectonics) and post-Laramide times, stress trends and fracture patterns were different in these two structures. These results suggest that the distance to the orogenic front influenced the fracture patterns but not the foreland stress magnitudes, which were likely controlled by the structural style. [Copyright &y& Elsevier]