THE FRACTURE AND FLUID HISTORY OF THE BIG ELK ANTICLINE IN THE IDAHO PORTION OF THE NORTHERN WYOMING SALIENT

Understanding the development of fracture connectivity and fluid evolution in growing anticlinal structures is important for modeling hydrocarbon migration during folding. In this study we examine the Big Elk anticline in the Idaho portion of the northern Wyoming salient. The fold is in the hanging wall of the Absaroka thrust, has a structural amplitude of ~3km, and was deformed during the Cretaceous Laramide orogeny. The rocks are moderately to highly fractured with one or more stages of blocky calcite filling the veins. Vein samples were taken from the carbonate rocks of the Giraffe Creek, Leeds Creek, and Watten Canyon Members of the Jurassic Twin Creek Formation and the Lower Triassic Thaynes Formation.

In the Big Elk anticline, fractures are typically filled with one or more stages of calcite. Fluid inclusions are uncommon. Oxygen and carbon stable isotope analyses are used to constrain the fluid history. Host rock samples in the Thaynes Formation have -1.10 to -3.9‰ δ¹³C VPDB and -8.0 to -12.6‰ δ¹⁸O VPDB. In the Giraffe Creek Member host rocks have 1.37 to 1.86‰ δ¹³C and -10.87 to -11.79‰ δ¹⁸O. In the Watten Canyon and Leeds Creek Members host rocks have 2.5 to 3.26‰ δ¹³C and -7.4 to -8.3‰ δ¹⁸O.

Two subvertical vein sets are present on both limbs and are primarily pre-folding to early syn-folding as poles to veins rotate to near vertical upon unfolding. One set strikes 080±10° and the other strikes 045±5°. The 045±5° striking cross fold veins display a closed isotopic fluid system with only two of eleven veins with a greater than 2‰ separation in δ¹⁸O. The 080±10° vein set is a closed system except in the Thaynes Formation where there is up to a 4‰ variation in δ¹⁸O. Bed parallel veins exhibit ~2‰ variation in δ¹⁸O in the Watten Canyon Member and Thaynes Formation, while in the Giraffe Creek Member there is a ~9‰ variation in δ¹⁸O indicating an open fluid system. Wedge faults display a similar habit as the bed parallel veins with 2‰ variation in δ¹⁸O. The cleavage parallel veins are less than 0.3‰ variation in δ¹⁸O, indicating a closed fluid system. A shear zone vein in the Thanes Formation has a 6‰ variation in δ¹⁸O and a 3.8‰ variation in δ¹³C. In summary, the Thaynes Formation is markedly more open isotopically than the overlying Twin Creek Formation. This is likely due to the Thaynes being a thicker, more homogeneous limestone than the Twin Creek.