A COMPARISON OF THE FRACTURE AND FLUID HISTORY OF FOUR FOLDS IN THE IDAHO-WYOMING SALIENT

This study compares the fracture and fluid history of four folds in the Jurassic Twin Creek Formation in the Wyoming Idaho salient. In order to understand the difference in fracture formation and fluid connectivity in folds we examined 4 separate folds of various sizes from two separate structural domains in the Idaho-Wyoming salient. The folds examined in this study are the Fairview anticline, Kimbell Junction anticline, Lambs Canyon anticline, and the Summit Park anticline.

The Fairview anticline is a large scale, doubly plunging fold located in the central portion of the salient. The samples from this location were collected only on the northern plunge out of the 20 km long, 3 km wide fold. This fold has 3 fracture sets. The primary set strikes 066° ± 6°, and two subsidiary sets strike 336°± 6° and 045°± 5. The stable isotope data from the vein samples from this anticline have a δ¹⁸O range of -13.96 to -8.16 ‰, and a δ¹³C range of 1.61 to 2.73 ‰. All bed-normal veins exhibit closed system behavior, while a bed parallel vein set was mineralized in an open fluid system.

Lambs Canyon, Kimbell Junction, and Summit Park anticlines are located in the southern part of the salient, are smaller than the Fairview fold, and have moderate to steeply dipping fold axes due to the influence of Uinta uplift. The Lambs Canyon fold is 2.5 km wide and 4.6 km long, with a fold axis that trends 011° and plunges 58°. There are two sets of bed-normal fractures in this anticline, 091°±8° and 012°±12°. The ~E-W striking set has isotopic values similar to the host rock indicating a closed fluid system, whereas a wedge fault has a 9 ‰ difference in δ¹⁸O from the host rock, suggesting an open system

The Kimbell Junction fold is 300 m across with a fold axis that trends 018° and plunges 18°, and the Summit Park fold is the smallest fold (100m wide) with a fold axis that trends 297° and plunges 58°. Kimbell Junction has two sets of fractures, one set strikes 159°± 5° and the second set strikes 117°±10°. The Summit Park fold has a wide range of bed-normal fracture orientations with strike ranging from 160° to 095°. Both of these folds have mineralized veins that are ~9 ‰ different in δ¹⁸O form the host rock, indicating open isotopic systems.

In summary, fold scale plays little role in whether a fracture fluid system in open or closed to external fluid reservoirs.