Sunday Structural Sundries 001. This is a repost of a series I initiated a couple of years ago on LinkedIn and then slacked off—time to revive it, so here it is! Link is provided here as there are some great commentary.

Multi-scale examples of dual-fault fold-thrust systems. The dual-fault aspect results from fault(s) propagating through the anticlinal and synclinal cores. Early-stage out-of-the-syncline subsidiary deformation is common (e.g., Five Springs and small-scale examples) but with progressive deformation the intervening strata become overturned and thinned, especially in shales, as the structure evolves from parallel to similar style folding. Shale thinning, for example, has resulted in the juxtaposition or welding of the Madison and Tensleep units due to the removal of the Amsden unit. The Five Springs and EA examples are from the Rocky Mountain Foreland of Wyoming, U.S., the outcrop example from Himalayas near Narendranagar, Uttarakhand, India, and small-scale example from laboratory modeling at Texas A&M University by J. Chester (1985).

For some additional insight/commentary, R. Berg described the dual-fault fold-thrust geometry back in the 60s with a couple of AAPG Bulletin papers. The basic geometry as encountered by wellbores is shallow upright strata (or basement), fault(s) in anticlinal core, overturned thinned strata, fault(s) in synclinal core, and followed by upright subthrust strata. The EA thrust shown in graphic is a great example. This has now been observed in foreland deformation and thrust belt deformation around the world. I especially like the Five Springs example as US Hwy. 14A effectively serves as a “deviated wellbore” through a fold-thrust system—large scale with smaller-scale out-of-the-syncline subsidiary deformation—the basement island during Cambrian transgression is an awesome bonus!