RMS-AAPG: Fault, Fold, and Cleavage Duplexes of Sandy Hollow
**Offered as part of the 2026 Annual RMS-AAPG Annual Conference
Field Trip Leader: Stuart Parker (MBMG), Steven E Boyer*
*Co-author of field guide; not in attendance
Field Trip Fee: $50 for professionals; $20 for students
Field Trip departs from and returns to Montana Tech campus.
Field Trip Highlights: The incredible exposures of folded and faulted rocks within Sandy Hollow, located about 50 miles south of Butte, have long been used by structural geologists as analogs for the structures that characterize thin-skinned fold-thrust belts around the world. Understanding the geometry and deformation mechanisms associated with folds and faults is crucial when assessing hydrocarbon potential in these tectonic settings. The series of fault-propagation folds and associated thrust faults observed around Sandy Hollow are structural analogs to what occurs at depth within many oil and gas fields in foreland settings. On this field trip we will focus on the outcrop to meso-scale duplexes that commonly occur within these larger fault-propagation folds and thrust faults, in an attempt to gain a better geometric understanding of smaller-scale, more distributed deformation zones that may not be directly visible on seismic surveys. The field trip will highlight the wide array of deformation mechanisms that operate within duplexes, which may include faulting, folding, or pressure solution (cleavage). Field examples of fault, fold, and cleavage duplexes will be used to demonstrate how porosity and permeability may be either created or destroyed during progressive deformation. Understanding how strain evolved within these common but small-scale structures allows for detailed inferences to be made regarding porosity and permeability within larger potential hydrocarbon targets, specifically in anticlines above fault-propagation folds like those on display in Sandy Hollow.
Questions? Contact 2026 RMS-AAPG field trip co-chairs Mary Bitney and Jake Thacker (rmsaapgmeeting@gmail.com)