Structural and Stratigraphic Development of a Salt-Diapir Shoulder, Gypsum Valley Salt Wall, Paradox Basin, Colorado Joshua McFarland, Katherine A. Giles1, Richard Langford1, and Mark G. Rowan2 1 Institute of Tectonic Studies, Department of Geological Sciences, The University of Texas at El Paso, 500 West University Avenue, El Paso, Texas 79968, USA 2 Rowan Consulting, Inc., Boulder, CO, USA [email protected] Outcrop studies of the Gypsum Valley salt wall in Southwest Colorado have identified a salt shoulder formed by onlap and burial by the flanking Triassic Chinle Formation. A salt shoulder is a local geometric feature of some salt diapirs that refers to an abrupt upward narrowing of the diapir. Stratal anticlines above salt shoulders are exploration targets in producing salt basins such as the Precaspian. In Gypsum Valley, incision by the modern Dolores River through the salt shoulder has created a natural amphitheater for the 3D study of the feature. This has allowed us to build a three-dimensional reconstruction of the shoulder and the facies architecture of the onlapping Chinle Formation. This study: (1) defines the 3-D fluvial reservoir geometry and continuity proximal to the diapir shoulder; (2) provides an interpretation and model for the formation of the shoulder and associated structural-stratigraphic trap geometries; and (3) furthers our understanding of the relationship between the tectonic and stratigraphic evolution of fluvial depositional systems and passively rising salt bodies. The 3D model was developed using geo-referenced outcrop photos, using Agisoft Photoscan software in order to recreate the terrain in three dimensions. Modeling using Maptek and Move software was performed in order to position facies, input stratigraphic sections and input key faults. Field-based data include geo-referenced measured sections and mapping within the Chinle Formation, starting in pre-shoulder strata and continuing up to post shoulder strata. Sections were measured along the strike of the salt wall, as well as perpendicular to the salt wall, moving outward into the adjacent mini-basin. Chinle facies include both channel conglomerates and debris-flow breccias containing diapir-derived gypsum and carbonate caprock clasts, lower fine to medium grained, erosive based sandstones (horizontally laminated, massive, and ripple cross laminated), and clay dominated mudstones. Upsection variations in stratal geometry, facies architecture, and position and geometry of intraformational unconformities reflect a decreasing rate of diapiric rise relative to sediment accumulation. This, along with dissolution collapse of the diapir top contributed to the formation of the shoulder and associated anticline.