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Interpreting Amalgamation Processes of a Fluvial Sandstone of the Nacimiento Formation in the San Juan Basin, New Mexico

Abstract

Understanding geometries and internal structures of fluvial deposits is invaluable for future studies involving groundwater, hydrocarbons, and contaminants in the subsurface. Outcrop studies can provide insights into the internal structure of such sandstone bodies, and modern 3-D photogrammetric analysis can aid in the mapping of bounding surfaces and facies. The Paleocene “unnamed” member (Tnu) of the Nacimiento Formation (Tn), located in the San Juan Basin, New Mexico, is a fluvial multi-storey sheet sandstone deposited in the medial to proximal areas on a prograding distributive fluvial system. In an excellent double canyon exposure, we identified bounding surfaces and analyzed internal structures using alluvial architecture analysis on both standard photomosaic images and 3D outcrop models produced with structure-from-motion photogrammetry. Raw outcrop images for these models were acquired with both ground-based cameras and a Phantom 4 drone with gimbaled camera. We are testing four possible hypotheses related to amalgamation processes that may have produced this sandstone: (1) intra-channel-belt processes, such as bars or chute and neck cut-offs, (2) amalgamation of more than one channel-belt by a regional or nodal avulsive process, (3) amalgamation by incised valley where the channel-belt migrates within a container valley, or (4) some combination of any three. Bounding surfaces, internal structures, and paleoflow measurements were collected in the field. Adobe Illustrator was used to draft bounding surfaces on orthomosaic outcrop images. A high-quality 3-D outcrop model was created using Agisoft Photoscan software. Within the 3D model, we can project bounding surfaces across a single canyon and between adjacent canyons. This enables us to interpret the depositional processes responsible for amalgamation of the “unnamed” member multi-storey sheet sandstones. Internal structure and geometries of the amalgamated sandstone will be measured and may be used for permeability structure models in the future. Preliminary outcrop models in Agisoft Photoscan and drafting in Adobe Illustrator shows great potential for identifying and correlating bounding surfaces in 3-D space. Preliminary analysis of this medial-to-proximal transition sandstone indicates that amalgamation is due to multiple channel-belt sandstones combining to produce a multi-storey sheet sandstone.