107-14 Fan and Fill: Sedimentology and Stratigraphy of Syn- to Post-tectonic Deposits within the Safford Basin, Southeastern Arizona

Session: Sedimentary Geology Division/SEPM Student Research Poster Competition: Dynamics of Stratigraphy and Sedimentation

Presenting Author:

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Abstract:

The sedimentary deposits of the Safford Basin record the transition from closed, internally drained basins to integrated river systems associated with significant incision of basin fill deposits. The basin fill—a thick sequence of sands, muds, and carbonates—is capped by a thin layer of gravels, which has been attributed to, and interpreted as, evidence for a change in sedimentation due to a climatic shift approximately 5 Ma. Alternatively, this change could instead be explained by depositional processes, namely progradation of alluvial fan gravels over finer deposits caused by the decrease in accommodation space associated with waning basin subsidence as Basin and Range tectonics ceased. The relationship between these deposits has generally been observed as a sharp upsection change from fine to coarse throughout the basin. In the case of a progradational sequence, the coarse sediments would be present throughout deposition but confined to near the range front until subsidence ended and the alluvial fan spread across the basin.

This project explores the relationship between the alluvial gravels and basin fill by determining the geometry of the contact between the facies from key exposures along the flank of the Pinaleño Mountains. In order to test the fan progradation hypothesis, we looked for evidence of laterally adjacent coarse and fine deposits, which would imply that lateral facies migration brought fan gravels above fill deposits, as opposed to a strictly vertical change. Using a newly released LiDAR DEM and hillshade images (1 and 5 m resolution) of the area, we located potential exposures. Subsequent expeditions to these locations allowed the identification of sedimentological facies and determination of the stratigraphic architecture. While the contact often remained uncertain (due to lack of exposure), the interfingering of mud and sand deposits (likely lake or stream beds) with gravels suggests that fan progradation in response to declining subsidence rate is the most supported explanation rather than primarily reflecting a climate change. Future research in other basins in the region is planned and may offer additional insights into the basin level processes involved in landscape evolution and sedimentation patterns in response to tectonic and/or climatic changes.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7736

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