GUADALUPE MOUNTAINS EROSION: TRACING THE ORIGIN AND EVOLUTION OF STREAM CHANNEL SEDIMENTS

Spatial patterns of erosion are influenced by factors including rock type, rock strength, climate, tectonics, and surface processes. Tracing erosion patterns and identifying erosion controls help to answer a few questions on landscape evolution. The purpose of this research is to trace apatite mineral origins along stream channels and determine tectonic controls on rock type and hardness. This work also evaluates how stream sediments change over time from weathering, in both active channels and related terraces. We propose three hypotheses. First, we predicted that most apatite grains are from easily eroded bedrock. Second, we expect coarse-grained sandstones to yield larger apatite grains. Third, we hypothesize that gravels in active channels will be smoother from abrasion compared to semi-stationary coarser grains found in terraces influenced by chemical weathering. First, we determined quantity and quality of apatite minerals in different rock units for dating with (U-Th)/He methods, in which cooling age is affected by inclusions, zonation, and mineral sizes. The research also assessed gravel size and roughness of active and past deposits based on dimensional measurements and contour gage variability. Most recovered apatite minerals (95%) were anhedral, coarse or rounded. The mean diameter of apatite grains across six samples so far was 96.64 microns, above the optimal threshold of 70. We conducted a preliminary assessment of gravel smoothness using the Krumbein roundness scale. Sandstones in an active channel were more rounded (mean roundness of 0.6) than limestones (mean roundness of 0.4). Limestone gravels in the active stream channel were more rounded than limestone gravels in terraces (mean roundness of 0.2 and 0.3). Ongoing research will provide better understanding of spatial patterns of erosion throughout the uplift history of the Guadalupe Mountains. Understanding past erosion in historic conditions is the key to understanding potential future change.