New Mexico Geological Society Annual Spring Meeting — Abstracts

The San Juan River is a 616 km-long tributary to the Colorado River in Colorado, New Mexico, Arizona, and Utah, USA. Its incision history is complex, owing in part to baselevel fall at its confluence with the Colorado River at Glen Canyon, rapid uplift of its headwaters in the San Juan Mountains, and broad but variable epeirogenic uplift through its central section across the core of the Colorado Plateau. Recent studies have added data-rich constraints on incision rates along the main stem of the San Juan River and its largest tributary by discharge, the Animas River; both of which are heavily influenced by high elevation (>4 km) headwaters in the San Juan Mountains that underwent multiple Quaternary glaciations. The more areally extensive San Juan River-left tributaries draining the central Colorado Plateau, including Cañon Largo, the Chaco River, and Chinle Creek, which together comprise approximately 50% of the total drainage area of the San Juan River, have received far less study on their uplift or incision histories. Recent geologic mapping in the upper Chaco River drainage on the Navajo Nation allowed close inspection of complex geomorphic surfaces on the Chuska Mountain front. These surfaces suggest punctuated and repeating episodes of pedimentation, alluvial aggradation, and bedrock incision leading to a flight of pediment-terraces on the Chuska Mountains’ eastern front. Here I present new ages acquired from optically stimulated luminescence (OSL) dating of alluvial sediment capping two of these pediment-terraces. A pediment-terrace whose strath is 48 to 52 m higher than local baselevel yielded OSL ages of 52.32 +/- 5.12 ka and 50.88 +/- 5.82 ka. A lower pediment-terrace whose strath is 10 m above local baselevel yielded an OSL age of 37.56 +/- 3.62 ka. Bedrock incision rates derived from the older surface equal 1051 m/My and 983 m/My. The younger surface yields a bedrock incision rate of 266 m/My. Previously published long-term bedrock incision rates across the southern Rocky Mountains Colorado Plateau through the Quaternary average from 50 to 160 m/My, but rates of ~255 m/My have been reported in the Animas River near Durango—attributed to uplift of the San Juan Mountains relative to the Colorado Plateau—and in the San Juan River near its mouth—attributed to rapid baselevel fall due to Colorado River incision. The high incision rates reported from the older surface here (~1010 m/My) are much greater than any reported in regional river systems and almost certainly do not reflect long-term bedrock incision rates but rather record rapid adjustment of a headwater system to local geomorphic and/or climatic changes. If the OSL ages reported here are accurate, then they perhaps should lead to a more careful consideration of widely-reported long-term bedrock incision rates elsewhere with regard to the inherent episodicity of landscape development. The mapping and ages reported here highlight the need for more and better basin-wide investigation of river incision histories in order better to understand the complex relationships among climate, tectonics, sedimentation, and erosion in the San Juan basin—a need first pointed out by Kirk Bryan in his 1954 The Geology of Chaco Canyon, New Mexico.