A 15,500 YEAR RECORD OF LANDSCAPE RESPONSE TO CLIMATE CHANGE PRESERVED IN LATE-QUATERNARY ALLUVIAL FANS IN MIDCONTINENTAL NORTH AMERICA

Alluvial fans along a steep bio-climatic gradient that extends from the dry-subhumid and semi-arid shortgrass prairie of the west-central High Plains to the moist-subhumid forest-prairie border of the Central Lowlands harbor well-preserved records of sedimentation and soil formation produced by landscape response to bioclimatic change. Alluvial fan development was time-transgressive across the region. On the High Plains of western Kansas and Nebraska, slow sedimentation began to form fans about 13,500 ¹⁴C yr B.P. and was punctuated by multiple episodes of soil development between ca. 13,000 and 9,000 ¹⁴C yr B.P. Organic-rich cumulic soils dating to the Younger Dryas (11,000-10,000 ¹⁴C yr B.P.) are especially common and are mantled by thick early- through middle-Holocene fan deposits; late-Holocene fan deposits generally are absent. In the Central Lowlands, on the other hand, alluvial fan sedimentation began between 9500 and 8500 ¹⁴C yr B.P. and ended by ca. 2000 ¹⁴C yr B.P. Most fans in the Central Lowlands exhibit three major aggradation episodes; one from about 8500-6500 ¹⁴C yr B.P., a second from 6000 to ca. 4000 ¹⁴C yr B.P., and a final episode from 3000 to 2000 ¹⁴C yr B.P. This sedimentation pattern has produced a related soil stratigraphic pattern in those fans; buried soils dating to about 10,000, 8500, 6500, 4100, and 2500 ¹⁴C yr B.P. are common. The two fan aggradation episodes associated with the greatest sedimentation rates, and by inference the most delivery of sediment from tributary basins, began about 8500 and 6000 ¹⁴C yr B.P respectively.

We postulate that regional climate change forced time-transgressive geomorphic response of small drainage basins that is reflected in alluvial fan sedimentation patterns across the region. Fan sedimentation cycles were driven by interactions among climate-related changes in vegetation communities that altered ground cover, and changes in the magnitude, frequency, and seasonal distribution of precipitation. These bioclimatic changes altered erosion, runoff, and sediment delivery relationships in the basins feeding alluvial fans.