HOLOCENE PALEOCLIMATE AND LANDSCAPE RESPONSE FROM LACUSTRINE AND ALLUVIAL STRATIGRAPHY, EL FRESNAL BASIN, BORDERLANDS REGION, NORTHERN CHIHUAHUA, MEXICO

The El Fresnal basin is one of four sub-basins constituting the formerly extensive Pluvial Lake Palomas system of the Borderlands region, northern Chihuahua and southern New Mexico. This basin is highly responsive to minor climate changes because of its large drainage area and high elevation headwaters. Castiglia (2002) documented constructional beach ridge complexes (BRC’s) in the El Fresnal basin that record millennially spaced highstands during the Holocene. This chronology shows early, middle, and late Holocene highstands in the Fresnal Basin as well as a Little Ice Age (LIA) highstand in the adjacent Santa Maria basin. Using this framework, we describe multiple detailed stratigraphic sections and the geomorphic context of these BRC sediments, which comprise lacustrine sediments that interfinger with distal alluvial fan sediments. Radiocarbon dates of 4090±40 and 4160±40 14C yr B.P. from lacustrine pelecypod shells confirm the presence of an early Neoglacial highstand to ~1192 m and delta progradation for two to three centuries. Associated tufa, marl, and indurate sediments suggest alkaline conditions, possibly becoming more concentrated over time. A weakly developed buried soil formed in early Neoglacial lacustrine deposits is discontinuously topped by pebble gravels, which demarcate an erosional unconformity lasting ~2000 years. The end of the soil forming interval is bracketed by charcoal dates of 2450±40 and 1640±40 14C yr B.P. from near the base of two different superposed units. These dates also bracket the onset of ~1-2 m of pre-LIA aggradation. Radiocarbon dates from charcoal of 370±40 and 340±30 14C yr B.P. from lacustrine and fluvial deposits, respectively document the presence of a LIA highstand <1190 m and associated fluvial aggradation. The aggradation continued until at least 170±30 14C yr B.P. after which time 2 to 3 m of downcutting occurred, that preceded the construction of a railroad bridge in the early 1900’s.