THE PALEOCLIMATIC IMPLICATIONS OF HOLOCENE SHORELINE STRATIGRAPHY OF THE EL FRESNAL BASIN, NORTHERN CHIHUAHUA, MEXICO

The El Fresnal basin is one of four sub-basins constituting the formerly extensive Pluvial Lake Palomas system of 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 (BRCs) in the El Fresnal basin, that record millennially spaced highstands during the Holocene. This chronology shows early (8269±64, 8456±97 14 C yr B.P.), middle (6110±80 to 6721±68 14 C yr B.P.), and two late Holocene highstands (3815±52 to 4251±59 and 221±33 14 C yr B.P.). Using this framework, I describe the detailed stratigraphy and geomorphology of these BRC sediments to refine the BRC chronology. The BRC sediments comprise well-resolved lacustrine sediments that interfinger with distal alluvial fan sediments. The stratigraphic record of the early Holocene BRC shows two highstands punctuated by a soil-forming interval. A lacustrine silty clay unit coarsens to a lacustrine sandy loam unit, and is followed by a unit of small gravel filled channels and minor soil development. These units are indicative of a regressive sequence, and are overlain by a transgressive lacustrine unit and possible deltaic units. Then, as indicated by geomorphic evidence, a major period of erosion and shoreline incision is followed by a middle Holocene highstand. The middle Holocene highstand, during the supposed Altithermal, demonstrates the sensitivity of the basin to subtle climatic perturbations. These records have unique implications for the Chihuahuan Desert paleoclimate, especially when compared to regional lacustrine paleoclimate reconstructions.