LATE QUATERNARY GLACIATION AT COSTILLA MASSIF, NEW MEXICO: MORAINE BOULDER EXPOSURE AGES INFORM VALLEY GLACIER DYNAMICS AND ICE CAP MODELLING

Costilla Massif is a sequence of 10 low prominence mostly granitic summits extending ~13 km north/south and 2.5-4 km east/west from northern New Mexico into southernmost Colorado at elevations from ~3600 to 3900 m. The massif likely featured an ice cap during the late Quaternary which spilled into five glaciated valleys extending eastward toward the Costilla Vega valley floor and a sixth smaller valley extending northeastward at the northern tip. Here, we present ¹⁰Be terrestrial cosmogenic nuclide exposure dating of quartz monzonite boulders from a sequence of five terminal, lateral, and recessional moraines in Long Valley and from a terminal cirque moraine east of State Line Peak to develop a preliminary glacial chronology for the region. We use energy/mass balance models to evaluate the extents of and the paleoclimatic conditions necessary to maintain the valley glaciers that deposited the examined moraines and determine equilibrium line altitudes and ice thickness. We likewise model the extent of the ice cap covering Costilla Massif from MIS 6 to the present. We compare ice cap and valley glacier fluctuations with other systems throughout the Sangre de Cristo mountains that have been associated with the presence of minor ice caps. We then compare our moraine derived paleoclimatic fluctuations with similar records elsewhere in the Rocky Mountains to evaluate local and regional warming trends throughout the Late Quaternary. Based on preliminary soil and topographic relative age criteria, we hypothesize the oldest glacial moraines at Costilla Massif likely date to the Bull Lake (MIS 6) while a relative lack of soil development with limited formation of A horizons indicates the most recent cirque glaciers at Costilla Massif will be of latest Pleistocene to early Holocene age. We predict that valley glacier retreat at Costilla Massif likely initiated prior to that in the Mid to Northern Rocky Mountains. We therefore propose that local climatic and topographic conditions, the more northerly latitude, and the presence of a relatively stable and locally extensive ice cap led to slower rates of glacial retreat compared to the southernmost locations within the Sangre de Cristo mountains and permitted stabilization of cirque glaciation into the early Holocene.