Chronology of cirque glaciation, Colorado front range

Abstract

Moraines and rock glaciers in Front Range cirques record at least four, and possibly five, intervals of Holocene glacier expansion. The earliest and most extensive was the Satanta Peak advance, which deposited multiple terminal moraines near present timberline shortly before 9915 ± 165 BP. By 9200 ± 135 BP, timberline had risen to at least its modern elevation; by 8460 ± 140 BP, patterned ground on Satanta Peak moraines had become inactive. Although a minor ice advance may have occurred just prior to 7900 ± 130 BP, there is no evidence that glaciers or perennial snowbanks survived in the Front Range during the “Altithermal” maximum (ca. 6000–7500 BP), or during a subsequent interval of alpine soil formation (ca. 5000–6000 BP). Glaciers were larger during the Triple Lakes advance (3000–5000 BP) than at any other time during Neoglaciation. Minimum ages of 4485 ± 100 BP, 3865 ± 100 BP, and ca. 3150 BP apply to a threefold sequence of Triple Lakes deposits in Arapaho Cirque. After an important interval of soil formation and cavernous weathering, glaciers and rock glaciers of the Audubon advance (950–1850 BP) reoccupied many cirques, and perennial snowbanks blanketed much of the area above present timberline; although the general Audubon snow cover had begun to melt from valley floors by 1505 ± 95 BP, expanded snowbanks lingered on tundra ridge crests until 1050–1150 BP, and glaciers persisted is sheltered cirques until at least 955 ± 95 BP. Following a minor interval of ice retreat, glaciers of the Arapaho Peak advance (100–300 BP) deposited multiple moraines in favorably oriented cirques. Interpretation of Holocene glacial deposits in the Southern and Central Rocky Mountains has been hampered by (1) a heavy reliance upon relative-dating criteria, many of which are influenced by factors other than age; (2) the assumption that glacial advances in high-altitude cirques can be correlated directly with alluvial deposition in far-distant lowlands; and (3) the assumption that glacial advances have necessarily been synchronous throughout the Rocky Mountain region and the world. Although Holocene glacier fluctuations in the Front Range are believed to reflect changes in regional climate, the Front Range chronology does not have particularly close analogs in other parts of North America. Better-dated local sequences are needed before the hypothesis of global synchroneity can be adequately evaluated; until synchroneity has been proven, long-distance correlations and worldwide cycles of recurring glaciation will remain unconvincing.