Chronology of Pleistocene glacial advances in the central Rocky Mountains

Abstract

The distribution and ages of moraines in the Wyoming-type localities for pre-Pinedale Rocky Mountain glacial deposits are not well known. Through morphostratigraphic mapping, river-terrace longitudinal-profile analysis, and tephrochronology, we place the age of Sacagawea Ridge moraines in the Dinwoody Lakes type area at about 660 ka. Outwash and river terrace deposits in the Bull Lake type area, previously considered to be Sacagawea Ridge, are actually older by one or two glacial cycles. In the Bull Lake type locality, 15 individual moraines are divided into 4 groups on the basis of relative age criteria. Using 36Cl/10Be cosmogenic dating of boulders, the two older groups must be >130 ka; the younger groups are given age ranges of 130 to 100 ka and 120 to 95 ka. A terrace along Wind River can be morphostratigraphically traced to the youngest of these moraine groups; 36Cl dating of boulders on the terrace provides an age range of 125 to 100 ka. The combined relative age and cosmogenic nuclide chronology is consistent with previously published evidence from Yellowstone National Park demonstrating that mountain glaciers advanced not only prior to and during the later portion of marine isotope stage 6 but also during marine isotope stage 5d. In contrast with some other areas of the Rocky Mountains, we found no evidence for advances during marine isotope stage 4. At Bull Lake, Pinedale moraines that have a 36Cl/10Be age range of 23 to 16 ka suggest that glacial advances were limited to the later part of marine isotope stage 2. A river terrace that can be traced to these moraines has an age range of 23 to 16 ka. A possible glacial advance during early marine isotope stage 2 is indicated by a river terrace whose age range is estimated by incision-rate modeling at 46 to 28 ka, but no moraines were identified. Along Wind River, there are eight terrace levels capped with outwash that are older than Sacagawea Ridge; although the terraces are the record of early to middle Pleistocene climatic cycles, there are no correlative moraines preserved at the land surface.