THE GRAND CANYON, ARIZONA (U.S.) RECORD OF FULL-LANDSCAPE RESPONSE TO MIDDLE-LATE PLEISTOCENE CLIMATE CHANGE

Studies in Grand Canyon are building one of the best-dated and most-detailed non-glacial Quaternary landscape records. A combination of OSL, U-series, and cosmogenic-surface-exposure dating of the mapped hillslope and stream stratigraphy highlights the sedimentologic linkages, or lack thereof, between: a) hillslope, b) tributary catchment, and c) the mainstem Colorado River landscape components through time as a function of glacial-interglacial climate changes.

Grand Canyon has undergone major landscape aggradation episodes correspond roughly to the terminations of OIS 6 and 10, as well as a recent episode of valley-filling during both OIS 4 and 3. The recent part of the record argues against any simple relation of sedimentation-and-erosion cycles to climate. Relict hillslope colluvial mantles are laterally correlated to tributary fill terraces that formed from 50 to 30 ka (OIS 3). Tributary streams must have incised into this fill sometime during OIS 2. This stored sediment was remobilized into a subordinate tributary deposit from 12.5-6.5 ka.

In contrast to local catchments, there is a chronologic and stratigraphic disconnect between the local landscape and the "exotic" mainstem Colorado River. A massive fill deposit along the Colorado River formed 75-60 ka (correlating well to OIS 4) significantly predates tributary aggradation, as evident in stratigraphic relations. Records in the headwaters of the Colorado River suggest there should be a mainstem fill correlating to the Pinedale glaciation (OIS 2), but there is no field evidence for such a deposit. Other research in Grand Canyon leads us to suggest it may be beneath the channel of the modern river. Likewise, there is no notable early Holocene deposit of the Colorado River to match that of local tributaries.

We interpret these data to indicate that the greater Colorado River is relatively sensitive to climate forcing in its distant headwaters, whereas local catchments have sedimentologic responses that lag 10^4 ky after changes to full-glacial climate and are buffered in their response to increased bedrock weathering rates and relatively steady hydrology. The importance of climate transitions relative to full glacial or interglacial episodes is confused by this lag time; and these landscape responses to climate are likely distinct to weathering-limited drylands.