TRIBUTARY DEBRIS FANS AND THE LATE HOLOCENE ALLUVIAL CHRONOLOGY OF THE COLORADO-RIVER, EASTERN GRAND-CANYON, ARIZONA

Bouldery debris fans and sandy alluvial terraces of the Colorado River developed contemporaneously during the late Holocene at the mouths of nine major tributaries in eastern Grand Canyon, The age of the debris fans and alluvial terraces contributes to understanding river hydraul ics and to the history of human activity along the river, which has be en concentrated on these surface's for at least two to three millennia . Poorly sorted, coarse-grained debris-flow deposits of several ages a re interbedded with, overlie, or are overlapped by three terrace-formi ng alluviums, The alluvial deposits are of three age groups: the strip ed alluvium, deposited from before 770 B.C. to about A.D. 300; the all uvium of Pueblo II age deposited from about A.D. 700 to 1200; and the alluvium of the upper mesquite terrace, deposited from about A.D. 1400 to 1880. Two elements define the geomorphology of a typical debris fa n: the large, inactive surface of the fan and a smaller, entrenched, a ctive debris-flow channel and fan that is about one-sixth the area of the inactive fan, The inactive fan is segmented into at least three su rfaces with distinctive weathering characteristics. These surfaces are conformable with underlying debris-flow deposits that date from befor e 770 B.C. to around A.D. 660, A.D. 660 to before A.D. 1200, and from A.D. 1200 to slightly before 1890, respectively, based on late-19th-ce ntury photographs, radiocarbon and archaeologic dating of the three st ratigraphically related alluviums, and radiocarbon dating of fine-grai ned debris-flow deposits, These debris flows aggraded the fans in at l east three stages beginning about 2.8 ha, if not earlier in the late H olocene. Several main-stem floods eroded the margin of the segmented f ans, reducing fan symmetry, The entrenched, active debris-flow channel s contain deposits < 100 yr old, which form debris fans at the mouth o f the channel adjacent to the river, Early anti middle Holocene debris -flow and alluvial deposits have not been recognized, as they were evi dently not preserved adjacent to the river or are buried by younger de posits.