GEOARCHEOLOGY AND ALLUVIAL HISTORY OF SIEBER CANYON, COLORADO CANYONS NATIONAL CONSERVATION AREA, WESTERN COLORADO

Geological mapping, stratigraphic sections and 14C dating provide a basis for interpreting latest Pleistocene and Holocene paleoenvironments in Sieber Canyon, located on the Uncompahgre Plateau of western Colorado. The alluvial stratigraphy of the canyon fill suggests that the departure of prehistoric Fremont indians from the region was affected by changing paleoenvironmental conditions.

Mapping shows that Sieber Canyon contains a deeply incised arroyo that has cut through a 10 to 20-m-thick fill terrace. The fill terrace consists of beds of reddish fine sand, silty sand, and clayey silt. Disseminated charcoal fragments are common in the clayey silts. Sand and gravel lenses up to 4 m thick locally cross-cut finer-grained deposits. The sand and gravel lenses do not correlate with significant buried soils, and probably reflect limited sediment reworking by paleochannels rather than major episodes of arroyo incision and filling. The overall stratigraphy of the fill terrace deposits reflects continuous aggradation and canyon filling. Subsequent arroyo incision produced a series of discontinuous cut terraces that are inset into the main fill-terrace deposits.

14C dating of detrital charcoal fragments within the fill terrace indicates that the aggradation commenced prior to 11,160 to 10,750 cal. yrs BP, and continued until at least 1550 to 1380 cal. yrs BP. A prominent Fremont petroglyph panel is present 3-4 m above the surface of a cut terrace inset into the main fill terrace. The elevation of the petroglyphs is approximately the same as that of the surface of the fill terrace. This observation indicates that the initial incision of the main fill terrace occurred after initial Fremont occupation in the canyon. Regional studies show that Fremont departed from the area ca. 1300 AD (650 cal. yrs BP), which provides a maximum age for the commencement of arroyo cutting.

Canyon filling was probably triggered by an increase in the proportion of Sieber Creek’s sediment load relative to its discharge at the end of the Pinedale glaciation. Subsequent arroyo incision could be related to large floods, perhaps associated with upland wild fires. Arroyo incision appears to coincide with and probably hastened the departure of prehistoric Fremont indians from Sieber Canyon.