Cenozoic incision history of the Little Colorado River: Its role in carving Grand Canyon and onset of rapid incision in the past ca. 2 Ma in the Colorado River System

This paper documents a multi-stage incision and denudation history for the Little Colorado River (LCR) region of the southwestern Colorado Plateau over the past 70 Ma. The first two pulses of denudation are documented by thermochronologic data. Differential Laramide cooling of samples on the ­Mogollon Rim suggests carving of 70–30 Ma paleotopography by N- and E-flowing rivers whose pathways were partly controlled by strike valleys at the base of retreating Cretaceous cliffs. A second pulse of denudation is documented by apatite (U-Th)/He dates and thermal history models that indicate a broad LCR paleovalley was incised 25–15 Ma by an LCR paleoriver that flowed northwest and carved an East Kaibab paleovalley across the ­Kaibab uplift. Lacustrine strata of the lower Bidahochi Formation were deposited 16–14 Ma in the LCR paleovalley in a closed basin playa or marsh with a valley center near the modern LCR. There is a hiatus in the depositional record in the LCR valley from 12 to 8 Ma followed by aggradation of the 8–6 Ma fluvial upper Bidahochi Formation. Interlayered 8–6 Ma maar basalts that interacted with groundwater mark local base level for upper Bidahochi fluvial deposits; this was also a time of increased groundwater flow to Hualapai Limestone at the western edge of the Colorado Plateau. The paleo–base level in the central LCR valley remained stable (∼1900 m modern elevation) from 16 to 6 Ma. The third pulse of regional incision and denudation, most recent and ongoing, started after integration of the Colorado River (CR) through Grand Canyon. Thermochronology from Marble Canyon indicates that early CR integration took place across the Vermillion Cliffs at Lees Ferry after 6 Ma. The elevation of the paleoconfluence between the LCR and CR at 5–6 Ma is poorly constrained, but earliest CR integration is hypothesized to have reoccupied the East Kaibab paleocanyon. In the upper LCR drainage, topographically inverted basalt ­mesas have elevations and K-Ar dates indicating a transition from aggradation to incision ca. 6 Ma followed by semi-steady incision of 20–40 m/Ma. In the lower LCR, incision accelerated to 120–170 m/Ma after 2 Ma as indicated by 40 Ar/ 39 Ar dating of basalt, ash-fall, and detrital sanidine. A 1.993 ± 0.002 Ma sanidine age for a tuff in the White Mesa alluvium provides a breakthrough for LCR and CR incision studies. Post–2 Ma differential incision magnitudes (and rates) in the lower LCR and at the LCR-CR confluence were 280–320 m (140–160 m/Ma), about three times greater than the 40–80 m (20–40 m/Ma) in the LCR headwaters. The proposed mechanisms driving overall post–6 Ma differential incision of the LCR involve headwater uplift associated with the Hopi Buttes and Springer­ville volcanic fields plus base-level fall caused by CR integration to the Gulf of California. A proposed mechanism to explain the accelerated post–2 Ma differential incision in the central and lower LCR valley, but not in the head­waters, involves mantle-driven epeirogenic uplift due to NE-migrating vol­canism associated with the San Francisco volcanic field. Tectonically driven differential surface uplift mechanisms were likely amplified by changes toward more erosive climate at ca. 6 Ma and ca. 2 Ma.