Age, isotopic, and detrital zircon data on the Hualapai Limestone Member and Muddy Creek Formation (western United States) constrain the time of the first arrival of the Colorado River on the west side of the Grand Canyon to ca. 6–5 Ma. We propose a karst piracy mechanism, along with a 17–6 Ma western paleo–Grand Canyon, as an alternative explanation for how the Colorado River became integrated across the Kaibab uplift and for the progressive upsection decrease in δ18O and 87Sr/86Sr values of the Hualapai Limestone Member. An earlier Laramide paleocanyon, along which this western paleocanyon followed, can also perhaps explain why no clastic delta exists in the Grand Wash trough.
Karst piracy is a type of stream piracy where a subterranean drainage connection is made under a topographic divide. The process of karst piracy proceeds through five main stages: (1) establishment of a gradient across a topographic divide due to headward erosion into the low side of the divide, (2) leakage in soluble rock along the steepest gradient, (3) expansion of the leakage route into a cave passage that is able to carry a significant volume of water under the divide, (4) stoping and collapse of rock above the underground river, eventually forming a narrow gorge, and (5) widening of the gorge into a canyon. A karst piracy model is proposed here for the Kaibab uplift area that takes into account the structure and hydrology of that area. Other examples of karst piracy operating around the world support our proposition for integrating the Colorado River across the Kaibab uplift in the Grand Canyon.