Evaluating the Shinumo-Sespe drainage connection: Arguments against the “old” (70–17 Ma) Grand Canyon models for Colorado Plateau drainage evolution

Abstract

The provocative hypothesis that the Shinumo Sandstone in the depths of Grand Canyon was the source for clasts of orthoquartzite in conglomerate of the Sespe Formation of coastal California, if verified, would indicate that a major river system flowed southwest from the Colorado Plateau to the Pacific Ocean prior to opening of the Gulf of California, and would imply that Grand Canyon had been carved to within a few hundred meters of its modern depth at the time of this drainage connection. The proposed Eocene Shinumo-Sespe connection, however, is not supported by detrital zircon nor paleomagnetic-inclination data and is refuted by thermochronology that shows that the Shinumo Sandstone of eastern Grand Canyon was >60 °C (~1.8 km deep) and hence not incised at this time. A proposed 20 Ma (Miocene) Shinumo-Sespe drainage connection based on clasts in the Sespe Formation is also refuted. We point out numerous caveats and non-unique interpretations of paleomagnetic data from clasts. Further, our detrital zircon analysis requires diverse sources for Sespe clasts, with better statistical matches for the four “most- Shinumo-like” Sespe clasts with quartzites of the Big Bear Group and Ontario Ridge metasedimentary succession of the Transverse Ranges, Horse Thief Springs Formation from Death Valley, and Troy Quartzite of central Arizona. Diverse thermochronologic and geologic data also refute a Miocene river pathway through western Grand Canyon and Grand Wash trough. Thus, Sespe clasts do not require a drainage connection from Grand Canyon or the Colorado Plateau and provide no constraints for the history of carving of Grand Canyon. Instead, abundant evidence refutes the “old” (70–17 Ma) Grand Canyon models and supports a <6 Ma Grand Canyon.

Rise of the Colorado Plateau: A Synthesis of Paleoelevation Constraints From the Region and a Path Forward Using Temperature-Based Elevation Proxies

“…Numerous geologic, thermochronologic, and geomorphic investigations constrain CP paleotopography, providing context for evaluating paleoaltimetry estimates (for details, see reviews by Hunt, 1956;Young and Spamer, 2001;Cather et al, 2012;Karlstrom et al, 2014;Ranney, 2014;Karlstrom et al, 2017;Crow et al, 2019;Karlstrom et al, 2019;Leopold and Zaborac-Reed, 2019;Hill and Polyak, 2020;Karlstrom et al, 2020).…”

“…Cooling histories from canyon-rim and river-level samples track paleo-relief of GC (Flowers et al, 2008;Flowers and Farley, 2012;Flowers and Farley, 2013;Lee et al, 2013;Karlstrom et al, 2014;Winn et al, 2017) and constrain minimum plateau paleoelevation. A synthesis of thermochronologic datasets by Karlstrom et al (2020) indicates segments of the GC were incised at different times before becoming integrated 6-5 Ma; the Western GC was likely a paleo-canyon with substantial relief incised by a northeast-flowing river by ∼65 Ma, whereas the Eastern GC was likely incised by a NW-flowing paleo-river by ∼20 Ma. These trends are compatible with multi-stage uplift prior to ∼20 Ma.…”

“…Continental extension and normal faulting to the West and South (Basin and Range, Rio Grande Rift) caused a drop in base-level and aided drainage reversal to the southwest (blue arrows). Paleogeographic reconstructions were adapted from these compilations : Hunt, 1956;Potochnik et al, 2001;Davis et al, 2009;Cather et al, 2012;Young and Crow, 2014;Hill et al, 2016;Karlstrom et al, 2020. lithospheric mantle by the Farallon slab during the Laramide (Hernández-Uribe and Palin, 2019); lithospheric foundering (Bird, 1979) or hydration (Porter et al, 2017) in the mid-Cenozoic; and mantle upwelling (e.g., Moucha et al, 2009) or isostatic response to denudation (e.g., Lazear et al, 2013) in the late Cenozoic. Accurate paleoelevation constraints can evaluate hypothesized uplift mechanisms and provide context for CR evolution.…”

Detrital sanidine 40Ar/39Ar dating confirms <2 Ma age of Crooked Ridge paleoriver and subsequent deep denudation of the southwestern Colorado Plateau

“…Thermochronology sample locations shown in Figure 1 show that rocks currently along the San Juan River across the Monument uplift ( Fig. 12) and those in Marble Canyon along the Colorado River at Lees Ferry were still beneath 1-2 km of Mesozoic strata until after 6 Ma (Kelley et al, 2001;Flowers et al, 2008;Hoffman, 2009;Karlstrom et al, 2012bKarlstrom et al, , 2014Karlstrom et al, , 2017Karlstrom et al, , 2020Lee et al, 2013). Thus, any pre-6 Ma major ancestral river in these areas would have flowed at the level of the Mesozoic rocks above the Paria Plateau (at 2-2.5 km elevation; Fig.…”

“…Certainly, the elevations of Crooked Ridge, The Gap, and White Mesa are too low to be vestiges of this hypothetical river system. An alternative model is that the paleo-Little Colorado River carved the East Kaibab paleocanyon between 25 and 15 Ma (Karlstrom et al, 2014(Karlstrom et al, , 2017(Karlstrom et al, , 2020. Models for pre-6 Ma drainage patterns and future tests for the existence of ancestral Colorado and San Juan paleoriver-paleocanyon systems will benefit from additional thermochronologic data, estimates of isostatic responses to deep denudation (e.g., Lazear et al, 2013), and consideration of post-10 Ma differential uplift and tilting of the Colorado Plateau-Rocky Mountain region (Karlstrom et al, 2012b).…”