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

   https://doi.org/10.1130/GES02265.1  

   Karlstrom, Karl E.; Jacobson, Carl E.; Sundell, Kurt E.; Eyster, Athena; Blakey, Ron; Ingersoll, Raymond V.; Mulder, Jacob A.; Young, Richard A.; Beard, L. Sue; Holland, Mark E.; et al (October 2020, Geosphere)

   null (Ed.)

   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. 

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2. Depositional age of the Boleo Formation and marine flooding in the central Gulf of California Rift, Santa Rosalía basin, Baja California Sur, México

   https://doi.org/10.1130/2025.2563(13)  

   Darin, Michael H; Dorsey, Rebecca J; Niemi, Tina M; Salgado_Muñoz, Valente O; Henry, Matthew W; Pecha, Mark E (June 2025, Geological Society of America)

   ABSTRACT The Santa Rosalía basin (Baja California Sur, México) contains a rich record of late Cenozoic volcanism, faulting, and sedimentation that provides a crucial constraint on the timing of marine flooding from the Pacific Ocean into the nascent Gulf of California oblique rift, yet the precise age of the basin is uncertain. Previous studies used reconnaissance paleomagnetic data and a 40Ar/39Ar age of 6.76 ± 0.90 Ma on the intrabasinal Cinta Colorada tuff to estimate a depositional age of ca. 7.2–6.3 Ma for the marine Boleo Formation and initial flooding of the central Gulf of California. Here, we present a large (n = 2091) detrital zircon U-Pb geochronology data set from the Boleo Formation that indicates a maximum depositional age of 6.35 ± 0.21 Ma for pumiceous sandstone at the base (below the basal limestone), a revised age of 5.86 ± 0.06 Ma for the Cinta Colorada tuff in the middle, and a maximum depositional age of 5.70 ± 0.21 Ma for the top. Detrital zircon age spectra suggest a local provenance for the Boleo Formation involving recycling from underlying Oligocene–Miocene strata in proximal source areas. Integration of detrital zircon ages with existing paleomagnetic data suggests that the lower ~30 m of the Boleo Formation accumulated during normal-polarity subchron C3An.1n (6.27–6.02 Ma), and the middle to upper Boleo Formation was deposited entirely during reverse-polarity chron C3r (6.02–5.24 Ma). We therefore reassign the depositional age span of the Boleo Formation to ca. 6.3–5.7 Ma. Although not preferred, a minimum-duration depositional model from ca. 6.1 to 5.8 Ma is also permissible if a consistently high sedimentation rate of ~0.4– 1.0 mm/yr is inferred. This revised younger age for the Boleo Formation implies marine incursion in the central Gulf of California at ca. 6.3 Ma, ~1 m.y. younger than previously thought. We envision that regional marine flooding occurred during a very short (<100 k.y.) event that inundated a narrow tectonic trough over a distance of at least ~1000 km along the plate boundary from the central Gulf of California to the Salton Trough and reaching into the present-day Lower Colorado River Valley. This study also demonstrates the utility of large-volume and large-n detrital zircon studies in establishing the ages of sedimentary successions deposited over very short time spans (<1 m.y.) and/or during relative lulls in magmatism and geomagnetic reversals. 

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3. Grand Canyon provenance for orthoquartzite clasts in the lower Miocene of coastal southern California

   https://doi.org/10.1130/GES02111.1  

   Sabbeth, Leah; Wernicke, Brian P.; Raub, Timothy D.; Grover, Jeffrey A.; Lander, E. Bruce; Kirschvink, Joseph L. (October 2019, Geosphere)

   null (Ed.)

   Abstract Orthoquartzite detrital source regions in the Cordilleran interior yield clast populations with distinct spectra of paleomagnetic inclinations and detrital zircon ages that can be used to trace the provenance of gravels deposited along the western margin of the Cordilleran orogen. An inventory of characteristic remnant magnetizations (CRMs) from >700 sample cores from orthoquartzite source regions defines a low-inclination population of Neoproterozoic–Paleozoic age in the Mojave Desert–Death Valley region (and in correlative strata in Sonora, Mexico) and a moderate- to high-inclination population in the 1.1 Ga Shinumo Formation in eastern Grand Canyon. Detrital zircon ages can be used to distinguish Paleoproterozoic to mid-Mesoproterozoic (1.84–1.20 Ga) clasts derived from the central Arizona highlands region from clasts derived from younger sources that contain late Mesoproterozoic zircons (1.20–1.00 Ga). Characteristic paleomagnetic magnetizations were measured in 44 densely cemented orthoquartzite clasts, sampled from lower Miocene portions of the Sespe Formation in the Santa Monica and Santa Ana mountains and from a middle Eocene section in Simi Valley. Miocene Sespe clast inclinations define a bimodal population with modes near 15° and 45°. Eight samples from the steeper Miocene mode for which detrital zircon spectra were obtained all have spectra with peaks at 1.2, 1.4, and 1.7 Ga. One contains Paleozoic and Mesozoic peaks and is probably Jurassic. The remaining seven define a population of clasts with the distinctive combination of moderate to high inclination and a cosmopolitan age spectrum with abundant grains younger than 1.2 Ga. The moderate to high inclinations rule out a Mojave Desert–Death Valley or Sonoran region source population, and the cosmopolitan detrital zircon spectra rule out a central Arizona highlands source population. The Shinumo Formation, presently exposed only within a few hundred meters elevation of the bottom of eastern Grand Canyon, thus remains the only plausible, known source for the moderate- to high-inclination clast population. If so, then the Upper Granite Gorge of the eastern Grand Canyon had been eroded to within a few hundred meters of its current depth by early Miocene time (ca. 20 Ma). Such an unroofing event in the eastern Grand Canyon region is independently confirmed by (U-Th)/He thermochronology. Inclusion of the eastern Grand Canyon region in the Sespe drainage system is also independently supported by detrital zircon age spectra of Sespe sandstones. Collectively, these data define a mid-Tertiary, SW-flowing “Arizona River” drainage system between the rapidly eroding eastern Grand Canyon region and coastal California. 

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4. High-precision U-Pb geochronology links magmatism in the Southwestern Laurentia large igneous province and Midcontinent Rift

   https://doi.org/10.1130/G51786.1  

   Mohr, MT; Schmitz, MD; Swanson-Hysell, NL; Karlstrom, KE; Macdonald, FA; Holland, ME; Zhang, Y; Anderson, NS (January 2024, Geology)

   Abstract The Southwestern Laurentia large igneous province (SWLLIP) comprises voluminous, widespread ca 1.1 Ga magmatism in the southwestern United States and northern Mexico. The timing and tempo of SWLLIP magmatism and its relationship to other late Mesoproterozoic igneous provinces have been unclear due to difficulties in dating mafic rocks at high precision. New precise U-Pb zircon dates for comagmatic felsic segregations within mafic rocks reveal distinct magmatic episodes at ca. 1098 Ma (represented by massive sills in Death Valley, California, the Grand Canyon, and central Arizona) and ca. 1083 Ma (represented by the Cardenas Basalts in the Grand Canyon and a sill in the Dead Mountains, California). The ca. 1098 Ma magmatic pulse was short-lived, lasting 0.25−0.24+0.67 m.y., and voluminous and widespread, evidenced by the ≥100 m sills in Death Valley, the Grand Canyon, and central Arizona, consistent with decompression melting of an upwelling mantle plume. The ca. 1083 Ma magmatism may have been generated by a secondary plume pulse or post-plume lithosphere extension. The ca. 1098 Ma pulse of magmatism in southwestern Laurentia occurred ~2 m.y. prior to an anomalous renewal of voluminous melt generation in the Midcontinent Rift of central Laurentia that is recorded by the ca. 1096 Ma Duluth Complex layered mafic intrusions. Rates of lateral plume spread predicted by mantle plume lubrication theory support a model where a plume derived from the deep mantle impinged near southwestern Laurentia, then spread to thinned Midcontinent Rift lithosphere over ~2 m.y. to elevate mantle temperatures and generate melt. This geodynamic hypothesis reconciles the close temporal relationships between voluminous magmatism across Laurentia and provides an explanation for that anomalous renewal of high magmatic flux within the protracted magmatic history of the Midcontinent Rift. 

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

   https://doi.org/10.1130/GES02319.1  

   Heizler, Matthew T.; Karlstrom, Karl E.; Albonico, Micael; Hereford, Richard; Beard, L. Sue; Cather, Steven M.; Crossey, Laurie J.; Sundell, Kurt E. (February 2021, Geosphere)

   null (Ed.)

   Abstract Crooked Ridge and White Mesa in northeastern Arizona (southwestern United States) preserve, as inverted topography, a 57-km-long abandoned alluvial system near the present drainage divide between the Colorado, San Juan, and Little Colorado Rivers. The pathway of this paleoriver, flowing southwest toward eastern Grand Canyon, has led to provocative alternative models for its potential importance in carving Grand Canyon. The ∼50-m-thick White Mesa alluvium is the only datable record of this paleoriver system. We present new 40Ar/39Ar sanidine dating that confirms a ca. 2 Ma maximum depositional age for White Mesa alluvium, supported by a large mode (n = 42) of dates from 2.06 to 1.76 Ma. Older grain modes show abundant 37–23 Ma grains mostly derived ultimately from the San Juan Mountains, as is also documented by rare volcanic and basement pebbles in the White Mesa alluvium. A tuff with an age of 1.07 ± 0.05 Ma is inset below, and hence provides a younger age bracket for the White Mesa alluvium. Newly dated remnant deposits on Black Mesa contain similar 37–23 Ma grains and exotic pebbles, plus a large mode (n = 71) of 9.052 ± 0.003 Ma sanidine. These deposits could be part of the White Mesa alluvium without any Pleistocene grains, but new detrital sanidine data from the upper Bidahochi Formation near Ganado, Arizona, have similar maximum depositional ages of 11.0–6.1 Ma and show similar 40–20 Ma San Juan Mountains–derived sanidine. Thus, we tentatively interpret the <9 Ma Black Mesa deposit to be a remnant of an 11–6 Ma Bidahochi alluvial system derived from the now-eroded southwestern fringe of the San Juan Mountains. This alluvial fringe is the probable source for reworking of 40–20 Ma detrital sanidine and exotic clasts into Oligocene Chuska Sandstone, Miocene Bidahochi Formation, and ultimately into the <2 Ma White Mesa alluvium. The <2 Ma age of the White Mesa alluvium does not support models that the Crooked Ridge paleoriver originated as a late Oligocene to Miocene San Juan River that ultimately carved across the Kaibab uplift. Instead, we interpret the Crooked Ridge paleoriver as a 1.9–1.1 Ma tributary to the Little Colorado River, analogous to modern-day Moenkopi Wash. We reject the “young sediment in old paleovalley” hypothesis based on mapping, stratigraphic, and geomorphic constraints. Deep exhumation and beheading by tributaries of the San Juan and Colorado Rivers caused the Crooked Ridge paleotributary to be abandoned between 1.9 and 1.1 Ma. Thermochronologic data also provide no evidence for, and pose substantial difficulties with, the hypothesis for an earlier (Oligocene–Miocene) Colorado–San Juan paleoriver system that flowed along the Crooked Ridge pathway and carved across the Kaibab uplift. 

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