More Like this

1. 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. 

   more » « less
2. 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. 

   more » « less
3. Detrital zircon geochronology and provenance of the Middle to Late Jurassic Paradox Basin and Central Colorado trough: Paleogeographic implications for southwestern Laurentia

   https://doi.org/10.1130/GES02264.1  

   Ejembi, John I. ; Potter-McIntyre, Sally L. ; Sharman, Glenn R. ; Smith, Tyson M. ; Saylor, Joel E. ; Hatfield, Kendall ; Ferré, Eric C. ( July 2021 , Geosphere)

   Abstract Middle to Upper Jurassic strata in the Paradox Basin and Central Colorado trough (CCT; southwestern United States) record a pronounced change in sediment dispersal from dominantly aeolian deposition with an Appalachian source (Entrada Sandstone) to dominantly fluvial deposition with a source in the Mogollon and/or Sevier orogenic highlands (Salt Wash Member of the Morrison Formation). An enigmatic abundance of Cambrian (ca. 527–519 Ma) grains at this provenance transition in the CCT at Escalante Canyon, Colorado, was recently suggested to reflect a local sediment source from the Ancestral Front Range, despite previous interpretations that local basement uplifts were largely buried by Middle to Late Jurassic time. This study aims to delineate spatial and temporal patterns in provenance of these Jurassic sandstones containing Cambrian grains within the Paradox Basin and CCT using sandstone petrography, detrital zircon U-Pb geochronology, and detrital zircon trace elemental and rare-earth elemental (REE) geochemistry. We report 7887 new U-Pb detrital zircon analyses from 31 sandstone samples collected within seven transects in western Colorado and eastern Utah. Three clusters of zircon ages are consistently present (1.53–1.3 Ga, 1.3–0.9 Ga, and 500–300 Ma) that are interpreted to reflect sources associated with the Appalachian orogen in southeastern Laurentia (mid-continent, Grenville, Appalachian, and peri-Gondwanan terranes). Ca. 540–500 Ma zircon grains are anomalously abundant locally in the uppermost Entrada Sandstone and Wanakah Formation but are either lacking or present in small fractions in the overlying Salt Wash and Tidwell Members of the Morrison Formation. A comparison of zircon REE geochemistry between Cambrian detrital zircon and igneous zircon from potential sources shows that these 540–500 Ma detrital zircon are primarily magmatic. Although variability in both detrital and igneous REE concentrations precludes definitive identification of provenance, several considerations suggest that distal sources from the Cambrian granitic and rhyolitic provinces of the Southern Oklahoma aulacogen is also likely, in addition to a proximal source identified in the McClure Mountain syenite of the Wet Mountains, Colorado. The abundance of Cambrian grains in samples from the central CCT, particularly in the Entrada Sandstone and Wanakah Formation, suggests northwesterly sediment transport within the CCT, with sediment sourced from Ancestral Rocky Mountains uplifts of the southern Wet Mountains and/or Amarillo-Wichita Mountains in southwestern Oklahoma. The lack of Cambrian grains within the Paradox Basin suggests that the Uncompahgre uplift (southwestern Colorado) acted as a barrier to sediment transport from the CCT. 

   more » « less
4. Detrital zircon geothermochronology reveals pre-Alleghanian exhumation of regional Mississippian sediment sources in the southern Appalachian Valley and Ridge Province

   https://doi.org/10.1130/GES02427.1  

   McKay, Matthew ; Jackson, William T. ; Spurgeon, Derek ; Ionescu, Adelie ; Shaulis, Barry ( November 2021 , Geosphere)

   Abstract The Black Warrior foreland basin records sedimentation associated with the development of intersecting Ouachita and Alleghanian thrust belts along the southern margin of Laurentia. Mississippian–Pennsylvanian units in the Black Warrior basin are interpreted to be sourced from either the northern Appalachians and mid-continent or more regionally from the southern Appalachians or nearby Ouachita thrust belt. We present detrital zircon U-Pb ages and Th/U values from Paleozoic units that indicate zircon from the Mississippian Hartselle Sandstone are temporally and chemically compatible with being sourced from the southern Appalachians. Zircon mixing models suggest sediment was primarily recycled from Cambrian, Ordovician, and Devonian strata in the Appalachian Valley and Ridge, with minor influx from Piedmont units. A ca. 415 Ma zircon population requires additional input from the Maya Block of the Yucatan Peninsula or similar outboard terranes. We present zircon (U-Th)/He analysis and thermal history modeling of Paleozoic units, which detail pre-Alleghanian exhumation in the Appalachian Valley and Ridge. Both the Cambrian Chilhowee Group and Pennsylvanian Pottsville Formation exhibit (U-Th)/He dates ranging from 507 to 263 Ma with a Mississippian subset (353–329 Ma, n = 4), which indicates rapid cooling and inferred exhumation during Late Devonian–Early Mississippian Neoacadian tectonism. We propose a Mississippian drainage system that transported material along southern Appalachian structural fabrics to the juncture between Appalachian and Ouachita thrust belts followed by a sediment-routing rotation toward the Black Warrior foreland. This interpretation honors chemical-age zircon data, accounts for metamorphic grains in thin section petrography, and matches Mississippian–Pennsylvanian Black Warrior foreland lithostratigraphic relationships. 

   more » « less
5. Asynchronous trilobite extinctions at the early to middle Cambrian transition

   https://doi.org/10.1130/G46913.1  

   Sundberg, F.A. ; Karlstrom, K.E. ; Geyer, G. ; Foster, J.R. ; Hagadorn, J.W. ; Mohr, M.T. ; Schmitz, M.D. ; Dehler, C.M. ; Crossey, L.J. ( February 2020 , Geology)

   null (Ed.)

   Abstract Trilobites appeared and diversified rapidly in the Cambrian, but it is debated as to whether their radiations and extinctions were globally synchronous or geographically restricted and diachronous. The end of the early Cambrian is a classic example—it has traditionally been defined by the extinction of olenellid and redlichiid trilobites and the appearance of paradoxidid trilobites. Here we integrate the global biostratigraphy of these three trilobite groups with high-precision tuff and tandem detrital zircon U-Pb age constraints to falsify prior models for global synchronicity of these events. For the first time, we demonstrate that olenellid trilobites in Laurentia went extinct at least 3 Ma after the first appearance of paradoxidids in Avalonia and West Gondwana (ca. 509 Ma). They also disappeared before the extinction of redlichiids and prior to the base of the Miaolingian at ca. 506 Ma in South China. This indicates that these three trilobite groups (paradoxidids, olenellids, and redlichiids) and their associated biotas overlapped in time for nearly 40% of Cambrian Epoch 2, Age 4. Implications of this chronological overlap are: (1) trilobite transitions were progressive and geographically mediated rather than globally synchronous; and (2) paleontological databases underestimate the diversity of the early Cambrian. This ∼3 Ma diachroneity, at a critical time in the early evolution of animals, also impacts chemostratigraphic and paleoclimatic data sets that are tied to trilobite biostratigraphy and that collectively underpin our understanding of the Cambrian Earth system. 

   more » « less