skip to main content
US FlagAn official website of the United States government
dot gov icon
Official websites use .gov
A .gov website belongs to an official government organization in the United States.
https lock icon
Secure .gov websites use HTTPS
A lock ( lock ) or https:// means you've safely connected to the .gov website. Share sensitive information only on official, secure websites.

Explore Research Products in the PAR
It may take a few hours for recently added research products to appear in PAR search results.


Title: U-Pb dating of pedogenic calcite near the Permian−Triassic boundary, Karoo Basin, South Africa
We report U-Pb age determinations of carbonate nodules from an in situ paleosol horizon in the Upper Permian Balfour Formation and from several horizons of pedogenic nodule conglomerate (PNC) in the Triassic Katberg Formation, Karoo Basin, South Africa, using laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS). The paleosol sample yields an age of 252 ± 3 Ma, which overlaps with a previous high-precision U-Pb zircon date from a volcanic ash deposit 2 m above the paleosol. This relationship demonstrates the reliability of using LA-ICP-MS dating techniques on terrestrial pedogenic calcite. Two PNC samples collected at the base of the Katberg Formation within the same sandstone unit yield ages of 255 ± 3 Ma and 251 ± 3 Ma. The age of 251 ± 3 Ma overlaps with the high-precision U-Pb zircon date below the PNC and is a maximum age estimate of deposition for the base of the Katberg Formation. Our results show that reworked nodules in the same concentrated conglomerate lag can be of different ages, but that similarly aged nodules are spatially associated. In addition, two PNC samples collected higher in the section yield ages of 249 ± 3 Ma and 241 ± 3 Ma, providing maximum depositional ages for the lower to middle Katberg Formation for the first time. We demonstrate that pedogenic carbonate nodules can be dated with meaningful precision, providing another mechanism for constraining the age of sedimentary sequences and studying events associated with the Permian−Triassic transition in the central Karoo Basin, even though the extinction boundary may not be preserved in this area.  more » « less
Award ID(s):
1714759 1714928
PAR ID:
10451222
Author(s) / Creator(s):
; ; ; ; ; ;
Date Published:
Journal Name:
Geological Society of America Bulletin
ISSN:
0016-7606
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. ; ; ; (, Geochronology)
    Ascough, P.; Dunai, T.; King, G.; Lang, A.; Mezger, K. (Ed.)
    Detrital zircon geochronology by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) is a widely used tool for determining maximum depositional ages and sediment provenance, as well as reconstructing sediment routing pathways. Although the accuracy and precision of U–Pb geochronology measurements have improved over the past 2 decades, Pb loss continues to impact the ability to resolve zircon age populations by biasing affected zircon toward younger apparent ages. Chemical abrasion (CA) has been shown to reduce or eliminate the effects of Pb loss in zircon U–Pb geochronology but has yet to be widely applied to large-n detrital zircon analyses. Here, we assess the efficacy of the chemical abrasion treatment on zircon prior to analysis by LA-ICP-MS and discuss the advantages and limitations of this technique in relation to detrital zircon geochronology. We show that (i) CA does not systematically bias LA-ICP-MS U–Pb dates for 13 reference materials that span a wide variety of crystallization dates and U concentrations, (ii) CA-LA-ICP-MS U–Pb zircon geochronology can reduce or eliminate Pb loss in samples that have experienced significant radiation damage, and (iii) bulk CA prior to detrital zircon U–Pb geochronology by LA-ICP-MS improves the resolution of age populations defined by 206Pb/238U dates (Neoproterozoic and younger) and increases the percentage of concordant analyses in age populations defined by 207Pb/206Pb dates (Mesoproterozoic and older). The selective dissolution of zircon that has experienced high degrees of radiation damage suggests that some detrital zircon age populations could be destroyed or have their abundance significantly modified during this process. However, we did not identify this effect in either of the detrital zircon samples that were analyzed as part of this study. We conclude that pre-treatment of detrital zircon by bulk CA may be useful for applications that require increased resolution of detrital zircon populations and increased confidence that 206Pb/238U dates are unaffected by Pb loss. 
    more » « less
  2. ; ; ; ; ; ; ; (, Geology)
    Abstract Despite being a prominent continental-scale feature, the late Mesoproterozoic North American Midcontinent Rift did not result in the break-up of Laurentia, and subsequently underwent structural inversion. The timing of inversion is critical for constraining far-field effects of orogenesis and processes associated with the rift's failure. The Keweenaw fault in northern Michigan (USA) is a major thrust structure associated with rift inversion; it places ca. 1093 Ma rift volcanic rocks atop the post-rift Jacobsville Formation, which is folded in its footwall. Previous detrital zircon (DZ) U-Pb geochronology conducted by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) assigned a ca. 950 Ma maximum age to the Jacobsville Formation and led researchers to interpret its deposition and deformation as postdating the ca. 1090–980 Ma Grenvillian Orogeny. In this study, we reproduced similar DZ dates using LA-ICP-MS and then dated 19 of the youngest DZ grains using high-precision chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS). The youngest DZ dated by CA-ID-TIMS at 992.51 ± 0.64 Ma (2σ) redefines the maximum depositional age of the Jacobsville Formation and overlaps with a U-Pb LA-ICP-MS date of 985.5 ± 35.8 Ma (2σ) for late-kinematic calcite veins within the brecciated Keweenaw fault zone. Collectively, these data are interpreted to constrain deposition of the Jacobsville Formation and final rift inversion to have occurred during the 1010–980 Ma Rigolet Phase of the Grenvillian Orogeny, following an earlier phase of Ottawan inversion. Far-field deformation propagated >500 km into the continental interior during the Ottawan and Rigolet phases of the Grenvillian Orogeny. 
    more » « less
  3. ; ; (, Geological Society of America)
    The Kootenai Formation of Western Montana records the Aptian- Albian (121.4Ma-100.5Ma), a significant interval in Earth’s history. The Early Cretaceous is notable for a multitude of changes in both the geologic and biotic realm. Significant events that occurred during this time include the tectonic evolution of the Western Interior Basin (WIB) and the displacement of gymnosperms by angiosperms. Given the significance of this time, previous and ongoing research seek to better understand the timing and interactions between these changes. The focus of this study is to refine stratigraphic constraint of the Kootenai Formation using carbon isotope chemostratigraphy. The depositional age of the lower clastic unit of the Kootenai formation has been debated over the past decade. Detrital zircon U-Pb analyses by Laskowski et al. (2013) indicated an Albian age with a U-Pb detrital zircon maximum depositional age (MDA) of 109Ma. However, more recent studies (Finezl and Rosenblume, 2020 and Rosenblume et al. 2021) using LA-ICP-MS-generated detrital zircon U-Pb analyses indicate MDAs of the lower clastic unit as old as Valanginian to Aptian (MDAs ~135-115Ma) with the upper units of the Kootenai having MDAs from Albian (~105 Ma). Detrital zircon U-Pb analyses have generally been limited in the lower units of the Kootenai particularly because syndepositionally formed zircon grains are not common in the lower units (Quin et al. 2018, Finzel and Rosenblume 2020).Additionally, previous flora in the Kootenai suggests predominately Aptian and older ages(Brown 1946). Given the limited geochronologic constraint of the lower clastic unit of the Kootenai formation, addition data is needed. For this study, approximately 60 samples from just above the basal conglomerate to the top of the lower clastic unit were collected and processed to determine bulk organic carbon isotope values. The prior MDAs suggest C isotope excursions such as those associated with OAE1a and even as old as the Valanginian Weissert event could be preserved in the strata of the lower clastic unit. The new stable isotope data will provide an opportunity to refine the age of these Cretaceous units leveraging the existing U-Pb data. 
    more » « less
  4. ; ; ; ; ; ; ; (, Journal of the Geological Society)
    Periods of cessation, resumption and enhanced arc activity are recorded in the Cretaceous igneous rocks of the Antarctic Peninsula. We present new geochronological (laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U–Pb) analyses of 36 intrusive and volcanic Cretaceous rocks, along with LA-ICP-MS apatite U–Pb analyses (a medium-temperature thermochronometer) of 28 Triassic–Cretaceous igneous rocks of the Antarctic Peninsula. These are complemented by new zircon Hf isotope data along with whole-rock geochemistry and isotope (Nd, Sr and Pb) data. Our results indicate that the Cretaceous igneous rocks of the Antarctic Peninsula have geochemical signatures consistent with a continental arc setting and were formed during the interval c. 140–79 Ma, whereas the main peak of magmatism occurred during c. 118–110 Ma. Trends in ε Hf t (zircon) combined with elevated heat flow that remagnetized rocks and reset apatite U–Pb ages suggest that Cretaceous magmatism formed within a prevailing extensional setting that was punctuated by periods of compression. A noteworthy compressive period probably occurred during c. 147–128 Ma, triggered by the westward migration of South America during opening of the South Atlantic Ocean. Cretaceous arc rocks that crystallized during c. 140–100 Ma define a belt that extends from southeastern Palmer Land to the west coast of Graham Land. This geographical distribution could be explained by (1) a flat slab with east-dipping subduction of the Phoenix Plate, or (2) west-dipping subduction of the lithosphere of the Weddell Sea, or (3) an allochthonous origin for the rocks of Alexander Island. A better understanding of the geological history of the pre-Cretaceous rocks of Alexander Island and the inaccessible area of the southern Weddell Sea is required. Supplementary material: A description of the methods used in this study and the complete dataset are available at https://doi.org/10.6084/m9.figshare.c.6089274 
    more » « less
  5. ; (, Geological Society of America)
    The Permian-Triassic time is a significant stage in the Paleozoic continental amalgamation and Cenozoic orogenic reactivation of southern Central Asian Orogenic Belt (CAOB). Field, petrographic, and detrital zircon U-Pb geochronological data of the uppermost Carboniferous–Lower Triassic sandstones from 3 sections in Bogda Mountains, greater Turpan-Junggar basin, NW China, are used to decipher the tectonic history. The sections are Tarlong-Taodonggou (TT) and Zhaobishan (ZBS) in the south and Dalongkou (DLK) in the north, 100 km apart and ~7,000 m in total thickness. Four petrofacies of 229 sandstones and U-Pb dates of 3505 zircons of 35 sandstones form the basis for interpretation. During Gzhelian–Asselian, andesite and basalt are the major source lithologies in TT. Zircon ages peak at ~300 Ma. During Sakmarian–Kungurian, basalt and andesite are the main source rocks in TT and ZBS; and zircon ages of both areas peak at ~300 Ma. The Roadian–Wordian is represented by a regional unconformity. The Guadalupian source lithology and zircon date show a major change. Andesite is the common and rhyolite and basalt the minor source lithologies for TT and DLK; but rhyolite for ZBS. A unimodal peak at ~305 Ma occurs in TT; but two peaks at 305 and 455 Ma with common Precambrian dates in ZBS; and peaks of 310–295 Ma in DLK. During Wuchiapingian–mid Olenekian, andesite and rhyolite are the common source lithologies for TT and DLK; but rhyolite as the primary volcanic lithology for ZBS. In TT, Wuchiapingian- Induan samples have a major age peak at ~300 Ma, and an Olenekian sample has two peaks at ~300 and ~450 Ma. In ZBS, the age pattern is similar to that of the Guadalupian sample. In DLK, samples have a major peak at ~310 Ma and a minor peak at ~450 Ma. The comparable age clusters identified by multi-dimensional scaling indicate that North Tianshan is the source for TT and ZBS during the latest Carboniferous–early Permian. But in mid Permian, south Central Tianshan became the main source solely to ZBS. During late Permian–Early Triassic, both north and central Tianshan became the common sources to all three areas due to enhanced denudation. The source change in mid-Permian across a regional unconformity is synchronous with Paleo-Asian Ocean closure and arc-continent and continent-continent collisions, which occurred along the southern margin of Turpan- Junggar basin no later than Guadalupian. 
    more » « less
  • Citation Formats
  • MLA
  • APA
  • Chicago
  • BibTeX
  • Save / Share this Record
  • Send to Email