An official website of the United States government
While it remains uncertain whether excursions in the stable carbon isotopic composition of Ediacaran marine carbonate (δ13Ccarb) represent globally synchronous events (or a direct measure of ocean carbon cycling), the absence of widely distributed and readily preservable fauna, and the presence of several iconic carbon isotope excursions (CIEs), has sustained δ13Ccarb correlation as the primary means to establish relative time relationships for Ediacaran successions. Here we present an Ediacaran global δ13Ccarb composite built with a dynamic time warping (DTW) time-normalization algorithm that generates libraries of least-squares alignments between chemostratigraphic records of unequal length and distinct sediment accumulation rates. When developing a δ13Ccarb composite for each of 16 globally distributed Ediacaran paleo-depositional regions, we selected high Pearson r alignments that conformed with published geological guidance about the correlation of constituent sections. When applying DTW to align these regional algorithmic composites into one global δ13Ccarb stack, we selected alignments that allied the excursions that field workers have established (or speculated) are the Marinoan cap carbonate excursion, the Shuram excursion, and/or the basal Cambrian excursion. There are strengths and weaknesses to making explicit the temporal relationships (point-to-point correspondences) often left implicit in visual correlation. One strength is to extrapolate depositional ages by means of isotopic correlation, and here we explored this with a Bayesian Markov chain Monte Carlo age model that predicts a median age, and uncertainty, for every carbonate stratum in the global Ediacaran δ13Ccarb composite. Yet, one must caution against a false accuracy that can arise from selecting one alignment among many possibilities––the likelihood that time-uncertain time series can be stretched and squeezed into one unequivocal alignment is low. Thus, while these alignments are grounded in the expert assessment of the field worker, this global Ediacaran δ13Ccarb–Bayesian age model should be viewed as a working hypothesis to enrich, but not arbitrate, discussions of the correlation, synchrony, and completeness of Ediacaran successions.
more »
« less
Frenchman Mountain Dolostone: A new formation of the Cambrian Tonto Group, Grand Canyon and Basin and Range, USA
We describe, interpret, and establish a stratotype for the Frenchman Mountain Dolostone (FMD), a new Cambrian stratigraphic unit that records key global geochemical and climate signals and is well exposed throughout the Grand Canyon and central Basin and Range, USA. This flat-topped carbonate platform deposit is the uppermost unit of the Tonto Group, replacing the informally named “undifferentiated dolomites.” The unit records two global chemostratigraphic events—the Drumian Carbon Isotope Excursion (DICE), when δ13Ccarb (refers to “marine carbonate rocks”) values in the FMD dropped to −2.7‰, and the Steptoean Positive Carbon Isotope Excursion (SPICE), when the values rose to +3.5‰. The formation consists of eight lithofacies deposited in shallow subtidal to peritidal paleoenvironments. At its stratotype at Frenchman Mountain, Nevada, the FMD is 371 m thick. Integration of regional trilobite biostratigraphy and geochronology with new stratigraphy and sedimentology of the FMD, together with new δ13Ccarb chemostratigraphy for the entire Cambrian succession at Frenchman Mountain, illustrates that the FMD spans ~7.2 m.y., from Miaolingian (lower Drumian, Bolaspidella Zone) to Furongian (Paibian, Dicanthopyge Zone) time. To the west, the unit correlates with most of the Banded Mountain Member of the ~1100-m-thick Bonanza King Formation. To the east, at Grand Canyon’s Palisades of the Desert, the FMD thins to 8 m due to pre–Middle Devonian erosion that cut progressively deeper cratonward. Portions of the FMD display visually striking, meter-scale couplets of alternating dark- and light-colored peritidal facies, while other portions consist of thick intervals of a single peritidal or shallow subtidal facies. Statistical analysis of the succession of strata in the stratotype section, involving Markov order and runs order analyses, yields no evidence of cyclicity or other forms of order. Autocyclic processes provide the simplest mechanism to have generated the succession of facies observed in the FMD.
more »
« less
- Award ID(s):
- 1954634
- PAR ID:
- 10434374
- Date Published:
- Journal Name:
- Geosphere
- ISSN:
- 1553-040X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Cambrian–Devonian sedimentary rocks of the northern Canadian Cordillera record both the establishment and demise of the Great American Carbonate Bank, a widespread carbonate platform system that fringed the ancestral continental margins of North America (Laurentia). Here, we present a new examination of the deep-water Road River Group of the Richardson Mountains, Yukon, Canada, which was deposited in an intra-platformal embayment or seaway within the Great American Carbonate Bank called the Richardson trough. Eleven detailed stratigraphic sections through the Road River Group along the upper canyon of the Peel River are compiled and integrated with geological mapping, facies analysis, carbonate and organic carbon isotope chemostratigraphy, and new biostratigraphic results to formalize four new formations within the type area of the Richardson Mountains (Cronin, Mount Hare, Tetlit, and Vittrekwa). We recognize nine mixed carbonate and siliciclastic deep-water facies associations in the Road River Group and propose these strata were deposited in basin-floor to slope environments. New biostratigraphic data suggest the Road River Group spans the late Cambrian (Furongian) – Middle Devonian (Eifelian), and new chemostratigraphic data record multiple global carbon isotopic events, including the late Cambrian Steptoean positive carbon isotope excursion, the Late Ordovician Guttenberg excursion, the Silurian Aeronian, Valgu, Mulde (mid-Homerian), Ireviken (early Sheinwoodian), and Lau excursions, and the Early Devonian Klonk excursion. Together, these new data not only help clarify nomenclatural debate centered around the Road River Group, but also provide critical new sedimentological, biostratigraphic, and isotopic data for these widely distributed rocks of the northern Canadian Cordillera.more » « less
-
Abstract Neoproterozoic–Cambrian rocks of the Windermere Supergroup and overlying units record the breakup of Rodinia and formation of the northwestern Laurentian ancestral continental margin. Understanding the nature and timing of this transition has been hampered by difficulty correlating poorly dated sedimentary successions from contrasting depositional settings across Mesozoic structures. Here we present new litho- and chemo-stratigraphic data from a Cryogenian–lower Cambrian succession in east-central Yukon (Canada), establish correlations between proximal and distal parts of the upper Windermere Supergroup and related strata in the northern Canadian Cordillera, and consider implications for the formation of the northwestern Laurentian margin. The newly defined Nadaleen Formation hosts the first appearance of Ediacaran macrofossils, while the overlying Gametrail Formation features a large negative carbon isotope anomaly with δ13Ccarb values as low as –13‰ that correlates with the globally developed Shuram-Wonoka anomaly. We also define the Rackla Group, which includes the youngest (Ediacaran) portions of the Windermere Supergroup in the northern Cordillera. The top of the Windermere Supergroup is marked by an unconformity above the Risky Formation that passes into a correlative conformity in the Nadaleen River area. This surface has been interpreted to mark the top of the rift-related succession, but we draw attention to evidence for tectonic instability through the early-middle Cambrian and argue that the transition from rifting to post-rift thermal subsidence is marked by a widespread unconformity that underlies upper Cambrian carbonate rocks. This is younger than the interpreted age of the rift to post-rift transition elsewhere along the ancestral western Laurentian continental margin.more » « less
-
The Ediacaran-Cambrian transition interval is described for the west part of the Gondwana Supercontinent. This key interval in Earth’s history is recorded in the upper and lower part of the Tagatiya Guazú and Cerro Curuzu formations, Itapucumi Group, Paraguay, encompassing a sedimentary succession deposited in a tidally influenced mixed carbonate-siliciclastic ramp. The remarkable presence of cosmopolitan Ediacaran shelly fossils and treptichnids, which are recorded in carbonate and siliciclastic deposits, respectively, suggests their differential preservation according to lithology. Their distribution is conditioned by substrate changes that are related to cyclic sedimentation. The associated positive steady trend of the δ13C values in the carbonate facies indicates that the Tagatiya Guazú succession is correlated to the late Ediacaran positive carbon isotope plateau. Sensitive high-resolution ion microprobe U-Pb ages of volcanic zircons from an ash bed ∼30 m above the fossil-bearing interval in the Cerro Curuzu Formation indicate an Early Cambrian (Fortunian) depositional age of 535.7 ± 5.2 Ma. As in other coeval sedimentary successions worldwide, the co-occurrence of typical Ediacaran skeletal taxa and relatively complex trace fossils in the studied strata highlights the global nature of key evolutionary innovations.more » « less
-
null (Ed.)Abstract We present chemostratigraphy, biostratigraphy, and geochronology from a succession that spans the Ediacaran-Cambrian boundary in Sonora, Mexico. A sandy hematite-rich dolostone bed, which occurs 20 m above carbonates that record the nadir of the basal Cambrian carbon isotope excursion within the La Ciénega Formation, yielded a maximum depositional age of 539.40 ± 0.23 Ma using U-Pb chemical abrasion–isotope dilution–thermal ionization mass spectrometry on a population of sharply faceted volcanic zircon crystals. This bed, interpreted to contain reworked tuffaceous material, is above the last occurrences of late Ediacaran body fossils and below the first occurrence of the Cambrian trace fossil Treptichnus pedum, and so the age calibrates key markers of the Ediacaran-Cambrian boundary. The temporal coincidence of rift-related flood basalt volcanism in southern Laurentia (>250,000 km3 of basalt), a negative carbon isotope excursion, and biological turnover is consistent with a mechanistic link between the eruption of a large igneous province and end-Ediacaran extinction.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1130/GES02514.1
