Paleoceanographic significance of the Neoproterozoic Polanco Limestones Formation (Uruguay): stratigraphy, isotopic geochemistry and age

• Author / Creator

  Aubet, Natalie

• Carbon and Strontium isotope data for marine carbonates can provide a detailed record of isotopic variations in seawater through time and have proven to be a valuable tool for interpreting biogeochemical events and correlating Neoproterozoic sedimentary successions worldwide. The Neoproterozoic of Uruguay has been regarded as containing a significant geological and palaeontological record, which would make these successions critical to unraveling diverse aspects regarding the assembly of southwestern Gondwana and to understanding the conditions surrounding the rise of animal life in a period punctuated by drastic palaeoenvironmental changes. Particularly, the Polanco Limestones Formation is the only carbonate unit of the entire record, and therefore offers a unique opportunity to explore the palaeoenvironmental conditions during sedimentation. This thesis is built on previous chemostratigraphic studies reporting a negative carbon (!13Ccarb) isotope excursion to values as low as -4.5‰ and interpreted as recording the aftermath of a post-Gaskiers glacial event occurred elsewhere on southwest Gondwana and correlated with the Shuram-Wonoka-Johnnie carbon isotope anomaly (~551 Ma). In the following dissertation, a multi-proxy approach combining extensive fieldwork, geochronology and isotope geochemistry on the Polanco carbonates is presented with the aim of better constraining the environmental changes and their implications during this critical period of time. The stratigraphy and age of the Neoproterozoic record in Uruguay, including the Polanco Limestones Formation, is proved to be equivocal. The same is true for existing basin models and tectonic evolution, which show different and sometimes contradicting supporting evidence. From a tectonic perspective, the sequence of events begins with a period of tectonic quiescence and deposition of extensive mixed siliciclastic-carbonate sedimentary successions. This is followed by the development of small fault-bounded siliciclastic and volcaniclastic basins and the emplacement of voluminous granites associated with episodic terrane accretion. The deposition of mixed siliciclastic-carbonate sedimentary successions, which encompasses the Arroyo del Soldado Group -including the Polanco Limestones Formation- and the Arroyo de la Pedrera Group occurred between 650 and 1000 Ma. In contrast, the Ediacaran record consists of the volcano- sedimentary Maldonado Group, and the Tacuarí, Barriga Negra, Rocha and Sierra de Aguirre formations. The best available radiometric age constraints indicate that intense magmatic-tectonic activity occurred during the deposition of these units, between 600 and 560 Ma, incompatible with previous models suggesting a stable, Atlantic-type passive margin on this portion of southwestern Gondwana. By combining new U-Pb ages in limestones and detrital zircons as well as K-Ar ages in shales along with chemostratigraphic data, the Polanco Limestones Formation is constrained between ~680 and 650 Ma implying that the unit was deposited during the Cryogenian interglacial period, between the Sturtian and Marinoan glaciations. Carbon isotopic profiles from deep- to shallow-water facies of the Polanco Limestones Formation show a dominance of negative values in deep-water facies and a progressive rise towards positive values in shallow-water settings. This previously unrecognized trend suggests that deposition of Polanco carbonates occurred in a stratified marine basin where degradation of organic matter below the redox boundary led to lower !13C values at greater depths. Carbonates were analyzed for paired !13Corg and !13Ccarb. Their records reveal local changes in nutrient input and phytoplankton growth rates. This is supported by simultaneous changes in the concentrations of nutrients (P), lithophile elements (e.g., Al, Zr) and 87Sr/86Sr ratios, which also indicate a terrestrial source. However, the dominant control on the !13Corg and !13Ccarb curves is interpreted to reflect changing [CO2]aq, and possibly in global pCO2. The consistently small !13C values suggest low atmospheric pCO2 wherein, under CO2-limiting conditions, the utilization of HCO3- by planktic cyanobacteria lowered !13C and produced the precipitation of biogenic whitings. This study challenges the idea by which in a hard-snowball scenario subsequent rapid deglaciation resulted from the buildup of volcanic carbon dioxide during glaciation reaching high partial pressures of atmospheric CO2 (50-250 PAL). Alternatively, it accounts for independent evidence and supports recent studies on cap carbonates arguing for a rather low atmospheric pCO2 possibly as low as the current value even during deglaciation periods.

• Subjects / Keywords

  • Neoproterozoic
  • Stratigraphy
  • Uruguay

• Graduation date

  Spring 2016

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3GQ6R958

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.