Stratigraphic records of paleogeography and global change from two late Proterozoic basins

Abstract

As sediments and volcanic deposits accumulate on Earth's surface, they record information about Earth's climate, the motion of continents, and the evolution of the biosphere. Through the study of ancient stratigraphic sequences, we can gain a window into our planet's varied, and sometimes tumultuous, past. In this dissertation, I employ a combination of field observations, magnetic data, and chemostratigraphic data in the Keweenawan Mid-continent Rift of North America and the Amadeus Basin of Central Australia to study the paleogeography and paleoclimate during and after the transition between the Mesoproterozoic (1.7 to 1.0 billion-years ago) and Neoproterozoic Eras (1.0 to 0.54 billion-years ago). The supercontinent Rodinia formed at the boundary between the Eras and broke apart throughout the Neoproterozoic. Basins that developed as Rodinia rifted apart record large changes in the biogeochemical cycling of carbon and sulfur, the waxing and waning of low-latitude ice sheets, and the progressive oxygenation of the atmosphere that facilitated the evolution of animals.

I report high-resolution paleomagnetic data in stratigraphic context from Mamainse Point, Ontario--the most complete succession in the 1.1 billion-year-old Mid-continent Rift. The results demonstrate that previous suggestions of large non-dipolar geomagnetic field components at the time stemmed from low temporal resolution across geomagnetic reversals during a period of rapid plate motion. This result strengthens the framework for evaluating records of tectonics and climate across the Mesoproterozoic/Neoproterozoic boundary. Rock magnetic experiments on Mamainse Point lavas, paired with electron microscopy, demonstrate that a component of the magnetization in oxidized flows that is antiparallel to the characteristic remanence is a result of martite self-reversal. This component is the best resolved natural example of the experimentally observed self-reversal that accompanies the maghemite to hematite transition. This result allows the magnetizations of the lavas to be fully interpreted, and also suggests that this self-reversal phenomena may be more widespread than currently recognized--with its identification in this study being greatly aided by stratigraphic context during a period when North America was moving rapidly towards the equator.

Stratigraphic and stable isotope work on the Neoproterozoic Bitter Springs Formation of the Amadeus Basin demonstrates that the negative carbon isotope values of the "Bitter Springs Stage" are tightly consistent in carbonate rocks across more than 400 km. In addition to being present in the isotopic composition of the carbonate, organic carbon isotope values shift sympathetically into and out of the stage thereby supporting the interpretation that the stage is a record of primary changes to the carbon cycle. The stage is bound by sequence boundaries that provide evidence for changes in sea-level and climate. Previous work on correlative stratigraphy from the Akademikerbreen Group of East Svalbard (Maloof et al., 2006), revealed changes in relative sea-level and paleomagnetic directions that have were interpreted to have resulted from a pair of large-scale true polar wander events. In an effort to further test this hypothesis, and to remedy a lack of paleogeographic constraints for north Australia in the early Neoproterozoic, I present paleomagnetic data from more than 630 paleomagnetic samples of carbonates, siltstones and basalt flows from the Bitter Springs Formation. A new reliable pole from post-Bitter Springs Stage siltstones provides strong support for a recently published hypothesis that there was relative rotation between north and south+west Australia in the late Neoproterozoic (Li and Evans, 2011), and for the long-standing hypothesis that Australia and Laurentia were cotravelers in Rodinia into the mid-Neoproterozoic Era. The difference between the paleomagnetic poles of syn-Bitter Springs Stage carbonates and post-Bitter Springs Stage siltstones is likely a result of a Cambrian remagnetization of the carbonates. However, if the difference is a result of large-scale true polar wander the resulting paleogeographic reconstruction is a snug geologically consistent SWEAT-style connection between Laurentia and Australia.

Lastly, the results from the Amadeus Basin are combined with results from the Adelaide Rift Complex of South Australia to argue for a change in the dynamics of the carbon cycle when the icehouse period of the Neoproterozoic commenced. I argue that changes in deep ocean chemistry, climate and the physical dynamics of ice sheets may be related to the glacial removal of a long-lived continental regolith during the Sturtian low-latitude glaciation following ~1.5 billion years without large-scale ice sheets on Earth.