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• 1. [Article] Structure and stratigraphy of the Sheep Mountain area, Centennial Range, Montana-Idaho

  The thesis area includes 44 square miles of the Centennial Range in Beaverhead County, Montana, and Clark County, Idaho. The rocks exposed in the area range in age from Precambrian to Recent and have been ...

  Citation × Citation Citation Abstract Copy Title: Structure and stratigraphy of the Sheep Mountain area, Centennial Range, Montana-Idaho Author: Moran, John Lee The thesis area includes 44 square miles of the Centennial Range in Beaverhead County, Montana, and Clark County, Idaho. The rocks exposed in the area range in age from Precambrian to Recent and have been divided into 23 mapped units. Pre-Belt metamorphic basement rocks are overlain unconformably by 3,433 feet of Paleozoic strata included in eleven formations. These formations represent all of the periods of the Paleozoic except the Ordovician and Silurian. Nine formations represent all the periods of the Mesozoic and total approximately 3,127 feet in thickness. Cenozoic units present in the area are basalts and rhyolites, landslide debris, glacial deposits, and alluvium. The major structure of the thesis area is a fault block that is the result of uplifting and tilting of the east-west trending Centennial Range during the downwarp of the Snake River Plain in late Pliocene and Pleistocene time. No stratigraphic or structural conditions in the thesis area are favorable for the accumulation and entrapment of oil and gas. Phosphorite has been mined from the Phosphoria Formation in the area. Title: Structure and stratigraphy of the Sheep Mountain area, Centennial Range, Montana-Idaho Author: Moran, John Lee The thesis area includes 44 square miles of the Centennial Range in Beaverhead County, Montana, and Clark County, Idaho. The rocks exposed in the area range in age from Precambrian to Recent and have been divided into 23 mapped units. Pre-Belt metamorphic basement rocks are overlain unconformably by 3,433 feet of Paleozoic strata included in eleven formations. These formations represent all of the periods of the Paleozoic except the Ordovician and Silurian. Nine formations represent all the periods of the Mesozoic and total approximately 3,127 feet in thickness. Cenozoic units present in the area are basalts and rhyolites, landslide debris, glacial deposits, and alluvium. The major structure of the thesis area is a fault block that is the result of uplifting and tilting of the east-west trending Centennial Range during the downwarp of the Snake River Plain in late Pliocene and Pleistocene time. No stratigraphic or structural conditions in the thesis area are favorable for the accumulation and entrapment of oil and gas. Phosphorite has been mined from the Phosphoria Formation in the area. Close

• 2. [Article] Deep-sea sediment paleomagnetism : a case study from the North Atlantic

  Sedimentary records from the North Atlantic, instrumental in the development of modern paleo-geomagnetic concepts, show a highly variable field even during times of constant polarity. Yet, our understanding ...

  Citation × Citation Citation Abstract Copy Title: Deep-sea sediment paleomagnetism : a case study from the North Atlantic Author: Strano, Sarah Elianna Sedimentary records from the North Atlantic, instrumental in the development of modern paleo-geomagnetic concepts, show a highly variable field even during times of constant polarity. Yet, our understanding of how the magnetization is acquired in the sediments is poorly understood. Primary magnetizations preserved in deep-sea sediments are known to be acquired through a depositional or possibly, a post-depositional remanent magnetization (DRM or pDRM). A pDRM process implies that the magnetization is locked-in at depth creating an offset between the age of the magnetization and the age of sediment. The process is not currently accounted for in paleomagnetic records despite the wide use of magnetic records to elucidate the timing and rate of change of many paleomagnetic and environmental processes. This dissertation uses seven Northern North Atlantic (NNA) deep-sea sediment cores that were studied by alternating field demagnetization of natural and laboratory imposed remanence on uchannel samples, providing for detailed paleomagnetic and environmental magnetic records. These high-quality Holocene and deglacial magnetic data are combined with independent radiocarbon chronologies to better understand the: (1) magnetic acquisition process, (2) the NNA paleo-geomagnetic signal and (3) the influence of rock magnetic parameters on the sedimentary paleomagnetic record. Under the traditional paradigm of magnetostratigraphy, sediment deposition and magnetization are assumed to occur synchronously and with little to no signal attenuation. In Chapter 2, we compare independently dated Holocene paleomagnetic records from the seven deep-sea sediments cores across the North Atlantic with regional paleo-geomagnetic reconstructions derived from ultra-high resolution sediment records. We find variable delays between the timing of these records, consistent with a magnetization “locked-in” at depth and over an interval that results in smoothing of the geomagnetic signal. Optimization modeling of the post-depositional remanent magnetization (pDRM) accounts for both offset and some of this smoothing. It also demonstrates that the preserved magnetization is acquired ~20 cm below the sediment-water interface. Consistent with previous observations, this potentially ubiquitous process results in age offsets of 350-2000 years even in deep-sea sediment accumulation rates in excess of 10 cm/kyr that is rarely if ever accounted for in magnetostratigraphy or paleomagnetic records. In Chapter 3, we assume that the new pDRM-corrected chronologies developed for Chapter 2 more accurately represent each paleomagnetic record and create a NNA stack of both direction and intensity from ~15,000 years ago to present (NAPstack15). Uncertainty analyses and comparison to data derived from global field models at the same locations suggest that both, our directional and intensity stacks robustly capture the evolution of the mean geomagnetic field variations of the NNA. Broader regional comparisons with data from North America and Europe begin to define the evolution of the geomagnetic field during this time interval. Geomagnetic morphology and spatial/temporal variability can be roughly broken into three time intervals consistent with the evolution of global intensity and implicate the dynamics of the high-latitude Holocene flux patches as a source of this variability. We evaluate the effect of rock magnetic properties on the fidelity of the NNA paleomagnetic record from the Holocene through the last deglaciation. NNA records have been argued to consistently record high-quality paleomagnetic records over millennial to orbital time scales with little concern for lithologic variability resulting from glacial-interglacial environmental changes. We find that rock magnetic variability has little effect on the fidelity of the NNA's directional record, but has a variable and sometimes large influence on normalized remanence records, which are commonly used as a relative paleomagnetic intensity proxy. We find that the eastern NNA records are most affected by the use of different normalizers during the Holocene. The western NNA cores are more affected by the use of different normalizers during the deglacial period but to a lesser extent. The Iceland Basin cores are an exception, providing consistent normalized remanence records regardless of the normalizer during both the Holocene and deglacial interval. This likely reflects their proximity to Icelandic basaltic sources and the consistent magnetite grain-size regardless of physical grain-size that these sources provide. Title: Deep-sea sediment paleomagnetism : a case study from the North Atlantic Author: Strano, Sarah Elianna Sedimentary records from the North Atlantic, instrumental in the development of modern paleo-geomagnetic concepts, show a highly variable field even during times of constant polarity. Yet, our understanding of how the magnetization is acquired in the sediments is poorly understood. Primary magnetizations preserved in deep-sea sediments are known to be acquired through a depositional or possibly, a post-depositional remanent magnetization (DRM or pDRM). A pDRM process implies that the magnetization is locked-in at depth creating an offset between the age of the magnetization and the age of sediment. The process is not currently accounted for in paleomagnetic records despite the wide use of magnetic records to elucidate the timing and rate of change of many paleomagnetic and environmental processes. This dissertation uses seven Northern North Atlantic (NNA) deep-sea sediment cores that were studied by alternating field demagnetization of natural and laboratory imposed remanence on uchannel samples, providing for detailed paleomagnetic and environmental magnetic records. These high-quality Holocene and deglacial magnetic data are combined with independent radiocarbon chronologies to better understand the: (1) magnetic acquisition process, (2) the NNA paleo-geomagnetic signal and (3) the influence of rock magnetic parameters on the sedimentary paleomagnetic record. Under the traditional paradigm of magnetostratigraphy, sediment deposition and magnetization are assumed to occur synchronously and with little to no signal attenuation. In Chapter 2, we compare independently dated Holocene paleomagnetic records from the seven deep-sea sediments cores across the North Atlantic with regional paleo-geomagnetic reconstructions derived from ultra-high resolution sediment records. We find variable delays between the timing of these records, consistent with a magnetization “locked-in” at depth and over an interval that results in smoothing of the geomagnetic signal. Optimization modeling of the post-depositional remanent magnetization (pDRM) accounts for both offset and some of this smoothing. It also demonstrates that the preserved magnetization is acquired ~20 cm below the sediment-water interface. Consistent with previous observations, this potentially ubiquitous process results in age offsets of 350-2000 years even in deep-sea sediment accumulation rates in excess of 10 cm/kyr that is rarely if ever accounted for in magnetostratigraphy or paleomagnetic records. In Chapter 3, we assume that the new pDRM-corrected chronologies developed for Chapter 2 more accurately represent each paleomagnetic record and create a NNA stack of both direction and intensity from ~15,000 years ago to present (NAPstack15). Uncertainty analyses and comparison to data derived from global field models at the same locations suggest that both, our directional and intensity stacks robustly capture the evolution of the mean geomagnetic field variations of the NNA. Broader regional comparisons with data from North America and Europe begin to define the evolution of the geomagnetic field during this time interval. Geomagnetic morphology and spatial/temporal variability can be roughly broken into three time intervals consistent with the evolution of global intensity and implicate the dynamics of the high-latitude Holocene flux patches as a source of this variability. We evaluate the effect of rock magnetic properties on the fidelity of the NNA paleomagnetic record from the Holocene through the last deglaciation. NNA records have been argued to consistently record high-quality paleomagnetic records over millennial to orbital time scales with little concern for lithologic variability resulting from glacial-interglacial environmental changes. We find that rock magnetic variability has little effect on the fidelity of the NNA's directional record, but has a variable and sometimes large influence on normalized remanence records, which are commonly used as a relative paleomagnetic intensity proxy. We find that the eastern NNA records are most affected by the use of different normalizers during the Holocene. The western NNA cores are more affected by the use of different normalizers during the deglacial period but to a lesser extent. The Iceland Basin cores are an exception, providing consistent normalized remanence records regardless of the normalizer during both the Holocene and deglacial interval. This likely reflects their proximity to Icelandic basaltic sources and the consistent magnetite grain-size regardless of physical grain-size that these sources provide. Close