Depositional and Structural History of the Pavian and Kudu Nappes in the Naukluft Mountains, Namibia

Citation

Morris, Freya Kurt (2024) Depositional and Structural History of the Pavian and Kudu Nappes in the Naukluft Mountains, Namibia. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/ecfc-c202. https://resolver.caltech.edu/CaltechTHESIS:08132023-171059108

Abstract

The termination of the Marinoan Snowball Earth glacial epoch was one of the most extreme climate events in Earth history. Yet, the transition from global glaciation to an ice-free warmer climate is still poorly constrained. The Naukluft Nappe Complex of south-central Namibia contains several stratigraphic formations that record the environmental and tectonic transitions of the Neoproterozoic, including glaciogenic deposits and basal-Ediacaran cap carbonate of the Marinoan Snowball Earth. This stratigraphic record has the potential to provide a critical record of the climate, sea-level history, ocean chemistry, and time frames across the climate transition of the Marinoan Snowball deglaciation.

We first show a detailed study of the sedimentology and stratigraphy of the upper Blässkranz Formation and Tsabisis Formation cap carbonate to develop an environmental and sequence stratigraphic history spanning and following the deglaciation. In downdip areas Marinoan diamictite transitions upward into dolostone intermixed with sandstone and extrabasinal clasts that is gradually overlain by fine grained laminated dolostone. Updip localities show the diamictite is overlain by intercalated sandstones, gravels, and shales before an abrupt change to laminated dolostone of the cap carbonate. A succession of stromatolites, which become strongly elongate upward, prograde into the laminated dolostone in the updip localities. The stromatolites are overlain by laminated dolostone that grades upward into rhythmite with intercalations of shale. Near the top of the cap, rhythmites may be reworked into tabular intraclast conglomerate, locally intercalated with hummocky cross stratified sandstone, which passes upward into the shale and limestone members of the Tsabisis Formation. The lateral and vertical distribution of facies indicate a retreat of the shoreline and glacially sourced siliciclastics near the base of the cap carbonate, a shallowing succession to fair-weather wave base at the top of the stromatolite facies, and a second shallowing succession to storm wave base near the top of the cap carbonate. Maximum flooding occurred soon after the initiation of carbonate deposition and two sequence boundaries mark higher stratigraphic levels within the cap carbonate. With a sea-level history and chronological framework inferred from the sequence stratigraphy we can consider different mechanisms of sea-level change, which may reflect the timescale and synchronicity of deglaciation.

Next, we consider the structural and stratigraphic relationships between the Neoproterozoic units of the Naukluft Mountains to define and contextualize the extent of the terminal Marinoan geologic record. We show that the Northern Pavian Nappe, which includes the Marinoan-associated Blässkranz and Tsabisis formations, is stratigraphically succeeded by the dolostone dominated Kudu Nappe and is not correlated or genetically related to the nearby Southern Pavian Nappe. Additionally, the modified stratigraphic and structural relationships allow for a simplified nappe emplacement history that reduces the magnitude of shortening associated with convergence along the Damara Orogen.

Finally, we use sea-level modeling of the Naukluft Marinoan record to constrain the duration of global deglaciation. Using a range of reconstructed synchronous and continuous deglaciation models, we evaluate if the observed sea-level patterns of the Naukluft can be fully explained by glacial isostatic mechanisms driven by the deglaciation. Short Snowball deglaciation durations, on the order of ~2 kyr, result in exclusive sea-level rise, or sea-level rise followed by sea-level fall, but cannot drive two distinct phases of sea-level fall. However, for longer duration snowball deglaciations, of ~10-30 kyr, we can drive two distinct intervals of sea-level rise and fall across much of the width of a continental margin, consistent with the stratal patterns observed in Naukluft Mountains cap carbonate succession. Our spatially varying sea-level predictions resulting from longer duration deglaciations may be applicable in interpreting stratal patterns of other cap carbonate successions. Furthermore, this work underlines the need for better constraints on the areal distribution and volume of Marinoan ice sheets, including improved understanding of plausible deglacial durations using updated global climate models.

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