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Dissertation/Thesis Abstract

Sedimentology and stratigraphy of a deepwater transient fan on the continental slope: The Late Miocene Isongo Formation, equatorial West Africa
by Wolak, Jeannette Marie, Ph.D., Montana State University, 2011, 197; 3454532
Abstract (Summary)

Deepwater clastic deposits of the Late Miocene Isongo Formation, located 50 km northwest of Bioko Island, characterize a sand-rich transient fan system developed in response to incipient structural growth on the continental slope. Approximately 1200 ft (366 m) of conventional core, 21 wells, 3-D seismic (maximum 40-60 Hz), biostratigraphy, chemostratigraphy and dynamic production data from the 100 km2 study area are used to: (1) Define process-based sedimentological facies within the Isongo Fan; (2) Characterize porosity and permeability trends at three temporal and spatial scales; (3) Identify sedimentary bodies throughout cored and uncored intervals; and (4) Correlate stratigraphic sequences within a framework of adjustment, initiation, growth and retreat (AIGR). While the former two objectives capture small-scale geologic heterogeneities developed at the time of deposition, the latter two describe changes in fan geomorphology during coeval uplift and sedimentation.

Fifteen core-defined sedimentological facies reflect subaqueous depositional processes including turbidity currents, debris flows and pelagic settling. Pore space generated during deposition is correlated to grain size; pore connectivity is correlated to sorting. Sedimentation units and facies assemblages, which characterize depositional processes operating during a single sedimentation event, show patterns of accumulative flow in the confined, narrow portion of the Isongo Fan; depletive flow in the unconfined, southwestern portion. Erosional slope channel-levee systems in the northeast demonstrate very large sedimentation events, likely due to eruptive activity and uplift of Mount Cameroon 50 km updip. Partly confined to unconfined distributary channel-lobe systems in the southwest characterize a change in fan geomorphology off the flank of a growing structure.

Core-calibrated petrophysical facies and wireline log thicknesses allow identification of sedimentary bodies in uncored intervals throughout the Isongo Fan, a 2.3 million year episode of sand-rich deposition (3rd order sequence). High frequency 4th and 5th order sequences are used to describe changes in fan morphology over time relative to the growing anticline. A surface of adjustment marks the onset of syn-sedimentary growth, followed by 4th order phases of initial deposition that onlap the structure. Sand-rich fan growth, however, is greatest during a period of minimal uplift, followed by retreat of the Isongo depocenter to the north. Post-Isongo deposits suggest that renewed anticline growth resulted in avulsion of the system to the northwest after 8.2 Ma.

Indexing (document details)
Advisor: Gardner, Michael H.
Commitee: Bowen, David W., Mogk, David W., Reidy, Michael, Schmitt, James G.
School: Montana State University
Department: Earth Sciences
School Location: United States -- Montana
Source: DAI-B 72/07, Dissertation Abstracts International
Subjects: African Studies, Geology, Petroleum Geology, Sedimentary Geology
Keywords: Continental slope, Deepwater transient, Late Miocene Isongo, Niger delta, Sedimentology, Stratigraphy, Submarine fan, Turbidite
Publication Number: 3454532
ISBN: 978-1-124-64160-7
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