Date of Award

1997

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geology and Geophysics

First Advisor

Arnold H. Bouma

Abstract

Thin-bedded turbidites (TBTs, 5-60 cm) constitute a common facies in fine-grained deep-water clastic environments. Proper characterization of these deposits are lacking due to the scarcity of well preserved outcrops exposing TBTs and the difficulty in resolving them by conventional subsurface logging methods. The objective of this study is to characterize TBTs developed in a variety of laterally contiguous depositional environments in a submarine fan. These deposits are well exposed in the Permian Tanqua Karoo subbasin, South Africa. The subenvironments studied include upper mid-fan channel levee-overbank, mid-fan passive channel fill, lower mid-fan channel-sheet transition, and lower-fan distal sheet deposits. Field and laboratory approaches reveal systematic variations in grain-size, sedimentary structures, grain-orientation, bed-thicknesses and vertical gamma-ray patterns between different subenvironments that indicate variable depositional styles. Coarsest grain-size (very-fine sand) is present in distal sheet sandstones and the finest (medium silt) in passive channel fills. Levee overbanks and passive channel fills are dominated by base truncated Bouma Sequences $\rm(T\sb{c},\ T\sb{cd},\ T\sb{cde}),$ while channel sheet transition and distal sheet deposits are characterized by top-truncated Bouma Sequences $\rm(T\sb{a},\ T\sb{ab},\ T\sb{abc}).$ Grain-orientations based on ferro-magnetic grains using anisotropy of isothermal remanent magnetization (IRMA) are primarily flow perpendicular (active channel fill, channel-sheet transition, and distal sheet deposits) or flow parallel (levee-overbank, passive channel fill) reflecting active tractional reworking or lack thereof. Active channel fills and channel-sheet transition deposits have the highest percent anisotropies that indicate flow confinement. Unconfined flows in levee-overbank and distal sheet deposits have lower percent anisotropies. Bed-thickness distributions are monotonously skewed towards lower values with minor variations between different subenvironments. Associated with the study of TBTs, a conceptual temporal correlation has been proposed between spatially separated depositional elements (e.g., upfan channel-levee overbank complexes, downfan depositional lobes etc.). A primarily retrogressive or backstepping mode of deposition is observed within a depositional sequence. A higher resolution temporal correlation is proposed based on field observations than previously reported. The Tanqua subbasin by virtue of its high latitude paleo-position also represents an analog to deposition controlled by asymmetric levees as a result of Coriolis forcing.

ISBN

9780591532784

Pages

160

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