Multi-scale architectural evolution and flow property characterization of channelized turbidite systems [electronic resource]
Keywords: Geological and Environmental Sciences.

Channelized turbidite systems in the deep ocean are important conduits for clastic detritus and can serve as excellent hydrocarbon reservoirs. These systems are inherently complex and heterogeneous, and our knowledge concerning their development, architecture and evolution is continuously advancing. Turbidity currents, the flows that carve and sculpt submarine channel systems, and their hydraulic properties, have proven difficult to characterize due to their destructive power. Consequently, outcrops and remotely sensed data of the seafloor have repeatedly demonstrated their usefulness in conveying small- and large-scale data that characterize submarine channel systems and the turbidity currents that build them. The ambition of this thesis is twofold: 1) to contribute to the growing database of unique examples of submarine channels and their evolution in spatial and temporal terms, and 2) to estimate turbidity current flow properties and consider how variations in those properties influence the architectural evolution of channelized turbidite systems. In order to accomplish that mission, three studies were undertaken, forming the chapters of this thesis. Chapter 1 is the detailed characterization of a large, asymmetric, conglomerate-rich submarine channel complex of the axial channel-belt of the Cerro Toro Formation in the Magallanes retro-arc foreland basin, southern Chile. This low-sinuosity channel belt flowed southward down the axis of the elongate foreland basin during the Late Cretaceous. Excellent exposures of the axial channel-belt on Sierra del Toro reveals the 3.5 km wide, 300 m thick 'Wildcat' channel complex that displays highly asymmetric facies distribution. Over 2000 m of measured section and field mapping demonstrate that grain size, bed thickness, degree of amalgamation, and margin architecture vary drastically across the channel fill. The eastern side of the Wildcat complex is characterized by thick-bedded conglomerate, sandstone, and debris-flow deposits onlapping a single erosional surface adjacent to sandy overbank deposits, whereas the western side shows thin-bedded, sandy and muddy facies onlapping a composite margin adjacent to a predominantly muddy overbank. The Wildcat complex is interpreted to represent part of a gentle right-hand meander bend of the axial channel-belt, and the facies and architecture of the opposing margins indicate that the eastern and western sides constitute the outer and inner banks of this meander bend. Turbidity currents, due to flow momentum and centrifugal forces, responded to the meander bend by preferentially depositing coarse, amalgamated sediment near the outer eastern bank and in the adjacent overbank; finer and non-amalgamated sediment accumulated near the inner western bank. The absence of lateral accretion deposits suggests that the channel was entrenched and did not migrate during filling. However, divergent paleoflow directions and overbank deposition in the uppermost channel fill indicate that late-stage flows were only weakly confined. These observations have been incorporated into an evolutionary model of asymmetric submarine channel fill that demonstrates observed facies distributions and the contrasting architecture of the inner and outer banks. This model can be applied to other low-sinuosity submarine channels and can be modified for more highly sinuous channels. Lastly, the abundant data concerning channel asymmetry presented here can be used to refine flume experiments and numerical models of sinuous channel evolution as well as populate reservoir models of sinuous submarine channels. Chapter 2 presents results from a seismic-reflection based study of the long-term evolution of a submarine canyon system located on the continental slope offshore Equatorial Guinea, west Africa. During the Late Cretaceous, the margin was incised by a sand-rich, erosive submarine canyon system that indented the shelf edge and had a downslope submarine fan. This canyon system was abandoned and partially infilled during the Paleogene, but the relict topography was reactivated in the Miocene during submarine erosion associated with tectonic uplift. A subsequent decrease in sediment supply resulted in a drastic transformation in the canyon morphology, leading to the modern 'Benito' canyon system, which does not indent the shelf edge, is mud-rich and aggradational, and has no downslope sediment apron. Borehole and core data indicates that the Cretaceous canyon system was dominated by erosive, sand-rich, high-density turbidity currents, whereas hemipelagic deposition is the chief depositional process aggrading the Benito canyon system. The presence of intra-canyon lateral accretion deposits suggests that the Benito canyon concavity was maintained by thick (> 150 m), muddy, dilute turbidity currents. When a Benito canyon loses access to the shelf and these dilute currents, it is abandoned and eventually filled. Fluid escape related to compaction of hemipelagic mud causes the successive formation of 'cross-canyon ridges' and pockmark trains along buried canyon axes during canyon abandonment. The modern seafloor just south of the study area is cut by a shelf indenting, erosive, sand-rich canyon that is morphologically similar to the Cretaceous canyon system, including the presence of a downslope submarine fan, yet this canyon exists adjacent to the much different Benito canyon system. Based on comparison of the three aforementioned canyon systems, this study promotes a bipartite canyon classification scheme: 'Type I' canyons indent the shelf edge and are linked to areas of high coarse-grained sediment supply, generating erosive canyon morphologies, sand-rich fill, and large downslope submarine fans/aprons. 'Type II' canyons do not indent the shelf edge and exhibit smooth, aggradational morphologies, mud-rich fill, and a lack of downslope fans/aprons. Type I canyons are dominated by erosive, sandy turbidity currents and mass wasting, whereas in Type II canyons, hemipelagic deposition and muddy, dilute, sluggish turbidity currents are the main depositional processes. This morphology-based classification scheme can be used to help predict depositional processes, grain size distributions, and the petroleum prospectivity of any submarine canyon. Chapter 3 developed out of my interest in climbing ripples and climbing-ripple cross-lamination (CRCL), a beautiful bedform that 'stores' flow property data upon deposition. The combination of bedload transport and suspended load sedimentation forms climbing ripples, and the angle of climb is dependent on the ratio of these two processes. These flow conditions have strict boundary conditions and indicate specific depositional environments. Three areas of deep-water CRCL formation were studied: 1) Miocene outcrops of submarine channel deposits in the Taranaki basin, New Zealand, 2) Permian submarine fan outcrops in the Tanqua Karoo, South Africa, and 3) Lower Pleistocene core from the Magnolia Field, Gulf of Mexico. These three locales, with various basin settings and local depositional architectures, all exhibit thick-bedded CRCL deposits. From these locales, four morphology-based CRCL facies are identified and the products of many different flow types, from depletive, short-lived flows that deposit only one thin set of CRCL to flows that are long-lived and exhibit surging before finally collapsing, forming CRCL with increasing angle of climb. Facies distributions and local contextual information were used to interpret the depositional environment of each locale. Although particulars vary, all locales occupy 'off-axis' environments not far removed from axes of turbidity current transport. Forty-four sedimentation units containing CRCL were measured in detail for input into the TDURE model. Calculating flow properties of this number of natural turbidites is unprecedented. CRCL sedimentation rates average 0.15 mm/s and average accumulation time is 27 minutes. Sedimentation rates do not vary significantly between locales, suggesting that CRCL in each locale was the result of non-uniform flow likely caused by a reduction in flow thickness. A distinct temporal increase in sedimentation rates in the New Zealand is interpreted to be caused by the filling of a submarine channel and the resulting progressive unconfinement. Finally, the flow property data is compared to hindered settling velocities in order to estimate concentrations of the depositing turbidity currents.