Are all submarine lobes made equal? Comparing the sedimentological processes and depositional architecture of submarine lobes in different palaeogeographic and sequence stratigraphic positions

Submarine lobes have been identified within many geographic and stratigraphic positions in deepwater systems, including frontal or precursor lobes that form remnants beneath related submarine channel-levee system; intraslope or perched lobes interpreted to have accumulating in low gradient steps on the slope; channel-mouth lobes that form down-dip of rapidly expanding (depletive) flows; crevasse lobes where turbidity currents have breached confinement of constructional levees; and topographically confined lobes where inherited or dynamic bathymetry influences the shape of deposits.

Furthermore, the stratigraphic position of a lobe within the lowstand systems tract is an important consideration (e.g. Hodgson 2009). Differences in the nature of the supply slope (flow efficiency), the type of substrate (entrainment of fines leading to flow transformation and loading), flow magnitude arriving at the basin floor, and scale of topographic relief (inherited from previous lobe, or intrabasinal topography) are important controls on flow processes and therefore the characteristics of lobes through time. An obvious question, therefore, is how do ‘early’ lobes that form during fan initiation differ in character to ‘late’ lobes, deposited during fan retreat?

A wide range of lobe types have been identified in the Laingsburg and Fort Brown Fms. and these will be analysed in detail to establish sedimentological criteria to distinguish different lobe types. Unit A is an ideal system to address the question of stratigraphic variability in lobe character as it covers a wide range of hierarchical and temporal scales, with lobe strata exposed over 10’s of km.

Subsurface investigations have indicated that an important component of lobes from a hydrocarbon reservoir perspective are hybrid beds (Haughton et al. 2009). Hybrid beds include a range of bipartite and tripartite beds with characteristics of deposition by non-cohesive flows (graded clean sand and silt) and cohesive flows (argillaceous, poorly sorted, organic-rich). Turbidites with an overlying ‘linked’ debrite have been widely reported from the fringes of submarine fans (e.g. Ito 2008; Hodgson 2009).
Although there is progress towards understanding the predictive value of hybrid beds, it is clear that hybrid beds can play a significant role in assessment of the economic viability of a reservoir due to their wide range of poro/perm measurements. There is variability between beds, but also laterally along event beds, which requires outcrops to constrain and predict.
Fundamental characteristics of hybrid beds that can be quantified through tracing individual beds at outcrop include metrics such as their width: thickness, turbidite: ‘debrite’ thickness, nature of the division contacts, and petrography and grain-size range of the ‘dirty’ division. Petrographic analysis, particularly of the silt-grade component, will be trailed as a pilot project initially using thin sections from core and outcrop.

As this is an interdisciplinary studentship, the student will benefit from a variety of research training in particular field-based deep-water sedimentology/stratigraphy, and will be required to spend field seasons in remote locations. The student will fully participate in the School and University postgraduate training programme and will present work at national and international conferences, as well as interacting with geoscientists at several a major hydrocarbon companies.