Deepwater Gravity Systems
Sediment routing in deepwater gravitational fold belts
Examples from the Gulf of Mexico, Angola and Niger Delta
Examples of slope channels being diverted or deflected by growth folds, salt walls or thrusts are found both at the modern seabed and in the subsurface in deepwater fold and thrust belts. Sands can be ponded behind growing structures and slumps originating on the flanks of growing anticlines are common. Most previous descriptions of the interaction of deepwater depositional systems with growing structures have tended to be descriptive. In our research we attempt a quantitative analysis of both the geomorphic parameters of deepwater channel systems and structural growth rate of fold and salt structures with the aim of investigating whether there are any predictive relationships between structural parameters such as structural relief, growth rate and depositional patterns.
Figure 1. Seabed map from 3D seismic data in outer fold and thrust belt on the Niger Delta. Note channels deflected or diverted by active folds that have associatied seabed topography. Other channels, such as Channel 1, crosses fold B. Work by B. Jolly shows that the channel crosses Fold B at a strain minima. B.Jolly unpublished data.
We are examining buried systems offshore Angola and the Gulf of Mexico and a Pliocene-Recent system in the deepwater parts of the Niger Delta. In each area we observe slope channels which modify their downstream course as they interact with growing structures. Ponded deposits are common in each area and local slumping associated with structural relief is observed in all three areas. Our work shows that the Pliocene-modern Niger delta slope channel systems behave comparably to the older buried channels studied in deepwater Angola and Gulf of Mexico.
We have quantified the shortening rate through time of thrust-related folds, salt cored anticlines and salt-detachment folds in each area. Shortening rates in the salt-dominated Angolan and Gulf of Mexico systems are comparable, with maxima of up to ~100 m/Ma. The Niger delta thrust-related folds appear to have grown somewhat faster, by up to a factor of 2. In general, during periods of high growth, channels are forced to deflect or divert around growing structures. However, channels that already have established a route through a fold or salt wall (e.g. at a relay position or fold saddle) can continue to exploit that route down slope as long as the turbidity flows continue to have enough erosive power to keep pace with the growth of the structure. In any one area, variations in growth rate and structural relief both along the length of structures and between structures is the most important control on channel behaviour. For example channels exploit areas of low growth to get through or around structures and ponding occurs behind the parts of a structure where most growth has occurred.
Figure 2. Map of salt constrained minibasin offshore Angola, with location of active structures (salt-cored folds) and channel systems (bright colours; RMS amplitude extractions). Folds colour-coded according to shortening rate for amount of strain that accrued within interval in which channels deposited. Red= high shortening rate (>30mm/Ma); Orange =medium (10-30mm/Ma) and yellow = low (<10m/Ma)The channel systems can be see to be diverted around structures with highest values of shortening rates. From Jones et al. 2012.
Lidia Lonergan, Byami Jolly, Gemma Jones, Alex Whittaker (Imperial College); Mike Mayall and Steve Dee (BP)
Gemma Jones: “Influence of active salt tectonics on the development of deepwater slope channel systems, Offshore Angola and Gulf of Mexico”. NERC-CASE award with BP; November 2010-
Byami Jolly: “The interaction between deepwater channel systems and growing thrusts and folds, toe-thrust region of the deepwater Niger Delta” PTDF Nigerian Government Scholarship. November 2010-
Jones, G. Mayall, M., Lonergan, L. 2012. Contrasting depositional styles on a slope system and their control by salt tectonics - through-going channels, ponded fans and mass transport complexes. In 32nd Annual GCSSEPM Foundation Bob F. Perkins Research Conference “New Understanding of the Petroleum Systems of Continental Margins of the World”’ Conference proceedings, p.503-533 (DVD).
Mayall, M., Lonergan, L., Bowman, A., James, S., Mills, K., Primmer, T., Pope D., Rogers, L., Skeene,R. 2010 The response of turbidite slope channels to growth-induced seabed topography. AAPG Bull. 94, 1011-1030.