Honeycomb structures and other intriguing geomorphological features in the North Falkland Basin

The North Falkland Basin, a Mesozoic-aged sedimentary basin, located 40 km north of the Falkland Islands, is a rift system comprising a series of offset depocentres. The largest of the depocentres, the Eastern Graben, has a proven petroleum system hosting stratigraphic and combined structural-stratigraphic traps. In the shallow section overlying this significant sedimentary basin, there is a selection of unusual geomorphological features observable on 3D seismic data.

These features are observed at time-depths around 20–150 ms two-way-travel-time below the seabed, and are concentrated in three main geographical areas, covering in total more than 600 km². These oval to polygonal depressions are typically 350–650 m across and have c. 5–10 ms of relief. The depressions are delineated by an interconnected network of V-shaped cracks, which do not appear to have a preferred orientation, and seem to be limited stratigraphically within two reflectors. The features were initially attributed to polygonal faulting but, after further investigation, they appear to be very similar to honeycomb structures observed in the Great South Basin of New Zealand, that were attributed to diagenetic processes. Immediately above the honeycomb structure, there is a series of pockmarks that may be related to fluid expulsion from below. The shallow depths at which they are found and the evidence of fluid expulsion suggests these features could be due to the opal-A/CT transition.

Other intriguing features include a sequence of mounds that are typically 150–250 m wide and display 2–5 ms of height. In seismic profiles, the first few horizons directly below the mounds show small centres of disturbance of the reflection. The amplitude map of this reflector shows a strong amplitude contrast between the mounds and the surrounding areas. The vast majority of the mounds present acoustic shadow towards NW that can extend for a few hundred meters. The geometry and dimensions of these mounds are consistent with deep-water coral mounds and the observed acoustic shadow could result from the preferential accumulation of coral rubble NW of the mounds, which could be indicative of the predominant currents during that period.

This contribution will provide a detailed discussion of the morphology of these geomorphological features and their relationship to the overlying fluid expulsion structures, sedimentary setting as well as suggesting possible mechanisms for their formation.