--> How Variable are Submarine Basin-Floor Lobes in Space and Time?

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How Variable are Submarine Basin-Floor Lobes in Space and Time?

Abstract

Abstract

Multi-kilometer scale outcrops and near-outcrop research boreholes from fine-grained basin-floor lobes in the Karoo Basin (South Africa) allow their variability in sedimentary facies, architecture and flow processes to be constrained. Sixty-four outcrop logs (∼5 km total length) and 11 core logs (∼1 km) from Unit A, Laingsburg depocentre, and Fan 4, Tanqua depocentre, were integrated to compare basin-floor lobe deposits of different palaeo-seabed topographic and stratigraphic positions.

Lithofacies and architecture of lobe axis and lobe off-axis deposits are consistent across lobe complexes, being sand-prone and commonly amalgamated. However, two distinct lobe fringe settings, frontal and lateral, differ in their lithofacies and architecture. Frontal lobe fringes comprise structureless sandstone and hybrid bed deposits that may exhibit elongated finger-like shapes with abrupt sandstone pinchout. In contrast, lateral fringes are dominated by heterolithic traction-influenced deposits that gradually thin and fine in a simple taper geometry. Differences are interpreted to be related to primary flow processes: lateral lobe fringes are dominated by deposits from low-density turbidity currents, whereas frontal lobe fringes are dominated by deposits from high-density currents. Confinement is an important autogenic factor controlling dispersal and stacking patterns of lobes in lobe complexes. Successive lobe deposits in weakly confined to unconfined settings commonly exhibit compensational stacking driven by avulsion of distributive channels. Therefore, it is possible for lobe axes to abruptly overlie lobe fringes to form a complex 1D succession. Increasing confinement can change the dominant stacking pattern and lead to aggradationally and/or longitudinally stacked lobe deposits (e.g. Unit A). Lateral confinement enhances the difference between frontal and lateral lobe fringes and can lead to the stacking of lobe fringes at lobe, lobe complex and lobe complex set scales. Confinement can also act to modify lobe lithofacies. Lobe fringes that are stacked over several hierarchical scales show dominant climbing-ripple lamination and climbing (sigmoidal) bedforms; these bodies are ‘aggradational lobe fringes’.

New insights gained in this study will help to improve prediction of the stacking patterns and facies proportions of lobes during hydrocarbon field appraisal and development of submarine fans.