An Alternative Model for Producing Levee Topography in Deep-Water Systems Based on Field Studies from the Neoproterozoic Isaac Formation

Proximal to distal deep-water levee transects are commonly characterized on seismic images by a concave-up profile in which the steeper part occurs proximal to the channel. In modern deep-water levee systems like those on the Amazon and Indus fans and deep-water Niger Delta the steeper segment of the profile is typically of the order of 2 to 4°. It is generally thought that this morphology reflects the internal stratigraphy of the levee wherein individual beds in the proximal levee dip more steeply than their distal levee counterparts. Levee strata in the Neoproterozoic Isaac Formation, however, show no visible or measurable dip based on lateral bed-by-bed correlations over 100's of meters with respect to a local paleohorizontal datum. In fact, stratal dip is scarcely recorded in the rock record and levee strata typically crop out parallel to regional bedding (e.g. published examples from Karoo Basin, Cerro Torro and Rosario formations).

The objective here is to present an alternative model for the creation of levee topography based on field observations. In the Isaac Formation, medium-bedded strata in the proximal levee thin rapidly over 100's of meters, forming bed surfaces with subtle topography. Similar observations have been reported from other ancient levee systems and are presumed to be responsible for levee topography. In the Isaac Formation, however, this topography becomes infilled and therefore is likely not the major source of levee topography. Instead, thin-bedded turbidites create the topography. These beds have a tabular geometry and terminate abruptly instead of tapering laterally. The upward, progressively more channelward (backstepping) stacking of these thin beds forms a concave-up, lateral-thinning profile. This stacking pattern is a consequence of progressively diminished flow magnitude causing more limited lateral bed extent, which in turn reflects increased channel confinement related to levee growth and reduced overspill into overbank areas.