Abstract

The crest region of deep-water levees is commonly expressed as a mound-shaped element in seismic images made up of multiple dipping seismic reflectors. These dipping reflectors are generally thought to reflect actual stratal dip within levees. Basal contacts of levee strata in the Neoproterozoic Isaac Formation, however, show no discernible dip over hundreds of meters laterally or tens of meters vertically. Notwithstanding, medium- to thick-bedded (10–100 cm [4–40 in.]) strata in the proximal levee change in thickness laterally away from the channel, over hundreds of meters, causing their upper surfaces to form subtle topography. However, this topography becomes later infilled and, accordingly, the vertical stacking of these beds does not contribute to an upward increase in stratal dip.

An alternative model proposes that levee topography in the crest region of some systems is formed mainly by thin-bedded (<10 cm [<4 in.]) turbidites that have a tabular geometry and terminate abruptly instead of tapering laterally. The upward, progressively more channelward (backstepping) stacking of these thin beds is interpreted to form a lateral-thinning profile. This stacking pattern is a consequence of diminishing flow magnitude, causing more limited lateral bed extent (i.e., flow runout), which in turn reflects increased channel confinement related to levee growth and reduced overspill into overbank areas. This model was then used to generate a synthetic seismic model that forms dipping reflectors similar to those observed in the crest region of some modern deep-water systems. Importantly, the reflectors produced in the model reflect a lateral change in lithofacies and not stratal dip. Moreover, these reflectors crosscut stratigraphy.

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