The shelf edge: characterization and significance of the shelf-slope break threshold for the infilling of the mid-Jurassic Neuquén Basin, Argentina

Abstract

Sedimentary wedges that build out from the margin of deep-water basins often exhibit large-scale (hundreds of meters), topset-foreset clinoform geometries. Deepwater basins therefore have a shelf-margin morphology of flat to gently sloping shelf, together with a steeper deepwater slope (1-4 degrees) and a gently sloping to flat basin floor. The two inflection points along the margin morphology are the shelf-slope break (or shelf edge), and the toe of slope break. The significance of such clinoformal patterns in stratigraphy is that they demonstrate the time equivalence of the sandy shallow-water shelf deposits with the respective muddy deepwater slope deposits, and allow the position of the shelf edge or the shelf-slope break and the toe of slope break to be pinpointed in the stratigraphy. This typical shelf-slope-basin floor clinoform geometry is easily recognizable in seismic data sets, but difficult to determine (when not supported by seismic) in outcrops because shelf-margin clinothemsin exhumed basin are generally destroyed due to erosion and deformation. Only few places clinoforms can be recognized in outcrops, such as, for example, the Eocene Central Basin of Spitsbergen (Norway), where the exposures are large and continuous with sandstones punctuating the hundreds of meters muddy slope clinoforms. Although seismic data allow shelf-slope-basin floor deposits to be constrained within chronostratigraphic intervals, they do not provide the bed-scale resolution inherent in the refinement of geological models. Outcrop analogue studies provide the finer-scale resolution needed, but these are not without problems too because co-genetic shelf-slope-basin floor deposits cannot often be time-correlated in outcrops. It implies that little is known about the character of the linkage between the shallow-water shelf and the associated deep-water depositional systems, since this transition has rarely been seen in outcrop where the downslope facies changes can be described and quantified. The mid-Jurassic deposits of Challaco-Las Lajas-Los Molles formations of southern Neuquén Basin, in La Jardinera area, fill the basin by means of building shelf-slope-basin floor clinoforms. Exceptional outcrops expose the hundreds of meters thick clinoforms, and thus, enabled a time- correlation between the shallowwater shelf and the deepwater depositional systems, and the character of this transition to be described and quantified. This study provided a three-dimensional, bed-scale and architectural model for the shelf edge rollover deposits that can be used as an analogue for subsurface predictability of deepwater reservoir presence and de-risking of prospects in similar settings. This conceptual paleographic model bring important contributions to improve our understanding of how sediment moves from shelf to deepwater areas of shelf margins.