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Fundamentally Different Proximal and Distal Lobe Stacking Styles Within the Same Stratigraphic Interval: Upper Broto System, Jaca Basin, Spain

Abstract

Compensational, or longitudinal and aggradational stacking patterns are recognized in deep-water lobe systems, and are typically ascribed to unconfined and confined systems, respectively. In contrast, other systems comprise ‘isolated’ sheet-like beds that can be traced for 10's to 100's km, which do not stack to form lobes. Proximal, medial and distal localities of the Upper Broto System, Jaca Basin, Spain, are described over a 70km depositional dip transect. Previous work describes sheet-like stacking; however new data reveals that tracing of beds is problematic in both proximal and distal localities. Proximal localities preserve stacking patterns that reveal compensational geometries and facies changes from thick- to thin-bedded turbidites over tens to hundreds of meters in both dip and strike oriented sections. Meter-scale scours and coarse-grained bed-top-lags indicate bypass of large volumes of sediment to distal parts of the basin. Correlation between distal localities is also challenging, especially at a bed scale, due to the disparate nature of the outcrops and facies variability. However, anomalously thick (meter-scale) sheet-like beds are traceable for several kilometres along depositional dip, but to a lesser extent along strike. Distal localities show an abrupt increase in the proportion of hybrid event beds (HEBs), up to 53%, compared to 0% and 1.9% at proximal and medial localities respectively. Similarly, bed tops in distal locations are often associated with an upper division that is carbonate mud-rich, which is absent in proximal locations. Sole structures indicate a primary paleoflow to the northwest; however ripple crests suggest a secondary flow component to the north. The abrupt appearance of HEBs, carbonate divisions and divergent paleoflow indicators are suggestive of distal HEB generation through flow deflection off the coeval southern carbonate-rich ramp. Larger volume flows deposited more sheet-like beds, whereas smaller flows deposited bed-scale compensational-like geometries. The overlap of these stacking patterns creates the complicated geometries observed. Therefore, a range of stacking patterns and bed architectures can exist within the same stratigraphic interval, and flow deflection off confining slopes can complicate the predicted architecture of submarine fans. HEBs generated from confining slopes can act as baffles between net-reservoir sands, dramatically altering reservoir performance compared to proximal locations.