Abstract

Submarine lobe and related deposits are amongst the largest discrete sandbodies on Earth, and can be significant hydrocarbon reservoirs. In outcrop and core-based studies, tools such as analysis of bed-thickness and grain-size distributions have been used to improve the understanding of the composition and architecture of such sandbodies. Analysis of sediment-gravity-flow (SGF) processes have also proved to be a useful tool in understanding the evolution of submarine lobes. In this paper, based on outcrop studies of submarine lobe and related deposits in the middle Eocene Jaca Basin, Spanish Pyrenees, a revised interpretation of the depositional environments of the lobe and related deposits and a new model for their architectural evolution is presented. This model is based on an analysis of bed-thickness, grain-size distribution, and a qualitative and quantitative study of the distribution of supercritical-flow deposits (SFDs) in these environments. The interpretation of lobe and related environments is mainly based on sandstone content and the distribution of sedimentary facies. The main supercritical-flow sedimentary structures recognized in the Jaca Basin, are unstable and stable antidunes, upper plane beds and backset-laminated beds.

This study demonstrates that seafloor topography, strongly controlled by both syndepositional tectonics and the accumulation of mass-transport complexes, likely exerted a significant influence on lobe architecture and the distribution of SFDs. Local increase in bed thickness, together with a progressive decrease in grain size and little variation in the proportion of SFDs in proximal-to-distal and axial-to-lateral directions, can be explained by: i) an increase in basin confinement of the distal part of the Jaca Basin due to tectonically induced narrowing, ii) enhanced local lateral confinement due, at least in part, to “carbonate megaturbidites” present in the distal part of the Jaca Basin and creating topography. Thus, basin confinement is introduced as a new parameter playing a role on flow criticality. There is a decreasing proportion of SFDs between the submarine channels and canyons of the Ainsa Basin and the submarine lobes of the Jaca Basin, the last basin being the focus of this paper. This confirms previous studies showing that channel confinement and slope gradient likely played an important role in flow criticality.