Sedimentation in deep-sea lobe-elements: Implications for the origin of thickening-upward sequences

The frequency and origin of thickening-upward packages in the sediments of deep-sea environments has been a topic of much recent debate. Excellent bed-scale exposures in the Carboniferous Ross Formation, western Ireland, allow single surfaces to be traced laterally and the detailed architecture of whole packages to be evaluated using multiple vertical logs. The deposits comprise architectural elements including bed-sets, lobe-elements and composite lobes, with the lobe-elements being arranged in thickening-upward depositional packages. Results show that these packages are the result of the progradation of each lobe-element, whereby each package records a vertical trend from distal to proximal deposits accompanied by an increasing frequency of megaflutes and ultimately the development of broad erosional surfaces. We propose a six-stage model for lobe-element evolution that documents successive phases of deposition, sediment bypass, erosion and lobe abandonment. This new model provides a mechanism for lobe-element switching and explains the development of thickening-upward successions. This allows the re-examination of existing process models for these sedimentary packages in deep-sea sediments.