Sedimentology: Tempestite facies variability and storm‐depositional processes across a wide ramp: Towards a polygenetic model for hummocky cross‐stratification

Congratulations to Mads E. Jelby, Sten‐Andreas Grundvåg, William Helland‐Hansen, Snorre Olaussen and Lars Stemmerik, for having one of the top downloaded papers in recent publication history of the journal Sedimentology!

The paper “Tempestite facies variability and storm‐depositional processes across a wide ramp: Towards a polygenetic model for hummocky cross‐stratification” was among the 10% most downloaded between January 2018 and December 2019. It was first published online in October 2019.

Abstract: 

The hydrodynamic mechanisms responsible for the genesis and facies variability of shallow‐marine sandstone storm deposits (tempestites) have been intensely debated, with particular focus on hummocky cross‐stratification. Despite being ubiquitously utilized as diagnostic elements of high‐energy storm events, the full formative process spectrum of tempestites and hummocky cross‐stratification is still to be determined. In this study, detailed sedimentological investigations of more than 950 discrete tempestites within the Lower Cretaceous Rurikfjellet Formation on Spitsbergen, Svalbard, shed new light on the formation and environmental significance of hummocky cross‐stratification, and provide a reference for evaluation of tempestite facies models. Three generic types of tempestites are recognized, representing deposition from: (i) relatively steady and (ii) highly unsteady storm‐wave‐generated oscillatory flows or oscillatory‐dominated combined‐flows; and (iii) various storm‐wave‐modified hyperpycnal flows (including waxing–waning flows) generated directly from plunging rivers. A low‐gradient ramp physiography enhanced both distally progressive deceleration of the hyperpycnal flows and the spatial extent and relative magnitude of wave‐added turbulence. Sandstone beds display a wide range of simple and complex configurations of hummocky cross‐stratification. Features include ripple cross‐lamination and ‘compound’ stratification, soft‐sediment deformation structures, local shifts to quasi‐planar lamination, double draping, metre‐scale channelized bed architectures, gravel‐rich intervals, inverse‐to‐normal grading, and vertical alternation of sedimentary structures. A polygenetic model is presented to account for the various configurations of hummocky cross‐stratification that may commonly be produced during storms by wave oscillations, hyperpycnal flows and downwelling flows. Inherent storm‐wave unsteadiness probably facilitates the generation of a wide range of hummocky cross‐stratification configurations due to: (i) changes in near‐bed oscillatory shear stresses related to passing wave groups or tidal water‐level variations; (ii) multidirectional combined‐flows related to polymodal and time‐varying orientations of wave oscillations; and (iii) syndepositional liquefaction related to cyclic wave stress. Previous proximal–distal tempestite facies models may only be applicable to relatively high‐gradient shelves, and new models are necessary for low‐gradient settings.

Reference:

Jelby, M.E., Grundvåg, S.-A., Helland-Hansen, W., Olaussen, S. & Stemmerik, L. (2020) Tempestite facies variability and storm-depositional processes across a wide ramp: Towards a polygenetic model for hummocky cross-stratification. Sedimentology (2020) 67, 742–781. DOI: 10.1111/sed.12671  [intranet]