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

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 stormwave-generated oscillatory flows or oscillatory-dominated combined-flows; and (iii) various storm-wave-modified hyperpycnal flows (including waxingwaning 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, metrescale 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 742 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.

The role of shelf morphology on storm‐bed variability and stratigraphic architecture, Lower Cretaceous, Svalbard

“…Sandstone storm deposits form an important part of many ancient shelf-shoreface successions and have received a considerable amount of attention in the literature for the last few decades (Dott & Bourgeois, 1982;Duke, 1985;Duke et al, 1991;Cheel & Leckie, 1993;Myrow & Southard, 1996;Dumas & Arnott, 2006;Quin, 2011;Jelby et al, 2020). This is mainly due to the ongoing discussion on how sand is transported across shelves during storms (Swift et al, 1987;Leckie & Krystinik, 1989;Lamb et al, 2008;Basilici et al, 2012a;Collins et al, 2017) and the origin of hummocky cross-stratification (HCS), which commonly occurs in storm deposits (Quin, 2011;Morsillli & Pomar, 2012;Jelby et al, 2020). Because many ancient shelf successions are dominated by thick-bedded isotropic HCS sandstone beds, characterized by no preferred lamina dip-orientation ( Fig.…”

“…It has therefore been debated whether or not pure oscillatory currents are capable of transporting sand onto and across shelves. Due to the wide range of storm-bed architectures reported from the stratigraphic record (Jelby et al, 2020), cross-shelf transport by geostrophic currents (Leckie & Krystinik, 1989;Duke, 1990;Midtgaard, 1996), combinedflows (Nøttvedt & Kreisa, 1987;Dumas et al, 2005;Quin, 2011) and storm surges (Mount, 1982), as well as various density-driven and wave-enhanced gravity flows (Myrow et al, 2002;Lamb et al, 2008), have all gained support. Although a combination of these processes most likely governs deposition during most storms, the dominance of isotropic HCS sandstones reported in many ancient shelf-shoreface successions remains enigmatic (Brenchley et al, 1993).…”

“…Although these models are valid for distally deepening shelves with moderate to steep gradients, they do not take into account the gently sloping nature and shallow water depths of many ancient ramptype shelves typical of epeiric seas where the role of offshore-directed, gravity-driven flows is reduced (Jelby et al, 2020). Instead, epeiric seas are characterized by frequent storm-wave reworking of deposited sediment, even in shelf settings located several hundred kilometres from the shore, as evidenced by the presence of laterally extensive sandstone sheets dominated by isotropic HCS in some ancient examples (Brenchley et al, 1986(Brenchley et al, , 1993Runkel et al, 2007;Jelby et al, 2020). Thus, shelf morphology and physiography appear to impose a strong control on storm-bed variability, which is inherently related to the stratigraphic architecture of the resulting storm-dominated shoreface-shelf succession.…”

“…Because of their extensive regional distribution and limited lateral facies variations, the wedges have been attributed to deposition on a low-gradient, ramp-type shelf (Fig. 4;Nagy, 1970;Dypvik et al, 1991a;Midtkandal & Nystuen, 2009;Jelby et al, 2020).…”

Dinocyst stratigraphy of the Valanginian–Aptian Rurikfjellet and Helvetiafjellet formations on Spitsbergen, Arctic Norway

“…S1). The sedimentological profile of the outcrop is provided in Grundvåg et al (2019) and Jelby et al (2020).…”