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

Abstract

The dominance of isotropic hummocky cross-stratification, recording deposition solely by oscillatory flows, in many ancient storm-dominated shorefaceshelf successions is enigmatic. Based on conventional sedimentological investigations, this study shows that storm deposits in three different and stratigraphically separated siliciclastic sediment wedges within the Lower Cretaceous succession in Svalbard record various depositional processes and principally contrasting sequence stratigraphic architectures. The lower wedge is characterized by low, but comparatively steeper, depositional dips than the middle and upper wedges, and records a change from storm-dominated offshore transitionlower shoreface to storm-dominated prodeltadistal delta front deposits. The occurrence of anisotropic hummocky crossstratification sandstone beds, scour-and-fill features of possible hyperpycnalflow origin, and wave-modified turbidites within this part of the wedge suggests that the proximity to a fluvio-deltaic system influenced the observed storm-bed variability. The mudstone-dominated part of the lower wedge records offshore shelf deposition below storm-wave base. In the middle wedge, scours, gutter casts and anisotropic hummocky cross-stratified storm beds occur in inferred distal settings in association with bathymetric steps situated across the platform break of retrogradationally stacked parasequences. These steps gave rise to localized, steeper-gradient depositional dips which promoted the generation of basinward-directed flows that occasionally scoured into the underlying seafloor. Storm-wave and tidal current interaction promoted the development and migration of large-scale, compound bedforms and smaller-scale hummocky bedforms preserved as anisotropic hummocky cross-stratification. The upper wedge consists of thick, seaward-stepping successions of isotropic hummocky cross-stratificationbearing sandstone beds attributed to progradation across a shallow, gently dipping ramp-type shelf. The associated distal facies are characterized by abundant lenticular, wave ripple cross-laminated sandstone, suggesting that the basin floor was predominantly positioned above, but near, storm-wave base. Consequently, shelf morphology and physiography, and the nature of the feeder system (for example, proximity to deltaic systems) are inferred to 196

Supplemental Material: Magnetostratigraphy of U-Pb–dated boreholes in Svalbard, Norway, implies that magnetochron M0r (a proposed Barremian-Aptian boundary marker) begins at 121.2 ± 0.4 Ma

“…Detailed facies analysis in Engen (2018) concluded that the depositional environments of DH1 changed upward into an interdistributary bay facies in the uppermost part (Engen, 2018;and shown in Grundvåg et al, 2019, their fig. 11, based on data from the DH cores); thereby representing deposition under an overall increase in relative sea level and supporting the interpretations of trends described by other investigations (Nemec, 1992;Midtkandal and Nystuen, 2009;Grundvåg et al, , 2020. The coastal plain during the deposition of the Helvetiafjellet Formation was subject to several relative sea level oscillations and/or shifting depocenters of sediment inputs (Midtkandal et al, 2007).…”

“…The Lower Cretaceous basin fill in Svalbard accumulated as a response to long-term regional subsidence interrupted by crustal updoming with resulting uplift in the north caused by the opening of the Amerasian Basin and the onset of the High Arctic Large Igneous Province (HALIP) to the northeast (e.g., Gjelberg and Steel, 1995;Maher, 2001, Midtkandal andNystuen, 2009;Grundvåg et al, 2019Grundvåg et al, , 2020. The Festningen Member of the lowermost Helvetiafjellet Formation is dominated by coarse-grained quartzitic cross-bedded sandstones that are interpreted as braidplain deposits (Midtkandal and Nystuen, 2009;Grundvåg et al, 2019).…”

“…The lower ca. 10 m of the Carolinefjellet Formation is an organic-rich shale as a regional flooding transformed the Helvetiafjellet Formation coastal plain into a shallow-shelf depositional setting (Grundvåg et al, 2019(Grundvåg et al, , 2020.…”

Sedimentary deposits and bioturbation in an Early Cretaceous subarctic stormy greenhouse shelf environment

“…The Carolinefjellet Formation has been divided into ve subunits, referred to in their ascending stratigraphic order as the Dalkjegla, Innkjegla, Langstakken, Zillerberget and Schønrockfjellet members (Fig. 2; Parker 1967; Nagy 1970; Dallmann 1999; Grundvåg et al 2021). These units have conformable boundaries and differ chie y in their relative proportion of mudshales and sandstones, but also in some other distinctive features.…”

“…The Aptian lower part of the formation (Fig. 1D) is generally regarded as a continued record of the punctuated regional marine transgression initiated by deposition of the underlying Glitrefjellet Member of the Helvetiafjellet Formation (Nemec et al 1988; Gjelberg and Steel 1995;Dypvik et al 2002), whereas the Albian upper part is considered to represent a punctuated normal regression, mainly aggradational (Dallmann 1999;Dypvik et al 2002; Maher et al 2004;Midtkandal 2007; Grundvåg et al 2019Grundvåg et al , 2021.…”