Architecture and recognition criteria of ancient shelf ridges; an example from Campanian Almond Formation in Hanna Basin, USA

Abstract

Shelf ridges are sedimentary bodies formed on the continental shelf due to transgressive reworking (tidal or storm) of lowstand deposits. Common on modern shelves, they are underrepresented in the geological record due to a lack of recognition criteria and facies model. This article This article is protected by copyright. All rights reserved. proposes a new facies and architectural model for shelf ridges, linked to their inception-evolutionabandonment cycle and the process regime of the basin. The model is mainly based on new outcrop data and interpretations from three sandstone bodies of the Almond Formation, an overall transgressive interval during the infill of the Campanian Western Interior Seaway. Building from the case study, and ancient and modern examples, six characteristics are proposed for the recognition of ancient shelf ridges. Shelf ridges: (i) are encased between thick marine mudstone intervals; (ii) have a basal unconformity that erodes into marine muds or into the remnants of a previous shoreline; (iii) have a non-erosional upper boundary that transitions into marine muds; (iv) are characterized by clean and well-sorted sandstones often cross-bedded; (v) contain fully marine ichnofauna; and (vi) present compound architectures with large accretion surfaces and lower order structures. Although shelf ridges have been described in previous studies as generated exclusively by either tidal or storm currents, it is clear, from modern examples and the case study, that these two processes can be recorded and preserved in a single shelf ridge. The stratigraphy of these sandstone bodies is therefore much more complex than previously recognized, bearing the signature of changing tidal and storm intensity through time. Because they are developed during transgressions, shelf ridges are commonly subject to strong changes in process regime as sea-level changes can easily affect the oceanographic conditions and the morphology of the basin. For this reason, shelf ridges can provide the best record of shelf process variability during transgressions.

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

“…Offshore bars typically develop from storm-related and tidal reworking of relict shoreline and deltaic deposits in transgressed coastal areas (including isolated lowstand systems; e.g. Plint, 1988;Leva L opez et al, 2016) or down-flank of major river distributaries (Olariu et al, 2012), and they commonly exhibit other sedimentary characteristics than those documented in the present study (for example, various clinoform geometries, sharp erosive bases and cross-bedded sandstone beds; Leva L opez et al, 2016). Considering the predominance of storm-wave-generated structures, the systematic parasequence stacking and their cumulative thickness (>100 m; Figs 7 and 8), and the abundance of sandstone in all of the facies associations (Fig.…”

Changes in hydrodynamic process dominance (wave, tide or river) in foreland sequences: The subalpine Miocene Molasse revisited (France)

“…Since the 1990s, modern mixed‐energy coasts have been investigated and processes are better constrained (Short, ; Masselink & Short, ; Li et al ., ; Yang & Chun, ; Anthony & Orford, ; Yang et al ., ; Masselink et al ., ; Vaucher et al ., ,b). Ancient mixed‐energy coastal environments are being increasingly documented in the literature (Vakarelov et al ., ; Ahokas et al ., ; Leva López et al ., ; Rossi & Steel, ; Vaucher et al ., ; Peng et al ., ; Zhang et al ., ).…”

Hyperpycnal delivery of sand to the continental shelf: Insights from the Jurassic Lajas Formation, Neuquén Basin, Argentina

“…Overall, the lack of tidal mud drapes, the low abundance of cross‐stratification, and the abundant parallel laminations deposited within a regressive sequence suggests that these sandstones are distinct from previously described shelf sandstone ridges (Fig. ; Leva López et al ., ).…”

“…Even considering the possibility that the sandstones in this study are features from a minor transgression within an overall regression, the existing sand ridge models cannot explain the extensive thick parallel laminations characteristic of the Lajas sandstones (Figs and ). Tidal currents play a strong role in shaping and reworking sediment in tidal sand ridges, and the extensive compound cross‐bedding typically associated with these processes (Dalrymple, ; Olariu et al ., ; Leva López et al ., ) is not observed in the study area. Furthermore, sand ridge models proposed by Snedden et al .…”

“…Previous work on shelf sand bodies has primarily focused on reworked transgressive features or relict lowstand deposits (e.g. Houbolt, 1968;Bergman & Snedden, 1999, and references therein;Leva L opez et al, 2016). However, relatively few studies document shelf sand bodies deposited by hyperpycnal currents in the rock record or on modern shelves (Mutti et al, 2003;Lamb et al, 2008;Warrick et al, 2013;Steel et al, 2016).…”