Storm-Dominated Siliciclastic Shelf Deposits: 24-Pet-A1 Principles of Stratigraphy and Sedimentation, May 2013, Q9

Question 9: Storm-Dominated Siliciclastic Shelf Deposits (10 marks)

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A storm-dominated shelf is normally below fair-weather wave base (FWWB), where day-to-day conditions are quiet enough for mud to settle and be bioturbated, but is periodically reworked down to storm wave base (SWB) by episodic, high-energy storm events – producing a sediment record dominated by discrete "tempestite" event beds within an otherwise fine-grained background.

Shelf profile showing fair-weather wave base (FWWB) and the deeper storm wave base (SWB): HCS/SCS tempestite sand beds accumulate in bioturbated mud between the two, while offshore mud below SWB carries only thin, sparse graded storm layers.

Main characteristics

Hummocky and swaley cross-stratification (HCS/SCS) – low-angle, gently undulating cross-sets produced by the combined-flow regime of a storm: oscillatory orbital wave motion superimposed on a unidirectional storm-return (geostrophic) current. HCS/SCS is the single most diagnostic sedimentary structure of this environment.
Graded, sharp-based "tempestite" storm beds – erosively based (often with sole marks), normally graded sand or silt beds representing a single storm event, frequently amalgamated where storms recur before the intervening mud fully accumulates.
Bioturbated mud between storm beds – the fair-weather background sediment, extensively burrowed because low, sustained energy allows benthic colonization between storms.
Coarsening-upward parasequences – progradation shoals the profile over time, so tempestite beds thicken and amalgamate upward as the section shallows toward the shoreface.

Bathymetric implications

Because HCS specifically requires water depth between FWWB and SWB – shallow enough for storm-wave orbital motion to reach the bed, but deep enough that fair-weather waves do not rework it away – its mere presence brackets relative paleo-water depth without any other evidence. Vertically, the character of storm beds records a shoaling or deepening trend directly: thin, sparse, fine-grained storm layers within otherwise undisturbed offshore mud indicate deposition near or below SWB (deeper, more distal shelf); thicker, coarser, more frequent and increasingly amalgamated HCS beds indicate progressive shoaling toward FWWB and the shoreface (more proximal shelf) – making storm-bed stacking patterns a practical tool for reconstructing relative bathymetry through a shelf succession.

Topic. Storm-dominated shelf sedimentation and its use as a paleobathymetric indicator.

Key relations. HCS/SCS forms only between FWWB and SWB; bed thickness/frequency increases shoreward (shoaling) and decreases basinward (deepening).

Why this works. HCS results from a specific hydrodynamic regime (combined oscillatory + unidirectional flow) that exists only within a bounded depth window relative to two independently defined wave bases, so its presence is a direct physical constraint on relative water depth rather than an indirect inference.

Common pitfall. Confusing HCS with dune-scale trough cross-bedding (Question 10) – HCS sets are low-angle (typically <10–15°), have variable, non-unidirectional dip and internal erosional truncation surfaces, whereas trough cross-bedding from unidirectional dune migration shows consistent downstream dip.

Source. Reading (ed.), Sedimentary Environments, 3rd ed., ch.9 (shelf/storm processes); Boggs, Principles of Sedimentology and Stratigraphy, 5th ed., ch.9.

Question 9 of 15: Storm-Dominated Siliciclastic Shelf Deposits

Question 9: Storm-Dominated Siliciclastic Shelf Deposits (10 marks)

Main characteristics

Bathymetric implications