Subaqueous Bedforms – Grain Size and Flow Velocity: 24-Pet-A1 Principles of Stratigraphy and Sedimentation, May 2013, Q10

Question 10: Subaqueous Bedforms – Grain Size and Flow Velocity (10 marks)

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For a given grain size, progressively increasing unidirectional flow velocity over a sandy bed produces an ordered succession of bedforms, summarized on a bedform-stability diagram (flow velocity on one axis, grain size on the other). The sequence reflects the changing balance between the boundary shear stress the flow exerts and the bed's resistance to erosion/transport, and each bedform leaves a diagnostic internal sedimentary structure once preserved.

Schematic bedform-stability diagram: with increasing flow velocity, a sandy bed passes through no-movement, current ripples, dunes, plane bed (upper flow regime), and antidunes; coarse sand can also show a separate lower-flow-regime plane-bed field at low velocity, before the ripple field is reached.

Bedform sequence with increasing velocity (lower flow regime → upper flow regime)
Bedform	Flow regime	Preserved structure
No movement	–	Original bed structure preserved (no transport)
Current ripples (fine sand only, <≈0.6 mm)	Lower	Ripple cross-lamination
Dunes (megaripples)	Lower	Trough/planar cross-bedding (the dune's foreset, at the scale of the whole bedform)
Plane bed	Upper (also a separate lower-regime field for coarse sand at LOW velocity)	Horizontal/planar lamination, often with parting lineation
Antidunes	Upper (highest velocity)	Low-angle, upstream-dipping backset lamination; rarely preserved because falling-stage erosion typically destroys it

Grain size shifts the whole diagram: coarse sand cannot form current ripples at all (ripples are restricted to sand finer than ≈0.6 mm) and instead passes directly from no-movement to a lower-flow-regime plane bed, then to dunes, at higher velocities than an equivalent fine sand needs to reach the same bedform – which is why the diagram must be read as a joint function of BOTH variables, not velocity alone.

Topic. The bedform-stability diagram and its control on preserved sedimentary structures.

Key relations. Increasing velocity: no movement → ripples (fine sand only) → dunes → upper-regime plane bed → antidunes; each has a diagnostic internal structure once lithified.

Why this works. Bedform geometry is the bed's dynamic response to the boundary shear stress the flow exerts, which is itself a function of both velocity and grain size (through the grain Reynolds number and Shields criterion); because the internal structure of a bedform is preserved essentially unchanged when it is buried, that structure is a direct, permanent record of the paleo-flow conditions that built it.

Common pitfall. Reading trough cross-bedding as automatically indicating "high energy" – dunes occupy the LOWER flow regime, and a genuinely higher-energy flow instead produces the flatter, non-cross-bedded upper-regime plane bed; cross-bed scale must be compared against grain size, not treated as a standalone energy proxy.

Source. Boggs, Principles of Sedimentology and Stratigraphy, 5th ed., ch.2 (sediment transport and bedforms); Southard & Boguchwal bedform-phase-diagram studies.

Question 10 of 15: Subaqueous Bedforms – Grain Size and Flow Velocity

Question 10: Subaqueous Bedforms – Grain Size and Flow Velocity (10 marks)