Deposition in different environments, the formation of sedimentary structures (bedding, cross-bedding, graded bedding, ripple marks) and how these are used as way-up indicators and palaeoenvironment evidence.

What this dot point is asking

WJEC wants you to link depositional environment to the sedimentary structures it produces, and to use those structures both to tell which way up a deformed sequence originally was (way-up or younging) and to reconstruct the ancient environment. These are core Component 1 and Component 2 skills: you read structures from photographs and from the rock record and infer the conditions of deposition.

The answer

Deposition and environment

Sediment is deposited when the transport agent loses energy and can no longer carry its load. The depositional environment (river, delta, beach, shallow sea, deep sea, desert, glacier) controls the grain size, sorting, composition and structures of the resulting rock. Because each environment leaves a characteristic signature, the rock record lets us reconstruct ancient surface conditions, an application central to the themes studied later.

The main sedimentary structures

Bedding (stratification) is the layering produced by successive episodes of deposition, the most fundamental sedimentary structure, recording changes in supply or conditions over time.

Cross-bedding is inclined layering within a bed, formed as sediment avalanches down the lee face of a migrating ripple or dune. It records current or wind direction (the beds dip downstream) and is a way-up indicator: the cross-beds are truncated sharply at the top and curve to meet the lower boundary tangentially at the base.

Graded bedding is a bed that fines upwards from coarse at the base to fine at the top, formed as a waning current (often a turbidity current) drops its largest grains first. It is a reliable way-up indicator: fining upwards means the right way up.

Ripple marks form on a bed surface as a current or waves move sediment. Asymmetrical ripples record one-way (current) flow and point downstream; symmetrical ripples record back-and-forth wave action. Both indicate shallow, agitated water.

Desiccation (mud) cracks form when mud dries out and shrinks, giving polygonal cracks that taper downwards. They indicate periodic exposure (a floodplain, tidal flat or drying lake) and act as way-up indicators because the V points to the older rock.

Using structures to reconstruct environments

A geologist combines grain size, sorting, composition and structures to deduce the environment. Well-sorted, well-rounded, cross-bedded sandstone with no fossils suggests a desert dune field; fine, graded beds (turbidites) suggest a deep-sea fan; fossil-rich, symmetrically rippled limestone suggests a warm, shallow shelf sea.

Examples in context

Devonian Old Red Sandstone. Cross-bedded, red, desiccation-cracked sandstones across Wales and the Welsh Borders record ancient rivers and floodplains under an arid climate, an environment read straight from the structures. Turbidite sequences. Repeated graded beds in deep-water successions record turbidity currents sweeping sediment down the continental slope, each bed a single flow event. Tidal flat mudcracks. Polygonal mudcracks preserved in shales record repeated exposure and drying on a tidal flat, fixing both the environment and the way-up of the sequence.

Try this

Q1. State how cross-bedding indicates the direction of the depositing current. [1 mark]

• Cue. The cross-beds dip (incline) in the downstream or downwind direction of flow.

Q2. A bed fines upwards from pebbles at the base to mud at the top. Name the structure and state the way-up. [2 marks]

• Cue. Graded bedding; the fining-up direction is the way-up, so the pebbly base is the older, lower part.

Q3. What environment is suggested by desiccation cracks in a mudstone, and why? [2 marks]

• Cue. A setting subject to periodic drying out, such as a floodplain, tidal flat or drying lake, because the mud must be exposed to air to shrink and crack.