Cross-Bedding
Introduction
Cross-bedding is one of the most abundant and recognisable sedimentary structures in ancient sandstones, and it is almost always a sign that bedforms were migrating at the time of deposition. The internal laminae record not only the existence of those bedforms but also the direction the current was moving - making cross-bedding one of the most useful paleocurrent indicators available to sedimentologists.
How Cross-Bedding Forms
Cross-bedding forms primarily when ripples and dunes migrate through water or air. Migration causes sediment to avalanche or settle from suspension onto the downstream (lee) face of the bedform, building a series of inclined layers called foreset laminae. [1]
The geometry of those foresets depends on how sediment arrives at the base of the lee slope. When bedload sediment is too coarse to travel in suspension, it avalanches straight down the face and the resulting laminae are steep and straight. These nontangential foresets meet the nearly horizontal bottomset laminae at a sharp angle close to the angle of repose. When suspended sediment is abundant, or when the lee slope is short relative to flow depth, fine material accumulates fast enough at the base to keep pace with the avalanche. The lower parts of the foresets then curve outward to meet the bottomset asymptotically - the laminae are tangential. [1]
The tangential/nontangential distinction is not a minor point. It reflects the ratio of bedload to suspended load in the original flow - straight nontangential foresets imply bedload-dominated transport, while curved tangential foresets imply that suspended load was significant. Both types are preserved in ancient rocks.
Why Cross-Bedding Preserves Well
Cross-bedding is preserved far more readily than the bedforms that created it. Ripple and dune crests tend to be planed off by later current or wind erosion before burial, but the internal foreset laminae, which lie deeper within the structure, survive. This means cross-bedding is very common in ancient sedimentary rocks even when the original ripple or dune surfaces are gone. [1]
Cross-stratification can also be produced by processes other than bedform migration - by filling of scour pits and channels, by deposition on the point bars of meandering streams, and by deposition on the inclined surfaces of beaches and marine bars. [1]
Scale and Environmental Complexity
Because of its diverse origins, cross-bedding formed under very different environmental conditions can look almost identical in outcrop. Distinguishing fluvial, eolian, and marine cross-bedding is often difficult in ancient rocks. [1]
Cross-beds occur in sets. Sets less than about 5 cm thick are called small-scale cross-bedding; those thicker than 5 cm are large-scale cross-bedding. [1]
The Two Principal Types: Tabular and Trough
Cross-beds are divided into two principal types - tabular and trough - on the basis of their overall geometry and the nature of the bounding surfaces of the cross-bedded units. [1]
Tabular Cross-Bedding
Tabular cross-bedding consists of units that are broad laterally relative to their set thickness and have essentially planar bounding surfaces. The foreset laminae are also commonly planar, though curved tangential laminae occur as well. [1]
Tabular cross-bedding forms mainly from the migration of large-scale, straight-crested ripples and dunes, so it is a lower flow regime structure. Individual beds range in thickness from a few tens of centimetres to a metre or more, but thicknesses up to 10 m have been observed. [1]
Trough Cross-Bedding
Trough cross-bedding consists of units in which one or both bounding surfaces are curved. The sets are trough-shaped - an elongate scour filled with curved foreset laminae that commonly have a tangential relationship to the base. [1]
Trough cross-bedding can originate both from migration of small current ripples - producing small-scale cross-bed sets - or from migration of large-scale, trough-shaped dunes. Where large-scale ripples are the source, the resulting beds commonly reach a few tens of centimetres in thickness and range in width from less than 1 m to more than 4 m. [1]
The contrast between the two types goes beyond appearance. Tabular cross-bedding implies straight-crested bedforms moving under steady lower-regime conditions; trough cross-bedding implies scour by sinuous or lobate bedforms and is the structure most commonly preserved from trough-shaped dune migration. Distinguishing them requires attention to both the bounding surfaces and the foreset geometry.
Related Topics
Bedforms
Bedforms are the repeating surface features that develop on a sandy bed under fluid flow - ripples, dunes, flat surfaces, and the upstream-migrating antidunes that form under the most energetic...
Ripples
Ripples are the smallest bedform produced by fluid flow, and they are among the most widespread sedimentary structures in both modern environments and the ancient rock record. They form in...
Sedimentary Structures
Sedimentary structures are large-scale features of sedimentary rocks - including parallel bedding, cross-bedding, ripples, and mudcracks - that form as a direct result of depositional or...
References & Citations
- 1.Principles of Sedimentology and Stratigraphy Boggs, Sam Jr.
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