Cross-bedding (or cross-stratification) is a primary sedimentary feature characterized by layers that intersect at an angle with each other through planar erosional surfaces that truncate inclined beds and laminae. This structure is the result of the migration of bedforms, such as dunes, ripples, and megaripples, produced by wind or water currents in sand-rich sediment. The scale of these structures is different, but their mechanisms of formation is similar [see in detail: bedforms]. Sand grains pushed by a current accumulate in small mounds or ridges, known as ripples. The current pushes ripples ahead eroding sand from their stoss side and depositing it on the lee side, the steeper side of the bedform leaning in the direction of the current. The accretion of sand on the lee side forms foreset laminae in small ripples up to inclined beds in larger bedforms such as dunes. Foreset laminae are inclined at about 34° (angle of repose of sand) with respect to the horizontal.
Bedforms are continuously produced and destroyed by currents, evolving continuously over time. What remains of them in the geological record are the foreset laminae, witnessing the migration of the bedform over time, and erosional surfaces separating different sets of ripples or dunes. The preservation of foreset beds as inclined beds and laminae in cross-bedded sequences is, hence, an extremely useful paleocurrent indicator, because these laminae are inclined in the same direction of the current that produced them. In the example above, the current flows from left to right. Therefore, in the cross-bedded sequence below…
… the current was also flowing from left to right. All the beds above are inclined in the same orientation. Therefore, we might say that the dominant winds in the area (these are fossil dunes) were blowing from left to right in the time interval witnessed by these layers.
Above: cross-bedding in the Coconino Sandstone (Walnut Canyon, Arizona). Which way was the wind blowing?
Cross-bedding and cross-lamination
Bedforms comes in all shapes and sizes. There is a remarkable difference in size, for example, between sand ripples (a few centimeters) and sand dunes (meters to tens of meters high). Both can be preserved in the geological record but the scale at which the cross-bedding is visible is very different. Migrating sand dunes produce cross-bedded sequences alternating at the meter scale. On the other hand, ripples produce cross-laminations, intersecting at the scale of a few centimeters.
Planar and trough cross-bedding
In nature, both ripples and dunes may show different shapes, depending on various flow parameters of the current generating them (more details here). In general, low velocity currents (or elevated depth) favor the formation of ripples with straight crests. On the other hand, faster currents (or lower water depth) produce various types of irregular ripples with sinuous to strongly curved crests. Cross-beds produced by bedforms with different shape show different geometries. Straight crested-ripples produce planar cross-stratifications, characterized by inclined laminae separated by flat erosional surfaces, which can be tabular or wedge-shaped.
Ripples with curvilinear crests (sinuous, lunate, linguoid etc.) are associated with irregular, curved troughs where foreset laminae accumulate during the migration of ripples. The resulting trough cross-bedding is characterized by curved planar erosional surfaces separating different sets of foreset laminae. In sections parallel to the current (see below), foreset laminae are inclined toward the direction of the current. On the other hand, in sections perpendicular to the current foreset laminae appear concave and concordant with the basal erosional surface.
Wave ripple cross-lamination
The cross-lamination produced by wave ripples is consistently different from the cross-laminae produced by unidirectional currents. Wave ripples produce oppositely dipping foreset laminae associated with ripples with symmetric shape, visible in cross section. There can be situations where laminae are undulated, with symmetric shapes resembling that of the ripples, or they may occur as sets of foreset laminae with opposite dips cross-cutting each other. Some very nice examples can be found here.
- Bedforms: ripples and dunes – Ripples, dunes, antidunes are all bedforms, structures that form in sand when it is moved by water or wind. Bedforms are ubiquitous on our planet. It is very common to see ripples, undulatory structures in sand, under shallow waters close to seashores or along riverbanks. And deserts, they are commonly covered with large sand dunes, in turn sprinkled by smaller… Leggi tutto »Bedforms: ripples and dunes
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