Defining Cross-Bedding
Cross-bedding, also known as cross-stratification, is a sedimentary structure characterized by internal layers (laminae) that are inclined at an angle to the main horizontal bedding plane. These inclined layers are formed by the migration of ripples, dunes, or larger bedforms as sediment is deposited from a flowing fluid, such as water or wind.
How Cross-Beds Form
The formation process involves sediment particles being transported up the gentle, upstream (stoss) side of a ripple or dune. Upon reaching the crest, these particles cascade down the steeper, downstream (lee) side, accumulating in angled layers. As the entire bedform migrates, these inclined layers are preserved, creating a series of non-parallel internal strata that provide direct evidence of the direction and energy of the ancient current.
A Practical Example of Cross-Bedding
Consider a sandy riverbed where water currents constantly reshape the bottom. Small sand ripples form and migrate downstream. As each ripple moves, sand grains roll up its upstream slope and deposit on its downstream side. If this process continues and the sand eventually hardens into sandstone, the internal angled layers within the rock will represent the preserved 'footprints' of these migrating ripples, showing the direction the river once flowed.
Importance in Geological Interpretation
Cross-bedding is a vital tool for geologists to reconstruct ancient environments (paleoenvironments) and interpret geological history. By analyzing the orientation and angle of the cross-beds, scientists can determine the direction of ancient wind or water currents (paleocurrents), the depth of the water, and the type of depositional setting, such as a river, desert, or marine shelf. This information is crucial for understanding sediment provenance, basin analysis, and locating economic resources like petroleum reservoirs.