Cross-bedding forms primarily through the downstream migration of bedforms such as ripples or dunes in a flowing fluid.
Understanding Cross-Bedding Formation
Cross-bedding, also known as cross-stratification, is a common sedimentary structure characterized by layers deposited at an angle to the main bedding plane. This angled layering is a direct result of how sediment, typically sand or silt, is transported and deposited by currents in environments like rivers, deserts, or shallow seas. The underlying mechanism involves the continuous movement of sediment features like ripples and dunes across the bed surface.
The Process of Cross-Bedding Formation
The formation of cross-bedding is a dynamic process driven by the interaction between a flowing fluid (like water or air) and loose sediment.
Here's how it works, based on the migration of bedforms:
- Initiation of Bedforms: Under sufficient fluid flow, sediment on the bed surface begins to move and accumulate into wave-like shapes called ripples (smaller) or dunes (larger). These bedforms have an upstream side (stoss side) and a downstream side (lee side).
- Sediment Transport: As the fluid flows over the bedform, it carries sediment grains. The fluid flow causes sand grains to saltate – they are lifted, carried short distances, and then dropped.
- Movement Up the Stoss Side: The saltating sand grains are transported up the gently sloping stoss (upstream) side of the ripple or dune by the force of the fluid.
- Collection at the Peak: The transported sediment grains collect and pile up at the crest or peak of the bedform.
- Reaching the Angle of Repose: Sediment continues to accumulate at the peak until the slope of the downstream face becomes too steep and reaches the angle of repose. This is the maximum angle at which unconsolidated material is stable.
- Downstream Avalanching (Migration): Once the angle of repose is exceeded, the sediment on the downstream (lee) side becomes unstable and slides or avalanches down the slope in thin layers. This slumping causes the peak of the bedform to effectively move forward in the downstream direction.
This continuous cycle of sediment transport up the stoss side, accumulation at the peak, and avalanching down the downstream side causes the entire bedform to migrate downstream. The angled layers deposited on the downstream side during this migration are the cross-beds. As the bedform moves, it leaves behind these inclined layers, creating the characteristic cross-bedded structure we see in ancient and modern sediments. Over time, if conditions change (e.g., flow stops or reverses), the top of the migrating bedform might be eroded, truncating the angled layers and creating the distinct boundaries between sets of cross-beds.
