CROSS-BEDDING is a feature that occurs at various scales, and is observed in conglomerates and sandstones. GRADED BEDDING means that the grain size within a bed decreases upwards. Cross bedding is very common in beach deposits, sand dunes, and river deposited sediment. A few examples of Sedimentary Structures. GRADED BEDDING Turbidites:The lower part (a-b-c) of a Bouma sequence: the interval a is constituted by massive bedding; the interval b by parallel, horizontal lamination; the interval c by cross-bedding and convolute lamination.
Some sedimentary structures are created by the water or wind which moves the sediment. Graded bedding results when a sediment-laden current (such as a turbidity current) begins to slow down. Hence, cross-beds may be used as indicators of ancient current directions. Sedimentary Rocks: Depositional Environments and Structures. They may contain numerous sedimentary structures, including cross-bedding, ripples, graded beds, footprints, mudcracks, and raindrop impressions. Ripple marks are undulating surfaces on bedding planes caused by waves or currents while the rocks were being deposited. Mud cracks in Cambrian rocks in southwestern Virginia. These large cross beds probably formed in an offshore sandbar Small cross-beds like these, from Scotland, are often called festoon crossbeds. The landslide flows scour flutes in the underlying sediment, then come to rest as graded beds of sand and silt that fill in the sculpted bottom.
Sedimentary structures include features like bedding, ripple marks, fossil tracks and trails, and mud cracks. They conventionally are subdivided into categories based on mode of genesis. Examples include bedding or stratification, graded bedding, and cross-bedding. Sedimentary structures that are produced shortly after deposition and as a result of compaction and desiccation are called penecontemporaneous sedimentary structures. Which type of sedimentary structure can be used to determine current direction? Hint fossils and mud cracks ripple marks and cross-bedding. Trough cross bedding is produced by the downflow migration of lunate dunes in both subaqueous and subaerial environments. Cross-sectional view of ripple cross-laminated sandstone, showing bi-directional cross laminae indicative of a wave origin. Graded beds, flame structures. Mudcracks can become very large, which may make them difficult to recognize in small outcrops.
Chapter 5, Part 7, Chapter Tutorial, The Sedimentary Archives, Levin 2005: The Earth Through Time, 8th Edition, Wiley
Graded bedding is caused by the differential settling by size and weight of suspended particles in water. Cross-beds are formed in beach, river and sand-dune environments. The photograph shows mud cracks in red siltstone from central Newfoundland. Ripple Marks. Flame structures are formed when a denser sediment, (typically sand) is deposited on top of a less dense sediment (typically mud). Some forms of deposition produce graded beds: for example, turbidity currents produce beds which grade upwards from coarse to fine material. Depositional environment: Places where sediments are deposited. Sedimentary structures: The second key to the identification of sedimentary facies and environments is small scale physical features they display. Graded bedding occurs when sediment loaded currents(turbity currents) experience a relatively quick drop in velocity causing the sediments to deposit. Small scale cross bedding creates ripple marks. Mud cracks are another common sedimentary structure.