Sedimentary structures and bedding style (part II)

Graded bedding. Graded bedding results from a rapid decrease in flow velocity that causes sediment to drop out of suspension. Larger particles settle fastest, therefore they accumulate at the bottom of the bed. Houcheng Formation, Jurassic, Hebei Province, China.

Graded beds, flame structures. Rapid sediment fallout from suspension often loads underlying fine-grained sediment to the point of failure, causing foundering of the overlying sediment and formation of structures termed flames (for obvious reasons!). Permian, Inyo County, California.

Flame Structure. The flame structures underlying this thick sandstone bed are evidence for rapid sand depostion; these sandstone beds are in fact sublacustrine turbidites. Cretaceous, Hebei Province, China.

Imbrication. Disk-shaped gravel clasts that are transported by rolling often "shingle" themselves to form a fabric termed imbrication. Individual clasts dip upflow. Current in this photo is from left to right (click to enlarge!). Owens Valley, California.

Graded and imbricated bed. OK, now I'm just getting redundant, but here you are anyways. Trail Canyon alluvial fan, Death Valley, California.

Flute casts. Flutes are caused by erosional eddies at the base of a turbulent flow. The deepest scour is on the upstream end of the flute, and the scours widen and become shallower downflow. Shiguai Formation, Inner Mongolia, China.

Gutter casts. Gutter casts are formed by entrained particles dragging along the base of a bed and leaving a track in their wake. These gutters were formed by wood fragments draggeing along the base of a river bed.Chengde Formation, Jurassic, Hebei province, China.

Dessication cracks. Mudcracks can become very large, which may make them difficult to recognize in small outcrops. Person for scale. Jixian Formation, Proterozoic, China.

Dessication cracks. A modern example. These are up to 1 m deep, and form polygonal networks that are often difficult to see except from the air! Panamint Valley, California.

Linguloid Ripples. These mud-draped ripples are linguloid in form, indicating shallow, rapid flow within the ripple stability field.Permian, Inner Mongolia, China.

Debris flow deposit. This type of non-erosive, unstructured and unsorted bed is typical of cohesive debris flow deposits. Coffin Canyon Fan, Death Valley, California.

Debris flow deposit. Oversized clasts "floating" in a mud-rich matrix are also typical of such flows. Coffin Canyon Fan, Death Valley, California.

Slump Deposit. Chaotic deposits such as these indicate slope failure. Cretaceous, Hebei province, China.

Sedimentary structures and bedding style (part I)

another picture of sedimentary structure and bedding style:

Wedge bedding. Successive stack of erosive-based channels creates wedge-shaped bedding cross-sections. Shiguai Formation, Inner Mongolia, China.

Channelized bedding. Erosive scours that backfill with sand may give rise to isolated channelized sandbodies such as this. Mesa Verde Group, Utah.

Tabular cross-bedding. These steep foresets are typical of eolian deposition. Late Jurassic, Liaoning Province, China.

Trough cross-bedding. Trough cross bedding is produced by the downflow migration of lunate dunes in both subaqueous and subaerial environments. Triassic, Hebei Province, China.

Trough cross-bedding. This unusual view is of mud-draped gravel foresets cut by a joint that is oriented parallel to bedding. We are looking down at a cut-away view of an originally horizontal surface. The arcuate surfaces shown were mud drapes on the downstream sides of gravel lunate dunes. Hammer handle points downflow. Note also the wood fragment oriented parallel to flow (just to the left of the hammer handle), and a second set of mud-draped forsets that is cut off by the bottom edge of the photograph.

Wave Ripples. Cross-sectional view of ripple cross-laminated sandstone, showing bi-directional cross laminae indicative of a wave origin. Entrada Formation, Jurassic, San Rafael Swell, Utah.

Ripple Marks. A pile of eroded, rippled beds that all contain gorgeous ripple marks. If you click on nothing else, click to enlarge this one!Carmel Formation, Utah.

Herrigbone cross-stratification. Bi-directional cross beds such as these are indicative of a tidal origin. Curtis Formation, Jurassic, Utah.

Gilbert Delta foresets. These are composite, large-scale foresets that indicate depostion into still water. The height of the foresets indicates the still water depth. Mecca Formation, California.

Climbing current ripples. Ripple foresets that "climb" on the backs of their predecessors are indicative of waning flow conditions and rapid sediment fallout, such that sediment drops out of suspension as fast as it can be molded into a bedform. Entrada Formation, Jurassic, Utah.

Climbing current ripples and convolute lamination. The result of rapid sediment fallout is often instability due to liquefaction, leading to disruption of laminae by water escape. Such disruption is termed convolute lamination. Modern Colorado River, Utah.

Sedimentary Structures

some kinds of sedimentary structures:

1. Deformational Structures

- Load cast: are rounded or bulbous protrusions formed during compaction. Where these grooves and depressions become filled with sediment, especially if the surface is mud, the marks may be preserved.

- Convolute Bedding: appears as highly contorted, folded and disrupted layers.

- flame structure: deformed clay or silt laminations with curved, pointed ends that project into the overlying bed and resemble a flame blown in the wind.

2. Current Flow Structures

- Parallel Lamination : (where the upper and lower contacts are approximately parallel) can result from

• settling from suspension
• deposition in a low energy environment
• laminar flow (water flowing in a planar way)

- Ripple Marks : formed when the current flow energy increases.

- Cross Bedding : continuous erosion of the stoss side of the ripples and deposition on the leeward side causes

- Graded Bedding: As sediment is carried from a high energy environment into a low-energy environment, the heaviest particles will be the first to settle out, followed by progressively lighter and lighter particles.