Graded Bedding

Introduction

Graded bedding is one of the most distinctive and informative primary sedimentary structures. It records a systematic change in the energy of the depositing agent - from high to low as a depositional event wanes, most often. Because the pattern of grading is tied to a specific physical process, graded beds are among the most reliable environmental indicators available to sedimentologists.

Definition and Physical Description

Graded beds are sedimentation units characterized by distinct vertical gradations in grain size. They range in thickness from a few centimeters to a few meters or more and commonly have sharp basal contacts. ^[1] Beds that show gradation from coarser particles at the base to finer particles at the top are said to have normal grading. ^[1]

The sharp basal contact is significant: it records the abrupt onset of a high-energy event - a sudden influx of coarse sediment - that then gradually waned as finer material settled out. This vertical pattern from coarse to fine is the fingerprint of a depositional system that started energetically and died down progressively.

Normal Grading

Normal graded bedding can form by several processes. Deposition from suspension clouds generated by storm activity on the shelf, or settling during the final phases of a heavy flood, can produce normally graded beds. However, the origin of most graded beds in the geologic record has been attributed to turbidity currents. ^[1]

Differences in the rate at which particles of different sizes settle from suspension during the waning stages of turbidity current flow appear to account for the grading, though the exact manner in which the grading process operates is not well understood. ^[1] The graded materials may be mud, sand, or, more rarely, gravel.

As a turbidity current decelerates, its competence - the largest grain size it can carry - falls progressively. The heaviest, coarsest particles drop out first, followed by progressively finer material as the current weakens. The result is a bed that fines upward, with the coarsest fraction locked at the base and the finest at the top.

Some graded turbidite units display an ideal complete sequence of sedimentary structures called a Bouma sequence, but more commonly the sequence is truncated at the top or bottom. The basal A division may be present, but some or all of the overlying divisions may be absent, or the A division itself may be missing. Normally graded turbidite beds commonly occur in thin, repetitious successions referred to as rhythmic bedding. ^[1]

Rhythmic bedding - the repeated stacking of graded units one on top of another - tells a story of recurring turbidity current events, each depositing a fresh graded bed on top of the last. This repetition is characteristic of deep-marine slope and basin-floor settings where turbidity currents periodically flush sediment from the shelf.

Reverse Grading

Reverse grading - where grain size increases upward rather than decreasing - can also occur, but it is much less common than normal grading. ^[1] Reverse grading has been attributed to two mechanisms: dispersive pressures and kinetic sieving. ^[1]

Dispersive pressures are believed to be proportional to grain size. In a sediment of mixed grain size, the higher dispersive pressures acting on the larger particles tend to force them upward into the zone of least shear - a region of lower velocity gradient where large grains can travel without being pushed back down. This effectively concentrates coarser grains toward the top of the moving sediment mass.

The kinetic sieve mechanism works differently. In a mixture of grains undergoing agitation, smaller grains can fall down through the spaces that open up between larger grains as those larger grains move. Over time, small grains migrate downward and large grains are left at the top - the inverse of what settling alone would produce. Overall, reverse grading is a relatively rare phenomenon, and its origin is still poorly understood. ^[1]

References

1. Boggs, S. Jr. (2012). Principles of Sedimentology and Stratigraphy, 5th ed. Pearson Prentice Hall.

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