Massive Bedding

Introduction

Massive bedding is, in a sense, a structure defined by the absence of structure. It describes beds that look perfectly uniform - no laminae, no size gradation, no visible internal differentiation. Because such beds resist easy interpretation, they are both rare and genuinely puzzling when encountered. Understanding massive bedding requires understanding the conditions under which internal organization can either fail to form in the first place, or be erased shortly after deposition.

Definition and Recognition

The term massive bedding describes beds that appear to be homogeneous and lacking in internal structures. ^[1] The appearance of homogeneity is not always real: X-radiography techniques or etching and staining methods often reveal that apparently massive beds are not truly structureless but rather contain very faintly developed structures that are invisible to the naked eye.

Nonetheless, some beds - particularly thick sandstone beds - genuinely have no internal structures recognizable even with X-ray or staining techniques. Such truly structureless beds are rare, which is significant, because they are very difficult to explain. Reported occurrences include both graded turbidite units that may lack internal structures other than size grading, and certain thick, nongraded sandstones. ^[1]

The rarity of truly massive beds matters in practice. When a geologist encounters a bed that looks structureless, the first instinct should be to check whether better investigation - X-radiography, staining, polished slabs - reveals hidden structures. Only after those tools are exhausted does the problem of genuine massiveness arise.

Origins

Two broad explanations have been proposed for massive bedding, one involving the destruction of existing structure and one involving the failure of structure to form at all.

Liquefaction of sediment owing to sudden shock or other mechanisms shortly after deposition has been proposed as a means of destroying original stratification. On this view, the sediment had structure when it was deposited, but a disturbance - a seismic shock, for example - caused the pore water pressure to spike, suspending the grains momentarily and letting them resettle without preserving the original arrangement. ^[1]

Otherwise, it is assumed that lack of stratification is a primary feature - that no structure formed in the first place. This would occur in the absence of traction transport, resulting from very rapid deposition from suspension or deposition from very highly concentrated sediment dispersions during sediment-gravity flows. The commonly invoked image is of sediment dumped very rapidly without subsequent reworking, forming a more or less homogeneous mass. ^[1]

A third possibility has been proposed: massive turbidite deposits could also form by gradual aggradation of sand beneath sustained steady or quasi-steady, high-density turbidity currents. ^[1] On this model, sand is added grain by grain in a continuous sheet beneath a powerful sustained current, with no opportunity for lamination to develop because deposition is never interrupted. This is a fundamentally different mechanism from the “dumping” model - it is slow and gradual rather than instantaneous, yet still produces a structureless deposit.

All three mechanisms remain viable explanations. The difficulty of studying truly massive beds - they are rare, and once preserved they offer few clues about their origin - means that the problem of massive bedding remains incompletely solved.

References

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

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