Bedding and Lamination

Bedding is the most fundamental structural characteristic of sedimentary rocks. ^[1] A bed is a tabular or lenticular layer of rock with sufficient lithological, textural, or structural distinctiveness to be separated from the strata above and below. The bounding surfaces between beds are called bedding planes or bounding planes. The upper and lower surfaces of each bed mark where depositional conditions changed enough to produce a recognizable difference in the rock.

Beds vs. Laminae: Thickness Classification

Beds are defined as strata thicker than 1 cm; layers thinner than 1 cm are called laminae. ^[1] Within the bed category, descriptive thickness terms distinguish very thin beds (1-3 cm), thin beds (3-10 cm), medium beds (10-30 cm), thick beds (30-100 cm), and very thick beds (>100 cm). Laminae are likewise classified as thin laminae (<3 mm) and thick laminae (3-10 mm).

Importantly, many beds defined by this criterion contain multiple true sedimentation units - layers of sediment deposited under a single constant condition - because a single bed might record several fluctuations that are too subtle to create a sharp visual break. The 1 cm threshold is therefore a practical observational criterion, not a rigorous process-based one.

Internal Subdivisions of a Bed

A single bed may contain several distinct internal features. ^[1]

• Structural subdivisions: distinct laminae or ripple sets within the bed.
• Compositional intraclasts: thin lenses of different composition, texture, cementation, or color - such as a pebble lens or a chert band.
• Amalgamation surfaces: marked discontinuities (commonly erosional) between two beds of similar composition that are treated as one combined unit.

The term layer is used informally for any bed or stratum, but its formal use is inconsistent across the literature.

Bedding Surfaces and Bed Geometry

Bedding planes most commonly represent periods of non-deposition, abrupt compositional change signaling a shift in depositional conditions, or an erosion surface. ^[1] Some bedding surfaces form after deposition by diagenetic processes or weathering. The geometry of the bed as a whole depends on how the top and bottom surfaces relate to each other. ^[1]

Surfaces may be parallel or nonparallel, and each surface may be even, wavy, or curved. These combinations generate a range of bed geometries: uniform-tabular, tabular-lenticular, curved-tabular, wedge-shaped, and irregular. Internal laminae parallel to the bounding surfaces produce planar laminated bedding; laminae deposited at angles to the bounding surfaces are called cross-strata or cross-laminae, and beds containing these are cross-beds.

Bedsets: Groupings of Beds

Beds are sometimes organized into higher-order groupings called bedsets. ^[1]

• Simple bedset: two or more superimposed beds sharing similar composition, texture, and internal structures.
• Composite bedset: a group of beds that differ in composition, texture, or structure but are genetically associated - representing a recurring depositional succession (for example, a fining-upward turbidite cycle).

A bedset is bounded above and below by its own bounding surfaces, making it a mappable unit at the scale above the individual bed.

Lateral Continuity and Bed Termination

Some beds can be traced laterally for many kilometres; others die out within a single outcrop. ^[1] Lateral termination occurs by three mechanisms: (1) pinch-out, where the upper and lower surfaces converge and merge; (2) gradational extinction, where the composition changes laterally until the bounding surfaces disappear; and (3) truncation by a cross-cutting channel, fault, or unconformity.

Time Represented by a Bed

Individual beds form under essentially constant physical, chemical, or biological conditions. ^[1] The time required to deposit a single bed spans an enormous range. A flood may produce a sand bed in hours or days; grain-flow laminae on a dune slip face can form in seconds; fine-clay suspension settling may take months or years to accumulate a single lamina-thick deposit. Bedding planes therefore record pauses in deposition, erosional events, or abrupt environmental changes - not continuous accumulation.

Beds are frequently destroyed before they can be preserved in the geologic record. The preservation potential is greatest for beds deposited by high-magnitude events such as major floods, which rapidly bury and protect earlier deposits. Small-scale, low-energy beds are far more susceptible to destruction by the next erosional episode.

Planar Bedding Types

Planar beds - those bounded by parallel, nearly flat surfaces and lacking cross-laminae - are divided into three subtypes based on internal structure. ^[1]

• Laminated beds: contain internal parallel or near-parallel laminae.
• Graded beds: show vertical gradation in grain size from base to top.
• Massive beds: appear structureless, lacking visible internal layering - particularly common in very thick sandstone units.

Planar beds may range in thickness from about 1 cm to several metres.

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