Chapter 4 Sedimentary Structures

Bed: Tabular or lenticular layers of sedimentary rock that have lithologic, textural, or structural unity that clearly distinguishes them form strata above and below. Bedding planes (Bounding planes): The upper and lower surfaces of beds. Sedimentation units: thickness of sediments deposited under essentially constant physical conditions. Laminae: layers less than 1 cm thick. Subdivision: an informal unit arising from distinctive associations of sedimentary structures such as plane or ripple laminae. Lens: Lenticular shaped deposit of different composition, texture, cementation or color than that of the surrounding material. Band: Similar to lens but a intermittent, linear deposit that differs from the surrounding material. Amalgamation surface: a marked discontinuity between two beds of similar composition. Amalgamated bed: a bed separated by an amalgamation surface. Layer: an informal reference to any bed or stratum of rock.

Terms: Beds Bedding planes Sedimentation Units Laminae Subdivision Lens Band Amalgamation surface Amalgamated bed Layer

Bedding Surfaces

Bedsets (Groups of similar beds – Simple or Composite)

Graded bedding: sedimentation units characterized by distinct gradations in grain size. These can be normal (larger grains on the bottom gradually changing to smaller grain on the top, or the opposite and less-common, reverse grading.

Bedform development as a function of water flow velocity

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Pebble Orientation and Current Direction

Imbrication

Ripple formation in a flume…

Symmetric ripples (Oscillation ripples) are formed by wave action.

Sediment Transport within an antidune

Tabular cross-bedding: cross-bedded units that are broad in lateral dimensions with respect to set thickness and that have essentially planar bounding surfaces. Trough cross-bedding: cross-bedded units in which one or both bounding surfaces are curved.

Flaser bedding: ripple bedding in which thin streaks of mud occur between sets of cross-laminated or ripple laminated sandy or silty sediment. More sand than mud.

Lenticular Bedding: a structure formed by interbedded mud and ripple cross-laminated sand in which the ripples or sand lenses are discontinuous and isolated in both a vertical and horizontal direction. More mud than sand.

Irregular Stratification Convolute bedding and lamination: a structure formed by complex or intricate crumpling of beds or laminations into irregular, generally small-scale anticlines and synclines.

Flame Structures: wavy flame-shaped tongues of mud that project upward into an overlying layer, which is commonly sandstone.

Ball and pillow structures: present in the lower part of sandstone beds and less common in limestone beds, that overlie shales. They consist of hemispherical or kidney-shaped masses that show internal laminations.

Synsedimentary Folds and Faults (slump structures) Décollement structure

Convolute lamination

Dish and pillar structures Scour and fill structures

Bedding-plane markings

Groove and Flute casts

Sole markings: owing to erosion of a mud bottom followed by the deposition of coarser material.

Index Fossils

Trace Fossils

Bedding-plane markings Syneresis cracks, precipitation imprints, mudcracks, rill marks & parting lineation. Syneresis cracks Precipitation imprints

Mudcracks Rill marks Parting lineation

Ichnofacies are trace fossil assemblages that indicate a specific environment. The type of trace fossils that may be encountered within ichnofacies include: Repichnia: crawling or walking traces (any trace made during locomotion. Fodichnia: feeding structures (usually infaunal burrows made by deposit feeders that systematically mine for food.) Domichnia: burrows used primarily for dwelling as opposed to feeding. Cubichnia: behavioral traces including resting or nesting traces. Pascichnia: traces made by grazing herbavores, usually at the sediment/water interface.

Schematic representation of the relationship of the characteristic trace fossils to sedimentary faces and depth zones in the ocean.

Stromatolites: organically formed laminated structures composed of fine silt or clay-sized sediment, or more rarely, sand-size sediment.

Paleocurrents