Fundamentals of geology I. (lithosphere) 1 1. The formation of the Earth 1
Pict. 2.4. Unbedded reef limestone Pict. 2.5. Secondary unbedded structure in sand
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- Picts. 2.6., 2.7.
- Pict. 2.6. Parallel bedded sand and clay Pict. 2.7. Laminite Cross-stratification
Pict. 2.4. Unbedded reef limestone Pict. 2.5. Secondary unbedded structure in sand Planar stratification Planar lamination forms when the flow is strong enough that the beds flatten out. The momentum of the transported grains and fluid are high enough that they tend to move horizontally, eroding any irregularities in the bed. This zone of planar lamination is called “upper flow regime”. A special type of planar lamination is rhythmite or varve deposit. any form of repetitive sedimentary rock stratification, either bed or lamination, that was deposited within a one-year time period. This annual deposit may comprise paired contrasting laminations of alternately finer and coarser silt or clay, reflecting seasonal sedimentation (summer and winter) within the year. Varved deposits are to be distinguished from rhythmites, the latter also being made up of paired laminations or beds but with an annual cyclicity that cannot be proved. Varved deposits are usually associated with fine-grained sediments, the muds or mudrocks, which include both silt- and clay-grade materials. Laminations in many mudrocks are both thin and laterally persistent over large areas (Picts. 2.6., 2.7.). 
Pict. 2.6. Parallel bedded sand and clay Pict. 2.7. Laminite Cross-stratification Cross-strata are layers of sediment that are inclined relative to the base and top of the set in which the inclined layers are grouped. Each group is called a set of cross-strata or a cross-stratified bed. Individual cross-strata can be classified as cross-laminae (<1 cm thick) or cross-beds (>1 cm thick). In general, each set of cross-strata is deposited by a migrating bedform. Thin sets are deposited by small migrating bedforms sueh as ripples, small dunes., or small antidunes, and thick sets are deposited by larger dunes, antidunes, bars, or other large bedforms. Cross-strata are a natural record of transported sediment and arc therefore useful for understanding the behaviour of modern bedforms and for interpreting environments in ancient deposits. Cross-bedding can be subdivided according to the geometry of the sets and cross strata into subcategories. The most commonly described types are tabular cross-bedding and trough cross-bedding. Tabular cross-bedding, or planar bedding consists of cross-bedded units that are large horizontal wise with reverence to set thickness and that have essentially planar bounding surfaces. Trough cross-bedding, on the other hand, consists of cross-bedding units in which the bounding surfaces are bowed (Fig. 2.5.). 
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