Plane bedding (or parallel bedding) is the simplest sedimentary structure. It occurs when bedding planes are parallel to each other. In undisturbed (non deformed) sedimentary sequences, plane bedding continues laterally as horizontal beds at the scale of kilometers to hundreds of kilometers. Beds end against the margins of the sedimentary basin or gradually fades into progressively thinner beds, moving away from the source area of the sediment. Plane bedding is common in marine environments (especially deep marine environments), where it may form as the result of slow deposition of suspended, pelagic sediments or the rapid deposition of layers due to a fast hydrodynamic event (i.e. turbidity currents). In shallow waters, plane bedding may mark alternating periods of slow deposition and storms that deposit coarser sediments. In pelagic carbonates and cherts, formed by the settling of microorganisms on the seafloor, bedding may mark cyclical changes in the productivity of these organisms, for example related to fluctuations in the Earth’s orbit (astronomical changes).
Plane lamination or parallel lamination is defined by small scale parallel bands (< 1 cm) of different lithology or grain size. In lacustrine deposits close to glacier, small, seasonal changes in sediment supply (summer/winter cyclical changes in melting of glaciers) produce a special type of parallel laminae called glacial varves. In most cases, however, parallel laminae are the result of strong, tractive currents (upper flow regime) that drag sediment at the bottom of a basin, forming finely alternating laminae.
- Bedforms: ripples and dunes – Ripples, dunes, antidunes are all bedforms, structures that form in sand when it is moved by water or wind. Bedforms are ubiquitous on our planet. It is very common to see ripples, undulatory structures in sand, under shallow waters close to seashores or along riverbanks. And deserts, they are commonly covered with large sand dunes, in turn sprinkled by smaller… Leggi tutto »Bedforms: ripples and dunes
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