Structural Geology Geomorphology Primary structures 3 rd year

Structural Geology & Geomorphology Primary structures 3 rd year –Geology/Biology Class Faculty of Education-2020/2021

Primary structures • A primary structure is defined as, "any structure that develops prior to or during the formation of the rock • Primary structures are non-tectonic, meaning they form during sedimentary deposition, or in the case of metamorphic rock, during crystallization

Sedimentary structures primary structures 1 - Bedding (stratification) » It forms as a direct consequence of » Steno's law of lateral continuity, that holds that a unit of sediment will extend laterally to the physical margins of the basin it is filing: "Material forming any stratum were » continuous over the surface of the Earth unless some other solid bodies stood in the way. " Stratification: horizontal bedding is » most common structure in sedimentary rocks

2. Cross-bedding: • It is an inclined stratification. • Cross-bedding forms during deposition on the inclined surfaces of bedforms such as ripples and dunes; it indicates that the depositional environment contained a flowing medium (typically water or wind).

Cross beds can tell geologists much about: • What an area was like in ancient times. • The direction the beds are dipping indicates paleocurrent, the rough direction of sediment transport. • The type and condition of sediments can tell geologists the type of environment (rounding, sorting, composition…).

2. 1 Tabular (planar) cross-bedding Tabular (planar) cross-beds consist of cross-bedded units that have large horizontal extent relative to set thickness and that have essentially planar bounding surfaces. The foreset laminae of tabular cross-beds have curved laminae that have a tangential relationship to the basal surface.

2. 2 Trough cross-bedding Trough cross-beds have lower surfaces which are curved or scoop shaped and truncate the underlying beds. The foreset beds are also curved and merge tangentially with the lower surface. They are associated with sand dune migration.

2. 3 Ripple marks • Ripples- undulating bedforms produced • Ripple marks are sedimentary structures (i. e. bedforms of the lower flow regime) and indicate agitation by water (current or waves) or wind. • Ripple marks are characteristic of shallow water deposition. They are caused by waves or winds piling up the sediment into long ridges.

Asymmetrical ripple marks can give an indication of current direction when formed in water, and when formed by wind, give wind direction.

3. Mudcracks » These structures result from the drying out of wet sediment at the surface of the Earth. The cracks form due to shrinkage of the sediment as it dries. » The presence of mudcracks indicates that the sediment was exposed at the surface shortly after deposition, since drying of the sediment would not occur beneath a body of water.

2. 5 Graded Bedding • Indicate sediment deposited underwater by turbid flow • Coarse-grained sediment at bottom, finer toward up.

• As current velocity decreases, the larger or more dense particles are deposited first, followed by smaller particles. • Graded bedding is usually explained as the. result of a large amount of mixed sediment being discharged into quiet water.