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summing-up 1Stratigraphy: rocks as the pages in a book • Stratigraphy provides the tools to originated in an environment that was stable over a long period of time. A strata is the smallest unit of • sedimentary rock formation: it is, in general, of modest thickness but large expanse. Facies indicates the set of a rock’s • lithological characteristics and depends on the formation environment. By determining the facies of a rock, it is possible to trace back to the environment in which it was formed. Amongst sedimentary facies there • are common continental facies (glacial, fluvial, desert etc.), transition facies (marsh, estuary, coastal, etc.), and marine facies (coastal, neritic, pelagic, etc.). 2Principles of Stratigraphy Stratigraphy has developed some • water from previously flooded areas, and ingression (or transgression), the advance of the sea on continental areas. Transgression gives rise to an • unconformity which may be: – simple, if the strata or layers prior to the transgression and successive ones are all parallel; – angular, if the layers prior to the transgression form a certain angle with successive ones. For an unconformity, which • describes a geometric aspect, there is a corresponding sedimentation gap. The presence, inside a series of rocks, of an unconformity with a sedimentation gap indicates that the following events occurred in succession in the area: –deformation and uplift of the crust, resulting in the emergence of a new land mass (regression); –erosion of the new land mass; and –return of the sea with resumption of sedimentation (transgression). 3Tectonics: why rocks deform Tectonics is the discipline that • consequence of movements in the Earth’s crust. Rock formations appear more or less • deformed (translated and/or changed in form). Deformations are successive to rock • formation and are due to the forces acting within the Earth’s crust. 4The results of deformation: faults and folds Faults are lacerations of the crust • There are different types of faults: • – normal faults, which result in an enlargement (extension) of the crust; – reverse faults, which on the contrary, lead to a shortening of the crust. If rocks, with an original planar • structure, such as sedimentary beds, has been bent, without breaking, they form folds (anticlines, fold upward, or synclines, fold downward). The folds may be upright, inclined, or overturned. 5Complex deformation: systems of faults and slabs A set of normal faults may cause a local • rift, in which the elevated areas of crust that remain are called pillars. If a sector of the Earth’s crust, • sliding along a fault, thrusts over a neighbouring sector thrust sheet takes place. In an override, the part overridden is • called the slab or covering blanket. 6The Geological cycle The stratigraphic sequence of an • area describes the vertical sequence of the formations involved. The reading and interpretation of • stratigraphic series reveals any cyclical repetition of events. The geological cycle (or Hutton • cycle) typically includes: – rock formation, usually on the bottom of the sea; – tectonic deformation of these rocks due to movements of the crust, with the development of magmatism and metamorphism, and with the uplifting of mountain ranges; and –the demolition and erosion of mountains produced by tectonic deformation, up to the landscape evolves into lowlands and plains. Afterwards, in the same area, a new • geological cycle may develop, beginning with a marine transgression and the formation of new sedimentary rocks, unconformally on the older ones. Over billions of years, the traces of numerous cycles, similar to that described, can be seen in the rocks in the Earth’s crust. trace the original spatial arrangement of rocks and the order in which they were formed. • The object of study in Stratigraphy is the geological formation. A geological formation is a rocky body, of a generally uniform nature, that principles for establishing, based on field observations, the relative chronology according to which different rocky structures were formed: –The principle of original horizontality; –The principle of stratigraphic superposition; –The principle of intersection. The study of successive facies over • time sheds light on the phenomena of regression, the withdrawal of studies the deformation that affects rocks after their formation, as a along which some rock formations glide slide with respect to adjacent ones. collapse of the crust, called tectonic Lupia Palmieri, Parotto Osservare e capire la Terra - edizione azzurra © Zanichelli 2012 unità 2•La giacitura e le deformazioni delle rocce 1