Unformatted text preview: lcanic bombs or volcanic blocks: > 64 mm Accretionary Lapilli: are special kinds of Lapillisize particles that form as moist aggregates of ash in eruption clouds, by rain that falls through dry eruption clouds or by electrostatic attraction Armored Lapilli form when wet ash becomes plastered around a solid nucleus such as crystal, pumice or lithic fragments during a hydro volcanic eruption Pyroclastic breccia is a consolidated aggregate of blocks containing less than 25% Lapilli and ash. Volcanic breccia applies to all volcaniclastic rocks composed predominantly of angular volcanic particles greater than 2 mm in size. Ash Tephra Bombs Vol. Breccia Lapilli 13 Geology & Geophysics in Oil Exploration Mahmoud Sroor Geology & Geophysics in Oil Exploration Ch2: Depositional Environment Sedimentary depositional environment describes the combination of physical, chemical and biological processes associated with the deposition of a particular type of sediment and, therefore, the rock types that will be formed after lithification, if the sediment is preserved in the rock record. In most cases the environments associated with particular rock types or associations of rock types can be matched to existing analogues. However, the further back in geological time sediments were deposited 1‐Continental deposits: A‐Terrestrial deposits: • Desert deposits: Sediments accumulated by (Wind blown sediments, Wash from upland slopes, Ephermal streams). Most sediments are etched & polished. Aeolian sediments are characterized by Wedge‐shaped cross bedded units • Glacial deposits: A glacier is a perennial mass of ice which moves over land. A glacier forms in locations where the mass accumulation of snow and ice exceeds over many years. Glacial deposits are composed of different amounts and shapes of till. Till is a general term used to describe all the unsorted rock debris deposited by glaciers. Till is composed of rock fragments ranging clay to boulder size. Till is generally identified by being unsorted (all the rock is jumbled toget...
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- Spring '10
- Geology, Volcano, Tephra, Volcanic Ash, Geology & Geophysics, Mahmoud Ahmed Sroor