Department of Petroleum Geology & Sedimentology, Faculty of Earth Sciences, King Abdulaziz University, Jeddah, Saudi Arabia
EPS 421 CLASTIC SEDIMENTA...
Department of Petroleum Geology & Sedimentology, Faculty of Earth Sciences, King Abdulaziz University, Jeddah, Saudi Arabia
EPS 421 CLASTIC SEDIMENTARY ROCKS Lecture 14: Volcaniclastic sediments; composition, classification and diagenesis Prof. Dr. Mahmoud A. M. Aref
Volcaniclastic sediments
Volcaniclastic sediments: are those composed chiefly of grains of volcanic origin, derived from contemporaneous volcanicity. EPS 321 Lecture 14
Problems in the study of Volcaniclastic rocks 1) It is often difficult (and dangerous) to observe modern volcanic processes and mostly the eruptions that can be studied are relatively small-scale. 2) The techniques used for studying Recent volcaniclastics, such as sieving for grain-size analyses are not applicable to their indurated ancient equivalents. 3) Diagenesis is a major factor in altering volcanic glass and minerals, destroying depositional textures and creating matrix. 4) The weathering of volcanic material is also very rapid; in a few years sandgrade ash particles can be reduced to clay. 5) There is also a problem with the preservation of the volcanoes themselves; being topographic highs, they are easily eroded, and also hyrothermally altered.
EPS 321 Lecture 14
Volcaniclastic sediments and rocks are produced from: Lava delta
1. Weathering of lava (cooled magma flows). 2. Ejected pyroclastic material or tephra that can be subdivided into different compositional categories: •
Mineral grains
•
Lithic fragments
•
Vitric material (volcanic glass or pumice)
EPS 321 Lecture 14
Classification of Volcaniclastic Sediments according to mode of formation: – Epiclastic sediments: volcanic fragments that are produced by erosion of volcanic rocks by wind, water, and ice. – Pyroclastic sediments: particles broken by volcanism, or the products of explosive volcanism. – Hyaloclastic sediments: the products of the granulation of magma-water interactions. – Autoclastic sediments: formed by mechanical or gravitational movement of lava flows and/or domes
EPS 321 Lecture 14
Classification of volcaniclastic grains and sediments, based on grain size Grain size
Volcaniclastic grains (tephra)
Volcaniclastic sediments
bombs—ejected fluid
agglomerate
blocks—ejected solid
volcanic breccia
lapilli
lapilli-stone
coarse ash
volcanic sandstone (tuffs)
> 64 mm
2 - 64 mm
0.06 – 2 mm
< 0.06 mm
fine ash
EPS 321 Lecture 14
volcanic mudstone (tuffs)
New classifications of Volcaniclastic sediments ODP scheme Grain size
