Feisic tuffaceous rocks of Malani Igneous Suite are generally considered to be air-borne, however, water-borne tuffs are reported from a few places. I need to understand the differences between them such as presence/absence of any structure, chemical dissimilarities etc. What are the differences between them in terms of field observations, petrography and geochemistry?
There are three principal types of pyroclastic deposits which have to be distinguished from epiclastic volcaniclastic deposit that follow the same hydrodynamic mechanism as siliciclastic sediments reworked in aquatic regimes.
Air fall tuffs are air-borne, flows are running down the slopes in a more or less fluidized way of a gas-liquid-solid mixture (subaerial) while surge deposits show all the textures and structures, e.g., cross bedding, antidunes bedding like their non-magmatic siliciclastic fluvial deposits (see textbooks on sedimentology).
The contact with aquatic regimes may it be groundwater heated up or lake and seawater provoke chemical reactions mostly resultant in glass and a series of alteration minerals such as zeolites, clay minerals (smectite) and in submarine environments bridges the gap into pillow structures.
For a more detailed information I recommend the textbook on pyroclastic deposit published by Fisher and Schmincke and the papers issued by Suthren which contain very informative tables and cartoons to make any distinction of the three types of pyroclastic deposits in the field feasible.
H.G.Dill
Туфы вулканические - продукты эффузивной магматической деятельности, формирующиеся, как правило, из разнообломочного вулканического материала - пепел, песок, "бомбы" и ксенолиты окружающих жерло вулкана пород. Отложение их в субаэральной обстановке приводит к формированию массивных плохо отсортированных, и текстурно практически неслоистых или скрытослоистых туфопесчаников, туфо-алевролитовб пелитов и туфо-брекчий. При этом физических границ/литологических в ввиде слоёв между разнообломочным материалом наблюдаются с трудом, либо не наблюаются. Минералогически и геохимически в них будут преобладать окислы и гидроокислы, с сохранением слабо окисляемых силикатов магматического происхождения. Часто туфовый материал накапливается во внутрикратрных бассейнах, куда он сползает по склонам кратера и гравитеционно осаждается после вулканических взрывов. В водной среде происходит его дифференциация и гидродинамическое перемещение, что приводит в формированию различных типов слоистости, в зависимости от литофациальной обстановки - возникает отчётливая слоистость, чато сложная комбинационная, как правило, туфовые толщи приобретают отчётливую расслоенность и дифференцированность, проявляющуюся в ритмичности их строения. Довольно типичны текстуры оползания - сложные мелки складки и седиментационные разрывы оползаэщих слоёв. При сходной минералогии всё же в составе субаквальных туфов будут преобладать гидроокислы, гидратации подвергнуться и слоистые силикаты. Пример субаквальных туфовых образований - туфово-брекчиевые комплексы кимберлитовых палеовулканов мелового возраста в Анголе - трубки Катока, КАТ-115, Камитонгу
, Луэле и др.
Volcanic tuff - products of effusive magmatic activity, which are formed, as a rule, from variously fragmented volcanic material - ashes, sand, "bombs" and xenoliths surrounding the rock volcano vent. This leads to the formation of massive poorly selected, texture-practically non-layered or hidden-layered tuff sandstones, tuff-siltstones, pelites and tuff breccias. Within these physical boundaries, not only observations are observed, but also observations of the population.
Mineralogical and geochemical, oxides and hydrooxides will prevail in them, with the conservation of weakly oxidized silicates of magmatic origin. Often tuff material accumulates in the intracrateral basins, it slides along the slopes of the crater and gravitationally precipitates after volcanic explosions. In the aquatic environment, its differentiation and hydrodynamic movement occurs, which leads to the formation of various types of layering, depending on the lithofacial environment - the emergence of desperate layering, both complex and complex.
Quite typical creeping textures are complex creases and sedimentary fractures of creeping layers. With similar minerals, the body is dominated by hydroacids, hydrates and layered silica.
A typical example of subaerial tuffs are rocks of the "tuff ring" - KTB-1 complex, and a subaquatic tuffs - KTB-2 complex of the kimberlite paleovolcano of Catoca pipe (Northeast Angola).
Volcanic tuff - products of effusive magmatic activity, which are formed, as a rule, from variously fragmented volcanic material - ashes, sand, "bombs" and xenoliths surrounding the rock volcano vent. This leads to the formation of massive poorly selected, texture-practically non-layered or hidden-layered tuff sandstones, tuff-siltstones, pelites and tuff breccias. Within these physical boundaries, not only observations are observed, but also observations of the population.
Mineralogical and geochemical, oxides and hydrooxides will prevail in them, with the conservation of weakly oxidized silicates of magmatic origin. Often tuff material accumulates in the intracrateral basins, it slides along the slopes of the crater and gravitationally precipitates after volcanic explosions. In the aquatic environment, its differentiation and hydrodynamic movement occurs, which leads to the formation of various types of layering, depending on the lithofacial environment - the emergence of desperate layering, both complex and complex.
Quite typical creeping textures are complex creases and sedimentary fractures of creeping layers. With similar minerals, the body is dominated by hydroacids, hydrates and layered silica.
A typical example of subaerial tuffs are rocks of the "tuff ring" - KTB-1 complex, and a subaquatic tuffs - KTB-2 complex of the kimberlite paleovolcano of Catoca pipe (Northeast Angola).
Dear all: surely any volcanic rock, lava or pyroclastic, deposited under water will be extensively altered to zeolites, smectites, and so on, but it is quite common that they develop hyaloclastites too. So, check for hyaloclastites, as they will indicate sub-acqueous emplacement of the volcanic rocks. Usually, as I've seen in samples here in Venezuela, basaltic hyaloclastites are cemented by a host of minerals which fill the porosity and even create meniscus cement, such as analcime, saponite, calcite and chlorite. There's a sequence according to the degree of diagenesis from one mineral to another. Surely, hyaloclastites are not pyroclastic rocks, they are not produced by explosions in felsic-intermediate volcanic centers, but they could help you to understand what happened in your stufy zone. Regards, Sebastián.
Dear Atanu,
There is a book called "Volcanic Successions, Modern and Ancient". Surely, you can find it very useful into answering your question or at least, part of it. Hope this is helpful for you. Regard, JC.
Look for syn sedimentary structures. They may give a very direct clue for sub aqueous deposits. Since you talked about Malani Group, Sindreth volcanics (type section) has beautiful syn sedimentary structures in tuffs as described in many papers.
Archi, you have correctly identified Malani volcanic tuffs deposited in aqueous conditions in Sindreth Region, western Rajasthan . and described in the paper.
- Badges
- Science topic
- Similar topics
- Geoscience
- Atmospheric Sciences
- Atmosphere
- Air