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Fiamme Formed by Diagenesis and Burial

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Fiamme Formed by Diagenesis and Burial Journal of the Geological Sociew, London, Vol. 147, 1990, pp. 919-922, 4 figs Printed in Northern Ireland SHORT PAPER layer can be traced laterally into a deposit of undeformed pumice clasts. Fiamme formed by diagenesis and Weathering of trachyte pumice can take place at an burial-compaction in soils and early stage in the Azores during soil formation. For ex- subaqeuous sediments ample, a substantial soil, upto 80cm thick, overlies the Fog0Aplinian deposit (Walker & Croasdale 1971), M. J. BRANNEY’ & R. S. J. SPARKS* erupted 4800 years BC, which covers much of the central ‘Department of Earth Sciences, University of and eastern parts of Sao Miguel. The upper 20cm of the Liverpool, PO Box 147, Liverpool L693BX, UK pumice deposit has been incorporated into the soil which ’Department of Geology, Wills Memorial Building, occupies pore spaces between pumiceclasts. In many places the pumicesin the soil have been altered to clay University of Bristol, Bristol BS8lRJ, UK minerals and can be squeezed flat by hand. 7 A halloysite and probably allophane were determined by X-ray diffractionin weathered Fog0 A pumice. Inthe pumice deposits at Vigia the white alteration products show the Pumice CUI be diageneticallyaltered at low temperatureand presence of 10 halloysite andtabular/rolled mor- flattened by compaction to form fiamme. Examples are described A from trachytic pumice fall layers alternating with palaeosoilson Sa0 phologies in X-ray diffraction patterns. These observations Migael in the Azores and from Ordovician lacustrine volcanicl~stic indicate thatthe alteration of glassy trachyte pumice to sedimentaq rocks from the English Lake DiPtriet. clay minerals was facilitated by activity of humic, phenolic and carbolicacids in the process of soil formation. The Vigiapumice deposits are overlainby at least 25m of The Occurrence of strongly flattened pumice clasts to form overburden including a 10 m thick basalt lava. The fiamme fiamme has been widely considered to indicate high- are thus interpreted as a product of burial compaction of temperature welding in pyroclastic deposits. Fiamme in altered pumice. whichpumice clasts are flattened into a bedding-parallel foliation (eutaxitic fabric) have beendocumented in Flattened pumice in Ordovich turbidites, English Lake ignimbrites (Smith 1960), pyroclastic fall deposits (Sparks & District. At Petts Quarry (Fig. 3) 3 km NNE of Amble- Wright1979; Mahood 1984), and pyroclastic deposits side in northwest England, diagentically flattened pumice reheated by overlying lavas (Christiansen & Lipman 1966; clastsoccur in subaqueous sedimentary rocks of the up- Schmincke1967). Fiamme also occur in some lava flows, per, post-caldera part of the Ordovician Borrowdale Vol- where they form during viscous shear accompanying canic Group (Branney 1988). The pumiceoccurs within vesiculation and autobrecciation (Pichler 1981) or during volcaniclastic turbidites which record syn-eruptive sedim- devitrification and hydrothermalalteration (Alien 1988). entation by continuous, surging density flows (Branney et Bedding-parallel foliations similar to welding-compaction al. 199Oa). The turbidites are andesitic in composition, fabricshave beeninterpreted elsewhere as recording although their mineralogy (feldspar + chlorite + sericite + preferential orientation of originally lenticular pumice clasts carbonate + opaques f epidote) reflects a Palaeozoic his- during sedimentation (Fiske 1969; Cas & Wright 1987). This tory of diagenesis, hydrothermal activity,burial and Aca- paperreports pumice clasts, in bothmodern and ancient dian tectonic compression (Branney 1988). Primary tex- deposits, which have been altered and compacted to form tures and sedimentary structures are mostclearly dis- fiamme during burial. The term ‘fiamme’ is used to describe played on cleavage-parallel surfaces. The turbidites are theappearance of therock, not to implyany particular entirely composed of coarse gravel to mud-grade andesitic origin of flattening. lithics, scoria, pumice and irresolvable fines. The density flows are thought to have been triggered by movement of Flattened pumice in Recent soils, Azores. Sea cliffs near andesitic magma into wet unlithified subaqueous sediments Vigia on the south coast of Sao Miguel expose analter- because the turbidites contain andesite hydro-clasts re- nating succession of basalt lavas and trachytic pumice fall worked from hot peperitic sheets of vesicular andesite, deposits (Fig. 1). The basalticlavas are derived from the such as are exposedin Petts Quarry (Branney & Suthren central monogenetic lavafield of Sao Miguel and the 1988). trachytic plinian and sub-plinian fall deposits are derived Matrix-supported pumiceclasts (Fig. 4a) upto 2cm from the long-lived Sete Cidadescaldera (Booth et al. across,occur in beddedand laminated poorly sorted 1978). Several white pumicefall layers areseparated by sandstone and well-sorted siltstone and mudstone. They brownsoils. The fine-grained, clay-richsoils have been occur both scattered throughout these beds and as pumice strongly bioturbated. The upper parts of each pumice fall concentrations at the base, centre or top of beds (Fig. 4). layer have been weathered, infiltrated withsoil and the Locally,individual pumices, within fine tops of normal- pumice clasts have beenconverted to white, soft clay- graded beds, appear to have sunk down through watery mud grade material (Fig. 2a). Thealtered pumice clasts have from the base of an overlying pumice-rich bed. been flattened to form white lenticles or fiamme (Fig. 2a). The pumice is sparsely porphyritic and is now altered to The textures are stronglyreminiscent of those in welded sericite + chlorite + carbonate + feldspar. Its original pumi- tuffs. Fiamme texturesare formed predominantly atthe ceous nature isdiscernible on polishedslabs from fibrous top of apumice layer, but also form atthe base where textures representing tube vesicles. Some beds contain underlyingsoil has infiltrated intothe pumice layer (Fig. pumice clasts with widely varying bedding-parallel flattening 2b). Development of thetexture is laterally variable. ratios, upto 24: 1. The flattened pumiceclasts are Figure 2c showsa pumice layer which has been entirely commonly compacted aroundequant lithicclasts and weathered in one place to form fiamme texture, but the crystals(Fig. 4b). Poorly-flattened pumices have ‘frayed’, 919 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/147/6/919/4890591/gsjgs.147.6.0919.pdf by guest on 26 September 2021 920 M. J. BRANNEY & R. S. J. SPARKS Fig. 1. Location map of Sao Miguel showingVigia outcrop and three active volcanic calderas which produce trachytic pumice. brush-like edges,but the more compacted ones have terminations which each thin to asingle long, wispy chloritic thread. The bedding-parallel flattening of the pumice cannot be (C) due to welding-compaction because the deposits are clearly Fig. 2 (continued) sedimentaryrather than pyroclastic, andbecause the ~~~ .- -- ~~~ - flattened pumice clasts occur within thin,subaqueously deposited mudstone beds and laminations. Vertical flatten- ingprofiles areirregular and dissimilar to typical welding-compaction profilesin pyroclastic rocks (Smith 1960). For example, flattening within some thick pumiceous lapillistone beds is most marked within the top 5 cm rather than towards the base or centre of the bed where maximum heat retention typically occurs in hot pyroclastic deposits. This suggests that flattening resulted from burial following diagenesis within thetop few centimetres of aporous substrate.Lithic clasts are not flattened presumably because, being less porous and less glassy, they were more resistant to diagenesis and collapse. Discussion. The occurrence of fiammewith adiagenetic origin within non-welded pumiceous deposits in the Azores and the English Lake District raises questions on (b) Fie. 2. (a) Upper contact of a trachytic pumice deposit showing unaltered and undeformed pumicelapilli in lower half of photograph and flattened white streaksof altered pumice in a brown soil matrix in the upper half. Lens capis 5 cm across. (b) Lower contactof a pumice deposit displaying flattened white streaks of altered pumice at the base.(c) Pumice deposit mixed with soil in which all the pumice clasts have been altered and Fig. 3. Location mapof Petts Quarry in the Borrowdale Volcanic flattened. Group (BVG), UK. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/147/6/919/4890591/gsjgs.147.6.0919.pdf by guest on 26 September 2021 SH O RT PAPER SHORT 921 although Fiske doesnot comment on the origin of the flattening. In ancient volcanic successions fiamme cannot be used in isolation to establish welding. Insome instances how- ever, welding can be firmly established in ancient fiamme- bearing pyroclastic rocks by associations of fiammewith criteria that show that tuffaceous matrix was once hot and glassy. Forexample, perlitic cracks andspherulites can only develop in vitrophyre; in ancient welded tuffs these are commonly pseudomorphed by quartzo-feldspathic mosaics, and they provide unambiguous proof of welding. Other characteristics, suchas columnarjoints, lithophysal no- dules, rheomorphic lineations andfolds, evidence of rhe- omorphic autobrecciation, (e.g. contemporaneous hydrocl- astic brecciation), of explosive interaction between hot tuff and wet substrate,and systematic vertical andlateral compaction profiles (e.g. Smith 1960) can also be used in associationwith flattened pumice to infer welding in an- cient tuffs. Combinations
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