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Zones and Zonal Variations in Welded Ash Flows Zones and Zonal Variations in Welded Ash Flows GEOLOGICAL SURVEY PROFESSIONAL PAPER 354-F 1 ~ ! ' ... Zones and Zonal Variations in Welded Ash Flows By ROBERT L. SMITH SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY GEOLOGICAL SURVEY PROFESSIONAL PAPER 354-F A concept of zonat£on £n ash flows based on degree of weld£ng and type of crystall£zat£on •· ..... UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1960 f UNITED STATES DEPARTMENT OF THE INTERIOR FRED A. SEATON, Secretary GEOLOGICAL SURVEY Thomas B. Nolan, Director _..,· For sale by the Superintendent of Documents, U.S. Government Printing Office Washington 25, D.C. I \ CONTENTS Page Abstract __________________________________________ _ 149 The zones~Continued Page Introduction ______________________________________ _ 149 Zone of partial welding _________________________ _ 154 Acknowledgments __________________________________ _ 150 Zone of dense welding __________________________ _ 154 Bruption and empln.cement _________________________ _ 150 Zones of crystallization _________________________ _ 155 ? vVelcling __________________________________________ _ 151 Cooling unit ______________________________________ _ 157 Crystallization ____________________________________ _ 151 Composite sheet ___________________________________ _ 158 1'he zones _________________________________________ _ 153 Influence of buried topography ______________________ _ 158 Zone of no welding _____________________________ _ 154 References cited _________________________________ -_- 158 ILLUSTRATIONS PLATln 20. Diagrams showing zones of some ash-flow cooling units------------------------------------------------ In pocket 21. Zones of no welding, partial welding, and dense welding, and their approximate crystalline equivalents __ Facing page 152 m ... SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY ZONES AND ZONAL VARIATIONS IN WELDED ASH FLOWS By RoBERT L. SMITH ABSTRACT Horizontal separation of compound cooling units into separate Welded tuffs are recognized as special parts of ash flows, cooling units suggests the existence of a hypothetical unit, the other pyroclastic flows, or, more rarely, air-fall deposits. Ash composite sheet, which could have time-stratigraphic signift­ flows may be emplaced at any temperature below a maximum cance. eruption temperature. Those emplaced above a minimum Buried topography can have a profound influence on the welding temperature may show any and all degrees of welding zonation of certain ash-flow cooling units. Very hot ones, be­ and crystallization. cause of their high compactability potential, will show less zonal change than colder ones, although they will show greater Three basic zones are recognized, the zones of no welding, surface expression of the buried topography. Abrupt changes partial welding, and dense welding. In sheets where the zone in relief, especially if the buried topographic high rises to the of dense welding occurs, the zones of no welding and partial top of the zone of dense welding, can cause horizontal changes, welding will have upper and lower counterparts. in a short distance, that may be difficult to interpret. Cooling of welded ash flows may result in sheets that range from completely glassy (except for phenocrysts and inclusions) to mostly crystalline. INTRODUCTION Crystallization superimposed on the zonal patterns produced by welding results in another set of zones that is controlled in In this report, welded tuffs are considered to be spe­ part by degree of welding. cial parts of ash flows, other pyroclastic flows, or more Three principal types of crystallization that. take place dur­ rarely, air-fall deposits (not to be confused with fused ing the cooling history are recognized. These in order of fre­ tuffs, p. 155). ~lost students of welded tuffs have rec­ quency of occurrence, are devitriftcation, vapor-phase crystal­ lization, and granophyric crystallization. The zone of devitri­ ognized vertical variations of texture, specific gravity, flcation is common to most crystallized welded tuffs and fre­ color, mineralogy, and other properties within the de­ quently occupies most of the zones of dense welding and partial posits. Horizontal variations are rarely emphasized welding. The zone of vapor-phase crystallization, where pres­ because complete ash-flow sheets have not been mapped, ent, occupies the porous parts of the welded tuff sheets and particularly in detail. Mapping of complete sheets has reaches its maximum development in the upper zone of partial welding where it overlaps the zone of devitri:fication. The not been done because of the generally large areal ex­ zone of granophy'ric crystallization is probably confined to cool­ tent of the sheets, they may be of only casual interest ing units several hundreds of feet thick where it will divide to the mapping problem, or because erosion, and cover the devitri:fied zone into upper and lower parts. Fumarolic by younger rocks, limits their area of exposure. alteration may be found in the upper zone of no welding. The uniform and unsorted character of ash-flow de­ Single ash flows may cool with the formation of a basic pattern of zones. This pattern is illustrated and described posits has been cited as a criterion for their recogni­ both for those sheets which remain glassy and for those in tion. Although this is generally true, the complex which crystallization has occurred. These flows are called emplacement and cooling history of many such de­ simple cooling 1tnits. posits may produce various textural and mineralogical Successive ash flows, emplaced quickly enough to bridge the facies that bear little or no resemblance to the original cooling gap between flows, will form a stack of flows that may also cool to form the zonal pattern of a simple cooling unit. materials of the ash flows. These variations are zonal The gaps or hiatuses, here called partings, may contain or and normally show a consistent pattern of transitions ·~· consist of lump pumice layers, flow surfaces reworked by wind both vertically and horizontally in uniform ash flows or water, airfall pyroclastic material, other deposits, 9r minor which have had an unimpeded cooling history. Once erosional unconformities. this zonal pattern is clearly understood for simple cool­ Compound cooling 1tnits consist of multiple flow units with or without visible partings and which show zonal patterns ing units, progress can be made in interpreting aber­ that depart from the patterns of the simple cooling units. rant patterns· in more complex deposits. 149 M922o-60 150 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY The purpose of this report is to describe the zones this difficulty may be overcmne if the geologist has a and the simple zonal patterns by means of 4 simplified clear understanding of the normal characteristics of diagrams (pl. 20 A-D) and a brief text. The dia­ the unaltered rocks. The zonal patterns will be ex­ grams are not intended to represent specific welded tremely important for detailed geochemical studies. ash flows but are simply hypothetical models. They were constructed to illustrate, in part, the writer's ACKNOWLEDGMENTS concept of simple ash-flow cooling units. The scales The writer is indebted to Clarence S. Ross with used give an order-of-magnitude relation among the . whom he has studied welded tuffs for many years and different zones that is probably realistic for some nat­ with whom he has written a more comprehensive re­ ural sheets. The most difficult characteristics to gen­ port (Ross and Smith, 1960). Much of the material eralize are the horizontal changes, because a cooling in the present report is an outgrowth of this earlier unit may range from less than a mile to many tens of study, although the writer is solely responsible for the miles in length, or from less than a square mile to organization of the data and theory as presented here. hundreds and perhaps thousands of square miles in The writer is especially indebted to Roy A. Bailey, area. The true nature of the distal ends is generally also a close working companion of many years, who problematical. They are rarely preserved or recog­ has been a most helpful critic. Of the many other nized in prehistoric sheets and there appears to be a Geological Survey colleagues who have aided the writ­ wide variation in historic "nuee ardente" deposits, er's studies special thanks are due C. A. Anderson and frmn "snub noses" to thin tapered ends often measur­ Harry W. Smedes for their constructive criticisms. able in inches. The diagrams were constructed based on the assump­ ERUPTION AND EMPLACEMENT tion that smne cooling took place in the direction of Much could be written about the eruption of pyro­ thinning. Although this is probably a common condi­ clastic materials that are emplaced as hot sheetlike tion it is by no means necessarily true for all flows. bodies and whose slow cooling may result in deposits Some widespread ash-flow cooling units show only that show striking physical and chemical differences slight effect of cooling or thinning over tens of miles from the initially erupted material. However, the whereas others show marked effects in only a few miles. main purpose of this report is to discuss the more obvi­ The direction of thinning may be related to the surface ous characteristics of the deposits after they have underlying the flow or it may reflect a change away cooled. It is well recognized that these deposits were, from the source area. These facts, among others, indi­ for the most part, emplaced as hot avalanchelike cate the need
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