Volcanic Ash Layers in Coal - American Chemical Society

7 Volcanic Ash Layers in Coal: Origin, Distribution, Composition, and Significance

Downloaded by FUDAN UNIV on November 18, 2016 | http://pubs.acs.org Publication Date: April 2, 1986 | doi: 10.1021/bk-1986-0301.ch007

D o n Triplehorn

1,2

and Bruce Bohor

2

1

Department

2

U . S . Geological Survey, Federal Center, Denver, CO 80225

of Geology and Geophysics, University of Alaska, Fairbanks, AK 99701

Volcanic ash that falls into a coal-forming environment stands a good chance of being preserved and contributing to the mineral matter in coal. Such layers are relatively common in some coals although they are commonly not recognized as volcanic in origin even by geologists. The original material usually consists of fine-grained glass, crystals, and rock fragments, but there is a great variety in texture and composition. After burial the glass, in particular, is readily altered so that with increasing geologic age secondary clay minerals become abundant and evidence of the volcanic origin becomes less obvious. Kaolinite is most abundant, but smectite is not uncommon; occasionally unusual minerals occur, such as aluminum phosphates with notable amounts of Sr and rare earths. In North America, volcanic ash layers are most important in western coals of Tertiary and Cretaceous ages; they are generally uncommon in eastern coals or those of greater age. The main purpose o f t h i s paper i s t o c a l l a t t e n t i o n t o v o l c a n i c e r u p t i o n s as a s o u r c e o f m i n e r a l m a t t e r i n c o a l . Volcanic material i s apt t o have m i n e r a l and c h e m i c a l c o m p o s i t i o n s , as w e l l as p a t t e r n s o f d i s t r i b u t i o n , d i f f e r e n t from t h e more u s u a l types o f m i n e r a l matter i n c o a l . Recognition of volcanic material requires some u n d e r s t a n d i n g o f t h e p r o c e s s o f o r i g i n , an awareness o f t h e c o n s i d e r a b l e v a r i e t y o f m a t e r i a l s i n v o l v e d , and an a p p r e c i a t i o n o f the tendency o f v o l c a n i c m a t e r i a l s t o undergo s u b s t a n t i a l a l t e r a t i o n so t h a t t h e i r g e n e s i s i s o b s c u r e d . M i n e r a l m a t t e r i n c o a l most commonly c o n s i s t s o f mud c a r r i e d i n t o the c o a l - f o r m i n g environments by streams as overbank d e p o s i t s d u r i n g times o f f l o o d . The m i n e r a l s a r e m a i n l y q u a r t z , f e l d s p a r s and c l a y m i n e r a l s , and t h e s e a r e d e p o s i t e d as s h a l y p a r t i n g s t h a t become t h i n n e r and f i n e r - g r a i n e d away from the s t r e a m c h a n n e l s . By c o n t r a s t , t h e v o l c a n i c ashes t h a t o c c u r i n c o a l s tend t o be o f t h e highly s i l i c e o u s v a r i e t y associated with explosive eruptions, during which a s h i s c a r r i e d h i g h i n t o the atmosphere and t r a n s p o r t e d f a r downwind. These form t h i n , u n i f o r m , widespread p a r t i n g s i n c o a l s , 0097-6156/ 86/0301 -0090506.00/ 0 © 1986 American Chemical Society

Vorres; Mineral Matter and Ash in Coal ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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Volcanic Ash Layers in Coal

commonly on the o r d e r of an i n c h t h i c k and d i s p l a y i n g l i t t l e l a t e r a l change i n t h i c k n e s s or c o m p o s i t i o n . Such s i l i c i c (70% S I 0 > v o l c a n i c ashes tend t o be m o s t l y composed of d u s t - s i z e d p a r t i c l e s of g l a s s , w i t h l e s s than a few p e r c e n t of m i n e r a l g r a i n s such as q u a r t z , v a r i o u s f e l d s p a r s , b i o t i t e , z i r c o n , and h o r n b l e n d e . Glass i s h i g h l y u n s t a b l e i n g e o l o g i c terms, and a l t e r s r a p i d l y t o c l a y ; thus the o r i g i n a l v o l c a n i c ash t e x t u r e and c o m p o s i t i o n a r e not e v i d e n t i n the f i n a l p r o d u c t .


2

Some comments on t e r m i n o l o g y are n e c e s s a r y . " V o l c a n i c ash p a r t i n g s " here r e f e r s to s e d i m e n t a r y u n i t s bounded by o r g a n i c - r i c h m a t e r i a l ( c o a l , l i g n i t e , or o r g a n i c s h a l e ) and which were d e p o s i t e d as a i r - f a l l v o l c a n i c ash w i t h e s s e n t i a l l y no subsequent t r a n s p o r t and m i x i n g w i t h t e r r i g e n o u s d e t r i a l s i l i c a t e s ( c l a y , mud and s a n d ) . No s p e c i f i c g r a i n s i z e or c o m p o s i t i o n i s i m p l i e d . The term " t e p h r a " i s now commonly used f o r modern, u n a l t e r e d , uncompacted m a t e r i a l , w h i l e the term " t u f f " i s used f o r the compacted ( r o c k ) e q u i v a l e n t . A v o l c a n i c o r i g i n i s o b v i o u s where t h e r e i s g l a s s , v o l c a n i c p h e n o c r y s t s , a l i m i t e d m i n e r a l s u i t e c h a r a c t e r i s t i c of v o l c a n i c r o c k s , or absence of t e r r i g e n o u s d e t r i t u s and l a c k of s e d i m e n t a r y s t r u c t u r e s formed by moving w a t e r . With i n c r e a s i n g age, however, t h e r e i s p r o g r e s s i v e a l t e r a t i o n and l o s s of r e c o g n i z a b l e v o l c a n i c f e a t u r e s ; the p r o d u c t i s g e n e r a l l y some k i n d of c l a y u n i t . As a g e n e r a l term they might be c a l l e d a l t e r e d t u f f s . I f m a i n l y composed on m o n t m o r i l l o n i t i c ( s m e c t i t i c ) c l a y t h a t i s l i g h t - c o l o r e d and s t i c k y when wet, t h e s e are commonly c a l l e d b e n t o n i t e s and m a i n l y o c c u r i n marine s h a l e s . In o l d e r c o a l s t h e y are u s u a l l y k a o l i n i t i c , o c c u r as f i r m r o c k s r a t h e r than s o f t c l a y , and weather t o l i g h t e r c o l o r s than the e n c l o s i n g c o a l . These are commonly c a l l e d " t o n s t e i n s " , and o l d term f o r such f i n e - g r a i n e d , h a r d c l a y p a r t i n g s t h a t are r e l a t i v e l y abundant i n European C a r b o n i f e r o u s c o a l s . T o n s t e i n was o r i g i n a l l y a p p l i e d as a p u r e l y d e s c r i p t i v e term ( " c l a y r o c k " ) but i n r e c e n t y e a r s an o r i g i n as an a i r - f a l l v o l c a n i c ash has become i n c r e a s i n g l y a c c e p t e d . We p r e f e r the g e n e r a l term " a l t e r e d v o l c a n i c a s h " because t h e r e i s c o n s i d e r a b l e range i n p h y s i c a l appearance, o r i g i n a l and s e c o n d a r y m i n e r a l o g y , as w e l l as i n the type of e n c l o s i n g s e d i m e n t . ORIGIN The environment of c o a l d e p o s i t i o n p r o v i d e s one of the b e s t p l a c e s f o r d e p o s i t i o n and p r e s e r v a t i o n o f v o l c a n i c a s h . The r e c e n t e r u p t i o n of Mt. S t . H e l e n s , f o r example, s p r e a d v o l c a n i c ash a c r o s s s e v e r a l w e s t e r n s t a t e s . On l a n d , almost a l l of the v o l c a n i c ash w i l l be eroded by wind and water, mixed w i t h t e r r i g e n o u s sand and c l a y , and u l t i m a t e l y d i s p e r s e d i n t o l a k e s or the ocean. In marshes or swamps, the g e n e r a l s e t t i n g i n which p l a n t m a t e r i a l accumulates to e v e n t u a l l y become c o a l , an ash f a l l has a good chance of r e m a i n i n g u n d i s t u r b e d because of the s h a l l o w water, low s t r e a m g r a d i e n t s , and l a c k of r e l i e f . The s e d i m e n t - b a f f l i n g e f f e c t s of v e g e t a t i o n m i n i m i z e s p r o c e s s e s t h a t c o u l d cause r e w o r k i n g o f the v o l c a n i c ash. P o r t i o n s of such swamps may be so d i s t a n t from major streams t h a t they r e c e i v e l i t t l e or no mud from overbank f l o o d s . Thus i t i s l i k e l y t h a t most ash f a l l s i n swamps would be b u r i e d i n o r g a n i c d e b r i s ( c o a l ) and remain f r e e of n o n - v o l c a n i c m a t e r i a l from f l u v i a l sources. O n l y v o l c a n o e s c h a r a c t e r i z e d by p a r t i c u l a r l y v i o l e n t e r u p t i v e s t y l e s are l i k e l y to be i m p o r t a n t i n p r o d u c i n g the t h i n , w i d e s p r e a d


MINERAL

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MATTER

A N D A S H IN

COAL

u n i t s of ash most commonly p r e s e r v e d i n the g e o l o g i c r e c o r d . The e x p l o s i v e n e s s of e r u p t i o n i s r e l a t e d to s e v e r a l f a c t o r s , i n c l u d i n g gas c o n t e n t and geometry of the v e n t . Of most importance h e r e , however, i s the s i l i c a c o n t e n t : s i l i c a i n c r e a s e s the v i s c o s i t y of magma and the tendency toward e x p l o s i v e e r u p t i o n s r a t h e r than q u i e t flows. Thus v o l c a n i c ash p a r t i n g s i n c o a l s are p r i m a r i l y the p r o d u c t of such s i l i c a - r i c h e r u p t i o n s ; t h i s has i m p o r t a n t consequences i n terms of the c o m p o s i t i o n of the a l t e r e d ash, as w i l l be d i s c u s s e d l a t e r .


FIELD APPEARANCE OF

VOLCANIC ASH

PARTINGS IN COALS

Most v o l c a n i c ash p a r t i n g s i n c o a l are t h i n , r a n g i n g from 1 mm to a few cm. A few, however, a t t a i n t h i c k n e s s e s of more than 1 m. Many are u n i f o r m i n t h i c k n e s s and have e i t h e r s h a r p or g r a d a t i o n a l b o u n d a r i e s w i t h the e n c l o s i n g c o a l . Some, however, p i n c h and s w e l l r a p i d l y and may c o n s i s t of a s e r i e s of l e n s e s r a t h e r than a c o n t i n u o u s bed, a f e a t u r e they s h a r e w i t h some C a r b o n i f e r o u s t o n s t e i n s ( W i l l i a m s o n , 1970). L i g h t shades of g r a y , brown, o r y e l l o w a r e most common, r e f l e c t i n g a lower o r g a n i c c o n t e n t t h a n the a d j a c e n t c o a l . B l a c k and d a r k brown p a r t i n g s a l s o o c c u r , and sometimes t h e s e become o b v i o u s o n l y a f t e r w e a t h e r i n g ; the o x i d a t i o n of o r g a n i c m a t t e r then r e s u l t s i n a l i g h t - c o l o r e d s u r f a c e l a y e r of siliceous material. F i g u r e 1 shows the most common f i e l d appearance; i n t h i s case f o r a C r e t a c e o u s example from s o u t h e r n Alaska. To a l a r g e e x t e n t the g r a i n s i z e and degree o f i n d u r a t i o n of t h e s e p a r t i n g s depends upon the amount of p o s t - d e p o s i t i o n a l a l t e r a t i o n ; t h i s i n t u r n i s l a r g e l y a f u n c t i o n of age and d e p t h of burial. For example, l i g n i t e s 4-5 m.y. o l d i n s o u t h e r n A l a s k a c o n t a i n p a r t i n g s t h a t are l o o s e and sandy; i n f a c t , t h e y are c l e a r l y r e c o g n i z a b l e as v o l c a n i c ash c o n s i s t i n g m o s t l y o f g l a s s s h a r d s . In c o n t r a s t , p a r t i n g s i n 100 m.y. Kentucky b i t u m i n o u s c o a l are h a r d and v e r y f i n e g r a i n e d ( S e i d e r s , 1965; Bohor and T r i p l e h o r n , 1982). The l a t t e r p r o b a b l y was s i m i l a r to the A l a s k a n example at f i r s t , but has c o m p l e t e l y a l t e r e d to a compact v a r i e t y o f k a o l i n known as f l i n t clay. COMPOSITION C o m p o s i t i o n of the p a r t i n g s i s a f u n c t i o n of b o t h the o r i g i n a l m a t e r i a l s and c o n d i t i o n s , and the k i n d and e x t e n t of post-depositional modification. Therefore i t i s necessary to c o n s i d e r the p r i m a r y c o m p o s i t i o n ( o r i g i n a l ) s e p a r a t e l y from the secondary composition ( a l t e r e d ) . Primary Composition The s o l i d p r o d u c t s of e x p l o s i v e v o l c a n i s m i n c l u d e i n d i v i d u a l g l a s s f r a g m e n t s , i n d i v i d u a l m i n e r a l c r y s t a l s ( p h e n o c r y s t s ) , and a g g r e g a t e s of t h e s e known as r o c k f r a g m e n t s . G l a s s i s perhaps the most abundant component but t h e r e i s a g r e a t v a r i e t y i n the s o l i d components e j e c t e d from modern v o l c a n o e s . A g i v e n v o l c a n o may e j e c t d i f f e r e n t m a t e r i a l over i t s e r u p t i v e l i f e s p a n . Even a s i n g l e e r u p t i o n , l a s t i n g perhaps o n l y a few d a y s , may i n v o l v e changes i n ash composition. V o l c a n i c ashes are c h a r a c t e r i z e d by a l i m i t e d s u i t e of m i n e r a l components. By f a r the most abundant are q u a r t z , s a n i d i n e and


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p l a g i o c l a s e f e l d s p a r s , c e r t a i n pyroxenes and amphiboles, magnetite, a p a t i t e , b i o t i t e , and z i r c o n . The p r e s e n c e of t h e s e m i n e r a l s and the absence o f o t h e r s c o n s t i t u t e e v i d e n c e o f a v o l c a n i c o r i g i n . C e r t a i n m i n e r a l c r y s t a l forms, such as the b e t a form o f q u a r t z and hexagonal prisms of b i o t i t e , a r e p a r t i c u l a r l y u s e f u l i n d i c a t o r s o f volcanic origin. S i m i l a r l y , the presence of s a n i d i n e , the h i g h - t e m p e r a t u r e form of p o t a s s i u m f e l d s p a r , s u g g e s t s a v o l c a n i c origin. On the o t h e r hand, the p r e s e n c e of n o n - v o l c a n i c m i n e r a l s such as m u s c o v i t e , e p i d o t e and g a r n e t i n d i c a t e s a n o n - v o l c a n i c o r i g i n , o r a t l e a s t some admixture of n o n - v o l c a n i c ( p r o b a b l y fluvial) material. O r i g i n a l g r a i n s i z e of an ash p a r t i n g r e f l e c t s the t e x t u r e of the m a t e r i a l produced by a g i v e n v o l c a n o p l u s the p r o g r e s s i v e l o s s of c o a r s e r and denser components as an ash c l o u d moves downwind. In o t h e r words, t e x t u r e i s i n p a r t a f u n c t i o n of d i s t a n c e from the source volcano. As noted e a r l i e r , the most common ash p a r t i n g s i n c o a l a r e t h i n , u n i f o r m and w i d e s p r e a d . These p r o b a b l y were d e r i v e d from d i s t a n t v o l c a n o e s and c o n s i s t e d o r i g i n a l l y o f s i l t - and c l a y - s i z e d p a r t i c l e s c a r r i e d by h i g h - a l t i t u d e w i n d s . Conversely, where ash p a r t i n g s a r e c o a r s e - g r a i n e d , t h i c k , and abundant, the v o l c a n i c s o u r c e i s assumed t o have been r e l a t i v e l y c l o s e . Secondary C o m p o s i t i o n The p r i m a r y c o m p o s i t i o n d i s c u s s e d above i s i m p o r t a n t i n t h a t i t determines i n p a r t the f i n a l p r o d u c t s of a l t e r a t i o n . I t s h o u l d be n o t e d , however, t h a t the f a c t o r s c o n t r o l l i n g the degree and d i r e c t i o n of a l t e r a t i o n have not been t h o r o u g h l y s t u d i e d . Chemical a l t e r a t i o n i n the environment of d e p o s i t i o n and the l e n g t h o f time i n v o l v e d may be as i m p o r t a n t as o r i g i n a l c o m p o s i t i o n i n d e t e r m i n i n g the f i n a l p r o d u c t . F o r example, both k a o l i n i t i c and s m e c t i t i c p a r t i n g s o c c u r i n T e r t i a r y c o a l s near C e n t r a l i a , Washington ( R e i n i n k - S m i t h , 1982) and i t i s not known i f t h i s r e f l e c t s the d i f f e r e n c e s i n o r i g i n a l ash c o m p o s i t i o n o r some d i f f e r e n c e s w i t h i n the c o a l - f o r m i n g environment t h a t r e s u l t e d i n the f o r m a t i o n o f d i f f e r e n t c l a y m i n e r a l s from the same ash p a r e n t . T r i p l e h o r n and Bohor (1981) r e l a t e d d i f f e r e n c e s i n the m i n e r a l o g y o f a l t e r e d v o l c a n i c ash p a r t i n g s , i n a c o a l bed i n Utah, a t l e a s t i n p a r t , t o d i f f e r e n c e s i n t h i c k n e s s o f the o r i g i n a l ash p a r t i n g s which a f f e c t e d the e f f i c i e n c y of l e a c h i n g . The main secondary p r o d u c t s a r e c l a y m i n e r a l s , e i t h e r s m e c t i t e or k a o l i n i t e . These c l a y m i n e r a l s a r e d e r i v e d m a i n l y and most r e a d i l y from g l a s s , but f e l d s p a r s , amphiboles, pyroxenes, and b i o t i t e a l s o a l t e r i n p a r t t o c l a y m i n e r a l s . The a l t e r a t i o n of v o l c a n i c ash t o k a o l i n i t e i n v o l v e s removal of Ma, Ca, Mg, Κ and Fe as w e l l as c o n s i d e r a b l e s i l i c a . Pure k a o l i n i t e c o r r e s p o n d s t o a m i x t u r e of subequal amounts of s i l i c a and alumina p l u s a s m a l l amount of water and v i r t u a l l y n o t h i n g e l s e — t h i s i s thus the p r o d u c t of v e r y i n t e n s i v e l e a c h i n g . S m e c t i t e on the o t h e r hand r e q u i r e s some Mg and more s i l i c a than k a o l i n i t e . I t i s thus the p r o d u c t o f l e s s i n t e n s i v e l e a c h i n g than t h a t which produces k a o l i n i t e . I t appears t h a t v o l c a n i c ash can sometimes a l t e r d i r e c t l y t o k a o l i n i t e ; i n o t h e r cases a s m e c t i t i c i n t e r m e d i a t e s t a g e may be involved. Where a l t e r a t i o n has gone t o c o m p l e t i o n , t h a t i t , no g l a s s remains and o n l y k a o l i n i t e i s p r e s e n t , i t may be i m p o s s i b l e t o


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determine d e t a i l s o f e a r l y s t a g e s o f the a l t e r a t i o n p r o c e s s . Even though the p r o c e s s o f a l t e r a t i o n o f t h e b u l k o f a p a r t i n g (presumably g l a s s ) may n o t be c l e a r , i n d i v i d u a l m i n e r a l s such as f e l d s p a r s and b i o t i t e can be observed a l t e r i n g d i r e c t l y t o k a o l i n i t e w i t h o u t any s m e c t i t e i n t e r m e d i a r y . Whatever t h e d e t a i l s , i t i s c l e a r t h a t a major amount o f s i l i c o n somehow must be removed i n s o l u t i o n ( a l o n g w i t h l e s s e r amounts o f such s o l u b l e elements as Na, K, Mg, Ca, and F e ) . Because s i l i c a , and most o f t h e o t h e r components c i t e d , do not appear as s e c o n d a r y m i n e r a l s i n a d j a c e n t r o c k u n i t s , they a p p a r e n t l y were c a r r i e d out o f t h e system i n s o l u t i o n by passage o f groundwater. T h i s i s i n c o n t r a s t t o some marine b e n t o n i t e s , where s i l i c a produced by a l t e r a t i o n o f v o l c a n i c ash o f t e n appears nearby as s i l i c a cement i n s i g n i f i c a n t q u a n t i t i e s . A v a r i e t y o f secondary m i n e r a l s i n a d d i t i o n t o c l a y m i n e r a l s may o c c u r i n a l t e r e d v o l c a n i c a s h p a r t i n g s . Some o f these may be r e l a t e d t o m o d i f i c a t i o n o f t h e p r i m a r y c o n s t i t u e n t s , but o t h e r s a r e more l i k e l y i n t r o d u c e d by ground water t a k i n g advantage o f t h e higher p e r m e a b i l i t i e s of v o l c a n i c ash l a y e r s r e l a t i v e to the a d j a c e n t o r g a n i c m a t e r i a l (now c o a l ) . Carbonate m i n e r a l s i n q u a n t i t i e s t o o l a r g e t o have been d e r i v e d e n t i r e l y from the p r i m a r y v o l c a n i c m a t e r i a l a r e n o t uncommon. S i d e r i t e i s most abundant, a l t h o u g h d o l o m i t e sometimes i s p r e s e n t . Abundant secondary c a r b o n a t e may obscure t h e v o l c a n i c o r i g i n o f t h e o r i g i n a l l a y e r because such well-cemented p a r t i n g s a r e s i m i l a r i n appearance t o t h e p u r e l y s e d i m e n t a r y c a r b o n a t e l a y e r s t h a t a r e v e r y commonly associated with c o a l s . R e c e n t l y o c c u r r e n c e s o f u n u s u a l aluminum phosphate m i n e r a l s have been found w i t h a s h p a r t i n g s i n c o a l s ( T r i p l e h o r n and Bohor, 1983). S i n c e t h a t r e p o r t we have found a number o f a d d i t i o n a l o c c u r r e n c e s i n A l a s k a and one i n t h e a p p a l a c h i a n a r e a . These m i n e r a l s were grouped by P a l a c h e ( e t . a l . , 1951) as t h e plumbogummite s e r i e s , w i t h t h e g e n e r a l f o r m u l a X A l ^ P O ^ K i O H ^ t ^ O . End members o f i n t e r e s t here i n c l u d e g o y a z i t e (where X - S r ) , g o r c e i x i t e ( B a ) , c r a n d a l l i t e ( C a ) , and f l o r e n c i t e (Ce, U, and o t h e r r a r e e a r t h s ) . Some o f t h e s e l a y e r s i n A l a s k a a r e s u f f i c i e n t l y r a d i o a c t i v e t o g i v e a d i s t i n c t r e a d i n g on hand-held r a d i a t i o n d e t e c t o r s i n the f i e l d . DISTRIBUTION Because many g e o l o g i s t s a r e not aware t h a t v o l c a n i c a s h p a r t i n g s o c c u r i n c o a l s , i t i s d i f f i c u l t t o i n t e r p r e t the absence o f published reports regarding t h e i r occurrences. The abundance o f such p a r t i n g s i s p r o b a b l y much g r e a t e r than p r e s e n t l y r e c o g n i z e d . The f o l l o w i n g g e n e r a l i t i e s r e g a r d i n g t h e i r d i s t r i b u t i o n a r e m a i n l y l i m i t e d t o N o r t h America and a r e based p r i m a r i l y on o u r own o b s e r v a t i o n s , d i s c u s s i o n s w i t h o t h e r s , and i n t e r p r e t a t i o n o f published r e p o r t s . S p e c i f i c references to recognized v o l c a n i c ash p a r t i n g s i n c o a l s a r e r e l a t i v e l y few and r e s t r i c t e d m o s t l y t o t h e past few y e a r s . I n s i m p l e terms a s h p a r t i n g s a r e r e l a t i v e l y abundant i n C r e t a c e o u s and T e r t i a r y c o a l s o f the West and r a r e i n C a r b o n i f e r o u s c o a l s o f the E a s t . We a r e l e s s c e r t a i n o f t h e G u l f Coast T e r t i a r y l i g n i t e s but they appear t o have a t l e a s t a moderate abundance o f



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such p a r t i n g s . To a degree t h i s apparent d i s t r i b u t i o n i s r e l a t e d t o the f a c t t h a t the v o l c a n i c o r i g i n of the younger ash beds i s more a p p a r e n t , w h i l e o l d e r p a r t i n g s commonly appear as k a o l i n i t i c c l a y beds w i t h l i t t l e e v i d e n c e of t h e i r v o l c a n i c h e r i t a g e (see T r i p l e h o r n , 1976; Bohor and T r i p l e h o r n , 1981). Even so, t h e r e i s no q u e s t i o n t h a t the a b s o l u t e f r e q u e n c y i s h i g h e r i n the West. L o c a l l y the abundance o f p a r t i n g s can be h i g h l y v a r i a b l e . Where many c o a l s are p r e s e n t , ash p a r t i n g s may be d i s t r i b u t e d s p a r s e l y but u n i f o r m l y or c o n c e n t r a t e d i n j u s t a few c o a l s . Figure 2 shows an example i n s o u t h w e s t e r n Washington, i n t h i s case i n c l u d i n g both k a o l i n i t i c and s m e c t i t i c p a r t i n g s (Reinink-Smith, 1982). In the West, where most of our work has been done, v o l c a n i c ash p a r t i n g s are known from numerous c o a l s i n U t a h , New Mexico, C o l o r a d o , Wyoming, Montana and Washington (Bohor, £ t . a l . , 1978; T r i p l e h o r n and Bohor, 1981). A g a i n , the f r e q u e n c y of p a r t i n g s i s h i g h l y v a r i a b l e . I n d i v i d u a l c o a l s i n Montana and C o l o r a d o c o n t a i n up t o twenty or more ash p a r t i n g s , w h i l e the u n u s u a l l y t h i c k Paleocene c o a l s of the Powder R i v e r B a s i n c o n t a i n almost none. In A l a s k a , v o l c a n i c ash p a r t i n g s are p r e s e n t i n a l l o f the C r e t a c e o u s and T e r t i a r y c o a l s we have examined, but appear to be p a r t i c u l a r l y abundant i n the Cook I n l e t a r e a . I n the A p p a l a c h i a n c o a l b a s i n and the E a s t e r n I n t e r i o r B a s i n , where we have l e s s e x p e r i e n c e , ash p a r t i n g s appear t o be r a r e (Bohor and T r i p l e h o r n , 1982). The l a c k of ash p a r t i n g s i n C a r b o n i f e r o u s c o a l s of e a s t e r n N o r t h America c o n t r a s t s w i t h t h e i r abundance i n European c o a l s of the same age. The l a t t e r have been s t u d i e d f o r over a c e n t u r y , a l t h o u g h t h e i r v o l c a n i c o r i g i n was not g e n e r a l l y a c c e p t e d u n t i l the l a s t few decades. Bouroz, et^. a l . (1983) p r o v i d e s a good r e c e n t summary of some of t h i s work i n E n g l i s h . SIGNIFICANCE The g e o l o g i c importance of v o l c a n i c ash p a r t i n g s i n c o a l s has been summarized p r e v i o u s l y by T r i p l e h o r n ( 1 9 7 6 ) . Of g r e a t e s t importance i s t h e i r use i n c o r r e l a t i o n , the p r o c e s s of d e t e r m i n i n g the time r e l a t i o n s h i p s among r o c k s exposed a t d i f f e r e n t l o c a l i t i e s . The s i m p l e s t use i s as marker beds, where i n d i v i d u a l ash f a l l s can be r e c o g n i z e d and d i s t i n g u i s h e d from o t h e r s on the b a s i s of some t e x t u r a l or c o m p o s i t i o n a l a s p e c t . Beds c o n t a i n i n g the same ash l a y e r (whether i n c o a l or any o t h e r r o c k t y p e ) a r e the same age, a l t h o u g h the a b s o l u t e age ( i n y e a r s ) i s not i n d i c a t e d . The a b s o l u t e age can sometimes be d e t e r m i n e d by r a d i o m e t r i c age d a t i n g of c e r t a i n m i n e r a l s i n the a s h e s . P o t a s s i u m - a r g o n d a t i n g i s used f o r such m i n e r a l s as f e l d s p a r and h o r n b l e n d e , w h i l e f i s s i o n - t r a c k d a t i n g may be used f o r z i r c o n and a p a t i t e ( T r i p l e h o r n et_. aj^., 1977; T u r n e r e t . a l . , 1980). I t may be of i n t e r e s t here to note t h a t W i l l i a m s o n (1970) mentioned the h i g h r a d i o a c t i v i t y of c e r t a i n ash p a r t i n g s (he c a l l e d them t o n s t e i n s ) t h a t made them u s e f u l i n b o r e - h o l e s t u d i e s because t h e y appeared as sharp maxima on gamma-ray l o g s of c o a l beds. Such maxima are c o n s p i c u o u s because c o a l s are g e n e r a l l y known f o r t h e i r absence o f r a d i o a c t i v i t y . He a s c r i b e d the h i g h r a d i o a c t i v i t y o f t h e s e p a r t i n g s to an u n u s u a l abundance of z i r c o n s . We have no s p e c i f i c knowledge of t h e s e o c c u r r e n c e s , but suggest t h a t the h i g h


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F i g u r e 1. Alaska.

L i g h t weathering

k a o l i n i t e v o l c a n i c ash i n c o a l ,

southern

F i g u r e 2. M u l t i p l e s m e c t i t i c and k a o l i n i t i c ash p a r t i n g s i n the B i g Seam, s o u t h e a s t e r n Washington.



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radioactivity might instead be related to uranium-bearing phosphate minerals. Thus far geologists have paid l i t t l e attention to the significance of ash partings as indicators of processes and conditions in the coal-forming environment. For example, thin widespread partings lacking penetrating plant material suggest that these originated as ashes that f e l l into shallow standing water. Thick ashes should have affected the kind and amount of vegetation, and indirectly the nature of the coal immediately overlying thick ash beds. As yet there is l i t t l e data in these aspects because geologists have not recognized the potential value of such studies. For those interested in mineral matter in coal, an awareness that some partings may be of volcanic origin may be useful in explaining the distribution of some of these layers and the occurrences of some unusual components, such as strontium, phosphate, or uranium. Volcanic ash partings are likely to be more widespread and uniform in texture, composition and thickness than the more common partings of fluvial origin. They are also more likely to show marked differences from layer to layer, and to contain exotic mineral or chemical components. REFERENCES Bohor, B . F . , R.M. Pollastro, and R.E. Phillips, 1978, Mineralogical evidence for the volcanic origin of kaolinite partings (tonstein) in Upper Cretaceous and Tertiary coals of the Rocky Mountain region (abstr.): The Clay Mineral Soc., 15th Ann. Mtng., 27th Ann. Clay Mineral Conf. Bloomington, Indiana, Program and Abstracts, p. 47. Bohor, B . F . , and Triplehorn, D.M., 1981, Volcanic origin of the Flint Clay parting in the Hazard No. 4 (Fire Clay) coal bed of the Breathitt Formation in Eastern Kentucky: Guidebook, Geol. Soc. Am. Annual Meeting, Coal Div. Field trip, Coal and Coal-Bearing Rocks of Eastern Kentucky, Kentucky Geol. Survey, p. 49-54. Bouroz, Α., D.A. Spears, and F. Arbey, 1983, Review of the formation and evolution of petrographic markers in coal basins: Societe Giologique du Nord, Memoires Tome XVI, 115 pp. Palache, C . , H. Berman, and C. Frondel, 1951, Dana's System of Mineralogy, 7th ed.: Wiley, New York, 1124 pp. Reinink-Smith, L., 1982, The Mineralogy, Geochemistry and Origin of Bentonite Partings in the Eocene Skookumchuck Formation, Centralia Mine, S.W. Washington. Unpublished M.S. Thesis, W. Washington Univ., 1/9 pp. Seiders, V.M., 1965, Volcanic origin of flint clay in the Fire Clay coal bed, Breathitt Formation, Eastern Kentucky; in Geological Survey Research, Chapter D: U.S. Geological Survey Professional Paper 525-D, p. D52-D54.


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Triplehorn, D.M., 1976, Volcanic ash partings in coals: characteristics and stratigraphic significance, The Neogene Society, Spring, 1976, Pacific Section, Society of Economic Paleontologists and Mineralogists, p. 9-12. Triplehorn, D.M., and B.F. Bohor, 1981, Altered volcanic ash partings in the C-coal, Ferron Sandstone member of the Mancos Shale, Emery County, Utah, U.S. Geol. Surv. Open File Report 81-775, 49 pp.


Triplehorn, D.M., and B.F. Bohor, 1983, Goyazite in kaolinitic altered tuff beds of Cretaceous age near Denver, Colorado: Clays and Clay Minerals, 31//4, p. 299-304. Triplehorn, D.M., D.L. Turner, and C.W. Naeser, 1977, K-Ar and fission-track dating of ash partings in Tertiary coals from the Kenai Peninsula, Alaska. A radiometric age for the Homerian-Clamgulchian stage boundary: Geological Society of Am. B u l l . , 88, pp. 1156-1160. Turner, D . L . , D.M. Triplehorn, C.W. Naeser, and V.A. Wolfe, 1980, Radiometric dating of ash partings in Alaskan coal beds and upper Tertiary paleobotanical stages: Geology, 8, p. 92-96. Williamson, I . Α . , 1970, Tonsteins--their nature, origin, and uses: The Mining Magazine, 112, p. 119-125, 200-209. RECEIVED August 26, 1985


Volcanic Ash Layers in Coal - American Chemical Society

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