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Eiszeitalter

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Öhringen/Württ.,

31. Oktober

1969

Development and Stagnation of the last Pleistocene icecap in the Yellowstone Lake Basin, Yellowstone National Park, USA ) 1

by GERALD M. RICHMOND, Denver, U . S. Geological S u r v e y W i t h 3 figures A b s t r a c t . In late Pleistocene (Pinedale) time, the last of a series of Pleistocene icecaps developed in the Yellowstone Lake Basin. Initiated by a large glacier that entered the lake basin from the upper valley of the Yellowstone R i v e r , the icecap g r a d u a l l y thickened until, at the glacial maximum, it had a maximum thickness of about 925 m, covered about 6,200 k m , and comprised about 975 k m of ice. The cap overrode the basin divides in a l l directions, including the high crest of the Absaroka Range to the east. 2

3

After the maximum, the ice receded a n d then readvanced before stagnation set in. A sequence of discontinuous käme terraces record the gradual stagnation of the ice from a height of 280 m above the present level of Yellowstone L a k e (2,360 m altitude) down to 33 m above the present lake, at which time the icecap was entirely surrounded by water. An ancestral Yellowstone Lake began to form at a height of 180 m above the present l a k e level northeast of the icecap and opened gradually southwestward into the basin at successively lower levels against the receding ice margin. It became an open body of w a t e r 21 m above present lake level about 11,000 years ago when the ice disappeared entirely. Zusammenfassung. Während des Jungpleistozäns (Pinedale) stieß als letztes Glied einer Serie von Vergletscherungen ein großer Gletscher aus der südlichen Absaroka-Range durch das hochgelegene T a l des Yellowstone R i v e r in nordwestlicher Richtung in das Seebecken vor. Während des Maximalstandes nahm der Gletscher 6200 k m e i n ; seine größte Mächtigkeit betrug etwa 925 m, so d a ß sich die gesamte Eismasse auf etwa 975 k m belaufen haben dürfte. Das Eis überschritt den R a n d des Beckens in allen Richtungen und gelangte in die benachbarten Täler, auf die angrenzenden Plateaus und Gebirgsränder, die ebenfalls Eiskappen aufwiesen. Eine Verzah­ nung des Hauptgletschers mit den Lokalgletschern ist an mehreren Stellen nachzuweisen. 2

3

Nach dem M a x i m u m des Vorstoßes taute der Gletscher teilweise ab, um später erneut vor­ zurücken, jedoch nicht bis zum Maximalstand. Der dann einsetzende Abtauprozeß, der in meh­ reren Phasen erfolgte, ist durch eine Folge von nicht zusammenhängenden Rückzugsbildungen (Kames-Terrassen, See- und Beckenabsätze) zwischen 280 und 33 m über dem jetzigen Seespiegel (2360 m) markiert. In einer Höhe von 180 m begann sich nordöstlich der abtauenden Eismasse zum ersten Mal ein Ur-Yellowstone-See zu bilden, der sich bei ständig sinkendem Wasserspiegel durch den zurückweichenden Eisrand allmählich in südwestlicher Richtung vergrößerte. Vor etwa 11 000 Jahren, als das Eis vollständig verschwand, bildete sich 21 m über dem heutigen Seespiegel zum ersten Mal ein See mit offener Wasserfläche. Introduction Y e l l o w s t o n e N a t i o n a l P a r k , l o c a t e d s o u t h of t h e 4 5 ° p a r a l l e l of l a t i t u d e , m o s t l y i n n o r t h w e s t e r n W y o m i n g , encompasses a b r o a d v o l c a n i c p l a t e a u a t a n a l t i t u d e of a b o u t 2,500 m. To the west, this p l a t e a u descends about 700 m to the S n a k e R i v e r P l a i n . To the n o r t h , east a n d s o u t h t h e p l a t e a u is s u r r o u n d e d b y m o u n t a i n s ( F i g u r e 1) t h a t rise as much as 1,000 m a b o v e i t . T h e p l a t e a u is m a d e u p of l o w e r a n d m i d d l e P l e i s t o c e n e w e l d e d tuffs a n d r h y o l i t e flows ( B O Y D 1 9 6 1 , C H R I S T I A N S E N , O B R A D O V I C H a n d B L A N K 1 9 6 8 ) . S t r u c t u r ­

a l l y , the p l a t e a u lies a t t h e i n t e r s e c t i o n of t h e n o r t h e a s t - t r e n d i n g d o w n w a r p of t h e S n a k e R i v e r P l a i n — u n d e r l a i n b y u p p e r C e n o z o i c v o l c a n i c rocks — w i t h s e v e r a l n o r t h - a n d n o r t h w e s t t r e n d i n g f a u l t block m o u n t a i n s — u n d e r l a i n b y P r e c a m b r i a n c r y s t a l l i n e rocks a n d P a l e z o i c a n d M e s o z o i c s e d i m e n t a r y rocks. In t h e e a s t e r n p a r t of t h e P a r k is t h e A b s a r o k a R a n g e , m a d e u p p r e d o m i n a n t l y of a n d e s i t i c v o l c a n i c sediments a n d l a v a flows of Eocene a n d O l i g o c e n e a g e ( P R O S T K A 1 9 6 8 ) . l ) Publication authorized by the Director, U . S. Geological Survey.


Development and Stagnation of the last Pleistocene icecap

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Fig. 1. Index map showing location of Yellowstone National Park and adjacent features mentioned in text. Y e l l o w s t o n e L a k e B a s i n , in the s o u t h - c e n t r a l p a r t of t h e P a r k , is b o r d e r e d on the south a n d east by the E o c e n e v o l c a n i c r o c k s of the A b s a r o k a R a n g e , a n d is i r r e g u l a r l y filled to the north a n d w e s t b y Q u a t e r n a r y r h y o l i t e tuffs a n d flows. T h e b a s i n contains Y e l l o w s t o n e L a k e , one of the largest b o d i e s of w a t e r in t h e w e s t e r n U n i t e d S t a t e s . Y e l ­ l o w s t o n e R i v e r enters t h e l a k e from t h e s o u t h through a b r o a d c a n y o n w h i c h h e a d s high in t h e A b s a r o k a R a n g e to the southeast, a n d flows n o r t h w a r d out of the l a k e t h r o u g h the f a m o u s G r a n d C a n y o n of the Y e l l o w s t o n e R i v e r , l o c a l l y 4 0 0 m deep. Older P l e i s t o c e n e g l a c i a t i o n s E v i d e n c e of three e a r l y a n d m i d d l e P l e i s t o c e n e g l a c i a t i o n s — from o l d e s t to y o u n g e s t , the W a s h a k i e Point, C e d a r R i d g e , a n d S a c a g a w e a R i d g e G l a c i a t i o n s — h a s been found in s e v e r a l r a n g e s of the R o c k y M o u n t a i n s ( R I C H M O N D 1 9 5 7 , 1 9 6 5 ) . T h e i r t y p e deposits a r e i n t h e W i n d R i v e r M o u n t a i n s , o n l y 1 3 0 k m from Y e l l o w s t o n e N a t i o n a l P a r k ( R I C H ­ MOND 1 9 6 2 , 1 9 6 4 ) . T h e s e g l a c i a t i o n s a r e c o n s i d e r e d to be c o r r e l a t i v e w i t h t h e N e b r a s k a n , K a n s a n , a n d Illinoian G l a c i a t i o n s , r e s p e c t i v e l y , of the U . S . c o n t i n e n t a l i n t e r i o r ( R I C H ­ MOND 1 9 5 7 ) . In Y e l l o w s t o n e P a r k , d e p o s i t s of o n l y one of these g l a c i a t i o n s , b e l i e v e d to represent the youngest o r S a c a g a w e a R i d g e G l a c i a t i o n , h a v e been found. It is possible t h a t t h e l o w e r and m i d d l e Pleistocene r h y o l i t e tufts a n d flows m a y c o n c e a l deposits of the e a r l i e r g l a c i a t i o n s . In e a r l y late P l e i s t o c e n e time, the B u l l L a k e G l a c i a t i o n , of e a r l y W i s c o n s i n a n age, f o r m e d a thicker a n d m o r e extensive i c e c a p in the Y e l l o w s t o n e L a k e b a s i n t h a n t h a t f o r m e d d u r i n g the last o r P i n e d a l e G l a c i a t i o n of late W i s c o n s i n a n a g e , w i t h which this p a p e r is p r i n c i p a l l y c o n c e r n e d . The o l d e s t deposits of the B u l l L a k e G l a c i a t i o n rest on a r h y o l i t e flow that h a s a n a g e of 1 5 0 , 0 0 0 y e a r s B. P. ( P o t a s s i u m - a r g o n d a t i n g b y J . D. O B R A D O V I C H , written c o m m u n . 1 9 6 8 ) . T h e a g e of a second a d v a n c e of t h i s g l a c i a t i o n , as d e t e r m i n e d from the e s t i m a t e d rate of h y d r a t i o n and thickness of h y d r a t i o n r i m s a l o n g f r a c t u r e s in obsidian p e b b l e s from e n d m o r a i n e s , is 8 0 , 0 0 0 ± 2 0 , 0 0 0 y e a r s (IRVING


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Gerald M. Richmond

F R I E D M A N , w r i t t e n c o m m u n . 1 9 6 7 ) . W o o d from recessional l a k e deposits of t h i s g l a c i a t i o n y i e l d s a r a d i o c a r b o n a g e of >42,000 y e a r s B . P. ( W - 2 1 9 7 ) ( M E Y E R R U B I N , w r i t t e n com足 mun. 1968). D e v e l o p m e n t of t h e last P l e i s t o c e n e i c e c a p A b o u t 2 5 , 0 0 0 y e a r s a g o , in l a t e P l e i s t o c e n e t i m e , d u r i n g the P i n e d a l e G l a c i a t i o n of l a t e W i s c o n s i n a n a g e , a c o m p l e x of r e l a t e d icecaps f o r m e d in a n d n e a r Y e l l o w s t o n e N a 足 t i o n a l P a r k : in the Y e l l o w s t o n e L a k e B a s i n itself, in the W i n d R i v e r M o u n t a i n s to the south, in the h i g h p a r t s of the A b s a r o k a R a n g e to the s o u t h e a s t , in the S n o w y R a n g e a n d d r a i n a g e of the L a m a r R i v e r to the n o r t h , on the B e a r t o o t h P l a t e a u to the n o r t h e a s t , a n d on the Pitchstone P l a t e a u in the s o u t h w e s t e r n p a r t of the P a r k . V a l l e y g l a c i e r s formed in n e i g h b o r i n g r a n g e s a b o v e 3,000 m in a l t i t u d e . L a t e P l e i s t o c e n e o r o g r a p h i c s n o w l i n e , t a k e n as t h e m e d i a n a l t i t u d e b e t w e e n t e r m i n a l m o r a i n e s a n d the highest p o i n t s on c i r q u e h e a d w a l l s in the r e g i o n , w a s a b o u t 2,500 m . In the A b s a r o k a R a n g e east of Y e l l o w s t o n e L a k e , s m a l l g l a c i e r s f o r m e d in the cirques a n d b e g a n to a d v a n c e w e s t w a r d d o w n t h e v a l l e y s t o w a r d t h e l a k e . S o u t h e a s t of the l a k e , v a l l e y g l a c i e r s from the h i g h e r p a r t of the A b s a r o k a R a n g e c o a l e s c e d in t h e b r o a d deep u p p e r v a l l e y of t h e Y e l l o w s t o n e R i v e r to form a s i n g l e l a r g e g l a c i e r w h i c h flowed into Y e l l o w s t o n e L a k e B a s i n . T h e l o w e r e n d of this g l a c i e r a p p e a r s to h a v e s e r v e d as a nucleus for the d e v e l o p m e n t of a s e l f n o u r i s h i n g i c e c a p t h a t u l t i m a t e l y filled t h a l a k e basin a n d o v e r f l o w e d its r i m . T h i s conclusion is b a s e d on s e v e r a l l i n e s of e v i d e n c e ( R I C H M O N D a n d PIERCE 1968):

( a ) S o m e t r i b u t a r y v a l l e y s east of the l a k e c o n t a i n e r r a t i c s of i g n e o u s rock d e r i v e d f r o m the h e a d w a t e r s of the Y e l l o w s t o n e R i v e r . (b) The l o w e r r i d g e s a n d v a l l e y s l o p e s of the A b s a r o k a R a n g e a r e not g r o o v e d a n d scoured in a w e s t e r l y d i r e c t i o n as w o u l d be e x p e c t e d if v a l l e y g l a c i e r s h a d flowed west across t h e m . R a t h e r , t h e y a r e o n l y s l i g h t l y modified a n d a r e c o v e r e d b y a t h i n veneer of g l a c i a l rubble, m o s t l y l o c a l l y d e r i v e d . (c) T h e t i l l in m a n y v a l l e y s east a n d south of the l a k e o v e r l i e s g r a v e l or l a k e silt deposits in p r o g l a c i a l l a k e s d a m m e d b y ice in the l a k e b a s i n a n d then o v e r r i d d e n b y it. (d) V e r y f e w e r r a t i c s from the A b s a r o k a R a n g e o c c u r w e s t of the l a k e b a s i n , and no e r r a t i c s from the r h y o l i t i c v o l c a n i c p l a t e a u west of the b a s i n occur on its e a s t e r n slopes. (e) F r a g m e n t s of silicified l a k e silt a n d sand of B u l l L a k e a g e , d e r i v e d from deposits b e n e a t h P i n e d a l e T i l l a l o n g the n o r t h shore of the l a k e , a r e a b u n d a n t in P i n e d a l e T i l l to the n o r t h . G r a d u a l l y the ice in the l a k e b a s i n t h i c k e n e d a n d w h e n the u p p e r v a l l e y of the Y e l 足 l o w s t o n e R i v e r h a d been filled to a d e p t h of n e a r l y 8 0 0 m the ice o v e r f l o w e d w e s t w a r d across the T w o O c e a n P l a t e a u ( F i g u r e 2 ) . H e r e it s c o u r e d s m a l l l a k e b a s i n s e l o n g a t e p a r a l l e l to the d i r e c t i o n of ice m o v e m e n t , r o u n d e d off t h e crests of c i r q u e s m a r g i n a l to the p l a t e a u , a n d d e p o s i t e d a thin g l a c i a l r u b b l e c o n t a i n i n g e r r a t i c s of b a s a l t , d e r i v e d from the A b s a r o k a R a n g e to the east, on t h e p l a t e a u surface. W e s t of the T w o O c e a n P l a t e a u the ice flowed south i n t o the d r a i n a g e of the S n a k e R i v e r , w h e r e it w a s 3 0 0 to 325 m thick a n d its e r o s i v e effects, in the f o r m of s t r e a m i n g , g r o o v i n g , a n d m o l d i n g of the l a n d s 足 cape, are very m a r k e d . A l o n g the east side of Y e l l o w s t o n e L a k e , the i c e c a p u l t i m a t e l y a t t a i n e d a thickness of a b o u t 925 m a n d o v e r f l o w e d e a s t w a r d across the crest of the A b s a r o k a R a n g e . M a n y s u m m i t s w e r e s h a p e d b y the ice i n t o l a r g e roches m o u t o n e e s o r i e n t e d to east, a n d the n o r m a l l y s h a r p crests of c i r q u e h e a d w a l l s w e r e r o u n d e d b y the e a s t w a r d o v e r r i d i n g ice. A few e r r a t i c s of r h y o l i t e tuff, from t h e l o w e r west slopes of the r a n g e , o c c u r on a n d east of the crest.


Development and Stagnation of the last Pleistocene icecap

Fig. 2 . M a p showing m a x i m u m extent, inferred surface contours and areas projecting surface of last Pleistocene ice c a p in the Yellowstone Lake Basin.

199

above

N o r t h e a s t of the l a k e b a s i n , about 6 t o 8 k m north of t h e b a s i n rim, the i c e c a p c a m e in c o n t a c t w i t h the L a m a r G l a c i e r ( F i g u r e 2 ) . F r o m here t h e b o u n d a r y of t h e i c e c a p e x t e n d e d n o r t h w e s t w a r d , a c r o s s the G r a n d C a n y o n of the Y e l l o w s t o n e R i v e r , a n d a l o n g the south f l a n k of the W a s h b u r n R a n g e . I n t h e s e a r e a s the ice w a s about 150 m thick. W e s t of the l a k e b a s i n , t h e ice o v e r f l o w e d the crest of t h e C e n t r a l P l a t e a u , w h e r e it p r o b a b l y w a s o n l y a b o u t 1 0 0 m thick, c r o s s e d the geyser b a s i n s a l o n g the F i r e h o l e R i v e r to the w e s t a n d o v e r l a p p e d t h e m a r g i n of t h e M a d i s o n P l a t e a u . S o u t h w e s t of the l a k e b a s i n , the ice o v e r f l o w e d the d r a i n a g e d i v i d e a n d filled the basins of S h o s h o n e a n d L e w i s L a k e s to a d e p t h of about 3 0 0 m . F r o m S h o s h o n e L a k e the ice e x t e n d e d southwest a c r o s s the s a d d l e s e p a r a t i n g the P i t c h s t o n e a n d M a d i s o n P l a t e a u s a n d d o w n t h e d r a i n a g e of t h e Bechler R i v e r , w h e r e it w a s c o n s i d e r a b l y a u g m e n t e d b y ice from a l o c a l c a p on the Pitchstone P l a t e a u . F r o m L e w i s L a k e , the ice f l o w e d s o u t h a n d j o i n e d a l a r g e r mass w h i c h e x t e n d e d s o u t h w e s t from the Y e l l o w s t o n e L a k e b a s i n across h i g h m o u n t a i n s in the s o u t h - c e n t r a l p a r t of t h e P a r k a n d d o w n the d r a i n a g e of t h e S n a k e R i v e r . S p e c t a c u l a r g l a c i a l g r o o v e s occur o n s e v e r a l of the s u m m i t r i d g e s . In d e e p v a l l e y s s e p a r a t i n g t h e high m o u n t a i n s the ice w a s o v e r 5 2 0 m thick, b u t in the v a l l e y of t h e S n a k e R i v e r a t t h e P a r k b o u n d a r y , it w a s o n l y a b o u t 300 m thick. P a r t of this ice flowed south


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Gerald M. Richmond

a n d j o i n e d g l a c i e r s f r o m the Teton R a n g e to form a l a r g e p i e d m o n t g l a c i e r in the b a s i n of J a c k s o n L a k e ; a n o t h e r p a r t flowed w e s t , along the s o u t h side of the Pitchstone P l a t e a u , to j o i n the t o n g u e f l o w i n g a r o u n d t h e n o r t h side of t h e p l a t e a u a n d t e r m i n a t e in a b r o a d i r r e g u l a r a r c just s o u t h w e s t of the P a r k . This ice w a s a l s o c o n s i d e r a b l y a u g m e n t e d b y ice f r o m the local c a p on t h e Pitchstone P l a t e a u . A t the g l a c i a l m a x i m u m , the Y e l l o w s t o n e L a k e 6 , 2 0 0 k m ( F i g u r e 2 ) a n d c o m p r i s e d a b o u t 975 k m i n c l u d e p e a k s a b o v e 3 , 0 0 0 m a l o n g t h e crest of the b e t w e e n 2,500 a n d 3 , 0 0 0 m in the s o u t h e r n p a r t b o u n d a r y . S o m e h i g h l a n d s at a b o u t 2 , 7 0 0 m in the a l s o projected a b o v e t h e ice. 2

3

B a s i n i c e c a p c o v e r e d a n a r e a of a b o u t of i c e . A r e a s p r o j e c t i n g a b o v e the ice A b s a r o k a R a n g e a n d several summits of t h e P a r k a n d s o u t h of the P a r k n o r t h e r n a n d w e s t e r n p a r t s of the c a p

S t a g n a t i o n of t h e P i n e d a l e i c e c a p F o l l o w i n g t h e P i n e d a l e g l a c i a l m a x i m u m , t h e r e b e g a n a c o m p l e x h i s t o r y of d o w n w a s t i n g , r e a d v a n c e , a n d final s t a g n a t i o n of the i c e c a p ( R I C H M O N D 1 9 6 8 a ) . A t first, t h e t h i n w e s t e r n p a r t of t h e c a p d o w n w a s t e d a n d , as u p l a n d s w e r e i n c r e a s i n g l y exposed, t h e ice s e p a r a t e d i n t o s e v e r a l s t a g n a n t m a s s e s a r o u n d w h i c h e x t e n s i v e i c e - c o n t a c t fluvioglacial s a n d a n d g r a v e l w a s d e p o s i t e d . A t t h i s t i m e , the a c t i v e ice front w a s a t l e a s t 5 to 25 k m f r o m its former m a x i m u m p o s i t i o n . T h i s e a r l y r e c e s s i o n w a s f o l l o w e d b y a series of m i n o r a d v a n c e s d u r i n g which d i s c o n t i n u o u s end m o r a i n e s , o u t w a s h g r a v e l p l a i n s , a n d t h i c k e m b a n k m e n t s of h u m m o c k y t i l l w e r e deposited. T h e m a x i m u m e x t e n t of these se­ c o n d a r y r e a d v a n c e s is s h o w n in p a r t in F i g u r e 3. T o t h e n o r t h , the ice w a s still in c o n t a c t w i t h the L a m a r G l a c i e r ; to the n o r t h w e s t it t e r m i n a t e d i r r e g u l a r l y a l o n g the crest of t h e C e n t r a l P l a t e a u ; to t h e southwest, it l a y in the d r a i n a g e of the Bechler R i v e r about 10 k m f r o m its former s t a n d . T o the south, it w a s still in c o n t a c t w i t h p i e d m o n t ice from t h e T e t o n R a n g e in the b a s i n of J a c k s o n L a k e . Subsequent d o w n w a s t i n g a n d s t a g n a t i o n of the c a p , t h o u g h c o n t i n u o u s , m a y be d i v i ­ d e d into four p h a s e s on the basis of t h e a l t i t u d e s of d r a i n a g e outlets l e a d i n g a w a y f r o m t h e ice, a n d the a l t i t u d e a n d c h a r a c t e r of i c e - m a r g i n a l f l u v i o g l a c i a l d e p o s i t s . For c o n ­ v e n i e n c e , a l t i t u d e s a r e expressed in t e r m s of height a b o v e the present e l e v a t i o n of Y e l l o w ­ stone L a k e ( 2 , 3 6 0 m ) . F e a t u r e s a s s o c i a t e d w i t h each p h a s e a r e s h o w n o n F i g u r e 3. P h a s e 1. — A s the c a p d o w n w a s t e d from its s e c o n d a r y a d v a n c e , u p l a n d s i n c r e a ­ s i n g l y projected t h r o u g h the ice a n d t h e Y e l l o w s t o n e L a k e B a s i n d i v i d e ( F i g u r e 3) b e g a n to be exposed. E m e r g e n c e of the h i g h T w o O c e a n P l a t e a u c a u s e d s t a g n a t i o n of ice to t h e s o u t h w e s t . I c e m a r g i n a l f l u v i o g l a c i a l deposits in passes l e a d i n g across t h e p l a t e a u s h o w t h a t m e l t w a t e r s f r o m thick a c t i v e ice i n the upper v a l l e y of the Y e l l o w s t o n e R i v e r f l o w e d w e s t t h r o u g h t h e m o n t o s t a g n a n t i c e . These passes, w h i c h r a n g e in a l t i t u d e from 5 7 0 m to 4 0 0 m a b o v e p r e s e n t Y e l l o w s t o n e L a k e , p r o b a b l y s e r v e d as o u t l e t s for o n l y a short time. P h a s e 2. — After the ice h a d w a s t e d b e l o w t h e l e v e l of the h i g h passes across t h e T w o O c e a n P l a t e a u most of the d r a i n a g e w a s p r o b a b l y on the ice itself u n t i l l o w e r s e c ­ t o r s of the Y e l l o w s t o n e L a k e B a s i n d i v i d e b e c a m e e x p o s e d a n d the i c e c a p w a s l a r g e l y confined to the b a s i n itself. D r a i n a g e o u t l e t s across t h e d i v i d e b e c a m e n u m e r o u s a n d i c e c o n t a c t f l u v i o g l a c i a l deposits f o r m e d e x t e n s i v e l y on t h e u p p e r slopes of the basin. T o t h e east, v a l l e y g l a c i e r s in the A b s a r o k a R a n g e s e p a r a t e d from the c a p a n d f l u v i o g l a c i a l d e p o s i t s f o r m e d in t h e i n t e r v e n i n g v a l l e y sectors a g a i n s t s t a g n a n t ice d o w n s t r e a m (Fig. 3 ) . T h e highest of these deposits a r e f r o m 2 8 0 m to 190 m a b o v e present Y e l l o w s t o n e L a k e , a n d m e l t w a t e r s f l o w e d east t h r o u g h t h e lowest pass a c r o s s the r a n g e ( F i g u r e 3 ) . N o r t h ­ e a s t of the l a k e b a s i n , m e l t w a t e r s d r a i n e d north o n t o t h e L a m a r G l a c i e r t h r o u g h passes 2 6 0 m, 2 1 7 m, a n d 1 8 0 m a b o v e p r e s e n t l a k e level- E x t e n s i v e k ä m e t e r r a c e deposits f o r m e d


Development and Stagnation of the last Pleistocene icecap

201

in v a l l e y r e - e n t r a n t s , a n d l a m i n a t e d silts w e r e deposited in a n i c e m a r g i n a l l a k e , a n c e s t r a l to Y e l l o w s t o n e L a k e , t h a t d r a i n e d n o r t h t h r o u g h the pass 1 8 0 m a b o v e p r e s e n t l a k e l e v e l . T h e a r e a of the G r a n d C a n y o n of the Y e l l o w s t o n e R i v e r w a s s t i l l c o v e r e d w i t h ice. W e s t of the l a k e basin, i c e - c o n t a c t deposits a l o n g the l a k e b a s i n d i v i d e l e a d w e s t t h r o u g h o u t l e t s r a n g i n g from 2 2 5 m t o 140 m a b o v e p r e s e n t l a k e l e v e l . O n e o u t l e t l e a d s s o u t h w e s t to k ä m e t e r r a c e s a n d l a c u s t r i n e deposits f o r m e d a g a i n s t s t a g n a n t ice in t h e b a s i n s of S h o s h o n e a n d L e w i s L a k e s . A l o n g the d i v i d e south of Y e l l o w s t o n e L a k e , a b r o a d h i g h k ä m e 2 2 5 m a b o v e the p r e s e n t l a k e l e v e l m a r k s a n i c e - e n c l o s e d d r a i n a g e l e a d i n g south across t h e d i v i d e onto s t a g n a n t ice in the b a s i n of H e a r t L a k e a n d the v a l l e y of the S n a k e R i v e r . I n the upper v a l l e y of the Y e l l o w s t o n e R i v e r , k ä m e t e r r a c e s w e r e f o r m e d a l o n g

•^Z DRAINAGE OUTLET. NUMBER REFERS TO PHASE OCCUPIED *»• ICE-CONTACT DEPOSITS OF PHASES i c e c o n t a c t d e p o s i t s o f phase 3 gl

l a c u s t r a l dep05it5.number r e f e r s t o phase depos i t e d area

above or b e y o n d

maximum

of

re advance

Fig. 3. M a p showing m a x i m u m extent of secondary readvance of last Pleistocene icecap, and deposits and drainage outlets of its stagnation phases.


202

Gerald M. Richmond

the v a l l e y w a l l s a n d in t r i b u t a r y r e - e n t r a n t s b e l o w a n u p p e r l i m i t of 1 9 0 m a b o v e present Y e l l o w s t o n e L a k e . M e l t w a t e r s d r a i n e d w e s t t h r o u g h the p a s s south of T w o O c e a n P l a t e a u ( F i g u r e 3) u n t i l the ice w a s t e d b e l o w this t h r e s h o l d , 115 m a b o v e p r e s e n t l a k e level. P h a s e 3. — F u r t h e r d o w n w a s t i n g of the ice c a u s e d s u c c e s s i v e l y l o w e r k ä m e t e r ­ r a c e s to be d e p o s i t e d a r o u n d the n o w t o t a l l y s t a g n a n t ice m a s s in the b a s i n . D r a i n a g e to the east, west, a n d o n t o the L a m a r G l a c i e r to the n o r t h h a d ceased. T h e last d r a i n a g e a c r o s s the d i v i d e to t h e south into the b a s i n of H e a r t L a k e w a s 76 m a n d t h e last into t h e b a s i n of L e w i s L a k e w a s 6 7 m a b o v e p r e s e n t Y e l l o w s t o n e L a k e . T h e o l d e s t m e l t w a t e r channel l e a d i n g i n t o the G r a n d C a n y o n of the Y e l l o w s t o n e R i v e r , w h i c h p r o b a b l y w a s still filled w i t h ice, w a s 82 m a b o v e p r e s e n t Y e l l o w s t o n e L a k e . A n c e s t r a l Y e l l o w s t o n e L a k e c o n t i n u e d to e n l a r g e s o u t h w e s t w a r d a t g r a d u a l l y l o w e r l e v e l s a g a i n s t the s h r i n k i n g ice m a s s . Thick d e p o s i t s of l a m i n a t e d silt b e t w e e n 115 m a n d 33 m a b o v e present l a k e l e v e l o u t l i n e the e x t e n t of the l a k e d u r i n g this p h a s e . S i m i l a r deposits b e t w e e n 75 m a n d 3 3 m a b o v e p r e s e n t l a k e l e v e l o c c u r in v a l l e y r e - e n t r a n t s a l o n g the east side of the l a k e b a s i n . T h e absence of s i m i l a r l a c u s t r a l deposits on the w e s t side of the b a s i n suggests t h a t the ice l i n g e r e d a g a i n s t t h a t slope in the shelter of the C e n t r a l P l a t e a u . P h a s e 4. — T h e b e g i n n i n g of p h a s e 4 is m a r k e d b y a p r o m i n e n t t e r r a c e , 33 m a b o v e p r e s e n t l a k e l e v e l t h a t e x t e n d s e s s e n t i a l l y a r o u n d the e n t i r e l a k e b a s i n . A n i c e - m a r g i n a l m e l t w a t e r channel l e a d s i n t o the G r a n d C a n y o n ( F i g u r e 3) a t the same h e i g h t . This t e r r a c e s h o w s t h a t the r e m n a n t ice mass w a s e n t i r e l y s u r r o u n d e d b y at least a n a r r o w b a n d of w a t e r a t this l e v e l . S i m i l a r b u t d i s c o n t i n u o u s t e r r a c e s o c c u r a t 28 m a n d 24 m a b o v e p r e s e n t l a k e l e v e l . These t e r r a c e s a r e l o c a l l y k ä m e t e r r a c e s , m a d e u p of p o o r l y sorted s a n d a n d silt, c o m m o n l y c o v e r e d b y b o u l d e r y s l u m p e d t i l l o r solifluction d e b r i s a n d l o c a l l y c h a r a c t e r i z e d b y d e a d ice depressions. E l s e w h e r e t h e y a r e c a p p e d b y beach deposits or l a c u s t r a l silt f o r m e d in i c e - m a r g i n a l l a k e s . In a few p l a c e s t h e y a r e cut on rock. In contrast a r e the c h a r a c t e r i s t i c s of a w i d e s p r e a d t e r r a c e a b o u t 2 1 m a b o v e present l a k e level t h a t m a r k s the end of p h a s e 4. Its deposits ( F i g u r e 3) a r e c o m m o n l y w e l l - s o r t e d s a n d a n d g r a v e l . In p l a c e s , these d e p o s i t s a r e thin, a r e c h a r a c t e r i z e d b y t y p i c a l beach c r o s s b e d d i n g , a n d rest on a bench cut i n t o o l d e r fluvioglacial or l a c u s t r a l d e p o s i t s , or rock. E l s e w h e r e , t h e y f o r m l a r g e spits a n d b a r s , c h a r a c t e r i z e d b y s t e e p l y d i p p i n g foreset b e d ­ d i n g , t h a t c o u l d o n l y h a v e f o r m e d in a l a r g e b o d y of o p e n w a t e r subject to p r e v a i l i n g w e s t e r l y w i n d s a n d s t r o n g l o n g s h o r e c u r r e n t s . A l t h o u g h s m a l l d e a d ice depressions occur in deposits of this t e r r a c e a t the south e n d of the l a k e , the d e v e l o p m e n t of t h e t e r r a c e o n i s l a n d s in the l a k e a n d t h e w i d e s p r e a d d i s t r i b u t i o n of the spits a n d b a r s c l e a r l y show t h a t t h e i c e c a p no l o n g e r e x i s t e d . F i n a l d i s a p p e a r a n c e of t h e ice is b e l i e v e d to h a v e been between 10,000 a n d 12,000 y e a r s ago. If the P l e i s t o c e n e e n d e d a b o u t 1 1 , 0 0 0 y e a r s a g o , the final d i s a p p e a r a n c e of the i c e c a p f r o m the Y e l l o w s t o n e L a k e B a s i n a n d the c o m p l e t e o p e n i n g of Y e l l o w s t o n e L a k e a t a l e v e l 2 1 m h i g h e r t h a n a t present p r o b a b l y c o i n c i d e d w i t h the b e g i n n i n g of H o l o c e n e t i m e . T h e l a k e a t the 2 1 m l e v e l m a y h a v e been d a m m e d a t first b y ice c o n t a c t deposits a n d s t a g n a n t ice r e m n a n t s in the v a l l e y of the Y e l l o w s t o n e R i v e r n o r t h of t h e l a k e . S u b ­ s e q u e n t l o w e r i n g of the l a k e a n d the d e v e l o p m e n t of l o w e r l a k e t e r r a c e s a t 14, 10, 8, 5 a n d 1.5 m a b o v e its p r e s e n t l a v e l h a v e been c o n t r o l l e d b y erosion of h a r d l a k e silts of B u l l L a k e a g e a t the o u t l e t . H o l o c e n e g l a c i a t i o n h a s been confined to c i r q u e s a n d v a l l e y s in the h i g h e r m o u n t a i n s , m a i n l y east of the l a k e . T h r e e g r o u p s of a d v a n c e s a r e r e c o r d e d ( R I C H M O N D 1 9 6 8 b ) . M o r a i n e s of the o l d e s t a d v a n c e s ( c u r r e n t l y classified a s l a t e P i n e d a l e ) b e t w e e n 9,000 a n d 7 , 5 0 0 y e a r s a g o reach a n a v e r a g e l o w e r a l t i t u d e of a b o u t 2 , 6 0 0 m. T h o s e of the n e x t y o u n g e s t g r o u p of a d v a n c e s ( T e m p l e L a k e S t a d e of N e o g l a c i a t i o n ) , a b o u t 3,700 to 9 5 0


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y e a r s a g o , reach an a v e r a g e l o w e r l i m i t of a b o u t 2,875 m , a n d those of t h e y o u n g e s t g r o u p of a d v a n c e s ( G a n n e t t P e a k S t a d e of N e o g l a c i a t i o n ) , a b o u t 4 0 0 y e a r s a g o to the present, r e a c h a n a v e r a g e l o w e r l i m i t of a b o u t 2 , 9 7 5 m .

References BOYD, F . R.: Welded tuffs and flows in the rhyolite plateau of Yellowstone Park, Wyoming. — Geol. Soc. Amer. Bull., 72, 387—426, 1961. CHRISTIANSEN, R. L . , OBRADOVICH, J . D., and BLANK, H . R., J r . : Late Cenozoic volcanic strati­ graphy of the Yellowstone Park region — a preliminary report (Abstract). — Geol. Soc. Amer., Rocky Mtn. Section, Program, 21st Ann. Mtg., 29—30, 1968. PROSTKA, H . J . : Facies of Eocene volcanics in northeastern Yellowstone National P a r k and their relations to eruptive centers (Abstract). — Geol. Soc. Amer., Rocky Mtn. Section, Program, 21st Ann. Mtg., 62—63, 1968. RICHMOND, G. M.: Three pre-Wisconsin glacial stages in the Rocky Mountain region. — Geol. Soc. Amer. Bull., 68, 239—262, 1957. — : Three pre-Bull Lake tills in the Wind R i v e r Mountains, Wyoming. — U. S. Geol. Surv. Prof. Paper 450-D, D132—D136, 1962. — : Three pre-Bull Lake tills in the Wind R i v e r Mountains, Wyoming-a reinterpretation. — U. S. Geol. Survey Prof. Paper 501-D, D104—D109, 1964. — : Glaciation of the Rocky Mountains, in The Quaternary of the United States, H . E. WRIGHT and D. G. FREY, editiors, Princeton Univ. Press, 217—230, 1965. — : Stagnation of the Pinedale icecap in the Yellowstone Lake Basin, Wyoming (Abstract). — Geol. Soc. Amer., Rocky Mtn. Section, Program, 21st Ann. Mtg., p. 65, 1968 [ 1 9 6 8 a ] . — : Late Pinedale and Neoglacial Glaciation in the Absaroka Range, east of Yellowstone Lake, Wyoming (Abstract). — Geol. Soc. Amer., Rocky Mtn. Section, Program, 21st Ann. Mtg., 65—66, 1968 [1968b]. RICHMOND, G. M., and PIERCE, K. L.: Late Pleistocene center of ice-accumulation over Yellow­ stone Lake area. In: Abstracts for 1967, Geol. Soc. Amer. Spec. Paper 1 1 5 , p. 4 4 1 , 1968. Manuscript came in 2. 10. 1969. Address of the author: Dr. G. M. Richmond, U . S. Geological Survey, Denver, Colorado 80225.

Quaternary Science Journal - Development and stagnation of the last Pleistocene icecap in the Y...  

Während des Jungpleistozäns (Pinedale) stieß als letztes Glied einer Serie von Vergletscherungen ein großer Gletscher aus der südlichen Absa...

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