1. THE MISSOULA FLOODS Sierra Turpen

2. Abstract Were the Missoula floods a single event or a series of multiple flood events?  Multiple flood events: Richard Waitt (1983) “Periodic Jökulhlaups from Pleistocene Glacial Lake Missoula—Evidence from Varved Sediment in Northern Idaho and Washington”  A single flood event: John Shaw (1999) “The Channeled Scabland: Back to Bretz?”

4. Brief History The Clark Fork river was damned by an ice dam. As water from the Rockies and melt water from the glacier accumulated a large lake formed to the east of the dam. The Purcell Trench lobe dammed the river creating glacial Lake Missoula. Once the pressure was too strong the water discharged beneath and through the dam. The water flowed south east through Washington and Oregon towards the ocean.

5. Periodic Jökulhlaups from Pleistocene Glacial Lake Missoula— Evidence from Varved Sediment in Northern Idaho and Washington –Richard B. Waitt, Jr.  Series of brief jökulhlaups seperated by quiet glaciolaucustrine and subaerial conditions  Occurred in the Late Wisconsin period  Glacial Priest lake was dammed by Purcell trench lobe. Varved muds here are separated by 14 sand beds  Sand beds are similar to slack water sediment in southern Washington  Varved muds are also found in glacial lake Spokane, in Latah Creek valley and other areas of northeastern Washington  20-55 silt-to-clay varves  Lacustrine environments persisted in the area for 2-6 decades between glacial floods

6. Introduction  40 rhythmites each individually represent a separate flood event  Graded beds repeats a sedimentary motif  Varved lacustrine beds separate coarse back flood sediment which shows that reoccurring glacial outbursts were decades apart.

8. Purcell Trench Lobe and Priest River Valley  A moraine was built by a distributary of the Purcell Trench lobe  Glacial priest Lake was created in the lower Priest River valley on top of beds of varved silt and clay.  1.5 km up valley of the moraine has beds of varved silts and clay punctuated by beds of fine sand spaced at regular intervals of 20-50 cm. Above the sand bed mud beds consists of 20-50 varves each 1-4 cm thick  Varves punctuated by 14 laterally fine-sand beds  These have variable sedimentary structures; ripple-drift laminae near base or middle of most beds with up valley dips

9. An example of graded sand bed in the Priest valley. Ripple-drift are up valley to the left

12. Latah Creek Valley: lower valley  A Spokane River tributary south of Spokane  16 thick graded beds of gravel and sand dipping south  Foreset beds are nonbasaltic crystalline-tock toped with a layer of mud 0-20cm thick with~55 regular “clayey” varves  Sand and gravel beds 1-4m thick containing basaltic, sandstone and siltstone boulders s large as 90 cm in diameter

13. Latah Creek Interpretation  Usual sedimentation in the late Wisconsin lake Spokane was quiet and distant from sediment sources  Interrupted at least 16 times by a violent influx of coarse sediment  After each event there was a quiet glaciolacustrine sedimentation for several years  28 estimated separate flood events in this area  Of which 16 are topped by varved-clay beds

14.  Pictures

15. Spokane Drainage Basin North of Spokane  Valleys veneered by gravel and sand  Foreset beds dip west, southwest or south  Deposits contain huge boulders associated with giant current dunes or huge bar-like forms  High velocity discharge event  Most gavel beds overlie each other and have no intervening fine sediment  As many as four are separated by intervening clay-silt beds (some have 35 varves)  Each bed of varved clay represents decades of quiet followed by a separate flood event

16. Conclusion  There were at least 40 flood events that repeated regularly  It takes weeks or months for clay particles to settle and accumulate centimeters thick  The Priest River section evidenced glaciolacustrine with carved silt and clay layers 50-100cm thick  Glacial Lake Missoula drained as a catastrophic jökulhlaup every few decades

17. The Channeled Scabland: Back to Bretz? –John Shaw  The channeled Scabland is the area of Washington State eroded by the drainage of glacial Lake Missoula  Proximal and distal rhythmic beds are explained from multiple pulses within a single flood  Drainage from the cordilleran trunk valleys was most likely the cause of the pulses

18. 1. Ninemile Creek 2. Sage Trig 3. Starbuck 4. Burlingame Canyon

19. Ninemile Creek Section  Rhythmic sequences of silt overlain by clay.  (Waitt interpreted as lacustrine sedimentation between drainage events)  Thick silt bed represents rapid sedimentation. Turbidity currents from jökulhlaups created these deposits  You would see normal varve sedimentation  The clay above the silt represents high sediment input during the jökulhlaups.

20. Varved lake sediment. This is what you would expect near Glacial Lake Missoula. Each silt bed is ~30 cm thick

22. Sage Trig Section  Sand at the base of rhythmic beds  Waitt interpreted as back-flooding deposits with 15 suggested events  Basaltic clasts are absent  Powerful water flows coming from the north were responsible for sand and gavel cross-beds, diapiric injections of silts and clay rip-up clasts, and soft- sediment-deformation structures at Sage Trig.

24. Starbuck Section  Three rhythmic sequences fine and thin upward (2m at the base to ~20cm near the top)  Unit A: poorly sorted angular to subangular, mostly basaltic cobbles with finer components  Unit B: moderately sorted, granular cross-bedding with multiple normal and reverse graded cross-strata  Unit C: thin (<40cm) parallel-bedded fine sand and silt. Suspension and bedload transported  Clastic dikes were observed cross cutting the rhythmites

25. Burlingame Canyon Section  60m of rhythmically bedded sand and silt  Waitt interpreted as flood surge deposits  Each bed represents a separate flood event followed by decades of exposure  Each of the ~40 rhythmic sequences is ~10cm to 2m thick  Unit A: plane-bedded coarse sand and granules  Unit B: fining-upward ripple drift cross-lamination with low-angle climbing ripples at the base  Unit C: massive silt, argued that the climbing tipple drift accreted within hours  Subaerial exposures following deposition: Mt. St. Helen’s ash. The ash was deposited from a water column subsequent to air fall  Dikes suggest pore-water pressure increased in depth and the cross cutting occurred after the formation of the sequence

26.  Pictures Clastic dike cuts through rhythmites at Burlingame Canyon. The dike is composed of silts and darker, coarse sand.

27. Conclusions  Deposits at the Ninemile section represent hundreds of years of normal lake sedimentation in Glacial Lake Missoula and the thick silt beds record episodic jökulhlaups from upstream glaciers  The other three locations are explained by sedimentation in backwater deposits in a single flood event with several meltwater pulses  Its estimated that the volume of water held in the reservoir is 10^5 km^3 and would have a discharge period of about 100 days

28. Comparison  They both looked at different sections of the Scablands  Time period:  Shaw noted that the discharge period would be about 100 days  Waitt noted that clay layers take weeks to months to accumulate and there were layers 50-100cm thick  Sedimentation represents melt water pulses or seperate flood events

29. Were the Missoula Floods a single flood event or a series of floods?

30. Sources:  http://www.iafi.org/images/floodsmap_lg.jpg http://www.iafi.org/images/floodsmap_lg.jpg  Waitt, Richard B., Jr. “Periodic Jökulhlaups from Pleistocene Glacial Lake Missoula – New Evidence from Varved Sediment in Northern Idaho and Washington.” Quaternary Research (1984): 46-57.  Shaw, John; Mandy Munro-Stasiuk; Brian Sawyer; Claire Beaney; Jerome-Etienne Lesemann; Alberto Musacchio; Bruce Rains; Robert R. Young. “The Channeled Scabland: Back to Bretz?” GEOLOGY (July, 1999): 605-608