Deep Coulee deposit

1
Deep Coulee deposit

• Horizontal, sheet-like units no sharp contacts

• 11 meters (35 ft) thick

2
Deep Coulee
Rhythmically bedded silt and fine sand
with sheets of chaotically-oriented gravel
3
Deep Coulee

• No buried soils
• No significant scour and fill or ripples

4
Model for deposition of Deep Coulee sediments

• Alternating beds of silt and sand were deposited
  into a relatively shallow lake

• During lowstands, sheet-floods carried the local
  supply of gravel onto the shallow lake

• Hydrologic and physical controls cause
  semi-regular surges of water into and out of the
  lake

5
Cat Creek deposita delta remnant

• gt24 meters (80 ft) thick

6
Cat Creek

• Well-sorted, imbricate gravels
• Finely laminated sands

• Giant cross-beds throughout
• Dip is 20-30o parallel to the long axis

• No glacial erratics in or on the deposit
• Local intrusives, cherts and limestones

7
Cat Creek deposit

• Post-glacial topographic inversion left the
  valley fill deposit standing out in relief on the
  landscape

• Formed as a valley fill delta triggered by
  downstream damming of Lake Mussel-shell

8
Conclusions and future work

• Both Deep Coulee and Cat Creek are consistent
  with a dynamic, fluctuating lake
• Suggests a spillway under or up against the ice

• There is evidence for at least three stages of
  Lake Musselshell
• Deep Coulee 2,600 feet
• Cat Creek 2,850 feet
• Highest erratics 3,050 feet

• Cosmogenic and bone collagen dates will help
  constrain the ages of the lakes

9
Erosion rates??