| Home | Announcements | Syllabus | Calendar | Lecture Notes | Grades |
Geology 370: Glacial Geology

GLACIAL MELTWATER DEPOSITION & LANDFORMS

A. Introduction

Supra-, en-, and sub-glacial water flow

Supra- and sub-glacial deposition

Ice marginal deposits

Lacustrine deposits

Meltwater landforms

---

B. Supraglacial Deposition

• inhibited by steep slopes and smooth ice surfaces

• promoted by drops in discharge

C. Subglacial Tunnel Deposition

• stratified sands and gravels

• may find dunes, ripples

• characteristics dependent on flow velocity

  • lower velocities: only finer material or upper layer moves

  • moderate velocities: everything moves, but coarser particles stay closer to the bed

  • high velocities: everything moves and all sizes mix together

• deposition occurs when velocities drop

• slow drop in velocity produces sorted, stratified beds

• quick drop in velocity may produce a non-sorted, non-stratified bed if the original velocity was high

---

D. Ice Marginal Deposition

• very cold dense water allows for high sediment loads compared to non-glacial rivers

• water discharge and deposition highly variable

  • seasonal: high percentage of total sediment may be deposited in a very short time period (late spring/early summer)

  • diurnal: greater discharge and sediment during the day

Proximal zone deposits

• controlled by:

  • geometry and position of ice margin

  • availability of sediments, particularly melt-out till

  • seasonal flow regime

• result:

• till sediments redistributed; no stratification or sorting

• rapid discharge fluctuations may result in sudden deposition of all sediments; minimal stratification and sorting of surface sediments only

• difficult to distinguish from melt-out till

Medial zone deposits

• braided river pattern (multiple impermanent bars and channels)

• sediment grain size decreases with distance from the glacier

  Image credit: Mark Emery. US Fish & Wildlife Service Digital Library System http://images.fws.gov/default.cfm?library_id=none&CFID=3302570&CFTOKEN=37370443

Distal zone deposits

much finer grained

more likely single-thread than braided

less temporal variation in flow

---

E. Lacustrine Deposition

• ice marginal lakes; supraglacial lakes

• depositional pattern determined by density stratification

  • compare density of water inflows to lake water density

  • epilimnion: surface layer of less dense (warmer) water

  • hypolimnion: lower layer of denser (colder) water

  • well-developed stratification in summer

  • overturning of water in fall

• deposition from:

-meltwater flows -direct deposition from glacier -iceberg melt-out -settling of suspended sediments

-resedimentation by gravity flows -reworking by currents -shoreline sedimentation -biological sedimentation

• lakes contain two main facies:

• lake marginal

• lake floor

• lakes dominated by ice-rafted debris

• much coarser sediments

• wider range of sediment sizes; diamicton (but not till)

Image credit: ©Bruce Molnia, Terra Photographics. Image source: Earth Science World Image Bank http://www.earthscienceworld.org/images

• lakes dominated by settling of suspended sediments

• rhythmites: laminations or layers

• suspended sediments: fine layers of silt and clay

• turbidity currents or underflow: coarser laminations

• varves: rhythmic sediments that reveal an annual cycle

---

F. Meltwater Landforms

• Factors affecting morphology

  • presence or absence of buried ice

  • when buried ice melts

• Outwash fans: stationary ice margin

USGS topographic map: Arnott, Wisconsin

• Outwash plains
  Image credit: NASA Visible Earth http://visibleearth.nasa.gov/view_rec.php?id=2267

  • merging of outwash fans into a braided river system (sandur)

  • steady ice margin retreat

  • pitted outwash

• Kame terrace - fluvial deposition between:

  • a retreating glacier and the backside of an outwash fan

  • a glacier and a valley wall

• Kames

  • deposition in an ice-walled channel or depression

  • primarily sand and gravel; some stratification

  • possible folding or faulting

  • lacustrine or mass flow deposits may be present

  • clasts less well-rounded than those in other fluvial deposits, but more rounded than subglacial sediments

  • moulin kames

• Kame and kettle topography: mounds and ridges separated by depressions

  • depressions and river channels on and around ice-cored ridges fill in with fluvial sediments

  • ice-cored riges melt, turning into low spots on the landscape (kettles)

  • former depressions and river channels turn into high spots on the landscpae (kames)

  • may form along the terminus of a glacier or on kame terraces

• Ice-walled lake plains

  • flat-topped mounds

  • may have a raised rim formed from till flowing down sides of the lake basin

  • silts found in center of basin

  • may contain rhythmites or varves

• Eskers and crevasse fills

• slightly sinuous ridges with varying heights

• poorly sorted sand and gravel core

• well-rounded, sorted sands and gravels above core

---

G. Summary