| Geol 370 Home | Calendar | Syllabus | Grades | Announcements | Lnks | Geog/Geol Dept | UWSP |

A. Introduction

Deposition location

• supraglacial & subglacial

• ice marginal

• lacustrine

Meltwater landforms

B. Supraglacial Deposition

• inhibited by steep slopes and smooth ice surfaces

• promoted by drops in discharge (e.g. ice-walled lakes)

C. Subglacial Tunnel Deposition

May find:

• stratified sands and gravels

• 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 bed

• high velocities: everything moves and all sizes mix together

Deposition occurs when velocities drop

• slow velocity drop produces sorted, stratified beds

• quick velocity drop may produce non-sorted, non-stratified beds if original velocity was high

D. Ice Marginal Deposition

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

Proximal zone deposits

• controlled by:

  • geometry and position of ice margin

  • availability of sediment, particularly melt-out till

  • flow regime

• flow regime highly variable

  • seasonal: high percentage total sediment deposited in short time period (late spring/early summer)

  • diurnal: greater discharge and sediment during the day

• rapid discharge fluctuations may cause sudden deposition of all sediment with minimal stratification or sorting

• 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

Density stratification

• determines depositional pattern

• compare water inflow density to lake water density

• lake stratification:

  • 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

• iceberg melt-out

• settling of suspended sediment

• resedimentation by gravity flows

Lakes dominated by ice-rafted debris

• much coarser sediment

• wide 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 sediment

• rhythmites: laminations or layers

• suspended sediments: fine layers of silt and clay

• turbidity currents: coarser laminations

• varves: rhythmic sediments that reveal an annual cycle

F. Meltwater Landforms

Morphology affected by:

• 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

• deposition between retreating glacier and backside of outwash fan

• deposition glacier and valley wall

Kames

• deposition in 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 rounded than those in other fluvial deposits, but more rounded than subglacial clasts

• 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 sediment

• ice-core melts, turning former ridges into low spots (kettles)

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

• may form along glacier terminus 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 of varying heights

• poorly sorted sand and gravel core

• well-rounded, sorted sands and gravels above core

G. Summary