GEOL 462: Principles of Sedimentology and Stratigraphy

Lecture 09: Tuesday, February 10, 2009

Topics
1. Fluvial Environments: processes
2. Types of rivers, environments, and deposits
3. Typical river sequences and identifying features
4. Eolian sand dunes in fluvial settings
5. Lake environments: processes, deposits, and sequences

Fluvial Environments: processes
In a fluvial environment, several processes may produce identical features:
a) River channels: currents and bedforms
b) Channel migration: erosional features, vertical sequences
c) Flooding events
d) Soil formation and plants in overbanks, bars, and floodplain, coal formation
e) Evaporation, exposure, etc.
f) Animal traces

Types of rivers, environments, and deposits
• Braided rivers, meandering rivers, and anastomosing rivers have different energy conditions
• Straight rivers may be locally developed, but may be temporarily
• Owing to the different water energy and morphological features, different types of rivers may have varying types of sediment supply and loads
• Braided rivers: lower (shorter) sinuosity (channel length to valley length)
       Occurrences: distal alluvial fans, glacial outwash plains, mountainous river reaches
       Sediment load: abundant sediments
       Energy conditions: rapid large sporadic variations in water discharge
       Vegetation: little vegetation
       Geological occurrences: more common in pre-Devonian sequences before well-developed land plants
• Meandering rivers: confined to single channel, higher sinuosity
       Occurrences: often downstream of braided streams
       Sediment load: less sediment load, less variation in discharge
       Energy conditions:
       1) low flow conditions--maximum velocity at concave outer parts of meander;
       2) high flow conditions--maximum velocity at convex inner parts of meander;
       3) erodes outer meander and deposits at inner edge of meander;
       Vegetation: Sometimes heavy vegetation in flood plains (forming coal beds)
       Geological occurrences: Common through geological history
       Microenvironments: 1) channel - imbricated gravel lag deposits;
       2) point bars - upward fining, plane lamination into dunes into ripples;
       3) natural levees - outer meander wall tops, climbing ripples;
       4) floodbasin and Oxbow-lakes - thin mud in shallow lakes behind levee;
       5) crevasse-splay - graded Bouma-like depsoits.

Typical river sequences and identifying features
• Braided rivers: Generally fining upward sequences, relatively coarse sediments, less floodplain deposits and paleosol
• Meandering rivers:
        1) strong upward fining trends from lateral facies shifts;
        2) hydrologic shifts can also cause reversed grading (less common);
        3) coarse basal channel lag gravels;
        4) parallel -laminated to trough crossbedding (dunes) sands;
        5) ripple cross-laminated sands (including climbing ripples);
        6) alternating sand and mud overbank and levee deposits;
        7) shale, siltstone, coal, roots, paleosol at the top (floodplain and overbank or oxbow lake deposits.

Eolian sand dunes in fluvial settings
• Eolian sand sheets and interdune area (water activities)
• Migration of sand dunes and sand sheets forming vertical sequences
• Vertical sequences: Eolian sand deposits alternate with floodplain and water deposits

Lake environments: processes, deposits, and sequences
•Basic information about lakes:
       Lakes make up 1% of the earth land surface;
       Largest modern lake surface area in the world: Caspian Sea (436,000 sq. km.);
       Deepest (greatest volume) modern lake: Lake Baikal (1700 m depth);
       Origins of lakes:
       1) tectonic motion, faulting and rifting - groundwater flow and springs, e.g., Lake Bonneville, East African rift lakes, Caspian;
       2) glacial processes, e.g., Great Lakes;
       3) landsides and mass movements (smaller ones);
       4) volcanics - ponded flows and caldera, e.g., Yellowstone, Creede;
       5) eolian deflation and interdunes areas;
       6) fluvial migration - oxbow and levee lakes.
• Physical processes:
       Wind, river inflow, atmospheric heating and stratification, surface barometric pressure, gravity, etc.
• Chemical processes:
       Evaporation and precipitation, humidity and runoff water chemistry, weather condition (dry lake or wet lake), etc.
• Biological processes:
       skeletal precipitation, respiration/photosynthesis, bioturbation, plant remains, geomicrobiology, etc.
• Lake depositional system:
       Hydrologically open lakes systems: inflow and precipitation balanced by outflow;
       Hydrologically closed lake systems: no major outflow, evaporation and infiltration exceed inflow;
       Other characteristics:
       a. low wave energy, so all coarse sediments confined to shallows;
       b. 10X greater sedimentation rates than marine environments;
       c. no tides;
       d. climates: varves - dark => cold high OM; light => warmer and higher sed input
       e. Organic matter and CO2 sinks.
• Facies and sequences
       See slides and figures in the Chapter.