Sedimentary Environments: places and conditions under which sediment can accumulate. Often leave particular signatures, including:

• Particular sedimentary structures
• Particular lithologies
• Particular physical relationships between lithologies

In particular, we are concerned about the combinations of facies in a package of sediment. From the Latin for "face" or "appearance", facies (singular is also "facies") are the sets of characteristics (lithologies, sedimentary structures, suites of fossils, etc.) that represent a particular depositional environment. When we talk about "depositional environments", though, we often mean broad regions; in the case of facies, these can be micro-environments of very small spaces (for example, the channel of a small stream; the banks of a lake; etc.)

Paleosols - Soils

• Formed by in situ erosion & weathering (no lateral transport); develop directly on (and from) host rock
• Type determined by climate, biological activity, and host rock composition
• Some important paleoenvironment indicators from paleosols:
  • Caliche and Calcrete: Often indicate seasonally arid environments
  • Laterites & Bauxites: Indicate very wet, generally hot environments
  • Root traces
  • Large animal burrows (e.g., Daemonelix)
• Soils generated in many regions, but are self-destroying.
• So, paleosols are rare

Lacustrine - Lakes

• Opposite of soils: smaller area of Earth surface, but by definition are areas of deposition, so well represented in rock record.
• Tend to have limited extent (although Lake Baikal/Great Lake-sized paleo-lakes can leave huge deposits)
• Sediments become finer towards topological center.
• Low energy, poor oxygenation: bottom sediment generally poorly disturbed, so laminae (very fine layers) common: preserved as shales
• Fine grain size and limited disturbance allows excellent preservation of fossils.
• Lake shales may be interrupted by bursts of bigger grain size ( tempestites or "storm deposits")
• Edges of lake deposits will have bigger grain size (silts & sands), bi-directional ripple marks, mud cracks, trackways, etc.

Paludal - Swamp

• HIGH amount of biological activity, so rocks of high organic content
• Mixed muds & coal/peat/lignite, with some fluvial sediments cutting through
• Often associated with other environments (deltaic, fluvial, coastlines, etc.)
• Low energy

Glacial - Glaciers

• Vary in frequency throughout Earth history: very rare some periods, very common others (i.e., ice ages).
• MASSIVE erosive capability, MASSIVE transport ability.
• Produces till (mixture of all particle sizes, from rock flour to boulders). Till may be organized into moraines or spread out as outwash. If tills become lithified, become a conglomerate called tillite.
• Leaves striations: grooves in bedrock.
• Glacial lakes: contain annual/seasonal varves (alternating sand and larger during melting, clays & organics during cold).
• In ocean sediments or larger lakes, leave dropstones (indicate icebergs)
• Loess - glacially-produced extremely fine grained material transported by wind to another region

Desert environments

• Mostly eolian (wind transport), dominant lithology is sandstone
• Sand dunes advance along lee-ward side, but shifting winds produce trough cross-stratification
• Playas - desert lakes, often with poorly-sorted sediments and evaporites
• Alluvial fans - Often form breccia

To Next Lecture.
To Previous Lecture.
To Syllabus.

Last modified: 26 January 2014