Sedimentary Rocks & Environments of Deposition

WHY STUDY SEDIMENTARY ROCKS?

• They record ENVIRONMENTS OF DEPOSITION & are archives for FOSSILS
  • Geologists can reconstruct PALEOGEOGRAPHY (distribution of mountain belts, stable continental interiors [Cratons], & continents themselves) & PALEOCLIMATES from this information

FEATURES OF SEDIMENTARY ROCKS USED TO INTERPRET ENVIRONMENTS OF DEPOSITION

• LITHOLOGY
  • Composition & Compositional Maturity
    • Reflects type & degree of weathering in source area, subsequent transport and degree of reworking for terrigenous sediments (maturity = greater weathering intensity or longer transport path)
      • quartz - increasing quartz content indicates increasing maturity in coarser sediments
      • clay minerals- increasing clay content indicates increasing maturity in finer sediments
      • feldspar & rock fragments - abundant feldspar & rock fragments content indicates immaturity in coarser sediments
  • Texture & Textural Maturity
    • Aspect of rocks including SIZE, SORTING, SHAPE, & ORIENTATION of sedimentary grains that gives clues to CONTINUITY, TURBULENCE & VELOCITY of flow (low energy/high energy; ice/water/wind; continous/episodic flow) at the site of deposition
      • SIZE: larger size = faster velocities (floods [large size] vs. lagoons [small size])
      • SORTING (range of particle sizes): well sorted = small range; poorly sorted = large range
        • wind & beach deposits: well sorted = continuous flow
        • glacial deposits: poorly sorted = glaciers pick up everything
        • alluvial fans: poorly sorted = rapidly deposited, episodic
      • textural maturity reflects relationship between matrix & framework
        • matrix - clay and silt sized grains
        • framework - sand and gravel sized grains
        • increasing sorting & rounding of framework indicates increasing textural maturity
        • matrix present in texturally immature sediments
• SEDIMENTARY STRUCTURES
  • Features formed during or shortly after deposition; extremely useful for deducing environments of deposition
    • 1. MUD (dessication) CRACKS = exposure to air & drying out (subaerial) nonmarine & transitional environments
    • 2. GRADED BEDDING = usually deep-marine (from turbidity currents)
    • 3. CROSS-BEDDING & RIPPLE MARKS = deposition from a current
      • i. symmetrical ripples = waves, tides; asymmetrical ripples = rivers
      • ii. can get paleocurrent directions

ENVIRONMENTS OF DEPOSITION

• NONMARINE (CONTINENTAL) ENVIRONMENTS
  • Fluvial - meandering rivers &. braided streams
    • BRAIDED STREAMS - coarse with horizontally-bedded gravel & cross-bedded sand with little mud
    • MEANDERING RIVERS - channel deposits (point bar) = cross-bedded sand to gravel; levees = silt & fine sand, floodplains = clays; crevasse = fine sand
      • produce elongate sand bodies surrounded by mud
  • Desert
    • SAND DUNES (eolian) = cross-bedded sand
    • PLAYA LAKES = clays & evaporites
    • ALLUVIAL FANS = coarse sediments from braided streams
  • Glacial
    • TILL - unsorted, unstratified drift deposited & in front of the ice (moraines) & under the ice
    • GLACIAL LAKE SEDIMENTS - with seasonal varves (alternating light & dark layers: light = warm w/silt & clay, dark = winter w/clay & organics)
    • OUTWASH - deposited by braided streams from melting glaciers
  • Lacustrine - lakes: fine-grained laminated sediments with freshwater fossils
• TRANSITIONAL ENVIRONMENTS
  • Deltas - river delivers sediment faster than marine processes can redistribute it - prograding; lobe switching
    • DELTA-PLAIN DEPOSITS - uppermost part of delta, many subenvironments, essentially flat lying
    • DELTA-FRONT DEPOSITS - inclined layers of sand & silt
    • PRODELTA DEPOSITS - essentially flat lying silts & clays
    • Exact configuration depends on interplay between river supply, tides & waves
  • Barrier islands - form where tidal ranges are small (<1 m)
    • BEACHES, DUNES, & WASHOVERS = sand, mostly exposed
    • TIDAL "DELTAS" = sand on either side of tidal inlet, mostly submerged
    • LAGOONS = mud, under water
• MARINE ENVIRONMENTS
  • Shallow-marine continental shelves (<200 m)
    • TERRIGENOUS (DETRITAL) SHELVES - sandy inner shelf with large sand waves, muddy outer shelf
    • CARBONATE PLATFORMS
      • barrier reefs & oolite shoals form marginal topographic high
      • lagoons (muddy) & patch reefs lie between barrier & shore
      • tidal flats (muddy) form the shore [sabkhas in arid regions]
  • Deep-marine ocean basins (>200 m)
    • CONTINENTAL MARGINS (slopes & rises) = terrigenous mud & turbidite sand in submarine fans
    • OCEAN BASINS (abyssal plains) = eolian clay, turbidite sand & SiO₂ ooze
    • TOPOGRAPHIC HIGHS (Mid-Ocean Ridge & seamounts) = CaCO₃ ooze

Study Questions

1. How can quartz, clay and feldspar/rock fragment content be used to establish environments of deposition?

2. How can large versus small size be used to establish environments of deposition?

3. How can well sorted versus poorly sorted sediments be used to establish environments of deposition?

4. What is cause & significance of mud cracks, graded bedding & cross-bedding/ripple marks?

5. What are examples of environments of deposition associated with each?

6. What are the major environments of deposition?