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

SEDIMENTARY ROCKS

• Form by lithification of sediments
  • Lithification = Compaction and Cementation
    • COMPACTION - IMPORTANT ONLY IN FORMATION OF SHALE (MUDS COMPRESS UP TO 40%)
    • Cementation - material precipitated from water into pores
• CLASTIC SEDIMENTARY ROCKS
  • Mechanical breakup of other rocks
  • Transported to site of deposition by water, ice or wind under the influence of gravity
  • Sedimentary particle sizes
    • GRAVEL - >2 mm
    • SAND - 0.064 to 2 mm
    • SILT - 0.004 to 0.064 mm [4-64 microns] also called
    • CLAY - <0.004 mm [<4 microns] MUD
  • Shale = clay- + silt-sized sediments deposited in quiet water
  • Sandstone = sand-sized sediments
  • Conglomerate = gravel-sized sediments
• CHEMICAL SEDIMENTARY ROCKS
  • Organic or inorganic precipitation of sediments
  • Limestone - CaCO3
    • REEF LIMESTONE - built by corals (and other critters in other eras)
    • COQUINA - poorly cemented large shells and shell fragments
    • CHALK - poorly cemented microscopic shells
  • Dolostone - altered limestone composed of dolomite [MgCa(CO3)2]
  • Chert - hard, dense rock consisting of microcrystalline silica (SiO2)
  • Evaporites - form during evaporation of sea water (rock salt - mostly halite [NaCl] & rock gypsum - mostly gypsum [CaSO4])
  • COAL - carbon rich

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
    • Color - mostly due to presence of carbon & iron sulfides & oxides, & indicates the oxidation state of the environment of deposition
      • 1. BLACK = carbon or iron sulfides, reducing conditions (high organics, due to rapid sedimentation, stagnant currents) - restricted basins, lagoons, tidal flats
      • 2. GREEN = magnetite (Fe3O4), more oxidizing conditions, but moderate amounts of organics - continental margins
      • 3. RED = iron oxide (hematite - Fe2O3), well-oxygenated conditions, nonmarine & arid, or deep sea
  • 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
• GEOMETRY - 3-DIMENSIONAL SHAPE of a sediment body
  • Elongate - rivers, beaches
  • Sheet-like - trangressive shoreline
• 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
• FOSSILS & TRACE FOSSILS
  • Organisms are adapted to specific environments, for example ECHINODERMS = normal, open marine, or BLUE-GREEN ALGAE (mats) = abnormal salinities

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
  • Swamp - fine-grained sediments with lots of organic matter
• 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
  • Tidal flats - form where tidal ranges are large (>2 m)
    • ESTUARIES = mud, under water
  • 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 black & red coloration be used to establish environments of deposition?

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

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

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

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

7. What are the major environments of deposition?

8. What are the important subenvironments of each major environment of deposition & the lithologic

characteristics of the sediments deposited in each?

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