Sedimentary Rocks Notes

Sedimentary Rock Formation

Origin of Sedimentary Rocks

  • Sedimentary rocks originate from sediment and chemical precipitates. These are the products of weathering and erosion.
  • These sediments and precipitates accumulate in sedimentary basins, such as lakes or oceans.
  • Over time, these materials are converted into sedimentary rock.

Erosion and Transportation

  • Sediment is transported to depositional basins through erosion.
  • The main agents of erosion:
    • Wind
    • Water
    • Ice
    • Gravity
  • Humans significantly contribute to erosion through modern civilization.

Sedimentary Environments

  • Sediment and chemical precipitates accumulate in various sedimentary environments and basins.
  • Continental Environments
    • Lake
    • Rivers
    • Desert lake
    • Desert
    • Glacier
  • Shoreline Environments
    • Delta
    • Beach
    • Tidal flat
  • Marine Environments
    • Continental margin/slope
    • Continental shelf
    • Organic reef
    • Deep sea
    • Turbidity currents

Conversion to Sedimentary Rock

  • Sediment transforms into sedimentary rock through three main processes:
    1. Accumulation: Sediment gathers in a sedimentary basin (e.g., a deltaic environment).
    2. Burial and Compaction: As more sediment layers accumulate, the underlying sediment is compacted.
    3. Cementation: The sediment is bound together by cementing agents that are dissolved in groundwater.

Lithification and Cementing Agents

  • Lithification involves compaction and cementation.
  • Cementing Agents:
    • Silica
    • Fe-oxide (Iron oxide)
    • Calcite
    • Clays
    • Tar (organics)

Identifying Cementing Agents

  • The identification of specific cementing agents in rock.
  • Determine the predominant cementing agent of a sandstone by observing its reaction with HCl acid and its color.

Lithification of Shale

  • Process:
    • Wet mud compacts under the weight of new sediment.
    • Splitting surfaces form.
    • Water is lost due to compaction.
    • Further cementation turns the compacted sediment into shale.

Sediment Grain Size

  • Sediment comprises various grain sizes, ranging from clays to boulder-sized clasts.
  • Grain size indicates the energy of the transport regime.
  • The size of cobbles in a stream provides insight into the stream’s energy regime during the year.

Clast Lithology and Imbrication

  • Conglomerates form from river channel deposits.
  • Clast lithology reveals information about the eroded source rock.
  • Imbrication (piggy-backing) of clasts indicates the direction of paleo-stream flow.

Grain Size and Transport

  • Grain size, sorting degree, and clast angularity relate to:
    • Transport agent (ice, water, wind, mass wasting)
    • Energy of the transporting agent
    • Distance of transport

Inorganic Clastic Sedimentary Rocks

  • Characterized by fragmental (clastic) texture
  • Types Include:
    • Conglomerate
    • Breccia
    • Sandstone
    • Siltstone
    • Shale

Chemically and/or Organically Formed Sedimentary Rocks

  • Types include:
    • Chert
    • Halite (Rock Salt)
    • Gypsum (Rock Gypsum)
    • Dolostone
    • Limestone
    • Coal
    • Oil shale

Clastic Sedimentary Rocks

  • Composed of cemented pieces of weathered rock deposited in a sedimentary basin.
  • Subdivided based on grain size.
  • Conglomerates have grain sizes ranging from gravels (>2 mm) to boulders.

Sedimentary Breccia

  • Similar grain sizes to conglomerate but with angular clasts.
  • Limited transport and erosion.
  • The degree of rounding or angularity indicates the distance of transport.

Sandstone

  • Clastic sedimentary rock composed of cemented, sand-sized (0.05 – 2 mm) clasts.
  • Cementing agent identification is important.

Sandstone Maturity

  • Mineralogically mature sandstones mainly contain quartz.
  • Immature sandstones (arkosic) contain minerals like feldspar and micas, which weather over time.
  • Sandstone maturity indicates the distance of sediment transport.

Siltstone

  • Forms in low-energy environments like lakes or marine basins.
  • Often laminated (deposited in thin layers).

Shale and Mudstone

  • Deposited in the lowest energy environments, typical of marine settings far from the continental margin.
  • Fine-grained texture with clay minerals (< 0.0039 mm) too small to see without magnification.

Chemical Sedimentary Rocks

  • Precipitate directly from aqueous solutions.
  • Example: Halite (common table salt) from evaporite deposits.

Salt Deposits

  • Large salt deposits accumulate in restricted marine basins with high evaporation.
  • Salt domes form important stratigraphic traps for oil.

Limestone

  • Mostly biogenic, derived from calcite formed by biological processes.

Inorganic Limestone

  • Precipitates in warm waters because calcite solubility is directly proportional to CO_2 content.
  • Warm water holds less CO_2, reducing calcite solubility.

Chalk Formation

  • Chalk forms from microscopic calcareous marine organisms accumulating on the ocean floor.

Calcium Carbonate Compensation Depth (CCD)

  • The CCD is the transition to increased dissolved CO2 in ocean waters, increasing CaCO3 solubility.
  • Limestone doesn't precipitate below this depth; silica (SiO_2) precipitation (chert formation) dominates.

Oolitic Limestone

  • Forms from wave oscillation of sand grains or shell fragments.
  • Concentric rings of calcite precipitate around a nucleating particle.
  • Wave agitation releases dissolved CO_2, reducing calcite solubility.

Chert

  • Composed of almost pure silica (SiO_2), precipitating in deep ocean basins below the CCD.
  • Can be formed from inorganic precipitation or accumulation of microscopic radiolaria (silica exoskeletons).
  • Breaks along conchoidal fractures.

Coal Formation

  • Forms from terrestrial organics accumulating in sedimentary basins.
  • Organic production exceeds decomposition.
  • Organics accumulate to form peat, then convert to lignite and bituminous coal with compaction and pressure.
  • Anthracite coal forms under the highest heat and pressure.

Stratification of Sediment

  • Sediment becomes stratified into depositional layers.
  • Layers signify the end of one depositional event and the start of another.
  • They signal changes in sediment or energy shifts.

Cross-Beds

  • Sedimentary layers deposited at an angle to underlying beds.
  • Formed by moving currents like rivers, streams, or wind.

Cross-Bed Formation

  • Sediment is deposited by moving currents, such as river, streams, or wind.
  • Sand grains saltate (bounce) along the stoss- face and then avalanche down the steep, lee face.
  • Paleo-wind direction can be determined by cross-bedding.

Mudcracks

  • Form when shrink-swell clays dry out during a period of desiccation, such as an ephemeral lake.
  • Development:
    1. Fine-grained sediment (mud) accumulates.
    2. Water evaporates, and mud shrinks.
    3. Mudcracks develop.
    4. Basin refills, and sandy deposits fill the cracks.

Deltas

  • Form when rivers flow into a still body of water and deposit sediment.
  • Subdivided into three distinct layers:
    1. Topset beds
    2. Foreset beds
    3. Bottomset beds

Post-Glacial Deltas

  • Formed when melt water streams flowed from the Cascades into a proglacial lake dammed by the Puget Ice Sheet.

Ripples

  • Form from currents (wind and water) or wave oscillation.
  • Asymmetric ripples form from currents; symmetric ripples from wave oscillation.

Graded Beds

  • Form because larger sediment grains settle faster than smaller grains.
  • Turbidity currents create chaotic mixtures of sediment that settle out with larger grains on the bottom.

Turbidite Sequences

  • Multiple turbidity currents result in turbidite sequences with many units of graded beds.

Fossils

  • Common in fine-grained clastic sedimentary rocks or biogenic sedimentary rocks.
  • Fine-grained rocks are more likely to preserve fossils due to low energy and absence of oxygen.

Categories of Fossil Types

  • Trace fossil: Indirect evidence (footprints, burrows, fossilized feces).
  • Molds and casts: Impression of an organism filled with sediment.
  • Replacement: Original material replaced with mineral crystals.
  • Petrified/Permineralized: Empty pore spaces filled by minerals.
  • Amber: Preserved tree sap that traps and preserves entire organisms.
  • Original material: Mummification or freezing preserves original organism material.

Fossil Formation

  • Requires:
    1. Rapid burial of the life form in sediment.
    2. Proper environmental conditions for fossilization.
    3. Dissolved minerals in groundwater replacing original organic matter or filling void space.