Sedimentary Rocks Formation

Sedimentary Rocks

• Sedimentary rocks are generally softer compared to igneous rocks.
• The rocks form by deposition in continental and marine environments.
• Sedimentary rocks include:
  • Sandstones
  • Conglomerates
  • Limestone
  • Mud-rocks or shale
  • Coal

Sedimentary Rock Formation Factors

• Weathering
• Erosion
• Transportation
• Deposition
• Compaction
• Lithification
• Diagenesis of terrestrial rocks

Main Sedimentary Environments

• Continental
• Marine

Clastic/Detrital Sedimentary Rocks

• Formed by weathering, erosion, transportation, deposition, compaction, cementation, and lithification.
• The rocks are classified by particle size:
  • Pebbly
  • Sandy
  • Silty
  • Argillaceous muddy

Chemical/Biochemical Sedimentary Rocks

• Formed by precipitation from seawater, evaporation, or dehydration.

Clastic (Detrital) Sedimentary Rock Classification

• The classification is based on particle size.
• These rocks are formed by weathering, erosion, transportation, deposition, compaction, lithification, and diagenesis of terrestrial rocks.
• Clastic Rocks are subdivided based on size of particles into:
  • Pebbly
  • Sandy
  • Silty
  • Argillaceous muddy

Pebbly Deposits

• Clasts are greater than 2mm in diameter.
• Fragments are cemented by a finer matrix of sand or clay.
• Two main types include:
  • Conglomerate
    • A clastic sedimentary rock with rounded pebbles.
  • Breccia
    • Made up of coarse angular fragments .
    • Can be formed by crushing of rocks along fault zones or by explosive eruptions.

Sandstone

• Clast size ranges between 2mm and 1/16 mm in diameter.
• Sand grains are compacted and cemented by silica, iron oxides, calcite, and clay into cohesive solid sandstone.
• Classified by cementing material:
  • Orthoquartzite (pure quartz-sand)
  • Ferruginous SS (iron oxide cement)
  • Calcareous SS (calcite cement)
  • Argillaceous SS (clay cement)
• Sandstones fall into several groups based on their mineralogy and texture:
  • Quartz arenite: > 90% quartz, well-sorted, and well-rounded.
  • Arkose: > 25% feldspar; sand grains are poorly rounded with poor sorting.
  • Lithic sandstone: rock-fragment-rich with fine-grained clasts of preexisting rocks.
  • Greywacke: made up of angular clasts quartz, feldspar, and rock fragments embedded in a fine-grained clay matrix.

Siltstone & Shales

• Siltstone: Clast range is between 1/16 to 1/256 mm.
• Shales and Clays: Grains are less than 1/256 mm in diameter.
  • Clasts are compacted into mudrocks in lakes
  • Pore-water is squeezed out
  • The increasing weight of overburden and mud forms shale. Marl is a calcareous shale or clay.

Limestone

• Consists of calcite (CaCO3) dolomite CaMg (CO3)2 with minor quartz SiO2.
  • Bedded rocks contain fossils (Fossiliferous Limestone)
  • Chalk is a soft, white very fine calcite with minute fossils
  • Ferruginous lmst contains Fe in cement.
  • Oolitic lmst features deposition of successive coats CaCO3 around nucleus of oolites.
  • Dolomitic LS and Dolomite {CaMg(CO3)2] occur by replacement of Ca by Mg from seawater during diagenesis.

Siliceous Deposits

• Form by deep-sea chemical or biogenic precipitation of silica.
• Siliceous nodules are deposited from seawater containing dissolved silica (Flint, Chert, and red Jasper).

Evaporites

• Salt deposition by evaporation of high salinity seawater.
• Gives gypsum, anhydrite, halite, or rock salt.

Coals

• Formed by dehydration of ancient vegetation covered by silt and mud by T&P
  • Peat (low grade coal, 60%C)
  • Lignite (more compact than peat, 70%C)
  • Sub-bituminous & Bituminous coal (80-90%C)
  • Anthracite (high-grade coal, >90% C, and very low moisture content).

Residual Deposits

• Occur as hydrated ferric oxide deposits (Laterite), and hydrated aluminum (Bauxite).
• Results from weathering under tropical conditions.

Phosphate Rock (Phosphorite)

• Composed of calcium phosphate deposited from phosphate-rich seawater.

Sandy Deposits' Varieties through Cementing Material

• Orthoquartzite (pure quartz-sand)
• Ferruginous Sandstone (iron oxide cement)
• Calcareous Sandstone (calcite cement)
• Argillaceous Sandstone (clay cement)

Sandy Deposits' Varieties through Mineralogy & Texture

• Quartz arenite: > 90% quartz, well-sorted and well-rounded sandstones.
• Arkose (feldspar-rich): > 25% feldspar; sand grains are poorly rounded without good sorting.
• Lithic sandstone: rock-fragment-rich with fine-grained clasts of preexisting rocks.
• Greywacke: made up of angular clasts quartz, feldspar, and rock fragments embedded in a fine-grained clay matrix.

Processes of Sedimentary Rocks for Clastic Rocks

• Weathering
• Erosion
• Transportation
• Deposition or sedimentation
• Burial
• Lithification and Diagenesis

Weathering

• Process by which rocks are broken down at the earth’s surface to produce sediment particles.
  • Physical mechanical processes are involved
  • Chemical rocks are chemically altered or dissolved.

Erosion

• Process by which weathered rocks are loosened and transported downstream away from the source

Transportation

• Process by which weathered rocks are transported downstream to sink areas.
  • This involves transportation agents e.g. water, wind & gravity

Sedimentation or Deposition

• Process by which sediments will settle down in an environment

Burial

• Process by which settled loose sediments accumulate together, are compacted, and progressively buried within the sink

Compaction

• Reduces pore space

Cementation

• Chemical precipitation of mineral material between grains (SiO2, CaCO3, Fe2O3) binds sediment into hard rock

Lithification & Diagenesis

• Physical and chemical alterations caused by pressure and heat at depth.
• Sediments are lithified and converted into a sedimentary rock.
  • Gravel forms to Conglomerate
  • Sand to Sandstone
  • Mud to Mudstone or shale

Grain Size

• 2-256 mm Pebble
• 2-1/16 mm Sand
• 1/16-1/256 mm Silt
• < 1/256 mm Mud

Sedimentary Structures

• Stratification
• Ripple marks
• Mud cracks
• Fossils
• Graded bedding
• Cross-bedding
• Bioturbation

Stratification

• Arrangement of sedimentary particles in distinct layers, beds, or layered arrangements of strata
• Graded (coarse to fine) and cross bedding (sand dunes).
• Stratum is a distinct layer of sediment accumulated at the Earth's surface.

Ripple marks

• Curved or undulatory surfaces produced by stream or tidal currents
• Useful in determining the direction of movement of paleo-currents.

Mud cracks

• These form due to the alternating wet/ dry environments.
• These form due to environments like shallow lakes/ desert basins.

Fossils

• Remains of once living organisms are preserved in sedimentary rocks
• These are important in correlating rocks of similar age but different places.

Graded bedding

• Occurs when particles are sorted according to size and grade upwards from coarser to finer
• Sizes vary gradually from coarse at bottom of the bed to progressively finer grains towards the top.
• This includes gravel-sand-silt within the same layer with larger clasts of gravel at its bottom.

Cross-bedding

• A series of thin, inclined layers within a larger bed of rock.
• The cross-beds form a distinct angle to the horizontal bedding planes of the larger rock unit.

Bioturbation

• Rough cylindrical tubes a few centimeters in diameter extending vertically through beds
• Relates to digestion digestion of sediments reworked by organisms such as worms.

Continental Environment (of Deposition)

• Alluvial deposits-river
• Desert deposits- eolian sandstone (wind)
• Lake deposits-lacustrine clays
• Glacial deposits- varved clays

Shoreline Environment (of Deposition)

• Delta deposits- rivers and waves
• Beach deposits- waves and tides
• Tidal Flats deposits- currents

Marine Environment (of Deposition)

• Continental Shelf deposits-waves and tides
• Continental Margin deposits- ocean currents
• Deep sea deposits- turbidity currents