Part 2

• Sedimentary Rocks


  Formed by the accumulation and consolidation/cementation of sediments, that have been weathered and transported by wind, water, or ice
• Sediment includes


  • Particles of Minerals
  • Rock Fragments
  • Organic Matter
• Classification of Sedimentary Rocks

  • Organic
  • Chemical
  • Biochemical
  • Clastic
• Organic matter


  Carbon-based material that was once living (plant and animal)
• Biological debris


  Physical remains of organisms (shells, bones, teeth)
• Organic Sedimentary Rocks


  • Bituminous Coal
  • Lignite
  • Peat
  • Anthrcite
• Chemical Sedimentary Rocks


  • Chert
  • Rock Salt
  • Dolostone
  • Banded Iron Formations
• Biochemical Sedimentary Rocks


  • Fossiliferous Limestone
  • Coquina Limestone
  • Chalk
  • Micrite (microscopic calcite mud)
  • Biogenic Chert
• Clastic Sedimentary Rocks


  • Coarse-Grained Clastic Rocks (≥2mm): Breccia (Angular Clasts) & Conglomerate (Rounded Clasts)
  • Medium-Grained Clastic Rocks (63 µm to <2mm): Sandstone: Arenite (sand grains + cement) & Wacke (>15% fine matrix)
  • Fine-Grained Clastic Rocks (< 63 µm): Shale (breaks into thin layers) & Mudstone (breaks into blocks – no layering)
• Depositional Environments of Sedimentary Rocks

  • Terrestrial: Glacial, Alluvial, Fluvial, Aeolian, Lacustrine, Evaporite
  • Marine: Deltaic, Beach, Tidal, Reef, Shallow Marine, Lagoonal, Submarine Fan, Deep water
• Geological Principles


  • Original horizontality
  • Superposition
  • Inclusions
  • Faunal succession
• Sedimentary Structures


  • Stratification/Beds/Laminations
  • Dunes and Ripple Marks
  • Cross-Bedding
  • Graded Bedding
  • Mud Cracks
  • Raindrop Impressions
  • Sole Marks
  • Unconformity
• Stratification/Beds/Laminations


  Arrangement of layers in any rock. Laminae = <1cm thickness, Strata = >1cm, Beds: Thick beds = >100 cm, Moderate beds = 10 - 100 cm, Thin beds = 1 - 10 cm
• Symmetrical Ripple Marks


  Symmetrical limbs = both sides of the ripple dip at about the same angle. Caused by bidirectional flow i.e. back-and-forth motion. Formed by wave or tide action shaping sediment on the ocean floor.
• Asymmetrical Ripple Marks


  Unequal limbs, with a shallow side and a steeper side. Caused by unidirectional flow i.e. one direction, like a river. Sediment moves up the shallow side and settles on the steep side.
• Cross-Bedding


  Forms when ripples/dunes migrate due to wind or water current. Steep side of a ripple always angles downward toward the direction the water or wind was moving. Common in depositional environments like eolian, shoreface, tidal, and fluvial.
• Description: Cross-bedding from ancient sand dunes in Coyote Gulch, part of the Canyons of the Escalante
• Modern asymmetrical ripples


  • From the Bahamas
• Asymmetrical ripples in sandstone

  • From Colorado, USA
• Precambrian asymmetrical ripples in quartzite

  • From Wisconsin
• Sedimentary Rocks
• Cross-Bedding


  • Forms when ripples/dunes migrate due to wind or water current
  • Steep side of a ripple always angles downward toward the direction the water or wind was moving
  • Common in depositional environments: eolian, shoreface, tidal (if strong enough), and fluvial (in point sequence where velocity increases)
  • Two main types: Planar/Tabular cross-bedding and Trough cross-bedding
• Cross-bedding from ancient sand dunes


  • In Coyote Gulch, part of the Canyons of the Escalante, Utah
• Herringbone Cross Stratification


  • Symmetrical tide
  • Asymmetrical tide – flood dominant
  • Flood dominant – no ebb deposit
• Graded Bedding


  • Grain size changes within a single sedimentary bed
  • Coarse particles are mainly found at the bottom (older)
  • Particles progressively smaller towards the top (younger)
  • Represent depositional environments in which transport energy decreases over time
  • Example: changing water velocity in a river
  • Rapid depositional events, such as turbidity currents, can also lead to the formation of these patterns
• Graded bedding


  • From the Precambrian of Tennessee, USA
• Mud Cracks


  • Wet sediment, often clay-rich, dries out and develops cracks, forming surface polygons
  • Modern mud cracks - along river margins / flood-prone desert valleys
  • Can be filled with new sediment
  • Cracks are wider at the top and narrower towards the bottom
  • Cross-section view of a crack serves as a useful way-up indicator
  • Helps determine the original deposition orientation
• Mud cracks


  • In limestone (Makgol Formation, Ordovician; South Korea)
  • Modern mud cracks (White River Badlands, South Dakota, USA)
  • In sandstone in the Cambrian of Maryland, USA
  • On base of sandstone bed (Kayenta Formation or Navajo Sandstone, Lower Jurassic; Utah, USA)
• Raindrop Impressions


  • Small, concave marks made by rain on soft sediment
  • Useful for determining the correct orientation of the rock layer
  • Viewed from the bottom, they appear as raised bumps (convex)
  • Fine-grained rocks like siltstones and shale
  • Coarser-grained sandstones
  • Scattered rather than widespread across the surface
  • To be preserved, the impressions must be filled with sediment before the next rainstorm
• Raindrop imprints


  • In sandstone in the Cretaceous of Colorado, USA
  • In siltstone from the Permian of New Mexico, USA
  • Modern raindrop imprints in the Leucite Hills of Wyoming, USA
• Sole Marks


  • Appear as impressions or grooves in sediment
  • Cast (the raised bump) is at the bottom
  • Mold side (the impression) is filled with sediment
  • Good way-up indicators since the cast side is facing down
  • Examples: Flute Casts, Tool Marks, Groove Casts, Load Casts
• Sole marks


  • Turbidite bed with flute casts on its base (Laingsburg, South Africa)
  • Tool marks
  • Groove casts of fault plane slickenside
  • Load casts with flute marks on the upper surface of shale
• Unconformity


  • Gaps or breaks in the geological record = periods no sediment was deposited or where erosion occurred
  • Formed by tectonic activity and changes in sea level etc.
• Unconformity


  • De Chelly-Shinarump contact (disconformity)
• Weathering


  • Physical weathering more pronounced where temperature and rainfall are low
  • Chemical weathering more intense in regions of high temperature and rainfall
• Sandstone


  • Clastic sedimentary rock composed mainly of sand-sized mineral particles or rock fragments
  • Involves understanding the various landforms and processes associated with the formation, erosion, and modification of sandstone landscapes
• Formation of Sandstone


  • Forms when sand grains accumulate and are later compacted and cemented together
  • Deposition Environments: Beaches, Deserts, Riverbeds, And Dunes
  • Lithification: Process in which the grains are compressed and bound by minerals like silica, calcium carbonate, or iron oxide
• Structural Features of Sandstone

  • Beds, Folds, Joints, Faults
  • Related to Compression, Tensional, and Shear forces
• Compression Forces


  • Push crustal rock together
  • Cause Folds: Ductile deformation = Bending, Limbs, Hinge zone, Axial surface, Axial trace
• Fold Classification


  • Symmetrical, Asymmetrical, Overturned, Recumbent, Parallel limb