GEO-sedimentary rock

Sedimentary Rock Activity Lecture

  • Focus: Relationships between mineral chemistry and textural characteristics of sedimentary rocks.

  • Recommended to explain concepts using proper terminology. Review the ‘sedimentary rock cycle’ and probable essay questions.

Sedimentary Rock Vocabulary Terms

  • Sedimentary rock: Rocks formed on Earth's surface from weathering processes.

  • Grain size: Average diameter of sediment fragments.

  • Groundmass: Smaller sediment grains.

  • Chemical sedimentary rock: Rocks that precipitate from mineral-saturated water.

  • Clastic/detrital sed rock: Formed from pieces of bedrock/sediment via mechanical weathering.

  • Sorting: Range of grain sizes within a sedimentary rock.

  • Arkose Sandstone: Sandstone with >25% feldspar.

  • Organic sedimentary rock: Derived from lithified remains of living organisms.

  • Lithification: Process turning loose sediments into sedimentary rocks (involves compaction, cementation, and recrystallization).

  • Well sorted: Narrow range of grain sizes.

  • Quartz Sandstone: Composed mainly of quartz grains.

  • Evaporites: Minerals precipitated from evaporating saline solutions.

  • Deposition: Accumulation of sediments after transport.

  • Poorly sorted: Wide range of grain sizes within the sediment.

  • Greywacke: Sandstone with a muddy matrix or lithic fragments.

  • Stratigraphy: Study of layered sedimentary rocks in terms of origin, composition, and geological timescale.

  • Diagenesis: Chemical alteration of sediments under heat and pressure.

  • Rounding of sediment: Smoothening of angular corners in sand grains.

  • Fissile: Separating into thin sheets.

Sedimentary Rock Cycle Steps

  1. Weathering: Breakdown of rocks into sediments.

  2. Transportation: Movement of sediments from the source.

  3. Deposition: Sediment accumulation in new locations.

  4. Unconsolidated sediment: Loosely arranged sediments before lithification.

  5. Lithification: Conversion of sediments to solid rock.

  6. Sedimentary rock: Result of lithification process.

Categories of Sedimentary Rocks

  • Detrital (clastic): Fragments of bedrock.

  • Chemical: Precipitated minerals.

  • Organic (bioclastic/biochemical): Remains of biological organisms.

Common Detrital Rocks by Grain Size

  • Conglomerate > Sandstone > Siltstone > Shale.

Classification of Sedimentary Rocks

  • Detrital/Clastic: Classified by grain size and mineral composition.

  • Chemical: Classified by lacking grains and mineral composition.

  • Organic: Classified by fossils and organic material presence.

Texture and Geologic Implications

  • Detrital Sedimentary Rocks: Clastic texture, made from rock/mineral pieces.

  • Chemical Sedimentary Rocks: Crystalline texture, composed of crystallized minerals.

  • Organic Sedimentary Rocks: Clastic texture but derived from biological remains.

Hydrochloric Acid Test (HCL)

  • Useful for testing chemical sedimentary rocks due to reaction with carbonate minerals, indicating presence of minerals like calcite.

  • Organic and chemical sedimentary rocks can react with HCL as they contain calcium carbonate.

Insight from Sedimentary Rocks

  • These rocks provide insights into the geological environments where they were formed based on their textures and compositions.

Relationship Between Sorting and Roundness

  • Sorting: Indicates transport distance and energy; well-sorted and rounded grains mean longer distance and higher energy; poorly sorted and angular grains indicate shorter distances and lower energy.

  • Arkosic Sandstone vs. Quartz Sandstone: Arkosic has more feldspar, indicating shorter transport and younger age than quartz sandstone.

Sedimentary Structures and Their Environments

  • Bedding Planes: Formed in various marine settings.

  • Bioturbation: Associated with shallow, marine environments.

  • Vugs: Can be found in various environments as small voids in rocks.

  • Graded Bedding: Forms with decreasing energy levels.

  • Mudcracks: Indicative of environments transitioning from water to air (e.g., tidal flats).

  • Cross-bedding: Found in deserts and coastal areas from dunes.

Sedimentary Depositional Environments

  • Abyssal Plane: Fine mud, chalk, chert, clay.

  • Submarine Fan: Graded Bouma sequence sediment, clastic rocks like shale, sandstone.

  • Lacustrine: Fine-grained, shale, limestone.

  • Continental Slope: Mud, siltstone, and limestone.

  • Aeolian: Poorly sorted sediments, sandstone, conglomerate.

  • Littoral (Beach): Well-sorted, sandstone.

  • Glacial: Poorly sorted, contains conglomerate.

  • Reef: Coral structures, limestone.

  • Lagoon: Fine-grained with bioturbation, shale.

  • Delta: Channelized sand/mud, clastic rocks.

  • Fluvial: Coarse sediments with structures indicative of channel flow.