Sedimentary Rocks and Depositional Environments

Wars, Geology, and Lecture Overview

  • Geology is at the forefront of discussions involving conflicts and resources, influencing potential defense deals related to mineral rights.

  • The lecture will cover weathering examples, sedimentary rock types, sedimentary structures, and depositional environments (around 50 slides).

  • A review game (Jeopardy) is planned for the metamorphic lecture next week.

Physical Weathering

  • Physical weathering is a dominant factor. Key examples include:

    • Jointing: Results in block and rectangular formations in sandstone and exfoliation in granites/igneous rocks.

    • Exfoliation: Occurs in igneous rocks due to pressure release as they are exposed, allowing them to expand.

    • Frost wedging, salt wedging, and root wedging: Physical removal of material via expansion within cracks.

    • Thermal expansion: Common in desert environments due to temperature fluctuations.

    • Animal attacks/burrows & human activities

    • Wind

Chemical Weathering

  • Involves the destruction of minerals through contact with solutions or air, leading to alteration of mineralization.

    • Dissolution: Dissolving of mineral grains in a solution (e.g., limestone erosion by acidic water).

    • Hydrolysis: Chemical reaction between water and minerals, breaking them down (e.g., water molecule breaking down into hydroxide OH and hydrogen, forming cations in clays).

    • Oxidation: Iron losing electrons ( Fe^{2+} to Fe^{3+} ), resulting in rust formation.

    • Hydration: Minerals incorporating water molecules (H_2O) into their crystalline structure (e.g., leading to gypsum formation).

    • Physical weathering increases surface area, enhancing chemical weathering processes.

Clastic Sedimentary Rocks

  • Clastic (detrital) rocks are classified by grain size:

    • Gravel: Coarser grains (larger than 2mm), including boulders, conglomerates, and breccias.

    • Sandstone: Sand-sized grains, varying in maturity:

      • Mature: Pure quartz, well-worked, well-rounded, and of similar composition.

      • Immature: Less pure (arkose or graywacke), less worked, closer to the source.

    • Fine-grained: Clay stones and mudstones.

Chemical and Biochemical Sedimentary Rocks

  • Primarily limestones (carbonate rocks) formed in shallow, warm water environments at lower latitudes.

  • Chert and rock salt:

    • Chert: Cryptocrystalline (crystalline structures too small to distinguish even with microscopes).

    • Rock salt: Halite formed in large evaporative environments.

    • Carbonate rocks identified by acid fizzing test; clastic rocks by grain size and maturity

Sedimentary Structures and Depositional Environments

  • Sedimentary rocks form at or near the Earth's surface, with surface features reflecting the paleoenvironment.

    • Bedding: Layering within the rock showing individual layers and their arrangements.

    • Beds: Linear, close-to-horizontal layers.

    • Strata: Groups of beds separated by bedding planes.

    • Changes in bedding indicate changes during deposition (e.g., silt layers alternating with gravel layers indicate flood events).

Formations and Geologic Maps

  • Members: Grouping based on depositional changes.

  • Formations: Grouping within the same environment, used on geologic maps.

  • Geologic maps display formations with different colors, which can pinch out or erode in different locations.

  • Examples

    • Jurassic Aztec sandstone marked in red

Bedding Features

  • Planar bedding: Not typically emphasized in quizzes.

  • Lamination: Very thin bedding.

  • Cross-bedding: Indicates wind direction in dunes or water direction in ripples.

    • Stoss Slope (windward side) and slip face (leeward side) where grains topple down.

    • Ripple marks are small features, while sand dunes are massive.

    • Ripple marks can be symmetric (beach environment) or asymmetric (river system).

  • Mud cracks: Indicate alternating wet and dry conditions, common in floodplains due to fine-grained particles (mud-sized) being deposited during floods.

  • Raindrop impressions: preserved indicate alternating dry and wet conditions

Turbidity Currents and Glacial Deposits

  • Turbidity currents: Create graded beds (coarse to fine) from slope failures in underwater environments.

  • Glacial deposits: Poorly sorted, angular, striated; glaciers transport the widest range of sediment sizes.

    • Drop stones: Rocks dropped from glaciers into fine-grained sediments, indicating past glacial extension.

  • Fossils: Frequently in sedimentary rocks, burrows, tunnels can be found

Depositional Environments and Sea Level Changes

  • Depositional environments inferred from surface features (ripples, mud cracks) and bedding/lamination.

  • Sedimentary rocks form from weathering, transportation, deposition, and lithification.

  • Example: Sandstone-shale-limestone sequence indicates a sea level rise (transgression).

  • Transgression: Sea level rise.

  • Regression: Sea level fall (caused by water trapped in ice or plate tectonics).

Terrestrial Depositional Environments

  • From glaciers to mountain streams to rivers, influenced by energy and sediment load.

  • Glaciers: Transport the widest range of sediment sizes (boulders to clay), poorly sorted, angular sediments.

  • Mountain streams: Carry boulders to smaller sizes due to seasonal changes and flash flood events.

    • Conglomerate and breccia: Common rock types.

Alluvial Fans

  • Sediment pours out of mountains, loses velocity, and forms a fan-like wedge.

  • Breccias, conglomerates and shales are prevalent.

  • Arcose: Immature sandstone close to the source rock (mountain).

  • Graded bedding occurs due to loss of energy as the flow leaves the channel.

River Environments

  • Vary depending on flow, flooding season, and water source.

  • Conglomerates indicate high energy environments, mud forms in outside floodplain areas.

  • Sand dunes: Arid environments, well-sorted sandstone from wind action.

Lake and Delta Environments

  • Lakes: Calm environments, fine-grained sediments, muds, and shales.

  • Deltas: Where rivers meet larger bodies of water (e.g., oceans), sediments accumulate around the river mouth, turbidity with water and slope builds up.

Beach and Offshore Marine Environments

  • Beaches: Very pure, well-rounded quartz sandstone with symmetrical ripple marks.

  • Offshore marine: Lower energy, muds and silts, mollusks, marine fossils.

  • With the silica input, clastic rocks of mudstone and shale can arise.

  • In the absence of silica input, carbonate reef systems and limestone form in warm, shallow marine environments.

Deep Sea Environments

  • Fine-grained chert formed from plankton remains.

  • Chert cliffs exposed due to uplift and glacier periods.

Economic Considerations and Local Examples

  • Geological processes require millions of years to form resources (gas, mineralization).

  • Local examples:

    • Limestone quarry provides material for local housing construction.

    • The local landfill powers 10,000 homes using methane capture.

    • Las Vegas Red Rock company crushes rock for construction projects.

    • Gypsum quarry in Blue Diamond provides material for drywall production.

Salt Mines, Groundwater, and Contamination

*Salt can be mobile and forms salt domes.

  • Groundwater content and contamination are important considerations; methane can randomly travel with water. The methane comes from the decomposition of trash.

  • Fracking can cause gas to move into the water supply.