Depositional Basins

Sedimentary Basins

• Basin: a depression in the crust covering a large area; deeper part in crust or ocean.

• Tectonic activity is the most important control.

Subsidence

• Subsidence: the sinking of land.

• Lowering of land due to weight of deposits.

• Along the margins or from continued deposition can cause the land to further depress.

  • Allows more sediment to deposit there and create another layer.

Determination of Sedimentary Rock

• Tectonic setting (applied stress)

• Type of rock (resistance)

• Sediment transport medium (flow like wind, water, or glaciers)

• Active processed in depositional environment (physical, chemical, or biological processes)

• Climate

• Natural agents that change sediment to solid rock (pressure, cement)

• Time

Long-Term Sedimentary Rock Distribution

• Controlled by tectonic activity

• Craton: stable interior of a continent (like the nucleus of a continent); where we will find the oldest rock we can find on the planet.

  • Shield: old crystalline basement rock that is exposed to the surface.

  • Platform: surrounds shield where ancient sediments were accumulated.

• Orogenic belts: Elongated regions bordering craton; was deformed by compressional forces.

• The prefix Oro- means mountain building.

Depositional Environment

• Depositional environment: The geographic location where sediments accumulate.

• Refers to environmental factors under which sediment is deposited.

• Using uniformitarianism

Shallow Marine

• Continental shelf: submerged edges of continents.

• Larger grains are heavier than smaller ones

  • Above wave base, waves are active and break things down - sandstone

  • Below wave base, siltstone and mudstone

Turbidity Currents and Turbidites

• Turbidity current: a fast-moving, sediment-laden flow of water that travels down underwater slopes, driven by gravity and density differences, often resulting in the deposition of graded beds.

• Triggered by earthquakes, tsunamis, etc.

• Common in deep water environments

• Graded beds: A sedimentary layer characterized by a gradual change in particle size from coarse at the bottom to fine at the top.

Deep Marine

• As you move away from continental slope, sea ooze deposit from single celled organisms.

• When they die, they sink to the bottom and create limestone.

Reefs

• Biological in origin: algae, sponges, and other marine invertebrates.

• Warm shallow tropical regions.

• Carbonate: anything related to calcite carbonate.

  • Commonly found in minerals such as calcite and dolomite, as well as in sedimentary rocks like limestone.

  • Example: corals

Fluvial (Continental)

• Depending on energy and type, rivers can carry sizes from clay to boulders.

• Sand and gravel fill concave-up channels.

• Fine sand, silt, and clay are deposited on flood plains.

• Resulting rocks: sandstones, mudrocks, conglomerates

Alluvial Fans (Continental)

• Mostly in desert environments

• Conglomerates, breccia, arkose

Lake Environments (Continental)

• Lacustrine - lake

• Accumulate fine grained sediment and algae, resulting in mudrocks and shale.

• Calm conditions

Glacial Deposits (Continental)

• Glaciers: Large groups of rocks that move down and carves out our landscape and then deposits the sediments as it travels.

• Till: Unsorted and unstratified sediment deposited directly by a glacier, consisting of a mixture of clay, silt, sand, gravel, and boulders; massive poorly sorted material; angular due to lack of weathering.

• Tillite: A sedimentary rock formed from the consolidation of glacial till; angular and poorly sorted, not influenced by water.

Eolian Faces (Continental)

• Aired deserts (wind blown sediments)

• Sandstones and siltstones

Beach and Lagoon (Transitional)

• Quartz sandstone, mudstone, sandstone

Delta (Transitional)

• River meets the ocean

• Shape and sediments controlled by the balance between marine destruction processes and sediment supply

• Sandstones, siltstones, mudstones, possibly carbonates, coals.

Describing Sedimentary Rocks

• The rock record of depositional environments, including all possible elements that can be used for recovering the physical, chemical, and biological processed occurred during deposition.

  • As a direct consequence to these processes, we describe sedimentary rocks by their facies.

  • Use color, texture, composition, and sedimentary structures.

Color

• Red/reddish rock - iron that has been oxidized

• Greenish rock - may contain iron that has been reduced

• Brown/black/very dark gray rocks - may contain large concentrations of organic matter

Sediment Textural

• Textural maturity: sorting, roundness and sphericity.

• Well sorted sediments: uniformly-sized grains imply constant reworking by water or wind.

• Roundness and sphericity: sediment grains are rounded during weathering and transportation.

• Most mature will be the most rounded and same material.

Common Siliciclastic (Felsic) Rocks

• In felsic rocks, grains are commonly cemented by either calcite, iron oxides, or silica.

• Features of cement may require observation under microscope.

• Gravel → Sand → Silt → Clay

Sediment Composition

• Compositional maturity: the amount of stable minerals

• Quartz is the most stable and very resistant

• Sediments composed of predominantly quartz are compositionally mature sediment

• Sediments containing quartz, feldspar, rock fragments are compositionally immature sediments

• Composition of Sandstone

  • Quartz sandstone: mature

  • Graywacke sandstone: immature

Changes in Grain Size and Maturity

• Decrease of grain size towards the ocean.

• Longer transportation distance and higher energy → better compositional and textural maturity.

Structures: Mud Cracks

• When clay dries out, it contracts inward, which allows materials to deposit in the cracks.

• Tells us rock formed in dry environment

Structures: Ripples

• Sand moved by wind, streams, and coastal waves.

• Helpful to determine current direction (direction of flow)

Cross-Bedding

• Aka cross-stratification

• Arrangement of beds or laminations in which one set of layers is inclined relative to others

• Formed via wind or water

• Direction in which beds slope is an indicator of current direction

Cross-Bedding in a Delta

• The succession of inclined forest beds is deposited over bottom-set beds that laid down earlier.

• Top-set beds are deposited by the stream above the forest beds

Graded Bedding

• Results when flowing water sorts particles by size

• Clay settles last

Lithofacies

• Lithofacies: A facies is part of a rock body that has characteristics from which we can infer depositional environment

• Litho → lithological (means stone or rock)

• Bio → fossil assemblage

Transgression (Sea Level Rise)

• Sedimentation during a transgression event produces an on-lap relationship in which finer offshore lithofacies overlie nearshore facies.

• Nearshore facies are progressively displaced away from a marine point of reference, and older beds are protected from erosion by younger beds.

Sea Level Change and Spatial Variations

• Dependent on vertical change

• Effects of rising or falling sea level on different coasts

Geological Maps and Cross Sections

• Contents of a geologic map

  • Basic topographic features

  • Rock units: Group, Formation, Member

  • Time constraints: Cambrian, Silurian, etc.

  • Strikes, Dips, Faults, and Folds

  • Scale and legends

• Cross section

  • Topographic profile

  • Extension of layers towards subsurface