Geol 102 Exam 2

Sedimentary Rocks

• Clastic and Biochemical sedimentary rocks

  • Clastic

    • lithification of transported sediment

    • classified by grain size

    • matrix: the fine-

    • grained material between clasts

    • Quartz sandstone = 90% quartz

    • arkose = 25% feldspar

    • lithic sandstone = 90% quartz

  • biochemical

    • precipitation of material in solution or lithification of organic material

    • classified by composition

    • carbonates are limestone. Examples being micrite, tufa, dolostone

    • evaporites are precipitates from solution in arid climates, examples being rock gypsum and rock salt

  • silica quartz is chert and diatomite

• Sorting and rounding

  • well-sorted grains imply consistent energy like beaches or sand dunes

  • Poorly sorted implies inconsistent energy like a glacier or landslide

  • rounding is formed due to abrasion by waves, streams or wind. Indicates distance traveled or length of time.

• Sedimentary structures: what they indicate and which indicate paleocurrent

  • Bedding: depositional layering in sedimentary rocks

  • Laminations are bedding that is less than 1cm thick

  • graded beds: grain size gets smaller as you move up

  • reverse graded beds: the grain size increases as you move down

  • cross bedding: inclined bedding surfaces, are tangent on the bottom

  • tabular cross bedding: very horizontal cross-bedded units, essentially flat at an angle; planar basal contacts

  • trough cross-bedding: curved contacts meaning they end at a point, common on stream channels

  • imbrication: the preferred orientation of clasts

  • ripple marks: formed by movement of wind or water

  • wave formed ripple marks are symmetrical due to oscillating currents

  • current ripple marks are asymmetric due to current flowing in one direction

  • mud cracks: clay shrinks when it dries indicating wet or dry conditions

  • trace fossils: tracks or burrows

  Depositional environments and common characteristics in sed rocks

  • fluvial(rivers and streams): meandering streams create point bars, making trough beds and current ripples

  • glacial outwash creates braided streams carrying sand and gravel

  • glacial lakes have dropstones

    • drop stones: rocks that fall to the bottom of the lake from ice

  • glacial lakes: varves are laminated mud deposits

  • moraines have till: poorly sorted sediment dropped by ice

  • playa lakes = evaporite deposits and mudcracks

  • transitional environments: delta (ripples and crossbeds), tidal flats (alternating cross-beds and wave-formed ripples), swamp, beach/barrier islands

  • continental shelf: sand near shore with offshore mud

  • continental slope: underwater avalanches deposit mud and sand (graded bedding)

  • reefs: fossiliferous limestone

  • deep ocean: micrite