Sedimentary Rocks

Weathering, Erosion, Deposition, and Lithification

Layered Rock on Mars may indicate sedimentary rock

Importance of sedimentary rocks. • Source of many construction materials. Red Beds • Sediments can store information on indicate past climate. oxygenation of early • Type of rock can indicate environment Earth of deposition. • The only rock type to preserve fossils.

1 How to make a sedimentary rock. Weathering • Weathering = the breaking down of • Chemical rock into smaller pieces (sediment). – Rock dissolved (in water) into individual • Erosion = the movement of molecules. – Especially effective if the water is acidic or sediment from one place to another. hot. • Deposition = the placement of • Mechanical sediment at a new location. – Rock is physically broken into smaller pieces. – Happens from impact, thermal expansion, or • Lithification = turning loose sediment frost wedging. into a sedimentary rock.

Erosion by water causes sediment Agents of erosion to be transported as: 1. Bed load 1.Water – Rolls or bounces along the bottom 2. Suspended load 2.Wind – Turbulence in water keeps smaller sized particles in the water and off the bottom 3.Gravity 3. Dissolved load – Rock material is dissolved into individual 4.Ice molecules and transported in solution.

Deposition 1. Chemical deposition – Dissolved molecules dissolved in water reaches saturation and begins to precipitate out of water. – Makes a chemical sedimentary rock. 2. Clastic (detrital) deposition – Rock pieces being eroded without being dissolved in water get deposited. – Makes a clastic sedimentary rock.

2 Lake Siltation

Clastic sediment size Clastic sedimentary rocks • Clay (mud) Mud sized = mudstone or shale • Silt Silt sized = siltstone • Sand Sand sized = sandstone • Pebble Rounded pebbles = conglomerate • Cobble Sharp angular pebbles = breccia • Boulder

3 4 Environments of deposition clastic • Need high energy to move large particles, and need low energy to deposit small particles. • Pebble sized and larger deposited in floods, debris flows, and mountain streams • Sand sized found at the base of mountainous areas, fast moving stream beds, and beaches. • Silt must have slow moving or still water to deposit, usually near shore. • Clay must have still water to drop out, usually deep calm water far from shore.

5 Lake Bonneville flood gravels underlying Lake Missoula Flood Deposits

6 Environments of deposition Chemical Sedimentary Rocks chemical • Carbonates • Evaporites require a body of water with –CaCO3 Æ Limestone high evaporation rate. –(Ca,Mg) Æ Dolostone – Usually need a “closed basin” where water • Silicates can run in but can’t run out.

–SiO2 Æ Chert (including flint, jasper, and • Limestone forms very slowly and will get agate) overwhelmed by any clastic particles • Evaporites – Need water with no clastics either far from – Rock Salt shore or in areas with no clastic input. – Rock Gypsum

7 8 What happened to Lake Peigneur?

Sedimentary particle texture • Sorting + –Poorly sorted, moderately sorted, well + sorted + + + •Size + + –Clay, silt, sand, pebble, cobble, boulder + + + + + + + • Roundness + + + + + + + + + + –Angular, subangular, subrounded, rounded, well-rounded

Particle sorting Degree to which particles are of the same size. • Well sorted = particles all of the same size • Moderately sorted = particles within a relatively narrow range of sizes • Poorly sorted = particles of very different sizes. – Bimodal sorting = particles of two distinct sizes.

9 Information from sorting • Well sorted = constant energy level • Moderately sorted = energy fluctuates within a certain range • Poorly sorted = energy levels fluctuates drastically over time or the material has been dropped all at once (as by melting ice).

Information from rounding and size • Rock clasts usually start out large and angular. • As they are eroded, sharper edges get worn down quickly. • The farther clasts have been eroded from the site of weathering, the rounder smaller they tend to be.

Lithification/Diagenesis 1.Compaction 2.Cementation 3.Recrystallization

10 Sedimentary patterns and Limestone structures Shale • Rhythmic layers = repeated sequences of Sandstone sedimentary rock • Cross bedding = lineation inside layers of Shale rock that go across the rock • Ripples Limestone • Graded bedding = layer of sedimentary rock with coarse clasts at the bottom fining Shale upward • Raindrop impressions Sandstone • Mudcracks Conglomerate

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