Week 4 - Sedimentary rocks

Sediments

unconsolidated fragments of pre-existing rocks

like, shells, mineral precipitates, organic material (mud, sand, shell, gravel, concrete)

sedimentary rocks

form from compactions and cementation of sediments together

precipitation of minerals from solution

growing organic skeletal material

Weathering and Erosion

(step 1 of process)

weathering is when materials are broken down and divided into smaller particles (mechanical = physically or chemically = oxidation, hydrolysis, etc.)

erosion happens when these rock particles are picked up and displaced to someplace else

Transportation

(step 2 of process)

Sediment pieces are moved by wind, water, ice, or gravity

Energy of transportation system affect is ability to carry sediment (e.g. wind can only move smaller particles, like sand, while water can move bigger sediments like large rocks or boulders)

Deposition

( step 3 of process)

when sediment settles out of the transporting agent

Usually occurs when speed of transportation changes

Lithification

(step 4)

Process by which sediment become solid rocks

a) Compaction

• pressure, burial reduces pore space (smushed together, rearrangement of granular structure, consolidation

b) Cementation

• filling of pore space by precipitation of cement

After deposition

Bedding and stratification

arranged in horizontal layers

laterally continuous for long distance s

composition layers on top each other (like lasagna)

Unconformities

erosional surfaces

disconformity — uneven layers between parallel layers of sedimentary rocks

nonconformity — between sedimentary rocks and older, eroded metamorphic/igneous rocks

angular unconformity — between horizontal sedimentary rocks and older tilted and eroded layers s

paraconformity — strata are parallel, hard to detect erosional surface

Mechanical weathering

Physical break down of rocks (smashing against each other, no chemical change the rocks are just in smaller pieces)

Types:

frost/ice, root wedging (trees), salt wedging (by ocean), shrinking/swelling clays, thermal expansion, animal activity (burrows) or abrasion

These all happen outside of the rock, meaning the rock itself isn’t’ changing but the appearance is.

Chemical weathering

Chemicals in rock interact w outside elements, penetrating the rock, completely changing

hydration

absorption of water into crystal structure of some minerals

hydrolysis

processes where minerals react with water and break down

oxidation

addition of atmospheric oxygen in the near surface

causes rock to “rust” — turns reddish color

Dissolution

rock material dissolved directly into solution

Water, often acidic, breaks down minerals in rocks transforming them into soluble ions and reshaping landscapes

structures - cross-beds

rivers, tidal dominated settings, shallow marine settings

sediment piles up and then slips down steep surface

structure: ripples

indicate agitation by water

asymmetric ripples - unidirectional flow

• short, steep down-current ripples

symmetric ripples - wave oscillation

• sharp ridges and concave-up troughs

• from back-and-forth swash

structures: dunes

same concept as ripples but larger concept w sand

structure: graded beds

coarse to fine-grain as you move up the bed

called fining upwards or normal grading

glacial environments

till - glacial transport

from ice wedging - slope deposits

mountain streams

high-gradient - high energy

braided gravel channels

conglomerate rocks

alluvial fans

stream gradient drops - mix of sediments deposited in fan-shape

sediment dump

conglomerates

aeolian environments

bare ground - strong winds entrain sand

sometimes see distal losses deposits (silt)

cross-bedded sandstones

lacustrine environments

low energy

any coarse sediment introduced to system will settle along shore

fine clays deposited in lake center

riverine deposits

fine gravels, sands, coarse silts tumble along river channel

fine sediments entrained in flow - settles floodplains

in-channel sed rocks: sandstone

marine deltaic deposits

where river mouth empties to sea

very complex deposits

thick sequences where deep water sediment prograde into fluvial sediments in a shoreward direction

coastal deposits

high energy

lagoonal - lower energy generally (fine grained seds)

becah - well sorted seds

shallow marine clastics

energy gradient drops as you move seawards from beach

sed rocks - finegrained, clay stones,