phy sedimentary rocks

Varve

Evidence of lake presence

• annual sediment layer: altering silt and clay

• forms in any lake that freezes in the winter

• the ice makes the water extra calm

• melting season: summer layer: light colored silt, variable thickness

• non-melting season: winter: dark colored clay, uniform thickness

Turbidity current

Underwater landslide

• dense sediment laden flow driven by gravity

• flow for considerable distances, slowing and dropping sediment only when they reach gentler slopes

Graded bedding

Results when a sediment laden current (like a flood) begins to slowdown

• As transportation begins to slow down, smaller sediments deposit last

• the grain size is larger at the bottom and finer at the top

Fossils

Preserved evidence of ancient organisms

• indicate types of organisms of the time

Mud cracks

Wet mud dries and then shrinks and cracks

• polygonal shape

• form when clay/mud is wet, then dry

Indicative of

• tidal flats

• desert environments

• river flood plain

Cross-beds or ripples

Indication of currents (wind or water)

• if they are buried, may be preserved and form into rock

• the shape/orientation can indicate speed or flow

• symmetrical = beach

• asymmetrical = rivers, desert, dunes

Glacial deposits

Polished, striated, grooved bedrock surface

• striated and flattened cobbles

• abrasion smooths rocks and shapes surface to make groves, striation

Dropstones

Stones dropped to the bottom of a lake or ocean from a melting body of ice afloat on the water surface

Glacial erratic

Glaciers can transport very large boulders, but when the glacier melts and recedes, these boulders are left behind

Clastic

Created from broken pieces of source rock from physical weathering

Chemical

Ions dissolved in water from chemical weathering

Biologic

Skeletal debris from organisms

Lithification

The making of a rock

• compaction

• cementation

Breccia

Base of mountains, angular, large

Conglomerate

In rivers or near mountains, rounded rock clasts

Sandstone

Sand-sized particles, clastic, quartz rich

• beaches, deserts, rivers, lakes, deltas

Mudstone/shale

Fine clasts of silt and clay

• found in slow water like tidal flats, lagoons, rivers, lakes

Biochemical

Derived from the shells of once-living organisms

• deposited as they settle to the sea floor, lithifying

Limestone

Almost entirely calcite, commonly used to make seashells

• preserves shells of fossil organisms

• organic reefs, warm, shallow oceans

• less than 4 km deep

Chert

Made of quartz derived silica, skeletons of certain plankton

• deep sea environments deeper than 4km

Evaporites

Derives from evaporation of large volumes

• halite and gypsum

Travertine

Precipitated from groundwater

• cold springs, hot springs, lakes, caves

• layered

Organic

Made of organic carbon and soft tissues

• coal

• swamps, wetlands