Sedimentary Rocks Lecture

What are sediments? • Loose grains and chemical residues of rocks, minerals, plants or animals and aqueous solutions. – Water/other chemicals from dissolution/chemical decay 

Chemical Weathering • Decomposition or dissolution of materials – feldspar/mica mineral grains  clay minerals – calcite  calcium and bicarbonate ions (in H20) – Olivine  iron oxide residues  – Halite  dissolves to form ______? 

Mechanical Weathering • Cracking, crushing, abrasion, scratching, etc. -Rain -Wind -Ice wedging -Glaciers -Temperature

Change

Describing Sedimentary Rocks: Texture • Transportation: wind, water or ice



– Scratched, broken, abraded  grain shape – How far was a grain carried? Angular  Rounded  Well-rounded

• Energy (i.e. wind, water) – Poorly sorted  Well-sorted 

Crystals precipitate from aqueous solutions to form……. • Crystalline texture – clearly visible crystals

*

Microcrystalline texture

Describing Sedimentary Rocks: Composition • Describe the abundance and kinds of grains that comprise the rock….

Biochemical (Bioclastic)

-Paleoenvironment -Preserved organism or traces of organism within the rock

Chemical (Precipitates) • Forms when iron and oxygen combine in solution and deposit sediment (Hematite)!

Detrital (Siliciclastic) • Mostly clasts of silicate minerals (i.e. quartz, feldspars, micas, clay minerals….) • Shale – composed of clay minerals

Precipitation • Can form a mass from intergrown crystals directly as they precipitate from aqueous solutions. Ocean water – Rock salt, rock gypsum -evaporation sequence -Aragonite – -Gypsum 50-75% -Halite 90%

PART II

Part II: Sedimentary Structures!

• Strata • <1cm – Laminations • >1cm – Beds

• Bedding Planes – surface of exposure between 

each event

• Cross-bedding 



– Hints at paleo-environment! *Current ripple marks – wind or water in one direction *Oscillatory ripple marks – waves barely touch bottom, tidal

• Graded beds – Coarse at the bottom to fine on top • Slowing current!

R a in Im p re ssio n s 

 M u d cra cks

Tra ce Fo ssils 