Sedimentary Rocks

What is a sedimentary rock? Sedimentary rocks are products of mechanical and chemical weathering They account for about 5 percent (by volume) of Earth’s outer 10 kilometers Contain evidence of past environments Provide information about sediment transport Often contain fossils

What is a sedimentary rock? Sedimentary rocks are important for economic considerations because they may contain Coal Petroleum and natural gas Sources of iron, aluminum, and manganese

Gravel Beach

Talus Cones

delta

Owens River

Meandering River

Alluvial fans

Sand Dunes

Death Valley salt pan

Playas

Deep Springs Lake

fringing reef

Sandy Beach

Spits

Swamps

Successive stages in coal formation

coal

Sedimentary Rocks Sedimentary Rocks

Fossil Fish - 50 Million Year Old Lakes in southern Wyoming Sedimentary Rock made of fine-grained mudstone. These fish tell us the Wyoming climate 50 million years ago.

Sedimentary Rocks Sedimentary – form in water Generally formed by the deposition, burial, compaction, and cementation of sediments (pieces of other rocks) 3 Types 1.Clastic 2.Chemical (crystalline) 3.Organic (Bioclastic)

3 Types of Sedimentary Rocks: Clastic (also called Detrial)—made of broken pieces of other rocks

Organic—remains of plants and animals are deposited in thick layers Examples Fossil rich limestone is made from the shells of ocean animals; used to make chalk 3 Types of Sedimentary Rocks:

Chemical—minerals dissolved in lakes, seas, or underground water 3 Types of Sedimentary Rocks: Mineral crystals are made as the shallow water that has flooded the bottom of Death Valley evaporates. Click on image for full size (66K JPG) Courtesy of Martin Miller, University of Oregon

Detrital sedimentary rocks The chief constituents of detrital rocks include Clay minerals Quartz Feldspars Micas Particle size is used to distinguish among the various types of detrital rocks

Classification of sedimentary rocks Two major textures are used in the classification of sedimentary rocks Clastic –Discrete fragments and particles –All detrital rocks have a clastic texture Nonclastic –Pattern of interlocking crystals –May resemble an igneous rock

Detrital sedimentary rocks

Common detrital sedimentary rocks (in order of increasing particle size) Shale Mud-sized particles in thin layers that are commonly referred to as laminea Most common sedimentary rock

Lake Bed Deposits

Shale containing plant remains

Detrital sedimentary rocks Sandstone Composed of sand-sized particles Forms in a variety of environments Sorting, shape, and composition of the grains can be used to interpret the rock’s history Quartz is the predominant mineral

B. Descriptions of individual clastic rocks Arkose Graywacke Quartz Sandstone

World’s Biggest Rock The Ayers Rock is made up of arkose, a course- grained sandstone rich in feldspar at least 2.5 km thick. Uplifting and folding between mya turned the sedimentary layers nearly 90 degrees to their present position. The surface has then been eroded.

Graywacke

Detrital sedimentary rocks Alternating sequences of shale and sandstone exposed in the Grand Canyon. Shale cannot support steep cliffs or form erosion

Detrital sedimentary rocks Conglomerate and breccia –Both are composed of particles greater than 2mm in diameter –Conglomerate consists largely of rounded gravels –Breccia is composed mainly of large angular particles

B. Descriptions of individual clastic rocks Breccia Conglomerate

Breccia

breccia1

Characteristics of sedimentary rocks Sorting – the distribution of grain sizes in a rock Sorting depends on properties of the depositing agent

Graded Beds

Sorting Sand dune – well sorted Glacier deposit – poorly sorted

3 Types of Sedimentary Rock 1.Clastic Formed from the deposition, burial, compaction, and cementation of sediments (fragments of other rock) Examples Grain Size Composition

3 Types of Sedimentary Rock 2.Chemical (crystalline) Precipitates – minerals fall out of solution when the water chemistry changes Evaporates – minerals left behind when water evaporates

Chemical sedimentary rocks Consist of precipitated material that was once in solution Precipitation of material occurs in two ways Inorganic processes Organic processes (biochemical origin)

Chemical sedimentary rocks Common chemical sedimentary rocks Limestone (calcareous deposits) –Most abundant chemical rock –Composed chiefly of the mineral calcite –Marine biochemical limestones form as coquina (broken shells), and chalk

Coquina

Fossiliferous limestone

Chemical sedimentary rocks Common chemical sedimentary rocks Dolostone (siliceous deposits) –Typically formed secondarily from limestone Chert (siliceous deposits) –Made of microcrystalline quartz –Varieties include flint and jasper (banded form is called agate)

Sedimentary Rocks A.Quartz (SiO 2 ) “flint”, “chert”

Chert nodules

Chemical sedimentary rocks Common chemical sedimentary rocks Evaporites (saline deposits) –Evaporation triggers deposition of chemical precipitates –Examples include rock salt and rock gypsum

C.Chemical and Biochemical Rocks Evaporites (Halite and Gypsum)

salt flats in western Utah

Dessication Cracks

C.Chemical and Biochemical Rocks Limestones Micrite Calcarenite

Limestones Chalk Coquina Fossiliferous

Limestones Oolitic

Oolites

C.Chemical and Biochemical Rocks Limestones Dolomites

Sinkhole: a circular depression in a karst area, commonly funnel- shaped.

This is what lies below that sink hole! And yes my feet were standing on air!

Examples of Sedimentary Features and Landforms The Queen’s Throne- Utah Narracoorte Caves, southeastern SA Stalactites (of mineral calcite). Biochemical and inorganic sedimentary limestone rocks.

Stalactites hanging (tight) to the ceiling of a cave.

This is what a cave typically looks like.

A speleothem made of the mineral gypsum, common in dry caves.

Chemical sedimentary rocks Common chemical sedimentary rocks Coal (Carbonaceous deposits) –Different from other rocks because it is composed of organic material –Stages in coal formation (in order) –1. Plant material –2. Peat –3. Lignite –4. Bituminous

coal

Sedimentary environments A geographic setting where sediment is accumulating Determines the nature of the sediments that accumulate (grain size, grain shape, etc.)

Sedimentary environments Types of sedimentary environments Continental –Dominated by erosion and deposition associated with streams –Glacial –Wind (eolian) Marine –Shallow (to about 200 meters) –Deep (seaward of continental shelves)

Sedimentary environments Types of sedimentary environments Transitional (shoreline) –Tidal flats –Lagoons –Deltas

Continental (left) and marine (right) depositional environments

Sedimentary environments Sedimentary facies Different sediments often accumulate adjacent to one another at the same time Each unit (called a facies) possesses a distinctive set of characteristics reflecting the conditions in a particular environment The merging of adjacent facies tends to be a gradual transition

Sedimentary facies

Characteristics of Sedimentary Rocks  May stratified because the sediments are laid down in horizontal layers called strata. ( one layer is called stratum)  May also contain fossil i.e: remains, prints or other indications of plants & animals found buried in rocks Dipping sedimentary layers of rock, Rocky Mountains, Canada.

Sedimentary structures Provide information useful in the interpretation of Earth history Types of sedimentary structures Strata, or beds (most characteristic of sedimentary rocks) Bedding planes that separate strata

Fossils: Evidence of past life By definition, fossils are the traces or remains of prehistoric life now preserved in rock Fossils are generally found in sediment or sedimentary rock (rarely in metamorphic and never in igneous rock)

Fossils: Evidence of past life Geologically fossils are important for several reasons Aid in interpretation of the geologic past Serve as important time indicators Allow for correlation of rocks from different places

Features of Sedimentary Rocks Stratification (bedding) is when layers of sedimentary rocks form stacked on top of each other

Features of Sedimentary Rocks Ripple Marks are sand patterns formed by the action of winds, streams, waves, or currents

Features of Sedimentary Rocks Mud Cracks develop in clay when it dries out and hardens into rock.