Sedimentary Rocks Chapter 6

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 miles 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

Turning sediment into rock Many changes occur to sediment after it is deposited Diagenesis – all of the chemical, physical, and biological changes that take place after sediments are deposited Occurs within the upper few kilometers of Earth’s crust

Turning sediment into rock Diagenesis Includes Recrystallization – development of more stable minerals from less stable ones Lithification – unconsolidated sediments are transformed into solid sedimentary rock by compaction and cementation Natural cements include calcite, silica, and iron oxide

Lithification

Types of sedimentary rocks Sediment originates from mechanical and/or chemical weathering Rock types are based on the source of the material Detrital rocks – transported sediment as solid particles Chemical rocks – sediment that was once in solution

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

Detrital (or clastic) sediments They have a clastic (broken or fragmental) texture consisting of: Clasts (larger pieces, such as sand or gravel) Matrix (mud or fine-grained sediment surrounding the clasts) Cement (the glue that holds it all together), such as: calcite iron oxide silica

Clasts and matrix

Detrital sediments are classified according to their texture (grain size): Gravel: Grain size greater than 2 mm (Conglomerate & Breccia) Sand: Grain size 1/16 to 2 mm (Sandstone) Silt: Grain size 1/256 to 1/16 mm (gritty) (Siltstone) Clay: Grain size less than 1/256 mm (smooth) (Shale) Shale and siltstone are called mudrocks

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 laminae Most common sedimentary rock

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

Sandstone

Arkose The presence of feldspar suggests that little chemical weathering has occurred – this is associated with arid climates

Sandstone Close-ups Quartz sandstone (left) Arkose (sandstone with >10% feldspar)

Outcrop of Sandstone

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

Conglomerate

Outcrop of Conglomerate

Interbedded sandstone and conglomerate

Breccia

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 Most abundant chemical rock Composed chiefly of the mineral calcite Marine biochemical limestones form as coral reefs, coquina (broken shells), and chalk (microscopic organisms) Inorganic limestones include travertine and oolitic limestone

Coquina

Fossiliferous limestone

Oolitic Limestone

Travertine forming

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

Chemical sedimentary rocks Common chemical sedimentary rocks Evaporites Evaporation triggers deposition of chemical precipitates Examples include rock salt and rock gypsum

Rock Salt

Bonneville Salt Flats

Layered gypsum

Chemical sedimentary rocks Common chemical sedimentary rocks Coal 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

Successive stages in coal formation

Classification of sedimentary rocks Sedimentary rocks are classified according to the type of material Two major groups Detrital Chemical

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

Identification of sedimentary rocks.

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 Fluvial - dominated by erosion and deposition associated with streams Glacial Eolian (Wind) Marine Shallow (to about 200 meters) Deep (seaward of continental shelves)

Sedimentary environments Types of sedimentary environments Transitional (shoreline) Deltas Estuaries Barrier island system Beaches Lagoons Tidal flats and marshes

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

Marine Transgressions A marine transgression occurs when sea level rises with respect to the land During a marine transgression, the shoreline migrates landward the environments paralleling the shoreline migrate landward as the sea progressively covers more and more of a continent

Marine Transgressions Each laterally adjacent depositional environment produces a sedimentary facies During a transgression, the facies forming offshore become superposed upon facies deposited in nearshore environments

Marine Transgression The rocks of each facies become younger in a landward direction during a marine transgression One body of rock with the same attributes (a facies) was deposited gradually at different times in different places so it is time transgressive meaning the ages vary from place to place younger shale older shale

Transgressions and Regressions

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 Cross-beds – formed by currents

Grand Canyon showing strata

Bedding Sedimentary rocks generally have bedding or stratification Individual layers less than 1 cm thick are laminations common in mudrocks Beds are thicker than 1 cm common in rocks with coarser grains

Sedimentary structures Types of sedimentary structures Graded beds - an upward gradual decrease in grain size Ripple marks- can be symmetrical or asymmetrical; formed by currents or waves Mud cracks – polygonal pattern of cracks produced on the surface of mud as it dries

Graded Bedding

Graded Beds

Wave Ripples

Ripple marks

Recent mudcracks

Ancient mudcracks

Asymmetrical ripples and cross-bedding

Cross-Bedding To view this animation, click “View” and then “Slide Show” on the top navigation bar. 61

Cross-stratification, upper Paleozoic rocks, Birmingham, AL

Great Sand Dunes National Monument, Colorado What is the prevailing wind direction?

X-beds in Navajo Sandstone What formed these x-beds? In what direction was the wind blowing? Wind

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 almost 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

Natural casts of ammonites

Natural cast of a trilobite.

Dinosaur footprint in limestone

End Chapter 6