Sedimentary Rocks What is a sedimentary rock Sedimentary

Sedimentary Rocks

What is a sedimentary rock? • Sedimentary rocks result from mechanical and chemical weathering • Comprise ~ 5% of Earth’s upper crust • About 75% of exposed rocks • Contain evidence of past environments • Record how sediment is transported • Often contain fossils

What is the economic importance of sedimentary rocks? • They are important for economic reasons Remember this because they contain when we talk about correlation. Note • Coal how beds pinch out or are offset by • Petroleum and natural gas faults • Iron, aluminum, uranium and manganese • Geologists use them to read Earth’s history

How does sediment become rock? • Diagenesis – chemical and physical changes that take place after sediments are deposited • Diagenesis varies with composition Chesapeake Bay from Skylab False color image www. nasa. gov

Turning sediment into rock – Diagenesis includes: – Recrystallization – growth of stable minerals from less stable ones – Lithification – loose sediment is transformed into solid rock by compaction and cementation Natural cements: calcite, silica, and iron oxide. Formed from ions in solution in water.

Lithification • Compaction: As more sediments are piled on top, compaction drives out the excess water. • Cementation: Precipitation of chemicals dissolved in water binds grains of a sediment together. • Remember where the dissolved chemicals come from?

Types of sedimentary rocks • Chemical rocks – sediment from ions that were once in solution • Detrital rocks –sediment transported as solid particles

Detrital sedimentary rocks • Rocks made of grains • Constituents of detrital rocks can include • Clay minerals • Quartz • Feldspars • Micas • Particle size is used to distinguish among the various types of detrital rocks

Ions weather out of rock, are transported by groundwater to sediment layers below Sediment grains moved to ocean by streams 6_11 Ions transported to lake or Water enters pore ocean spaces between sediment grains Ion-rich groundwater Dissolved ions precipitate to form cement between sediment grains

Detrital sedimentary rocks • Mudrocks: less than. 06 mm – 1. Mud: small particles easily kept in suspension – Settles in quiet water – Includes Shale: mud-sized particles <. 004 mm deposited in thin bedding layers called laminae Most common sedimentary rock 2. Larger mudrock grains called silts silt-sized particles. 004 -. 06 mm Gritty grains can be felt

Detrital sedimentary rocks • Sandstone – Made of sand-sized particles. 064 – 2 mm – Forms in a variety of environments – Sorting, angularity and composition of grains can be used to interpret the rock’s history – Quartz is the predominant mineral (due to its durable nature)

Classifying Sandstones Plagioclase grain Photomicrograph of quartz rich sandstone (Arenite) Grains subangular Making thin sections to subrounded, sandstone is poorly sorted

Types of Sandstone • Quartz Arenite >90% quartz grains – Beach and dune deposits • Arkoses >25% feldspar, angular, poor sort. – Transform boundaries; exposed granites – Any felsic rock eroded, not transported far • Graywackes Quartz, feldspar, volcanics – Port sorted, angular – Erosion of Island Arcs – Rift Valley Sediments

Detrital sedimentary rocks • Conglomerate and breccia – Both composed of particles > 2 mm in diameter – Conglomerate consists largely of rounded clasts. Rounded pebbles in high velocity areas – Breccia is composed of large angular particles Breccia is made of shattered rock that accumulates at the base of a cliff

Energy • Coarse sediments are deposited in high energy (fast water) environments such as under breaking waves at the beach, or in the beds of fast streams. • Fine sediments are deposited in low energy environments, e. g. the slow water of deep lagoons, the abyssal plain, etc.

Outcrop of conglomerate with cobble-sized clasts interbedded with sandstone Conglomerates are fastwater sediments “High Energy” K. E. = 1/2 mv 2 In fast water, smaller sizes swept away

Chemical sedimentary rocks • Precipitated material once in solution • Precipitation of material occurs two ways: • Inorganic processes: the minerals precipitate out of water • Organic processes: animals and plants precipitate the minerals to use as shells or skeletons http: //www. ucmp. berkeley. edu/collections/micro. html

• Common chemical sedimentary rocks • Limestone –Most abundant chemical rock –Made of the mineral calcite Ca. CO 3 –Marine biochemical limestones form as coral reefs, coquina (broken shells), and chalk (microscopic organisms) –Inorganic limestones include travertine (caves) and oolitic limestone (Bahamas)

http: //www. ndsu. nodak. edu/instruct/ashworth/coursework/g 410/evaporites/saltbeds. jpg • Common chemical sedimentary rocks • Evaporites –Evaporation triggers deposition of chemical precipitates –Examples include rock salt and rock gypsum

Chalk Outcrops in SE USA Chalk Hand Specimen

Inorganic Chemical Sediments Ooids under microscope Oolitic Limestone - Bahama Shoals Oolitic Limestone - Hand Specimen

• Other chemical sedimentary rocks • Chert – Made of microcrystalline quartz – Usually deposited as siliceous ooze in deep oceans (can be diatomaceous) chert Diatomaceous chert

• Other chemical sedimentary rocks – Dolostone (made of mineral Dolomite) – Like Calcite, but some Ca is replaced by Mg The Dolomites, sediments thrust up when the Alps formed

Evaporation How to make Dolomite 6_19 Limestone Seawater enriched in Mg 2 -rich seawater circulates through porous limestone Dolostone Mg 2 replaces some of the Ca 2 in limestone

Chemical Sediments: Coal

Sedimentary environment determines roundness sorting, mineral diversity Character of detrital sediments depends on time, 6_5 distance, and energy. For example, in streams: Particles are large and irregular, and consist of a variety of lithologies, including the least resistant. Particles are mid-sized and of intermediate sphericity, and include resistant and nonresistant lithologies. HIGHLANDS LOWLANDS Particles are small and nearly spherical, and consist mainly of the most resistant lithologies, such as quartz. NEAR-COASTAL

• Sedimentary Facies • Different sediments accumulate next to each other at same time • Each unit (called a facies) possesses a distinctive characteristics reflecting the conditions in a particular environment • The merging of adjacent facies tends to be a gradual transition

Some Facies in an oversimplified drawing Abyssal Ooze Stillwater muds Nearshore sands

A picture glossary of sedimentary Turbidite: underwater landslide = graded bedding environments

Sedimentary structures • Tell us something about past environments • Types of sedimentary structures • Strata, or beds (most characteristic of sedimentary rocks) -bedding planes that separate strata caused by variation in deposition

Strata- Bedding Planes

Fine Scale Bedding- Lamellae

Graded bedding Floods change the local conditions 6_6 Fine-grained sediment On floodplain Older sediment 1 Pre-flood Bounders on bottom, sands and muds suspended Flood water Erosion of uppermost fine-grained sediment 2 Flood stage Waning flow Fine-grained above Coarse-grained below Bedding plane 3 Post-flood

Graded Beds – grains fine upward Note: Beds were tilted from horizontal after deposition

Recognizable Sedimentary Structures • Ripples • Irregularities in bottom sediment lead to ripples • Asymmetric types indicate flow direction. • Symmetric types formed in tidal areas

Slabs of eroding sandstone with ripple marks

Cross Beds are ripples in cross section • Irregularities lead to ripples, dunes, sand bars. • In cross section these look like lines at an angle to the horizontal – “cross beds” • Ripples can indicate direction of air or water flow if asymmetrical, a tidal environment if symmetrical. Size and shape indicate fluid velocity.

Cross bedding in Sand Dune deposits Navaho Sandstone deposited in ancient sand dunes Frosted Grains, well sorted

Mud Cracks: clay layer shrinks during drying, curls upward; cracks fill next flood. Useful for right-side up

Sedimentary Environments • Sediments are formed in many different environments • Each have characteristic appearance today, features that allow them to be recognized in the geologic record

Fresh Water Facies Streams (includes big Rivers), with floodplains and levees, called fluviatile. The Point Bar Sequence is typical for meandering streams. Cutoffs generate Oxbow deposits. • High gradient streams with high sediment load are Braided. • Lake deposits called lacustrine, generally still waters, often varved deposits if winters cold http: //hays. outcrop. org/gallery/rivers/arid_meander? full=1

Point-bar Sequence: Meandering Stream Deposits Associated with Meandering Streams Ox. Bow Floodplain http: //hays. outcrop. org/gallery/rivers/arid_meander? full=1

Point Bar Sequence Fines of Floodplain Crossbeds of Bar Gravel of bed Erosion

Terms for Marine (i. e. Ocean) Environments 6_27 and some characteristic sediment facies Continental shelf Shallo w marin e Continental slope Abyssal Plain Dee p ma rine Submarine volcanoes

Facies changes due to rising sea level - water getting deeper everywhere River 6_29 Direction of migration of shoreline, and landward shift of sedimentary facies Time B Shoreline at time B Time A Shoreline at time A Shallow marine Beach River Sea level rising Deep marine Shallow marine Beach Comparison of sediments deposited Deep marine Shallow marine Deposited at time A Deposited at time B REMEMBER: the facies follow the shoreline

Fossils are traces of prehistoric life generally preserved in sedimentary rock

Dinosaur footprint in mudstone

End of Sedimentary Rocks