Chapter 6 Sediments and Sedimentary Rocks GEOL 101 Introductory Geology Chapter 6 Sediments and Sedimentary Rocks

Sediments Collective name for loose, solid particles originated from weathering and erosion, thus Unconsolidated. Classification of sediments based on sizes Gravel > 2mm Sand between 1/16 and 2 mm Silt between 1/256 mm and 1/16 mm Clay < 1/256 mm

Change from Sediments to Sed Rxs Transportation Roundness: round – distance from source Sorting: well sorted - distance from source Deposition: settle of particles Preservation

Change from Sediments to Sed Rxs Lithification Compaction – reduction in pore space Cementation – seal out the pores Crystallization – growth of crystals from solution.

Sedimentation Process

What is Sedimentary Rock? Sedimentary rocks are products of mechanical and chemical weathering Account for about 5 % (by volume) of Earth’s upper crust Contain evidence of past environments Provide information about sediment transport Often contain fossils Economic importance Coal Petroleum and natural gas Sources of iron, aluminum, and manganese

Sedimentary Rocks Rock types Classification Sedimentary Environments Fossils

Sedimentary Rock Classification Classified according to material type Clastic Chemical Two major textures are used in the classification of sedimentary rocks Discrete fragments and particles All detrital rocks have a clastic texture Nonclastic Pattern of interlocking crystals May resemble an igneous rock

Sedimentary Rocks Types Sediment originates from mechanical and/or chemical weathering Rock types are based on the source of the material Clastic 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 Particle shape and sorting give indication of transport distance (maturity)

Particle Size Classification

Sorting, Shape, Setting Sorting Sphericity Roundness Depositional

Clastic Sedimentary Rocks Common Clastic 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 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

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

Shale containing plant remains

Sandstone

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) Common chemical sedimentary rocks

Chemical Sedimentary Rocks Common Types Limestone Dolostone Chert Evaporites Coal

Limestone Most abundant chemical rock Composed of calcite carbonate CaCO3 Marine biochemical limestones form as coral reefs, coquina (broken shells), and chalk (microscopic organisms) Inorganic limestones include travertine and oolitic limestone

Coquina

Fossiliferous Limestone

Chemical Sedimentary Rocks Dolostone Composed of calcite, magnesium carbonate CaMgCO3 Typically formed secondarily from limestone Chert Made of microcrystalline quartz Varieties include flint , jasper and agate (banded form) Evaporites Evaporation triggers deposition of chemical precipitates Examples include rock salt and rock gypsum

Chert Agate – Banded Chert

Coal Common chemical sedimentary rocks Different from other rocks because it is composed of organic material Stages in coal formation (in order) Plant material Peat Lignite Bituminous

Coal

Coal Formation Stages

Sedimentary Environments A geographic setting where sediment is accumulating Determines the nature of the sediments that accumulate (grain size, grain shape, etc.) Identified by sedimentary facies, fossils, and structures

Sedimentary Environments Continental Dominated by stream erosion and deposition Glacial Wind (eolian) Marine Shallow (< 200 meters) Deep (seaward of continental shelves) Transitional (shoreline) Tidal flats Lagoons Deltas

Depositional Environments

Depositional Environments Continental Marine

Turbidity Currents

Turbidity Currents Graded Bedding

Evaporite Formation

Sedimentary Facies Different sediments often accumulate adjacent to one another at the same time Facies: possesses a distinctive set of characteristics reflecting the conditions in a particular environment Merging of adjacent facies tends to be a gradual transition

Sedimentary Facies

Sedimentary Structures Useful for interpreting sedimentary environment Strata: or beds, layers of similar sediment (most characteristic of sedimentary rocks) Lamina: thin layers (< 1cm) Bedding planes separate strata Cross-bedding Graded beds Ripple marks Mud cracks

Bedding

Bedding Structures Graded bedding Lamina

Cross Beds

Ripple Marks unconsolidated sediment lithified rock

Mud Cracks

Grand Canyon o

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)

Dinosaur footprint in limestone

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 Studied in detail in Historical Geology (GEOL 102), offered in Spring 2004

Chalk Cliffs of Dover, England Tiny planckon shells Deposited on sea floor Later uplifted