Sedimentary Rocks n A sedimentary rock is formed

Sedimentary Rocks n A sedimentary rock is formed by the accumulation and compaction of sediments (rock pieces, minerals, animal parts, or chemical precipitates)

n Sediments are derived from weathering of rocks u Chemical weathering – Dissolution or decomposition of minerals grains due to chemical reactions

Bowen’s Reaction Series: The last minerals to crystallize are the most resistant to weathering. Susceptible to weathering Resistant to weathering

u Mechanical weathering – Breakage of rock due to cracking, scratching, crushing, freezing, thawing, and abrasion

n There are 3 different types of sediments 1) Detrital (clastic) sediments – Pieces of eroded rock fragments and minerals grains. Sediments of this type form Detrital Sedimentary Rocks

n There are 3 different types of sediments u Detrital (clastic) sediments – Pieces of eroded rock fragments and minerals grains. Sediments of this type form Detrital Sedimentary Rocks u Chemical sediments – Minerals precipitated from water due to chemical reactions (ex. calcite, gypsum, halite). These sediments form Chemical Sedimentary Rocks

n There are 3 different types of sediments u u Detrital (clastic) sediments – Pieces of eroded rock fragments and minerals grains. Sediments of this type form Detrital Sedimentary Rocks Chemical sediments – Minerals precipitated from water due to chemical reactions (ex. calcite, gypsum, halite). These sediments form Chemical Sedimentary Rocks u Biochemical sediments – Mineral remains of organisms or minerals precipitated due to biological processes (ex. shells, plant fragments, carbon). Sediments of this type form Biochemical Sedimentary Rocks

n Detrital sedimentary rocks are classified according to their grain characteristics: u Size of the grains: refers to the range of sizes of the individual sediment pieces found within the rock. Can be divided into three categories: t coarse-grained • Boulder, cobble, pebble, granule t medium-grained • Sand t fine-grained • Silt, clay

Classification of Clastic Sedimentary Rocks based on grain sizes Category: Sediment: Coarse Medium Boulder | Cobble | Pebble | Sand Size (mm): 256 Rock Name: Conglomerate 64 2 Fine | 0. 062 Sandstone Silt | Clay 0. 0039 Siltstone Shale

u Grain shape refers to the roundness of the minerals. Grains that are well-rounded indicate they have traveled a long distance Angular Rounded Well-Rounded

u Grain sorting refers to the amount of similar size pieces within a sample Well-Sorted Poorly-Sorted

n Chemical and Biochemical sedimentary rocks are classified by their chemical composition n Common chemical sedimentary rocks include: 1) Crystalline Limestone (calcite = Ca. CO 3)

2) Chert (quartz = Si. O 2) 3) Rock Salt (halite = Na. Cl)

4) u Rock Gypsum (gypsum = Ca. SO 4 · 2 H 3 O) Common biochemical sedimentary rocks include: 1) 2) Fossiliferous Limestone (well cemented calcite body parts)

2) Coquina (poorly cemented calcite shells) 3) Bituminous Coal (Carbon compounds from partially decayed plant matter)

n A depositional environment is any place where sedimentary rocks form n Environments can be characterized as high, medium or low energy

n The larger the pieces being moved, the higher the energy of the environment u u n Mountain river, alluvial fan = high energy Swamp, deep ocean = low energy The more the pieces are being moved about, the higher the energy of the environment u u Swash zone of beach = high energy Floodplain, playa lake = low energy

n Sedimentary structures are features preserved in rocks that give information about the depositional environment. u Common examples include: t (a) cross-beds,

t (b) ripples, and t (c) bioturbation