Lithification and Classes Sedimentary Rock Lithification and Classes

The Rock Cycle What processes produce sediment? What types of rocks are the sources of sediment? What processes produce sedimentary rocks?

Sedimentary Rocks Sediments are produced by the deposition of existing rocks – any type of rock - that have undergone weathering and erosion These sediments can then be compacted and cemented into sedimentary rock

Deposition Deposition – settling of weathered and eroded materials Commonly in a water environment (but not always) Environment of deposition is related to the type of sediments that are deposited For example – small sized sediments are deposited very slowly since they need very little energy to carry them Therefore sediments like these will only be deposited in deep and/or still water (lakes)

Walther’s Law Walther’s Law relates environment of deposition of several rock units Can be used to correlate sea transgressions

Sediments Deposited sediments have porosity Porosity measures the gaps between the particles Large, irregular shaped particles have a higher porosity

Sorting Depending on the type of erosion, sediments will be either well or poorly sorted. Well sorted sediments all have (nearly) the same size Poorly sorted materials have very different sized sediments Very poor sorting is a good indication of glacially formed sediments!

Sorting Poorly sorted Well sorted

Compaction After sediments have been deposited and buried for a long time, they are sometimes compacted Compaction is due to the weight of the overlying material Removes any trapped water from the sediment and closes the pore space

Cementation Cementation occurs when minerals “cement” the sediments together into a rock Common cements are silica, iron oxide, and carbonates VERY LONG process – can take thousands of years! Entire process of weathering, erosion, deposition, compaction, and cementation is referred to as Lithification

Classes of Sedimentary Rocks

Clastic Sedimentary Rocks Clastic sedimentary rocks form from the compaction and cementation of particles that do not dissolve in water Silica is a major component and is often the cement Classified based on their texture (grain size) and composition (minerals)

Clastic Sedimentary Rocks Textural classification compares grain sizes and is based on the Wentworth Scale of Particle Sizes Particles over 2 mm are called “gravel” Other categories are sand (2 mm – 1/16 mm), silt (1/16 mm – 1/256 mm) and clay (less than 1/256 mm)

Clastic Sedimentary Rocks The largest grained sedimentary rocks are further divided based on the shape of the gravel Rounded gravel – conglomerates Angular gravel – breccia Different shapes indicate the length of time it took for cementation Breccias form relatively quickly – no time for gravel to become weathered

Clastic Sedimentary Rocks

Clastic Sedimentary Rocks In order of decreasing grain size, clastic sedimentary rocks include: Conglomerate/breccia Sandstone Siltstone Shale Clastics are then described by the composition For example – a Red Shale contains Iron; an Arkose Sandstone contains feldspar

Clastic Sedimentary Rocks

Clastic Sedimentary Rocks As noted earlier, the environment of deposition determines the type of rock formed Conglomerates/breccias are formed in high-energy environments (rivers, oceans, glacier deposits) Sandstones are formed in moderate energy environments (mature rivers, beach, desert) Siltstone form in low energy environments (old rivers, estuaries, lakes, nearshore) Shales form in very low energy environments (deep lakes, offshore, flood plains, deltas)

Clastic Sedimentary Rocks

Clastic Sedimentary Rocks Geologists use the type of rock present to reconstruct the past For example – when sandstone is present, it indicates a former beach (or desert) environment Geologists then use other clues (fossils or sedimentary structures) to narrow the possibilities

Non-clastic Sedimentary Rock Non-clastic rocks are the result of chemical or biochemical processes Chemical processes include evaporation and chemical precipitation Chemical precipitation is where a solid forms spontaneously in a liquid Biochemical processes include the compaction of living materials (shells, leaves, etc)

Non-clastic Sedimentary Rock Chemical sedimentary rocks include Halite (Rock salt) Gypsum (Rock gypsum) Chert Limestone Dolostone Biochemical sedimentary rocks include: Chalk Coquina Coal

Non-clastic Sedimentary Rock

Non-clastic Sedimentary Rock

Sedimentary Structures Features of the rock unit that were produced as it formed Very useful in determining the environment of deposition! Includes: Layering Surface Structures

Sedimentary Structures - Layering Very obvious feature! Thickness of the layers indicates the stability of the environment Thinner layers are from a rapidly changing environment Layers can be of different sized sediments, different colored sediments, or different types of sorting

Sedimentary Structures - Layering Cross-Bedding occurs where there was frequent changes in wind/water movement Common in beach, river, and dune deposits

Sedimentary Structures - Layering Graded Bedding occurs when sediment size changes from large to small Indicates a delta environment

Sedimentary Structures – Surface Structures Surface structures indicate water currents, climate, and wind directions Surface structures include: Ripple marks Mud cracks Fossils Rain drops

Sedimentary Structures – Surface Structures