Sedimentary Rocks

• Sedimentary Rocks
• » lithification of sediments
• sediments - unconsolidated particles from the weathering of the earth’s surface
• Lithification
• Process whereby sediment is converted to rock through compaction and cementation.

• Common Cements
• Silica - SiO₂
• Calcium carbonate - CaCO₃
• Iron oxide - Fe₂O₃•H₂O

Types of Sedimentary Rocks

• Types of Sedimentary Rocks
• Clastic (Detrital) - composed of particles derived directly from rock weathering and transported to site of deposition
• Organic - lithified remains of plants and animals (coal, chert)
• Chemical - abiotic precipitation of dissolved ions (evaporites)
• Bioclastic - clasts of biological origin (originally shell fragments)

• Clastic Sedimentary Rocks
• >90% of all sediments
• Composed of chemically resistant mineral and rock fragments
• The composition and texture of a detrital rock are determined by its history

• Texture
• Grain Size
• most important
• Grain Shape
• rounded vs. angular
• Sorting
• distribution of grain sizes

• Grain shape
• Grain sorting (see transparency)

• poorly sorted
• intermediate sorted
• well sorted

• Composition
• composed of minerals with slow rates of chemical weathering, or the products of chemical weathering
• Composition varies with weathering intensity or length of transport
• low - qtz, feldspar (mostly K-spar), rock fragments, mica, amphibole, pyx (?)
• medium - qtz, K-spar, mica, clays, rock fragments
• high- qtz, clays, K-spar, and clay minerals dominate

• Sediment transport
• moving water, wind, or ice
• fluid flow velocity - energy - controls ability to move particles (competency)
• slowing causes coarser particles to drop out (i.e., be deposited)
• as rivers enter lake or ocean, coarsest particles drop out at mouth; finer particles carried further

• Clastic Sedimentary Rocks
• conglomerate - contains granule-sized grains and larger; grains rounded
• breccia - angular granule-sized and larger grains
• sandstone - 20% of all sedimentary rock
• quartz sandstone - mostly quartz; well sorted, well rounded
• arkose - angular grains; feldspars and rock fragments common

• graywacke - “dirty sandstone”; contains quartz, feldspar, rock fragments and clay
• siltstone - mostly silt
• mudstone - silt and clay; no shaly parting (tendency to split along bedding)
• shale - most common sedimentary rock (70%); mostly clay, shaly parting
• color give clue to environment of deposition:
• red - oxygen present (oxic)
• gray or black - oxygen absent (anoxic)

• Mature vs immature sediments - determining environment of deposition

• Terrestrial
• alluvial (rivers)
• lake
• desert (wind)
• glacial
• Marginal marine
• beach (waves, tides)
• delta (rivers, tides)