Sedimentary Rocks The products of deposition and lithification of weathered and eroded debris and chemical precipitants. BY FAR, most of the rocks exposed on the Colorado Plateau are sedimentary!

Weathering: the physical breakdown and chemical alteration of rock at or near the Earth’s surface. Mechanical weathering: from physical forces (e.g. frost wedging, thermal expansion) Chemical weathering: chemical alteration of rock (e.g. dissolution, oxidation) Erosion: the physical removal and transportation of weathered material by water, wind or ice.

Weathering Erosion

Types of sedimentary rocks: Clastic (detrital): from deposits of detritus (loose sediment) Are lithified by compaction and cementation and tend to be more permeable with larger grain sizes Chemical: from reprecipitation (and recrystallization) of minerals Crystals grow until they fill all of the pore space, so these rocks tend to be relatively impermeable until they are exhumed (brought towards the surface) or fractured

Weathered material (sediment) has to go somewhere… So, Gravity carries the sediment down until it settles out of air or water Lithification: the process by which unconsolidated sediments are changed into solid sedimentary rock

Lithification Processes for clastic rocks: Compaction: compression of sediment from the weight of the overlying material Cementation: A chemical change that involves the precipitation of mineral among the original grains Most common cement types: Silica: creates a harder, more resistant sedimentary rock Calcite: creates a softer rock and will effervesce with HCl Iron oxides: causes a red color on weathered surfaces from oxidation

Common minerals in detrital rocks Clay: most abundant product of chemical weathering from feldspars (clay is a mineral name and a size term) Quartz: most resistant mineral –when granite is weathered, chemical weathering attacks the other minerals and the quartz grains are freed Less common: Feldspars: break down easily from chemical weathering processes Micas: Soft, breaks down easily from mechanical weathering processes

Clastic (Detrital) Rocks Conglomerate/Breccia Sandstone/Arkose Siltstone Mudstone Claystone Grain Size

Grain Size: gives information about the energy of the depositional system

Sorting: The degree of similarity in particle size Gives us information about the depositing current (e.g. wind blown sand = well sorted, mountain stream deposits = poorly sorted Very well sorted Well sorted Moderately sorted Poorly sorted Very poorly sorted 0.35 0.5 0.7 2.0

Composition of detritus Rounding Tells you the distance material was transported Composition of detritus Tells us about rapidness of burial and length of transport, because less stable minerals will weather quickly at the surface

Classification by % matrix Sediment size Sediment Name Sedimentary Rock Less than 0.0004 cm   Clay Shale 0.0004 cm – 0.006 cm Silt Siltstone 0.006 cm – 0.2 cm Sand Sandstone Mixed Conglomerate (rounded sediments) Breccia (angular sediments)

Shale Most common sedimentary rock Comprised of silt and clay sized particles Form gradual in a relatively calm environment Compaction: clay minerals wrap around silt particles Darker= more organics Low porosity from tightly packed parallel alignment of clay and silt

Quartz Sandstone Second most common sedimentary rock Contains sand sized particles of quartz Form gradually in a relatively calm environment Often contains bedding Grains cemented together Greater permeability than shale

Conglomerate/ Breccia Usually poorly sorted Contains particles larger than sand size Conglomerate has rounded clasts while breccia has angular ones From a high energy system near the sediment source Can be from glacial activity Are rarer than Ss and Sh because they are usually deposited in a system that has not reached equilibrium Breccia Conglomerate

Carbonates Organic carbonates Inorganic Carbonates Limestone makes up ~10% of all sedimentary rocks Made up predominantly of calcite and dolomite Can form by inorganic means or biochemically Although they are produced under a variety of conditions, those with a marine, biochemical origin are the most common Organic carbonates Inorganic Carbonates

Sedimentary Environments Continental Fluvial (stream) Alluvial Fan Eolian Playa Lake Marine Shallow Marine Deep Marine Transitional Beach Tidal flat Lagoon Delta