Soils, Sediment, Weathering and Sedimentary Rocks
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Soils, Sediment, Weathering and Sedimentary Rocks Sedimentary Rocks Rocks that form by the cementing of grains or fragments of pre-existing rocks, or by the precipitation of minerals out of a solution Form a veneer over basement rocks (igneous and metamorphic) Sedimentary layers (cover) over the Vishnu schist (basement) in the Grand Canyon, AZ
Weathering and Erosion • Sediment – loose fragments of rocks or minerals broken off of bedrock, minerals that precipitate directly out of water, and shells of organisms. • Deposition / Sedimentation – occurs when sediment settles out as winds/water current die down, or as glaciers melt. • Lithified – sediment that has been cemented together by geologic processes to form a rock • Rocks are broken down and turned into sediments by two main processes – Physical Weathering • Plumbers snake – Chemical Weathering • Liquid Drain-O Highly weathered sandstone in Bryce Canyon N.P., UT
How do Sedimentary Rocks Form? The following applies to Clastic/Detrital rocks; biochemical and chemical sed rocks are different • 1) Erosion – mobilizes particles by weathering, most commonly by rainfall & gravity. • 2) Transportation – Occurs when currents of wind, water, and moving ice (glaciers) transport particles to new locations (downhill or downstream) • 3) Deposition – sediment is deposited when the transporting current slows to the point that it can no longer carry its load. • 4) Burial – As layers of sediment accumulate, the layers accumulate in sedimentary basins. Older, previously deposited sediments are compacted. These layers remain at depth until either erosion or tectonic processes act on them. • 5) Diagenesis & Lithification – Refers to the physical and chemical changes that lithify sediment into rock. Includes pressure, heat and chemical reactions
The Sedimentary Stages of the Rock Cycle Weathering Erosion carries breaks down away particles. rocks. Transportation moves particles downhill. Glacier Delta Desert Playa Deposition occurs lake when particles settle out or precipitate. Sedimentary Burial occurs rocks as layers of Metamorphic sediment rocks accumulate. Plutons Diagenesis lithifies the sediment to make sedimentary rocks.
Detritus & Grain Size • Physical weathering breaks rocks into chunks called detritus • Detritus is classified by size (diameter) » Boulder > 256 mm Visible grain size » Cobble - between 64 and 256 mm Coarse-grained » Pebble - between 2 and 64 mm Medium-grained » Sand - between 1/16 and 2 mm Fine-grained » Silt - between 1/256 and 1/16 mm Microscopic grain size Fine-grained » Mud < 1/256 mm
Physical/Mechanical Weathering Rocks can be physically weathered by: • Jointing – Formation of cracks in rocks. Joints – form in rocks due to, stretching, or cooling (contraction) • Exfoliation joints – when deep rocks are exposed, the removal of overburden causes sub-horizontal cracks to form causing the rock to easily peel away like layers of an onion. Also called sheet joints or unloading joints and are common in exposed batholiths.
Joints in the Field • Below: Joints in sedimentary rocks in Brazil. Near vertical joints in sedimentary rocks are common. • Above: Exfoliation / Sheet / Unloading joints in the Sierra Nevada Batholith (granite) in California.
Physical/Mechanical Weathering • Frost wedging: water fills cracks, freezes, expands, and forces cracks to open causing them to grow. Can lift large blocks. • Root wedging: same as frost wedging except that roots pry open the cracks. • Salt wedging: salt crystals form when evaporating water flows through rocks. The salt crystals pry open the cracks
Results of Physical Weathering • Eventually, mechanical weathering processes create an apron or pile of debris at the margins of slopes called talus
Chemical Weathering Chemical weathering is typically strongest in warm wet climates Types include: • Dissolution: – Primarily affects carbonates and salts…when a chemical reaction breaks down minerals into new compounds • E.g. CaCO3 (Calcite) + H2CO3 (carbonic acid) Ca2+ (aq) + 2HCO-3 (bicarbonate) • Hydrolysis: – Water acts to ‘loosen’ chemical bonds to break down minerals. Works faster in slightly acidic water • E.g. H2O (acidic) H+ + OH- • E.g. H+ + KAlSi3O8 (K-feldspar) Al2Si2O5(OH)4 (Kaolinite) + K+ (aq) • Kaolinite is a clay mineral • Oxidation: – When an element loses an electron…commonly when it bonds with oxygen. • E.g. 4Fe2+ + 3O2 2(Fe3+)2O3 (iron lost an electron and went up in charge) • Hydration: – Absorption of water into some minerals (mainly clays) causes them to expand
Dissolution • Chemical weathering processes act on rock/mineral surfaces – Results in rounding of edges
Surface Area and Weathering • All other things being equal, the ratio of surface area to volume of a material controls the rate of weathering
Weathering & Roundness • Weathering tends to round off corners and leave things more smooth in shape. – Angular detritus = not very weathered – Rounded detritus = very weathered
Weathering and Bowen’s Reaction Series Minerals that form early (high temp) in Bowen’s reaction series are least stable. Exceptions are calcite and halite, which are highly susceptible to chemical weathering. Typically, mafic minerals weather by oxidation, felsic minerals weather by hydrolysis, carbonates weather by dissolution, and oxides don’t weather at all. Dissolves Follow’s Bowen’s reaction series!!
Differential Weathering Differential weathering – a primary control on the shape of our physical landscape. Under the same set of climatic conditions, not all minerals and not all rocks will weather at the same rate. Some develop more joints and fractures, some undergo faster dissolution, some remain more-or-less intact. Weak layers weather more quickly, leaving behind more resistant layers. This process occurs on small scale – such as an outcrop of rock And at a large scale, such as entire valleys and mountain ranges. e.g. El Capitán is the product of differential erosion (Show valley and ridge in GoogleEarth) El Capitán – Guadalupe Mtns (SW Texas)
Soil • “Soil consists of rock and sediment that has been modified by physical and chemical interaction with organic material and rainwater, over time, to produce a substrate that can support the growth of plants.” • Soil-forming processes require long periods of time. • Soil may be easily destroyed by human activities. • Soil is a crucial natural resource in need of protection.
Soil Thickness • Weathering breaks rocks down into detritus, which forms regolith • Regolith: any unconsolidated material that covers bedrock – Soil is a regolith • What controls the thickness of soil? 1- Age: The longer a surface has been exposed, the thicker the soil
Soil Thickness 2- Slope: The steeper the slope the less soil will accumulate bowl-shaped areas, such as basins will accumulate thick soils
Soil Thickness 3- Bedrock Type: Thick soils will form over bedrock that is easily weathered, such as carbonates, evaporites, or muds. The soil composition will partly reflect the composition of the bedorock
Soil Thickness 4- Climate: Thick soils will form in warm wet climates; deserts will form little or no soil
Providence Canyon, SW Georgia • Formed due to poor farming techniques in the 1800’s – Trees and natural land cover was removed for agriculture – Fast rates of soil erosion created deep gullies – Up to 150 feet deep
Sedimentary Rock Types • Detrital / Clastic – Cemented fragments of pre-existing rocks. Arkose – E.g. sandstone, mudstone • Biochemical – Rocks made of cemented shells of organisms – E.g. coquina, limestone Coquina • Chemical – made from minerals that precipitate out of water solutions. – E.g. travertine, various evaporites Evaporites near a salt lake
Clastic Sedimentary Rocks • A clastic/detrital rock forms in five stages: 1- Weathering/Erosion 2- Transportation 3- Deposition 4- Burial 5- Lithification and Diagenesis (compaction+cementation) Grain size is reduced as sediment is transported
Classifying Clastic Sedimentary Rocks Clastic/Detrital rocks are classified by: 1. Clast size 2. Clast composition 3. Angularity and Sphericity 4. Sorting 5. Type of cement
Common Clastic Rock Types • You should know most of these from lab, but if not, then make sure to know the basic characteristics of these types of clastic rocks.
Biochemical and Organic Sedimentary Rocks • Biochemical Limestone – A biochemical or chemical rock made of calcite, which is made of the remains of shells of organisms that secrete calcite or aragonite (a polymorph of CaCO3) shells. – Fossiliferous limestone – lots of fossils! Limestone – Micrite – made of calcite mud – Chalk – made of plankton shells • Biochemical Chert – made of cryptocrystalline (microscopic crystals) quartz, formed from the shells of plankton that sat on the sea floor and Folded Chert Beds dissolved into a silica rich ooze. • Organic Rocks – Coal, black shale, made of organics derived from plants and animals • We’ll talk about this stuff more in Chapter 14. A Coal Seam
Chemical Sedimentary Rocks • Evaporites – products of the evaporation of water – Main minerals: gypsum, halite • Travertine (chemical limestone) – limestone that precipitated out of groundwater – E.g. stalactites, stalagmites, also found at gysers & hot springs • Dolomite – chemical alteration of limestone CaCO3 into dolomite CaMg(CO3)2. – Happens when Mg bearing groundwater reacts with limestone. • Replacement Chert – When chert (SiO2) replaces minerals in a rock. – E.g. petrified wood Travertine at Mammoth Hot Springs, MT
Formation of Evaporites • Evaporites can form anywhere where a large body of water evaporates – E.g. the Great Salt Lake, or the Dead Sea • Evaporites can also form when an inland sea has limited circulation – E.g. the Persian Gulf, or the Mediterranean Sea
Sedimentary Structures Many sedimentary rocks have features/structures that help indicate how they formed • Bed – A single layer in a sedimentary rock with a definable top and bottom – Bedding plane – the boundary between two beds; forms when depositional conditions change • Strata – A group of several beds – Bedding/Stratification – The overall arrangement of strata • Bioturbation – When burrowing organisms and/or plant roots destroy bedding
Cross Bedding • Cross Bedding: Internal laminations within a bed that are not parallel to the larger bedding orientation. Usually caused by deposition in a moving current of wind/water. Note: bedding planes and laminations are not always parallel http://www.ed.ac.uk/
Bedforms • Cross bedding – When internal laminations (thin layers) are not parallel to the boundary of the main sedimentary layer • Ripple Marks – a type of cross bedding that forms due to deposition in moving water. Typically < a few cm high Asymmetric – one flow direction Symmetric – two flow directions
Ripple Marks asymmetric ripple marks in a beach Symmetric ripple marks in 1.5Ga quartzite
More Bedforms: Dunes • Dunes - form and grow just like ripple marks except that they are deposited by wind (usually) and can be many meters tall • Since deposition occurs on the slip face dunes migrate with time • The inclination of the cross beds can be used to tell the direction of wind
Ancient Dunes: Zion N.P.
Turbidity Currents and Turbidites • Turbidity Current – An underwater moving cloud of water and sediment. Moves downhill because it is more dense than water. Forms when sediment on a slope in the ocean floor is disturbed (e.g. earthquake or storm). Can carry large boulders, snap phone lines, move sunken ships. • Turbidite – A rock formed by turbidity currents
Turbidites and Graded Beds • Graded bedding – when the grain size in a single bed varies consistently from coarse grained at the base to fine grained at the top. – This is a diagnostic feature of turbidity currents
Turbidites and Graded Beds A typical turbidite with graded bedding
Bed Surface Markings • Mud Cracks – when wet mud dries it contracts and forms pseudo-hexagonal plates that curl up on their edges • Scour Marks – when currents flow over a sedimentary surface, the current may scour out small troughs, which can be preserved • Fossils – Evidence of past life including shells, plant material, and footprints. Scour Marks Mud Cracks Mud Cracks
Rain Spots Mud Cracks
Formations and Groups • Formation – A particularly thick bed or a sequence of alternating rock types, sometimes called a stratigraphic formation • Group – A group of stratigraphic formations • Sedimentary Basins – A region where the surface of the Earth has subsided (gone down) and leaves a space for sediments to fill. Stratigraphic formations in the Grand Canyon, AZ, note that the sandstones and limestones form cliffs, whereas the shales form eroded slopes
Sedimentary/Depositional Environments • Where do sedimentary rocks form? – Marine Environments: Terrestrial Red Beds deposited under the ocean – Terrestrial Environments: deposited on land • Red Beds - Exposure to air can cause oxidation of iron leading to a red color in some terrestrial beds. Marine Limestone
Terrestrial Environments • Glacial – mix of everything boulders-mud. Very poorly sorted. Deposits: Till • Mountain Streams – coarse grained conglomerates and sandstones. Poorly sorted. • Alluvial Fans – deposited at the mountain front by mountain Mountain Stream Glacial Till streams in arid regions. Poor to moderate sorting. Deposits: Arkose, Sandstone, Conglomerate • Sand Dunes – in arid regions, wind blows sand and silt around. Well sorted sandstones and siltstones. • Lakes – low energy environment forms laminated lacustrine deposits of mudstone and shale Alluvial Fan
Terrestrial Environments • Rivers – complex environments that deposit a variety of sedimentary rocks, from mudstone to conglomerates. In general, sorting increases with distance traveled. Also, the farther the sediment has traveled, the greater percentage of quartz will be present. Rivers deposit fluvial sedimentary rocks.
Marine Environments • Beach – well sorted mature sandstones with well rounded grains, ripple marks common • Shallow Marine – well sorted siltstones and mudstones often with lots of fossils. Can also produce limestones and other carbonates if enough organisms are present. Carbonate environments are usually in shallow warm water (reefs). • Deep Marine – Fine grained mudstones, chalks if enough plankton shells present. Only non-fine grained deposits are those of turbidites. Chalk, Up Close The Chalk Cliffs, England
Carbonate Environments • Tropical carbonate environments yield a diverse array of sedimentary rocks.
Transgression & Regression • Transgression – when sea level rises, the coast line moves inland • Regression – when sea • Changes in sea level falls, the coast level are common line moves seaward. throughout geologic – Not well preserved time. due to erosion • Linked to • This means that a Sedimentation regional unit may not form laterally at the same time (Show transgression animation)
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