Soils, Sediment, Weathering and Sedimentary Rocks

Transcription

1 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

2 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

3 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

4 The Sedimentary Stages of the Rock Cycle Weathering breaks down rocks. Erosion carries away particles. Transportation moves particles downhill. Glacier Delta Desert Playa lake Deposition occurs when particles settle out or precipitate. Sedimentary rocks Metamorphic rocks Plutons Burial occurs as layers of sediment accumulate. Diagenesis lithifies the sediment to make sedimentary rocks.

5 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 Medium-grained Fine-grained Fine-grained» Pebble - between 2 and 64 mm» Sand - between 1/16 and 2 mm» Silt - between 1/256 and 1/16 mm» Mud < 1/256 mm Microscopic grain size

6 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.

7 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.

8 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

9 Results of Physical Weathering Eventually, mechanical weathering processes create an apron or pile of debris at the margins of slopes called talus

10 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. CaCO 3 (Calcite) + H 2 CO 3 (carbonic acid) Ca 2+ (aq) + 2HCO - 3 (bicarbonate) Hydrolysis: Water acts to loosen chemical bonds to break down minerals. Works faster in slightly acidic water E.g. H 2 O (acidic) H + + OH - E.g. H + + KAlSi 3 O 8 (K-feldspar) Al 2 Si 2 O 5 (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. 4Fe O 2 2(Fe 3+ ) 2 O 3 (iron lost an electron and went up in charge) Hydration: Absorption of water into some minerals (mainly clays) causes them to expand

11 Dissolution Chemical weathering processes act on rock/mineral surfaces Results in rounding of edges

12 Surface Area and Weathering All other things being equal, the ratio of surface area to volume of a material controls the rate of weathering

13 Weathering & Roundness Weathering tends to round off corners and leave things more smooth in shape. Angular detritus = not very weathered Rounded detritus = very weathered

14 Follow s Bowen s reaction series!! 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

15 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)

16 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.

17 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

18 Soil Thickness 2- Slope: The steeper the slope the less soil will accumulate bowl-shaped areas, such as basins will accumulate thick soils

19 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

20 Soil Thickness 4- Climate: Thick soils will form in warm wet climates; deserts will form little or no soil

21 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

22

23 Sedimentary Rock Types Detrital / Clastic Cemented fragments of pre-existing rocks. E.g. sandstone, mudstone Biochemical Rocks made of cemented shells of organisms E.g. coquina, limestone Arkose Coquina Chemical made from minerals that precipitate out of water solutions. E.g. travertine, various evaporites Evaporites near a salt lake

24 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

25 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

26 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.

27 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 CaCO 3 ) shells. Fossiliferous limestone lots of fossils! Micrite made of calcite mud Chalk made of plankton shells Limestone Biochemical Chert made of cryptocrystalline (microscopic crystals) quartz, formed from the shells of plankton that sat on the sea floor and dissolved into a silica rich ooze. Folded Chert Beds 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

28 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 CaCO 3 into dolomite CaMg(CO 3 ) 2. Happens when Mg bearing groundwater reacts with limestone. Replacement Chert When chert (SiO 2 ) replaces minerals in a rock. E.g. petrified wood Travertine at Mammoth Hot Springs, MT

29 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

30 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

31 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

32 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

33 Ripple Marks Symmetric ripple marks in 1.5Ga quartzite asymmetric ripple marks in a beach

34 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

35 Ancient Dunes: Zion N.P.

36 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

37

38 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

39 Turbidites and Graded Beds A typical turbidite with graded bedding

40 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

41 Rain Spots Mud Cracks

42 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

43 Sedimentary/Depositional Environments Terrestrial Red Beds Where do sedimentary rocks form? Marine Environments: 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

44 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 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 Mountain Stream Alluvial Fan Glacial Till

45 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.

46 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

47 Carbonate Environments Tropical carbonate environments yield a diverse array of sedimentary rocks.

48 Transgression & Regression Changes in sea level are common throughout geologic time. Linked to Sedimentation (Show transgression animation) Transgression when sea level rises, the coast line moves inland Regression when sea level falls, the coast line moves seaward. Not well preserved due to erosion This means that a regional unit may not form laterally at the same time