UNLV GEOL101 Lecture Outline Fall Week 9/26/11

Transcription

1 Week 9/26/11 I. Sedimentary Rocks A. definition 1. rocks formed from the weathered products of other rocks a) compacted fragments (clastic rocks) b) chemical precipitates 2. rocks formed from compacted organic matter B. importance 1. a small percentage of the earthʼs crust 2. most rocks exposed on the continental crust 3. important economically (e.g., oil, coal, salt, gypsum, etc.) II. Clastic Sedimentary Rocks A. definition of clastic rocks 1. rocks assembled from solid particles 2. solid particles arise from weathering B. particle size and sedimentary rocks 1. primary means of classifying clastic sedimentary rocks 2. indicative of the depositional environment a) high energy is required to move big particles b) smallest can only settle out in very low energy C. mineralogy of sedimentary rocks 1. most common particles are clay minerals and quartz a) clay is the most abundant weathering product of silicate minerals b) quartz is both abundant and durable 2. feldspars and micas may also be present in quantity D. sedimentation 1. process by which sediments ʻsettle outʼ of water 2. sediment accumulates 3. material near the bottom compacts E. lithification 1. process through which sediments become rock 2. consists of compaction and cementation 3. compaction a) compresses the sediments into a smaller space b) forces out some of the water and gasses 4. cementation a) water moving between grains may carry a cement b) calcite, silica, and iron oxide are the most important F. conglomerates 1. well rounded, gravel sized (or larger) particles

2 2. often cemented with a matrix of finer material 3. transport of large particles indicates a high energy environment 4. if the particles are angular rather than rounded, the rock is called breccia G. sandstone 1. composed primarily of sand sized grains 2. description a) sorting b) grain shape c) mineral type 3. sorting a) well-sorted - all grains are about the same size b) ʻpoorly -sorted - variety of grain sizes are present c) sorting is indicative of the means of transport (1) wind and water sort grains by size (2) ice, mass wasting do not 4. grain shape a) round grains (1) particles were transported a long distance (2) wind or water, not ice b) sharp angular grains indicate a short travel distance 5. primary mineral type a) quartz in the primary component of sandstone b) if significant amounts of feldspar are present, the rock is called arkose H. fine-grained 1. clay and silt sized particles a) not necessarily made of clay b) deposited in quiet marine environments 2. types a) shale - clay sized particles, splits into thin layers b) mudstone - clay sized particles, breaks into clumps c) siltstone - silt sized particles I. greywacke 1. dark gray rock 2. heterogeneous mixture of particle sizes 3. large pebbles in a mud matrix 4. commonly associated with turbidity currents 5. may also represent post depositional weathering III. Chemical Sedimentary Rocks A. most form when dissolved ions precipitate 1. can occur form either inorganic or organic (biochemical) processes 2. some chemical rocks form from organic matter B. limestone

3 1. the most abundant chemical sedimentary rock 2. composed primarily of calcite (CaCO 3 ) 3. usually organic in origin, but can be inorganic 4. coral reefs a) created by small invertebrates (1) absorb Ca, CO 2 from sea water (2) secrete a calcite external skeleton (3) skeletons are linked to others b) currently form in tropical seas (1) Florida, Bahamas, Australia 5. coquina a) formed from accretions of shell fragments b) shells belong to fairly large critters c) can see shell fragments in coquina 6. chalk a) also formed from shell fragments, b) shells belong to microscopic marine organisms c) in the open ocean there is little clastic deposition d) microscopic shells accumulate without being buried 7. travertine a) inorganic limestone b) natural waters contain dissolved CO 2 c) water changes pressure or temperature d) dissolved CO 2 leaves groundwater e) limestone precipitates (1) common in caves (2) scale in the coffee pot 8. oolitic limestone a) inorganic precipitation in shallow seas b) water is supersaturated with calcite c) little grains roll around on the sea floor d) grow in size and stick together C. dolostone 1. composed of dolomite, (Ca,Mg) CO 3 rather than calcite 2. dolomite is not actively forming in the world today 3. dolostone probably results from alteration of limestone a) magnesium replacement of calcium b) Mg carried by flowing water D. chert 1. microcrystalline quartz(sio 2 ) a) flint, jasper, agate are examples b) chert is very hard and exhibits conchoidal fracture 2. found in bedded layers or nodules 3. forms by chemical or sedimentary activity rather than igneous

4 4. chert nodules often occur within limestone, and are believed to be of inorganic origin 5. bedded chert is thought to be from the hard parts of micro-organisms like diatoms and radiolarian E. evaporites 1. water carrying dissolved minerals (ions) evaporates from shallow seas 2. leaves minerals behind in sedimentary layers a) gypsum (CaSO 4 *2H 2 O) b) halite (NaCl) c) potash (K(Mg)Cl) F. coal 1. definition a) coal can be a rock, but not a mineral b) formed from compressed plant matter (1) lots of plants (2) stagnant water (no oxygen) (3) burial 2. degree of compression determines type of coal a) plant matter to peat b) peat to lignite (1) soft brown coal (2) still contains obvious plant fragments c) lignite to bituminous coal (1) most common black coal (2) little resemblance to original plants (3) bituminous to anthracite coal (a) lots of heat and pressure (b) hard black coal (c) really a metamorphic rock IV. Sedimentary Structure A. definition of sedimentary structure 1. patterns and features in the rock 2. formed prior to lithification 3. during sedimentation or after B. bedding 1. horizontal bedding a) sediments deposited in deep water b) form layers that are often preserved in rock c) bedding planes separate layers d) layers may be mm or meters thick e) each typically represents a new episode of deposition 2. varves a) found in lakes that freeze during winter

5 b) alternating bands of coarse and fine sediment c) fine materials only settle under the ice 3. cross-bedding a) layers are tilted b) reflect downwind side of sand dunes c) sand doesn't stay on the upwind side d) that sand is always moving e) stacked up cross-beds with different tilts mean a change in wind direction C. ripples 1. wind or water 2. are asymmetrical if flow is one direction 3. symmetrical for oscillating flow ( beaches) 4. burial can preserve the ripples in the rock 5. tells us about currents that were flowing D. bioturbation 1. worms live in ocean bottom muds 2. they "eat" the mud for organic matter 3. output a slightly different mud 4. differences are often preserved in the rock 5. since worms are soft we don't find worm fossils we have to study their burrows E. graded bedding 1. happens locally in streams and big time in lakes/oceans 2. fast moving water suddenly slows a) flood enters a large body of water b) turbidity current 3. larger particles settle faster 4. creates an upwards fining sequence V. Sedimentary Environments A. sedimentary rocks tell us about their history 1. environment that existed at the time of formation 2. type of rock, mineralogy, particle shape, and particle size 3. chalk forms in a very different environment than conglomerate B. basin subsidence 1. large scale subsidence of weak zones in the Earth's crust 2. weakness can form from rifting a) splitting of the crust to form a new diverging boundary b) always splits into three arms c) not all arms may fully develop (failed rift) 3. crust stretches and thins in the rift a) weaker b) lower than the surrounding regions

6 4. sediments wash into the rift 5. weight of the sediments causes the weak crust to subside 6. can generate thick sequences of sedimentary rocks a) conglomerates at the bottom b) calm water deposits (limestone, shale) higher up 7. Oklahoma, East African have failed rifts 8. thermal sag a) If the rift grows into a divergent boundary b) edge of the rift moves away from the heat c) sags downwards as it cools 9. subsiding basins are great places for oil C. platform carbonates 1. shallow zones in open ocean 2. formed from microscopic shells 3. critters take CaCO3 from ocean to make shells 4. shells fall when the critter dies 5. far from sources of clastic sediment 6. Bahamas, and off Florida D. deltas 1. rapid energy loss a) fast water in the stream b) slow in the lake or ocean 2. forms graded beds 3. coarse particles settle out first 4. then the finer ones 5. layers are tilted 6. E. reef islands 1. coral reefs can only grow in shallow water a) ft b) light must penetrate 2. we find coral islands in the open ocean 3. coral colonizes an extinct volcano a) some of the volcano erodes b) rest of it sinks a under weight of the coral 4. coral keeps growing upward to stay in the light (shallow water) 5. Darwin hypothesized this, proved much later VI.