GEOLOGY 333 LAB 8. Sedimentary Rocks

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1 GEOLOGY 333 LAB 8 CLASTIC SEDIMENTARY ROCKS Sedimentary Rocks Sedimentary Rock = Rock that forms by the accumulation and lithification (hardening) of sediment (loose grains). Importance of Sedimentary Rocks: 1) Most abundant rock type at the Earth s surface. 2) Major economic resource: e.g., sandstone and limestone are used for building stone as cut blocks or slabs; crushed limestone and dolostone are used for aggregate, which is used in asphalt paving, mixed with cement to make concrete, or used alone as a base for roads, sidewalks or railroads; ground up limestone and shale are used to make cement; shale is used to make bricks 3) Geologic research - study of fossils in sedimentary rocks give evidence of life forms over geologic time; sedimentary rocks give clues to local geologic history, including environment where sediment accumulated, i.e., depositional environment 1

2 Sedimentary Rocks Classification of Sedimentary Rock: Commonly divided into three categories based on their origin. 1) Clastic (or detrital) sedimentary rocks form by transportation and deposition of layers of sediment (loose individual grains of a preexisting rock) that is compacted and cemented. Shale and sandstone are examples of clastic sedimentary rocks, which we will study this week. 2) Chemical sedimentary rocks form by chemical precipitation where dissolved ions combine to form solid mineral grains. Examples of chemical sedimentary rocks include limestone and rocksalt. 3) Organic (or biochemical) sedimentary rocks form by accumulation of remains of plants and animals, such as calcite shells. Examples of organic sedimentary rocks include fossiliferous limestone and coal. **Commonly there is overlap among these three categories.** Clastic Sedimentary Rock Classification Classification = based on grain size (Table 6.1) Conglomerate = large rounded grains (>2mm, gravel), coarsegrained texture Breccia = large angular grains, coarse-grained texture Sandstone = medium grained texture, mainly sand-size grains ( mm) Siltstone = fine grained ( mm) Shale = very fine grained (<0.004 mm) and fissile (splits into thin sheets) Mudstone = very fine grained (<0.004 mm) and massive (does not split) 2

3 Clastic Sedimentary Rock Classification Sandstone Classification: Sand-size grains of sandstone are called framework grains and consist mainly of quartz, feldspar (plagioclase and K-feldspar) and lithics (rock and mineral fragments of any type other than quartz and feldspar). Space between sand grains is called pore space, which can be empty or filled with mineral cement (forms by chemical precipitation) or matrix (fine grained mud). (framework grains) or matrix Clastic Sedimentary Rock Classification Sandstone Classification: Sandstones are subdivided by abundance of framework grain type and abundance of matrix (mud in pore space). Arenite = Sandstone with <5% matrix (empty or cement-filled pore space. Wacke = Sandstone with >5% matrix. 3

4 Clastic Sedimentary Rock Classification Step 1: Determine if pore space is empty, filled with mud, or filled with cement. Step 2: Determine the % quartz, % feldspar, and % lithic fragments. Step 3: Plot percentages on the appropriate triangle to determine rock name. Quartz Framework grains or cement Low relief No cleavage Low birefringence M&A p.115 4

5 Calcite Cement Cement only 3 rd order birefringence Usually won t see calcite cleavage when cement M&A p.127 Clay Matrix Fine-grained Medium birefringence Looks like crud! M&A p.123 5

6 K-Feldspar Framework grains Low relief and 1 st order birefringence Commonly altered to clay (cloudy in PPL, highly birefringent mica impurities in XPL) Variable appearance in XPL: May be twinned (criss-cross pattern) May have exsolution lamellae. M&A p.55 Plagioclase Framework grains Low relief Polysynthetic twinning 1 st order birefringence Commonly altered to clay (cloudy in PPL, highly birefringent mica impurities in XPL) Will appear orange- to peach-colored in stained slides. M&A p

7 Feldspar Often altered in sedimentary rocks. Look for twinning and cruddy appearance M&A p.119 Lithic Fragments Refers to any mineral other than quartz and feldspar 7

8 Textural Terms Roundness = degree of smoothness of grain surfaces, related to sediment transportation, greater transportation produces more rounded sediment grains. Textural Terms Sorting = similarity of grain sizes, related to sediment transportation type where wind and moving water usually produce well sorted sediment (narrow range in grain size), landslides and glacial ice produce poorly sorted sediment (broad range in grain size). 8

9 Textural Terms Induration = cohesiveness of rock; poorly indurated = loose and crumbly due to weak cement and compaction, well indurated = rock resists crumbling because it is well cemented and compacted Mature Sediment = well sorted, well rounded and mainly quartz with little matrix; indicates extensive physical and chemical weathering and lengthy transport by wind or moving water Immature Sediment = poorly sorted, angular and abundant feldspar, lithics and matrix; little physical and chemical weathering and little transport by wind or moving water Step 1: Pore Space: Empty pore space, Quartz or calcite cement, Muddy matrix (>5%) Step 2: Framework Grains: % Quartz: % Feldspar: % Lithic Fragments: Step 3: Rock Name: 9

10 Step 1: Pore Space: Empty pore space, Quartz or calcite cement, Muddy matrix (>5%) Step 2: Framework Grains: % Quartz: % Feldspar: % Lithic Fragments: Step 3: Rock Name: Step 1: Pore Space: Empty pore space, Quartz or calcite cement, Muddy matrix (>5%) Step 2: Framework Grains: % Quartz: % Feldspar: % Lithic Fragments: Step 3: Rock Name: 10