More sedimentary rocks

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Today’s Lecture: Types of sedimentary rocks Chapter 7: Sedimentary Rocks Concepts of textural & mineralogical maturity Nature and classification of detrital sedimentary rocks Turning sediments into sedimentary rocks Stratification features of sedimentary rocks ◆ Detrital (or “clastic”) Sedimentary Rocks ❖ Made up of fragments (clasts) of pre-existing rocks ❖ In terms of composition, the most mature contain: - Clay minerals (weathering of feldspar) - Quartz (resistant to weathering) ❖ Presence of unstable minerals (mafics, feldspars) indicates immaturity: - limited weathering - rapid transport and deposition near source ❖ For detrital rocks, particle (clast) size: - most basic distinguishing factor for naming rocks Sedimentary rock classification is based on texture and composition Rock type Main criterion Detrital ◆ Particle size Chemical ◆ Mineral composition Classification of detrital (or “clastic”) sedimentary rocks Based on size of clasts (particles): Sediment name ❖ Gravel Size larger than mm (1/10 inch) ❖ Sand sand-sized ❖ Silt, Mud, Clay very fine-grained Rock name Conglomerate or Breccia Sandstone Shale or Mudstone Decrease in grain size with increasing transport Inferring Depositional Processes Sedimentary rocks contain many clues about the processes that formed them Clast Size Current Velocity Transport Distance Proximity to Source ◆ Large clasts Stronger Shorter Deposited Nearby ◆ Small clasts Weaker Composition of clasts: Longer Deposited Far Away Tell us about the nature of the source rocks Loose Sediments gravel sand silt clay Sedimentary rocks conglomerate sandstone siltstone shale Detrital Sedimentary Rocks ❖ Other important textural features: Grain shape (Roundness, sphericity) angular intermediate rounded Angular Well-rounded Shapes of sand grains Detrital Sedimentary Rocks ❖ Other Important Textural Properties: Grain size sorting Range of particle sizes in a sediment or rock Very poorly-sorted moderately sorted Very well-sorted Two minute in-class exercise Describe the grain sorting and rounding of the quartz sand above What does it take to get a sediment like this? Turning sediment into rock Process of transforming unconsolidated sediment into rock is called “lithification” How does this happen? Sand to sandstone: How? Turning sediment into rock Two important processes: ◆ Compaction Sediments accumulate, one layer on top of another to great thincknesses Weight of overlying sediments compresses deeper buried sediments ◆ Cementation Dissolved minerals in groundwater, Precipitate in the spaces between grains, “cementing” them together The most common cements are calcite, silica, & iron oxide Ledge-forming beds are better cemented and more resistant Nature of Sedimentary Deposits Most sedimentary deposits are layered or “stratified”  Sedimentary layers were originally deposited ~horizontally Horizontal layering Color variation of layers So what happened here? Nature of Sedimentary Rocks Each bed or stratum is unique in showing differences in color, texture (grain size & sorting) and internal structures Classic example of layered sedimentary rocks: Grand Canyon of Arizona Nature of Sedimentary Rocks Each layer (bed, or “stratum”) is bounded by an upper & lower bedding plane Nature of Sedimentary Rocks Each layer (bed, or “stratum”) is separated by bedding planes Nature of Sedimentary Rocks Each layer (bed, or “stratum”) is bounded by bedding planes The shape of the beds and nature of bedding planes provides important information clues about the processes that originally deposited the sediments How does the shape of this bed differ from the others? Why is this bed different? It was deposited in a channel! [...]... (grain size & sorting) and internal structures Classic example of layered sedimentary rocks: Grand Canyon of Arizona Nature of Sedimentary Rocks Each layer (bed, or “stratum”) is bounded by an upper & lower bedding plane Nature of Sedimentary Rocks Each layer (bed, or “stratum”) is separated by bedding planes Nature of Sedimentary Rocks Each layer (bed, or “stratum”) is bounded by bedding planes The... are calcite, silica, & iron oxide Ledge-forming beds are better cemented and more resistant Nature of Sedimentary Deposits Most sedimentary deposits are layered or “stratified”  Sedimentary layers were originally deposited ~horizontally Horizontal layering Color variation of layers So what happened here? Nature of Sedimentary Rocks Each bed or stratum is unique in showing differences in color, texture... - flooding rivers - streams near mountain source areas (steep) - mudlfows Detrital Sedimentary Rocks Breccia ❖ Course like a conglomerate, but with angular grains ❖ Short transport! ❖Deposited close to the source area for sediment ◆ Detrital sedimentary rocks Sandstone ❖ Composed of sand grains ❖ 2nd most abundant sedimentary rock ❖ Deposited by moderately active transport processes: - Running water... quartz Wisconsin obsidian Hawaii reef carbonate Australia “Oolites” Cancun, Mexico skeletal Oregon Example: Well-sorted Poorly-sorted ◆ Detrital Sedimentary Rocks Shale & Siltstone ❖ Silt & clay-sized particles (mud, clay, silt) ❖ Over 1/2 of all sedimentary rocks ❖ Particles too small to identify w/ naked eye ❖ Deposited in quiet (slow moving) water - deep ocean & continental slope - lakes - floodplains...Fig 7.18 Upstream, nearer the source, clasts are larger& more angular (texturally immature) They also contain a greater proportion of unstable minerals & rock fragments (compositionally Immature) Fig 7.29b Fig 7.28c © Martin Miller Products of long-term weathering and erosion: Quartz and clay Detrital Sedimentary Rocks Conglomerate ❖ Composed mostly of pebble to boulder-sized clasts
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