Chapter 6. Sediments and Sedimentary Rocks

Chapter 6 Sediments and  Sedimentary Rocks Hurricane Ivan: 09-16-04, 15:15 UTC Image source: NOAA-GEOS-12 (1 km) rock cycle! sediment: (sedimen...
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Chapter 6

Sediments and  Sedimentary Rocks Hurricane Ivan: 09-16-04, 15:15 UTC

Image source: NOAA-GEOS-12 (1 km)

rock cycle!

sediment: (sedimentum: settling )! • fragments of solid material that settle and accumulate! • fragments settle in layers after being transported!

fragments are products of weathering and erosion! easy to see sedimentation in action!

sedimentary rock: “lithified” sediment; “lithos” -- rock! • 75% of all rock exposed on Earth’s land surface! • composition is weathering products of rocks or biological matter! • principal source of coal, oil, natural gas, cement, Al, Fe, groundwater!

3 types of sediments and sedimentary rocks:! detrital sediment: fragments of pre-existing rocks!

chemical sediment: precipitated from dissolved ions in water!

organic matter: formed from accumulation of biological debris (swamps)!

relationship to Earth’s systems! •  atmosphere ! -- most sediments produced by weathering in air! -- sand and dust transported by wind!

•  hydrosphere ! -- water is primary agent in sediment production, ! transportation, deposition, cementation, and ! formation of sedimentary rocks!

•  biosphere ! -- biological activity key to formation! -- petroleum and coal resources have biological origin!

sediment: loose, solid particles! originate from:! -- weathering and erosion of pre-existing rocks! -- chemical precipitation from solution, including ! "secretion by organisms in water!

sediment is classified by particle size! • boulder > 256 mm! • cobble: 64 to 256 mm! • pebble: 2 to 64 mm! • sand: 1/16 to 2 mm! • silt: 1/256 to 1/16 mm! • clay: < 1/256 mm!

turning sediment into sedimentary rock! transportation!

movement of sediment away from its source,! typically by water, wind or ice! rounding occurs by abrasion during transport! “particles knocking against each other”!

sorting occurs as sediment is separated by size! by transporting agents, such as water! --sediment size decreases with increased transport distance--!

sediments may be angular or rounded!

rounded beach boulders! angular fragments!

particle size sorting by rivers ! as flow slows down, river can no longer carry larger particles!

current velocity decreases!

headwaters!

mouth! grade = change in elevation/length!



Mountains

River Gravel





Plain



Mountains

River Gravel



Plain



Sand



Mountains

River Gravel



Plain



Sand

Silt and clay

gravel/boulders at headwaters!

normally! deposited! in or near! steep! mountains! at river! headwaters

sand/silt downstream, closer to mouth of river!

Distance from Source

sediments may be poorly sorted or well sorted! wind: most selective transporting agent! "-- only picks up sand thus deposits are well sorted!

windblown sand forms dunes!

note well sorted and rounded grains!

glaciers and flooding rivers are unselective transporting agents! flooding rivers -- fast currents! …transport all sizes…!

poorly sorted river sediment!

poorly sorted glacial sediment!

turning sediment into sedimentary rock! deposition! • settling of transported material! • accumulation of chemical or organic sediments,! "particularly in water! sediment transported! from canyon and! deposited at ! canyon mouth! at the! base of the slope!

environment of deposition is setting of deposition!

sediments and sedimentary rocks form in many settings!

turning sediment into sedimentary rock! preservation! • sediments must be preserved to turn into rock! • burial by additional sediments deposited on top!

turning sediment into sedimentary rock! lithification! • sediments must be hardened into rock! • compaction and cementation occur!

water is expelled from pore space!

other fluids cement grains!

Basic Clastic Rock Types •  Breccias: angular particles! •  Conglomerates: rounded particles! •  Sandstones! – Quartz sandstone: dominated by quartz grains! – Arkose sandstone: composed of qtz & fsp grains! – Graywacke: dominated by lithic (rock) fragments! •  Shales: mud and silt particles!

Coarse Breccia NB Angular fragments

Conglomerate

NB Rounded particles & dissimilar lithologies

Quartz Sandstone - > 90% qtz

Arkose Sandstone - Feldspar + Qtz

Graywacke - lithic fragments in dark fine grained matrix

Poorly sorted, “dirty” sandstone Sorting can be based on particle size and/or composition; It yields important constraints on provenance of the clastic particles.

Shale outcrop, Pittsburgh, PA

Shale hand specimens

NB very fine grained and thin laminations

Lithification and compaction of shale

NB volume loss during compaction

Chemical Sedimentary Rocks •  Carbonate Rocks (e.g. CaCO3) •  Chert (e.g. SiO2) •  Evaporites (e.g. NaCl; CaSO4•2H2O)

Carbonate atoll, Bora Bora!

Patch Reefs

Global Reef Distribution

Carbonate Beach - Lagoon Environments

Bioclastic Limestones

Clearly visible fossils

Fine grained carbonate mud

Coralline algae - Lagoon Environment

Fossil Rich Limestone - Coquina

Marl - Chalk (pelagic ooze)

SEM images of forams and coccoliths (CaCO3)

Photo credits: V. Pariente, J. Lundquist, D. Greig

Carbonate Bank - Shallow Currents

Oolitic Limestone

Nodular Chert  (poorly crystalline SiO2)

Redwall limestone, Grand Canyon, AZ

Bedded Chert

SEM images of diatom fustules (SiO2) - unicellular plants

100 m

100 m From: http://www.uni-mainz.de/FB/Geøpalaeontologie/griessdiato.html

Evaporites

Bonneville salt flats, UT

Organic Sedimentary Rocks •  Coal!

Sedimentary Structures! Environments of Deposition

Coal Bed - Trinidad, CO Composed of decayed plant remains - compressed peat bogs

Sedimentary Structures •  Bedding - series of visible layers in sedimentary rocks! –  Principle of Original Horizontality! –  Superposition! –  Cross-cutting relationships!

•  •  •  • 

Grading! Mud Cracks! Ripple Marks! Fossils

Horizontal Bedding

Cross Bedding

Cross Bedding Models

Normal Grading

Grading Model

Turbidity Currents

Mud Cracks

Preserved mud cracks

Ripple Marks Model

Symmetric

Asymmetric

Tidal Flat Ripples

Current Direction

Preserved Ripple Marks

Preserved Bivalve Fossils

Vertebrate (Fish) Fossil

Dinosaur Footprint

Interpretation of Sedimentary Rocks! •  Source Area! •  Environment of Deposition! •  Plate Tectonics and Sedimentary Rocks  

Sedimentary Formation Definitions •  Body of rock of considerable thickness! •  Characteristics must be distinguishable from adjacent units! •  Usually composed of one or more beds! •  Often based on rock type (lithology or lithofacies)! •  Contacts are the boundary surfaces between two rock units of different lithology or age

Upper Grand Canyon Sequence

Lithofacies - Paleocurrent Maps

Fluvial Environments

Alluvial Fan Deposits - Baja, CA Coarse channel deposit (conglomerate)

Convergent Margin Environment

Rift Sedimentary Environment

NB that this model only is applicable for sub-aerial environments

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