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Mudrocks Introduction Mudrks mostly silt & clay Sometimes called argillites Make up 65% of sed rks Difficulties studying mudrocks • • • • Recessive F. grained Clay alteration Hard to get to modern analog • Mineral i.d. difficult (qtz vs. felds) • Sed structure not common as in sandstone • Thus problem w/ strat. column Organic rich mudrocks -economically imp. Thin section of mudrock. Hard to distinguish grains Recessive Mudstone Overturned Mississippian Lisburne Formation (resistant carbonate) in depositional contact with overturned Permian Echooka Formation (recessive mudstone), on the south face of Atigun gorge, Alaska. (photo: Alan Carroll) More Recessive Mudstone Contact between lower, light brown sandstone and dark brown silty mudstone within Imperial Formation on a tributary to the Arctic Red River, Northwest Territories. www.nwtgeoscience.ca photo shows the character of bedding at a scale of a few meters. The thicker sand beds are typically a little coarsergrained and tend to be more resistant and stick out of the cliff. The finer-grained material is commonly in thinner beds and more recessive. clasticdetritus.com/.../ Clays most abundant • Kaolinites [Al2Si2O5(OH)4] formed in warm moist climates where Ca, Na, and K ions leached and removed by weathering. kaolinite clays indicates a source in a humid tropical climate. • Smectites Are expanding clays. Expand by taking in water between layers. Montmorillinite(½Ca,Na)0.7(Al,Fe,Mg)4Si,Al)8O20(OH)4.n H2O is a good example. Form from weathering of Fe -Mg rich ign & meta rocks in temperate climates Most abundant clays in modern sediment. • Illites - K1-1.5Al4Si7-6.5Al1-1.5O20(OH)4 Formed by weathering of feldspars in temperate climates and by alteration of smectite clays during diagenesis. Have structure similar to muscovite. • Mixed layer clays Interlayering between smectites like layers and illite like layers in same crystal Common in modern sediment. • More illite w/time i. 80% clay minerals in Paleozoic rks is illite ii. Reasons: increased volcanism; increased plant life,., climatic changes, diagenetic processes Mudrock compositions http://soils.missouri.edu/tutorial/page8.asp Mudstone Composition Continued Qtz • Mostly silt-size, angular Feldspars • Low concentrations Other • Muscovite, calcite (skeletal & diagenetic), pyrite, glauconite, hematite, etc. Classification Grain Size Description >2/3 silt Abundant silt sized grains Silt-shale visible with a hand lens Siltstone >1/3, <2/3 silt Feels gritty when chewed Mudshale Mudstone >2/3 clay Feels smooth when chewed Clayshale Claystone Fissile Rock Nonfissile Rock Depends on grain size & if rk fissile or not Fissile rock tends to break along sheet-like planes nearly parallel to bedding planes Fissility caused by clay minerals deposited with sheet structures parallel to depositional surface. Texture Grain Shape • Clays and quartz usually angular Not much rounding because grains small & carried in suspension Thin section; cross polarized. Scale: each tick mark = 1 mm geohistory.valdosta.edu Texture Continued Fissility—Depends on • Abundance of clay-more clay more fissile • Orientation of clays Clay grains adhere to one another Adhesion of grains called flocculation • Also depends on salinity & organic matter=more = more flocculation Bioturbation • Destroys orientation of clays Diagenesis • Aligns grains perpendicular to max stress direction Get slaty cleavage and foliation in metamorphic rocks geology.uprm.edu Structureless Mudstone geology.about.com Describing Mudrocks Fissility--part parallel to bedding Bioturbation-massiveness? Flocculation inhibits fissility Laminations Lamination vs bed? • 1 cm Origin of lamina • • • • a. productivity variation b. grain size c.composition d. biochemical No laminations = massive (bioturbation/redeposition) Laminations due to textural differences Sandlaminated dark grey mudstone from unit MMa, Tom ore deposit, Paleozoic, Northern Canada gsc.nrcan.gc.ca/.../ sedex/tom/index_e.php Laminated Phospatic Mudstone, Monterey Fm, Mussel Roc Cross laminated mudrock, Brazil Describing Mudrocks Concretions • Nodular or stratiform • Some Form immediately after deposition; Evidence? Cannonball Concretions, New Zealand More Concretions, North Dakota Describing Mudrocks Colors • Gray to black, generally > 1% o.m. • Conditions favorable for o.m. preservation Little oxygen Rapid sedimentation Low temperatures of water Low permeability Oxygen present, o.m. goes to water & carbon dioxide • 3. Red, brown, yellow, green--iron present Reflect oxidation state of Fe Oxidizing conditions the most Fe = Fe+3 • • • • • • Give rock red, brown, orange colors Hematite (Fe2O3) = red color Iron hydroxide [FeO(OH)] (geothite) = brown color Limonite gives sediment yellow color Lack of iron then green (illite, chlorite, & biotite) Use color for descriptive purposes Color of Mudrocks: Green-oygenated environment Black-Organic-rich, low oxygen Depositional Environments A. Major mudrock types • Residual--weathering & soil formation on pre-existing rock i. Preservation potential? • Detrital--erosion, transportation & deposition • Weathering & alteration of volcanic deposites B. Residual • Calcretes (caliche)--common where evap>precip C. Detrital • Marine/non-marine • Distinguishing features: Fossils, bioturbation to laminated Deposition below active wave base May pass shoreward to sandstones May be organic rich Local example is Monterey Fm. Residual Soil http://blass.com.au/definitions/resid ual%20soil Raymond Wiggers Dropstone in laminated mudstone, Brazil Mudcracks in red-brown mudstone, Watahomigi Formation. Red from hematite. Courtesy USGS Depositional Environments Continued Non-marine • Common in river floodplains, assoc. w/s.s. • Lacustrine environments--varved Glacial lakes = coarse = spring melting, winter= fines Non-glacial lakes--opposite- why? Volcaniclastic derived mudrocks • Volcanic material alters to clay • If alteration is to montmorillonite then mudrock known as bentonite • How identify volcaniclastic origin of mudrock? Marine Sediments Most ocean floor covered by marine sediments • Sediment thickness is thinnest at mid-ocean ridge and thickest at continental margins Sediment Accumulation Rates Cm/1000yrs Continental Margin • Shelf• Slope • Fraser River Delta 15-40 20 700,000 Deep Sea • Coccolith Ooze • Clays 0.2-3.0 0.03-0.8 Types of Ocean Sediments • Terrigenous – “rock-derived •Biogenous – “life-derived” • Hydrogenous – “water-derived” • Cosmogenous – “cosmic-derived” Lithogenous Sediments • Derived from the weathering of rocks – continents or volcanic islands • Transported by rivers, glaciers or wind • Most deposited on continental margins • Covers about 45% of ocean floor Composed mostly of quartz sand and clay Lithogenous Sediments - Deltas Lithogenous sediments added to marine environment by deltas Delta common features Pelagic and Neritic Defined Pelagic sediments deposited in deep ocean away from shelf processes influences • Usually clays, unless turbidites – other gravity flows, ice rafting Neritic sediments deposited in shallow water over shelves. Pelagic sediments in abyssal plains most red clays Growing anthropogenic contribution –factory dust, plastic (PCBs), time markers Lithogenous Sediment Examples Mississippi River Sahara Desert Mt. Pinatubo Red Clays – Terrigenous from rivers, dust, and volcanic ash – Transported to deep ocean by winds and surface currents – Common in deep oceans, clays most common – Accumulates 2 mm (1/8”) every 1,000 years Red Clays--Pacific Lacks calcium carbonate material Note siliceous materials— Diatoms & sponge spicules Paula Worstell Sediment Distribution • Calcareous and Siliceous Oozes Biogenous Sediment Biogenic ooze – greater than 30% biogenous sediment • Composed mostly of hard skeletal parts of once-living organisms • Two main compositions of hard parts: 1. Calcium Carbonate (CaCO3) a)Coccolithophore (phytoplankton) b)Foraminifera (zooplankton) c)Pteropod--molluscs 2. Silica (SiO2) a) Diatoms (phytoplankton) b) Radiolarian (zooplankton) • Distribution depends on chemistry, ocean productivity Biogenous –Foraminifera Calcareous Examples Composed of CaCO3 Foraminifera www.noc.soton.ac.uk • Widespread in relatively shallow areas Coccolithophore Biogenous – Siliceous Examples Radiolarians Diatoms • Composed of SiO2 • Base of food chain • Like forams Benthic ones better survive Sediment Distribution – Calcareous/Siliceous Biogenous – Siliceous Ooze Covers 15% of ocean floor • Distribution - areas of high productivity (zones of upwelling) • Dissolve more slowly than calcareous particles •Seawater undersaturated wrt silica, siliceous particles should dissolve •Surface waters more depleted •Bottom waters colder, most dissolution on seafloor • Diatoms common at higher latitudes • Radiolarians common at equatorial regions Siliceous Oozes How do planktonic organisms get to bottom? Lightweight, drift Biopackaging—marine snow, feacal pellets Biogenous – Calcareous oozes Cover greater than 50% of ocean floor • Distribution controlled by dissolution processes • Calcium Carbonate Compensation Depth (CCD) – the depth at which the rate of accumulation of calcareous sediments equals the rate of dissolution • Cold bottom waters undersaturated with respect to CaCO3 – slightly acidic ( CO2) – readily dissolves CaCO3 Lysocline = depth at which dissolution of carbonate material begins Most dissolution takes place on seafloor, only pass short distance through corrosive zone Depth of CCD depends on degree of undersaturation, productiviy, & flux faculty.uaeu.ac.ae/ Paleoclimatology/Productivity • Diatomaceous Rocks Monterey, Sisquoc Fm Increased Miocene Oceanic Productivity Miocene sealevel changes • Phosphatic Rocks o.m. content 4-30 high productivity low oxygen levels in oceans • Oxygen Isotopes & Mudrocks O2 isotopes in shells in deep marine rocks Construct isotope curves Show changes in ocean temp. Tie to sea level curve • Carbon Isotopes & Mudrocks Reflect changes in productivity, continental runoff, ocean circulation, atmospheric gsc.nrcan.gc.ca/.../ sedex/tom/index_e.php Laminated Monterey Formation
