 A. Siliceous, dense, --qtz, chalcedony or opal B. <1% of sed rks  1. present from PreCam-Quat. C. Cherts assoc. w/ deep water seds e.g. shales & turbidites D. Common in ophiolites & subduction complexes E. Chert nodules & stringers in shallow water l.s.-- diagenetic replacement F. Economically important--silicon in semiconductor & computer industries, glass  1. petroleum also assoc. siliceous rks I. Intro

 Chert Example Chert (jasper) on opiolite,Newfoundland 001/13ophiol.htm

 A. qtz=primary mineral 1. other SiO2 minerals--chalcedony, amorphous silica (opal A), disordered cristobalite (opal- CT) 2. Opal-CT = low temp cristobalite disordered by interlayered tridymite lattices B. Opal A -- mainly biologic in origin 1. converts to opal-CT then to qtz w/time & Temperature increase C. Can get opal A, opal-CT, & quartz chert in same rocks II. Mineralogy & Texture

 Chalcedony Chalcedony is fibrous. V_pQWolu5oA/TjL0KauRXTI/AAAAAAAAAKo/_GZmEpoYD6s /s1600/W-28-2-I-10x-xp1.JPG

 D. Many contain remains of siliceous organisms 1. e.g. radiolarian, diatoms, silicoflagellates, sponge spicules 2. some contain clay & other siliciclastics plus athegenic minerals--silica, clay, hematite, pyrite E. Mostly SiO 2 but contains Al, Fe, Mn, Ca, Na, K, Mg, Ni, Cu, Ti, Sr, & B 1. SiO 2 varies from 99% to 65% 2. Si from organism, Ca maybe rom calcareous organism 3. detritus contributes most of remaining elements 4. High Mg & K maybe from volcanic areas e.g. backarc basins II. Mineralogy & Texture

  A. Flint - synonym for chert & as chert type e.g. nodules in Cretaceous chalk  B. Jasper- red chert due to hematite  C. Novaculite--dense, f.grained, even-textured--mid Paleo rks of Arkansas  D. Porcellanite--f. grained silceous rk w/texture of unglazed porcellanite Chert Types

 A. Bedded--layers of nearly pure chert 1. shale laminae--mm thick, interbedded 2. sed structure include grading, x-bedding, ripple marks, sole marking, convolute layers & soft sed folds 3. sed structures indicate mechanical transport 4. assoc. w/ ophiolitic rocks Bedded and Nodular Chert

 1. Diatomaceous--fossil diatomaceous oozes i. marine (Monterey/Sisquoc Fm), non marine (lacustrine--Green River) ii. up to hundreds meters thick--Monterey up to 2000 m 2. Radiolarian = radiolarian chert - Franciscan Fm i. assoc. w/tuffs, mafic volcanic rks ii. some assoc w/ l.s. deposited m 3. Siliceous spicules--spicules from sponges i. spicular chert ii. mostly deposited in shallow water--assoc. w/ glauconite 4. Bedded cherts lacking siliceous skeletal remains i. may be radiolarian cherts 5. Nodular chert i. form nodules, irregular lenses or layers - few cm - 10's cm ii. most lack internal structure -- green to black iii. found in carbonate rocks, but also diatomites, s.s., mudrks iv. mostly diagenetic origin -- replace fossils B. Type of Bedded Cherts

 A. Solubilities 1. qtz 6-10 ppm 2. cristobalite ppm 3. opal A ppm B. Increase temp, increase solubilities C. Increase pH>9 sharp increase in solubility D. Silica transported in river waters as silicic acid (H 4 SiO 4 ) E. Silica added to seawater through reaction of seawater w/hot volcanic rocks along m.o.r. & by alteration of oceanic basalts & detrital silicate particles 1. and from silica-enriched pore water of pelagic seds F. Surface seawater=<.01 ppm SiO2--biologic silica uptaike 1. below 2km = 11ppm 2. oceans average 1ppm VI. Chert Deposition

 A. Seawater (1ppm) 1. hard to xlize qtz even if [silica] >6-10ppm 2. might precipitate if have Silica saturated water due to dissolution of volcanic ash 3. therefore, how get thin-bedded chert B. Silica contributed by silica-secreting organisms 1. radiolarians--Camb/Ord-Holocene 2. diatoms (Jur?)-Holo 3. silicoflagellates (Cret-Holo) 4. result in production of thick deposits VII. Precipitation of Chert

 A. Silica-rich sediments common in upwelling regions B. Nutrients in upwelled water result in diatom blooms 1. diatoms settle to sea floor 2. how get to sea floor if so light 3. on seafloor protective coating dissolves--and dissolution begins 4. high sedimentation rates= preservation of siliceous test and formation of siliceous oozes (>30% diatoms) VIII. Upwelling & Siliceous Organisms

 Siliceous sediments deposited 1. burial = increase in temp. 2. Opal A dissolves i. dissolution leads to saturation w.r.t. opal-CT ii. opal-CT precipitates iii. solution-precipitation mechanism 3. With increased temp., opal-CT goes to quartz chert B. Other factors influence silica diagenesis 1. calcareous material--dissolve & increases alkalinity favoring opal- CT precipitation i. also speeds up rate of transformation opal-CT to quartz 2. detritus i. transformation from opal-CT to quartz retarted IX. Silica Diagenesis--The Monterey Example

 Opal A, CT, Quartz Transformations Images from D. Lee

 Opal A, Opal CT Opal Ct in diatom pores, Opal A

 Behl, 2012 Chert is diatomaceous rocks, Palos Verdes PeninsulaPhotos by Anne Hargreaves

 Behl, 2012

  L. Jurassic to Cretaceous  Rock include  Radiolarian Chert, greywackes, shales, basalt, serpentinites, blueschist metamorphics. Franciscan Complex Franciscan Chert, Mount Diablo, North Coast Geological Society. Franciscan Chert on Pillow Basalts,

 Franciscan Complex

 A. Some chert free of biologic remains,e.g. Arkansas Novaculite 1. could have formed of biologic remains--but since destroyed 2. PreCam. cherts problematic i. siliceous organism not known to exist ii. some possible sightings iii. if no skeltal remains, where did silica come from X Non-biogenic Cherts Banded Iron Formation, Australia Banded Iron formation, Australia

 A. Occur in PreCam-Neogene 1 common in Jr-Neogene, Dev & Carboniferous 2. low in Silurian & Cambrian B. Most concentrated 0-30 o latitude 1. here are upwelling regions C. Some not assoc. with low latitudes 1. different oceanographic circulation patterns in past D. Ribbon chert found in continental margin setting in orogenic belt sequences 1. back-arc basins--Japan, Bering, Phillipines 2. silled basins- e.g. California 3. spreading basins--Gulf of CA XI Bedded Cherts & Time