Transcript Chert - Cal State LA - Instructional Web Server

I. Intro

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
Chert Example

Chert (jasper) on
opiolite,Newfoundland
https://instruct.uwo.ca/earth-sci/200a001/13ophiol.htm
II. Mineralogy & Texture

A. qtz=primary mineral
1. other SiO2 minerals--chalcedony, amorphous
silica (opal A), disordered cristobalite (opalCT)
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
Chalcedony

Chalcedony is fibrous.
http://3.bp.blogspot.com/V_pQWolu5oA/TjL0KauRXTI/AAAAAAAAAKo/_GZmEpoYD6s
/s1600/W-28-2-I-10x-xp1.JPG
II. Mineralogy & Texture

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 SiO2 but contains Al, Fe, Mn, Ca, Na, K,
Mg, Ni, Cu, Ti, Sr, & B
1. SiO2 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
Chert Types

 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
Bedded and Nodular
Chert

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
B. Type of Bedded Cherts

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 200-1000m
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
VI. Chert Deposition
A. Solubilities

1. qtz 6-10 ppm
2. cristobalite 25-30 ppm
3. opal A 120-140 ppm
B. Increase temp, increase solubilities
C. Increase pH>9 sharp increase in solubility
D. Silica transported in river waters as silicic acid (H4SiO4)
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
VII. Precipitation of Chert
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
VIII. Upwelling & Siliceous
Organisms

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)
IX. Silica Diagenesis--The Monterey Example
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 opalCT precipitation
i. also speeds up rate of transformation
opal-CT to quartz
2. detritus
i. transformation from opal-CT to
quartz retarted
Opal A, CT, Quartz Transformations

Images
from D. Lee
Opal A http://www.sjvgeology.org/geology/microfossils.html#diatoms
Opal A, Opal CT

Opal Ct in diatom pores, http://www.sjvgeology.org/geology/microfossils.html#diatoms

Chert is diatomaceous rocks,
Palos Verdes PeninsulaPhotos by
Anne Hargreaves
Behl, 2012

Behl, 2012
Franciscan Complex
Franciscan Chert, Mount Diablo,
North Coast Geological Society.
 L. Jurassic to
Cretaceous
 Rock include

 Radiolarian
Chert,
greywackes,
shales, basalt,
serpentinites,
blueschist
metamorphics.
Franciscan Chert on Pillow Basalts,
http://www.marin.edu/~jim/ring/images/thermal.gif
Franciscan Complex

X Non-biogenic Cherts
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

Banded Iron formation, Australia
i. siliceous organism not
known to exist
ii. some possible
sightings
iii. if no skeltal remains,
where did silica
come from
Banded Iron Formation, Australia
XI Bedded Cherts & Time

A. Occur in PreCam-Neogene
1 common in Jr-Neogene, Dev & Carboniferous
2. low in Silurian & Cambrian
B. Most concentrated 0-30o 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