Conglomerates - Cal State LA

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Transcript Conglomerates - Cal State LA

Conglomerates
geology.about.com/.../lagunaconglom.htm
San Onofre Breccia, Laguna Beach
Introduction
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Rocks mostly of > 2mm
clasts (gravel)
 Also called rudites
 Common, not abundant
Important for tectonics,
provenance, depositional
environments, economics
Grains = mostly rk
fragments
May contain matrix--clay
or sand
 matrix is finer material
which infills the spaces
between grains
Example of Matrix
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Matrix is not
cement
It is deposited with
rest of sediment
Miocene San Onofre Breccia, Aliso Beach
County Park, www.geoscapesphotography.com/ca-alisobeach.htm
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Study mostly in field--hand sample not
truly representative of outcrop
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Need to see large scale variations in
composition & textures
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Varies
 Folk - 30% gravel; less =
gravelly mud/sand
 Gilbert - 50% gravel
 < 50% = conglomeratic
ss/mudstone
 Also called pebbly
ss/mudstone
 Gilbert uses this
term at <25%
 Is pebbly correct
term?
 Confusion led to term
“diamictite”
 Poorly sorted, large
particles in mud
matrix
 So, have conglomerates
with/without matrix
Definition
Diamictite
Boulder of diamictite of the Precambrian Mineral Fork Formation. It
is lithified glacial till. Along the Elephant Head Trail, Antelope Island,
Utah. Scale bar on notebook is 10 cm. Taken 2002.
Conglomerates Defined Using
Fabric
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Clastsupported
(framework
supported)
Matrix
supporteddiamictite
memoirs.gsapubs.org/content/198/105/F7.large.jpg
Breccias
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Angular, gravel-sized particles
sublimereddit.aesptux.com
Origin of Breccias
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Sedimentary--mass
flows--triggering
mechanism
Non-sedimentary-tectonic (faults, folds);
volcanic
When is rk a
breccia?--is any
fragmented rk a
Miocene San Onofre Breccia, Aliso Beach
breccia
County Park,
www.geoscapesphotography.com/ca-alisobeach.htm
Intraformational
Conglomerate/Breccia
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Intraformational=
redeposited
fragments. of weakly
consolidated beds
Thin, short term
events e.g. landslides
Rounded to angular
fragments. common
in shales & l.s.
http://www.ing.pan.pl/Sediment_2009/03-B1_Trips.htm
http://www.depauw.edu/acad/geosciences/fsoster/Mudcracks.htm
Epiclastic (extraformational)
Conglomerates
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Generated outside basin thru breakdown
of older rks
Framework clasts
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Oligomictic (e.g. qtz, arenite-stable
fragments.)
Polymictic (many dif types of fragments)
Polymictic conglomerate. w/-unstable clastsshale, L.S., basalt -Petromict conglomerate
Epiclastic Conglomerate Continues
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Recycling leads to oligmictic conglomerate,
but depends on source of rock (qtzite, qtz
arenite, chert etc.)
Composition may reflect sorting
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shale breaks down, metaqtzite= coarser
durable clasts
volcanic –bigger clasts survive; finer clasts
may not survive transport
Matrix Supported, Polymictic
Conglomerate, New Zealand
http://flexiblelearning.auckland.ac.nz/rocks_minerals/rocks/conglomerate.html
Oligomictic Breccia, New
Zealand
Matrix of Epiclastic Conglomerate
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Clay & sand-size material
May have qtz, calcite, hematite, clay
cement
Conglomerate Textures
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Matrix content & fabric support
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High energy transport process=gravels w/little
sand
Glaciers=matrix supported=diamictites
Typically, 2 size modes gravel and mud to
sand
1998 Debris flow deposits in Chalone Creek, Pinnnacles
National Monument, San Benito County, California
USGS
Debris flow deposits partly cover a road in Stevens Creek
Canyon (Santa Clara County). The debris flow occurred
during a storm in late January 2003
Structures
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Massive—lacks internal stratification
Crude to good planar horizontal or inclined
stratification
Tabular & trough x-beds , less common
than S.S
May have normal & inverse grading
Fluvial Conglomerate, France
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Conglomerates and
sandstones
representing bars
deposited in the
channel of a braided
river. Note the
lenticular bed geometry
and crude stratification.
The northward dip (to
the left) is a result of
folding during Pyrenean
earth movements.
http://www.virtual-geology.info/sedshots/sedshot023.html
Crudely stratified Conglomerate/breccia,Protestant Cathedral, Ireland
Note alignment of the clasts
Classification
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Lack adequate system
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use informal names--qtzite cong (comp.); fluvial
conglomerate (dep env.); pebble conglomerate. (clast
size
probably because cong. not easily class; don’t have 3
principle clast types
Classify Clast Stability
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ultra stable clasts (>90) = qtzose cong/diamictite--multicycle
meta/unstable clasts (> 90)= petromict
conglomerate/diamictite; First cycle
Classification Continued
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Classify Clast Lithology
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clast stability class--little info of comp
meta-clast, ig-clast, sed clast, polymict
conglomerate
tells about provenance
also used for diamictite
Classification Continued
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Clast Size
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cobble-rich qtzose rudite;boulder-rich
petromict diamictite
Qtzose Conglomerate
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Metaqtzite, vein qtz,
chert--sed, meta, ig
rk
 least stable
lithology destroyed
by weathering,
erosion,
transportation
 vein
quartz=erosion of
ig/meta rk
 implies intense
weathering or
vigorous transport
 mostly fluvial
magicmomma.smugmug.com/.../1/790534557_A6f
9J
Massanutten Formation ,Silurian, Virginia
www.nvcc.edu/.../massanutten_conglomerate.jpg
Petromict Conglomerate
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have metastable clast, various source rks
plutonic not as common--maybe due to
weather.
first cycle
can be extensive
imply rapid erosion, elevated highlands
limited transport--fluvial to deep marine
Depositional Environments
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textures & structures related to
environmental conditions
reconstruct ancient environments
genetic class. desirable but difficult
fluvial, glacial, marine gravel
process-related sheetflood, streamflow,
wave-worked--this is favored
Genetic Classification
Paleocene-Eocene Submarine Fan Conglomerate & Turbidites, German Rancho
Fm, Salt Point, San Francisco
http://eqdoc.home.netcom.com/salt2.html
Conglomerate Textures
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Shape and orientation
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Gravel rounded in short distances
Therefore, fluvial conglomerate. may have
rounded gravel in angular sand
Pebble Imbrication
geolojay.com
Conglomerate Textures
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Preferred orientation to elongate clasts
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Transverse to flow, imbricated
Pebble Imbrication,Quaternary Aluvial Fan
Deposit, Spain
More Imbrication, Truckee River
Nevada
(c) 2006 Andrew Alden
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Form (sphericity) related to parent rock
fissile/schistose fabric=tabular, massive rocks
more equant
http://doktori.bme.hu/bme_palyazat/2012/hallgato/honlap/szabo_timea_en.htm