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
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
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
Study mostly in field--hand sample not
truly representative of outcrop
Need to see large scale variations in
composition & textures
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
Clastsupported
(framework
supported)
Matrix
supporteddiamictite
memoirs.gsapubs.org/content/198/105/F7.large.jpg
Breccias
Angular, gravel-sized particles
sublimereddit.aesptux.com
Origin of Breccias
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
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
Generated outside basin thru breakdown
of older rks
Framework clasts
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
Recycling leads to oligmictic conglomerate,
but depends on source of rock (qtzite, qtz
arenite, chert etc.)
Composition may reflect sorting
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
Clay & sand-size material
May have qtz, calcite, hematite, clay
cement
Conglomerate Textures
Matrix content & fabric support
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
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
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
Lack adequate system
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
ultra stable clasts (>90) = qtzose cong/diamictite--multicycle
meta/unstable clasts (> 90)= petromict
conglomerate/diamictite; First cycle
Classification Continued
Classify Clast Lithology
clast stability class--little info of comp
meta-clast, ig-clast, sed clast, polymict
conglomerate
tells about provenance
also used for diamictite
Classification Continued
Clast Size
cobble-rich qtzose rudite;boulder-rich
petromict diamictite
Qtzose Conglomerate
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
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
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
Shape and orientation
Gravel rounded in short distances
Therefore, fluvial conglomerate. may have
rounded gravel in angular sand
Pebble Imbrication
geolojay.com
Conglomerate Textures
Preferred orientation to elongate clasts
Transverse to flow, imbricated
Pebble Imbrication,Quaternary Aluvial Fan
Deposit, Spain
More Imbrication, Truckee River
Nevada
(c) 2006 Andrew Alden
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