SEDIMENTARY PETROLOGY
INSTRUCTOR: PEDRO C. RAMIREZ
Fall ‘14
CONGLOMERATES AND BRECCIAS
Conglomerates and breccias, which are also termed rudites, volumetrically, represent a small (<1% by weight)
portion of the stratigraphic sequence. However they are especially important because they provide important
clues to provenance (the source area or the area from which the materials in the sediments were derived).
Conglomerates/breccias consist of coarse particles, which typically have not been transported relatively great
distances. These coarse particles usually are composed of rock fragments, and it is these rock fragments which
provide the key to the provenance interpretation. Sandstones mostly consist of individual mineral grains
generated through the break down of rock particles. Consequently, reconstructing source area lithologies using
sandstone compositions is more difficult. Conglomerates also provide excellent clues to the tectonic history of
an area and to changes in sea level. Tectonic uplift of an area results in increased erosion and to the deposition
of conglomerates. The occurrence of these conglomerates in the stratigraphic sequence may mark these periods
of tectonic uplift. Conglomerates may also mark sea level changes. Conglomerates are more likely associated
with drops in sea level. As you can surmise, conglomerates are very important for reconstructing source area
compositions and determining the tectonic history of an area and sea level changes impacting a region.
Conglomerates, in addition to their subdivision into conglomerates and breccias, are subdivided into
orthoconglomerates (framework supported conglomerates) and paraconglomerates (matrix supported
conglomerates). Compositionally, coarse clastics are subdivided in polymictic (clasts of varying compositions)
and oligomictic (clasts of very restricted compositions).
Working with hand samples of conglomerates in the lab is not the always the best thing to do. Most of your
data gathering should be in the field. A hand sample may not be representative of the whole outcrop, and
sedimentary structures present in outcrop may not be evident in hand sample. Nevertheless, we will use the
samples listed below to help get you get started on describing conglomerates in preparation for our field trip.
Laboratory
I) Identify the samples of the sedimentary, igneous and metamorphic
1. If it is a sedimentary rock indicate whether it is a sandstone, siltstone, mudstone, shale, chert, etc.
2. If it is an igneous rock indicate whether it is
a. Intrusive or extrusive
b. If it is extrusive determine if it a basalt, andesite, rhyolite, etc
c. If it is intrusive determine if it is granite, diorite, gabbro, etc.
3. If it is a metamorphic rock determine if it a slate, phyllite, schist, gneiss, etc.
II) For each of the samples listed below:
1. Make a representative sketch
2. Indicate current direction
3. Indicate top/bottom direction
4. Determine if it is an epiclastic or intraformational conglomerate/breccia
5. Determine if it is an ortho- or paraconglomerate/breccia
6. Determine if it is an oligomictic or polymictic conglomerate/breccia
7. Name the conglomerate e.g. epiclastic oligomictic orthoconglomerate
8. If matrix is present, I) what is the range in grain size e.g. coarse to fine sand, II) What is the average
9. Identify the composition of the clasts
10. Give range and average size of clasts
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11. Answer any question posed in the write-up for each sample
Sample IPC 3109C. Breccia, Bedford Canyon, Jurassic (?), southern California. The finer grained material
between the gravel is termed matrix. In this sample the matrix is fine sandstone. Note that most of the coarser
particles are in contact with each other, forming a framework of support for the rock.
1. Can you see any evidence of bedding? If so, show it in your sketch. Look for grain alignments. You'll
see that there is an alignment to the long axis of the grains. Generally, this alignment is parallel to
subparallel to bedding.
Sample 4693. Precambrian conglomerate (center of sample), Formation (?), Cadiz area, California. Some point
contacts are evident. This sample is probably best referred to as a metaconglomerate. Some of the matrix and
the clasts appear to be recrystallized.
1. Do you see any evidence of grain alignment?
2. What are the three ways by which a conglomerate consisting of only quartzite is generated?
Sample 1. Formation (?), Age (?), Locality (?). Note the nature of the contacts in this sample. The contacts
evident include point, tangential, and penetration contacts (see Fig. 2.8, p 18 of your text). Penetration contacts
indicate that the sediment has undergone significant compaction. This is important because it gives you an
insight into the diagenetic history of the conglomerate. Find the volcanic clasts in the conglomerate. One of the
ways to identify the volcanic clasts in the rock is to look for small euhedral plagioclase grains in the clasts.
Note that the rock contains a fine to medium grained matrix. The presence or absence of a matrix is important
to note because it tells you something about the depositional processes. Usually when strong currents (e.g. high
energy fluvial and beach environments) act on sediments the finer materials are removed and only the coarser
clasts are deposited. Conglomerates deposited with matrix usually result from sediment gravity flow processes
such as debris flows. One can imagine a debris flow moving downslope. This debris flow is a mixture of gravel
and sand or mud and water. A debris flow is usually matrix supported and poorly sorted. One must not
overlook studying the matrix for composition, texture, etc. Note that some of the filled fractures in the rock
extend across the clasts and into the matrix. This indicates that the fracturing occurred after lithification and
that the grains were not fractured prior to deposition (it is difficult to transport fractured grains, they break
apart).
VB-1. Sample 2 Formation (?), Age (?), Locality (?). This rock is composed of volcanic material. The question
is whether this is a sedimentary rock or volcanic rock. The distinction between the two is not always clear-cut
as you will see when you read the chapter on volcaniclastic rocks. There are a couple of things that I want to
point out in this rock: 1) note again that fractures cut across grains and into the matrix. Again this indicates that
fracturing occurred following deposition. 2) Observe the milky white material that occurs between the grains.
This material is a cement that binds the rock. Cements in conglomerates range from calcium carbonate to quartz
to clays.
1. What is the cement in this sample?
Metaconglomerate. This rock has been metamorphosed.
1. What is the nature of the grain contacts?
2. What are some of the diagenetic changes that took place?
3. What is a possible depositional mechanism?
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Sample 3634 sandy conglomerate, age, locality, and formation unknown. Most of the clasts in this
conglomerates are cherts and quartzites (make sure that you can identify each). Find the volcanic clasts in this
conglomerate.
1. Is there bedding in this conglomerate.
2. Describe rounding of the matrix grains. Is it angular, subangular, subrounded, rounded,or well rounded?
3. How does the texture of the matrix compare with that of the coarser clasts? In other words is the degree
of rounding of the matrix similar to that of the coarser grains?
Commonly, the matrix material is more angular than the larger grains. The larger grains have a tendency to
round much quicker than sand-sized material. For example, large (up to 3' in radius) boulders found right at
the base of the San Gabriel Mountains are well rounded even though they have traveled a relatively short
distance from the source. Conversely, the sand grains in the streams where these boulders are found are very
angular.
4. What is the nature of the cement in this conglomerate?
Sample 2674-2 sandy conglomerate, age, locality, and formation unknown.
1. Observe the laminated sandy interval in the sample. These laminae are typical of those produced by
currents, and consequently imply that the conglomerate was deposited by moving water. Note that the
laminations wrap around or mimic the shape of the clasts.
2. Which way is up? The clues provided by the laminae indicate that the sandy conglomerate was
deposited and currents subsequently deposited the laminated sandy interval. Another conglomeratic
interval was deposited following the deposition of the laminated interval.
Polished Sample on Counter
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Conglomerates Checklist
1. Identify whether conglomerate or breccia is:
a. Extraformational/intraformational
b. Oligomict/polymict
c. Ortho/paraconglomerate
2. Composition
a. Overall percentage of clasts & matrix
b. Composition of clasts
c. Composition of matrix (e.g. sand, silt, clay)
3. Size and shape of clasts
4. Fabric
a. Alignment of grains
b. Type of grain contacts
5. Sedimentary structures
6. Fossils
7. Cements
8. Post-depositional features
a. Fractures in grains (intergranular = confined to grains; intragranular = fractures cut across grains
and matrix)
b. Cements
c.
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