EES 2021
LAB 3 – CARBONATE ROCKS
Name________________________________________________
Date_________________
Carbonate rocks are fundamentally different than siliciclastic rocks in terms of their depositional
environments and chemistry. To deal with these differences, geologists have developed classification
schemes and facies models specific to carbonate rocks. While the study of carbonate rock depositional
environments and subsequent diagenetic alteration are subjects for at least a full semester class, we will
attempt to cover some of the basics here.
Components of Carbonate Rocks
Carbonate rock grains, called allochems, can be divided into 4 broad categories: skeletal allochems,
nonskeletal allochems, matrix, and cement. We will deal with skeletal, nonskeletal allochems and matrix
compositions for this lab.
Non-skeletal allochems
Non-skeletal allochems are components which are not derived from the physical remains of a living
organism (shell, coral bits, etc.). These include intraclasts, oolites, and pellets. Intraclasts are lithified
(or at least partially lithified) aggregates of carbonate sediment. Rip-up clasts are an example of an
intraclast grain. Oolites are spherical, concentric grains of carbonate sediment which coat a central grain
which acts as a nucleus. Wave agitation on shallow shoals promotes the growth of oolites much like a
snowball being rolled around in snow. Pellets are non-spherical, oblong shaped grains, which are
usually the product of fecal material.
Skeletal allochems
Skeletal allochems are transported bits and pieces of marine organisms. This includes (but is not limited
to) brachiopod values, pelecypod (bivalve) valves, gastropods, echinoderms, corals, and bryozoans. The
degree of articulation (how “together” the organism is), abrasion, and diversity of skeletal allochems
must be considered when interpreting a depositional environment. For example, a rock containing many
broken brachiopod and coral fragments must have been deposited in a high-energy depositional
environment, such as a reef margin or breaker zone. Conversely, a rock containing exclusively wellpreserved delicate organisms (such as bryozoans) must have been deposited in a low energy depositional
environment, such as a lagoon. Sedimentary structures must also be incorporated into interpretations
regarding depositional environments.
Matrix
Carbonate matrix composition can be divided into 2 classes: micrite and sparite. Micrite is essentially
lime mud (analogous to the muddy matrix in a greywacke sandstone). Micrite appears “muddy” in hand
sample and is a dull brownish color in thin section. Sparite has a crystalline texture (interlocking grains)
and appears clear in thin section when viewed in plain polarized light.
Classification
Because the nature of the origin of carbonate rocks is almost unequivocally different than that of
siliciclastic rocks, carbonate rocks are classified based on different characteristics than siliciclastic
rocks.
Folk’s (1959, 1962) scheme focuses on the composition of carbonate rocks. In Folk’s classification
scheme, the type of allochem serves as a prefix to the cement type in the rock. For example, a rock
containing mostly ooids in a sparry matrix would be classified as an oosparite. Folk’s classification
further separates rocks which have formed in-situ. This is the biolithite category and refers to reef rocks
and stromatolites. Rocks lacking allochems are simply called micrites or sparites (depending on the
matrix). Rocks with cavities (usually micrites with sparite filled cavities) are referred to as dismicrites.
Depositional Setting: Input the number that corresponds with the number on the figure. There are extra numbers
than there are depositional environments, so be careful on which answer you select.
SAMPLE #
ROCK NAME
DEPOSITIONAL SETTING
1
2
3
4
5
6
7
8
9
1
10
2
52
6
1
7
3
8
4
EES2021 – Sedimentary Environments
COASTAL MARSH STRATIGRAPHY LAB
Stratigraphy is a study of sediment layers (strata) that enables scientists to reconstruct the
sequence of events or environmental changes as reflected in changing sediment types and vegetation
patterns. In this part of the exercise, you will use the information collected from two areas (A and B)
in a saltmarsh to reconstruct the past 200-500 years of environmental change.
Materials: pencil and graph paper or you can use ant Graphics software.
1. Using the data from page 3, draw two geological cross-sections on the sheet of graph paper Area A on top, Area B below. Plot the cores as vertical columns by using the distance to each
core from the next page (one square = 1 m of distance). Assume that the water table is
horizontal and adjust all core elevations accordingly.
2. Draw cores A-1 and A-2 on the Area A section by using different symbols or colors for each
layer (one square = 10 cm of depth). Write brief description of each layer next to the core
section. Put cores B-1, B-2, B-3, and B-4 on the Area B section below using the core
distances from the road (zero distance) as you did above.
3. Correlate the strata by connecting similar layers with lines. See the cross-sections below for
an example of correlation (dashed lines).
dune scar p
5
mar sh
dune
RO-2
RO-1
0
-5
5
MHW
beach/washover
0
V.E. = 4
RO-3
0
20
m
dune
RO-4
mar sh
tidal flat
-5
MHW
heavy m ineral
concentr ations
beach/washover
RO-13
marsh
?
Page 1
N
EFFECTS OF STORMS
ON COASTAL
BARREIRS
LANDWARD
SAND TRANSFER
storms:
n
unian)
flood-tidal delta
breach
washovers
SALTMARSH ZONATION
(modified after John Norton)
Page 2
Area A - marsh behind an old inlet
Depth (cm)
Description
Vegetation
from ground
surface
color, sorting, grain-size,
Sp - Spartina patens
Sa - Spartina alterniflora
(most abundant listed
first)
Core A-1
50 m from road
0-18
18-40
Core A-2
dark-brown peat with S.p.
gray, well-sorted medium
sand
70 m from road
0-18
18-68
68-73
73-90
water table depth - 5 cm
Sp
none
water table depth - 0 cm
brown muddy
peat
dark brown peat
brown muddy
peat
gray, well-sorted medium
sand
Sa, Sp
Sp
Sa, Sp
none
Area B - marsh behind a narrow barrier
Saltmarsh starts 17 m from the road
Core B-1
10 m from road
0-50
50-70
Core B-2
20 m from road
water table depth - 50 cm
dark gray medium sand
brown coarse sand
poison ivy
none
water table depth - 34 cm
0-8
8--70
brown-gray peaty sand
brown coarse sand
Sp
none
Transitional saltmarsh starts 23 m from the road
Core B-3
30 m from road
water table depth - 0 cm (water is at the surface)
0-20
20-36
36-40
Core B-4
40 m from road
sandy brown peat
peaty sand
brown coarse sand
Sp, Sa
Sp
none
water table is 5 cm above core (this core is underwater)
0-22
22-40
40-50
brown fibrous
peat
dark brown sandy peat
brown coarse
sand
Sa
Sa, Sp
none
Page 3
COASTAL MARSH STRATIGRAPHY LAB – ANSWER SHEET
REMEMBER TO ATTACH YOUR CORRELATED STRATIGRAPHIC SECTIONS
Name___________________________________________
Date_________________
Area A – a saltmarsh behind a closed tidal inlet
1. What is the sandy feature underlying marsh peat?
______________________________
2. The marsh that is represented by a mixture of Spartina patens and S. alterniflora plants is
called a transitional marsh and occupies the boundary between the high and low marsh.
What is the elevation of this type of marsh relative to tidal levels? (circle one of five answers)
(Legend: MHT-mean high tide, MLT – mean low tide, MSL-mean sea level, SHT – spring high tide)
MLT to MHT
MHT
MLT
MSL to MHT
MHT to SHT
Area B – a saltmarsh behind a narrow barrier (not an old inlet site)
1. What is the sandy feature penetrated by all cores?
______________________________
2. Briefly explain why S. alterniflora peat overlies a mixed (transitional) peat in core B-4 (for the
same reason, mixed peat overlies S. patens peat in core B-3):
(Hint: what must be happening to cause intertidal areas to take over supratidal ones?)
_____________________________________________________________________________
Page 4
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