Historical Geology
Lab #2
Carbonate Rocks
INTRODUCTION
Carbonate rocks are either limestones composed of calcite (CaCO3) or
dolostones composed of dolomite (CaMgCO3). These rocks form through biological
and biochemical processes and through inorganic precipitation from seawater.
Carbonate rocks occur throughout the world and in every geological period beginning
with the Cambrian. Most of the carbonate sediments found in carbonate rocks form in
warm waters. For example, corals and algae are quite common in carbonate rocks, and
they are mostly found in abundance in tropical to subtropical latitudes or from about 30º
north and south latitudes. Most of the carbonate sediments are generated in the photic
zone. The photic zone is that part of the ocean waters where light penetration is at its
maximum. Maximum light penetration in the oceans occurs between 0 and 100-200
meters.
Unlike sandstones, most carbonate grains are formed in the environment in
which they are deposited. Detrital grains in sandstones are usually derived from outside
the environment in which they were deposited. For example, sand grains usually
develop through weathering and are transported to such environments as a continental
shelf or a beach. Carbonate grains are usually reworked and deposited in the same
depositional environment.
Due to geochemical considerations, carbonate sediments do not usually occur
below depths of about 3500 -4000 meters in the oceans. Below these depths carbonate
sediments will dissolve. The depths at which carbonate sediments dissolve in the
ocean is called the carbonate compensation depth or CCD. Therefore, when we study
the origin of carbonate rocks we know that most of them form at relatively shallow
depths in warm waters and that they do not usually form in very deep water.
In this lab we will learn to identify the various components of carbonate rocks,
and we will also learn to classify them. Two principal classification schemes are used
for limestones. One is termed Folk's classification scheme, which is named after Robert
Folk, and Dunham's classification scheme, which is name after Robert Dunham. Folk's
classification scheme is based on the presence or absence of allochems (carbonate
particles or grains), matrix (limestone clay called micrite), and the type of cement (spar
is a cement composed of coarse crystals). Dunham's classification scheme is based on
the texture of the rock. i.e. whether the rock contains matrix, whether the carbonate
grains float in the matrix or whether they are in contact with each other, and whether the
rock has any carbonate grains.
LAB
For this lab I have selected carbonate rock samples that are listed below. For
each specimen, name the rock according to the two classification schemes and it is
important that you become familiar with the components of each rock.
Additionally, I have provided some background information on some of the
components present in the samples. These components provide clues to the origin of
the carbonate rocks.
SOME CARBONATE COMPONENTS:
ooids--Ooids are spherical grains formed by calcite precipitation around a nucleus such
as a shell fragment, a quartz grain, etc. They are less than 2mm in diameter and they
typically form in shallow, warm, agitated, and carbonate-saturated waters such as those
near the Bahamas.
pisolites--Pisolites are ooids greater than 2mm in size
stromatolites--Stromatolites are laminated carbonate sediments composed of mats of
blue-green algae and layers of sediment. Algae, which forms the mats, is a plant and
requires sunlight to survive. Therefore, stromatolites generally form in warm shallow
waters. The algae are "sticky" and it grow filaments. This sticky and filamentous algae
trap sediment brought in by currents. Consequently, a layer of algae forms then a layer
of sediment is deposited on top of the algae. Subsequently, the alga grows through the
sediment to form another mat and the cycle begins again. Ultimately, a layered rock
composed of alternating algal mats and sediments is produced. Stromatolites form in
quiet, hypersaline waters with little animal life which destroy the mats.
oncolites--Oncolites are stromatolites rolled in a ball.
micrite--Micrite is fine grained (less than 4 microns) carbonate material. Essentially, it
is limestone clay. And like clastic sediments, fine-grained carbonate material generally
accumulates in quiet water environments from tidal flats and lagoons to the deep ocean.
corals--Corals have a symbiotic relationship with dinoflagellate algae called
zooanthellae. Algae are plants. Therefore, in order for corals to survive they generally
have to be within the photic zone (the zone of maximum light penetration in the ocean).
Plants require sunlight for photosynthesis. In the process of photosynthesis, plants
produce oxygen and consume CO2. Corals are major reef formers today and in the
past. However, keep in mind that some corals do occur in deep waters.
Intraclasts--Semi-consolidated carbonate material ripped up and incorporated within
rock.
dolostone--A dolostone is a rock compose of 90-100% dolomite.
CARBONATE SAMPLES
Name each rock using both Folk and Dunham classification scheme and answer
associated questions
5026 - Oolitic sand, 5024--oolitic limestone;--Examine sand with hand lens and name
the rock that would form if the sand lithified. Be careful not to lose the oolitic sand
sample. What type of environmental information can you obtain from ooids. In other
words, where do ooids form?
Folk? Dunham? Assume that the oolitic sand would eventually be cemented by spar
cement.
# 3533, 3558--corals. Name sample 3558. Where do corals form?
Folk? Dunham?.
4594--What are the principle components of this rock? Where did this rock form and
what evidence in the rock indicates that the carbonate components have been
transported?
Folk? Dunham?
1170U and 2362--chalk. Chalks are formed by carbonate mud and greater than 30%
by weight biogenous material (hard parts of organism, e.g. shells). Where did this rock
form?
Folk? Dunham? Assume that micrite cements the rock and that the rock consists
entirely of microscopic animals called foraminifer.
1170W—micritic limestone. This rock is composed of micrite. Note the fine grained
nature of rock. Name the rock.
Folk? Dunham? Fossils are not present in this rock
3754--stromatolite--Under what conditions do stromatolites form.
Folk? Dunham?
1772--pisolitic limestone—Sketch the pisolites. What are pisolites?
Folk? Dunham? Look for milky colored spar cement between the grains.
3133—algal limestone--Where did this rock form? Also, examine loose algal sample
from Catalina Island for comparison of modern and ancient algae.
Folk? Dunham? Look for milky colored spar cement between the grains.
3907--Name this rock
Folk? Dunham?
1170 AD and 4023--oncolitic limestone. What are oncolites?
Folk? Dunham?
NAME THE YELLOWISH MYSTERY ROCK (3657/58/3600, Sample has all these
numbers)
Dunham Classification Scheme for Limestones
Folk Classification Scheme for Limestones