Sandstone Types
This lab focuses on sandstone types. Before proceeding with the lab, make sure that you examine Fig.
2.57 on pg. 49 of your text. The figure shows Pettijohn's classification of Sandstone, the classification scheme
we will be using in this mind. Keep in mind that this is only one of many classification schemes dealing with
sandstones. Make sure that you know this classification scheme well. Before you begin the lab, plot the
following composition on a triangular plot : (a) Qtz 97, Felds. 3, Lithics 0 ; (b) Qtz. 55, Felds. 25, Lithic 10,
biotite 5 ; (c) Qtz. 25, Felds. 5, Lithic 70.
Quartz Arenite -- Quartz Arenites are sandstones composed of greater than 95 % quartz. Quartz-rich
sandstones are probably the easiest to work with in thin section. They are compositionally and typically
texturally mature, and, as a result, do not possess compositionally variable grain types. Yet these rocks are
perplexing. What circumstances could lead to the generation of such texturally and compositionally mature to
supermature sandstones? To answer this question think about distance of transport, reworking of sediments,
climate, etc.
A common problem that students have when working with quartz-rich rocks is distinguishing between
quartz arenites and quartzites. Some people feel that when a well-cemented quartz-rich rocks fractures across
the grain it should be considered a quartzite. However, imagine a quartz arenite cemented by quartz cement.
When broken open with a hammer, the rock will fracture across the grains, but this rock is not a true quartzite.
Another criteria that people use to distinguish quartz arenite from quartzite is whether one can see individual
grains in the rock, then the rock should be called a quartz arenite. A quartzite is a recrystallized quartz arenite
and many times individual grains will not be evident.
Examine the rocks and sediment samples supplied. Learn to identify quartz arenites and quartzites,
determine textural and compositional maturities of these rocks, and learn how to identify the quartz and
hematite cement . For the samples with asterisk I would like you to provide the following for the samples listed
below:
1. Textural Description
a. Size (range; average) in millimeters and Wentworth size scale (e.g. fine to coarse grained, average)
b. rounding (range, average)
c. sorting
d. matrix (if present) percent, size, rounding, sorting, composition
e. fabric
2. Compositiona. bulk composition
b. normalized for quartz, feldspars, and rock fragments)
c. QFL plot
3. Cement Type
4. Textural maturity
5. Compositional maturity
6. Explain origin of textural maturity—why you feel rock is texturally mature or immature eg. Distance of
transport and what happens to texture of sediment with increasing transport, transport mechanism e.g.
wind, water currents or debris flow and what types of textures result
7. Explain origin of compositional maturity—why you feel rock is compositionally mature e.g. distance of
transport include first cycle or multicyle interpretation, interpretation of whether source area a high area
undergoing rapid erosion
8. Diagenetic history--What happened to rock after deposition eg. cementation, fracturing, compaction.
9. Rock name e.g. poorly sorted, coarse grained, subangular quartz arenite
*Quartz sand in trays -- make sure you can identify quartz grains, size, rounding and sorting
*# 401 Quartz Arenite
*# 402 Quartz Arenite
*# 3629
*# 3627
poorly indurated quartz arenite
Quartz Arenite
1795 . The sand in the tray was derived from sample 1795. Would you classify the gains in the tray as
having undergone one cycle of erosion, transport, and deposition of multiple cycles fo these.
3620 Describe how this sample differs form the other quartz-rich rocks in the trays. What is the name
of the rock?
Sed Pet IPC. What is the name of this rock.
Arkoses -- Arkoses contains significant (25%) feldspar. For arkoses to form, a source rock must contain
significant quantities of feldspar, and the feldspar generated from the source rock must be preserved. Generally,
the preservation of feldspar requires rapid uplift, erosion, and rapid sedimentation. Climate also plays a role. In
a humid climate the changes of feldspar presentation are poor because the feldspar is altered to kaolinite.
However, if rapid uplift, erosion, and rapid sedimentation occurs then the feldspars might be preserved.
Examine the following samples of Arkoses. Make sure that you can identify the quartz and the feldspar
and that you are comfortable classifying these arkoses. Also examine the sediments in the trays.
Sediments in trays C, D, and E. The material in the trays is composed of granitic fragments. The angular,
poorly sorted material in the trays is the type from which arkoses are generated. Typically, the source rock for
an arkose is a granitic rock or gneiss that contain significant amounts feldspar. Learn how to identify the
granitic rock fragments and the feldspar and quartz that comprise the fragments.
Sample 466—Igneous rock stained for potassium feldspar (yellow) and plagioclase feldspar (pink). The quartz
is gray. Examine the unstained half and make sure that you can identify the quartz, plagioclase and potassium
feldspar. Arkoses are derived from these types of rocks.
1. Textural Description
a. Size (range; average) in millimeters and Wentworth size scale (e.g. fine to coarse grained, average)
b. rounding (range, average)
c. sorting
d. matrix (if present) percent, size, rounding, sorting, composition
e. fabric
2. Compositiona. bulk composition
b. normalized for quartz, feldspars, and rock fragments)
c. QFL plot
3. Cement Type
4. Textural maturity
5. Compositional maturity
6. Explain origin of textural maturity—why you feel rock is texturally mature or immature eg. Distance of
transport and what happens to texture of sediment with increasing transport, transport mechanism e.g.
wind, water currents or debris flow and what types of textures result
7. Explain origin of compositional maturity—why you feel rock is compositionally mature e.g. distance of
transport include first cycle or multicyle interpretation, interpretation of whether source area a high area
undergoing rapid erosion
8. Diagenetic history--What happened to rock after deposition eg. cementation, fracturing, compaction.
9. Rock name e.g. poorly sorted, coarse grained, subangular quartz arenite
Sample 10 Hand samples of arkoses. Note again that the feldspars have also been altered. Examine the
textures and determine the compositions and plot on triangular diagram. Use the checklist provided to describe.
52—Typical arkose. Determine compositions and plot on triangular diagram. Use the checklist provided to
describe.
Lithic Sandstones--Lithic sandstones are important because the rock fragments in the rocks are excellent
indicators of the rock types present at the source areas. For these rocks to occur conditions must such that
unstable material is produced and preserved. Generally, rapid uplift and burial meet such conditions. Alluvial
sandstones deposited on the craton meet the aforementioned conditions.
Sample IPC 3111 What are the lithic fragments that are in the rock. Estimate the percentage of lithic
fragments, quartz and feldspar and plot on a triangular diagram
Sample 48 Most of the lithic fragments were derived from volcanic rocks. Identify the volcanic rock fragments
and estimate their percentages.
Green Sand
What is the green mineral?
What is the provenance of this sand?
Will the sand survive weathering?
Has it been transported great distances? Explain
Purple Sand
What is the purple mineral?
What is the provenance of this sand?
Will the sand survive weathering?
Has it been transported great distances? Explain