© Pamela J. W. Gore, 1995-2004
Georgia Perimeter College
Principle tectonic elements of a continent:
- craton - stable interior of a continent; undisturbed by mountain-building events since the
- shields (large areas of exposed crystalline rocks)
- platforms (ancient crystalline rocks are covered by flat-lying or gently warped
- orogenic belts - elongate regions bordering the craton which have been deformed by
compression since the Precambrian.
Theory promoted by James Hall (1859) and D. Dana (1873) resulting from their work on the
Appalachians. Sediments thicken at continental margins (to the east in the Appalachians).
- Miogeosynclines (miogeoclines)
- Depositional setting = nearshore, continental shelf
- Quartz sandstone (beach, shallow sea)
- Limestone (warm, shallow sea)
- "Red beds" - oxidized terrigenous clastics - conglomerate to clay textures ( rivers,
deltas, coastal plain deposition)
- Sometimes called "molasse"
- Eugeosynclines (eugeoclines)
- Depositional setting = deeper water sediments farther from the craton, submarine
fan environments may be included, associated with volcanics
- Black shales
- Dark, dirty sandstones (graywackes)
- Volcanic rocks, pillow lavas
- Sometimes called "flysch"
Origin of Sedimentary Rocks
Depositional basin - a place where sediments accumulate
Sedimentary rocks may be:
- Extrabasinal in origin - sediments formed from the weathering of pre-existing rocks
outside the basin
- Intrabasinal in origin - sediments form inside the basin; includes chemical, biochemical,
and organic sedimentary rocks
The goal in Historical Geology is to try to interpret the conditions under which the sediments
Source area - the land which weathers and erodes to form terrigenous sediments (extrabasinal).
Terrigenous sedimentary rocks are a product of:
- Source area
- Conditions in the depositional basin such as energy levels
Texture as an indicator of energy levels
Waves, currents, moving water = high energy
Quiet water = low energy
How do you determine energy levels?
- Grain size
- coarse sediments indicate high energy (sandstone, conglomerate)
- fine sediments indicate low energy (shale, micrite)
- Grain shape
- Roundness is the absence of sharp corners
- Roundness is due to abrasion in streams or waves
- Roundness takes both energy and time
- Large grains round faster than small grains - in a conglomerate, the larger
grains are rounded, but the smaller grains may be angular
- Sphericity is equal dimensions
- Chart of roundness vs. sphericity - examples of high and low roundness and high
and low sphericity
- Textural maturity
- Three steps
- Winnowing or washing out of fines
- Sorting of grain sizes
- Good sorting implies consistent energy (washing)
- Poor sorting implies inconsistent energy (dumping)
- Muddy sediment is immature
- Poorly sorted sediment with no mud is submature
- Well sorted sediment with no mud is mature
- Well sorted and rounded sediment with no mud is supermature
- Textural inversions
- An unusual association of textures, such as rounded sand (implies high energy) in
clay (implies low energy). Cannot be fit into the textural maturity classification
Sandstones are classified on the basis of the composition of their grains.
Three components are considered:
- Quartz grains
- Feldspar grains
- Rock fragment grains
Three major types of sandstone:
- Quartz sandstone (also called quartz arenite)
- Litharenite or lithic sandstone (commonly but imprecisely called graywacke)
Composition is an indicator of time in the basin of deposition (among other things).
Remember Bowen's Reaction Series and the Goldich Stability Series
- Dominated by quartz (very stable sedimentary mineral)
- Not mature
- Not dominated by quartz
- Contains unstable mineral and rock fragments
Each type of sandstone implies something about depositional history:
- Quartz sandstone implies a long time in the depositional basin.
- Arkose implies a short time in the depositional basin (because feldspar typically
weathers quickly to clay). Also implies rapid erosion, arid climate, tectonic activity,
- Litharenite implies rapid erosion, temperate or arid (not humid) climate
SANDSTONE INTERPRETATION GUIDE
Sandstone textures and compositions may be used to interpret many things about the history of
the sand, including source area lithology, paleoclimate, tectonic activity, processes acting in the
depositional basin, and time duration in the basin. Remember that the source area is the land
which is weathering and eroding to supply terrigenous debris to the depositional basin.
Source area lithology
Composition gives the key information (minerals or rock fragments present). Remember
that quartz sandstone or quartz arenite is dominated by quartz grains; arkose is dominated by
feldspar grains (usually potassium feldspar); and graywacke is dominated by rock fragment grains.
- Sand-sized quartz grains could come from the weathering of source area rocks such as
granite, gneiss, or other sandstones which contain quartz (recycled sandstones).
- Sand-sized feldspar grains could come from the weathering of source area rocks such as granite or gneiss.
- Sand-sized rock fragment grains come from the weathering of fine-grained source
rocks. Possibilities include shale, slate, phyllite, basalt, rhyolite, andesite, chert, and
possibly schist. Limestones would not be included usually because they dissolve so
Paleoclimate refers to the climate which existed in the source area. We are particularly
concerned with weathering rates here. Remember that in humid climates, feldspar weathers to
clay by hydrolysis. Other minerals also weather to clay (with associated iron oxides), such as olivine, pyroxene, and amphibole.
Also remember the difference between weathering (BREAKDOWN of rock by
hydrolysis, dissolution, oxidation, exfoliation, frost wedging, or freeze thaw), and erosion
(TRANSPORTATION of particles).
- If feldspar is present in your sand, it indicates that the climate was probably arid. (Orthat erosion rates were very rapid, and that tectonic activity was extremely high - lots of
uplift,and steep slopes.)
- If quartz is the dominant mineral in the sand, the climate was probably humid (all of the feldspars weathered away to clay).
- If rock fragments are present in your sand, it helps to know what lithology they are. If
they are rock types which would weather rapidly (such as basalt or limestone fragments),
the climate was probably arid. If they are rock types which would be relatively stable
(shale, slate, or chert), the climate may have been temperate to humid. (remember
Bowen's Reaction Series and the Goldich Stability Series to determine what is stable or
unstable). If rock fragments are present and no rock types are given, a good compromise
answer would be temperate climate.
Tectonic activity in the source area
We are basically classifying tectonic activity as "active" or "passive". For a good model,
consider the west coast of the US as tectonically active - steep slopes, mountains close to the sea, lots of earthquakes, tectonic uplift, and volcanic activity. On the other hand, consider the east coast of the US as tectonically passive - broad, flat coastal plain, few or no earthquakes, no uplift, and no volcanic activity.
- If a sand has a lot of feldspar or rock fragments, it probably indicates high tectonic
- If a sand has a lot of quartz, it probably indicates low tectonic activity - a passive
Tectonic activity also influences sorting, time duration in the depositional environment
(and to some extent, compositional maturity). High tectonic activity might produce rapid
dumping of sediments into the basin with little or no time for sorting. Low tectonic activity
means little uplift, low erosion rates, and therefore little sediment supplied to the basin; what
sediment that is there is likely to wash around for a long time and become well sorted and
rounded, and grains other than quartz are likely to be destroyed (by abrasion or chemical
Processes acting in the depositional basin
This question refers to energy levels ("high" vs. "low") and consistency of energy.
Texture gives the key information.
- Coarse sediments generally indicate high energy, and fine sediments indicate low energy.
- Well sorted sediments indicate consistent, fairly high energy levels. (Winnowing.)
- Poorly sorted sediments indicate inconsistent energy levels - rapid dumping (which might
involve short episodes of high energy), followed by low energy conditions.
Time duration in the depositional environment
Both mineralogy and texture can be used to determine time in the depositinal
A compositionally mature sediment (abundant quartz grains) suggests a long time in the
depositional environment. Quartz is more resistant to abrasion than feldspar or rock fragments.
A compositionally immature sediment (abundant feldspar or rock fragment grains)
suggests a short time in the depositional environment.
Textural maturity is also useful in interpreting time in the depositional environment.
Immature or submature sediments probably spent only a short time in the basin before
burial. Mature or supermature sediments were probably rolling around in the basin for a
long time before burial. Roundness is a good clue to a long time in the depositional
environment. Rounding of grains takes a long time; it is more likely in a tectonically
passive situation. Desert sands are often well rounded because of the "sandblasting"
process of wind transport. Hence, in an arid desert, it is possible to get a well-rounded
For each of the following, interpret:
- Source area lithology (rock type from which it was derived)
- Paleoclimate (humid? arid?)
- Tectonic activity (high or low tectonic activity)
- Energy levels (high or low, consistent or inconsistent energy levels)
- Time (long or short time in depositional basin)
- Quartz sandstone, well sorted, well rounded
- Arkose, poorly sorted, poorly rounded
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© 1995, Pamela J.W. Gore
Georgia Perimeter College
Created September 24, 1995
Document last modified October 9, 1996
Typo corrected October 1, 1997
Modified February 3, 1999
Modified December 12, 2003
Links updated August 15, 2009