Sedimentary Rocks - Geol 102

Tectonic Setting

• craton - stable interior of a continent; undisturbed by mountain-building events since the Precambrian
• shields (large areas of exposed crystalline rocks)
• platforms (ancient crystalline rocks are covered by flat-lying or gently warped sedimentary rocks)
• orogenic belts - elongate regions bordering the craton which have been deformed by compression since the Precambrian.

Geosynclines

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
• Chert
• Dark, dirty sandstones (graywackes)
• Volcanic rocks, pillow lavas
• Sometimes called "flysch"

Origin of Sedimentary Rocks

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 accumulate.

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

Quiet water = low energy

How do you determine energy levels?

1. Grain size
1. coarse sediments indicate high energy (sandstone, conglomerate)
2. fine sediments indicate low energy (shale, micrite)
2. Grain shape
1. Roundness
1. Roundness is the absence of sharp corners
2. Roundness is due to abrasion in streams or waves
3. Roundness takes both energy and time
4. Large grains round faster than small grains - in a conglomerate, the larger grains are rounded, but the smaller grains may be angular



2. Sphericity
1. Sphericity is equal dimensions
3. Chart of roundness vs. sphericity - examples of high and low roundness and high and low sphericity

3. Textural maturity
1. Three steps
1. Winnowing or washing out of fines
2. Sorting of grain sizes
1. Good sorting implies consistent energy (washing)
2. Poor sorting implies inconsistent energy (dumping)
3. Rounding
2. Muddy sediment is immature
3. Poorly sorted sediment with no mud is submature
4. Well sorted sediment with no mud is mature
5. Well sorted and rounded sediment with no mud is supermature
4. Textural inversions
1. 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 above.

Sandstone composition

Three components are considered:

1. Quartz grains
2. Feldspar grains
3. Rock fragment grains

Three major types of sandstone:

1. Quartz sandstone (also called quartz arenite)
2. Arkose
3. Litharenite or lithic sandstone (commonly but imprecisely called graywacke)

Composition is an indicator of time in the basin of deposition (among other things).

1. Mature
1. Dominated by quartz (very stable sedimentary mineral)
2. Not mature
1. Not dominated by quartz
2. Contains unstable mineral and rock fragments

Arkose

Each type of sandstone implies something about depositional history:

1. Quartz sandstone implies a long time in the depositional basin.
2. Arkose implies a short time in the depositional basin (because feldspar typically weathers quickly to clay). Also implies rapid erosion, arid climate, tectonic activity, steep slopes.
3. 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 readily.

Paleoclimate

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 activity.
• If a sand has a lot of quartz, it probably indicates low tectonic activity - a passive setting.

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 weathering).

Processes acting in the depositional basin

Grain size:

• 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

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 (supermature) arkose.

Examples

For each of the following, interpret:

1. Source area lithology (rock type from which it was derived)
2. Paleoclimate (humid? arid?)
3. Tectonic activity (high or low tectonic activity)
4. Energy levels (high or low, consistent or inconsistent energy levels)
5. Time (long or short time in depositional basin)

1. Quartz sandstone, well sorted, well rounded
2. Arkose, poorly sorted, poorly rounded
3. Others

© 1995, Pamela J.W. Gore
DeKalb College
pgore@gpc.edu
Created September 24, 1995
Document last modified October 9, 1996
Typo corrected October 1, 1997