Surface processes II: Sedimentology
Sedimentary rock - rock composed of the transported remains of pre-existing rocks, i.e. sediment.
On Earth, where weathering, transport, and deposition of sediments is a constant process, these are a very important part of the rock record because they:
- Form the major part of Earth's surface
- create an extensive record of Earth surface conditions, recording ancient environments and the critters that inhabited them.
Sediment: material derived from the weathering of preexisting rock.
General life history of sediment: In order to make a sedimentary rock, four things need to happen:
- Weathering: The mechanical and chemical breakdown of rock into:
- transportable fragments or
- ions in solution.
Microcosm of sedimentation on Santa Monica beach, CA.
- Transport: Physical movement of the products of weathering by some agency. Material may be transported as fragments or in solution.
Tiny delta in outwash stream of Worthington Glacier, AK.
- Deposition: The process of depositing the sediment. For solid fragments, this involves deposition is some sort of topographic basin. For materials in solution, this involves precipitation.
Cementation of carbonate grains from Balcones Escarpment - University of Texas.
- Cementation: The binding of grains through the precipitation of mineral cements (or, sometimes, through chemical reactions between the grains, themselves, that cause them to bind).
Granite weathering from Geograph.
Agents of transport: On Earth, the following processes typcially move sediment:
- Mass wasting: Wherever there is a topographic slope, the is the potential for loose material to slide down it under the influence of gravity. This applies whether the slope is on land or in the ocean basins.
Savage River, Denali National Park, AK.
- Fluvial transport: On the continents, the primary movers of sediment are flowing streams and rivers. Note: These deliver their sediment load to the shores of standing bodies of water like lakes and oceans but not far into them. Thus, a common depositional feature is the delta, a fan of sediment deposited at a river mouth.
Oblique waves power long-shore current between Smithfield and Port Douglas, QLD.
- Shoreline transport: Where the interface of air and water meets the shore, wave energy transports sediment along the shoreline.
Sediments deposited at toe of Matanuska Glacier, AK
- Glacial trasport: The ultimate, indiscriminant mover of sediment fragments of all sizes.
White Sands, NM
- Aeolian transport: The wind is major transporter of small fragments. On Earth, it lofts dust-sized particles far into the atmosphere and bounces sand grains along the surface, where they can collect in dune seas if they encounter some kind of topographic trap.
Landslide scar and deposit at Resurrection Bay, AK.
Alluvial fan and source rock at Anza Borrego State Park, CA.
"A Rock is the record of the environment in which it forms."Each of these processes leave their signature on the resulting rock, with the result that we can learn a great deal about:
- The source rock from which sediments were weathered.
- How far and by what means they were transported: Different agents of transport favor different grain sizes and modifies those grains in different ways. E.G.:
- Sorting: The wind strongly sorts sediment grains by size. Glaciers don't discriminate between grains of dust and boulders the size of houses.
- Rounding: Fluvial and shoreline sediments, having been rolled for long distances over a solid surface tend to be rounded and polished. Aeolian sediments are "frosted" by sand-blasting. Gravity and glacier-transported sediments may be angular.
- In what environment they were deposited: Each depositional environment has characteristic fingerprints. E.G.:
- Aeolian environments are marked by large "cross-beds" that preserve the ancient slip-faces of sand dunes.
- Ancient fluvial environments also preserve much smaller cross beds.
Evorites in dry lake bed from Lake Scientist
- The beds of ephemeral lakes are preserved as crusts of minerals precipitated from solution.
Limestone etched by fresh water of the Li River near Guilin, China
- Diagenesis: The physical changes that have occurred in the depositional environment since deposition, including the dissolution of minerals. At right: limestone etched and dissolved by fresh water on the Li River near Guilin, China. (Note, in some landscapes, dissolution releases lots of ions into solution, leaving "karst" landscapes full of caves and remanent towers. E.G. the region near Guilin.)
Comparative Sedimentology in the Solar System:
Earth seems to be the Solar System champ for diversity and frequency of sedimentary processes, but there are other players:
80 km landslide scar and deposit on Iapetus from Space.com
Smaller bodies:Mass wasting seems to occur on all solid planetary bodies. The image of Iapetus at right shows where the collapse of a 15 km high basin wall has caused a giant landslide roughly 120 km long. A particularly spectacular example of a process that occurs on many worlds at smaller scales.
Beyond this, there really are only two contenders.
Martian dust dunes from NASA - JPL
Mars:On contemporary Mars, aeolian processes dominate. Indeed, any flat surface seems to support ripples and dunes of reddish dust and black sand. Aeolian transport of dust is what gives Mars' sky most of its color.
Gullies in Martian crater from University of Hawaii - Planetary Sciences Recent Discoveries
Opponents such as Pilorget and Forget, 2015, cite landslides and debris-flows triggered by the accumulation of winter CO
Ancient delta deposit in Eberswalde Crater Malin Space Science Systems
Cross-beds in Meridiani Planum seen by Opportunity from Steven Earle - Vancouver Island University
- Cross beds! (arrow)
- Countless small concretions, termed "blueberries."
Where did it go? The fate of all this Martian surface water is a major issue in Mars science, and a primary justification for the current MAVEN mission.
Glacial valley on flanks of Arsia Mons
Huygen's one and only image of Titan's surface from
Titan:Your text was published before good information about Titan was available. Contemporary Titan looks like an analog to ancient Mars, with an ongoing hydrologic cycle and standing bodies of liquid. At a mean surface temperature of 94 K, this cycle is based on ethane and methane. We have already spoken of the branching channels, lakes, and cumulus clouds observed by Cassini. Examination of the Huygens lander's image adds some detail:
- Rounded pebbles like we might expect from a stream channel
- Deposits of sand around them that appears to have been sculpted by a flowing liquid.
We now know that Great Lakes sized lakes exist on Titan. One would expect these to experience shoreline transport, also. (See Visit Ontario Lacus.) So far, however, radar images of Titan's lakes show them to be glassy still, a possible consequence of a prolonged season of still air or their liquid being highly viscous.
Magic Islands: And for something completely different, consider Hofgartner et al.'s 2014 report of an ephemeral island-like feature that appeared briefly in the lake Ligaea Mare during 2013 then vanished. Foaming waves? Bubbles? Floating material? Really, no clue.
Sikkun Labyrinthus from Biblioteca Pleyades
Titanian eolian dune sea from NASA
Nitrogen ice sheet in Sputnik Planum on Pluto fromWikipedia
But wait! Pluto:
But in summer 2015, New Horizons revealed that Pluto supports flowing glaciers as well. In this case, the ice is frozen nitrogen and carbon monoxide. Perhaps these transport sediments of rock-solid water and ammonia ice.
Key concepts and vocabulary:
- Sedimentary rock
- Mass wasting
- Fluvial transport
- Shoreline transport
- Glacial transport
- Aeolian transport
- Cross beds
- Amy planetary body with a solid surface
- Mass wasting
- Aeolian dark dunes and red dust
- Gullies carved by water or CO2 (or both)
- Ancient fluvial transport
- Meridiani Planum cross beds and concretions (blueberries)
- Fluvial transport in methane/ethane streams
- Shoreline transport
- Magic islands (WTF?)
- Karst environments
- Aeolian transport in huge dune seas
- Glaciers of nitrogen and carbon monoxide
- Stephen Grasby, Bernadette Proemse, and Benoit Beauchamp. 2014. Deep groundwater circulation through the High Arctic cryosphere forms Mars-like gullies. Geology, 42(8):651-654.
- J. D. Hofgartner, A. G. Hayes, J. I. Lunine, H. Zebker, B. W. Stiles, C. Sotin, J. W. Barnes, E. P. Turtle, K. H. Baines, R. H. Brown, B. J. Buratti, R. N. Clark, P. Encrenaz, R. D. Kirk, A. Le Gall, R. M. Lopes, R. D. Lorenz, M. J. Malaska, K. L. Mitchell, P. D. Nicholson, P. Paillou, J. Radebaugh, S. D. Wall, and C. Wood. 2014. Transient features in a Titan sea. Nature Geoscience 7, 493-496.
- M. Masse, S. J. Conway, J. Gargani, M. R. Patel, K. Pasquon, A. McEwen, S. Carpy, V. Chevrier, M. R. Balme, L. Ojha, M. Vincendon, F. Poulet, F. Costard, and G. Jouannic. 2016. Transport processes induced by metastable boiling water under Martian surface conditions. Nature Geoscience 9, 425-428.