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:

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:

Agents of transport: On Earth, the following processes typcially move sediment:


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:

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
Mars Gullies: There are some erosional features, however, that can't be blamed on the wind: Contemporary gullies are apparently being carved currently in the walls of craters and other steep slopes. These show regions of erosion uphill and depositional aprons downhill. What is the agent of transport? A majority opinion seems to be that salty groundwater percolating out of aquifers is the culprit. Masse et al., 2016, replicated the temperature and pressure of Mars' surface to demonstrate that brines emerging from the ground would last long enough before boiling away to do the deed. Their hypothesis is supported by spectroscopic evidence for salt deposits at the gullies.

Potential analogs on Earth are described from Iceland (Hartmann et al., 2003), Ellesmere Island (Grasby et al., 2014), and Alaska (Hooper and Dinwiddie, 2014).

Opponents such as Pilorget and Forget, 2015, cite landslides and debris-flows triggered by the accumulation of winter CO2 ice as the agent of transport. Are these even mutually exclusive propositions?


Ancient delta deposit in Eberswalde Crater Malin Space Science Systems
Mars channels: In the distant past, (4.5 - 3.5 ga) considerably more water flowed across Mars, forming channels with dendritic drainage patterns like we see on Earth. Whether these were semipermanent or intermittent is unclear. Most exciting is the possibility that Mars once possessed lakes or oceans with stable shorelines. Features like the apparent delta deposits at right (14 x 19 km) in Eberswalde crater don't make much sense otherwise.


Cross-beds in Meridiani Planum seen by Opportunity from Steven Earle - Vancouver Island University
Adding color to these observations are those made by by the rover Opportunity at Meridiani Planum.

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
from HIRISE
Glacial ice is not an important factor today, as most ice exists as static ice caps or subsurface ice, rather than as flowing glaciers. This has not always been so. Scanlon et al., 2014 document many features typical of volcanic eruptions beneath glacial ice on the slopes of Arsia Mons.


Huygen's one and only image of Titan's surface from
Wikipedia

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: Huygens seems to have landed in a dry stream bed. Indeed, Cassini has observed many features that look like gullies and dry river beds (although if they were currently "wet" that would be hard to determine.) Many of these are the size of full scale rivers. At least one seems to be permanently flowing.

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
Karst environments: Mitchell and Malarska, 2011 report that, beside Earth, Titan is the only world that might have karst landscapes, Thus, Sikkun Labyrinthus (right) might be formed similarly to the Guilin region) by widespread solution of bedrock. E.G.:


Titanian eolian dune sea from NASA
Aeolian environments: Ethane/methane rivers notwithstanding, Titan is an arid place dominated by aeolian processes. These manifest in a globe girdling sea of dunes. These dunes occupy low flat regions and flow around topographically upstanding "bedrock" (ice, actually) features. These seem to be made of sand-sized fragments of some unknown hydrocarbon compound the consistency of coffee grounds. The image covers roughly 220 x 179 km.


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:
  • Sediment transport in the Solar System
    Additional reading: