Above the Dox formation is the Cardenas Basalts. Volcanic rock, i.e. lava flows. Much of what you get from google on it conisists of demented creationist honking about how the dating for this series is inaccurate or whatnot.
Here's a quick description I managed to dig up:
"The Cardenas Lava is the name given to a series of basalt and basaltic andesite flows and sandstone interbeds that are above the Dox Formation, but below the Nankoweap Formation. The Cardenas Lava is only exposed in the eastern part of the canyon, where it ranges from 785 feet to 985 feet in thickness. The contact between the Cardenas Lava and the Nankoweap Formation is an unconformity. An unknown amount of the Cardenas Lava was removed before the deposition of the Nankoweap."
Sandstone and basalt, hmmm. Periodic coastal volcanic eruptions?
Andesite is a different type of lava. It has a different mineral composition to basalt (less silica, most significantly), and is generally a good deal thicker, leading to steeper volcanoes and more explosive eruptions.
It seems that the Cardenas Lava flows are between 685 and 985 feet thick.
The lower Dox formation we covered earlier is mostly shale (mud) and sandstone with indications of being made during a series growing and receding seas. It has areas that show ripple marks and also area of mud cracks.
While the Cardenas Lava flows are between 785 and 985 feet thick, the Dox formation is up to 3000 plus feet thick.
As we look at each of these layer we seem to find signs that each layer really was at one time at the surface; for example we found ripple marks, rain drop impressions and mud cracks in parts of the Hakatai Shale (about 400 to 100 feet thick), then the Shinumo Quartzite layers (1000 to 1300 feet thick) then the Dox formation that is a totally different set of compositions and, as stated about, ranging up to 3000 feet thick.
For reference, when we talk about a formation such as the Cardenas Basalt, we're not speaking of a single type of rock. We're actually talking of a whole series of layers of rock, representing a particular depositional environment, in this case sandstone being deposited in between volcanic eruptions. In general terms, formations are divided based on the change in depositional environment, and within the formation you will have several members or types of rock which define that environment. The Cardenas Basalt has several members such as a red sandstone layer and a basalt layer.
Formations are the basic units, I guess, of how you look at the rocks.
This from Wikipedia: ...Clays are distinguished from other small particles present in soils such as silt by their small size, flake or layered shape, affinity for water and high plasticity index."
I thought clays were also characterized by ionic polarity (like water, thus the affinity for water), while silt was not. Both, being very small particle size, settle slowly in water, and thus take a lot longer to accumulate than larger granular sediment (like sand (or shells)) when they have to settle out of the water column.
quote:Clay deposits are mostly composed of clay minerals (phyllosilicate minerals), minerals which impart plasticity and harden when fired or dried, and variable amounts of water trapped in the mineral structure by polar attraction.
Or am I mixing definitions:
quote:The distinction between silt and clay varies by discipline. Geologists and soil scientists usually consider the separation to occur at a particle size of 2 Âµm (clays being finer, silts being coarser), sedimentologists often use 4-5 Î¼m, and colloid chemists use 1 Î¼m. Geotechnical engineers distinguish between silts and clays based on the plasticity properties of the soil, as measured by the soils' Atterberg Limits.
Defining it by size alone seems a little arbitrary to me, especially when you can have particles above and below that limit that exhibit plasticity and particles above and below that limit that don't exhibit plasticity (or should we have 'plastic clays' and 'non-plastic clays'?).
quote:In the Udden-Wentworth scale (due to Krumbein), silt particles range between 1â„256 and 1â„16 mm (3.9 to 62.5 Î¼m), larger than clay but smaller than a sand. In actuality, silt is chemically distinct from clay, and unlike clay, grains of silt are approximately the same size in all dimensions; furthermore, their size ranges overlap. According to the USDA Soil Texture Classification system, the sand-silt distinction is made at the 0.05 mm particle size.
quote:Clay minerals are typically formed over long periods of time by the gradual chemical weathering of rocks (usually silicate-bearing) by low concentrations of carbonic acid and other diluted solvents. These solvents (usually acidic) migrate through the weathering rock after leaching through upper weathered layers. In addition to the weathering process, some clay minerals are formed by hydrothermal activity.
This implies that all clays take a long time to make, but I also thought that clays were formed by (mixing water with) volcanic ash?
quote:Volcanic ash consists of very fine rock and mineral particles less than 2 mm in diameter that are ejected from a volcanic vent. Ash is created when solid rock shatters and magma separates into minute particles during explosive volcanic activity. The usually violent nature of an eruption involving steam (phreatic eruption) results in the magma and perhaps solid rock surrounding the vent, being torn into particles of clay to sand size.
Which would be a fairly rapid process for making clay (mixed with larger particles) layers, and the different size particles could be separated by erosion and different settling rates ...
The chemistry of ash is very diagnostic and can be chemically characterized by looking at grain sizes, mineralogic compositions, and trace-element compositions. Each volcanic eruption, even if it's from the same volcano, will be slightly different, and so by studying these differences and mapping out the extents of the ash layers themselves, we can often trace them back to their source.
When deposited on land, these ash layers can remain relatively fresh retaining their primary mineralogy, though glass will devitrify (to clay) over time in the presence of water. When the ash falls on water, however, like the ocean or an inland sea, and is subsequently deposited in a subaqueous settings, the glass shards quickly devitrify into clay particles. (Note: devitrification means that the amorphous glass structure, which is unstable at surface conditions and free of water, will change into a more stable, orderly, and water-bearing form, and so in the case of volcanic glass, the glass converts to clay - typically bentonite.)
Some of the larger ash particles become clay in water?
The Nankoweap Formation is a 370-foot (113-meter) thick cryptic unit that comprises sandstone and lesser amounts of siltstone, shale (rare black shales), and mudstone (Elston and Scott, 1976). This formation has been informally subdivided into an upper and lower member. This is based both on variations in grain size (the lower member is finer than the upper member), but more importantly by an angular unconformity that separates the two members and signifies tectonic activity during Nankoweap time (Elston and Scott, 1976).
Above the Cardenas formation we find the Nankoweap. Once again we see a change in the types of rock
From what little I have learned so far, several thing jump out at me. First, it mentions that the lower part of the formation has a finer grain composition than the upper. That seems to indicate that the lower layers were deposited in a less active environment than the upper. I understood that when material is transported by both water and air, the larger particles get deposited first and the finer material is held in suspension longer. If the Nankoweap had been a single event I would have expected the larger coarser material to be lower with the finer sediment towards the top.
Precambrian from their stratigraphic position and complete lack of fossils. That makes them some of the oldest sedimentary rocks in the world. It's unusual to find supracrustal rocks (rocks that are deposited on the earth's surface like sediments or lava flows) this old that havn't been metamorphosed (deeply burried and altered mineralogically).
Do we have a sedimentary layer without (recognizable) signs of life near the bottom of the canyon, and a layer above that ...
Message 143 (Jar): Now for the big question. It seems that this last layer is the first to actually show signs of live, but only of cyanobacteria. Is this correct?
Message 144 (Rox): I believe the first signs of life in the Grand Canyon are visible in the Bass Limestone. I recall reading about stromatolites in the Bass.
I thought the Bass (3a?) was above the Vishnu Schist (1a) and the schist was metamorphic, and that only leaves group 2 below (did I miss something between the Vishnu and the Bass? (and is this where the layer(s) got "pinched"?)
I'd really like to at least work through the Super Group before we need to close this thread and even after we cover these last few formations, I have some really big questions to ask about what happened.
quote:From what little I have learned so far, several thing jump out at me. First, it mentions that the lower part of the formation has a finer grain composition than the upper. That seems to indicate that the lower layers were deposited in a less active environment than the upper. I understood that when material is transported by both water and air, the larger particles get deposited first and the finer material is held in suspension longer. If the Nankoweap had been a single event I would have expected the larger coarser material to be lower with the finer sediment towards the top.
Is that reasonable?
More or less, yes. I couldn't say 100% because I haven't actually examined the rock in detail.
quote:If I remember correctly from earlier parts of this thread, sandstone, siltstone, shale and mudstone each form under different conditions and have different compositions.
Can you or another of our geologists give me a short description of each of those just as a reminder?
Wikipedia has a very concise description on shales and sandstones:
Shale is a fine-grained sedimentary rock whose original constituents were clays or muds. It is characterized by thin laminae breaking with an irregular curving fracture, often splintery and usually parallel to the often-indistinguishable bedding plane. This property is called fissility. Non-fissile rocks of similar composition but made of particles smaller than 1/16 mm are described as mudstones. Rocks with similar particle sizes but with less clay and therefore grittier are siltstones.
Just to explain: fissility is the way that shales break into thin sheets rather than into blocks like other rocks.
Sandstone is a sedimentary rock composed mainly of sand-size mineral or rock grains. Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any color, but the most common colors are tan, brown, yellow, red, gray and white.
So overall, the visual difference between sandstone, siltstone, and shale/mudstone is how 'gritty' or coarse they are. And mudstone and shale differ only in that shale is fissile and mudstone is not.