The Lance/Hell Creek As a Flood Deposit

At Jar's request, I'd like to propose a thread that discusses the Upper Cretaceous Lance Formation in the western United States (mostly Wyoming, but also Montana and Colorado) within the context of The Flood. By that I mean to ask if someone can explain the geology we see in the Lance as a flood deposit. The Lance is often held up as the classic example of a meandering river deposit. This suite of rocks is characterized by lenticular, discontinuous sand bodies interbedded with mudrocks. There are also a good number of lignites/coals within the unit. Plants aren't common, but they exist. Dinosaurs are quite common (preserving a snapshot of the last days of the dinosaurian reign, this is the stomping ground of Tyrannosaurus, Triceratops, Ankylosaurus, and the like) as are freshwater crocs, turtles, birds, rays, etc. Could someone offer an explanation for this unit as a the result of a single global flood event. Since they're essentially the same package of rocks, we'll include the Hell Creek Formation herein as well.

Thread moved here from the Proposed New Topics forum.

First, can you link us to a map that shows the extent of that area?Next, just for those of us that don't really know what some of the terms mean, can you explain a few to us? What is a meandering river as opposed to some other kind of river? Does it mostly meander because the elevation is pretty much constant, not a great slope? Can you show us some images of a meandering as opposed to some other kind of river? Huh? Lenticular? Discontinuous?

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Immigration has been a problem Since 1607!

I have a .pdf with a map. What is the best manner by which to copy and paste that in here? There are several types of rivers. We'll worry only about meandering right now. These are rivers like the Mississippi, the Connecticut, the Missouri, the Columbia. Rivers like you probably think of when you think of rivers. Again, yes, if I can figure out how to put pictures in (I'm screwing something up).

Images have to be hosted on a site and linked. There is no way right now to upload and insert an image directly. I will email you and if you can send them to me will host them.

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Immigration has been a problem Since 1607!

When you think about bedded rocks, you probably tend to envision a layer cake with the beds just going on and on...heading off to the horizon. And so if I say interbedded sand and mud, you can probably picture a stacked sequence of sand beds (probably lighter colored than the mud--in the case of the Lance they're tan) and mud beds (in the case of the Lance it's dark brown) making a layer cake but where the layers are thin and go forever. The Lance is more like a mud cake with discontinuous lenses of sand within in it. The sand bodies are sometimes only 20-30 feet long (though often much larger) and their edges are rounded often...sometimes they taper.

Okay, I think I understand but let me see if I can explain it in other words and get it right.First, it would look something like the images on this page.You mentioned we were talking about sand and mud. If I remember correctly, one of the things that determines what size material gets deposited is the energy of the transporting medium. Faster moving flows deposit larger material and slower less energetic flows deposit smaller, finer material. So the sand would have been deposited by a more active flow than the mud.Would the sand deposits be something like the weathered crests of ridges from an active period and the mud from slower flows, similar to what we see in a tidal basin?

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Immigration has been a problem Since 1607!

Yes, but most like the "Comparing Bedding" little column for C, but on a HUGE scale. In general...yes, exactly. The muds are overbank deposits (accumulations on the flood plain, mostly through flooding, but also with a significant amount of building through soil development). The sands are channel deposits (bedload of the river, bars, etc.) and higher energy floodplain deposits (floods, breakouts from the channels, etc.). So yes, the sand is higher energy than the mud. Sort of/possibly. The sand AND mud might be sourced to some degree in highlands (and this can be traced sometimes), but it could also be an ancient beach deposit that the river cut into 300 km upstream and is redepositing. Meandering river deposits record "business as usual" for a river, which is moving sediment and water from point a to point b, and those times when the rivers do interesting stuff during periods of high flow. Where the sediment comes from is a complicated question.

Here is an example of a meandering river:
click
It's basically a river that takes a very windy, sinuous path through its flood plain. Left to its own devices, it will migrate around its flood plain as banks are eroded, and others built up with sediment. Lakes are often left where one part of the river cuts through its bank in to an adjacent part as shown hereYou can imagine how chaotic the sedimentary deposits would seem if the process above repeats itself many times all over the flood plain.Edited by The Matt, : No reason given.

Okay, so let me struggle with this a little more.Would it be likely that the river as seen in figure a would leave behind sand since it was a more active environment, then durning figure b and figure c we see a change to finer deposits? Edited by jar, : appalin spallin

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Immigration has been a problem Since 1607!

The river itself probably wouldn't change a great deal throughout, but you might see fine deposits and peats forming in the swamp in the old channel.

I'm trying to determine the mechanics that left the "discontinuous lenticular deposits".

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Immigration has been a problem Since 1607!

Mostly they're point bar deposits in meandering rivers, if I recall. They'll often be discontinuous because the river is always shifting and eroding some areas while depositing in others.

Pretty much all of the sand in a meandering system occurs in fairly discrete units. Bars are lobate structures (in both map and cross-section view) that are meters to hundreds of meters in scale. Splay deposits (which occur when channel waters break the levee at the edge of the channel and deposit (usually sand) out over the floodplain) tend to be localized. Even sandy sheet flood deposits (floods deposit sand and mud...usually one type of material dominates in an area because of hydraulics so you'll areas of sand deposition and areas of mud deposition) are fairly limited in areal extent (a couple of hundred meters). The bottom of the channel is usually sand (except where lags accumulate, which are usually conglomerates) and so isn't that discrete, but it shows up so in the rocks because rock exposures are slices through the system.So these are main sand dominated processes/units in the meandering system and all of these tend to produce discrete deposits, often with edges that are thinner than the middles (thus lenticular).If you look at the Seven Sisters image Matt threw up in message #9, you'll notice that the flood plain of the active channel is a mess of abandoned channels and oxbows. Each of these channels had bars and channel lags and other sandy deposits associated with it (also reference the diagram jar posted). Now imagine taking a slice through that Seven Sisters system in any direction. The edge you would see would be a mess of sand and mud deposits and a good number of the sand deposits would discontinuous and some would have edges that "pinched out" into a background of sand or mud.

Oops! The image isn't the seven sisters. I was looking for something completely different beforehand, and google must have saved my search terms. That's somewhere in the amazon.