Note: This document was prepared informally and contains jargon and unconventional abbreviations.

GEOL 2024, HISTORICAL GEOLOGY
SPRING 1996 FIELD TRIP #2

Note: References to "figures" in this text are references to houndouts that were given on the trip. The photos shown below were taken on the field trip and are included for web site viewers.

This field trip is designed for doing during an afternoon lab session. Take Rt 7 south to Dardanelle; then south of town catch Hwy 27 south to Danville. Just before entering the main part of Danville, take Hwy 10 west. Go through Havana and then about 7 more miles to Rt 309. Turn left (south) on 309 and go 0.8 mi. to the entrance to Waveland Recreation Area, where you turn right (west) into the Recreation Area. Off that road going south is a road that leads to the dam. Take that road and cross the dam to the south end and park.

Blue Mountain Dam outcrop of Early Pennsylvanian Atoka Formation

A write up of this outcrop is given as a handout (Cohoon and Vere, 1988). You may recognize the authors. I will also include some discussion based on another write up of this outcrop in Arkansas Geological Commission Field Trip Guidebook GB 77-1. It is difficult, however, for any amount of write-up to help you understand this outcrop, but without it I doubt you could ever figure this out. THIS IS COMPLEX GEOLOGY by anybodies estimation.

The complexity here is largely because the outcrop has both (late stage) tectonic deformation from the Ouachita orogeny and (early stage) syn-depositional, "soft-sediment deformation". The outcrop is on the south flank of the Ranger anticline, so regional strike is E-W and regional dip is to the south. A series of thrust faults cuts up this area also (not seen in the outcrop, but shown on the map in Cohoon and Vere[1988]). Although there is all this tectonic deformation, probably most, if not all, the meso-scale folds and faults seen here are from soft-sediment deformation, that is, from slumping-related deformation during the time of sedimentation. When there is tectonic deformation with shales involved, cleavage develops in the shales that mimics the orientation of axial planes of the folds. Rock cleavage does not seem to be associated with any of these folds, suggesting that the folds formed independantly of tectonic deformation. Also some of the features which appear to be faults are without signs of brecciation, gouge, or shear foliation associated with them; faults lacking these features are more common in soft-sediment deformation. Furthermore, many of the structures (folds, faults) are rather chaotic in nature, a feature more characteristic of soft sediment deformation than tectonic deformation. Finally, many places show structural relationships that are like angular unconformities between overlying sediments and underlying sediments (these are not, however, regional unconformities, but very local phenomena). Again this kind of feature suggests penecontemporaneous deformation of a set of sedimentary layers that were near-surface, followed by continued sedimentation.

Although there is a philosophical argument against using the following approach, perhaps it will be easier to understand this outcrop if we begin by discussing the way this outcrop is interpreted first. It is thought that this part of the Atoka Formation was at the time of deposition part of the North American continental slope. The shelf was to the north. The slightly older Bloyd Formation strata we saw at Pedestal Rocks show that by early Pennsylvanian the shelf was becoming dominated by clastic sedimentation instead of carbonates. In this interpretation, the Clarksville area (to the north of here) was on the shelf. Near Clarksville, Atokan rocks of similar age to those here are deltaic (see "Figure 1" of extra handouts). An outcrop at Paris Lake Dam (further south from Clarksville) with Atokan rocks of similar age demonstrates facies which exhibit structures suggestive of deeper water than Clarksville and more of a slope to the depositional surface. That area is interpreted as approximately the edge of the continental shelf of that time (again see Figure 1). The Blue Mountain Lake Dam outcrop here represents continental slope. Rocks of similar age at Chula, farther south, are finer grained and are thought to be more of a deep basin environment (see Figure 1).

I include additional handout "Figure 2" to show a couple of sketches representing shelf/slope/basin with submarine fan deposits on them. These of course do not exhaust the possibilities of configurations of fans, canyons, etc. The whole slope is probably actually a build-up of coalesced (combined, run-together) fans. Like alluvial fans on land, and like deltaic distributary systems, submarine fans build out their fan shape by changing the direction and positions of distributary channels from time to time, creating an overall radial pattern. [The canyon in Figure 2A is not drawn to represent a fan channel. In fact no distinct fan channels are drawn in that figure, but they are roughly represented by the radial lines drawn to "texture" the fans in the drawing.] At any one location on the fan, it may for a while receive sheet-like flow of turbidites (sediment that is "overbank" from any nearby channel flow), then at a later time a distributary channel may cut down into those sediments. Then later still that channel may fill, and the main local distributary channel switch to another location. At that time the location will return to finer-grained-dominated sedimentation with sheet-like turbidite sediments.

The canyon shown eroding through part of the shelf and slope in Figure 2A is not meant to indicate a universal thing. There may or may not be canyons. Also, such canyons, when present, may be as large as the Grand Canyon of the Colorado river, or may be smaller, or much much smaller.

A feature common with submarine canyons or with erosional structures from distributary channels, but not shown on the figure, is slumping. When canyon walls (or channel walls) become oversteepened, slumping can occur in the sediment that forms the canyon walls, causing sediment to slump toward the canyon axis. Also slumping may occur anywhere on a fan, unrelated to channels or canyons. This slumping is toward the basin (some small slumps are illustrated in Figure 2B). Slumping like this would be greatly facilitated by (1) the slope being present [a requirement for slumping], and (2) the mud-dominated nature of the sediments [mud being a weak material], and (3) the water-saturated nature of the sediments [further weakening the sediments].

I include as "Figure 3" a "borrowed" sketch of a slump on land because it illustrates some pertinent things. Slumps, even in soft sediment, develop along "normal" fault surfaces. Along with the faulting, folding of strata can and will occur. Quite often that folding is rather chaotic. Blocks of sediment can even be ripped apart (look at the detail in the longitudinal and transverse sections of the slump in Figure 3). If the slump illustrated in Figure 3 were developed in an area of continued sedimentation (like a submarine fan), the faulted and folded layers illustrated would become overlain by more sediments, with angular discordance between the younger and older layers. There could be multiple generations of slumps and sags in the same area.

With this background, let's try to look at the outcrop and see some of these features that led students of the Ouachitas to interpret the Atoka of this area as parts of submarine fans:

Brief discussion of outcrop:

Let us start on the simpler west end of the outcrop, near the pump station. The rocks here are interbedded shales, siltstones, and sandstones. We may be able to find graded bedding, and I know we can find flute casts and possibly other "bottom marks" on sandstone beds. The flutes have an asymmetry that indicates southward-directed turbidity current deposition of the sand. Cohoon and Vere (1988) illustrate a "sandstone dike" that occurs about 15 ft. up from the chain link fence. Sandstone dikes are also associated with soft sediment deformation processes, and are a common feature in and near distributary channels of submarine fans. One thing you do not see in these sandstones is cross bedding or ripple marks typical of shallow water deposition (though ripples like the ones in a Bouma turbidite sequence may be found). These sedimentary rocks are more like deep water sediments of continental slopes.

Above: Amalgamated turbidite beds. An interpretation of this outcrop is as follows: Just below the prominent reddish layer is part of the rippled "C" subdivision of a Bouma sequence. The overlying D and E subdivisions were eroded off by the subsequent turbidity flow. The reddish beds are the base of the next turbidite layer, much of which, in turn, was eroded by another flow.

On the left-hand side of the outcrop above are flute casts on the bottom of a turbidite bed (bedding dips into the hill side). This outcrop is stratigraphically above the outcrop seen in the previous photo.

Also in this area is what looks like an angular unconformity (but this is not a mapable unconformity, rather a local phenomenon). What is seen here could be explained by rotation of underlying sediments on a fault that is off to the west a little out of sight, followed by planing off of some of the sediments by turbidity currents, then deposition of the layers above it. That whole section is also folded into a syncline, and I don't know if that is part of the later tectonic deformation or the earlier soft sediment deformation. However, as mentioned earlier, there is not an axial plane cleavage in the shales, and also the orientation of the fold does not parallel the E-W trend of the Ranger anticline, the regional fold of which this is part.

In this general western part of the outcrop, there is supposed to be an isolated lens of sandstone well up on the slope. I did not see it on a short visit I made earlier this winter. This sandstone lense is considered to be a little slide block of sand related to soft sediment deformation.

Now walk to the east end of the outcrop. If you look at the outcrop as a whole, you can see that the strata strike roughly E-W and dip into the hillside. I forget, but there may be some more minor deformation features in here. I can remember that in this middle section down on the ramp that leads to lake level there are some hard-to-figure-out structures. Some look like what you would get if some sediment were layed down, then deformed, and later sedimentation filled in a low spot caused by the deformation of the surface of the underlying sediments. Other places have the "angular unconformity" look.

The east end of the outcrop is the most complex part of all. There is a deep channel-shaped body of sandstone cut into a sequence of sandstone and shale. The bedding to the left of the channel sand is nearly vertical, but a little further to the left it is more horizontal-looking (though, like most of the outcrop, dipping back into the hill toward the south). Let's discuss the channel sand, then the deformed beds below it.

Channel structure in the Atoka. Thin-bedded sediments on the left were probably slump-folded prior to the cutting of the channel. the channel filled in with sand (deepest part of channel positioned where red sign is). Irregular base of channel rises up to about at head level of the student on the left (John Hatchett) and continues to the right.

If you look up in the sand body, it becomes eventually a horizontal-looking structure (again, probably dipping back into the hill). That, I think, is the true regional orientation of strata here. What is seen could have formed by a channel being cut (by high energy turbidity flows concentrated along here) and then filled in. At the top sand spread out laterally on the overbank part of the channel. (I assume, that general flow of sand was toward the south [into the hill] based on those flute casts on the west end of the outcrop). Note that the sand in the channel is "massive", that is, essentially devoid of internal structure. This is a feature commonly found in modern submarine distributary channels.

The folded beds below the channel are again probably soft sediment deformation. They are very chaotic, especially if you examine them in detail. Layers are very hard to trace out; they commonly end in little hook-like folds, then you loose them. There is no consistent relationship of cleavage and axial planes of folds.

This whole outcrop is just a chaotic mess. Whatever the environment was, it must have been deep water, because no shallow water structures are present. Turbidity currents were involved, as indicated by the flute casts. There was a slope to the site as indicated by soft sediment deformation. It is thought to have been continental slope with channels, turbidites, and slumps developing during sedimentation.

[ Tech ]

GEOL 2024, HISTORICAL GEOLOGY SPRING 1996 FIELD TRIP #2

GEOL 2024, HISTORICAL GEOLOGY
SPRING 1996 FIELD TRIP #2