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Note: This document was prepared informally and contains jargon and unconventional abbreviations.
We will attempt to see many representative formations and features of the Ozark area that illustrate the kinds of rock types that characterize this area and other features that give evidence of the processes involved in their being formed.
To see more photos from this field trip, click here.
Planned Route: Out of Russellville, take Hwy 124 going NE to Hwy 105. Go north and join Hwy 27 near Hector. Continue N on 27 to Hwy 16 and go east to Clinton (alternate routs to Clinton are possible). Go through Clinton on 16 to Shirley and there catch Hwy 9 north to Mountain View. When pass "Ozark Folk Center", note milage; continue north on Hwy 9 toward Allison. Stop 1 is 3.5 mi. N of the Folk Center.
Stop 1
"Allison South" road cut on Hwy 9:
St. Peter Sandstone, Joachim Dolomite, Plattin Limestone
This stop is shown on "Plate I" handout near the western edge of Sec 14, T15N, R11W. There is much discussion of this and the next Stop in Craig and Deliz (1988) handout. We will try to start by finding the fault that juxtaposes the St. Peter Sandstone and the Joachim Dolomite, then work "up section". Some of the main features we will attempt to see include:
Normal fault-- downdropped to the north. Normal faults are one of the main deformational features in the Ozarks. Most are downdropped to the south, but not all.
St. Peter Sandstone-- A good, well rounded, clean quartz sandstone (quartz arenite). Interpreted as a shallow-water, nearshore sand complex. The St. Peter's sand is considered to have been derived from erosion of older sedimentary rocks because of its high degree of (quartz) purity and degree of rounding of grains. Upper part makes a gradational transition into overlying Joachim Dolomite (thru dolomitic sandstone, sandy dolomite) suggesting it is a conformable formation contact.
Joachim Dolomite-- The Joachim was probably originally deposited as limestones that were later dolomitized. The transition from St. Peter nearshore facies to Joachim carbonate facies (off shore) indicates marine transgression. However, carbonate deposition largely kept pace with submersion of this area because much of the Joachim has features suggestive of intertidal deposition. Some features to look for include:
Mottling in places from burrowing.
Dismicrite ("sparry" calcite spots in micrite) possibly from filling of voids left from burrowing creatures
Mud-cracked layers. Especially well developed in Subunit 4, where associated overlying rip-ups occur. These are strongest evidence for intratidal or supratidal environment.
Algal stromatolites, also near top of Joachim in Subunit 4.
May find calcite pseudomorphs after halite.
Plattin Limestone-- The contact is confused here by what is considered to be a low-angle fault with fault breccia and even some solution/precipitation along the fault. The Plattin is interpreted as being a similar environment as the Joachim (subtidal to intertidal to supratidal). Not being dolomitized, perhaps it was deposited under less dessicating conditions (more humid?) or was lower on the tidal flat.
Continue on to Stop 2: Continue north on Hwy 9 half a mile to Hwy 14. Turn west. Go 0.8 mi. from the intersection to a turnout on the left (south) side of road.
Stop 2.
"Allison West" road cut on Hwy 14:
(M.O.) Plattin and Kimmswick Limestones, (U.O.) Fernvale, and
(L.Miss.) Boone Formation
Going up hill and up-section, we'll start in upper Plattin and proceed up thru upper Ordovician limestones. Then there is a major unconformity (disconformity) with Lower Mississippian Boone Formation above. The Boone is a very widespread unit in northern Arkansas. Some of the features we will try to see include:
Plattin Limestone-- The Plattin here is a biomicrite (skeletal wackestone) with various fossils among which are colonial corals in growth position. WE ARE ASKED NOT TO COLLECT THESE FOR THE SAKE OF FUTURE SCIENCE. THIS IS ONE THING WE BRING CAMERAS FOR. (Is this therefore a "paleo-protected species"??)
Kimmswick Limestone-- The Kimmswick is a little coarser grained than the Plattin and the contact is sharp, but probably we we'll have a hard time to find it. (The bottom of a large modern solution cavity is near the contact [solution cavities like this are common features of "karst" topography, important in limestone terranes]).
The Kimmswick is interpreted to be subtidal limestone. It lacks the mud cracks that are common in the Joachim and Plattin. Subunits 1 and 3 are skeletal "packstones" and "wackestones". These are fossiliferous rocks with lime mud matrix. An abundance of mud indicates it was "protected" by some topographic barrier from open ocean (perhaps a shoals area). Parts are not micritic, suggesting migration of less protected facies at times.
Fernvale Limestone-- The Fernvale has a coarser look than most of the Kimmswick because it lacks lime mud (micrite) matrix. It is a crinoid biosparite (grainstone). It would represent a less protected environment, possibly shallower, with more current action winnowing out the mud.
Siliciclastic interval (shale/sandstone) and major unconformity-- A major unconformity occurs above the Fernvale, because after a thin siliciclastic unit the Mississippian Boone Formation overlies. Silurian and Devonian units are missing. The Craig and Deliz article discuss some interesting details of this interval used to sort out "what's what". Previously this unit was not assigned to a formation because it was uncertain to what units it belongs. These authors say that the shale has Late Ordovician conodonts (a kind of fossil) while the 3" sandstone below the Boone has Early Mississippian. Also the shale interval below that 3" sand has some fine angular sand which is common in the Cason Shale, the Upper Ordovician shale that normally occurs above the Fernvale. The 3" sand itself is medium-grained with well-rounded grains, the texture common in "basal Mississippian" sandstone underlying the Boone Formation elsewhere. The unconformity is thus at the base of the 3" medium sandstone.
Boone Formation-- The Boone is characterized by having abundant chert nodules and discontinuous beds. Chert in limestone like this is usually a post-depositional replacement phenomenon (during diagenesis).
Continue on to Stop 3: Continue west on Hwy 14 for five miles to Rt. 87. Turn S on 87 and go 3.5 miles south to South Sylamore Creek. Park across bridge on gravel bar. (NOTE: We did not do this stop, because it is on private land, and I was unable to locate the property owner prior to the trip).
Stop 3.
South Sylamore Creek at Rt. 87:
(U.O.) Fernvale Ls, (M.S.) Lafferty Ls, (U.D.) Chattanooga Shale,
(L.M.) Boone Fm
We will examine the rock exposure on the north side of the creek. The main objective here is to see the organic rich Chattanooga Shale. Also note how there is sporadic occurrence of Silurian and Devonian rocks in parts of northern Arkansas. The Chattanooga Shale and Lafferty (a finer grained limestone than the Fernvale) are not exposed every where, and there are other Silurian and Devonian units that are exposed elsewhere, but not here. There were evidently episodes of low sea level relative to land in this area during those pre-Mississippian times and different areas received different amounts of erosion, removing different amounts of sediment.
Note that with the Chattanooga Shale there is a basal sandstone (the "Sylamore sandstone member" of the Chattanooga Shale). These are considered a transgressive couplet.
Continue on to Stop 4/5: Return north on Rt. 87 to Hwy 14 and go west. (An older field trip guidebook that I have says that 0.2 mi from that intersection, the road to Blanchard Springs Cavern goes off to the north; and there, on the south side of the road is the surface expression of a "sink hole".) We will travel west on Hwy 14 for a total of 17.5 miles to where the highway approaches Big Creek (entering the Big Creek valley via the Bear Creek valley).
Stop 4/5.
Road cuts on Hwy 14 @ Bear Creek/Big Creek
Unconformable contacts of Plattin/Fernvale and of Everton/St.
Peter
On "Plate II" handout the outcrops we will examine briefly are shown as "Stop 4" and "Stop 5". These are both discussed in the Craig and Deliz (1988) handout as "Alternate Stop 1". These are in Sec 33, T16N, R14W.
"Stop 4" is said to be the contact of Plattin below Fernvale and that there is slight angular unconformity here. Thus the Kimmswick is missing. We will try to see if we can recognize these two rocks by comparison to what we saw at Stops 1&2 of our trip (we saw Plattin and Fernvale there). Another sign of the unconformable relationship to look for is the patches of Fernvale lithology (grainstone) in the upper part of the Plattin (wackestone/packstone) mentioned by Craig and Deliz (1988). These are considered infillings of Fernvale sediments in small solution cavities on the karsted surface of Plattin Limestone.
This unconformity shows that not only in the Silurian/Devonian interval, but also during the Ordovician there were processes at work producing unconformities. However, if one considers the shallow-water nature of these deposits, it would not take much tectonic uplift or sea-level fall to expose parts of this region to weathering and erosion from time to time and then vice versa for continued deposition.
"Stop 5" is said to be an "excellent view of the pre-St. Peter unconformity". The St. Peter is considered the base of a major continent-wide depositional cycle called the "Tippecanoe sequence", and the unconformity is a continent-wide unconformity. So this is a "famous" thing to see. We will look at the contact up close and also try to look from a little distance. I believe we will see an irregular upper surface to the Everton. Also we will look at the fault shown on the map. One one side the upper contact of the St. Peter Sandstone is exposed, and on the other the lower contact.
Continue on to Stop 6: Continue west on Hwy 14 about 3.5 miles to the junction with Hwy 27 (I think this is kind of a fork). Bear right (north) and stay on Hwy 14. Go about 10 miles to the first road on the right after the Buffalo River. Turn right (Wild Bill's and other canoe rental places right around there). Go about 3 miles to the Buffalo Point recreation area. Go to the gravel bar where the canoes are launched.
Stop 6.
(OPTIONAL STOP) Buffalo Point gravel bar:
Everton Formation/St. Peter unconformity
In your handout package is a photocopy of part of the Cozahome 7.5' quadrangle. The outcrop is along the western border of SW3, Sec 35, T17N, R15W. From the west side of the Buffalo at the gravel bar where canoe outfitters launch their canoes, looking east across the river you can see a bluff of limestone/dolomite/sandstone. A somewhat thinly bedded sequence of Everton carbonates are at river level. Up maybe 15 ft (I forget) is an irregular subhorizontal surface with a massive-bedded sandstone above. That sandstone is the St. Peter Sandstone. This is the same unconformity as seen at Stop 5. I include this simply because it is a beautiful site for looking at a great disconformity. Very photogenic in the fall when leaves are turning.
Continue to Stop 7: Return to Hwy 14, go south to the Hwy 27 intersection again, and continue SE on Hwy 27 toward Marshall. When reach US-67, turn left (south), and go 1 mile to a large road cut.
Stop 7. Hwy
65 road cut just south of Marshall:
(Upper Mississippian) Fayetteville Shale and Pitkin Limestone
The location of Stop 7 is shown as "Stop 6" on a map handout called "Plate III". The location is in the E2, SE3, SE3, Sec 31, T15N, R15W.
In the area for which the Fayetteville Shale is named the unit is almost entirely shale, but in north central Arkansas there are thin interbeds of dark limestone. We will look to see if we can find graded bedding in any of these thin lime layers. (I am told that a little further south near Leslie, very obvious graded beds of carbonate are abundant in the upper Fayetteville Shale). One idea on this facies of the Fayetteville is that during "Fayettevelle time" there was a high stand of sea level which made it too deep for the good limestones to form in this area; but during times of major storms, lime sediment in shallower water to the north (toward the craton) was stirred up and turbidity currents carried slugs of this sediment to deeper water. Thus the graded carbonate beds would represent short-term, sporadic events. Then during quiet very long periods of time, normal slow accumulation of "background" clay deposition occurred. That is little bits of clay just floating around gradually settling out deposited the shale layers between the lime layers.
Above the Fayetteville is the Pitkin Limestone. The contact is considered as conformable. Evidently a shallowing occurred and a carbonate environment built out over the shelf again. Let's look to see if there is evidence for whether the Pitkin here represents a high-energy or a low-energy carbonate depositional environment.
Continue on to Stop 8: Go back north on US-65 to Marshall. Continue through town and go about 5 mi. further to Rt 74, which goes off to the left. Turn left (SE) on Rt 74. About 7 mi. down the road is Snowball, where the road becomes Rt 377 going south to Witt Spring (11 mi. from Snowball). At Witt Spring bear right onto Hwy 16 which continues south. After 17 miles you come to Ben Hur. Continue on almost 3 more miles to the parking area on left with a sign for "Pedestal Rocks". [I think this is accurate!!]. Park there and take the trail (about 1 mile walk) south to Pedestal Rocks.
Stop 8.
Pedestal Rocks.
(Early Pennsylvanian [Morrowan]) Thick sandstone in middle of the
"Bloyd Shale"
Working from memory, I think I have found the right quadrangle and location for this stop. I give you a photocopy of part of the Sand Gap 7.5' quad. The site should be where I have labeled a cliff (see topo lines) in NW3, NW3, Sec 9, T12N, R19W.
The cliff is in a prominent sandstone member in the middle Bloyd Shale. This sandstone is informally called the Gaither Mountain Sandstone of the Bloyd Shale. This same unit forms bluffs all over the place in this neck of the woods, including Alum Cove with its natural bridge NW of Deer in Newton County.
This sandstone is considered to be fluviatile. The great abundance of cross beds is consistent with that interpretation. We may be able to find a few milky quartz pebbles in the sandstone. They are much more abundant in the same unit farther north. This is one piece of evidence that suggests that the rivers that deposited this sand had source in the north.
What really dominates ones attention here is the cave-like features in the face of this cliff. This unit has these in many places (again such as Alum Cove). The origin of these is more a "Geomorphology" question, but we'll spend some time speculating on this question.
Head for home: Walk back to parking lot. Continue west on Hwy 16 to Pelsor where 16 meets Hwy 7. Take 7 south to Russellville.
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