Petro98 field trip

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

PETROLOGY FIELD TRIP, SPRING 1998

Below is the itinerary used for the field trip. You may follow it to see the same sites if you please.

Click here to see additional photos of the trip.

Bring acid bottles. Bring large sample bags for students. Bring marking pens for students. Students bring notebooks for notes on samples.

Start with northern Arkansas lithologies

Route: Take SR 7 north to Lurton. Take SR 123 to Mt Judea. Continue to where 123 crosses the Buffalo River. Go up hill a little on the north side to first otc.

The bottom rock in the otc is Everton Fm (Ordovician) dolomicrite. Over it is a bed of coarse biosparite w/ echinoderm parts and sparse brachs. Prob Boone Fm (Mississippian). This constitutes an important disconformity! As go up is more Boone (St Joe Member, reddish). Further up is limestone w/ dark, hard chert: typical of Boone in lower parts, above St. Joe.

Route: Go south on SR 123 to SR 74. Go west. After a few miles come to a grade going down to Bear Creek(?). Large otcs on right.

Work down section. Start in rock typical of upper portions of the Boone with white, chalky chert. Going down get into Boone w/ hard, dark chert. Further down is chert-free St. Joe. Eventually get down to a possible St. Peter Ss (Ordovician) layer or maybe it is ss within the Everton. Somewhere in here is an unconformity. The layer that looks like St. Peter ss is, I think, actually part of the Everton. I say so, because Everton is said to have such layers in it, as well as dolomite and also limestone. Below this layer is a limestone with abundant round qz grains. (Acid test bears out that it is ls and not dol). Also above it is a coarse Acrystalline@-looking ls with qz grains in it. At first I thought it was Boone, but now I think both those are parts of the Everton Fm, because the round qz grains are so characteristic of Everton. There are at least 3 layers of ss in this part of the otc.

Route: Continue west on SR 74 to Jasper. Go south on SR 7. Going up grade somewhat after Fairgrounds turnoff is otcs on right that I think are ls/sh interbedded of the Fayetteville Shale Fm (look at). This would be like the sequence seen at Marshall on US 65. Further up, I think it is about at the second sign you come to warning trucks about the steep grade (orange signs, as viewed going down hill), is an outcrop of cross-bedded oolitic ls, the Pitkin Fm (Uppermost Mississsippian) (look at).

Further up hill come to the middle Bloyd ss, a massive ss. The Bloyd Fm is Lower Pennsylvanian in age. There is a good wide spot on the left right where there is a transition from massive ss to interbedded ss/sh above it. Stop there. Near the top of the ss one can see the poorer sorting of this ss w/ some qz granules (look at).

Further up at turnout where there is a small stone fence on left (past picnic area), look out and show the panorama which includes the middle Bloyd ss w/ rounded topo above it and also a large exposed quary face in the valley, the Smith(?) Quarry in Boone.

Except for pre-Everton dolomitic units (Powell and Cotter) and the Atoka which overlies the Bloyd, we have seen here a substantial part of the stratigraphy of the Paleozoic shelf of the southern midcontinent as expressed in northern Arkansas.

Route: Proceed to Beavers Bend State Park in Oklahoma. We=ll look at as many otcs as possible and then camp. Take Rt-7 to Rsvl and on to Dardanelle. Catch Rt-27 to Danville. After Danville take Rt-80 west toward Waldron. Before getting to Waldron, 4 mi past Union Hill, at fork in road take Rt-250 west. Catch US-71 and take it south toward DeQeen. Near Y-City (where US-270 intersects US-71?) is an outcrop of vertically bedded Jackfork Fm.

Jackfork Fm @ Y-City: Near Boles, a few miles to the north, we crossed the Y-City fault, which separates Ouachita-facies rocks from rocks of the Arkoma foreland basin. The Jackfork here is vertical in orientation and consists of sandstone interbedded with shale. Although the Jackfork Fm is often called the Jackfork Sandstone, it is not entirely sandstone, but has shale intervals as well. It is all considered to have been deposited by turbidity flows. Facies that were once part of fan lobes tend to be interbedded sandstones and shales. The sandstone here is a good quartz arenite. It is a cleaner sandstone than what is typical of the Atoka to the north, which can also have similar bedding characteristics.

Route: Take US-70 west into OK to the town of Broken Bow. Go N on US-259 for 7 mi to Beavers Bend State Park. Turn right (E) into the Park and loop through Park to see the outcrops described below.

Beavers Bend State Park: Paleozoic Ouachita facies rocks

Note: If it is getting late (less than a couple hours of daylight left) it would be best to see AStop 2 & 3" before camping and see AStop 1" in the morning.

The descriptions are given by Nielsen, KC (1988) in a field trip guidebook published as the GSA Centennial Field Guide. I describe here what I think we will be able to see based on his description. I put in italics sections of Nielsen's description that may be more apparent when we get there and will be used to try and find outcrops of some of the major geologic mapping units of the Ouachitas. By the way, all these units continue into Arkansas.

Stop 1 (Locality 2 of Nielsen): Take US-259A to the park and into it. Continue to the main park area (a few miles from US-259). Come to a stop sign west of the park museum; turn left and drive west. Proceed to a T intersection and turn right and cross a river. Park either by the swimming area to the west (left) or in the park area to the east (right). Refer to AFigure 8" (taken from Nielsen's Fig.4). Note: to see all of this series of outcrops you will need to contact the park before coming and arrange to have a gate opened in the outer fence of the power plant. Most of the Novaculite sequence is inside the fenced area.

To begin the traverse in the oldest units, walk about 800 ft west of US-259A. North of you there will be a small valley cut into Devil's Backbone. This valley has developed at the contact between the Blaylock Sandstone and Missouri Mountain Shale. Outcrops in the Missouri Mtn Shale (Upper Silurian) are visible in the cut behind the parking lot. At the break in slope apparent on the map where the strike and dip symbol showing the 211 E dip of the rocks occurs, there are outcrops of Blaylock Sandstone (Middle Silurian). This is not far from the western corner of the parking lot. If look carefully you can see that the Blaylock is very fine quartz arenite with some interbedded minor shale. As you work back east into the Missouri Mountain, you will find that the lower part of that formation is interbedded very fine sandstone and slaty mud and clay shale. The proportion if shale increases upward. This shows a conformable contact relation between the two formations.

The contact between the Arkansas Novaculite and Missouri Mountain Shale is fairly abrupt, with black siliceous shale overlain by the massively bedded white-to-gray chert. This contact can be seen along the slope north of the swimming area at this locality. Proceeding to the northeast corner of the parking area and just at the foot of the road bed, the first massive chert can be observed. Outcrops of the novaculite can ineed be found before reaching the road if you stay along the foot of the steep slope of Devil=s Backbone. However a conflicting statement occurs in another paragraph it says that the "Lower Division" of the novaculite is thin in this area and covered by the road.The contact can indeed be seen. I guess this statement is indicating that most of the lower division is covered. The Arkansas Novaculite is Devonian through early Mississippian in age.

Because of the shallow dip, the novaculite sequence is exposed along the full length of the cliff behind the power station. This is the part that, except for a very small part, is blockaded by the power station fence. Exposures of the Middle Division extend to the east side of the power station. East of the power station, the cliff exposure is mainly the upper division which is usually a thick-bedded novaculite, but here it is said to contain two carbonate units that are apparent because of cavities developed in the weathered surface, each being 10-20 ft thick. Walking to the east, take time to look closely at these units, which reveal very irregular replacement textures. The carbonate occurs as thin laminations, small elongate irregular pods or large nodules. We did not get to look at these things because of the locked gate. Even if you can get in through the gate, I do not know if the other end of the fence would block your way to the upper division part of the cliff exposure. Regarding the carbonates, it is not clear, but I think he is saying that the "replacement" is silica replacing carbonate. He says that the lower carbonate has only 20% carbonate, being more Asilicified@ than the upper, which has 50% carbonate. He also mentions massive sandstone layers, interbedded chert and shale, and thin conglomeratic layers being associated with these carbonate layers in the Upper Division, but does not make clear if this is somewhere else in the general area or right here at this locality.

At the eastern end of the low ridge, there is a wide area (1400 ft) of nearly horizontal black fissile shale. This shale is the basal Stanley Formation (Mississippian). Examination of the black shales behind the water tank above the power station reveals significant sulfide accumulation and evidence for soft-sediment deformation. The shales in the valley are visible, especially at a small shale pit that has been dug. To get to the exposure behind the water tower would require climbing the hill perhaps from the east end of the cliff exposure behind the power plant, or by going back to the road and walking up hill and then back to the east on top of the ridge. However, again there is a locked fence to get to the water tower.

Continuing the traverse toward the east, across the low area, the next sequence of rocks is the lowest of five major tuff sequences within the Stanley Formation. Two of the units are exposed here. Walking up the western flank of Rattlesnake Bluff, you first cross through the Beavers Bend Tuff and then, at the top of the hill, the Hatton Tuff. ...There is an interval between the tuffs with quartzose and feldspathic turbidites. Since we came when the fence at the power plant was locked, we had to do it a different way. One cannot go to Rattlesnake Bluff by following the river, that is, on the river side of the power plant. This is because a channel is put there for water from the hydroelectric plant. The best way to get to the tuffs is to go back to the road and go up hill, toward the north. There is a gravel road going off to the right (east) to the water tower. Walk it to the water tower area. From there you can see power lines with a cleared area below them heading generally east. Follow that down hill and cross a drainage at the low spot. Continue up the next hill then down. On the way down is a small old gravel road going off to the right at an angle. Follow it to where it comes to a flowing stream. Go right (south) and follow the stream toward the main river. After a few hundred yards you will come to a wash coming down the west flank of Rattlesnake Bluff (not a bluff here, but a steep hill). Go up the wash looking at rocks. The tuff will be found first as float. Nice feldspar phenocrysts are conspicuous; also there are quartz phenocrysts. Look for the in-place outcrops. We found apparently two tuffs below the Hatton tuff, which occurs nearer the top.

The Arkansas Novaculite and units below it are considered to have accumulated slowly in a deep Oceanic basin off the south margin of the Paleozoic continent. The Stanley Formation is considered to have been deposited more rapidly as a sudden deepening of the basin occurred because of downward flexure of the lithosphere as oceanic lithosphere to the south began to be subducted beneath an approaching continent. Volcanism from a continental magmatic arc that was above that subduction zone to the south is considered to be the source of the Hatton Tuff and the other related tuffs. These tuffs are of the same age as the lower part of the Stanley, that is, Mississippian. This lower part of the Stanley is also similar in time of deposition as just after the Boone and before the Fayetteville shale in northern Arkansas.

Stop 2 (Locality 4 of Nielsen): Get back on US-259A and continue on to the Dam. Cross the dam and stop at the Scenic Overlook on the east side of the road north of the dam. Look east across the lake and see at the soreline a sequence of tight overturned folds in the Blaylock Sandstone (Silurian). I know this is Petrology, but we can't pass up an opportunity to see such nice structures! Besides, it is important to point out that after the deposition of much of the Paleozoic sequence, a compressional event occured with folding and thrusting of the basinal facies rocks. This is considered to have occurred from continent-continent collision in the late Paleozoic to form Pangea.

Optional Stop 3 (Locality 5 of Nielsen): From the previous stop, continue W on US-259A.Go about a mile (I think). Cross over the spillway at Stephens Gap and park in the area below the spillway (NW corner of Sec 4). This location is said to be in a strike valley of the Polk Creek Shale (Upper Ordovician), trending E-W. To the north is the Bigfork Chert (Ordovician) and to the south are hills in the Blaylock Sandstone (Silurian). Cross the small stream south of the parking area and proceed south along the dirt road abou 1300 ft. The road will take you into an abandoned quarry in the Blaylock Sandstone. Sandstone is nearly vertical, trending E-W, and the quarry trends the same. The south wall of the quarry should have Abottom markings@ which indicate strat up to the south. Also indicate an east to west current direction. Internal stratification of the sandstones is supposed to have well-developed Bouma sequences. Folds in quarry are open w/ horiz axial surfaces.

Returning north along the road, you cross the gradational contact into the Polk Creek Shale. Some of the best outcrops of Polk Creek shale are exposed in the larger stream east of the parking area. (Graptolitic black shales w/ significant pyrite. Interbedded layers of chert.

I get the impression that there is a Asecondary road [leading] down the the lake@(which should be to the north) along which one can see the Bigfork Chert. It also says that one can see the Bigfork Chert by Aclimbing up the hill on the northeast side of the stream), but that it is not well exposed.

Cretaceous sediments (post Ouachita) exposed in Highland Gypsum Quarry

[This alternative can only be visited if you were able to get to all the Beavers Bend State Park the first day]

Route: From the last stop in Beavers Bend State Park, continue on US-259A, which finishes the loop back at US-259. Return south to Broken Bow and take US-70 east into Arkansas. At DeQueen, US-71 joins US 70. Continue east on this joint highway. About 8 miles out of town they diverge agin. Follow US-71 south 5 miles to SH-24 at Lockesburg. Go east 7 miles on SH-24; then take SH-26 NE through Center Point and on to Highland. At Highland look for the entrance to the Highland Gypsum Quarry on the left.

At the Highland quarry, the DeQueen Formation (L.K., Trinity Group) is exposed. This stop is discussed in detail in an article by J.D. McFarland of the Arkansas Geological Commission. Note here the contrast in the nature of these rocks with those just a little farther north in the Ouachitas. The rocks here are much less lithified than the Ouachita rocks, and also these lie horizontally. Drilling for water wells in this area would, at a fairly shallow depth, intersect hard Paleozoic rocks beneath these in the subsurface. There is an angular unconformity between these. This unconformity provides relative dating evidence that the Ouachita orogeny occurred sometime postdating the early Pennsylvanian, and predating at least the Cretaceous.

The DeQueen Formation consists of limestones, gypsum, mudstones, and minor sands. This mixture of carbonate/evaporite plus terrigenous siliciclastic sediments is interpreted to have been deposited in a broad, shallow lagoon just off the Cretaceous coast. This lagoon was protected by a reef called the Glenrose Reef. This reef is interpreted from rocks of similar age that occur now in the subsurface farther south. The mudstones, siltstones, and sandstones have brackish water fossils in them. Toward the top, some of the rocks have calcite pseudomorphs after halite. The gypsum beds in the DeQueen Formation are discontinuous when looked at over large areas. The layers have an overall lenticular geometry. These observations suggest that the gypsum was deposited in depressions of limited areal extent on tidal mud flats. The calcite pseudomorphs after halite indicate that there were time periods of supratidal (above sea level) exposure with drying out of saline waters. Both the halite pseudomorphs and the gypsum probably indicate supratidal brine pools. Note: some zones near the middle of the highwall contain celestite nodules.

Jackfork Formation Sandstone in the HMB Quarry

This alternative can be done if there is not enough time to go to the Highland gypsum mine.

Route: From Beaver's Bend State Park go south to Broken Bow. Take US-70 west to DeQueen. Continue east on US-70. A few miles east of DeQueen, before leave Sevier County, will be a sign on the left labeled HMB QUARRY. Take the gravel road to the quarry.

This is a huge quarry where the Jackfork was mined along strike for over a mile. In 1998 there was someone kind of wathching the place, but no other activity except for a piece of equipment that seemed to be gathering some fill material. About 200 yds from the eastern end we found an outcrop on the south side of the cut with good flute casts on the bottom, showing that the sequence was not overturned. Sandstone similar to what was was seen at the Y-City stop can be seen, only here it was much thicker.

Magnet Cove and vicinity

Route [from Highland Gypsum]: Go east on Route 26 to Route 27. Take 27 north to Murfreesboro and continue north to Kirby. At Kirby, US-70 joins Rt-27. Take US-70 to Glenwood and continue on it all the way to Hot Springs. On the west edge of Hot Springs take the US-270 bypass that bypasses Hot Springs by swinging south and then east to joing Aregular@ US-270 on the east side. Continue east to the junction with Rt-51. Take Rt-51 east (turning left) toward Magnet Cove. Go 0.75 mi and stop at the crest of ACook Mtn@, which is on the western edge of Magnet Cove. This is labeled as A2" on the map of Magnet Cove, which is taken from AGC Field Trip Guidebook 67-1 (note on that map the road is labeled as US-270 because it was made before they changed the designation of the road).

Route [from HMB Quarry]: Continue east on US-70. Directions after the Kirby vicinity given above are the same.

Stop 1 (#2 on map of Magnet Cove): We should be able to see the concentric disposition of some ridges in this area held up by some of the more erosion-resistant units of the ring complex. Look for exposures of sphene nepheline syenite in the road cut here. Also look for saprolite that pseudomorphically preserves the texture of the parent rock [could not see well now, although mentioned in the guidebook]. Look for garnet-pseudoleucite syenite outcrop about 575 ft further east. [Forgot to look for this in 1998].

Route: Continue east on Rt-51 another 0.8 mi. Stop near bridge at Cove Creek.

Stop 2: Magnet Cove Alkalic Intrusive Complex: carbonatite at Cove Creek/Highway 51

This stop is labeled as #4 on the map of the Magnet Cove intrusive complex. The Magnet Cove intrusion is a world-class alkaline complex, including the most unusual of all igneous rocks, carbonatite. On the north side of the road and on the west side of the creek, some white rock is exposed. This is composed mostly of calcite. There are also some accessory minerals in with the calcite. Across the road is some nepheline syenite pegmatite.

Route: Continue east on Rt-51 2 mi and turn left on the old barite mine access road. [This is just after the cemetery.] The Guidebook mentions after 0.3 mi a junction, and it says continue left. [We went off to the barite mine and saw the pit, now filled with water. Some modifications have been made, but the turnoff to go to Cove Creek has a sign mentioning some "church"]. About 1.5 mi after having left Rt-51 we should cross Cove Creek. Juacupirangite is said to crop out in the creek bed. [I am not sure of the milage, but after going down a steep grade (winiding) you cross two small creeks. Then after another 0.1 mile or so is a triangular intersection. You go right there.] At 1.7 mi from Rt-51 a molybdenite-bering vein occurs off to the right across the creek, which was prospected as a possible source of molybdenum and titanium. Stop at 1.9 mi from the Rt-51 turnoff. [We did not have our odometer right because of having driven around looking at the barite pit and also getting lost. We could not see outcrops in the creek from the road, and all of it had fences anyway. We did not find the jacupirangite. Later, with better directions, on another trip, we found it. The road to go north on off of Rt-51 is a few tenths of a mile west of the cemetery, across from a fairly new white house.]

Stop 3 (#6 on map): Jacupirangite at Cove Creek.

The guidebook says that the outcrop is along the creek here, and it is. The correct site is not marked against "tresspassing". Jacupirangite is a nepheline-rich pyroxenite. There are several dikes of other mafic and alkalic compositions in the outcrop.

Route: Return to Rt-51 and continue east to the junction with US-270. Turn right on 270 and go a little ways (a mile?) to the entrance to the Mid-State quarry (now Martin-Marietta). Turn right into the access road and proceed to the quarry office to check in.

Stop 4. Stanley Formation hornfels and Arkansas Novaculite at Mid-State Construction Materials Jones Mill Quarry (now Martin-Marietta): Contact metamorphism from the Magnet Cove intrusive complex

The main objective of this stop is to see intrusive contact relationships between the Magnet Cove pluton and the Paleozoic age country rock. We will look at Stanley Group Ashales@ and "sandstones" that have been baked by contact metamorphism into hard, brittle hornfels. This demonstrates the intrusive nature of the adjacent igneous complex, which is just over the hill from this quarry. Compare these rocks to what you saw of the Stanley in other places.

In addition to the hornfelsed nature of the Stanley Group rocks here, we will also see lamprophyre dikes and sills. Lamprophyre is a dark, mafic igneous rock that has ferromagnesian minerals as phenocrysts. These lamprophyres are evidently related to the alkaline intrusive complex. Since they cut across the Stanley Group rocks, this is additional field evidence for the post-Mississippian intrusion of this igneous complex. Actually, depending on where quarrying is at at any one time, dikes can be found cutting across folds as well. Thus the intrusion also post-dates folding. Since Pennsylvanian rocks (Atoka and Hartshorne) near home (Russellville) are also folded in the Ouachitas, the intrusion post-dates the Pennsylvanian based on field relationships we can see.

After looking at the hornfels, follow the access toward the northwest(?). At the top, just maybe 30 meters past the last highwall there is an outcrop of saprolite and spheroidally weathered nepheline syenite. We have crossed the contact, showing just how close to the intrusion the hornfels is. Perhpas in future years the contact will be exposed.

In an older part of the quarry, we can see Arkanas Novaculite that is intensely folded. The novaculite is no longer being quarried here.

Route: Go back east on US-270 to I-30. Go east on I-30 to the vicinity of Benton. Continue on interstate to the Bryant exit (Hwy 183). Go south on 183 through Bryant, a total of about 5 miles, to the vicinity of Bauxite. There, Rt 183 will bend sharply to the right (west). After bending west, go less than one mile to where railroad tracks are on a bridge over the road. Immediately after that, turn left at the Bauxite Post Office. You will pass some large buildings related to the old bauxite mining works here on your left. After about 0.6 of a mile, this road will join into another road coming in from the left. You will stay to the right here and join that road. After about 0.3 of a mile the road will fork and you stay to the left. About a 0.1 of a mile after the fork there is an intersection that we go through. About 2 mile further will be a white gate with a trail going off to the right to the location of the next site. Just past the white gate (along the main road) you can see a sign that says ATrucks entering road@.

Xenolith in Nepheline Syenite Pluton Near Bauxite, Ark.

This location was donated to the Arkansas Geological Commission by Alcoa Aluminum in 1990 for the sake of scientific education. The Geological Commission does not allow any kind of sampling from this outcrop without written permission.

The xenolith is of some form of layered rock, giving it a distinctly sedimentary appearance. The very angular shape of the xenolith suggests it had broken off in a brittle fashion from the wall rock or roof rock while the pluton was intruding. This xenolith, however, is very unusual in that the minerals in these sedimentary-looking layers are largely sodic pyroxene and alkali feldspar. It is thought that chemical reaction between the alkalic magma and this xenolith converted the original clay and silt of the sedimentary rock into these minerals. This is a process called metasomatism. The xenolith is thought to be composed of one of the Paleozoic rock units. The xenolith would indicate that the intrusion of the syenite came after the Paleozoic. Also in this outcrop, numerous dikes of various sizes can be found. These represent late-stage magmatic processes.

Route: "Home James": Retrace route back to I-30. Go east to Little Rock. Take I-430 north to I-40; take I-40 west to Russellville and ATU.

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