Structural Geology Field Trip

Note: The following is a rough sketch of the itinerary used for the field trip. This document was prepared informally and contains jargon and unconventional abbreviations. No serious attempt at editing for grammatical errors has been made.

PLANS FOR GEOL 3004, STRUCTURAL GEOLOGY, FIELD TRIP, FALL 1997

Left: Beautiful folds in sedimentary rocks of the cut on the west end of Blakely Mtn dam! Right: Fall is a great time to take this field trip. view from east end of the dam, looking west.

To see more photos from this trip, click here.

Leave ATU 12:30 pm, Thurs. 11/6.

(???Go to "Robinson Quarry" in Russellville & see channel structure...disconformity b/t Hartshorne and Atoka. Stop 2-1, p. 36 of AGC Guidebook 77-1, says upper Atoka here has ss, sh, & a thin coal bed and that these are cut by a channel of Hartshorne. ???)

(1) Go to Glenwood/Skyline Drive and see ss overlying sh w/ dip to south. Then go to back side of Dardanelle Rock to see same sequence dipping north. Then go for side view of Dardanelle Rock. Show my profile of the syncline and the connection of the Hartshorne here with the caprock atop Mt. Nebo.

(2) Go I-40 to Little Rock. Go S on I-430 to rocks just south of river. Look at road cuts along road that parallels the freeway on its west side (north side of Cantrell Rd).

(3) Continue S on I-430 to Col. Glenn Rd exit. See Bigfork Chert w/ weathered dikes.

(4) Continue S on I-430 to cut w/ Bigfork Chert, Ark Novaulite, wx dike, Tertiary gravels.

The rocks in this outcrop are intensely deformed, giving an excellent view of the style of deformation of the Ouachitas. We will examine the outcrop on the west side of I-430. The north end of that outcrop has Big Fork chert that is overturned. Bedding dips to the north. Stratigraphic up, however, is towards the south. Structurally underlying the Big Fork (but stratigraphically Aoverlying@) is the black shale and thin chert beds of the Polk Creek Shale. A lot of shearing occurs along the contact between the Big Fork Chert and the Polk Creek Shale. Further towards the south in this outcrop the Polk Creek is in contact with the ArkansasNovaculite. Since the novaculite is younger than the Polk Creek, and yet underlies it structurally, this demonstrates that the beds indeed are overturned. Progressing further towards the south along the outcrop, the very massive bedded lower part of the novaculite makes transition into the interbedded chert and shale of the middle division of the Arkansas Novaculite.

All these rocks are intensely sheared and folded. Note here that the structural vergence is towards the south. This is an anomaly in the Ouachitas that is difficult to explain when thrusting is considered to be from the south toward the north (which should produce northward vergence).

Another feature to look for in this outcrop is a dike of igneous rock that cuts through nearly vertically in this outcrop. This igneous rock has been weathered heavily and turned to clay. If lumps of the clay are dug up, sometimes a faint color difference within that clay gives one the impression of feldspar phenocrysts in a aphanitic matrix. This is a pseudomorphic replacement phenomena where the porphyritic original texture of the rock is preserved even though the rock has been totally turned into clay by weathering.

On the very top of the outcrop is an area where a gravel deposit occurs. This is thought to be part of the Tertiary-aged deposits. So this outcrop combines exposured of some of the oldest rocks in the orogen, intense deformation of the Ouashita orogeny, Cretaceous intrusives, and Tertiary sediments.

(5) Continue S to I-30W. Get off right away and drive to fence for Affiliated Foods. Look at angular unconformity w/ binoculars.

(6) Get back on I-30 going W to the Bryant exit (Hwy 183). Go S 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 soon on your left). After about 1 2 mile(?) further will be a trail going off to the right to the location of the next site. At that spot there is a white metal gate going across a wide pathway into woods. Just past the white gate is a sign on the paved road saying "Trucks Entering Road".

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

This is a Apavement outcrop@ of nepheline-bearing syenite with an awsome xenolith in it.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 (if my memory serves me right) sodic pyroxene (the dark layers) and Kspar (the light layers). It is thought that chemical reaction between the alkalic magma and this xenolith converted the original clay and silt of the sedimentary rock into pyroxenes and feldspar. 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.

CAMP: Alternatives

I-30 to Exit #97, first past US 270 (Malvern/Hot Springs exit). Go N on SH-171 12 miles to dead end at Lake Catheryn State Park. Can camp in Aoverflow@ as a group, which is picnic area, but avoid being near pavillion, for $1/head. Visitors center is open 8-5, so if get in later and leave earlier try to give money to camp host in site #39. There are bath houses w/ hot showers 1/4 mile from picnic area. To get to Jones Mill Quarry from there, must retrace steps back to I-30 & N to US 270 exit and then back west on US 270.
I-30 to US 270 (Malvern/Hot Springs exit). Go west to vicinity of Hot Springs National Park. Go NE (right) on US-70 (Grand Ave) a few miles to entrance to Gulpha Gorge on left (US-70B). Campground is a couple miles north on right. No showers. $8.00/site. Prob can put 2 tents on a single site, though technically 1/site is the rule. In morning, retrace back to Jones Mill (just a few miles).

(7) Friday morning go to Mid State Jones Mill Quarry and see folds in Stanley Fm. Large scale folds. Also lamprophyre dikes w/ cross-cutting relations. Describe folds in size, shape, orientation. Take strike and dip on limbs of fold to later plot axis with stereonet. Randy Wilkinson is the quarry Supt. His number @ Jones Mill Quarry is 501/337-1021 (622-8314 mobile).

(8) Continue E on US-270 to I-30 and go W to exit for SH-84. Go 7 mi W on SH-84 to SH-128 where you turn N. Go about 2.5 mi to outcrop in Ark Novaculite on E side of road just before 128 bends sharply west. See couple of faults.

(9) Continue N on SH-128 to SH290; turn left (W). Go to SH-7 and go N. Go W on US-270 (MLK bypass) to exit for McCleod Rd w/ road cut I have picture of in my office. Describe fault and folds. See veins also.

(10) Continue on US-270. A little tricky at end of divided hwy. Go N on 227 to Mountain Pine and on a mile or so to Blakeley Mtn Dam on Lake Ouachita to see complex folds in Blakely Mtn Sandstone. Including a sandstone dike. Take Mountain Pine Geologic Worksheet.

(11) Retrace back to US-270, back to where US-70 goes west toward Hempwallace. Continue on US-70 to Glenwood. Go N on SH-27 3.6 mi and turn left on a dirt road (this should be 2nd dirt road to left after west end of an abandoned landing strip that comes in at about 2.5 miles from Glenwood). Go to end of road (in small residential area). Walk west along unpaved road about 600 ft. Turn left and follow path south to RR tracks (about 260 ft).

See Stanley Fm in south-facing RR cut. NW dipping ss, sh, sltstn of upper part of lower Stanley. Flysh-type seds. Graded bedding, tool marks, flute casts, load stuctures, bedding-parallel burrows. South to SW paleocurrent. Measure strike and dip of bedding and trend and plunge of flutes and later do stereonet unfold for paleo current. (Assume E-W subhoriz fold hinge?).

There may be normal faults along with drag folds? (Notes from some description I had obtained from someone or a guidebook sometime in past).

(12) Continue N on SH-27 a few miles to Caddo Gap road cut in Arkansas Novaulite.

(13) From Caddo Gap go N on 27 through Mt Ida and on to Danville. Then take SH-10 W to Havana. There take SH-309 N across Magazine Mtn and on to Paris. At Paris take SH-22 E to Midway. Then take SH-109 N thru Scranton, Morrison Bluff, and across Arkansas River to New Spadra. Go W on SH-194 to US-64 thru Hartman to Coal Hill. Take SH-164 N thru Hunt and on to SH-103. Take SH-103 N to Oark. Just a mile before Oark there is many normal faults in horizontal strata of the Middle Atoka. One near river is on the Ark State Geol Map.

Return via SH-103 to Clarksville. Then I-40 to Russellville.

NOTE: We were unable to do stop 13 above. Time did not allow.

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