Report on July 2009 Badlands Field Expedition

Harmon Maher, Robert Shuster, Dept. of Geography and Geology, University of Nebraska at Omaha

Project abstract: This project is investigating the origin of chalcedony vein concentrations found in the White River Group of Nebraska and South Dakota. Specifically it is looking at the role that structural diagenesis played in their formation. Previous work indicates that clastic dikes are connected to chalcedony vein formation and/or to structural diagenesis, and so we are also studying these features. This summer’s field work was focused on the Badlands National Park area. Dr. Rachel Benton (Badlands National Park Chief Paleontologist), is acknowledged and thanked for helping us navigate the permit system and providing guidance on the Park geology. This is an undergraduate research project funded by the Petroleum Research Fund of the American Chemical Society.

Field expedition participants:

Participating students: Bart Cubrich, Nathan Hegdahl, Rachel Hines, Jon Kinkade, Rebecca Moshman, Brooke Nicholson, Brian Tipton
Faculty: Robert Shuster, Harmon Maher

From left to right: Broke Nicholson, Nathan Hegdahl, Rachel Hines, Bart Cubrich, Brian Tipton, Jon Kinkade, and Rebecca Moshma

 

Localities worked at


Chamberlin Pass: Work here focused on a stratiform chalcedony horizon that straddles  the Chadron/Brule formational contact. 14 samples of the mudstones were collected, a stratigraphic column of the sampled interval measured, and orientations of the associated chalcedony veins taken. Some samples of the chalcedony veins were also taken.  The chalcedony bearing horizon is discontinuous and is absent just to the south, possibly because of a channel complex that cuts down into this level.

Chambelin Pass sampling site: The greyish rock litter in the foreground is chalcedony vein material that has weathered out from the stratabound horizon that the veins occur in here. A channel sandstone complex can be seen as the well bedded material on the near ridge. Veins persist up to near the base of the channel complex, and fine-grained horizons were sampled from beneath the chalcedony horizon (somewhat below the student in yellow) to above the channel complex.


Cedar Pass: Work here mapped GPS positions of clastic dikes, acquired orientations, and collected observations. Information from over 150 dike localities was obtained (multiple localities per dike). This information will be added to previously collected data. The dikes show a very complex array of  internal geometries and fill histories. It is clear that there is an opportunity to capture the full complexity with several more days of data collection, and the plan is to this work in this area more this fall or next summer.

Clastic dike with green alteration zone. Bart Cubrich for scale. Note how the clastic branches vertically upward as it cuts through the conglomeratic channel complex above. The channel complext within the Sharps Formation has cut down into the Scenic Member of the Brule Formation.

A composite vertical clastic dike cutting through a nodular sandstone horizon with truncation of the sand and muddier layers in the dike. The geometry may provide clues as the relative timing of 'nodularization' and dike injection.

End of the field day at Chamberlin Pass.

Indian Creek Road: This was a chalcedony vein horizon site encountered while doing some recon to better understand the geology. This concentration is the stratigraphically lowest one found yet, well down in the Chadron Fm., and roughly equivalent to the position veins occur in at Toadstool, Nebraska. Chalcedony veins here display very well developed wall slickensides, and abundant vertical shortening features such as folds. Mainly observations were collected here.

Well developed slickensides on the walls of chalcedony vein material at the Indian Creek road site. Slickensides clearly and consistently indicate outside wall down movement. The pencil tip rests on a vein segment rotated around a horizontal axis but otherwise in place.

Sheep Mountain Dike: At this site a large clastic, composite dike cuts through chalcedony horizons. Eleven sample pairs of clay bearing lithologies were taken inside and outside the dike over a vertical distance of about 35 m, a stratigraphic column was measured of the interval sampled, and GPS and orientation data was collected on chalcedony veins in the area. Sampling started from below the level where chalcedony veins occur, up through the tiered concentrations of chalcedony veins. Samples of the chalcedony veins were also collected.

Looking along the trace of the dike sampled. The clastic dike can be traced to the other side of the ridge across the entrance road.

Students sampling dike and wall rock material at this site. From left to right: Nathan Hegdahl, Jon Kinkade, Rebecca Moshman, and Robert Shuster.

Sheep Mountain Entrance: At this locality in the lowermost Brule Formation, chalcedony vein fins are well exposed, allowing the map view geometry of the veins to be studied. Over 300 GPS vein positions and orientations were measured. Initial analysis indicates that here the strike pattern is non-random. No samples were collected here.

Sheep Mountain Upper: This site is quite close to the Sheep Mountain Dike area. The base of a stratibound chalcedony vein horizon is well exposed. 10 samples of clay bearing lithologies were collected, a stratigraphic column of the sampled interval was measured, and orientations of  chalcedony veins was collected. It was discovered that at this locality chalcedony veins occur as tiered stratabound horizons throughout most of the Scenic Member of the Brule Formation, and so the sample suite extends from the base upward, through only the lowest tier, which is bracketed by more sandstone rich horizons.

Chalcedony veins occur concentrated in a horizon from just below the lowest students pictured on this ridge up to the highest student picture, which was the interval sampled. This is approximately in the middle of the Scenic Member of the Brule Formation.

Sheep Mountain Valley: This area is just to the west of Sheep Mountain Upper site and is a larger area. The larger scale map distribution of chalcedony veins and clastic dikes was investigated. Three sandstone (channel) horizons were used as markers. Some very interesting interactions were found between the clastic dikes and chalcedony veins. This area will be a site of mapping next field expedition. Several samples were collected of composite dike/vein material.

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Future work during fall and spring semesters: Sediment samples collected will be analyzed for clay and silica phase content.   A thin section suite of chalcedony veins and siltstones will also be petrographically studied. Orientation data will be added to the existing GIS database, and analyzed for preferred orientations.

One of many thunderstorms we saw. For a dry place it rained a lot while we were there.