Geopic of the week: Rosie boulders

Rosie boulders

Pictured above are gigantic sandstone boulders in a gravel pit near Rosie Arkansas.  While these house-sized boulders are not unusual, they’re location is: there is no nearby source for them.  Some geologists speculate that they were transported  to this location by a great tsunami generated during a major meteorite impact.  Perhaps the same meteorite that caused that tsunami created the 110 mile diameter Chicxulub crater on the Yucatan peninsula about 65 million years ago. 

Geopic of the week: Turbidite deposits in Baumgartner Quarry

The Baumgartner quarry is located a little South of Kirby Arkansas along highway 27 in the Ouachita Mountains.  It exposes approximately 160 m. (590 ft.) of the upper Jackfork Formation which consists of interbedded sandstone and shale deposited  about 300 million years ago when the area that is now the Ouachita Mountains was a deep ocean basin.  Deep oceanic deposits such as these are the kind petroleum geologists explore for oil.  Because these deposits are exposed at the surface in southern Arkansas, geo-scientists from all over come here to study our rocks and gain a greater understanding of the deeply buried rocks they look for oil in in places like the Gulf of Mexico.  This is a view looking west along strike.

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Geopic of the week: well exposed fault

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The picture above shows a normal fault exposed in an old railroad grade just north of Greers Ferry lake in the Ozark Mountains of central Arkansas.  Two different lithologies are juxtaposed against one another: siltstone on the left and sandstone on the right.   Exposures such as this one are relatively rare in the Ozarks where most of the bedrock is covered by loose sediment and vegetation.

This fault is extensive and roughly parallels the northern portion of Greers Ferry lake.  The fault gives the northern portion of the lake its linear shape (see below).

Greers Ferry lake

Geopic of the week: Normal Fault

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Pictured here is a normal fault discovered this week on the Parma quadrangle by our field mapping crew.  It’s developed in the Witt Springs formation and offsets thick-bedded sandstone against thin-bedded sandstone.  There is about a one foot wide zone just north (left) of the fault plane where the bedding appears to parallel the fault.  This could be due to the sandstone of the footwall being dragged into the fault or the near-vertical “beds” may actually be joints that developed due to stress near the fault.

Geopic of the week: Recumbent folds

Tight recumbent folds in a fresh exposure of Bigfork Chert, Ouachita Mountains Arkansas.  Thin chert beds divided by siliceous shale beds are the basic lithology of this Ordovician age (540 to 490 Ma) formation.  Complicated structures like those pictured below are characteristic of the rocks that form the Ouachita Mountains.  The complex folds and faults resulted from plate tectonic forces that compressed the rock and caused many of the structures to rotate and overturn.

 

 

City Quarry Fox Pass6

Geopic of the Week: Soft-sediment deformation

2014-08-20 029Dewatering StructureThis photo, recently taken on the Prim quad, shows deformation of the bedding, probably due to dewatering of the sand prior to lithification.  This soft-sediment deformation typically occurs in areas where deposition rates are high such as a delta or near shore marine environment.  As sediment is deposited, water is trapped in the space between grains.  As sedimentation continues, pressure on the trapped water, due to compaction, causes rupture of the sediment and escape of the water to the surface.  Evidence for this are “tent” or “pillar” structures left behind in the rock as seen in the photographs above.  The black line on the right photo shows the probable path of the water from one bed to the next through what is now a sandstone outcrop of the Pennsylvanian Witts Springs Formation.

 

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Hello all! Sorry I haven’t blogged in awhile. I’ve been so busy trying to complete the maps for this year by the June 30th deadline.  But, I am proud to announce that the Geologic Maps for Shirley and Fairfield Bay are now published and available on our website!  Click below to view and download the maps.

http://www.geology.ar.gov/maps_pdf/geologic/24k_maps/Shirley.pdf http://www.geology.ar.gov/maps_pdf/geologic/24k_maps/Fairfield%20Bay.pdf

The process to create these maps takes an entire year. I kept you updated each field week from July to April last year, so I thought you might be interested to know how we take the raw data we collected in the field and use it to make a map. First of all, it’s a collaborative effort.  It takes a lot of people who specialize in various disciplines working together to make this product.  Basically, drawing the map starts with the notes we took in the field.  At each point, we tried to identify the rock formation exposed there.  Sometimes this was difficult, especially in the southern portion of the Boston Mountains Plateau where we worked this year. These rocks are all so similar–mostly sandstone and shale.  Nevertheless, if you cover as much ground as we did, you begin to discern similarities in the rock types and bedding structures, and can make formation calls based on those similarities.  Many of the points are taken on what we considered to be contacts between different formations.  These points are used to hand draw contact lines on a blank topographic base map. 2014-07-11 0032014-07-11 007 These lines are continued into areas where the contacts may not be exposed, because we assume lateral continuity of these units.  Many times there are topographic breaks along these contacts which can help us draw the lines in areas of poor exposure or in areas we just didn’t get to.  Structural lines are drawn along the trace of faults or other structures at the surface in areas where we saw the hallmarks of faulting such as deformation bands and non-vertical joints.  Also, the many strike and dip measurements we took were plotted on the map and helped determine orientation of faults and other structures, such as the axis of a monocline.  Once all the lines were neatly drawn on the topo, it was scanned into the computer and georeferenced to the grid of all quads in the state.  Next, each line was painstakingly digitized in ArcView by one of our cartographers, in this case Brian Kehner.  The digitized map was then added to a layout that Danny created in Adobe Illustrator. 2014-07-11 008

The layout includes descriptions of each formation developed from our field notes and are specific to each quad.  A correlation of map units, a generalized stratigraphic column, an inset map of the locations of the field points, a symbol chart, and a rose diagram of the frequency of each joint direction are also added to the layout.  A cross-section based on formation thickness is hand drawn, digitized, and placed along the bottom of the layout. Formations are symbolized by color and an abbreviation.  Sometimes photos are added to balance the layout.  Also plotted are any quarries or pits we found or were in the economic commodities database we keep at the Survey.

2014-07-11 012 After we have a reasonably good map, it’s printed and set out for staff review.  They really let us have it, but this editing process always greatly improves the maps.  After two or three revisions, we finalize it and send it to the USGS by June 30 to fulfill the requirements of the STATEMAP grant.  Whew!  What a relief!Geologic Map of Shirley red1 Geologic Map of Fairfield Bay red This year, as in years past, I have designed a commemorative STATEMAP t-shirt.  I’m taking orders until July 25th if anyone is interested.  They are available for the cost of the shirt you choose plus the printing.  Please email me at richard.hutto@arkansas.gov for details. AGS14_shirt_front (1)AGS14_shirt_back (1) Now we get ready to head back out again to our new field area.  This year we’ll be mapping the Parma, Prim, and Greers Ferry quads.  I’m breaking in a new field partner this time out.  Andrew Haner says he’s looking forward to seeing some of the Arkansas wilderness.   I just hope the snake count is low this year.  From what I’ve see so far, the ticks seem to be at an all time high.  I’ll try to keep you posted, but will be out of the office four days a week this year.  That will leave little time for blogging.  So until my next post, I’ll see you on the outcrop! Richard Hutto