Monthly Archives: October 2014

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.

For more views of the Rosie boulders look here

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