Category Archives: Sedimentary Features

Geo-pic of the week: A lively sea in North Arkansas

 

Starfish pit 2Starfish pit 1

Starfish pit 4Starfish pit 6

Starfish pit 5Starfish pit 3     

The above pictures are of sandstone beds from a quarry in North Arkansas developed in a rock formation called the Batesville Sandstone.  Though it formed in a marine setting, the Batesville is typically composed of fairly homogeneous, flat-bedded rock with little evidence of inhabitation.  This spot is an exception.  The pictures clearly indicate the depositional environment was teaming with sea life at the time the sediment was emplaced.  

The abundant trace fossils, which preserve the activity of organisms rather than their physical form, show a variety of behaviors common to marine invertebrate animals that lived in Arkansas more than 330 million years ago.  Remnants of grazing traces of various snail and worm-like critters (A), resting traces (starfish; B), Dwelling burrows (sea-anemone or bivalve?; C), and locomotion trails (D) are indicators of the conditions present in North Arkansas near the end of Mississippian time.   

Geo-pic of the week: Fracture-Fill at Shine-Eye

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The photo above shows a vertical dark rock in the center of flat-lying white rock. The dark rock is a sandstone deposit, probably Mississippian-aged, and the white rock is Silurian-aged limestone. If one were to follow the sandstone dike upward, it would lead to a sandstone bed sitting on top of the limestone. Since the limestone was deposited first, we can infer that it was exposed to weathering. The limestone was solutioned and deep fractures or cracks formed. Afterwards, sand was deposited in the area, filled the fractures in the limestone, and eventually lithified into sandstone. There are several of these sandstone-filled fractures present along the Buffalo National River in Silurian-aged limestone. The one pictured above is located at Shine-Eye.