Pictured above is approximately 450 million year old (Ordovician) St. Peter Sandstone photographed near the White River, north of Mountain View, Arkansas. At the center of the picture is a feature not uncommon in this formation: a “quicksand structure”. Though not well understood, this kind of structure is believed to be a preserved conduit through which ground water once flowed to the surface in a boiling spring.
A boiling spring is where ground water rises up through unconsolidated sediment such as sand, keeping the sediment in continuous motion so that it appears to be boiling. Because ground water contains dissolved minerals, the tunnel through which the water is rising can eventually become hardened by precipitated minerals. Today, as the sandstone that formed from that sediment is being eroded away, the conduit stands in relief because it’s a little harder than the surrounding sandstone due to that mineralization.
Clues like these “quicksand structures” offer us a glimpse into what the environment was like when the rock was being deposited.
This is a picture of sandstone and shale of the Maumelle chaotic zone that outcrops along highway 10 west of North Little Rock, Arkansas. The Maumelle chaotic zone is part of the Jackfork Formation which forms the bedrock around much of the Little Rock area. The chaotic zone is called that because of the disarray the rock is in there. In the example above, broken blocks of sandstone are interspersed with disorganized shale beds that have been rolled, squashed and otherwise deformed (rock hammer at center is for scale). The rocks weren’t deposited this way but were originally organized into horizontal beds on a deep-water ocean slope. Before they could be hardened into solid rock, the slope failed and the beds were transported down hill in a massive submarine landslide.
Note: Other interpretations for this zone have been proffered. The author of this blog prefers the above interpretation.
For more views of the Maumelle chaotic zone click here
Pictured above is the mineral cinnabar (red) on a nest of quartz crystals. Cinnabar is the mineral we extract mercury from. Cinnabar in Arkansas, which was commercially mined in the early 1930’s, is located in a linear band approximately 20 miles long in Pike, Howard, and Clark counties in the Ouachita Mountains of western Arkansas. It is in a linear band because it was deposited as fracture fillings in the highly fractured rock of a tightly folded anticline. The intense deformation that created the fold also caused fractures to open in the rock to relieve the pressure. While this folded rock was still deeply buried it came into contact with mineral rich hydrothermal fluids that were circulating deep in the earths crust. These hydrothermal fluids left deposits of mercury-rich cinnabar in the fractures.
In addition to the cinnabar, early prospectors also found fractures and cavities that were filled with free mercury or “quicksilver” as they called it. Thousands of pounds of this quicksilver were extracted from the deposit during World War 2. Unfortunately, due to poor health and safety measures at that time, and the toxicity of mercury, many of the miners eventually developed very serious health problems; mining of the cinnabar ended in 1945. The level of mercury in cinnabar is high enough that it’s recommended you handle it cautiously.