Highland gypsum mine
Bauxite Mines
Highland Gypsum Mine Chapter 8, Page 5
Current Status: Closed and reclaimed.
The Highland gypsum mines are about a 10-minute drive out AR Hwy 26 west from Murfreesboro, Pike County. A variety of minerals and fossils await the collector at both the active and abandoned mine sites.
Project GO, the Geology Orientation summer aegis program of outstanding students, braved blistering summer temperatures to visit the Higland gypsum mines. July temperatures soared above 105 degrees F. while the students hunted for satin spar, alabaster, celestite, salt hopper casts, fossil shells, and ripplemarked limestone beds.
An old shallow sea
Ripple-marked thin limestone beds are evidence of shallow-water conditions that existed in this area during the Early Cretaceous. The presence of massively bedded gypsum (alabaster) attests to the high salinity of the water which deposited these beds.
Immediately above the commercially valuable gypsum layers is an interstratified unit of dark clay and thin beds of fine-grained limestone. Many of the limestone units exhibit mud cracks or ripple marks. Some have been thoroughly reworked by organisms, creating bioturbated units which have lost all their original sedimentary structures. Some of the fine-grained limestones exhibit ripplemarks on their upper surfaces and salt-hopper casts on the bottom of the beds. This occurred when halite crystals were deposited in the clay as the clay sediments dried out.
Skeletal salt crystals pushed the mud aside as they grew on the sediment's surface. Then a thin limestone mud washed onto the clay surface, dissolving the salt crystals and filling the holes left in the dried out clay with the limestone mud. The mud later lithified to produce a flaggy bed with these features intact.
Satin Spar
Satin spar formed as the original seawater in the sediments was forced out during compaction. Satin spar consists of myriad fibrous gypsum crystals, all parallel in orientation. The direction of growth of satin spar is primarily vertical, gravity being the greatest force exerted on the crystals as they grew. Satin spar at the Arkansas gypsum mines may be as much as 2.5 inches thick. Satin spar seams may extend over 100's of feet horizontally even though they are a secondary depositional feature.
Dwarfed oyster shells are packed into thin layers. These oysters had a rough time surviving in the Cretaceous hypersaline waters of this basin marginal land-locked sea. Evaporation rates of the sea were often higher than the incoming ocean water, so the shallow margins would periodically dry up.
Ch 8 Page 5 
Rockhounding Arkansas revised Feb 2004
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