Geology of the Rush Mining District

Geologic Map by McKnight 1935

The zinc deposits of the Rush Mining District are closely associated with faults, especially the Rush Creek fault.  The Rush Creek fault underlies the valley of Rush Creek and is responsible for the straightness of the valley.  Where Rush Creek crosses the fault a modest spring, "Boiling Springs" erupts. Strata to the north of the valley floor have been uplifted up to 350 feet relative to those same beds to the south.  

Figure from Croneis 1930 illustrating the Rush Creek Fault but incorrectly giving the stratigraphy. The Everton Formation has been labeled Cotter.

Stratigraphy

Geologic Period	Stritgraphic Layer or Formation
Pennsylvanian	Not recognized in the immediate area of Rush but does crop out further up stream along the Buffalo River.
Mississippian	Pitkin Formation Millions of years ago the Pitkin Formation covered the Rush area but erosion has long sense removed it. Beautifuf outcrops of the Pitkin can be seen to the along US 65 just south of Marshall, Arkansas. The massive layers near the top of the Pitkin contain some very large crinoids. The thin layers below contain only a few small brachiopods and pelecypods.  Fayettville   Boone Formation: The Boone Fm. is represented in the Rush area as a series of limestone beds. These were formed from fragments of calcite producing marine animals and plants, especially crinoids, bryozoans, and trilobites. Some beds will have brown to gray colored chert nodules. Where the Boone has been greatly weathered all that remains will be the chert nodules. The photomicrograph at the right is a microscopic view of a typical sample of the Boone from an outcropping along the Rush Road. Most of the grains in the view are fragments of crinoids but near the top is a odd looking grain that is a fragment of a trilobite. St. Joe: The St. Joe represents the earliest deposits of the Mississippian in the Rush area. It forms a beautiful reddish brown limestone which separates into well formed beds making it an excellent building stone or dimension stone. The building in the photo was constructed at Buffalo Point by the Civilian Conservation Corps (CCC) in the 1930's using stone from the St. Joe.
Devonian	Not recognized in the immediate area of Rush.
Silurian	Not recognized in the immediate area of Rush.
Ordovician	Plattin   St. Peter Sandstone: The St. Peter Sandstone forms a prominent massive sandstone layer in the area. The rock is mostly made of well rounded quartz grains. It frequently is exposed as moss wildflower covered glades like the one in the photograph to the right. Everton Formation:  The Everton Formation consists of limestone, dolostone and sandstone. layers. Many of the limestone layers were formed by stromatolite, the remains of blue green algae. The Everton was deposited in a shallow warm sea that covered much of northern Arkansas, Missouri and the adjoining states. The zinc deposits of the Rush all occur within the Everton.
Cambrian	Not exposed in the Rush area but exists below the surface where is consists largely of sandstone layers that form excellent aquifers. The water wells in the area generally tap the trapped water in these beds.