Chapter 5, The St. Joe Formation

The St. Joe is treated by some authors as a ‘Formation’ in its own right, and by others as a lower (or older) member of the Boone Formation. The distinction is somewhat arbitrary in this instance, and I mention it only for the sake of clarity. It will be treated as a ‘formation’ in this text, as it is further subdivided into 4 ‘member’ units.

The St. Joe Formation is made up of 4 distinct sub-units in Eureka Springs, and these comprise the bluffs that characterize many of the more striking views in Eureka. An excellent example of these bluffs can be seen behind Basin Park. Like the overlying Boone Chert, but a little older, the St. Joe Formation dates from the Mississippian Era.

This rock unit also hosts the vast majority of the springs that give Eureka Springs its name.

https://pdfs.semanticscholar.org/70f9/0606d00e28a8b20a662ece2cb857dbba9d57.pdf

Fossils of the St. Joe Formation

Brachiopods

One of the more common fossil groups found around Eureka Springs are the spiriferida. They are an extinct group of brachiopods that existed from the Late Ordovician to the Middle Jurassic. The ones found in and around Eureka Springs, Arkansas, are from the earlier part of the Mississippian period, about 340 to 360 million years ago, and occur in the Boone Chert Formation and in the members of the St. Joe Formation, meaning from the bluffs to the hilltops..

Fossil brachiopod (a seashell) of the Order Spiriferida, found in crinoidal limestone of the Boone Chert Formation at Eureka Springs, Arkansas, dating from the Osagean Stage of the Mississippian period, or about 340 to 348 million years ago.

Fossil brachiopod of the Order Spiriferida in the crinoidal limestone of a member of the St. Joe Formation (the bluffs) at Eureka Springs, Arkansas.

Crinoids

Crinoids are a group of ancient sea creatures related to starfish, sand dollars, and sea urchins. Numerous related species still live in the world’s oceans. They are echinoderms, or members of the phylum echinodermata. Crinoids are found in the fossil record from the Ordovician (nearly 490 million years ago) to the present, but the ones in and around Eureka Springs all date from the Mississippian period, about 323-359 million years ago. They were so numerous in the Mississippian seas in this region that one occasionally encounters rocks known as crinoidal coquina. Coquinas are fossil containing rocks composed of more fossil material than of surrounding sediments such as sand or mud. Crinoidal coquinas are sometimes made up almost entirely of crinoid fragments, suggesting these creatures were extraordinarily numerous, and that their fragmented bodies must have at times covered the floor of the inland sea that rested where Eureka Springs now sits.

The crinoid fossils around Eureka Springs are almost always, if not always, fragments. Small disks and columns represent stems, smaller disks made up the feathery tops of the organisms, and one occasionally finds pieces of the rounded bases of the organisms as well. They occur in the Boone Chert Formation and Saint Joe Formation. These rock layers are both from the Mississippian period, and make up the hill slopes from the bottom of the bluffs in and around town, visible above Basin Park, to the hilltops. To the north, in southern Missouri, the same rocks that we call the Boone Chert are named the Burlington Keokuk Formation. The same fossils can be found there. Starting several miles south of town, some crinoids can also be found in younger rocks from the Pennsylvanian period.

To learn more about crinoids, you can read an excellent Wikipedia article at https://en.wikipedia.org/wiki/Crinoid