The fossil record in the St. Joseph’s Island limestone formation

Introduction

This paper looks at the relevant data and information concerning fossil deposits within the St. Joseph’s Island limestone formation. The island is located close to St. Mary’s River specifically near its mouth. It is specifically located in northwestern side of Lake Huron. St. Joseph Island was formed through erosion and deposition of debris mainly towards the end of the glacial period. It was formed when deposits in the form of rock and sand were deposited by melting glacier. The mode of its formation and the fossil deposits within its sedimentary rock structure provides rich data and information about fossil deposits.

Figure 1: Location of St. Joseph Island

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Figure 2: Aerial Photo of St. Joseph Island

Fossil Records

After several generations with people working in the area, the original structure has been tempered with but substantial data exists about the fossil records of the area. A sand pit from the area would yield many tax of fossils depicting both animal and plant remains that are dated for over 14,000 years. The fossils in the island are of both freshwater and terrestrial origin. The fossil deposits comprise of a diversified but coherent assemblage of various organisms that include ostracodes, mollusks, chironomidae, Turbellaria, Coleoptera, as well as some fragments of fish bones. These fossil deposits formed the oldest ever known postglacial geographical depositions within the island and in Ontario in general. On the other hand, paleoenvironemnts range from tundra to boreal forest as indicated by taxa. The site, including the whole of Ontario is known to have the richest fossil record. In this site, sediments are grouped into various units (LEHTOLA, 1973).

The units include glaciof fluvial sub-aquatic fan origin, the lobe tavistock Till outwash of Huron-Georgia Bay, fossiliferous sand, which is inter-bedded with fossiliferous clay and sand. The forth unit involve stratified sand. The fossiliferous sand and the stratified sand are associated with fluctuations of lake level within the lake basin. The fossiliferous sand unit consists of disturbed sand laminated clay and silt with specifically plant and shell remains. It also consists of massive sand that is rich in shells, and laminated clay and silt with plants. Large amounts of fossil deposits have been taken away from the surface through various pit operations. Other fossil deposits include shrub-ox bone found within the same area although they are hardly accessible especially after 2001. This inaccessibility occurred when the area was covered by aspects such as fill, slump, and landscaping (LEHTOLA, 1973).

James Hall was the first individual to discover and report rocks aged to the Middle Ordovician age within the area. These rocks extent to the Michigan are Upper Peninsula and contain a substantial amount of fossils. Hall correlated the rocks with the results of other researchers. This correlation was meant to depict his accuracy in establishing the types and degree of fossils within the sedimentary rocks of the island. Based on a description of lithology and fossils, there are vertebrate-baring rocks resulting from river formations. Most fossils collected by researchers such as Hussey are from the northeastern section of the island. The fossil records depict disarticulated plates having stellate tubercles. Their age is referenced to the middle Ordovician period. This period is also termed as Middle Wilderness times. The fossils are specifically assigned to genus Astraspis based on tiny and discrete plate that are disarticulated and having attachments of tubercles (Karrow, Morris, Mc Andrews, & Morgan, 2007). These tubercles are found to have central pulp cavities, which have laminated dentine that surround them (LEHTOLA, 1973). The tubercles are small, ranging from 0.25mm to 0.40mm in terms of diameter.

Ostracodems occur within almost pure limestone but with some traces of detrital quartz. In this case, tubercles are closely space although irregularly placed within the bony plates. The tubercles vary in size, mainly from 0.25mm to 0.40 mm. Every tubercle is stellate and has tiny apical know unique for each one of them depending on the size. Among all the examined specimens, mushroom-shaped tubercles have never been recorded. According to the records about tubercles, tubercles could have grown through a division of rays. As the size of tubercles increase, the rays increase as well. The records also show that all tubercles have pearly luster, which is definite (LEHTOLA, 1973). Large fragments are also not found in the area. The largest fragment found has sixteen tubercles in 3 rows, and it measures one millimeter by three millimeters in size. Single tubercles are also found in the site. However, records depict that tubercles mainly exist in groups of five, four, or three (Karrow, Morris, Mc Andrews, & Morgan, 2007). The tubercles in this area are generally soft. They also scratch very easily.

Conclusion

There is no full information about the all the fossils found in the St. Joseph’s Island. The available information is however adequate although there have been various changes and human activities since the fossils deposits were formed. The existence of fortuitous discoveries of fossil sediments on the island has contributed to a diversified assemblage of both plant and animal remains (Karrow, Morris, Mc Andrews, & Morgan, 2007). Most of the fossils depicted include macrofossils, molluscs, bugs, cases of flatworms, and caddisflies among other organisms. All these exist in the form of tubercles as depicted in the paper.

Reference

Karrow, P. F., Morris, T. F., Mc Andrews, J. H., & Morgan, A. V. (2007). A diverse late-glacial (Mackinaw Phase) biota from Leamington, Ontario. Canadian Journal of Earth Sciences, 44(3) , 287-296.

LEHTOLA, K. A. (1973). ORDOVICIAN VERTEBRATES FROM ONTARIO. CONTRIBUTIONS FROM THE MUSEUM OF PALEONTOLOGY, 24(4), 23-30.