Big Hill Formation

The Big Hill Formation is a geologic formation in Michigan. It preserves fossils dating back to the Ordovician period. A fossiliferous site on the Stonington Peninsula (in Delta County) includes a dolomite bed of the Big Hill Formation which has abundant and well-preserved fossils. The most common fossils are soft-bodied medusae (jellyfish), followed by linguloid brachiopods, algae, and arthropods (namely chasmataspidids, leperditiid ostracods, and eurypterids). This site is considered a Konservat-Lagerstätte, and is commonly referred to as the Big Hill Lagerstätte or Big Hill Biota.^[1][2]

Big Hill Formation
Stratigraphic range: Katian PreꞒ Ꞓ O S D C P T J K Pg N
Jointed limestone from the Big Hill Formation (Upper Peninsula of Michigan)
Type	Formation
Unit of	Richmond Group (geology)
Underlies	Manitoulin Dolomite
Overlies	Stonington Formation
Lithology
Primary	limestone
Location
Region	Michigan
Country	United States

Depositional environment

The Big Hill formation likely represents a shallow, sheltered basin within the photic zone, suggested by the completeness and lack of transport of the algal specimens. However, the lack of dessication cracks and adhesion structures suggests that there was no subaerial exposure and the absence of ripple marks suggests that the basin was at least several meters deep.

The basin was likely dysoxic, as suggested by the limited diversity and lack of bioturbation. In addition, the lack of rhynchonellate brachiopods and echinoderms as well as the abundance of lingulids suggests that the basin had a different salinity than normal marine environments; however, direct evidence of hypersalinity like evaporite molds is absent. Microbial mats provide evidence of ecological stress.

While the basin was largely sheltered, it was likely connected by an extent to open marine ecosystems, as shown by the presence of chasmataspidids, eurypterids, and scyphozoan medusae. The medusae represent different stages of decomposition, suggesting that they were not all deposited in a single event but rather accumulated over time on the seabed via waves or currents. In addition, the orthocones may have been empty shells that drifted into the basin.

Fossil content

Color key Taxon Reclassified taxon Taxon falsely reported as present Dubious taxon or junior synonym Ichnotaxon Ootaxon Morphotaxon		Notes Uncertain or tentative taxa are in small text; crossed out taxa are discredited.

Chelicerates

Chelicerata
Genus	Species	Images	Notes
Hoplitaspis[2]	H. hiawathai	Hoplitaspis hiawathai	A diploaspidid chasmataspidid.
Carcinosomatidae indet. [3]			A carcinosomatid eurypterid.
Dolichopteridae indet.[3]			A dolichopterid eurypterid.
Lunataspis[4]	L. gundersoni		A xiphosuran.

Trilobites

Trilobita
Genus	Species	Images	Notes
Isotelinae indet.[1]

Crustaceans

Crustacea
Genus	Species	Images	Notes
Leperditiidae indet.[1]			A leperditiid ostracod.

Lophophorates

Lophophorata
Genus	Species	Images	Notes
?Lingula[1]	?L. changi		A lingulid brachiopod.
?Trematis[1]	?T. rugosa		A discinoid brachiopod.
?Hallopora	?Hallopora sp.		A halloporid bryozoan.

Mollusks

Mollusca
Genus	Species	Images	Notes
Lophospira[1]	Lophospira sp.		A gastropod.
Actinocerida indet.[1]			A actinocerid cephalopod.
Pseudorthocerida indet.[1]			A pseudorthocerid cephalopod.
?Vanuxemia[1]	?Vanuxemia sp.		A bivalve.

Cnidarians

Cnidaria
Genus	Species	Images	Notes
Conchopeltis[1]	Conchopeltis sp.		A scyphozoan.

Algae

Flora
Genus	Species	Images	Notes
Archaeobatophora[1]	A. typa
Chaetocladus[1]	Chaetocladus sp.

See also

• Earth sciences portal
• Michigan portal
• Paleontology portal

• List of fossiliferous stratigraphic units in Michigan