Thunder Bay Limestone

Thunder Bay Limestone is a geologic formation in Michigan that preserves fossils dating back to the Middle Devonian and is the uppermost formation of the Traverse Group.^[1]

Thunder Bay Limestone
Stratigraphic range: Middle Devonian (Givetian) PreꞒ Ꞓ O S D C P T J K Pg N
Type	Geological formation
Unit of	Traverse Group
Sub-units	Partidge Point Member, Potter Farm Member, and Norway Point Member
Underlies	Kettle Point Formation and Squaw Bay Limestone
Overlies	Potter Farm Formation
Thickness	4.88 metres (16.0 ft)
Lithology
Primary	limestone
Other	shale
Location
Region	Michigan
Country	United States
Type section
Named for	Thunder Bay (Michigan)

Description

Stratigraphy of the Devonian deposits of the northern part of the Lower Peninsula of Michigan

The name for the formation originates from the description of strata near the "south cape of Thunder Bay" by C. C. Douglas in 1841, this area also represents the type locality of the formation. Due to negligence by multiple later workers, the area was later referred to as the Partridge Point Formation by Warthin & Cooper in 1935. It wasn't until 1943 that Warthin & Cooper would rename the formation back to its original name.^[2]

The type locality (Locality 30-8-11 SE) is the largest outcrop of the formation, exposing six units that have a total depth of about 4.8 meters. These units are largely made from light-colored limestones though the bottom-most unit has bluish limestone. The most unique unit would be unit 3 which mostly is made up of grey shales that grades into granular limestone at the surface.

The environment that the formation represents is a shallow carbonate platform with large amounts of coral and shelly fauna. Later formations show an increase in depth as water levels raised in the Eastern Interior seaway. Over time, the seaway would develop a stratified water column. Eventually, younger formations, like the Squaw Bay limestone, would show a deep basin environment.^[3]

Paleobiota

Anthozoa

Genus	Species	Notes	Image
Antholites[4]	A. alpenensis
Aulacophyllum[5]	A. hemicrassatum
Alveolites[6]	A. sp
Bethanyphyllum[5]	B. geniculatum
Cylindrophyllum[7]	C. grabaui
Cystiphylloides[8]	C. americanum elongatum
Drymopora[6]	D. sp.
Favosites[9][6]	F. mammillatus
	F. placentus
	F. romingeri
Heterophrentis[6]	H. sp.
Hexagonaria[10]	H. potterensis
Platyaxum[6]	P. sp.
Stereohma[11]	S?. sp. A.
Syringopora[6]	S. sp.
Trachypora[12]	T. alternans
	T. dendroidea
	T. perreticulata
	T. proboscidialis
	T?. reticulata

Brachiopoda

Genus	Species	Notes	Image
Athyris[6]	A. sp
Atrypa[6]	A. sp
Camarotoechia[6]	C. sp
Chonetes[6]	C. sp
Cranaena[6]	C. lincklaeni
	C. romingeri
Cytina[6][13]	C. hamiltonensis
	C. sp
Meristella[6]	M. sp
Mucrospirifer[6]	M. sp
Pentamerella[6]	P. sp
Orthospirifer[14]	O. traversensis

Bryozoa

Genus	Species	Notes	Image
Cyphotrypa[6]	C?. unua
Euspilipora[6]	E. serrata
Fenestrellina[6]	F. compacta
	F. longispinosa
	F. nodicula
	F. variifenestrula
Fistuliphragma[6]	F. spinulifera
Fistulipora[6]	F. acervulosa
	F. corrugata
	F. stellifera
	F. sulcata
Hederella[6]	H. cirrhosa
	H. compacta
	H. delicatula
	H. persimilis
	H. rugosa
Lioclema[6]	L. incompositum
	L. minutum
Polypora[6]	P. modesta
Scalaripma[6]	S. approximata
	S. separata
Semicoscinium[6]	S. approximatum
Sulcoretepora[6]	S. hadmiltonensis

Conodonta

Genus	Species	Notes	Image
Icriodus[15]	I. cymbiformis
	I. expansus
	I. latericrescens latericrescens
Polygnathus[15]	P. varcus

Echinodermata

Genus	Species	Notes	Image
Aorocrinus[6]	A. cassedayi
Atractocrinus[16]	A. campanulatus
Botryocrinus[17]	B. thomasi
Codaster[6]	C. gracile
Corocrinus[18]	C. pettyesi
Dactylocrinus	D. alpena
Dolatocrinus[19][6]	D. asterias
	D. barrisi
	D. triangulatus
Euryocrinus[6]	E. barrisi
Gennaeocrinus[18]	G. romingeri
Lipsanocystis[20]	L. traversensis
Megistocrinus[6]	M. concavus
	M. multidecoratus
	M. nodosus
	M. novus
	M. tuberatus
Nucleocrinus[6]	N. elegans ?
	N. meloniformis
	N. obovatus
Pentremitidea[6]	P. americana
	P. bassleri
	P. bassleri hastula
	P. milwaukeensis
Stereocrinus[6]	S. barrisi
	S. triangdatus
	S. triangdatus lirata
Synbathocrinus[21][6]	S. matutinus
	S. sp. cf. S. michiganensis

Mollusca

Genus	Species	Notes	Image
Cimitaria[6][22]	C. recurva
	C. sp
Modiomorpha[22]	M. mytiloides
Mytilarca	M. cf. M. oviformis
Plethomytilus[6]	P. sp

Placodermi

Genus	Species	Notes	Image
Ptyctodus[23]	P. sp

Stromatoporoidea

Genus	Species	Notes	Image
Stromatopora[6]	S. sp.

Tentaculita

Genus	Species	Notes	Image
Tentaculites	T. sp

Trilobita

Genus	Species	Notes	Image
Dechenella[24]	D. reimanni
Dipleura[24]	D. dekayi
Greenops[24]	G. alpenensis
Phacops[24]	P. iowensis
Proetus[24]	P. alpenensis

See also

• Earth sciences portal
• Michigan portal
• Paleontology portal

• List of fossiliferous stratigraphic units in Michigan