Prehistoric Tropical Reef Project 2007: Thunder Bay Island 6-29 to 7-8-07

Thunder Bay National Marine Sanctuary - In the footsteps of Carl Rominger.

A preliminary report from the field by Cranbrook Institute of Science Geologist John Zawiskiejzawiskie@cranbrook.edu

“The exuberance of fossils in the strata is most beautifully exhibited in the shoals of the lakenorth of Thunder Bay Island, where any one sailing over them can see for miles the whole

bottom paved with corals in convex lumps, from a few inches to some feet in diameter, theirwhite, sparry substance contrasting beautifully with the dark limestone that encloses them.”

Carl RomingerThe Geology of the Lower Peninsula of Michigan, 1873 –1876, Geological Survey of Michigan,vol. III. p 47

Working from the legendary former State Geologist Carl Rominger's classic 1874 geological survey

descriptions, a Cranbrook Institute of Science – Noble Odyssey Foundation team aboard the Research

Vessel Pride of Michigan (Captained by Luke Clyburn) was able to locate the upper portion of a remarkable

undescribed Middle Devonian stromatoporoid reef complex (~385 million years old). The reef bed was

exposed by low lake levels along the limestone ledges cropping out at the southern end of Thunder Bay

Island, within the Thunder Bay National Marine Sanctuary.

Stromatoporoid dominated reefs have been documented from the Petoskey Limestone, along Little Traverse

Bay on the western side of the state (Rominger, 1874, Fenton, 1931, Kesling et al, 1974) and a

Corals in storm layer in black Rockport Quarry Limestone, south Thunder Bay Island.

stromatoporoid biostrome is reported to occur in the middle of the Rockport Quarry Limestone at the type

section (Ehlers and Kesling, 1970); however, no such 3 dimensional stromatoporoid reef exposure like the

one at Thunder bay Island has been described from the Alpena area (Stum, 1969), arguably one of the best

Middle Devonian rock sequences in the world. Stromatoporoids (currently considered to be lime secreting

sponges with possible relationships to extant sclerosponges, Stock, 2001) dominate the Thunder Bay Island

reef in association with some tabulate and rugose coral mounds. Together these three groups were the

primary reef builders of the middle Paleozoic Era until a massive extinction of warm water species at the

end of the Devonian (one of the 5 largest extinction events in Earth's history).

Most of the large stromatoporoids at Thunder Bay Island are spheroidal to subsperoidal shapes or are

mound-like, in growth position and are commonly closely spaced or abut one another, with some individuals

being nearly 2 meters across. Some of the compound mounds exceed 5 meters in diameter and are

dissected by erosion to various levels, including some fully intact examples with full external morphology

preserved. The occurrence closely matches the description of stromatoporoid beds by Rominger (1874) in

the Petoskey Limestone in the bluffs along Little Traverse Bay.

“It is Stromatopora pustulifera in particular which is so abundant; it grows in strumose,globular masses, often several feet in diameter, tightly crowded together and sparinglyintermingled with other fossils.”

Carl RomingerThe Geology of the Lower Peninsula of Michigan, 1873 –1876, Geological Survey of Michigan,vol. III. P 55

The large filter-feeding stromatoporoids were likely growing below fair weather wave base along a shelf in a

moderately turbulent zone of a shallow tropical sea. This type of environment was inferred by Ehlers and

Kesling (1970) for the stromatoporoid-coral community type in their paleoecological - depositional model.

Eroded subspheroidal

stromatoporoid, south

Thunder Bay Island. Note

angled fringe of flank

sediment around its

base. Length of hammer

is 27 cm. Star marks a

center of growth withinthe structure.

The sedimentology of the enclosing layers shows how the Thunder Bay Island stromatoporoids withstood

battering during violent storm surges, until some were knocked over or planed off by ersoion and eventually

buried by sediments washed in from shallower-water more turbulent environments. These storm beds

(tempestites) are wonderful black limestones (mostly wackestone and packestone, interbedded with thin

black shale layers) containing white torn up corals, stromatoporoid fragments, crinoids and brachiopods

and/or with extensive networks of unlined burrows of marine organisms referable to the trace fossil genus

Planolites. We were able to document several periods of stromatoporoid burial and rejuvination in the

section.

Section in Rockport Quarry Limestone looking in northerly direction from shore at GPS locale:

N45°02.265’ W083°11.575’

The 3-meter thick outcrop of limestone ledges on Thunder Bay Island is best exposed near the lighthouse

(dating to 1832) and the bedding plane exposures extend over an extensive area just above the current

lake level. Water levels in 1874 when Rominger came to the island were roughly four feet higher than

today.

Observations, photos, video, rock and fossil samples taken by the Noble Odyssey – US Naval Sea Cadet

divers on a dramatic twenty foot submarine cliff just offshore and stratigraphically below the stromatoporoid

bearing land exposure confirms that this is the middle to lower part of the Rockport Quarry Limestone

Formation as inferred by previous workers (Keslng and Ehlers, 1970) based on the island's location and

Rominger's 1874 description of the strata. This stratigraphic position in the formation for the reef is also

supported by observations we made of the upper portion of the type section of the formation in the

remaining cut face of the abandoned Rockport Quarry, located north of Alpena.

NOF Diver (Kathy Trax) exploring lower Rockport Quarry Limestone along Thunder Bay Island.

Sea cadet diver collects fossil from the lower Rockport Quarry Limestone Formation.

The primary public education focus of the geologic research and filming in this area of the Thunder Bay

National Marine Sanctuary is focused on the crucial importance of limestone and related carbonate rocks

with emphasis on three areas:

1) Importance as a natural resource for the production of cement, aggregate and as flux in the

processing of iron ore;

2) The remarkable earth system history recorded in these rocks; ie. shallow tropical seas teaming

with coral reefs and other marine life from a time 385 million years ago when this region was in

the tropics just south of the equator as a result of plate tectonic movements;

3) The underappreciated fact that limestone, dolostone and related oceanic carbonate sediments are

the greatest reservoir of carbon in the earth system by several orders of magnitude. The

bicarbonate ion in calcite and dolomite ultimately came from carbon dioxide in the atmosphere and

it’s sequestering in rocks and sediments allows our planet to have atmospheric carbon dioxide

levels less than .05%, thus allowing for the existence of liquid water and life.

An exhibit on the Cranbrook-Noble Odyssey-US Naval Sea Cadet Prehistoric Reef Project in the main

lobby of the Institute will dovetail nicely in that space with an exhibit on modern reefs, Discovery Reef

from Mote Marine Lab that will run from the last week of August until October 5, 2007. A DVD of this

year’s CIS-NOF expedition is currently being compiled with an accompanying educational teacher guide.

Noble Odyssey Foundation and Great Lakes US Naval Sea Cadets Web Sites

http://www.greatlakesdivision.org/

http://www.nof.homestead.com/

Cited References

Ehlers, G.M. & Kesling, R.V., 1970, Devonian Strata of Alpena and Presque Isle Counties Michigan,

Guide Book, north-central section Geological Society of America.

Fenton, M. A., 1931, A Devonian Stromatoporoid Reef, American Midland Naturalist, Vol. 12, No. 7, 195-

202

Kesling, R.V., Segall, R.T., Sorensen, H.O. 1974, Devonian Strata of Emmet and Charlevoix Counties,

Michigan, Museum of Paleontology, Papers on Paleontology, no. 7 (Univ. of Michigan)

Martin, H., 1973, Glimpses into the Life of Carl Ludwig Romnger, Museum of Paleontology, Papers on

Paleontology, no. 5 (Univ. of Michigan).

Rominger, C. 1874, The Geology of the Lower Peninsula of Michigan, 1873 –1876, Geological Survey of

Michigan, vol. III.

Stock, C.W., 2001, Stromatoporoidea, 1926-2000 Journal of Paleontology; Nov 2001; 75, (6) 1079-89.

Stum, E.C., 1969, Devonian Bioherms of the Michigan Basin, Contrib. Mus. Paleont. U. Mich V.22, (18), p.

241-247

Wright, J.D. 1973, That Remarkable Man Carl Ludwig Rominger, State Geologist, Museum of

Paleontology, Papers on Paleontology, no. 4 (Univ. of Michigan).

*Project supported in part by:

MICHIGAN COASTAL MANAGEMENT PROGRAM

Michigan Department of Environmental Quality and theNational Oceanic and Atmospheric Administration