1 Prehistoric Tropical Reef Project 2007: Thunder Bay Island 6-29 to Thunder Bay National Marine Sanctuary - In the footsteps of Carl Rominger. A preliminary report from the field by Cranbrook Institute of Science Geologist John Zawiskie Corals in storm layer in black Rockport Quarry Limestone, south Thunder Bay Island. The exuberance of fossils in the strata is most beautifully exhibited in the shoals of the lake north 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, their white, sparry substance contrasting beautifully with the dark limestone that encloses them. Carl Rominger The Geology of the Lower Peninsula of Michigan, , 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
2 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). 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 within the structure. 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 sparingly intermingled with other fossils. Carl Rominger The Geology of the Lower Peninsula of Michigan, , 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.
3 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: N W 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.
4 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
5 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, 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 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, 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, , Geological Survey of Michigan, vol. III. Stock, C.W., 2001, Stromatoporoidea, Journal of Paleontology; Nov 2001; 75, (6) Stum, E.C., 1969, Devonian Bioherms of the Michigan Basin, Contrib. Mus. Paleont. U. Mich V.22, (18), p 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 the National Oceanic and Atmospheric Administration