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Earth has an astonishingly long geologic time span, but evidence of early life forms on our planet can still be seen today in the form of distinctive bodies of rock called stromatolites.
The Wyoming State Geological Survey (WSGS) at the University of Wyoming has published a new field guide, titled “Self-guided Walking Tour of the Paleoproterozoic Stromatolites in the Medicine Bow Mountains, Wyoming,” which provides a guided tour to many of the best outcrops found in the Medicine Bow Mountains, located west of Laramie.
“Our guided tour will take people to some of the best examples of ancient stromatolites in the world, found right here in southeastern Wyoming,” says co-author Don Boyd, professor emeritus with the UW Department of Geology and Geophysics.
The WSGS website features a stromatolite page containing the downloadable report, an interactive Google Earth map of the tour stop locations (with photos) and a video with additional photos of each stromatolite outcrop included in the guide.
The 24-page color guide includes photos, illustrations, maps and GPS coordinates to lead users to stromatolite outcrops dotting the high-alpine landscape. A stromatolite looks like a cross between a cauliflower and a rock. The delicate lamination and internal structure can be seen on the weathered surface of rocks.
“The picturesque patterns were brought into relief by weathering during thousands of years of exposure since the last glacial event,” Boyd says.
“We created this walking tour to satisfy the many people who have heard about these unusual life forms found in the rocks, but did not know where to look,” says co-author David Lageson, professor of geology at Montana State University.
Based on a comparison with similar features forming today and the work of geologists studying similar Precambrian structures, the co-authors conclude that the 2 billion-year-old Medicine Bow stromatolites were built by communities of bacteria and bacteria-like organisms that dominated a shallow marine environment long before an oxygen-rich atmosphere and the appearance of animals.
“In our interpretation, the distinctive layering of a stromatolite was created by repeated colonization of a sea-floor mound by microbial mats that both trapped sediment and precipitated cement,” Boyd says. “The unequal contribution of organic and inorganic processes produced a diversity of stromatolite shapes and sizes.”
The primary organism that built the stromatolites is believed to have been cyanobacteria, which are prokaryotic bacteria (domain of life Eubacteria). As photosynthesizers, they played a major role in oxygenating the Earth's oceans and atmosphere.
Boyd and Lageson’s field guide describes a wide variety of stromatolites.
“Some are classic microbial growth structures of various shapes and sizes, and are typical of similar forms found in Precambrian and younger rocks in other parts of the world,” Lageson says. Others, however, tell a different story. “It depends on the stromatolite being observed.”
Included in the field guide are directions to representative outcrops with descriptions of stromatolite features of interest at each location. The geology behind these ancient records of life on Earth is described.
Wyoming's Paleoproterozoic Nash Fork Formation, the major unit in which the stromatolitic beds occur in the Medicine Bow Range, is about 1.2 miles thick and consists of tan stromatolite-bearing dolomite with thick interbeds of pyritic black argillite and phyllite, and some quartzite. Stromatolitic zones are most common in the lower 700 meters of the Nash Fork Formation; they are found in massive dolomite and silicified dolomite intervals. The largest stromatolites (true giants) are found in the “silicified domal digitate stromatolite facies association” in the lower Nash Fork Formation between 100-200 meters and 300-400 meters from the basal thrust fault contact.
Well-known Wyoming geologist S.H. Knight extensively studied the Nash Fork Formation stromatolites, producing research that garnered major international attention. The walking-tour guide includes many of the outcrops illustrated in Knight's research paper published in 1968. Together with most of his contemporaries, Knight believed stromatolites exhibit the original size and shape produced by the organisms that built them.
While Boyd and Lageson agree that this is true for some of the Medicine Bow stromatolites, they describe abundant evidence at outcrops visited in the tour for major alteration of stromatolite shape and dimensions by post-depositional processes, such as soft-sediment deformation (sliding and slumping) and perhaps storm events.
“Our intent with this field guide is for users to ponder the evidence as they visit each stromatolite outcrop,” Boyd says.