STROMATOLITES

strōma, mattress, bed, stratum lithos, rock Means “ stony carpet ” Are considered as accretionary structure. Formed in shallow water by trapping, binding and cementation. Biofilm microorganisms (such as cyanobacteria) used as binding agent in cementation.

S TROMATOLITES AND T HROMBOLITES Buildups of microbial mats that form in limestone- or dolostone-forming environments. Together with oncoids (formerly called "algal biscuits" or "Girvanella"), they typically form by the baffling, trapping, and precipitation of particles by communities of microorganisms such as bacteria and algae.

O RIGIN

M ORPHOLOGY Exist in the following forms: 1. Conical 2. Stratiform 3. Branching 4. Domal 5. Columnar Occur mostly in the Precambrian and are rare today. Few contains fossilized microbes. While some are results of biological activity. Most were considered to form by abiotic precipitation.

F OSSIL R ECORD Abundant during the Precambrian Period. Archean fossil remains are presumed to be colonies of cyanobacteria. Younger (that is, Proterozoic) fossils maybe premordial of the eukaryote chlorophytes. Stromatolites are a major constituent of the fossil record for about the first 3.5 billion years of life on earth.

S TRATIGRAPHY Become dimishingly uncommon in the Archean. Have higher production during the Proterozoic than the Phanerozoic eon. The north, central U.S. and Wisconsin and Minnesota particularly have much old rock, and not surprisingly, considerable stromatolites and banded iron.

B ANDED IRON F ORMATION Are massive, laterally extensive and globally distributed chemical sediment deposits. That consist primarily of Fe-bearing minerals (iron oxides) and silica.

M ODERN O CCURRENCE Are mostly found in hypersaline lake and marine lagoons. Are only known to prosper in open marine environment. Also occurs in freshwater environment where the waters are rich in magnesium and those stromalites are made of hydromagnesite Mg 5 (CO 3 ) 4 (OH) 2.4H 2 O

Discovery of ancient stromatolites on Lake Superior's shore astounded the scientific world, reestablishing the age of the oldest known fossils. These stromatolites are visible as rings in an outcropping of Gunflint formation chert near Schreiber, Ontario.

Kona dolomite at Marquette, Michigan, clearly shows flat layers of small stromatolites about 1 cm tall.

Stromatolites, formed from layers of blue-green algae, are the world's oldest fossils.

Pre-Cambrian stromatolites (commonly referred to as "Mary Ellen jasper") on display at the North American Museum of Ancient Life. The fossil was originally excavated in the Iron Range of Minnesota.

Stromatolites are considered the most intriguing fossils that are our window into deep time on earth, the emergence of life, and the eventual evolving of the beautiful life forms from Cambrian to modern time.

In cross-section these stromatolites have a conspicuous curved layering

These compare well with excavated ancient stromatolites found around Marble Bar in Western Australia dated at 3.45 billion years:

An example of this BIF rock in which the red is rich in hematite

The first eukaryotic life forms may be as old as 2 billion years ago. Grypania spiralis has been found in ancient rocks in Michigan. This fossil is preserved because it formed formed simple shells:

Gracefully coiled filaments are easily seen by the naked eye in these 2.1- billion-year-old fossils from the Upper Peninsula's Empire iron mine. They are possibly the oldest "megascopic" formations of life forms ever found. TSU-MING HAN, CLEVELAND

A microscopic cross section of columnar stromatolite at the Mary Ellen Mine, Biwabik, Minnesota.

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