Giant stromatolites of the Eocene Green River Formation (Colorado, USA)

The Eocene Green River Formation in the Sand Wash Basin, Colorado (United States), contains the largest known lacustrine columnar stromatolites. Spectacular columns, as much as 5.5 m high with diameters of 7 m, occur over a broad area. The stromatolites are composed of laterally continuous centimeter-thick layers that can be traced from the base to the top of the column (synoptic relief), thus these stromatolites stood as much as 5.5 m above the lake floor. The layers consist of one to several different kinds of microbialites, which makes these large columns even more unusual. The giant columns were the result of a combination of factors including the lake transgressing a flooded woodland, in situ tree stumps providing elevated substrates above the lake floor, an abundant supply of calcium-rich spring and surface water that mixed with saline-alkaline lake water, and the subsidence rate of the nearshore lake environment not exceeding the rate at which the stromatolites grew.