Stromatolites are the most common megascopic fossils, contained within ancient rocks dating to 3.5 billion years in age.
In both the living and fossil form, they are created by the trapping and binding of sediment particles and the precipitation of calcium carbonate to the sticky surface of mat like filaments grown on a daily cycle by blue-green algae (also known as cyanobacteria).
Modern stromatolites are found throughout the world; they are of particular use in the creation of hydrocarbon reservoirs, in geologic mapping, and as indicators of paleo environments.
Distribution of Stromatolites
In the geologic record, stromatolites are themost abundant of fossils found in rocks dating to the Precambrian era, that period of time from the origin of Earth (approximately 4.6 billion years ago) up to 544 million years ago. The oldest fossil stromatolites are contained in the 3.3-billion- to 3.5-billion-year-old Warrawoona group of rocks in Australia.
Close in age are the 3.4-billion-year-old stromatolites of the Swaziland group of South Africa. Somewhat more removed are those associated with the 2.5-billion- to 2.8-billion-year-old Bulawayan Limestone, also in Africa.
All these examples originated in the Archean eon, the first recorded period of geologic history, extending from approximately 4 billion to 2.5 billion years ago. During the Proterozoic eon, immediately following the Archean eon and extending to 544 million years ago, stromatolites became prolific.
This Proterozoic expansion is probably reflective of the initial development of continental landmasses and associated warm, photic continental shelf regions, as plate tectonics became a controlling process in the early development of the earth’s crust.
Throughout the Archean and Proterozoic eons, blue-green algae underwent a steady and progressive state of biologic evolution, recognized today as the singular, common megascopic fossil of the Precambrian time period. For this reason, the Precambrian is often referred to as the “age of algae,” or, more specifically, the “age of blue-green algae.”
Throughout the Phanerozoic eon, defined as 544 million years ago to the present, blue-green algae underwent minimal evolution, probably because their evolutionary state had become adapted to a variety of environments, reducing the need for further diversification.
Stromatolitic-building algae maintained their dominance of the aquatic world during the Early Phanerozoic eon (544 million to about 460 million years ago).
With the rather abrupt appearance, however, of shelled, grazing, and cropping invertebrates in the early Phanerozoic eon, blue-green algae began to decline in significance. Today, as compared to their Precambrian domination, they have, on a relative scale, become endangered.
Geographically, fossil stromatolites are ubiquitous on every continent, especially within sedimentary carbonate rock sequences older than 460 million years.
On southeastern Newfoundland, stromatolites built by blue-green algae of the genus Girvanella are found in conglomerate and limestone strata of the Bonavista Formation (approximately 550 million years in age).
In nearby Zambia, algal stromatolites are closely associated with rock sequences containing economic levels of copper and cobalt.
Upper Permian (250million years ago) stromatolite horizons can be traced over an area of northern Poland exceeding 15,000 square kilometers. Miocene age (15 million years old) algae of the species Halimeda compose the limestone-forming rocks of the island of Saipan in the Mariana Islands of the Pacific Ocean.
In North America, fossil algal-bearing rocks include the 2-billion-year-old Gunflint (Iron) Formation of Ontario, Canada, and the well-developed stromatolitic horizons of Early Paleozoic era (450 million years ago) composing the Ellenberger Formation of Oklahoma and Texas.
Classification and Identification
The classification and identification of stromatolites are often concluded on the basis of overall morphology, particularly the size, shape, and internal construction of the specimen.
The relevant literature makes use of a variety of morphological terms, including the adjectives “frondose” (leaf-like), “encrusting,” “massive,” “undulatory” (wavelike), “columnar,” “laminar,” “domed,” “elliptical,” and “digitigrade” (divided into finger like parts). Through the study ofmodern blue-green algae, it is suggested that three environmental criteria are of importance in stromatolite geometry development.
These are direction and intensity of sunlight, direction and magnitude of water current, and direction of sediment transport. As an example, the extant elliptical stromatolites of Shark Bay, western Australia, are oriented at right angles to the shoreline as the result of strong current-driven wave and scour action.
Under certain environmental conditions, cyanobacteria growth surfaces are not preserved, producing fossil algal structures characterized by a lack of laminae (leaf blades). These structures are termed thrombolites, in contrast to laminar-constructed stromatolites.
While stromatolites are generally described as megascopic in size, discussion of specific dimensions relates both to laminae thickness and to overall size. Stromatolite laminae of the Precambrian-aged Pethei Formation, an outcropping along the shores of Great Slave Lake in the Northwest Territories of Canada, are both fine and coarse in dimension.
The coarse-grained layers, formed of lime-mud pellets and calcium and magnesium carbonate rhombs, are principally less than 5.0 millimeters in thickness. The fine-grained laminae, composed of calcium carbonate clay and silt-sized particles, are, on average, only 0.5millimeter thick.
Subspherical varieties of stromatolite-like structures, formed by the accretion of successive gelatinous mats of blue-green algae and generally less than 10 centimeters in diameter, are termed oncolites.
Stromatolitic complexes in the Great Slave Lake district measure 80 meters long by 45 meters wide by 20 meters in thickness and can be continuously traced for distances exceeding 160 kilometers.