Stromatolites

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 forms, they are created by the trapping and binding of sediment particles and the precipitation of calcium carbonate to the sticky surface of matlike filaments grown on a daily cycle by blue-green algae. Modern stromatolites are found throughout the world and are of particular use in the creation of hydrocarbon reservoirs, in geologic mapping, and as indicators of paleoenvironments.

Stromatolite-Producing Algae

Stromatolites (“layered rock” in Greek) are organosedimentary structures associated with certain types of the sedimentary class of rock; they develop through the metabolic processes of plant microorganisms. Stromatolites can also be considered to be fossils. In this sense, they do not represent the remains of actual organisms but rather the remains of material deposited and collected by a living organism in a manner such that the original size, shape, and morphology of the organism are preserved.

The organisms principally responsible for the development of stromatolites are photosynthetic cyanobacteria, commonly referred to as “blue-green algae.” Structurally, blue-green algae are among the most primitive of this class of life, lacking true stems, roots, and leaves. They grow on a daily cycle in response to the rising and setting of the sun. In response to this cycle, a sticky, filamentous, organic surface is produced in the form of wavy, matlike laminae. This mucus-coated mat traps and binds fine- and coarse-grained sediment to its surface during daylight hours, increasing the thickness of the laminae. During evening hours and nighttime, new algae growth penetrates the sediment-coated layer, producing a new gelatinous filament that in turn will trap and bind more sediment during the following daylight period. As this process continues, a layered, or laminated, structure is built.

Blue-green algae are prokaryotic (prenuclear) microorganisms, as they possess neither a cell nucleus nor a specialized cellular organelle (that portion of a cell that functions as an organ). Blue-green algae are also asexual and thus restricted in their degree of variability, resulting in many living species that are almost indistinguishable from species that lived more than 1 billion years ago. Because of this relative lack of evolution over geologic time, much of what is known regarding ancient stromatolites has been gained by the study of living stromatolites produced by blue-green algae. The mineral matter associated with extant stromatolites is primarily composed of calcium carbonate, although a few species are associated with siliceous material, as evidenced by stromatolites studied in Yellowstone National Park, Wyoming. In the fossil form, the original calcium carbonate content is sometimes altered through replacement by magnesium, iron, and silica.

Modern blue-green algae have a wide geographic distribution and grow in diverse aquatic environments, including salt water, fresh water, and even moist soils. Some blue-green species thrive in hot springs and geysers with temperatures only a few degrees below the boiling point, while others have been collected in Arctic and Antarctic regions. Regardless of their aquatic environment, all modern blue-green algae depend upon sunlight for growth and survival. Because of this need for light, it was formerly thought that marine varieties were restricted to shallow waters near shore. However, it is now known that the photic zone—that region of the ocean penetrated by sunlight—extends to depths approaching 150 meters (492 feet), allowing growth of deep-water varieties.

Stromatolite-producing algae are found in aquatic environments that are generally hostile to grazing and burrowing invertebrates. Shark Bay in western Australia, a classic site for the study of modern (living) stromatolites, is conducive to blue-green algal growth because the hypersaline environment limits the activity of grazing snails. In Yellowstone National Park, alkaline waters up to 59 degrees Celsius (128 degrees Farenheit) in temperature present an environment supporting the development of unique siliceous stromatolites produced by bacteria rather than by algae. In the shallow marine environment of the Bahama Banks in the Caribbean, blue-green algae are abundant in supertidal channels with current flow too strong for the effective colonization of stromatolitic-grazing invertebrates such as gastropods and ostracods.

Distribution of Stromatolites

In the geologic record, stromatolites are the most 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 less 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 by definition 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 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 (the Archean and the Proterozoic eons). 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 of shelled, grazing, and cropping invertebrates in the early Phanerozoic eon, however, 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 the Transvaal region of South Africa, delicately banded stromatolitic structures compose one of the most widespread of early Proterozoic shallow-water carbonate deposits in the world, extending over an area exceeding 100,000 square kilometers (62,137 square miles). In nearby Zambia, algal stromatolites are closely associated with rock sequences containing economic levels of copper and cobalt. Upper Permian (250 million 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 age (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” (leaflike), “encrusting,” “massive,” “undulatory” (wavelike), “columnar,” “laminar,” “domed,” “elliptical,” and “digitigrade” (divided into fingerlike parts). Through the study of modern 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. 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 millimeters (0.2 inches) in thickness. The fine-grained laminae, composed of calcium carbonate clay and silt-sized particles, are, on average, only 0.5 millimeter (less than 0.02 inches) thick.

In size, individual stromatolites can range from centimeters up to several meters. Fossils of the common Precambrian genus Conophyton occur in a range of sizes, from pencil-sized shapes to columns up to 10 meters (33 feet) in diameter. Subspherical varieties of stromatolite-like structures, formed by the accretion of successive gelatinous mats of blue-green algae and generally less than 10 centimeters (4 inches) in diameter, are termed “oncolites.” Stromatolitic complexes in the Great Slave Lake district measure 80 meters (263 feet) long by 45 meters (148 feet) wide by 20 meters (66 feet) in thickness and can be continuously traced for distances exceeding 160 kilometers (100 miles).

Principal Terms

algae: primitive, one-celled, chiefly aquatic plantlike organisms lacking stems, roots, and leaves

blue-green algae: any of the algae classified within the division Cyanophyta

cyanobacteria: algaelike bacteria commonly known as “blue-green algae”

fossil: the remains, trace, or imprint of any plant or animal that lived during the geologic past

oncolite: an organosedimentary rock structure, concentrically laminated, formed by blue-green algae and smaller in size than a stromatolite

organosedimentary structure: a sedimentary rock feature developed by the life processes of blue-green algae

prokaryote: an organism characterized by simple protoplasmic structure and lacking a nucleus

stromatolite: an organosedimentary structure formed by the trapping, binding, or precipitation of sediment upon the laminae surface of blue-green algae

thrombolite: a stromatolite-like fossil characterized by an obscurely clotted, unlaminated internal structure

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