Stromatolites

Introduction

Stromatolites are among the oldest biosignatures in the geological record - layered structures built by microbial mats that trapped fine sediment, grew upward, trapped more sediment, and repeated the cycle over millions of years. They are not body fossils and not pure sedimentary structures, but something in between: organosedimentary structures where biology and sedimentation are inseparable. Their presence in ancient rocks older than 3 billion years documents the early diversification of photosynthetic life on Earth, long before any animal could burrow through the sediment that stromatolites were building.

What Stromatolites Are

Stromatolites are organically formed, laminated structures composed of fine silt- or clay-size sediment, or more rarely sand-size sediment. Most ancient stromatolites occur in limestones, though they have also been reported in siliciclastic sediments. ^[1]

Stromatolites were once considered true body fossils, but they are now understood to be organosedimentary structures formed largely by the trapping and binding activities of blue-green algae (cyanobacteria). They are forming today in many localities, occurring mainly in the shallow subtidal, intertidal, and supratidal zones of the ocean. They have also been found in lacustrine environments. Because they depend on cyanobacterial photosynthesis, they are restricted to water depths and environments where enough light is available. ^[1]

How the Laminated Structure Forms

The laminated structure forms because fine sediment is trapped in the very fine filaments of algal mats. Once a thin layer of sediment covers the mat, the algal filaments grow up and around sediment grains to form a new mat that traps another thin layer of sediment. This successive growth of mats produces the laminated structure. The laminations are generally less than 1 mm thick and are caused by concentrations of fine calcium carbonate minerals, fine organic matter, and detrital clay and silt. Stromatolites composed of quartz grains have also been reported. ^[1]

Morphological Types and Energy Control

Stromatolitic bedding ranges from nearly flat laminations to hemispherical forms in which the laminae are crinkled or deformed to various degrees. The hemispherical forms range in shape from biscuit- and cabbage-like forms to columns. Three basic types of hemispherical stromatolites are recognised: (1) laterally linked hemispheroids, (2) discrete, vertically stacked hemispheroids, and (3) discrete spheroids or spheroidal structures. Laterally linked and vertically stacked hemispheroids can combine in various ways to create compound stromatolites. Structures that resemble stromatolites in external form and size but lack distinct laminations are called thrombolites. ^[1]

The shapes of the hemispheres are controlled by water energy and scouring in the depositional environment. Laterally linked hemispheroids tend to form in low-energy environments where scouring is minimal - the individual heads can grow toward each other and link because currents do not separate them. In higher-energy environments, scouring by currents prevents linking, so vertically stacked or discrete hemispheroids form instead. ^[1]

This energy-morphology relationship is one of the more elegant examples of form following process in sedimentology. The shape of a stromatolite is not arbitrary; it is a direct record of the hydrodynamic conditions at the growth site. A geologist reading a Precambrian carbonate can therefore use the stromatolite type not only to confirm biogenic origin but also to estimate the relative energy of the ancient environment.

Geological Record

Stromatolites are forming in the world’s oceans at the present time and have been reported in ancient rocks as old as 3.45 billion years. ^[1]

The 3.45 billion year record makes stromatolites some of the earliest direct evidence of life on Earth. Their abundance through the Archaean and Proterozoic, followed by a dramatic decline in the Cambrian when grazing animals diversified, provides one of the clearest connections between biological evolution and the sedimentary record. In the modern ocean, stromatolites persist only in environments hostile to grazing organisms - hypersaline lagoons and extreme tidal flats - where the conditions that allowed them to dominate the ancient Earth are still reproduced.

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