Stromatolites

This is the geological term used for a laminated benthic microbial deposit (Riding 1999), for which an origin from a bacterial mat is often inferred. In fossil stromatolites, the organic and trapped sediment layers are usually seen as alternating or intergrading pale and dark lamellae (Fig. 8.5). Preservation of filaments is rare but is known in silicified and fine-grained carbonate stromatolites. More often, only the sheaths or the upward gliding trails are preserved. In most cases, however, stromatolites preserve no relics of an organic origin.

The gross form of a stromatolite is controlled by a combination of factors including mat viscocity (e.g. from mucilage, which may reflect the biological components of the mat) and surface roughness of the mat (e.g. from grain size, which may reflect current energy and sediment supply). Precambrian palaeontologists often employ a binomial system of nomenclature in the description of stromatolites. Groups (= genera) are based on a general shape (planar, domed, columnar, oncolitic), mode of branching (straight, digitate), morphology of the 'wall' (i.e. marginal zone) and

Branched (digitate) columnar.

Straight columnar

Branched (digitate) columnar.

Straight columnar

Stromatolite Digitate Structure

Tangled calcareous tubes

Fig. 8.5 Cyanophyte sedimentary structures. (a) Stromatolite types in vertical section, xl. (b) Girvanella tubes in skeletal oncolite. (c) Ortonella tubes in skeletal oncolite. (d) Section through endolithic cyanophyte borings and skeletal envelopes (diagrammatic). Scale bars = 100 |m. ((d) Based on Kobluck & Risk 1977.)

Tangled calcareous tubes

Vacant boring '

Micrite filled boring7

Fig. 8.5 Cyanophyte sedimentary structures. (a) Stromatolite types in vertical section, xl. (b) Girvanella tubes in skeletal oncolite. (c) Ortonella tubes in skeletal oncolite. (d) Section through endolithic cyanophyte borings and skeletal envelopes (diagrammatic). Scale bars = 100 |m. ((d) Based on Kobluck & Risk 1977.)

geometry of the laminae (Fig. 8.5a). Forms (= species) are distinguished by microscopic textures and lamina geometry.

Skeletal stromatolites differ from the non-skeletal stromatolites in that the form of the cells or sheaths has been moulded in CaCO3, giving rise to micritic tubes within a micritic or sparry calcite internal filling. The tubes appear to represent calcification of the sheath, such as can occur during life in certain autotrophs because of CO2 uptake during photosynthesis. Postmortem calcification is also possible however, and this possibility is suggested by stratigraphical intervals during which such calcification seems to have been more widespread (Kazmierczak 1976). Such skeletal stromatolites are known from both freshwater and marine waters. In Girvanella (Fig. 8.5b, L. Camb.-Rec.) the tubes are tangled and unbranched and occur in both oncolites and thrombolites. Ortonella (Fig. 8.5c, L. Carb.-Perm.) is an oncolitic form with branched tubes.

Thrombolites differ from stromatolites in lacking the internal laminations, having instead a mottled or clotted microtexture. They were probably built by coccoid cyanobacteria (e.g. Renalcis, Fig. 8.3f) or by filamentous forms with wispy, tufted, branched or tangled growth rather than vertical growth. Thrombolites are typically found in sublittoral, often calcareous facies in association with reef-dwelling invertebrates.

Travertine develops in CaCO3-supersaturated waters in which coccoid and filamentous cyanobac-teria may become encrusted by physicochemical precipitation of CaCO3, forming hollow tubes. In this case, however, the crystals greatly exceed the diameter of the original organic sheath. The moulds left by such fossilization are of little taxonomic value. During the Phanerozoic, travertine stromatolites have largely been confined to fresh or extremely hypersaline waters. Many Early Precambrian (Archean) stromatolites are actually marine travertine. This suggests that there have been long-term changes in the chemistry of the oceans and atmosphere.

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