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Algal mats and algal-laminated sediments are present in the protected intertidal environment of the Gladstone embayment, a small reentrant on the east coast of Shark Bay, Western Australia. Three morphologic types of algal mats are recognizable—tufted, convoluted, and smooth. Thick sequences of flat-laminated sediments are formed beneath smooth algal mats, whereas sediments below other mat types are poorly laminated or unlaminated.

Algal-laminated sediments are composed of mechanically deposited particles of clastic origin which have been bound by a film of filamentous and unicellular algae. The algal-laminated sediments in the Gladstone embayment are essentially flat-laminated; they lack the regular development of large-scale structures which characterize algal stromatolites as defined by Logan.

Flat algal-laminated sediments develop several small- scale structural and textural features which form during or after deposition. Primary (depositional) features include algal-filament molds, graded bedding, cyclic bedding, and storm deposits. Secondary (postdepositional) features include structures formed by gas movement and accumulation, desiccation, and organisms.

Indurated sediments associated with algal-laminated sediments are classified as crusts and wall rock. Both indurated sediment types are characterized by aragoni- tic mineralogy, pelletal textures, and fenestral fabrics. Crusts in the form of pavements and “flat-pebble conglomerates” occur in the inner areas of the intertidal zone and in the supratidal zone, whereas wall rock forms “spur-and-groove” structures in the intertidal zone.

Criteria for the recognition of algal-laminated sediments are the combination of clastic texture, the presence of steeply inclined, vertical or overfolded laminae, algal-filament molds, and association with sediments with pelletal textures and fenestral fabrics.

An important environmental factor in the development of algal mats and laminated sediments in the Gladstone embayment and the southern basins of Shark Bay is the high salinity of the basinal waters. Salinity of water in these areas ranges between 55 and 65 ‰ in summer. The significance of algae as mat-forming and sediment- binding organisms in intertidal environments decreases toward the more open, northern waters of Shark Bay where salinity is lower.

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