Algal Mound Depositional Environments

Algal mounds composed of the green algae Halimeda form distinct bodies in Indonesia and Florida Bay. Similar algal mounds, common in the ancient record, are composed of different types of green algae. As the algae grows, it forms a baffle to mud, and the mud is trapped between the algae. When the algae die, their carbonate component is added to the sediment as sand.

Algal Mounds from Offshore Mahakam Delta in Indonesia

An excellent example of a modern analog to algal mounds was presented by Roberts et al. (1987) from the offshore area of the Mahakam Delta in Indonesia (see photos below). The mounds are tens of meters high and cover tens of square kilometers at the platform margin and in the platform interior.

Sparker data and side-scan sonar of Halimeda mounds (bioherms) from Indonesia. The mounds occur in the platform interior and are several tens of meters high. Data courtesy of Harry Roberts.

A soft-sediment core sample from the Halimeda mound showing accumulated Halimeda plates. Photograph courtesy of Harry Roberts. Click in the red box to see thin-section example.

Rock Types

Rock types that characterize this facies range form algal wackestone, packstone, grainstone, to bafflestones. The original mineralogy of the algae was aragonite, which undergoes dissolution during diagenesis, resulting in moldic porosity if the mold is not later filled with cement.

Pennsylvanian green algal bafflestone. Algal plates formed a protected environment in which the mud could settle and be preserved. Bar scale at left in inches.

NEXT