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Textures of Carbonate Sediment and Cement Associated with Ubiquitous Microbial Mats: Nearshore and Lacustrine Systems, Holocene of Kiribati


To better understand the nature and distribution of microbial mat-associated sediment and cement, this study explores Holocene lacustine and nearshore systems on two atolls in Kiribati, a central Pacific nation. On Aranuka (0.1° N) and Beru (1.3° S) atolls, both within the equatorial upwelling zone, extensive mats occur in two general settings: nearshore zones and coastal lakes. Unlithified mats occur on or proximal to the foreshore on both atolls, most commonly on the western sides of islands or slightly sheltered areas. Foreshore mats form gently (<5°) dipping surfaces 5-10 m wide, topographically below beaches and above bare or seagrass-covered sediment. These regions are covered with smooth, laminated mats, clasts of smooth mats, or pustular mats. Mats in the intertidal upper shoreface most commonly inhabit rocky substrates and form layers up to 5-10 cm thick that cover hundreds of m2. Mats in these settings have a diversity of morphology, from smooth to pustular to mudcracked. In both foreshore and shoreface examples, thin sections illustrate broadly parallel, close-packed silt with dark sheathes that alternates with more loosely packed laminae with no sheathes. Coastal lakes (∼1 and ∼3 km2 area) occur on Beru. These hypersaline shallow (< 2 m) lakes are not directly connected to the open ocean, but are covered with abundant unlithified microbial mats and evaporites. The morphology of mats varies from unstructured gelatinous ooze that binds the sediment surface to leathery to cohesive and weakly to moderately laminated. In one lake, the laminated mat substrate reveals a range of features and textures. In addition to local teepees and undulatory beds, cm-scale features include subparallel, arborescent, and arbustiform geometries. Petrographic and SEM analyses of each reveal mm-scale continuous to discontinuous alternations of micritic, peloidal, botryoidal “cementstone,” and spherulitic layers; calcified filaments are evident in some laminae. At the largest scale, microbial mats are ubiquitous, and occur in a spectrum of environments on these atolls, interpreted to be favored by equatorial upwelling and nutrient-rich waters. The close association of mats with sediment with calcified filaments and cement suggests a genetic link, although mechanistic details are presently being investigated.