Larger benthic foraminifera – faunal pioneers in mixed carbonate-siliciclastic systems

Larger benthic foraminifera are one of the most abundant and widespread organisms in shallow marine tropical
environments. Their role as rapid colonizers of new substrates has been suggested, but little is still known to support
this idea. To address this question, larger benthic foraminifera assemblages have been studied in the
paleoenvironmental reconstruction of a Langhian (Middle Miocene) shallow-water patch reef, as part of a broader multitaxon
fossil comunity analysis, including corals, coralline algae, and bryozoans. The studied outcrop (approx. 80 m
wide and 25 m thick) is located at the northeast margin of the Kutai Basin near Bontang (Indonesia).
The patch-reef was developed in mixed carbonate-siliciclastic depositional environment and despite the high
terrigenous input affecting water transparency, this reef contains a diverse marine biota. Five different facies types were
distinguished based on lithology and fossil content: foraminifera packstone, bioclastic packstone with foralgal
communities, thin-platy coral sheetstone, platy-tabular coral platestone, and shales. Excluding the fossil barren shales
facies, only larger benthic foraminifera occur in all four remaining facies types, showing the ability to rapidly adapt to
changes in environment. In the current study, high siliciclastic input caused fluctuations in light levels, but also resulted
in deposition of soft-bottom substrate. In newly developed conditions, the larger benthic foraminifera appear first and
mark the initiation of reef growth.
The persistence of LBF throughout the fossil reef succession indicates their high tolerance to terrigenous input and also
highlight the role of foraminifera as primary colonizers, proving themselves as important faunal pioneers, particularly in
mixed carbonate-siliciclastic systems.