Burdigalian turbid water patch reef environment revealed by larger benthic foraminifera

Ancient isolated patch reefs outcropping from siliciclastic sediments are a trademark for the Miocene carbonate
deposits occurring in East Kalimantan, Indonesia. They develop in transitional shelf sediments deposited between
deltaic and deep marine deposits (Allen and Chambers, 1998). The Batu Putih Limestone (Wilson, 2005) and similar
outcrops in adjacent areas have been characterized as shallow water carbonates influenced by high siliciclastic
input, showing low relief patch reefs in turbid waters. Larger benthic foraminifera (LBF) are excellent markers
for biochronology and paleoenvironmental reconstruction. This study aims to reveal age and paleoenvironment
of a shallow water carbonate patch reef developed in mixed depositional system by using LBF and microfacies
analysis.
The studied section is located near Bontang, East Kalimantan, and is approximately 80 m long and 12 m high.
It is placed within Miocene sediments in the central part of the Kutai Basin. Patch reef and capping sediments
were logged through eight transects along section and divided into nine different lithological units from which
samples were collected. Thin sections and isolated specimens of larger benthic foraminifera were analyzed and
recognized to species level (where possible) providing age and environmental information. Microfacies analysis
of thin sections included carbonate classification (textural scheme of Dunham, 1962) and assemblage composition
of LBF, algae and corals relative abundance. Three environmentally indicative groups of LBF were separated
based on test morphology, habitat or living relatives (Hallock and Glenn, 1986).
Analysed foraminifera assemblage suggests Burdigalian age (Tf1). With use of microfacies analysis nine
successive lithological units were grouped into five facies types. Paleoenvironmental reconstruction of LBF fossil
assemblage indicate two cycles of possible deepening recorded in the section. Based on high muddy matrix
ratio in analyzed thin-sections we still cannot conclude whether they were deeper water assemblage, or that they
occurred in shallower water and influenced by turbid conditions as the result of terrigenous input. According to
preliminary analysis and siliciclastic content in the sediments the later one should be more likely. Further work
will include additional fossil groups analysis (corals, algae and bryozoans), detailed petrographical analysis and
Strontium isotope stratigraphy.