Indonesia 2025. Day 16. Geologizing around Makassar

We boarded our Garuda Indonesia flight at 11 am and by 1 pm landed in Makassar, at the southwest corner of the island of Sulawesi. Waiting for us were our excellent friends form BMKG, who after the usual exchange of greeting left us in the hands of Pak Jamroni, the geologist/geophysicist who heads the Geophysical station here in Makassar (there is also the Meteorology station at the airport, and the Climatology station north of Makassar). 

Jamroni had enlisted the help of Hilmar (a born ProMod racer) to drive our expeditionary vehicle, so we were able to move through the heavy traffic going out of the city in record time. But first things first: Lunch. Since we had just arrived in this fair island, we had the regional dish, Coto Makassar, which is a tasty lamb soup that is accompanied by salsa, lime juice, and small rice tamales. It would be a great hit in Mexico!

Around here the big thing is the karst terrain that has developed on a thick sequence of limestones of late Oligocene to early Miocene in age, but we are putting that aside until tomorrow. The goal for this afternoon is to reach an outcrop of banded chert, which is another rare type of rock. We had to travel for nearly an hour and a half into the mountains, and in the meantime the weather turned menacing and we were in the middle of an equatorial downpour by the time we reached the top of a trail that descended to a stream where the chert was exposed among the rapids. It was a great outcrop, with the chert gently folded and adding its beauty to the Feng Shue of the running water. For a moment I was transported to my younger years, when my Field Geology Methods class traveled to the coastal jungle of eastern Mexico to map an area covered with heavy vegetation. The trick is to map the creeks, wading through the water from outcrop to outcrop to determine the stratigraphy and locate faults and fold axes.

So what is the big deal about banded chert? Well, it is a type of rock that forms on the deep ocean floor, and when we find it on land it indicates that rocks from the ocean floor have been accreted against continental rocks. In California this is observed in the Franciscan Melange, the tectonic mixture of rocks jammed against the subduction zone (disjointed blocks of shallow marine sandstone, pillow lavas, banded cherts, blueschists, and even eclogite). Jamroni had visited this particular chert outcrop as a geology student looking for pieces that could be polished to mount on class rings, and thought there were outcrops of schists nearby, and after Irvan asked Dr. Google we learnt that blueschists and eclogites have also been reported, so on the spot we decided we were looking at the Sulawesi Melange! By then we were sopping wet, so it was time to go back to search for a dry roof and a cup of coffee. I feel I can now call myself an Indio Necio, as my dear Raúl has so colorfully put it.