A Carboniferous world exposed
In the Bago Region of Myanmar, the digging of the Kyaukkyi Lead-Zinc Mine has provided a rare record of the deep past. The mine's tunnels slice through a thick sequence of sedimentary rocks known as the Lebyin Group, which formed during the Carboniferous Period. These layers of mudstone, slate, and greywacke were deposited on the floor of a shallow sea roughly 300 million years ago. At that time, this piece of Earth's crust was part of a small continental block called the Sibumasu Terrane. This terrane was located on the margin of the supercontinent Gondwana, in a position much farther south than its current location and within a cool-water environment. The layers of sediment, now hardened into rock, preserve a detailed record of this ancient marine environment.
The fossils that tell time
The mudstone layers within the Kyaukkyi mine are particularly full of fossils that confirm the age and environment of the deposits. The most significant are brachiopods, marine animals with two-part shells that were exceptionally abundant during the Paleozoic Era. Several species have been identified in the wider region's Carboniferous rocks, including those from the genera Spinomartinia, Aulacella, and Chonopectus. These creatures were filter feeders, attached to the seafloor, and their shells accumulated in the soft sediment after death. Because brachiopod species evolved and went extinct over specific time intervals, their presence allows geologists to date the rocks with considerable precision, a technique called biostratigraphy. The specific assemblage of fossils found here points to an Early Carboniferous (Mississippian) age for many of the deposits, confirming they were laid down when this part of the world was submerged beneath the sea.