Imagine that you are, here, 400 millions years ago. No road, no hills, no rocks...but water above and around you. You are on the floor of an ancient ocean!
On this oceanic floor, you are surrounded by strange living forms from 5 to 20cm high which looks like sponges. They are calcareous sponges characterized by skeletons composed entirely of calcium carbonate spicules (needlelike structures) and are part of the “Calcarea” sponge family.
The scientific name of these calcareous sponges is: “stromatoporoids”. Stromatoporoids were the primary reef - building organisms 470 millions years ago (Middle Ordovician) until almost being entirely wiped out 372 millions years ago in a great mass extinction (the so - called Late Devonian extinction which consisted mainly of two extinction pulses, one about 372 mya and the other about 359 mya) - a planetary event during which nearly 22% of all the 'families' of marine animals (largely invertebrates) were eliminated. Through time, the sediments of this ancient ocean were transformed in dark grey siliceous limestone which constituted this so special site of Cao Bang UNESCO Geopark. If you look more closely this dark limestone, you will see inside, small white laminae often connected together with vertical pilars, creating a global aspect of white red.
Calcarea
Stromatoporoid fossils
These are the fossil remains of the stromatoporoids's thick skeleton of calcium carbonate (calcite). More recently, the rocks in the area were fractured into zones up to several tens of meters wide and strongly dissected by recent movements of the movements of the earth's crust (Neotectonic activities).
Due to falling blocks, this site is not safe. So, please don't cross the road and observe only the rocks placed closed to this panel and try to meet this strange and fascinating fossil which was abundantly living, there, in an ocean 400 millions years ago!.
Cao Bang Geopark
