Hallstatt's Dachstein-Kalk Limestone Formation
By Heidi Henderson | May 2nd 2020 02:12 PM | Print | E-mail
The Hallstatt Limestone is the world's richest Triassic ammonite unit, yielding specimens of more than 500 ammonite species. Along with diversified cephalopod fauna — orthoceratids, nautiloids, ammonoids — we also see gastropods, bivalves, especially the late Triassic pteriid bivalve Halobia (the halobiids), brachiopods, crinoids and a few corals. For microfauna, we see conodonts, foraminifera, sponge spicules, radiolaria, floating crinoids and holothurian sclerites — polyp-like, soft-bodied invertebrate echinozoans often referred to as sea cucumbers because of their similarities in size, elongate shape, and tough skin over a soft interior. Franz von Hauer’s exhaustive 1846 tome describing Hallstatt ammonites inspired renowned Austrian geologist Eduard Suess’s detailed study of the area’s Mesozoic history. That work was instrumental in Suess being the first person to recognize the former existence of the Tethys Sea, which he named in 1893 after the sister of Oceanus, the Greek god of the ocean. As part of the Northern Limestone Alps, the Dachstein rock mass, or Hoher Dachstein, is one of the large karstic mountains of Austria and the second-highest mountain in the Northern Limestone Alps. It borders Upper Austria and Styria in central Austria and is the highest point in each of those states. Parts of the massif also lie in the state of Salzburg, leading to the mountain being referred to as the Drei-Länder-Berg or three-state mountain. Seen from the north, the Dachstein massif is dominated by the glaciers with the rocky summits rising beyond them. By contrast, to the south, the mountain drops almost vertically to the valley floor. The karst limestones and dolomites were deposited in our Mesozoic seas. The geology of the Dachstein massif is dominated by the Dachstein-Kalk Formation ("Dachstein limestone"), which dates back to the Triassic.