Fossils of the Carpathian Region

4. The Cretaceous

143 The last period of the Mesozoic era (denoted with a K in geological abbreviations), lasted for about 80 million years and was named after a special rock, “chalk” (from the German word Kreide), that formed in large amounts at this time. Although the specific Cretaceous “writing chalk” formed from the remains of masses of chrysophyte algae familiar across northwestern Europe is not found in the Carpathian Basin, formations of other Cretaceous-aged deposits are nevertheless common. This originally bi-, or tripartite (recently declared officially bipartite) period is usually divided into 12 stages by geologists. Most stages—just like those of the Jurassic —were named after (usually French) villages. The Cretaceous , following the relatively calm Jurassic, was one of Earth history’s most eventful periods from a tectonic point of view. Because of this activity, the Cretaceous layers in the Carpathian Basin are very diverse, and include rocks of continental and marine (both nearshore and offshore) origin. The biotas of the Cretaceous, thanks to this environmental variation, were also very rich and are evidenced in the fossil assemblages known from this period. The massive extinction that marks the end of this period was a milestone in the evolution of life on Earth. It marks not only the end of the Cretaceous and the end of the Mesozoic , but the beginning of the Cenozoic. During the Cretaceous, the drift and breakup of continents increased in speed. Parallel with the opening of a southern basin in the Atlantic Ocean, Africa began moving and rotating in a counterclockwise direction. Because of this movement, the Tethys seaway gradually became narrower and the formation of mountains in the Alp-Carpathian region also began: thick layers of rock laid down at earlier times were shortened, and thrusting (formation of geological structures called nappes) began in the Carpathian Basin. Also as a result of these movements , oceanic sea levels changed several times during the Cretaceous. With such diversity in environmental conditions, many kinds of sediment were deposited encompassing continental, shallow marine and deep marine (pelagic). The climate of the Cretaceous, although similar to that of the Jurassic, was warmer and experienced less fluctuation in temperature than we see today. THE MAJOR EVENTS OF AN EVENTFUL PERIOD The events described above resulted in spectacular changes to the western basin of the Tethys Ocean, which practically closed during the Cretaceous. In its place—the Western and Central European landmasses of today—a shallow ocean formed with an archipelago. As a result of global lowering of the sea level in the early Cretaceous, shallow marine platform sediments resurfaced in several places leading to the formation of bauxite deposits in the Villány Mountains of Hungary and in the Pădurea Craiului region of present-day Romania. This period is also characterized by a series of terrestrial and shallow marine environments, followed by rising sea levels and deposition of pelagic sediments. The remnants of the latter are found expressed, for example, in the thick, ammonite-bearing sedimentary sequences of the Gerecse Mountains of the Transdanubian Range. The so-called Urgon-type limestone , characterized by thick-shelled bivalves and gastropods , was also deposited during such a transgression in the Bakony Mountains, Transylvania, and the Carpathian region, for example. In some areas the Upper Cretaceous is represented by a thick succession of layers that were formed in a third cycle, which includes rocks (for example, bauxite) deposited on land, in coal beds formed in swamps and marshes, shallow marine reefal limestones, or pelagic marls and limestones. The most spectacular development of this third cycle— the Upper Cretaceous—found in Hungary is in the area of the Southern Bakony, between Sümeg and Magyarpolány. The relatively fast tectonic events of the Cretaceous that altered the Earth’s surface, drifting of continents and the formation of mountains, were also often accompanied by intensive volcanic activity. The most marked examples of this activity within the Carpathian Basin can be found in the Metaliferi Mountains and the Lower Cretaceous of the 4 The Cretaceous 144 t h e m e s o z o i c Mecsek Mountains. In the Mecsek, basaltic lava poured to the surface underwater leading to the formation of wellpreserved pillow structures. The circular reefs that evolved around the volcanic island —or islands—of the Mecsek were similar to modern atolls and hosted a rich fossil fauna. Parallel with thrusting and the formation of mountains, the erosion of the uplifting Carpathian Basin also began at this time; as a result, the Carpathian...