Fig. 1.16. Classification of the extrabasinal grains on the base of their shape and roundness (Szakmány 2008a)

2. Intrabasinal sedimentary rocks: Grains of intrabasinal sedimentary rocks are arise in the sedimentation basin. The most frequent rocks of this group are carbonates, like limestone and dolomite. Carbonate rocks are made of particles (composed >50% carbonate minerals) embedded in a cement. The particles are carbonate minerals, mostly calcite (CaCO₃) and dolomite (CaMg(CO₃)₂).Most carbonate rocks result from the accumulation of bioclasts created by calcareous organisms. Therefore carbonate rocks originate in area favouring biological activity i.e. in shallow and warm seas in areas with little to no siliciclastic input. In present day Earth these areas are limited to ±40 latitude in region away or protected from erosion-prone elevated continental areas. Limestone is composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate (CaCO₃). Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera. Limestone makes up about 10% of the total volume of all sedimentary rocks. Calcite can be evolve both continental and marine environments. The terrestrial carbonate rock called travertine. It formed by the precipitation of carbonate minerals from solution in ground and surface waters, and/or geothermally heated hot-springs. Similar (but softer and extremely porous) deposits formed from ambient-temperature water are known as tufa. Marine limestones have several types. Two classification schemes are in common use by those who work on carbonate rocks. Although you will use only the Folk classification in lab, you should also become familiar with the Dunham classification since it is widely used as well (Picts. 1.4-1.7.).