Sedimentary Rocks Part 3 - ERTH 209-02 Fall 2024 PDF

ERTH 209-02 EARTH Fall 2024 Sedimentary rocks_part 3 Generalities  Chemical or precipitated rocks form through precipitation from a supersaturated solution mostly at the surface, rarely in the superficial part of the crust.  Precipitation can be inorganic as in most...

ERTH 209-02 EARTH Fall 2024 Sedimentary rocks_part 3 Generalities  Chemical or precipitated rocks form through precipitation from a supersaturated solution mostly at the surface, rarely in the superficial part of the crust.  Precipitation can be inorganic as in most cases, or the result of the metabolism of some organisms and in this case they are referred to as biochemical rocks, biogenic rocks or biochemically precipitated rocks.  Carbonate rocks (dominant carbonates) and evaporite rocks (dominant halides and sulphates). Depositional environment  Tidal carbonates Mounds of the burrowing shrimp Callianasa; such bioturbation destroys the original textures and structures of the carbonate mud (Bahamas) Reefs and other organic buildups  Wave resistant buildups are commonly referred to as reefs.  Those organic buildups that are not wave resistant are referred to as banks or bioherms.  The use of the term ‘reef’ is somehow equivocal. Therefore, a distinction between ‘stratigraphic reefs’ and ‘ecological reefs’ should be made. Reefs and other organic buildups Large sized Tridacna bivalve in the Great Reef Barrier, off Queensland, Australia. Oolites, carbonate banks and shoals  Such sediments accumulated in areas with high-energy waters and particularly unstable substrate.  These areas include some shelf edges. If waters become supersaturated with respect to the calcium carbonate, ooids can form. Oolites, carbonate banks and shoals Oolitic sand; Abu Dhabi, United Arab Emirates. Oolites, carbonate banks and shoals Blackened ooid sands; Off Qatar. Chalks The chalks represent a form of deeper water carbonate sediments consisting mostly of algal debris. The best known are the Upper Cretaceous chalks. Cliffs at Dover; southern England. Carbonate classification after Dunham Carbonate types: boundstone Characteristics: original components are organically bound during the depositional process. Carbonate classification after Dunham Carbonate types: mudstone Characteristics: mud-supported, less than 10% allochems. Carbonate types: wackestone Characteristics: mud-supported, over10% allochems. Carbonate classification after Dunham Carbonate types: packstone Characteristics: grain supported, and with carbonate mud. Carbonate types: grainstone Characteristics: grain supported, and without carbonate mud. Carbonate classification after Dunham Evaporites - generalities  Evaporites are rocks originally precipitated from a saturated surface or nearsurface brine, one process that is mostly driven by solar evaporation.  Occurrences of evaporites in the geological record are indicative for the existence of a warm and arid paleoclimate.  Primary evaporites are precipitated by solar evaporation from a brine pool at the surface of the Earth. Secondary evaporites form subsequently through dissolution and re-precipitation of the primary ones. Primary evaporites: halite Primary halite from the northwestern coast of Gulf of California close to the air/water interface. Primary evaporites: halite Primary halite; Dead Sea shore (Israel).  Secondary evaporites Secondary halite & potash (Qatar).