Consider the following pairs with reference to sedimentary rocks and the process of their formation : 1. Chert - Chemically formed 2. Geyserite - Organically formed 3. Shale - Mechanically formed Which of the pairs given above is/are correctly matched ?

Sedimentary rocks are classified based on their mode of formation into mechanical, organic, and chemical types. Chert is primarily composed of microcrystalline quartz and can form through chemical precipitation when silica-rich solutions saturate or through diagenetic replacement. Geyserite, also known as siliceous sinter, is often classified as an organically formed sedimentary rock because its deposition around hot springs and geysers is frequently facilitated by the presence of algae and other microorganisms [1]. Shale is a classic example of a mechanically formed (clastic) sedimentary rock, produced through the lithification, consolidation, and compaction of fine-grained sediments like clay and silt [2]. While some sources note that chert can also be biogenic, standard geological classifications used in competitive examinations typically group it under chemical or organic depending on the specific context, and all three pairs provided align with established geological categories [1].

Sources

1. [1] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 13: Types of Rocks & Rock Cycle > 13.2. Sedimentary Rocks or Detrital Rocks > p. 171
2. [2] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 13: Types of Rocks & Rock Cycle > Some Examples of Metamorphosis > p. 174