Which one of the following sedimentary rocks has not been formed mechanically?

Sedimentary rocks are classified based on their mode of formation into mechanical, organic, and chemical categories. Mechanically formed (clastic) rocks result from the accumulation and lithification of physical debris from pre-existing rocks [2]. Sandstone, conglomerate, and loess are primary examples of mechanically formed rocks [1]. Sandstone consists of sand grains like quartz, while conglomerates are composed of coarser fragments [2]. Loess is specifically identified as a wind-deposited mechanical sediment [1]. In contrast, geyserite (siliceous sinter) is classified as an organically or chemically formed rock [1]. It forms through the precipitation of silica from hot springs or hydrothermal solutions, often involving biological activity or chemical supersaturation rather than the mechanical breakdown and deposition of rock fragments [1]. Therefore, geyserite is the only option not formed mechanically.

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] Certificate Physical and Human Geography , GC Leong (Oxford University press 3rd ed.) > Chapter 2: The Earth's Crust > SEDIMENTARY ROCKS > p. 19