Wackestone

Under the Dunham classification (Dunham, 1962^[1]) system of limestones, a wackestone is defined as a mud-supported carbonate rock that contains greater than 10% grains. Most recently, this definition has been clarified as a carbonate-dominated rock in which the carbonate mud (<63 μm) component supports a fabric comprising 10% or more very fine-sand grade (63 μm) or larger grains but where less than 10% of the rock is formed of grains larger than sand grade (>2 mm).^[2]

Fragmented bioclastic wackestone

A Wackestone in thin section (width of image is 10 mm)

The identification of wackestone

Schematic wackestone as seen in thin section under the petrographic microscope. Type of carbonaceous rock according to the depositional texture: Alloctonous carbonates – Original components not bound at the deposition time. Less than 10% of components larger than sand size (> 2 mm) Contains carbonate mud (micrite, silt/clay size <63 μm) Fabric supported by carbonate mud (micrite, <63 μm) 10% or more composed of 63 μm or greater grains Legend: Dotted background: micritic matrix. Curved blue particles: bioclasts (indeterminate fossils, e.g. bivalve fragments). Blue cones: bioclasts (e.g. fossils of gastropods) Black spheroids: bioclasts (pellets).

A study of the adoption and use of carbonate classification systems by Lokier and Al Junaibi (2016)^[2] highlighted that the most common problem encountered when describing a wackestone is to incorrectly estimate the volume of 'grains' in the sample – in consequence, misidentifying wackestone as mudstone or vice versa. The original Dunham classification (1962)^[1] defined the matrix as clay and fine-silt size sediment <20 μm in diameter. This definition was redefined by Embry & Klovan (1971)^[3] to a grain size of less than or equal to 30 μm. Wright (1992)^[4] proposed a further increase to the upper limit for the matrix size in order to bring it into line with the upper limit for silt (62 μm).