I'm not up to snuff on my USDA taxonomy, but here is an explanation of this horizon:

• Soils consist of three master horizons: A, B and C
• this soil is from the B horizon
• two nearly mutually exclusive pedogenic processes have occured to create this horizon:

1. Illuviation - the deposition of clays from the A horizon
2. Carbonate enrichment

Clays and carbonates both leach from the A horizon during pedogenesis, but carbonates move way faster and are long gone before the first clay particles arrive. Clays move much slower and don't go as far.

You can get carbonate enrichment from deeper in the profile though capillary rise, but it's almost always restricted to the C horizon. What you end up with is a very rich carbonate layer over top of the naturally calcareous layer of the soil.

In THIS case, evaporation is so strong it's brought these carbonates into the B (middle) horizon so you end up with a calcareous, clay enriched B horizon which really should not exist.

Soils are wild.

Bonus picture