Morphogenesis of Hollow Bulbous Gypsum Structures Along a Brine Spring in Northeast Alberta

The stromatolite tufa mound at La Saline Lake developed along the Athabasca River Valley in northeast Alberta consists of a 30 m-high structure with a multi-meter thick caprock of stratified gypsum. The gypsum caprock developed when the meteoric-charged groundwater channeled along shallowly buried Upper Devonian limestone was redirected deeper and encountered anhydrite beds of the Middle Devonian Prairie Evaporite Formation, only 175–200 m below. Discharge of the sulfate-saturated brine from the central vent of the gypsum caprock eventually ceased and the flow was redirected to the western lakefront bank of the tufa mound. This active brine spring, characterized by total dissolved solids level of ∼79,000 mg/L, is channeled along a 25 m gully toward La Saline Lake. The bottom sediment in each of the interconnected brine pools along the gully consists of a 2–4 cm-thick calcite-gypsum thrombolite and an overlying gypsum crust. This sulfate crust developed as densely packed arrays of hollow botryoidal to hemispheroidal and bulbous gypsum protuberances, each 0.5–1.5 cm long, that extend upward into the brine. This is the first documented example of bulbous protuberances of gypsum that developed within brine pools with hollow interiors. The unusual hollowness of these bulbous gypsum protuberances resulted from the rapid encasement of gas bubbles that ascended from the underlying thrombolite ooze and were trapped within the overlying microbial mats and meshwork of gypsum crystallites on the surface of the bottom pool sediment. Nanoscale biomineralization of gypsum developed along the parallel arrays of microbial stalks within the enveloping mat, resulting in a meshwork of parallel aligned crystallites that encased the surfaces of the trapped bubbles. Continued abiotic gypsum precipitation transitioned the abiotic crystallites into enlarged needle-form crystallites distributed as parallel arrays along curvilinear growth surface laminae. Sufficient rigidity on the bubble surfaces precluded implosion-collapse or detachment. Strontium adsorption widely stabilized the acicular crystals, inhibiting complete coalescence as gypsum spar.