SUBARTESIAN CARBONATE MOUND SPRING DEPOSITS OF THE UPPER JURASSIC MORRISON FORMATION OF CENTRAL MONTANA: PALEO-PRECIPITATION PROXIES

Recent investigations of the Morrison Formation in central Montana resulted in the discovery of 100 small (< 3 m diameter) carbonate buildups. The buildups are found in five distinct spatial clusters on the southern and western flanks of Spindletop Dome and are aligned with Jurassic-aged structural lineaments in the region. The encasing sediments are illitic mudstones with no associated sandstone or terrestrial carbonate beds. The buildups are distributed stratigraphically between 40‒52 m above the base of the Morrison Formation. Electrical resistivity tomography surveys suggest additional buildups in the subsurface.

The buildups are divided into two groups by dominant mineralogical composition: siderite versus calcium carbonate. The siderite buildups display prominent cone-in-cone structures from fracturing owing to crystallization, and/or fluid over-pressurization. The siderites formed as groundwater moved up fractures into the vadose zone and interacted with the organic- and iron-rich forest soils. Aided by iron-oxidizing betaproteobacteria, siderite precipitated in the subsurface.

The carbonate buildups, interpreted to be mound springs, lack many characteristics of groundwater-fed terrestrial deposits such as travertine and tufas. Typical macrophyte debris, hydrophytes, or charophytes are generally absent. However, partial bryophyte leaves (liverworts) are preserved in several buildups. Ten carbonate buildups have petrified wood in close proximity and three have incorporated petrified logs. An organic fabric discovered in some buildups, might be residue of the tufa-associated green algae Oocardium stratum. Petrographically, the carbonate buildups exhibit bacterially-precipitated, radial-fibrous spherulites.

Isotopically negative δ¹⁸O (‒15.5 to ‒16.9) and δ¹³C (‒4.3 to ‒6.5) values demonstrate the buildups were produced by meteoric waters in a high latitude (≈ 45^o) continental setting with a short subsurface residence time. The mound springs formed at the surface from carbonate-rich groundwater that migrated up the fractures at low discharge rates. The presence of deposits scattered throughout a 12 m stratigraphic section of the formation indicate periods of an elevated piezometric surface, signifying that the region experienced extended periods of increased rainfall.