TUFA-LIKE CARBONATE BUILD-UPS IMAGED THROUGH ELECTRICAL RESISTIVITY TOMOGRAPHY: A CASE STUDY FROM THE MORRISON FORMATION OF CENTRAL MONTANA

Research of the Upper Jurassic Morrison Formation of central Montana has yielded 34 siderite build-ups and 62 tufa-like carbonate build-ups exposed at the surface. The build-ups are present only in the upper third of the formation. Build-up surface exposures are commonly hemispherical in shape, with a radius of approximately one meter. Siderite build-ups are distinguishable from tufa-like carbonate build-ups by the presence of cone-in-cone structures. This feature suggests the siderite build-ups formed in the subsurface at the capillary fringe in the vadose zone. In some instances, the tufa-like carbonate build-ups have siderite cone-in-cone features at the base or along the flank. In such cases, a sharp contact exists between the siderite and carbonate build-ups, suggesting two phases of accumulation. The tufa-like carbonate build-ups likely formed when the phreatic zone was very near the surface and may have experienced low-flow rates sourced from a high ground water table along fracture zones. Spatial relationships of the tufa-like carbonate build-ups suggest they are related to the major lineaments of Montana. In one area, 30 siderite build-ups are surrounded by 35 tufa-like carbonate build-ups, suggesting variation in paleotopography. Petrified wood is preserved in, or in very close proximity to, eight carbonate build-ups.

To better understand the size and relationship of the surface build-ups, four electrical resistivity tomography (ERT) surveys were conducted. The ERT images define the lateral extent of the build-ups in the subsurface. In addition, they reveal thin (< 3 m) carbonate layers with limited lateral extent two to six meters below the surface build-ups. The images provide information about previously unknown stratigraphically lower carbonate layers. These layers are also interpreted to be carbonate build-ups. The surface build-ups and the subsurface carbonates are decoupled by a thin low-resistivity zone, which likely represents interbedded mudstone/shale. The stratigraphic variance of the build-ups implies repeated intervals of high ground water during a wet climatic period. The subsurface information obtained from the ERT improved the depositional facies model for these build-ups and provided additional evidence of repeated wet intervals in the Late Jurassic Morrison Formation.