GSA Annual Meeting in Indianapolis, Indiana, USA - 2018

Paper No. 250-4
Presentation Time: 9:00 AM-6:30 PM

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


RICHMOND, Dean R.1, BEHM, Michael1, LUPIA, Richard1 and HEIN, David G.2, (1)ConocoPhillips School of Geology and Geophysics, University of Oklahoma, Sarkeys Energy Center, Suite 710, Norman, OK 73019, (2)1021 Toole Circle, Billings, MT 59105

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.