Development of Lower Mississippian Cyclic Carbonates, Montana and Wyoming

Maya Elrick, J. F. Read

The Lower Mississippian Lodgepole/Madison formations of Wyoming and Montana consist of a 20 to 300-m upward-shallowing sequence of cyclic slope/basin, deep-ramp to shallow-ramp carbonate deposits. Shallow-ramp cycles (1-3 m) are composed of cross-bedded oolitic grainstone and pellet grainstone, overlain by rare algal laminite caps. Deep-ramp cycles (1-10 m) are characterized by thin-bedded, substorm-wave-base limestone/shale, nodular limestone/shale, and storm-deposited limestone overlain by hummocky cross-stratified grainstone caps. Average periods of the cycles range from 35,000 to 110,000 years. Slope/basin deposits are 10 to 20-cm thick couplets of even-bedded, micritic limestone and shale.

Computer modeling of the cycles incorporates fluctuating sea level, subsidence, depth-dependent sedimentation, lag time, and platform slope. Data from spectral analysis (basin/slope couplets), Fischer plots (shallow-ramp cycles), computer modeling, and field data suggest (1) subsidence rates across the 700-km wide platform range from 0.01 m/k.y. to 0.12 m/k.y., (2) high-frequency (10⁴-10⁵ years) sea level fluctuations with 15 to 25-m amplitudes affected the platform, and (3) shallow-ramp slopes were less than 2 cm/km and deep-ramp slopes were greater than 10 cm/km. Computer models produce stratigraphic sections (one-dimensional models) that graphically illustrate how imput parameters interact through time to produce the cyclic stratigraphic section. Two-dimension l models generate regional cross sections that illustrate the lateral and vertical relationships between cycles across the entire platform and how small-scale cycles are packaged within Vail-type sequences (tens to hundreds of meters thick). Preliminary results from both one-dimensional and two-dimensional modeling generate stratigraphic sections and cross sections that closely resemble those observed in the field.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.