QUANTIFYING THE RELATIONSHIP BETWEEN FORELAND BASINS PROCESSES AND THEIR PROGRADING CARBONATE FILL: TECTONOSTRATIGRAPHIC CO-EVOLUTION OF THE ANTLER FORELAND BASIN AND LOWER-MIDDLE MISSISSIPPIAN CARBONATES, SW MONTANA AND EAST-CENTRAL IDAHO

Initiation, growth and senescence of the Lower-Middle Mississippian Madison carbonate ramp in the northern US Rockies were closely related to the contemporaneous Antler orogeny, a relatively poorly understood sequence of tectonic events along the western boundary of North America. In this study, the disparate fields of carbonate sedimentology and sequence stratigraphy and lithosphere mechanics and geodynamics are combined to provide a coherent, self-consistent view of processes which led to the initiation, stabilization and ultimate destruction of a carbonate ramp in a flexural setting.

Current models of carbonate ramp development in foreland basins predict depositional systems which are areally and temporally restricted as a result high subsidence rates across, and high fluxes of siliciclastic sediments into, the basin. The Madison ramp, however, was a large (~1 x 10⁶ km²) and long-lived (~15 Ma) system which provided much of the basin infill (up to 1.2 km). The disagreement between modeled and observed filling has led authors to invoke a wide variety of possible models ranging from complex reactivation of preexisting fault systems to spatial variations in thrust loading; however, we favor a new interpretation for Madison development.

Outcrop analysis of this area has shown patterns of juxtaposed deep and shallow water facies, karsted unconformities, and slumping and mass wasting, as well as formation-scale facies reorganization, indicative of changing ramp physiography. These observations suggest patterns of syn-Madison tectonic subsidence and basin inversion consistent with flexural subsidence followed by viscoelastic relaxation of the lithosphere. Forward modeling of lithospheric flexure in response to orogenic loading and viscoelastic relaxation during periods of tectonic quiescence provides quantitative estimates of accommodation space development across a profile from orogen to craton. Modeling predicts regional deepening due to loading and initial landward migration of the flexural profile, while narrowing and deepening of the foreland basin and upwarp of the flexural forebulge occur in periods of tectonic abeyance. These processes are responsible for the evolution of the ramp and its ultimate demise.