GENETIC FACIES: AN APPROACH ILLUSTRATED BY ANALYSIS OF THE LOWER MESAVERDE GROUP, BIGHORN BASIN, WYOMING

Sedimentologists and stratigraphers on one hand, and groundwater hydrologists and petroleum engineers on the other, have for some time attempted to reconcile their approaches to the analysis of what they respectively call “facies” and “heterogeneity.” We suggest a more rigorous approach to facies analysis, based on recent studies of sediment transport dynamics, in order to facilitate this synthesis. A key observation notes that facies can be divided into gradational, process-related assemblages, separated by bounding surfaces cut during their formation (source diastems). Thus defined, the assemblages fit systematically into the larger-scale sequence stratigraphic framework. The approach reveals eight characteristic facies assemblages in the lower Mesaverde Group, a facies-rich, shallow marine succession. Several additional patterns intervene between these assemblages and third order sequences. The facies bodies themselves are composed of successions of 1-100 cm thick event strata (autocyclic strata), recording high-frequency storm, river flooding or tidal events. At larger time and space scales, river avulsion and pseudo-random (chaotic) shoreline changes cause autocyclic repetition of facies assemblages. Allocyclicity, due to variations in climate controlled by the earth’s orbit, results in repetition of paralic and inner shelf deposits over 1-5 m (“parasequences”) and repetition of facies assemblages over 1-20 m (high-frequency) sequences. Finally there is plastic distortion of the geometry and grain-size characteristics of facies assemblages through out the lower Mesaverde Group, due to variation through time of the basin variables (“Sloss variables;” rates of sea level change, fluid power available for sediment transport, and the supply rate and grain size of sediment). Numerical simulations of shallow marine stratal successions indicate that the genetic facies scheme is also compatible with the requirements of stratigraphic numerical models.