CYCLIC FACIES ARCHITECTURE AS A KEY TO DEPOSITIONAL CONTROLS IN A DISTAL FOREDEEP - CAMPANIAN MESAVERDE GROUP, WYOMING, USA

The Mesaverde Group consists of a thick wedge of fluvial, littoral-del taic and shallow marine clastics shed into the Cretaceous Western Inte rior Seaway of North America. The western parts of the seaway lay with in the strongly subsiding foredeep of the active Sevier fold and thrus t belt further to the west. The study area is located east of the axis of maximum subsidence and is thus in a favourable position to record competing effects of eustasy, sediment supply and thrust-load induced subsidence. Facies and sequence analysis carried out on high quality o utcrop and well log data led to the recognition of a complex depositio nal cycle hierarchy within the typical storm- and wave-dominated inner shelf/shoreface/strand plain and delta systems of the Mesaverde. Four th-order parasequences and parasequence bundles of estimated 100 - 400 ka duration are the best recognizable, ubiquitous and most useful str atigraphic units. Their arrangement with respect to sequence boundarie s, however, varies with their overall stratigraphic position and also differs from the Exxon models. Mesaverde progradation was interrupted by a major transgression that occurred out of phase with the aggradati onal to progradational stacking trend of third-order sequences. A prop osed genetic model relates large-scale (second-order) sequence archite cture to tectonics: a Sevier thrust event as well as Laramide uplift w ithin the foredeep controlled non-linear changes in the accommodation/ supply ratio. Parasequence stacking patterns and sequence boundary for mation, in contrast, were the product of (global?) eustasy enhanced by short-term, perhaps local, changes in the rates of subsidence and det rital influx.