Facies and sequence architecture of mixed carbonate-siliciclastic depositional systems during transforming sag to foreland basin geometries - “Sundance Basin”, Middle and Late Jurassic, western United States

Abstract

During the Middle Jurassic, the symmetric intracratonic “Sundance Basin” in the western portion of the North American continent was overridden by the approaching tectono-orogenic front of an early Cordilleran orogeny and transformed into an asymmetric foreland basin. In the Late Jurassic, the orogenic activity ceased and the basin reflexively regained its symmetric geometry. The basin transformation comprises three evolutionary stages and fundamentally influenced the facies evolution as well as the sequence architecture. The reorganization had a tremendous impact on distribution, character and geometry of economically significant sediment bodies in the carbonate-siliciclastic basinfill.<br /> These stratigraphic-sedimentologic relationships were investigated in an original case study. Furthermore, this investigation provides the first analysis of the entire “Sundance Basin”. The study is based on a grid of 35 outcrop sections in Wyoming, Montana, Utah, Idaho, and South Dakota. This data set was supplemented by stratigraphic sections, well data and research results from the present regional-geologic literature.<br /> More than 20 carbonate, siliciclastic and evaporitic facies types indicate basinwide depositional models describing homoclinal and distally steepening ramps. Basinwide discontinuities define five allostratigraphic units. Each unit represents the remnant of a transgressive-regressive second-order sedimentary cycle: First Marine Cycle (C I), Second Marine Cycle (C II), Third Marine Cycle (C III), “unnamed cycle” and Fourth Marine Cycle (C IV). Internally, the second-order sedimentary cycles are composed of third-order sequences. The sequence boundaries are recorded by transgressive deposits and/or erosional surfaces. The second-order sedimentary cycles and third-order sequences consist of transgressive and regressive systems tracts of differing hierarchy. The architecture of the sequences varies along the time axis.<br /> The different sequence types and stacking patterns correlate with the three stages of basin evolution. During the initial basin stage (“sag basin stage”) tabular sequences with a layer cake stacking developed. Wedge-shaped sequences with an aggradational to progradational stacking pattern evolved during the asymmetric basin stage (“foreland basin-style stage”). In the final evolutionary stage (“rebound stage”) simple stacked, tabular and truncated sequences were generated.<br /> Sea-level changes as a major controlling mechanisms are not eustatic, but regional and are controlled by regional-tectonic and climatic parameters. Moreover, the formation of sequence boundaries corresponds to tectono-orogenic phases of the early Cordilleran orogeny. The temporarily asymmetric subsidence behavior generated additional accommodation space, while the increasing input of clastic material from orogenic sources primarily regulated the sediment supply. This interplay influenced the carbonate factory in the subsiding, asymmetric portion of the basin. Low sediment supply, sufficient subsidence rates and a warm climate promoted the formation of thick, distal carbonate successions, while the increasing input of siliciclastics caused the termination of the carbonate factory. Ceasing orogenic activities and erosion of the evolved orogen produced low subsidence rates and initiated partial overfilling of the basin during the final evolutionary stage.<br /> The geometric transformation significantly influences the generation of potential reservoirs and seals. In a symmetric basin geometry (“sag basin stage”) these associations are developed as thin but widespread carbonate reservoir facies types. During an asymmetric basin geometry (“foreland basin-style stage”) potential reservoirs and seals occur either in shoreline-detached carbonate facies belts that fringe areas of increased subsidence or in continuous siliciclastic shoreface-foreshore successions of tectonically stable areas. For symmetric basin settings that undergo partial overfilling by increasing siliciclastic input (“rebound stage”) no significant reservoir and seal facies types were found due to the high degree of erosion and redistribution within the sedimentary system.