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Improving geological understanding of sedimentary basins through the analysis of key sequence stratigraphic surfaces

Abstract

Unconformities, by definition, correspond to non-depositional or erosive surfaces, separating older strata below, from younger rocks above, and encapsulate time-gaps. However, recent studies have emphasized the composite nature of some unconformities, as well as their heterochronous and diachronous character, which practically restricts the use of such a definition to single-borehole data.

This study analyses the nature of the regionally significant J-3 Unconformity, which separates the Middle Jurassic aeolian deposits of the Entrada Sandstone from the Upper Jurassic, tide-dominated, marginal-marine Curtis Formation (and laterally equivalent units) in east-central Utah (USA). Our detailed mapping and characterization indicate that the J-3 “Unconformity” is in fact a composite surface generated by erosion-related processes such as aeolian deflation and water-induced erosion, and/or by deformational processes. These multiple mechanisms interacted and overlapped in time and space, which demonstrates the composite, diachronous and non-unique nature of such boundaries. This contact has been historically interpreted as an unconformity, but our results show that it is in fact a time-transgressive flooding-ravinement surface that formed shortly before and during a series of transgressions that flooded the area during the Late Jurassic Period. Consequently, the regionally extensive, composite, heterochronous, and diachronous J-3 Unconformity does not fit the classic definition, after which an unconformity universally separates older from younger strata basin-wide.

This study, combined with the analysis of other Upper Jurassic key sequence stratigraphic surfaces of east-central Utah, further recognises a temporary overprint of autogenic processes into allogenically-driven, short-lived relative sea-level variations within the Curtis sea. Prior to the onset of the major transgression, the heterolithic sedimentary record consisted of muddy to very fine-grained, rippled cross-stratified siltstone and sandstone strata. After this, the shallow-marine (20 m < D < 50 m) sea entered into tidal resonance as the flooded basin reached an optimal length (ca. 800 km). This length corresponded to an odd multiple of one quarter of a tidal wavelength. The temporary, high-energy, resonant system resulted in deposition of amalgamated well-sorted, fine- to medium-grained tidal bars and dunes; this system overprinted the effects of otherwise-dominant allogenic forcing. By contrast, the neighbouring coastal systems continued to record allocyclic signals preserved as stacked aeolian sequences, and a cyclical pattern recognised within the supratidal part of the studied succession.

In conclusion, this study highlights the fact that one process can be represented by varying surface expressions in the stratigraphic record, and conversely many processes may result in the same stratigraphic expression. Also, by documenting a resonant stage in a tide-influenced basin that overprinted the otherwise dominant allocyclic processes, this study underlines the importance of assessing coeval depositional systems in order to build complete stratigraphic basin histories. Moreover, this investigation shows how the careful assessment of the sub-seismic character of stratigraphic surfaces helps improve timing and sediment budget predictions, which will ultimately increase the understanding of basin evolution and the quality of reservoir models.