In order to understand how an eolian sequence was generated, a 45-m-high, 54,000-m2 outcrop of Middle Jurassic Page Sandstone near Page, Arizona, was mapped and reconstructed within the context of a 9.5-km-long lateral traverse. Recognition of prominent bounding surfaces and distinctive sequences of cross-strata between surfaces allows the identification of nine complexes. The 'A' complex consists primarily of coarser grained, low- to moderate-angle wind-ripple laminae arranged in sets, but also shows a single set of high-angle grainflow strata. The complex is situated in a depression on the 3-2 unconformity at the top of the Navajo Sandstone, and is interpreted to represent deposits of isolated barchan dunes. slipfaceless bedforms, and ‘wedges’ of sand filling the depression. The ‘A’ complex may be part of the J-2 ‘event’, predating the Page per se and representing local deposition in an overall deflationary period, or it may herald widespread renewed deposition immediately preceding the Page system. The B' and 'C complexes are separated by a prominent surface, but both consist of large sets of trough cross-strata in which the directional spreads of the foresets define crescentic shapes, and the dominance of grainflow strata indicates deposition by transverse dunes. Complex ‘B’ also contains sets that are interpreted as the result of secondary air flow along flanks of crescentic draas. A wavy-laminated, silty, fine-grained, red standstone rests on a prominent surface that truncates the ‘C’ complex. Cross-strata above this surface are distinctly different from those of the underlying complexes. Complexes ‘D’ to T, each of which is separated by a surface with polygonal fractures, are similar to one another and consist of sets dominated by wind-ripple laminae with strongly developed cyclic stratifications, and are interpreted as oblique-dune deposits. The complexes differ, however, in interpreted dune size and whether the bedforms were simple dunes or draas. All complexes on the outcrop show a mean transport direction to the south, which corresponds to predominant northerly summer winds. Dunes represented by complexes ‘B’ and ‘C’ appear to have been little affected by winter winds from the northeast, but dunes in the overlying complexes were moderately modified by more variable winter winds.
Radically different interpretations of the complexes and prominent surfaces can be reasonably proposed. Interpretation of the surfaces as first-order surfaces envisions that the vertical and lateral Page sequence results from uninterrupted accumulation of a widespread erg, albeit under changing conditions with evolving dune types. Interpretations of the surfaces as super surfaces and the complexes as deposits of separate dune fields and ergs forces the Page sequence to be viewed as a complex amalgamation of overlapping segments of ergs and dune fields formed in a dynamic-basin setting with varying times, sites, and degrees of deposition and erosion. These alternate views are fundamental to the understanding of the genesis of eolian sequences at the basin level, and are presently best addressed at the regional and not outcrop scale.
Figures & Tables
The Three-Dimensional Facies Architecture of Terrigenous Clastic Sediments and its Implications for Hydrocarbon Discovery and Recovery
While there has been much interest in recent years in concepts of sequence stratigraphy, this book focuses on stratigraphic units that are, in general, an order of magnitude smaller than sequences. A knowledge of such architectural detail is of considerable significance in the development of detailed, scaled facies models for depositional environments, and is of paramount importance in the efficient design of advanced petroleum recovery projects. This book is the outcome of a SEPM Research Symposium held at the annual meeting of the Society in San Antonio, Texas, April 1989. The intent of the meeting was to bring together modern research on facies architecture, and to apply this research to the investigation of reservoir heterogeneities and production problems.