Stratigraphic and Structural Characterization and Evolution of Exposed Megaflaps Flanking Salt Diapirs

Cora Evelyn Gannaway Dalton, University of Texas at El Paso

Abstract

A Neoproterozoic salt system exposed in the Willouran Ranges of South Australia and a Mesozoic salt system in the south-central Pyrenees of Spain provide natural laboratories to investigate the stratigraphic and structural development of megaflaps flanking salt diapirs. Megaflaps are near-vertical to overturned, deep minibasin stratal panels that extend far up the flanks of steep diapirs or their equivalent welds. Megaflaps can form by halokinetic drape-folding, contraction, or a combination of these processes. The existing models of megaflap development are based primarily on seismic data, well penetrations, cross-section restorations, and numerical and analog modeling. Investigation of the sedimentology, stratigraphy, and sub-seismic scale deformation of megaflaps is limited by a scarcity of high-resolution seismic images at the salt margin and only one-dimensional well-bore datasets. Therefore, this dissertation, which utilizes comprehensive field-based studies, represents one of the first systematic characterizations of exposed megaflaps with respect to the integrated sedimentologic, stratigraphic, and structural evolution. The Witchelina diapir, which is an exposed Neoproterozoic primary passive salt diapir in the Willouran Ranges, is flanked by a halokinetic megaflap. Sedimentologic and stratigraphic analysis of the Witchelina megaflap and coeval growth strata on the opposite diapir flank reveal the influence of the inflating Witchelina diapir from the earliest stages on regional basinwide and local minibasin scales. Early erosion exposed Witchelina diapir to form a salt shoulder, which led to the evolution of asymmetric minibasin geometries typified by a halokinetic megaflap on one flank. Deposition and halokinetic deformation of the megaflap strata occurred with periodic erosional thinning and onlap progressively higher up the inflating diapir flank. The Aulet and Adons diapirs, which are exposures of Triassic Keuper evaporites in the south-central Pyrenees, are flanked by two contractional megaflaps. The diapirs have been previously interpreted as either salt rollers bounding extensional-rollover subbasins or passive diapirs flanked by salt-withdrawal minibasins. Integrated sedimentologic, stratigraphic, and structural analyses place constraints on the origin and evolution of the Aulet and Adons diapirs and preliminarily assess the relative amounts of gravity-driven extension, passive diapirism and salt evacuation, and contractional reactivation. The domains to the south of the diapirs, which contain the contractional megaflaps, appear to have evolved as extensional-rollover basins but with variable timing and amounts of stratal rotation accommodated by a fault-bounded remnant salt ridge, whereas the domain to the north of the diapirs had a component of passive diapirism and salt evacuation. The results of these field-based studies have important implications for petroleum exploration where megaflaps have typically been identified only post-drill. In the absence of abundant publicly available high-resolution seismic and well data, cataloguing and characterizing the depositional and structural evolution of exposed megaflaps aims to improve the predictability of megaflaps so that salt-flank trap potential and risks can be adequately assessed. More broadly, incorporating a megaflap in the reconstruction of a salt basin can drastically change or validate an interpretation of both the basin history and the regional tectonic influence. Megaflaps preserve the sedimentary record of the earliest salt movement, salt-sediment interaction, and halokinetic deformation, and as such, provide unique snapshots of the evolution of long-lived salt tectonic systems.

Subject Area

Geology|Sedimentary Geology|Petroleum Geology

Recommended Citation

Dalton, Cora Evelyn Gannaway, "Stratigraphic and Structural Characterization and Evolution of Exposed Megaflaps Flanking Salt Diapirs" (2019). ETD Collection for University of Texas, El Paso. AAI13879018.
https://digitalcommons.utep.edu/dissertations/AAI13879018

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