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Controls on reflux dolomitisation of epeiric-scale ramps: Insights from reactive transport simulations of the Mississippian Madison Formation (Montana and Wyoming)

Research output: Contribution to journalArticle

Original languageEnglish
Pages (from-to)85-102
Number of pages18
JournalSedimentary Geology
Volume345
Early online date14 Sep 2016
DOIs
DateAccepted/In press - 2 Sep 2016
DateE-pub ahead of print - 14 Sep 2016
DatePublished (current) - 1 Nov 2016

Abstract

Prediction of the geometry and petrophysical properties of dolomite geobodies depends on understanding both the hydrological system supplying reactive fluids and the chemistry of these fluids. However, patterns are complicated by the non-linear response of the diagenetic system to depositional texture, which controls both fluid flux via permeability architecture and reaction rate via effective surface area. This study explores interactions between extrinsic controls (spatial distribution of brine composition and temperature) and intrinsic controls (permeability and reactivity) using local and regional scale reactive transport models of sequential episodes of brine reflux that resulted in partial dolomitisation of the Mississippian Madison ramp.

Inter-well scale models show preferential early dolomitisation of fine grained, more reactive beds. Pervasive dolomitisation can occur most readily beneath the brine pool where flow is perpendicular to bedding, and is most rapid at high brine fluxes. Down-dip of the brine pool, bedding-parallel flow is focused in relatively permeable coarse grained beds, providing reactants for strongly preferential alteration of intervening more reactive fine grained beds. In contrast, thicker sequences of fine grained beds dolomitise more slowly, limited by the rate of supply of magnesium. Regional-scale models, with injection of brines of increasing salinity towards the ramp interior, reproduce the observed pattern of dolomitisation. However, more realistic simulations in which reflux is driven by lateral density contrasts, generate flow rates orders of magnitude too low for significant dolomitisation. Simulations suggest pervasive dolomitisation of epeiric-scale ramps by a platform-wide reflux circulation, as often invoked, is not feasible. Rather, dolomitisation of such extensive systems critically requires local-scale flow systems, such as may result from topographically-controlled variations in restriction of platform-top seawater circulation.

    Research areas

  • Reactive transport modeling, Madison Formation, Reflux, Dolomitisation, Epeiric ramp, Brine pools

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  • Full-text PDF (accepted author manuscript)

    Rights statement: This is the author accepted manuscript (AAM). The final published version (version of record) is available online via Elsevier at http://www.sciencedirect.com/science/article/pii/S0037073816300823. Please refer to any applicable terms of use of the publisher.

    Accepted author manuscript, 5 MB, PDF document

    Licence: CC BY-NC-ND

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