Fig 13 - uploaded by Gray Bebout
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(A) Paleogeographic map and the location of the Western Interior Seaway during the Late Cretaceous (Ericksen & Slingerland 1990). The frontal thrust of the Cordilleran Mountain Belt is shown and the study area is marked with a star in SW Montana. Modern North American shoreline is shown as reference. Climate models postulate that large cyclones blew across the seaway during the Cretaceous (e.g., Kump & Slingerland 1999). (B) Paleogeographic map of the Eocene (Scotese 2002), showing the location of the Pacific Ocean further to the west and the significant retreat of the Western Interior Seaway. The modern North American shoreline is shown for reference.
Source publication
Calcite veins and Mississippian carbonates from the Sevier thrust front
record syntectonic meteoric fluid infiltration and hydrocarbon
migration. The Tendoy and Four Eyes Canyon thrust sheets were emplaced
onto the western margin of the Late Cretaceous Western Interior Seaway
\{WIS\}. Low salinity \{Tice = -0.6° C to +3.6° C\}
and low temperature \...
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Citations
... Not to scale, see text for further discussion. values (Perry and others, 1983) and fluid inclusion analysis (Johnson, ms, 2002) support deformation temperatures of 80° to 120°C for the frontal thrust sheets. Intercalated ashes within proximal synorogenic foreland basin conglomerates of the Beaverhead Group constrain emergent thrusting along the thrust front to be Late Cretaceous (79 –76 Ma on Erdmont thrust near Bannack, Montana; Kalakay, ms, 2001). ...
... Carbon and oxygen isotope analyses were undertaken to explore the scale at which the thrust belt behaved as an open system during deformation. Samples were collected from massive layers, deformed zones, and several generations and types of veins at Willow Creek anticline and as done at other localities transecting the Montana recess (see Davidson and others, 1998; Bebout and others, 2001; Johnson, ms, 2002; A. Johnson, D. Anastasio, and G. Bebout, manuscript in preparation). Carbon dioxide was liberated from calcite using H 3 PO 4 following the method of McCrea (1950) and samples were analyzed using the Finnigan MAT 252 mass spectrometer at Lehigh University. ...
Finite strain and geochemical variations along strain gradients were used to study cleavage development in carbonates in the Lost River Range, Idaho. Deformation accommodating layer-parallel shear was partitioned into thin deformation zones during folding of Willow Creek anticline. Cleavage intensity is strong to very strong in deformation zones and weak in surrounding rocks. Strain magnitudes range from e�s�0.32 (1.15:0.93:0.73; x, y, z, principal axes of strain) outside, to e�s�0.64 (1.29:0.80:0.52) inside, deformation zones. We found a positive linear correlation between strain, cleavage development, and negative dilation. Volume loss, at a cubic centimeter scale, ranges from 12 percent to 49 percent. Cleavage selvages are depleted in Ca and 18O and enriched in other elements relative to microlithons and less deformed protolith carbonates. Mass-balance considerations indicate that cleavage was formed by incongruent pressure solution leading to a passive concentration of less soluble components during Ca loss and metasomatic additions of Si, Al, and K to
produce authigenic clay minerals in selvages. Data for the Willow Creek locality and elsewhere in the Lost River Range (Davidson and others, 1998), Pioneer Mountains, and the Tendoy Range (Bebout and others, 2001), show that across the Sevier orogen, fluid infiltration was heterogeneous at centimeter-kilometer scales and resulted from positive feedback between deformation and far-traveled surficial fluids. Metasomatic strain softening enhanced deformation zone development, which generated increased permeability as the thrust belt evolved from a porosity-based closed system to a discontinuity-based open system. Increased fluid infiltration and isotopic exchange was associated with volume loss, increasingly prolate strains, and crystallization of clays, to produce the cleavage selvages. Mass transfer was accommodated by diffusion early and advection later in the
deformation history. Mesoscopic structures (deformation zones, faults, veins) focused fluid flow and were kinematically related to larger-scale structures (faults, faultrelated folds). The inferred addition of surficial fluids to depths of >7 kilometers in the thrust belt implies a fluid regime involving significant topographically driven recharge. Deformed whole-rocks and microsamples are lower in �18O than undeformed samples which have O- and C-isotope compositions similar to those of marine carbonates. Veins are even lower in �18OV-SMOW, with minimum values of ��5 permil reflecting penetration of the crust by ocean-derived precipitation with �18O near to somewhat lower than 0 permil (range of �7.5 to �2.5‰ calculated for H2O in equilibrium with these veins). The inferred penetration, into the thrust belt, of nearshore meteoric waters is consistent with proximity to the reconstructed Western Interior Seaway. Later fluid infiltration locally lowered the �18O of carbonates even
further, based on the O-isotope compositions of veins related to younger compressional deformation and to the onset of crustal-scale extension in the Eocene. This progression toward lowered �18O of the surficial waters is compatible with the retreat of the seaway during emergence of the thrust wedge and Paleogene extension, uplift,
and subaerial volcanism.
Structural, petrographic, and isotopic data for calcite veins and carbonate host-rocks from the Sevier thrust front of SW Montana record syntectonic infiltration by H2O-rich fluids with meteoric oxygen isotope compositions. Multiple generations of calcite veins record protracted fluid flow associated with regional Cretaceous contraction and subsequent Eocene extension. Vein mineralization occurred during single and multiple mineralization events, at times under elevated fluid pressures. Low salinity (Tm = −0.6°C to +3.6°C, as NaCl equivalent salinities) and low temperature (estimated 50–80°C for Cretaceous veins, 60–80°C for Eocene veins) fluids interacted with wall-rock carbonates at shallow depths (3–4 km in the Cretaceous, 2–3 km in the Eocene) during deformation. Shear and extensional veins of all ages show significant intra- and inter-vein variation in δ¹⁸O and δ¹³C. Carbonate host-rocks have a mean δ¹⁸OV-SMOW value of +22.2 ± 3‰ (1σ), and both the Cretaceous veins and Eocene veins have δ¹⁸O ranging from values similar to those of the host-rocks to as low as +5 to +6‰. The variation in vein δ¹³CV-PDB of −1 to approximately +6‰ is attributed to original stratigraphic variation and C isotope exchange with hydrocarbons. Using the estimated temperature ranges for vein formation, fluid (as H2O) δ¹⁸O calculated from Cretaceous vein compositions for the Tendoy and Four Eyes Canyon thrust sheets are −18.5 to −12.5‰. For the Eocene veins within the Four Eyes Canyon thrust sheet, calculated H2O δ¹⁸O values are −16.3 to −13.5‰.
