Abstract

Fluid migration during various stages of deformation in the Absaroka sheet of the western overthrust belt (OTB) has been evaluated through a detailed structural and geochemical study of the Madison Group (Mississippian). In traveling over a major ramp in the Absaroka thrust fault east of Afton, Wyoming, this carbonate unit has deformed by: 1) early, bedding and transport normal (N-S) stylolitization and later fracturing (Group I) parallel to these cleavage surfaces; 2) bedding normal, transport parallel (E-W) fracturing throughout the history of ramping (Group II); and 3) late, transport parallel (E-W) bedding plane slip and veining (Group III). Surfaces associated with 1) through 3) are commonly filled by calcite and rarely by dolomite or quartz. All veins were produced before or during motion of the Absaroka thrust sheet.

In detail each major vein set preserves a record of intermittent veining or slip fiber growth. The oldest veins are dully luminescent and are cut by more luminescent, zoned veins. Group III veins, which often occur as coarse, calcite fibers oriented parallel to transport, commonly display two stages of slip mineralization.

Group I and II veins exhibit a range of oxygen compositions (-10.0 to -18.0 δ¹⁸0) that are strongly depleted relative to host carbonate (δ¹⁸O = -2.0 to -4.0). Calcite in Group I and II veins has a carbon composition similar to that of surrounding host rock (δ¹³C = +3.0 to +6.0). In contrast, bedding parallel veins (Group III) display small isotopic variation and are indistinguishable from surrounding host rock.

Based on several lines of evidence, Group I and II veins are interpreted to record the changing composition of fluids associated with early stages of thrusting. Group III vein calcite was derived almost exclusively from pressure solution and granulation of adjacent host rock. Because bedding parallel veins appear to have formed at a later stage in the deformation history of the Absaroka sheet, the difference in isotopic compositions between calcite in bedding normal (Groups I and II) and in Group III veins may also reflect cooling of vein fluids and equilibration with the Madison reservoir as thrusting progressed to completion.